first steps to limit NOx emission were introduced in the industry (Figure 3). ..... An incidents like "Volkswagen emissions scandal" increase the pressure.
WARSAW UNIVERSITY OF TECHNOLOGY Faculty of Materials Science and Engineering
Ph.D. THESIS Przemysław Dominik GACIA, M.Sc. Eng.
Development of single-step and low-temperature method of CuO nanopowders synthesis and their catalytic properties
Supervisor Professor Anna Boczkowska, Ph.D., D.Sc. Co-supervisor Katsuhiko Ariga, Ph.D
Warsaw, 2016
1
2
Acknowledgement I would like to express gratitude to the many people involved in various stages of development of this doctoral dissertation. Special gratitude I would like to refer to Anna BOCZKOWSKA (PW, WIM, Poland) and Katsuhiko ARIGA (NIMS, Japan) for their scientific supervision, as well as Noelia SANCHEZ-BALLESTER (NIMS, Japan), Gaulthier RYDZEK (NIMS, Japan) and Lok Kumar SHRESTHA (NIMS, Japan) for their everyday help and support. I would like to thank Hideki ABE (NIMS, Japan) and his team members, Tsubasa IMAI (NIMS, Japan) and Keiko MISHIMA (NIMS, Japan) for their knowledge, technical advice and help with measurements in the field of catalysis. Amir PAKDEL (NIMS, Japan), Qingmin JI (NIMS, Japan), Iwona CIEŚLIK (WAT, Poland) and Jonathan HILL (NIMS, Japan) I would like to thank for their invaluable advices and kindness on many relevant filed. And last but not least, I would like to thanks members of NIMS MANA Technical Support Team Makito NAKATSU, Kiyotaka IIYAMA and Isamu YAMADA for their professionalism, precision and patience to my mistakes.
I would like to thank also the institutions without whose involvement in this work would never come into being. National Institute for Materials Science (NIMS), Japan and Warsaw University of Technology, Poland for an internship award, as well as World Premier International Research Center Initiative (WPI Initiative), MEXT, Japan for covering part costs of used chemical supplies.
3
4
Abstract Air pollution is one of the most severe and discussed problem these days, that will only expand with the development of the world economy. The fight against air pollution is now severely limited by the costs associated with the use of expensive and rare metals such as platinum, ruthenium, and the rare earth elements. Therefore, important direction of research is the search for low-cost alternatives for this type of application. One of the potential candidates for this role is CuO. It is cheap, widely available and studied over 50 years. Application of CuO as catalyst could bring huge benefits and allow the introduction of catalytic converters into low-cost vehicles produced in developing countries. The aim of this doctoral dissertation was to develop heterogenic CuO catalyst with high specific surface area in single-step, low-temperature synthesis method. Such CuO nanopowder was expected to possess good catalytic performance in the lower temperature range, corresponding with the conditions of low power or high efficient engines. Detailed characterization of the structure and properties of CuO nanopowder was conducted in order to obtain a complete view of the material before and after catalytic tests. Morphology was monitored with use of scanning electron microscopy (SEM). Structural characterization was obtained by using powder X-ray diffraction (XRD) and transmission electron microscopy (TEM). Nitrogen adsorption/desorption measurements were performed to determine the surface areas. Temperature programed reduction (TPR) was used to determine the chemical properties of the material. The NO-remediation reaction was accomplished over the catalyst in a circulating-gas reactor equipped with a gas chromatograph at 100 and 150 [°C] in order to test catalytic properties, including selectively. Extensive research resulted in developing reaction with temperature dependent morphology of final product. Developed synthesis method was based on assumptions of green chemistry by applying hydrothermal methods with copper (II) acetate Cu(CH3COO)2·H2O and 2-piperidinemethanol (2PPM) as starting materials. Reaction products at low-temperature range are monoclinic CuO nanostructures with dendritic morphology, short nanorod-like substructures and exhibited large specific surface area up to 179 [m2/g] (for material synthesised at room temperature). With increasing reaction temperature dendritic morphology was gradually replaced by the cubic morphology. NO-to-N2 conversion conducted at temperature of 150 [°C] showed that CuO nanopowder synthesised at 50 [°C] catalyst exhibits high selectivity with over 80 [%] efficiency.
5
Streszczenie Zanieczyszczenie powietrza jest obecnie jednym z najpoważniejszych i szeroko omawianych problemów, który będzie się wzrastał wraz z rozwojem światowej gospodarki. Obecnie, walka z zanieczyszczeniem powietrza jest ograniczona z powodu wysokich kosztów związanych ze stosowaniem drogich i rzadkich metali takich jak platyna, ruten i pierwiastków ziem rzadkich. Dlatego ważne są badania nad poszukiwaniem alternatywnych, tanich zamienników dla tego typu aplikacji. Jednym z potencjalnych kandydatów jest CuO. Jest tani, powszechnie dostępny i badany od ponad 50 lat. Zastosowanie CuO jako katalizatora może przynieść ogromne korzyści i pozwoliłoby na wprowadzenie katalizatorów w tanich pojazdach produkowanych w krajach rozwijających się. Celem pracy doktorskiej było opracowanie heterogenicznego katalizatora na bazie CuO, o dużej powierzchni właściwej, z wykorzystaniem jednostopniowej, niskotemperaturowej, syntezy. Oczekiwano dobrych właściwości katalitycznych nanoproszku CuO w zakresie niskich temperatur, w odniesieniu do silników niskiej mocy lub dużej efektywności. Przeprowadzono szczegółową charakteryzację struktury i właściwości nanoproszku CuO przed i po próbach katalitycznych. Morfologię zbadano z użyciem skaningowego mikroskopu elektronowego (SEM). Charakteryzację strukturalną przeprowadzono stosując dyfrakcję rentgenowską (XRD) oraz transmisyjną mikroskopię elektronową (TEM). Pomiary stopnia rozwinięcia powierzchni wykonano przez adsorpcję/desorpcję azotu (BET). Redukcję programowaną temperaturą (TPR) użyto w celu określenia właściwości chemicznych materiału. Reakcję remediacji NO z wykorzystaniem badanego katalizatora prowadzono w reaktorze obiegowym, wyposażonym w chromatograf gazowy w temperaturze 100 oraz 150 [°C] w celu zbadania właściwości katalitycznych oraz selektywności katalizatora. Obszerne badania doprowadziły do opracowania reakcji, w której morfologia produktu zależna była od temperatury syntezy. Opracowany sposób syntezy wykorzystywał założenia „zielonej chemii”, metodę hydrotermalną, w której jako substraty zastosowano octan miedzi (II), Cu(CH3COO)2·H2O i 2-piperydynometanol (2PPM). Produktem reakcji w niskich temperaturach były cząstki CuO zbudowane z dendrytów o powierzchni właściwej do 179 [m2/g] (w przypadku materiału syntetyzowanego w temperaturze pokojowej). Wraz ze wzrostem temperatury reakcji, morfologia dendrytyczna była stopniowo zastępowana przez struktury sześcienne. Test konwersji NO do N2 prowadzony w temperaturze 150 [°C] wykazał, że nanoproszek CuO syntetyzowany w 50 [°C], wykazuje wysoką selektywność jako katalizator z wydajnością ponad 80 [%]. 6
Table of Content Acknowledgement ...................................................................................................................... 3 Abstract ...................................................................................................................................... 5 Streszczenie ................................................................................................................................ 6 Table of Content ......................................................................................................................... 7 1.
Introduction ......................................................................................................................... 9
2.
NOx pollution .................................................................................................................... 10
3.
2.1.
Scale of pollution problem ........................................................................................ 13
2.2.
Impact of pollution on environment .......................................................................... 18
2.3.
Impact of NOx on humans health .............................................................................. 20
2.4.
Impact air pollution on industry and politics ............................................................. 26
Currently used catalyst solutions ...................................................................................... 30 3.1.
The economic justification for the research of alternative catalytic materials .......... 32
3.2.
Overview of research on alternative materials for currently used catalysts .............. 35
4.
Features that the best catalysts should have ...................................................................... 40
5.
Introduction to NO - CO remediation ............................................................................... 43
6.
CuO as catalytic material .................................................................................................. 46
7.
Motivation and the aim of the studies ............................................................................... 55
8.
Materials and methods ...................................................................................................... 57 8.1.
The synthesis of the catalyst ...................................................................................... 57
8.1.1.
Raw materials ..................................................................................................... 57
8.1.2.
Experimental procedure ..................................................................................... 58
8.2.
Characterisation methods .......................................................................................... 59
8.2.1.
Scanning Electron Microscopy (SEM) .............................................................. 59
8.2.2.
High-Resolution Transmission Electron Microscopy (HRTEM) ...................... 60
8.2.3.
Energy-dispersive X-ray spectroscopy (EDX) ................................................... 60
8.2.4.
X-Ray Diffraction (XRD) .................................................................................. 61 7
9.
8.2.5.
Thermogravimetric Analysis (TGA) .................................................................. 61
8.2.6.
Brunauer, Emmett and Teller (BET) .................................................................. 62
8.2.7.
Dynamic Light Scattering (DLS) and Zeta-potential ......................................... 64
8.2.8.
Temperature-Programmed Reduction (TPR) ..................................................... 65
8.2.9.
NO-remediation catalysis test ............................................................................ 66
Results & discussion ......................................................................................................... 68 9.1.
Selection of starting materials and synthesis conditions ........................................... 68
9.1.1. 9.2.
Description of selected synthesis method .......................................................... 71
Characterisation of temperature controlled synthesis products ................................. 72
9.2.1.
The morphology of as prepared CuO ................................................................. 72
9.2.2.
Chemical composition of as prepared CuO ........................................................ 81
9.2.3.
Specific surface area (SSA) of as prepared CuO ............................................... 82
9.3.
Selection of potential catalyst .................................................................................... 85
9.4.
Characterisation of selected material before catalytic test ........................................ 86
9.5.
Characterisation of the material after catalytic test ................................................... 93
9.5.1.
The results of catalytic performance of the material .......................................... 93
9.5.2.
The changes in morphology of the material ....................................................... 95
9.5.3.
The changes in chemical composition of CuO................................................... 96
9.6. 10.
The results in the context of the physical chemistry of CuO surfaces ...................... 98 Conclusions and final remarks .................................................................................... 102
References .............................................................................................................................. 105 List of Figures ........................................................................................................................ 120 List of Tables .......................................................................................................................... 122
8
1. Introduction In the past few years, nanotechnology has expanded out of the chemistry department into the fields of medicine, energy, aerospace, even to computing and information technology. In the case of nanomaterials, number of atoms on the surface area can be even larger than in relation to the number of atoms in the bulk. When the particles are only 1 to 100 [nm] across, different properties begin to arise, making nanoparticles superior to their bulk counterparts. Potential advantage properties of nanostructures caused fast expansion of nanotechnology observed in past two decades. In the field of air pollution prevention, nanotechnological approach focuses on developing potentially new catalysts. Materials that do not originally exhibit catalytic properties in the nano-scale can often compete with expensive and well established materials such as Ruthenium and Platinum, while being from them several orders of magnitude cheaper. One of such materials is copper (II) oxide (CuO) [1,2]. CuO is good oxygen storage with band gap size dependent on the crystallographic orientation [3]. In effect, CuO is one of the most prominent catalysts. Its specific capacity for oxidation reactions of CO makes it suitable substitute of noble metals, extensively used in environmental catalysis [4]. Its additional advantages such as low price, high availability and ease of chemical modification well compensate its shortcomings. Lower than the noble metal chemical activity can be easily increased by development of its specific surface area. Many years of research on CuO nanostructures gave solid evidence of higher catalytic activity than CuO bulk or micro counterparts [5]. Despite all its advantages, CuO has not been widely applied in automotive catalytic conversion. An obstacle to this is lack of proper selectivity of developed CuO catalysts. Contained in the exhaust gas chemical compounds creates complex mixture of oxides on various oxidation stage. In particular two gases, NO and CO are most problematic. Their mutual neutralization is possible by a process known as remediation. However, there are other paths that kinetic of the reaction may follow. Application of CuO as catalyst could bring huge savings for the automotive industry. More than that, it would allow the introduction of catalytic converters in low-cost vehicles produced in developing countries. Nevertheless, it creates many challenges which must be overcome in order to obtain that goal. The most important of them is the production of stable, selective catalyst for NO-CO remediation. It is also essential that production method was simple and scalable, so that it could be easily introduced into industrial practice. Right here is the largest 9
challenge, since nanostructures are usually synthesized with the use of sophisticated technologies in very complex processes.
2. NOx pollution The NOx is general term for mono-nitrogen oxides. Nitric oxide (NO) and nitrogen dioxide (NO2) are considered as solely industrial pollutant since their main natural sources include bacterial and volcanic action, intrusion of stratospheric nitrogen oxides [6], and extreme heat released by lightning during thunderstorms [7]. Man-made NOx are produced from the reaction of nitrogen and oxygen gases during combustion. It occurs in the presence of nitrogen, for example in the case of nitrogen-bearing fuels such as coals and oil or in air-breathing engines. In high temperature the nitrogen bound in the fuel is released as a free radical and ultimately forms N2, or NOx [8]. The process which leads to the formation of those compounds was first studied and described by soviet physicist Y.B. Zeldovich in 1946 [9,10]. Zeldovich showed chemical pathway for nitric oxide formation in engine exhaust, during combustion at high temperature and pressure (Equations 1 and 2). To this day Zeldovich mechanism dominates NO formation under most engine conditions. However, in later years Lavoie, Heywood and Keck [11] extended his work by reaction with the hydroxyl radical (-HO). The hydroxyl radical mechanism is responsible for the formation of NO in the presence of hydrocarbons (Equation 3). 𝑁2 + 𝑂 → 𝑁𝑂 + 𝑂
(1)
𝑁 + 𝑂2 → 𝑁𝑂 + 𝑂
(2)
𝑁 + 𝑂𝐻 → 𝑁𝑂 + 𝐻
(3)
Nitrogen dioxide typically arises via further oxidation of NO [12] illustrated by equation 4. 2𝑁𝑂 + 2𝑂2 → 2𝑁𝑂2
(4)
The relative balance between all mechanisms makes impossible to eliminate the NO x compounds from exhaust gases.
10
Table 1. Selected properties of COx and NOx chemicals [13–17]. IUPAC1 name carbon monoxide nitric oxide carbon dioxide Formula
CO
NO
CO2
nitrogen dioxide NO2
Structural formula
Physiochemical properties 28.0101
30.0061
44.0095
46.0055
gas
gas
gas
gas
0.00115
0.00123
0.00184
0.00188
Melting point [°C]
-205.0
-163.6
-56.6
-11
Boiling point [°C]
-191.5
-151.7
-78.5
21
Molar mass [g/mol] Phase (at STP2) Density (at STP) [g/cm3]
Thermochemistry Specific heat capacity (C) [J·mol−1·K−1]
29.1
29.9
37.1
37.5
197.7
210.8
214
240
−110.5
91.29
−393.5
34
Standard molar entropy (So298) [J·mol−1·K−1] Standard enthalpy of formation (ΔfHo298) [kJ·mol−1]
1
IUPAC - International Union of Pure and Applied Chemistry. In chemical nomenclature, a preferred IUPAC name (PIN) is a unique name, assigned to a chemical substance and preferred among the possible names generated by IUPAC nomenclature. 2 STP - Standard Temperature and Pressure.
11
Toxicity properties Odour
odourless
acrid odour
odourless
acrid odour
100
45
27000
9
10
30
9000
5
Short-term exposure limit [mg/m3] Long-term exposure limit 3
[mg/m ] 3
RTECS classes
mutagen, reproductive effector, human data
mutagen
reproductive effector, human data
mutagen, reproductive effector, human data
Industrial labelling NFPA 7044
United
Oxidizing
Corrosive
Non-flammable gas
Non-flammable gas
Flammable
Nation (UN) & Europe Union (EU) classification
Hazardous to health
(GHS5)
Non-flammable gas Corrosive
Hazardous to health
Toxic
Toxic
Toxic
3
RTECS - Registry of Toxic Effects of Chemical Substances (RTECS) is a database of toxicity information compiled from the open scientific literature. 4 NFPA - National Fire Protection Association, this standard presents a simple, readily recognized, and easily understood system of markings of hasards. 5 GHS - Globally Harmonized System of Classification and Labelling of Chemicals
12
2.1. Scale of pollution problem Lifetime of NOx gases is short. It depends on height within the troposphere and time of the day. Generally, NOx lifetime varies from 1 day or less near ground level up to 5÷10 days in the upper troposphere [18]. Because of it, its level is localised relatively near of its source, and can be directly attributed to it. As it is presented in Figure 1, elevated and high NO2 levels are concentrated around huge human settlements area. The seasonal changes are caused by fissile fuels combustion processes which are major anthropogenic sources of NOx emissions. All those changes are directly linked to mankind energy demand [19,20].
Southeast Asia
NO2 in July 2015
NO2 in November 2014
Europe
Figure 1. NO2 tropospheric column over Europe and Southeast Asia. Satellite data obtained by Ozone Monitoring Instrument on Aura Satellite (Aura - OMI) by KNMI/NASA. Visualization with ESA TEMIS v2 [21,22].
Principal sources of NOx By industry, the largest contributors to NOx emissions are heating, power generation, and engines in vehicles and ships [23–25]. In all of those branches, the predominant fuels are oil and coal. According to European Environment Agency (EEA), for EU countries in 2011,
13
combustion of oil contribution in over exceeds 40 [%] of total emissions and over 55 [%] when combusting coal [26].
Commercial, institutional and households 12.80%
Energy use in industry 12.60% off-road transport 7.30%
Energy production and distribution 22.50%
Industrial processes 2.40% others 4.30%
Road transport 40.50%
Agriculture 1.80% Waste 0.10%
Figure 2. Sector share of nitrogen oxides emissions (EEA member countries) (2011) [26].
Currently, road transport is dominating sector responsible for 47 [%] of the total NOx emissions in 2011 [26]. However, overall emission has been falling down since 1979 when first steps to limit NOx emission were introduced in the industry (Figure 3). In the case of most European countries, emission from road transportation starts to drop after 1990 [25]. The greatest progress in reducing NOx level has been made in the last two decades. Only in years 2002-2011, total EU emissions felt by 27 [%]. In the case of Poland, annual emission in 2010 dropped to 23 [kg/capita] (-33 [%] change when compare to average from 1990-2010). For comparison, Japan emission for 2010 was 12 [kg/capita] (-16% change when compare to average from 1990-2010) [27]. Nonetheless, EU as a whole emitted in 2011 about 5% more NOx when compare to the emission ceiling set in the NEC Directive for 2010 [28,29]. Still, in global scale the NOx level rise from year to year. It happens mainly due to developing countries where air quality regulation does not exist or are ignored. As shown in Figure 4, an average annual NO2 level had dropped in almost all of Europe between 1996 and 2002. Mostly in North America, central and south Japan. Meanwhile, in rest of Asia and Sought Africa, average annual NO2 level had raised. The most dramatic situation is observed in
14
North-East China where in 2010 an annual NOx emission exceeded 26 [Mt] and further increase is expected [30].
3500
NOx/yr [kton] or [Gg]
3000 2500 2000
Poland Japan
1500
Germany Sweden
1000 500 0 1970
1975
1980
1985
1990 Year
1995
2000
2005
Figure 3. Selected national NOx emission from 1970 to 2008 [25].
Figure 4. Average annual NO2 changes 1996-2002 (ESA) [31].
Mentioned data and statistics do not include international sources of pollution such as cargo transport and people transit. Cargo ships and airplanes consume enormous amount of energy. In the case of shipping, due to highly concentrated emission sources elevated NOx levels mark 15
shipping lanes all over the world [32]. Several major international shipping lanes are clearly visible on Indian Ocean and South China Sea (Figure 5). This state is likely to continue in the future, as international trade increases from year to year (Figure 6).
Figure 5. NO2 tropospheric column over Ocean, averaged over 2005 - 2012. Satellite data obtained by Ozone Monitoring Instrument on Aura Satellite (Aura - OMI) by KNMI/NASA [33].
16000
NOx/yr [kton] or [Gg]
14000 12000 10000 8000
Int. Shipping Int. Aviation
6000 4000 2000 0 1970
1975
1980
1985
1990 Year
1995
2000
2005
Figure 6. Selected international NOx emission from 1970 to 2008 [25].
16
Prognosis of pollution problem growth According to OECD predictions, NOx emission is expected to continue to fall for OECD countries in the coming decades (Figure 7). A downward trend will be supported by international and national policies and regulation, as well as new technologies implementations [34]. In the case of Brazil, Russia, India, Indonesia, China and South Africa (the BRIICS) and rest of the world group (RoW), for the key emerging economies, 50 [%] rise of NOx emission levels in 2050 are likely to occur in the coming decades. For the BRIICS, emission growth will be driven by a growth in economic activity, especially in the energy sector. In the next two decades, NOx emission level is expected to slowly stabilise and stay around one and a half times of the 2000 emission level by 2050. Its reckon to increasing uptake of cleaner fuels and combustion technologies driven by rising income levels [34]. In the RoW group, a significant increase of NOx level is expected due to the increase of urban population and the consequent increase in demand for energy. The rise is unlikely to level off by 2050 (Figure 7) [34].
Emissions (index 2010 = 100%)
140 120 100 OECD
80
BRIICS 60
RoW
40 20 0 2000
2010
2020
2030 Year
2040
2050
2060
Figure 7. Prediction of baseline emissions NOx by region for 2010-2050 [35].
17
2.2. Impact of pollution on environment In natural conditions, NOx is present in atmosphere in very low concentration. Anthropogenic emission intensifies naturally occurring in the atmosphere chain reactions what have severe consequences for both nature and humankind. Acid rain One of the first spotted results was acid rain, which includes a mixture of nitric acid (HNO3), sulfuric acid (H2SO4) and carbonic acid (H2CO3). First connected with NOx and SO2 emission in mid-1980s, far greater then natural quantities of acidic compounds cause severe damages to environment and manmade structures. Natural rainfall is characterized by pH 5.6. Acidity below pH 4.4 is lethal to most fish [36]. Meanwhile, annual average pH of European precipitation ranges between 4.1 and 4.9 [37]. Rainfall at Mount Mitchell, NC (July 1986) had pH of 2.2. In 1982, the pH of a fog on the West Coast of the United States was measured at 1.8pH [36]. Problems ranging from killing freshwater fish, damaging crops, acidification of farmlands and forest vegetation [36,38,39] to eroding buildings and monuments [40]. However, sulfuric acid forms from gases released during volcanic eruptions and carbonic acid formation is part of carbon cycle. Nitric acid formation can be almost totally attributed to humankind activities. Two most important way of nitric acid formation are hydroxyl radical (OH-) termination reaction (Equation 5) and NO2 hydrolysis (Equation 6) [18,41,42]. The most dangerous is NO2 hydrolysis, since due to NO natural self-oxidation (Equation 4) it creates a closed cycle. 𝑁𝑂2 + 𝑂𝐻 − + 𝑀 → 𝐻𝑁𝑂3 + 𝑀
(5)
3𝑁𝑂2 + 𝐻2 𝑂 → 2𝐻𝑁𝑂3 + 𝑁𝑂
(6)
Ozone NOx presence is crucial for ground level or tropospheric ozone formation. NO2 under solar radiation is forced to release O(3P), highly reactive form of atomic oxygen radical [43]. Free oxygen radical reacts with molecular oxygen (O2). In effect tropospheric ozone (O3) is formed (Equations 7 and 8) [44–46].
18
𝑁𝑂2 + ℎ𝑣 → 𝑁𝑂 + 𝑂(3 𝑃)
(7)
𝑂(3 𝑃) + 𝑂2 → 𝑂3
(8)
Photochemical Smog Additionally, when both volatile organic compounds (VOCs) and nitrogen oxides are present in the air, react to sunlight and create photochemical smog. A mixture of pollutants creating a brown haze above cities [47,48]. Since reaction between precursor gases and sunlight tends to occur near the source and more often in summer when we have the most sunlight. In effect, it causes deterioration of air quality in already polluted areas [49]. Notably, in places there precursor gases pollutes the air mainly as a result of road traffic and energy production with use of old combustion technologies [48]. According to World Health Organization (WHO), only 12 [%] of the people living in cities where air quality is reported, and about half of the monitored urban population is exposed to at least 2.5 times higher air pollution levels then WHO recommendation. From standpoint of WHO statistic, India, Pakistan and Iran are locations some of the world's most polluted cities. The top four are located in India [50]. The annual mean of PM2.56 for Delhi was 153 [μg/m3] in 2013. For comparison, for famous from its smog Beijing (China), PM2.5 of 56 [μg/m 3] was reported in 2010. The most polluted air in China where in Lanzhou with PM2.5 of 71 [μg/m3]. In the case of countries with applied green policies and widespread advanced technologies the situation is much better [34]. For example, the most polluted air in Japan is in Urawa-ku (Saitama prefecture) with annual mean of PM2.5 of 14 [μg/m3]. In continental scale, situation in European countries is much better (Figure 1). In Warsaw in 2011, annual mean of PM2.5 was equal 26 [μg/m3]. Cracow and Zabrze registered PM2.5 of 40 [μg/m3]. The same time in Berlin PM2.5 level was 20 [μg/m3]. In year 2008 PM2.5 levels were respectively: Berlin 20.8 [μg/m3]; Cracow 35.5 [μg/m3] and Zabrze 40.4 [μg/m3] [51].
6
PM2.5 – Particulate matter with diameter of 2.5 [μm] or less.
19
2.3. Impact of NOx on humans health NOx gases are considered as pollution since both NO and NO2 are formed simultaneously during the same process. But, NO and NO2 have different impact on human health. In case of Nitric oxide (NO), blended with oxygen it was used as medicine in critical care to promote capillary and pulmonary dilation for neonatal patients with primary pulmonary hypertension [52–54]. Nitric oxide is currently not used out of neonates cases where it has not proper replacement. In case of adult patients with acute respiratory distress syndrome, acute lung injury, and severe pulmonary hypertension NO can improve hypoxemia. However, the effects are temporal and there is lack of research which could demonstrate improved clinical outcomes [55]. Direct influence of NO2 as a single agent is not considered as significant. However, studies conducted on human populations point out noticeable influence on the occurrence of health problems. Symptoms such as decrease lung function and increase risk of respiratory problems like acute bronchitis and cough and phlegm occurs particularly in children or people with preexisting lung disease [56–58]. Effects related to short-term exposure are disruptions of pulmonary function especially of people with asthmatics problems. Increase of hospital admissions, airway allergic inflammatory reactions and mortality rate [57]. It should be mentioned that in experiments on healthy humans without any pulmonary problems, symptoms have been provoked when healthy subjects were exposed to 4.0 ppm of NO2 concentration. While, the asthmatics did not show symptoms exposed to the same concentration [59]. In the same time, even without allergens in air, exposed asthmatic volunteers showed increase airway eosinophilic inflammation7 associated with NO2 dose-related manner [60]. Nevertheless, overall experimental evidence indicates that high concentration of NO2 increases bronchial responsiveness to inhaled allergens. Only 30 minutes of exposure to 500– 750 [µg/m3] concentrated NO2 was enough to show increase of airway allergic inflammation and sensitivity to allergen exposure for patients with mild asthma or allergic rhinitis [57,61– 64].
7
According to Wikipedia "Eosinophilic esophagitis (eosinophilic oesophagitis), also known as allergic oesophagitis, is an allergic inflammatory condition of the esophagus that involves eosinophils, a type of white blood cell. Symptoms are swallowing difficulty, food impaction, and heartburn." [379]
20
Additionally, studies considered by the WHO on experimental animals have shown a reduction in the daily production of sperm and general increases of lung susceptibility to bacterial infections [57,65]. However, it must be pointed that reported results showed a stronger response of experimental animals to NO2 than humans in general. But this can be attributed to concern for the life and health of the human test subjects. On the whole, according to clinical research and epidemiological studies there is no evidence for a threshold level of exposure to NO2 below which no effects on human health are expected [57,66]. Simultaneously it should be considered that nitrogen oxides strongly interact with other air pollutants. As mentioned earlier, nitrogen oxides in ambient air are involved in a series of reactions leading to the formation of complex mixture of much more dangerous for human health air pollutants [18]. Ozone (O3) is one of the most reactive forms of oxygen. Same as superoxide, singlet oxygen, hydrogen peroxide, and hypochlorite ions, ozone is a free radical that occurs in both natural and artificial systems and reacts directly with organic double bonds [67]. Mentioned direct reaction with carbon-carbon double bonds is directly responsible for ozone irritant properties for cells. In case of single cell organism, it results in anti-bacterial properties. However, in case of human body can cause temporal affection of the lungs, the respiratory tract, and the eyes. Short-term exposure to ozone had been related to adverse effects on pulmonary function and respiratory symptoms. This is due reaction of ozone with compounds lining the lungs. Products of that reaction are specific form of cholesterol-derived metabolites that are thought to facilitate by host. They build-up in the body and cause pathogenesis of atherosclerotic plaques. It has been proven that these metabolites naturally occurring in human body and are products of ozonolysis of cholesterol's double bond and aldolization [68,69]. Exposure to ground level ozone had been related to lung inflammatory reactions, increase risk of death from respiratory illness, especially for children [58]. It can cause increase in medication usage, hospital admissions and mortality rate [56]. Long-term exposure had been related to reduction in lung function development [56] and neural system degeneration [70]. As for particulate matter (PM), they complex formation process and a wide range of sources (both biological how anthropogenic) lead to a number of dangerous interactions. We recognize two types of particles separated according to size. Particles with a diameter of 10 [μm] or less (PM10) and particles with a diameter of 2.5 [μm] or less (PM2.5). Applied 21
distinction between them is justified by the different way the human body reacts to different sizes of pollution. In general, particles bigger than 10 [μm] are removed from body system by natural processes relatively easily. Below that value, around 75 [%] of particles 5 [μm] in size are deposited on head. Rest of body system is relatively well protected up to 2.5 [μm] (PM2.5) when tissues lose their ability to manage these size contaminants. According to W.G. Kreyling et al. around half of particles bellow 0.025 [μm] in diameter stays in lungs alveoli and around 35 [%] of 0.005 [μm] in bronchi [71]. All the rest is transferred to bloodstream and is deposited in soft body tissues. PM10 and
PM2.5 were designated by WHO and IARC 8 a Group 1 carcinogen and are
considered the deadliest form of air pollution [72,73]. In short-term, particle-tissue interaction is resulting in inflammatory type reaction. However in long-term, due to the fact that particles penetrate deep into tissues unfiltered, different kinds of particles have been shown to induce permanent DNA damage what may result in cancer if selective apoptosis of the damaged cells fails [74]. According to study from 2013 which involved 312 944 people from nine European countries showed 22 [%] increase of lung cancer rate for every additional 10 [μg/m 3] of PM10 particles in the air [75]. What is more, PM2.5 proved to be particularly deadly, with a 36 [%] increase per 10 [μg/m3]. Additionally, PM2.5 leads to creation of blood platelets deposits in arteries. The results are vascular inflammations and reduction of arteries elasticity, which can lead to heart attacks and other cardiovascular problems [76]. Research conducted in 2014, suggest a 13 [%] increase of heart attacks risk for every 5 [µg/m3] rise in estimated annual exposure to PM2.5 [77]. That leads to conclusion that there is no minimal level of PM under which this type of pollution should be considered as harmless [6,56,57,66]. Apart from how each of mentioned factors may affect our health individually, we must remember that every single minute our bodies are exposed for all of them. Human body response to synergistic exposure leads to much more severe health outcomes [78,79]. Complex interactions between body metabolism and air pollutants are analysed by searching for statistical significance in large population [80]. Such data associate elevated health problem and mortality rate with urban pollution [57,81]. This is especially important because according to WHO data in 2005 in Poland, 75.6 [%] of population was exposed to 20-40 [μg/m3] annual average concentration of PM10. For 12.2 [%]
8
International Agency for Research on Cancer (IARC) or French: Centre international de Recherche sur le Cancer (CIRC)
22
annual average were above 45 [μg/m3], mainly in big cities and industrial areas [82]. Further, the same dataset point out 36th maximum daily average value for Polish population as respectively 27.4 [%] and 21 [%] exposed to level of 50-65 [μg/m3] and above. In case of ozone, for 26th highest daily maximum 8‑hour value 72.0 [%] of Polish population was exposed to levels between 110–120 [μg/m3]. 13.5 [%] to levels between 120–140 [μg/m3] respectively [82]. At this point, it should be recalled that NO2 is the main source of nitrate aerosols. An important fraction of PM2.5 formed in the presence of ultraviolet light, of ozone. The significant role played by NOx in those pollutants formation caused that nitrogen oxides become a major target in the struggles for air quality improving. According to EEA, between year 2010 and 2012 in Poland over 1 [%] of population was exposed to NO2 levels over EU reference value of 40 [μg/m3] over the year [83]. In scale of Europe, that is relatively good result. However, as is shown in Figure 8 and Figure 9, the areas with NO2 level that threatening human health are mostly intensively urbanized areas or using outdated industrial installations. By close comparison of both maps, the expansion of the areas with the most harmful concentration is clearly visible. Also in the case of Poland where industrial point sources had the biggest (up to 70–80 [%]) share in emission [84]. During 2005 in Europe approximately 370,000 premature deaths were related to ambient air pollution [82]. In 2012, that number was estimated at 524,000. Around 47,300 premature deaths in that number happened in Poland [85]. S. S. Lim et. al. analised data colected by two dacades between 1990-2010 and estimated number of deaths due to air pollution at 3.22 million global in 2010 only [86].
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Figure 8. Nitrogen dioxide 2010 - Annual limit values for the protection of human health, European Environment Agency (EEA), 2012 [87].
Figure 9. Nitrogen dioxide 2012 - Annual limit values for the protection of human health, European Environment Agency (EEA), 2014 [88].
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Guideline values Air pollution from its very nature cannot be enclosure in a particular area or territory. Wind, seasonal changes in weather, long-distance movements of air masses, all these factors cause the spread of pollutants. This necessitates the use of complex, extensive solutions. Hence, it was necessary to adopt standards for assessing the effectiveness of counter measures that were taken [23,47]. Since complete removal of pollutants from the air is impossible. The activities of international organizations, especially the WHO and the European Union (EU), were aimed at limiting the risks inherent from exposure to pollution. The WHO in its studies determine the acceptable level of individual factor exposure. Despite the widespread lack of complete agreement as to the interpretation of the term "acceptable risk level", indicated values are understood as a target which, if achieved, significantly reduce the risks for acute and chronic health effects from air pollution can be expected [56,89,90]. Thus some health effects may occur below that level [57]. For each agent there are two types of exposure. Annual means and 1-hour means exposure. The first value allows to determine risk level in long-term perspective. The second value determinates the scale of incidental phenomena such as lack of airflow in the area for a long time. Usually, in such cases it comes to scale intensification of health problems that already occurring in the population [20,56,85]. This refers to the increased number of patients admitted to hospitals and health deterioration of the general population in a given period of time [91,92]. WHO periodically publishes guideline values of acceptable levels for each pollutant with proven negative impact on human health. These values are updated every time when new scientific studies results allow a better determination of the true scale of the threat. Current recommendations for long and short-term exposure with clarification of the time scale for each case are presented in Table 2. Table 2. Ambient air quality recommendation according to WHO [93]. Type of NO2 O3 PM2.5 PM10 exposure Long-term [μg/m3] Short-term [μg/m3]
40 annual mean
-
SO2
10
20
20
annual mean
annual mean
24-hour mean
200
100
25
50
500
1-hour mean
8-hour mean
24-hour mean
24-hour mean
10-min. mean
25
WHO recommendations are not obligatory on UN members. However, they are generally taken into account when developing national standards. The various permissible levels are adjusted in relation to the economic reality prevailing in specific country. In this way for example, in the case of China there are three permissible levels of pollutants. Therefore, the highest standard (China I) is in line with the recommendations of the WHO, where the most lax standard (China III) values represent about five times of that level [94]. In the case of the EU, where the accepted standards are much more unified, they have been adapted to the reality of all member states [95]. The values have been collected and presented in the Table 3. Importantly, these standards represent legal regulations. As well as, most of them are also consistent with WHO guidelines. Table 3. Ambient air quality standards in EU [95]. Type of NO2 O3 exposure Long-term [μg/m3] Short-term [μg/m ] 3
40 annual mean
-
200
120
1-hour mean
8-hour mean
PM2.5
PM10
SO2
25
40
125
annual mean
annual mean
24-hour mean
50
350
24-hour mean
1-hour mean
-
2.4. Impact air pollution on industry and politics Public awareness of the problem of air pollution dates back to Victorian times in England where the problem was widespread smog [96,97]. But the source of the problem was domestic furnace, also there was lack of alternatives that may provide a solution. In 1950 A. J. HaagenSmit published his finding about nature of Smog in Los Angeles [98,99]. It was a breakthrough because until then, the science did not understand why smog occurs in a place where coal was not used to heat spaces. E.J. Houdry, a French mechanical engineer involved with catalytic oil refining inspired by A. J. Haagen-Smit's works developed the first catalytic converter for which he received a patent in 1956 [100,101]. With the beginning of the 60's, Moyer D. Thomas issued several reports on the effects of air pollution on plants [102,103]. These works drew the attention of WHO. In the early 70's, air pollution problem was already quite well understood and accepted by industry and scientific community [104]. This brief historical introduction was necessary to introduce case of human lead poisoning, caused by tetraethyllead added to gasoline to reduce knocking in early piston engines. In 1963, C. C. Patterson published his data, which opposed to lob by industry "natural level of 26
lead in the environment" theory. That started public debate on the ban of lead gasoline [105]. Ultimately, it NO2 air pollution led to the prohibition of the practice. In 1973 R.C. Stempel (at the time, former CEO of General Motors) decided to begin implementation of the catalytic converter in automobiles. In year 1975, the United States government introduced regulation that forced installation of catalytic converters in cars and light-duty trucks by the manufacturers [106]. Lead is a heavy element, which is very easily deposited on almost any kind of surface. Presence of lead in exhaust gases passing through the catalytic converter causes formation of an insulating layer on active surface of the catalyst [107,108]. In the case of Polish economy which heavily based on usage of old vehicles, prohibition of leaded fuel usage was not introduced until 2005 [109]. The development of technologies applicable on an industrial scale for cars in early 70's coincided with the solutions development for power generation industry. W. C. Pfefferle developed a catalytic combustor for gas turbines. Pfefferle solved problem of Catalyst-Fuel mixture contact in high mass flow system. His solution allowed to ignite combustion process at lower temperature and with higher oxidation rate. That significantly reduced formation of nitrogen oxides and carbon monoxide while increase power output [110,111]. Because the catalytically aided process greatly increases the efficiency, it was economically justified. Thereby similar solutions rapidly spread in the industry of the highly-developed countries [111]. These early solutions led to a dramatic improvement in air quality. However, did not solve the problem. The major sources of anthropogenic emissions of NO2 are combustion processes related to heating, power generation, and engines in vehicles and ships. Therefore, as long as humanity will acquire the energy mainly from fossil fuels, it is impossible to eliminate the emission of pollutants. Currently used by industry solutions strongly rubbing against technical capabilities of economically viable solutions. Recent (September 2015) "Volkswagen emissions scandal" prove the point that low emission stays in contradiction with high performance of vehicle [112–114]. In that case, the end-user expectations about vehicle performance were different from achievable while meeting imposed by emissions standards limitations. The current and next generation of technologies for reducing emissions is too expensive to spread itself [115]. From the standpoint of market to allow for further emission reduction the drivers will have to give up the current driving style and the industry to accept much more expensive and more complex technologies. The supporting economical factor for changes are expected to be rising oil prices [115–120].
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Any changes resulting from beyond economic reasons must and will be the result of appropriate policy, legal and social arrangements. Along with spread of knowledge on the consequences of air pollution, society gets willing to take actions coordinated at national and international level. An incidents like "Volkswagen emissions scandal" increase the pressure on the political class to improve their policies in this regard [120,121]. Historical examples of such policies are the USA Federal Clean Air Act of 1970; Canadian Environmental Protection Act in 1999 or international law such as Convention on Long-Range Transboundary Air Pollution (LRTAP), Geneva from 1979. One of the most important is Kyoto Protocol from 1997, with 83 signatories and 192 parties [122]. Integral part of such policies are air quality monitoring, emission standards and control technology requirements. At the moment, worldwide is about 1,600 towns in 91 countries keep up to date report on levels of air pollution. Data collected in the years 2009-2012 recorded the 6 [%] increase of global emissions [50,123]. The only decreases of pollution in cities were recorded in Central and East Europe (-13 [%]) and high-income countries (-2 [%]). At the same time, in Africa were 26 [%] registered increase in emission. The burden of disease from ambient air pollution is expected to grow along with increasing populations of urban areas. OECD predicts that until 2025 year over 469,000 cities will accommodate more than 10 million people. By the year 2050 is expected that over 6.2 billion people will live in cities [34]. United Nations (UN), world population in 2050 is predicted to be 9.7 billion people which means that around 64 [%] will live in cities [124]. As a consequence, air pollution, which already causes problems in developing countries such as China, can only become more severe. On December 8, 2015 the Beijing authorities were forced to announce the first-ever red alert [125]. According to aqicn.org, the highest recorded air quality index (AQI)9 for PM2.5 in Beijing at that day where 400. Its equivalent of 350.4 [μg/m3], what is almost nine times of value recommended by WHO for long-term and almost double of short-term value [93]. Detailed studies conducted by OECD have estimated the so-called hidden costs of pollution. According to those studies, only in 2005 the additional medical costs of air pollution amounted to 380 million Euros. The cost of production losses were estimated at 3,060 million
9
Air Quality Index (AQI) - is a number use by government agencies in public communicates to inform about current air pollution level [380].
28
Euros, in Europe in 2005 alone [126]. Those numbers do not include WTP 10 parameters. Different study analyzed economic cost of deaths from ambient air pollution in year 2005 and 2010. Calculated losses were estimated at 1,470,487 million USD in 2005 and 1,571,170 million USD in 2010 for all OECD countries. In case of Poland those numbers were respectively 47,729 and 52,631 millions USD [127]. However, it should be mentioned that these values were based on the value of goods produced during the lost years of life. And performed calculations take into account the variation between the value of live in different countries. When we look at it from Years of life lost (YLL) perspective, these numbers were 7,940,439 [YLL] in 2005 and 7,312,212 [YLL] in 2010 for all OECD countries. For Poland, 527,605 [YLL] and 424,174 [YLL], respectively [127]. Nevertheless, from the point of view of economics, both the costs of taking action and measurable value potential benefits need to be considered. Such analyze has been carried out. S. Reis analyzed it based on studies related to ozone. In his assessment of the potential benefits can be valued at 14.3 billion Euros, while the cost of would be about 39.4 billion Euros [128]. However, it is hard to extrapolate it for full scale of problem since author was concentrated on EU-15 and costs of government policies. Similar study conducted by OECD predicted for year 2030, Benefits/Costs ratio for four scenarios in scale of all OECD countries. Predicted values were 142 for low cost program, 44 for mid cost program, 15.5 for high cost program and 3.8 for maximum technical feasible reduction [127]. In this case, however, researchers were concentrated only on health and transportation and took into consideration solutions like public transportation and infrastructural investments. They also used average value of statistical life.
10
Willingness-to-pay (WTP) - is understood as maximum amount of money that an individual is willing to sacrifice to procure a good result or avoid something undesirable.
29
3. Currently used catalyst solutions All the most widespread technologies to reduce air pollutants emission can be classified into one of three main categories [114,129–131]:
flame control,
catalytically guided reactions,
catalytic exhaust gas purification technology.
An additional category may be considered an industrial exhaust gas after-treatment facilities. However, this class is used only in specific applications, and most of the time those are highly specialized installations [132]. In many cases, solutions use several approaches to solve problem. As mentioned Catalytically-supported thermal combustion where we see both flame control and catalytically guided reactions technology [110]. By far, the most spread technology is catalytic converter with all its variants. In general, a catalytic converter is a vehicle emissions control device, used to convert toxic by-products of combustion process to less toxic pollutants by performing catalyzed redox chemical reactions [133]. The reactions type strongly depends from type of engine and fuel used in the process. Hence, catalytic converter has to be designed specifically for the engine it will work with. However, operating range of the catalytic converter is much broader than for industrial installations. Strict requirements that must be met on parameters such as cost, maintenance, size, operation and performance by which it is possible or not to use it in vehicles [114,132,134]. These requirements have strongly limited types of materials that could be used as a catalyst. At this moment the dominant elements are: platinum (Pt), palladium (Pd) and rhodium (Rh). Those three metals were chosen due to their excellent thermal stability, lower tendency to react with support materials and ability to process gas streams containing upwards of 1,000 [ppm] of sulphur (by weight) without being transformed to bulk sulphates [135,136]. Nevertheless, at the beginning of R&D over catalytic materials for converters, PGM 11 were not popular research subject. Mostly, because it concerns over the cost and availability of noble metals. However, it quickly became apparent that the base metals lacked the intrinsic reactivity, durability, and poison resistance required for automotive applications [135,137].
11
Platinum Group Metals (PGM) - noble metals considered as a group due to their similar physical and chemical properties. PGM contains ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os), iridium (Ir), and platinum (Pt) [159].
30
Additionally, other three PGM metals, ruthenium (Ru), iridium (Ir) and osmium (Os) all form volatile oxides what automatically disqualifies them as potential replacements [135,136]. Since the beginning of development over materials for catalytic converters, the biggest issue was selection of a suitable NOx reduction catalyst. In the classic approach, NOx control was performed in dual-bed converter arrangement. The NOx has to be reduced in the first bed to avoid NH3 reconversion back to NOx in the second bed, where CO and HC should be oxidized [114]. That first arrangement, reduction-oxidation stage bed was inherited in all subsequent solutions. However, it also created a new problem which was selectivity of the catalyst towards reduced to N2. When national policies started to require NOx control in 1979 the best known candidate was Ru, stabilized against vitalization, in the form of complex oxides (example: MRuO3, where M=Ba, Sr, La) [135,138,139]. Nevertheless, soon Rh became the catalytic element of choice due to its high selectivity [140]. In mid 80's industry switched to three-way catalysis. Mainly because new, more strict policies and limitation of double-bed conversion technology. The term of "three-way catalysis" comes from demand of simultaneous catalyzing of three types of reactions: NOx reduction, CO oxidation and hydrocarbon (i.e. HC) oxidation [141,142]. This approach remains in use to this day, as the dominant mainly thanks to the technology is well, low-cost and high reliability. With time, some materials recycling solutions appeared as well [143–145]. Most of the development that took place in that field between 80's and year 2000 was primarily intended to mineralize among of required PGM, as well as improve the properties of support materials. Therefore, in order to increase the reaction surface area of PGM it was necessary to manufacture thermally stable nanostructures. This proved to be difficult because at the nanometric scale Pt, Pd and Rh exhibit behaviours such as high reactivity and strong tendency to agglomerate. Along with reduction of size, advantageous of increased reactivity is limited by thermodynamic. Additional surface energy causes a lowering of the oxidation temperature and leads to formation of often volatile PGM compounds [146–148]. If support materials do not stabilize nanoparticles, elevated temperature increases the mobility and leads to strong loss of specific surface area. The result is permanent material damage and dramatic loss of the converter efficiency [149,150]. Additionally, the change in used particles size scale force to take account of quantum effects [151,152]. However, these types of issues are regarded both as the current obstacles and as opportunity for further improvements of process control. Currently used catalytic converters, both diesel and gasoline, are highly optimized, efficient piece of equipment. They have achieved an extremely high level of complexity. Both, 31
metallic substrate catalyst components and their working space have been developed and manufactured with optimized fluid dynamic and turbulent flow characteristics in mind. Current solutions keep in balance demands on performance, fuel efficiency, comfort, as well as regulatory requirements. Further development on that field is expected in the heat recovery and further reduce of manufacturing costs [153].
3.1. The economic justification for the research of alternative catalytic materials The global production of PGM slowly approaches to 1,000 [ton] per year [154,155], from which approximately 210 [ton] is Platinum itself [156]. This trend is expected to continue, mostly due to their scarcity in deposits [157,158], specific application in industry related to their rear properties and fact that its perceived by the market as a stable investment [159].
Figure 10. Relative abundance of elements in the Earth's upper crust [160].
Overall demand for PGM is built by all its markets what includes jewellery, electronics, chemical, petroleum and several others [161]. The broader market is the more stable prices of individual elements becomes (Figure 10 and 11). Such a case is Rhodium, which applications 32
are almost exclusively as a catalyst. Its market is small, therefore it often becomes a subject of stock market speculation [161,162]. The last such situation occurred in 2008, when during speculative bubble development a price of Rh almost reached 10,000 [USD/oz] (Figure 12). At the moment, the only possible path to reduce the cost of based PGM technologies is to increase supplies to the market.
Figure 11. World production of PGM and Cu between 1900 and 2013 [155,163].
However, primary suppliers from Canada already met the optimal level of delivering ability. Mining in USA is expected to rise to 170 [%] before stabilization. All possible and expected increase is concentrated in South Africa and Zimbabwe, where PGM mining industry is under development. For Russia, it may be possible to increase industry output. However, their deposits are located mainly in the polar regions through which the profitability of potential investments will strongly depend on the constantly rising prices [157,164,165]. In this situation, it is hard to expect a decline in raw material prices. Since the currently used technologies increasingly often verge on the boundaries defined by economics and technical capabilities, the only logical steps that can be performed is to change the material used in the manufacturing process. Evident step of action is to return to research from the early phases of the catalytic converters development. Some work suggest usage of gold and silver [166,167]. As encouraging they are, both metals suffer from exactly the same drawbacks as platinum. Materials such as 33
titanium, nickel, copper, iron, manganese and cerium seem to be reasonably cheap solutions. Unfortunately, all of them still suffer from a lack of intrinsic reactivity, durability, and are highly sensitive for catalytic poisoning [135,137,168]. The need to take account more sophisticated approach becomes obvious when take into consideration oxides, complex intermetallic and ceramic materials [169,170].
Figure 12. Historical market prices of Platinum(Pt), Rhodium (Rh), Palladium (Pd) and Copper (Cu) between 1960 and 2015 [163,171–173].
In this context, the copper becomes very attractive candidate. Copper is relatively inexpensive and its global production in 2013 was 18.3 million [tons] [163]. Mineral depositions are widely distributed around earth what is important from supply chain safety point of view [174]. All of these features combined with the fact that copper oxides are a class of materials with ability to selectively catalyse of NOx makes it perfect candidate for developing of PGM replacement [175].
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3.2. Overview of research on alternative materials for currently used catalysts Subject of materials catalytic properties is vast. Several books and monographs on individual elements, as well as material groups have been written. This chapter will introduce main achievements of development on most promising materials. Titanium Oxides The most important and valuable feature of titanium oxide (TiO2) is its ability to perform photocatalysis under ambient-temperature. First of all, photocatalysis occurs only when TiO2 is exposed to UV radiation [176]. Secondly, catalytic ability of TiO2 are oxidative whereby TiO2 is grate in destroying carbon-carbon bond [177]. It is able to degrade organic and inorganic particles in both, the air and aqueous environment. Additionally, if support material in which TiO2 has been incorporated posses some storage capacity for degraded hydrocarbon, then final material has limited ability to transform nitrogen oxides (NOx) into nitrates salts [178]. Unfortunately, research has shown that the use of pure TiO2 directly for hydrocarbons degradation in aqueous solution may leads to release of NOx [179]. Some progress has been made in this field, but it is related to the methods that can be used only in industrial conditions [180]. Oxidative nature and requirement of UV as reaction trigger made TiO2 suitable for second stage exhaust gas purifying catalyst if we assume that photocatalytic effect can be used effectively. Nonetheless, TiO2 already found several application in as ingredient of photocatalytic water purification, air purifying paints and cements [181–184]. Iron and Iron Oxides Theoretically, Iron (Fe) has the capacity to chemisorption of substances such as: O2; C2H2; C2H4; CO2; N2; H2 and CO. It is connected with its natural ability to oxidize by which is similar in the behaviour to elements such as: Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, Mo, W, Ru, Os [185,186]. This makes it a good candidate for a cheap replacement for many processes. Three highly chemically active crystallographic surfaces Fe (110), Fe (100) and Fe (111), in combination with FCC12 crystallographic structure type makes it easy to develop a material that could be used in the catalytic processes. Moreover, it does not form volatile oxides as in the case of Ru or Os [135,187]. Fe is mainly suitable for reductive catalyst. It is commonly
12
FCC - face-centered cubic
35
used in the production of ammonia (NH3), where high absorption capacity of the nitrogen atoms on the Fe (111) surface allows for effective implementation in industrial scale [188]. As regards the iron oxides, they were widely used in industry since 50's. The most popular catalyst from this group is Iron(III) oxide (Fe2O3), used for catalytic cracking. Often use as one of additive for silica and alumina to increase resistance against metal poisoning [189]. In opposite to pure Fe, Fe2O3 is oxidative catalyst. Is good, but not perfect candidate for CO oxidation process [190–192]. This is due to the ease of oxygen chemisorption that Fe2O3 posses on its surface [193,194]. Unfortunately, iron oxides are not suitable for efficient chemisorption of nitrogen. There were attempts to change that by using alumina-iron oxide catalysts for nitrogen monoxide chemisorption [195]. However, any proves of their continuation were not found in the literature. Nevertheless, development over iron and its oxides is far from end. Currently, one of main direction of development for it is in form of complex compounds with incorporated iron ions for specialized purpose such as water splitting [196]. Nickel and its compounds First known use of Nickel (Ni) as a catalyst is from 1902, for olefin hydrogenation in Oil refining industry. It is also good replacement for Pt or Pd for reformation of benzene to cyclohexane [188]. For this reason, it is used in production of nylon. In USA, Nickel was used at early stage of TWC13 development to counteract hydrogen sulfide (H2S) formation. Presence of Ni catalyse H2S to SO2, whereby the exhaust gases did not smelled like "rotten eggs" [197,198]. However, in Europe this solution was not accepted due to potential formation of poisonous nickel carbonyl. The reaction of Ni with carbon monoxide leads to formation of highly toxic nickel tetra carbonyl [198]. There are alternative sulphur interceptions currently that are not blocked to the legislation. As well, some research are conducted in that field, which includes usage of more stable forms such as Cu-Ni alloys over zeolite for NOx reduction [199]. Finally, the work on Ni-Natural Zeolite where on NOx removal from gasoline engine exhaust [200]. Nevertheless, these are individual works, not constitutes of a major trend.
13
TWC - Three Way Catalytic Converter
36
Manganese Oxides Pure Manganese (Mn) is much more limited as compared to Fe. It should be capable to chemisorption of gases such as O2; C2H2; C2H4 and CO [185]. It is one of the most abundant element in Earth's crust and soil (1000 [ppm] and approx. 440 [ppm]) [201]. It is cheap and easily available, but as happens in such cases, it is far from being perfect. Manganese oxides are rarely used alone in catalytic converters, mainly because of MMT14. MMT is nontoxic compound that is used in unleaded gasoline in the place of TEL15 since 70's [202,203]. In poorly designed catalytic converters, it can lead to deposition of complex compounds on the catalyst surface. That causes plugging of monolithic converters or TWC poisoning [204,205]. New research proved that incorporation of manganese oxides in catalytic converters can inhibit deposition of catalyst poisons, such as phosphorus and zinc on the surface of the catalyst [206]. Nevertheless, most of the time manganese oxides are cocatalysts where play the role of a cheap additive to store nitrogen for reaction [207–209]. In industrial application it is used to catalyse NOx neutralization by acceleration reaction with atmospheric oxygen and introduced ammonia [210,211]. Also, It is possible to use it for hydrocarbons neutralization at low temperature [212]. Cobalt Oxides Cobalt (Co) has two states of oxidation, +2 and +3. Therefore, it is able to form three different oxides. Two most basic are Cobalt (II) oxide (CoO) and Cobalt (III) oxide (Co2O3). The third one possesses Co atoms on both oxidation states, and form Cobalt (II,III) oxide (Co3O4) [201]. This spinel is most interesting from catalytic point of view. It inherit after Co ability to chemisorptions of O2; C2H2; C2H4; CO2; H2 and CO as well as its inability to bond with nitrogen [185,213]. In 70's there were attempts to improve nitrogen chemisorption by spotpromoted reduction, but this direction has been discontinued [214]. Nevertheless, it can enhance nitrogen storage ability of other non noble metals [215]. Virtually all practical applications of cobalt oxide focuses on the use of its natural capacity for chemisorption of oxygen. In this respect, it is an excellent replacement for platinumpalladium alloys [188,216]. The process of converting carbon monoxide over Co3O4 is complex and still is a subject of discussion [217,218]. But we know that it is affected by
14 15
MMT or MCMT - Methylcyclopentadienyl manganese tricarbonyl, (CH3C5H4)Mn(CO)3 TEL - Tetraethyllead, (CH3CH2)4Pb
37
relative position of the Co2+ and Co3+ ions [216]. The possible impact of the exposures of specific crystallographic planes is also taken into account [219]. As far, Co3O4 found numerous applications in oxidation catalysis. Mainly carbon monoxide neutralization in TWC or specialized installations [220]. In general, it is excellent oxidation promoter for hydrocarbons [221–223]. But in order to effectively use it, it is necessary to provide an efficient oxygen source. Therefore, in catalytic converters between second and third degree air is introduced to ensure adequate its availability [114,130,131,135]. However, the current trend in the development of these oxides is concentrated on mixed metal oxides and co-catalysts in order to fully exploit its potential. Studies show excellent cooperation of cobalt oxide with cerium oxides [224]. Tests of mixed oxides with Cu, Fe and Ni proving to be promising solutions for NOx with ammonia oxidation [225]. Similar work was performed for oxidation of CO and hydrocarbons [226,227]. Cerium Oxides The Cerium (Ce) is rare-earth element, but due to its wide use as a catalyst, it is described separately. Ce possesses unusual and interesting properties because of its variable electronic structure. Its outer (valence) electrons are nearly the same as that of the inner 4f level. For this reason, only small amount of energy is required to change the relative occupancy of these electronic levels. The effect is called dual valency states [186,201,228]. Ce by itself is relatively abundant in the Earth’s crust. Its 66.5 [ppm] should makes it an economically exciting alternative to noble metal. For comparison, Cu abundance in Earth’s crust is estimated to be 60 [ppm] [186,201]. Nevertheless, fact that is one of the Rear Earth Elements what greatly reducing the availability of its deposits. As a catalyst, Ce is always used in form of oxides. In its pure form it occurs on the third (Ce2O3) and fourth (CeO2) oxidation state, and found a number of applications which proved to be useful. It is very popular material in petroleum cracking [229], fuel cells[230], synthetic organic chemistry and finally in TWC where its oxygen storage capacity found good use [231,232]. Cerium oxides can efficiently interact with NOx and CO. By becoming a nonstoichiometric cerium oxide can give up oxygen without decomposing and then recover lacking oxygen from surrounding gases base on its ambient partial pressure of oxygen [233,234]. Due to semiconductive properties of cerium oxides, this material is very sensitive to doping. It can be used as a co-catalyst, usually in combination with oxides of Rh, Fe, Co and Ni or noble metals like Pt and Au [235]. Despite relative inaccessibility CeO2/Ce2O3 are still cheaper than PGM. Since ceria shown to be able for substantial reductions of especially Pt and Rh usage, it became very popular as 38
additive. Nevertheless, it is not able to completely replace noble metals. This is due to the ability of cerium oxide to disrupt the hydrogen-carbon bonds [236,237]. Additionally, cerium oxides require high temperatures of work which requires the use of heating systems [114]. For this reasons, complete reliance on this material could lead to formation of dangerous compounds or inability to work properly on a "cold start". It has places especially in case of heavy-duty engines [238]. Rare Earth Oxides There are seventeen chemical elements in the periodic table that we use to call rare earth element (REE). This group contains all fifteen lanthanides, as well as scandium and yttrium [239]. One of these elements, Cerium (Ce) was described above. Counter-intuitively, with the exception of the radioactive Promethium (Pm), all REE are relatively plentiful in Earth's crust [201]. The distribution of extractable and economically viable mineral deposits of these elements make their accessibility problematic. Despite the fact that these elements always occurs in deposits together, known deposits are dominated by Ce, Y, La and Nd [240–242]. This significantly affects the price of the already expensive elements [243]. All these factors influence the fact that the full REE potential is unexploited, because they are mainly used as additives to improve properties of other materials. All Rare Earth Oxides (REO) are characterised by small energy required for ionisation. Dielectric constants of REO decries for each next atomic number [244]. A similar relationship can be observed for basicity and CO2 desorption. As Sato et al. showed, the strength of both decreases with decreasing radius of the rare earth cation [245]. Virtually every REO has a stable version of its oxide, and the vast part of them have preferred in catalysis the multifaceted crystallographic structures. All REO exhibit resemblance to the behaviours of the alkaline metals oxides and are capable for chemisorption of hydrogen and carbon atoms [245,246]. Via atomic exchange and transportation ability REO can be used for selective catalysis of NOx in presence of hydrocarbon. This process has been tested for example on NO with La2O3 [247]. It is believed that in such cases an oxygen acts as active element in chemisorption by bonding with ingle vacancy and a lattice oxygen atom [248]. Nevertheless, this is one of the many proposed mechanisms [249], since direct decomposition was observed as well [250,251].
39
4. Features that the best catalysts should have On the way to find the solution of problem, we have to understand the source of that problem first. In addition, the first thing we must understand is that it will be necessary to make compromise between the amount of energy released and the amount of pollution formed. Perfect solution in this case is impossible, since combustion condition within pollutants forms are far from perfection. The reason for that is stoichiometry required for proper combustion and catalysis in catalytic converter [252]. During combustion, pollutants emissions are determinate by amount of available oxygen and temperature. As we approach perfect combustion condition with properly balanced air/fuel ratio, the temperature of process rise. At the same time additional heat causes formation of NOx what consumes both oxygen and energy. Thereby we cannot achieve perfect combustion condition in presence of atmospheric air [252,253]. Furthermore, if we look at the engine and exhaust as a whole, we notice that catalyst in converter cannot neutralize NOx unless the oxygen level in the exhaust is very low [253–255]. For this reason, excessive oxygen becomes undesirable for two reasons. Selected air/fuel mixture of 14.64:1 is the best compromise. It does not provide perfect combustion but it provides the lowest CO and HC levels achievable. Disadvantage of this is very high NOx level. It would be possible further reduce emission during combustion. Second emission minimum is attributable to 17.5/1 mixture. In this case, however, the exhaust gases would be too rich in oxygen to allow NOx neutralization in the first place [252]. At this point, it should be recalled that the CO and HC oxidation requires enough oxygen in the exhaust. In consequence, there is no fuel mixture we could use to allow for CO, HC and NOx catalysis with maximum efficiency. We artificially provide this condition by catalytic additives in fuel to promote low temperature combustion and by injecting air to rear bed of catalytic converters to properly oxidize CO and HC [135,198,253,256]. With that in mind, we can start working on the design of a suitable catalyst for the process. First, we must ensure the possibility of reaction occurrence at all. To do this we just look at how currently used catalysts do it. From section "Currently used solutions”, we know that Rh and Pt are elements of choice for NOx neutralization. In the case of Rh, we know that this element is particularly effective in the chemisorption of nitrogen. It is not less effective in bonding with both nitrogen and oxygen [216]. Thanks to this feature, Rh is core of first stage of catalytic process. Prior to the introduction of TWC technology, to catalyse the NOx we used Pt. Platinum is somehow universal catalytic element. In contrast to Rh, is capable of 40
catalysing both reduction and oxidation reactions. It is connected to the fact that Pt is particularly good at chemisorption of oxygen [148,257]. Secondly, when we design catalyst we need to ensure that reaction will proceeds place in direction that is preferred by us. Selective catalytic reduction (SCR) is necessary in the case of mixed complex composition gases such as exhaust fumes [114,256]. In this case, we want to selectively reduce NOx while compounds such as CO, CO2 and HC remain intact. Despite the fundamental importance for the whole catalysis process, we do not have effective tools to predict the behaviour of the proposed catalyst. Methodology based on past experience shows that in many cases the catalyst which theoretically should work poorly not at all, works perfectly and opposite [258]. Complex interactions that can occur in the catalytic converter require experimental verification of the material behaviour. In the case of Rh, it is assumed that its selectivity is related to its ability to tie both oxygen and nitrogen together [216,257]. However, there are many proposals for alternative mechanisms. Thirdly, all catalysts must have surface area as strongly expanded as possible. All heterogeneous catalytic processes occur at the interface surfaces [254]. By expansion of this area translates directly into the reaction speed. In the case of a good catalyst, it results in usage of smaller quantities of them [185,216,245]. For less active materials, a significant expansion of surface area will result in increased material activity proportionally to the expansion ratio [259,260]. Another important aspect is the ability of a material to develop and sustain surface point defects. Surface vacancies or atomic inclusions are important for such effects like free electron management, adsorption initiation, adsorbed gas molecules decomposition or long range transportation of adsorbed elements [244,246,261,262]. For PGM where the catalyst particles are composed out of only one type of atoms, it seems to not be so important phenomenon. But new research suggests that if the surface was too perfect, the catalytic effect would never occurred [258]. We have noted as well that when particles of catalyst approach to nano-scale, it may cause a significant stoichiometry loss and resulting in its decomposition. We see it in ionic solids and particularly in the case of some REO [233,234,246,263,264]. At higher dimension, the material may be unsuitable because of its thermodynamics. The exhaust fumes have average temperature of several hundred degrees Celsius when reaches the catalytic converter. Specific value depends on the motor and fuel. In the case of oxides, there is a risk of thermal decomposition of the material. It must be remembered that exhaust fumes contain highly reducing gases such as CO. Hence, if the operating temperature would be high enough, the material would be destroyed via reduction. This issue is gaining momentum along 41
with a reduction in the size of the material architecture approaches to nanometric scale. Increased mobility of atoms in combination with a high activity of the material can lead to agglomeration and destruction of structures responsible for its desired properties. The point is that, the material must be able to survive conditions in which it should work. As we are continuing over real condition, we must refer to supporting material. No catalytic material is working in suspension. Supporting materials are made out of highly resistant to temperature and chemicals substances, usually such as silica gel or γ-alumina (Al2O3) [265]. Catalyst attached to the supporting materials technology is already well established in the industry, for both metals and oxides [266]. Some advanced solutions used the so-called support effect where properly doped and prepared support material can improve catalyst performance [265,267]. Such material can artificially increase surface area of catalyst, for example by high dispersion or thin film [268]. It may improve thermal, chemical, and mechanical stability of catalyst [269]. Support active functions through chemical or electronic interactions between catalyst active centres and support [267,269]. In the opposite way, the active material of the catalyst may be dissolved in the support material or its active mechanism could be offset by supporting material [266,270,271]. Therefore, the development of a suitable catalyst has to consider that possibility as well. Finally, potential material should be affordable. As it was justified in section about catalytic materials economy, potential material must be attractive replacement of noble metals based technologies for developing countries. Summing up, the desirable features of the catalyst for the NOx remediation are:
capability of chemisorption of nitrogen and/or oxygen,
selective catalytic reduction,
highly expanded specific surface area,
stability despite presence of defects or not stoichiometric chemical composition,
thermal and chemical stability,
non-reactive with supporting material,
made from cheap and easily attainable materials.
Even incomplete meeting of these requirements would result in obtain catalyst that is competitive to existing solutions.
42
5. Introduction to NO - CO remediation The idea behind the process of catalytically-assisted remediation of NO + CO is simple. It consists of the reduction of adsorbed NO and then, use of released thus oxygen for the oxidation CO. If the reaction occurs spontaneously, its chemical notation would look like as follows: 𝑁𝑂 + 𝐶𝑂 → ½𝑁2 + 𝐶𝑂2
(9)
In fact, the process is much more complicated and takes several stages. The reaction may also have several products. For a perfectly clean gas mixture of NO + CO the real live equation would look like: 𝑁𝑂 + 𝑤𝐶𝑂 → 𝑦𝑁2 + 𝑧𝑁2 𝑂 + 𝑤𝐶𝑂2
(10)
The presence of N2O in this case may be inevitable since, as Cho et al. [272] and McCabe et al. [273] shown that N2O might be a transition step in the NO-CO remediation reaction. According to them, reaction over Rh involves three reactions: 𝑁𝑂 + 𝐶𝑂 → ½𝑁2 + 𝐶𝑂2
(11)
2𝑁𝑂 + 𝐶𝑂 → 𝑁2 𝑂 + 𝐶𝑂2
(12)
𝑁2 𝑂 + 𝐶𝑂 → 𝑁2 + 𝐶𝑂2
(13)
An important aspect here is the requirement of oxygen-poor reductive conditions. High reactivity of CO which makes gas so strong reducer make it to oxidize first [254]. Thus, further equations are true only for those oxygen pure conditions. The N2O case is relatively simple. Direct reaction with CO (Equation 13) and free carbon particles gives desired outcomes [249]. 2𝑁2 𝑂 + 𝐶 → 2𝑁2 + 𝐶𝑂2
(14)
However, at a temperature above 900[K] (626.85[°C]) it undergoes spontaneous decomposition. One of the products of this reaction is the oxygen that threatens reducing conditions. This is important because many potential catalytic materials require high temperatures. For this reason, it is undesirable reaction [249]. 𝑁2 𝑂 → 𝑁2 + ½𝑂2
(15)
𝑁2 𝑂 + 𝑁𝑂 → 𝑁2 + 𝑁𝑂2
(16)
2𝑁𝑂 + 𝑂2 → 2𝑁𝑂2
(17)
N2O can also react with NO or that release oxygen and form NO2 [249], which is one of the compounds responsible for the acid rain formation. This reaction occurs very effective over platinum, so it should be avoided to allow N2O left the first part of the catalytic converter, where it still can be neutralized [188]. To avoid this, we must carefully look at what 43
determines the success of the reaction. On the surface of catalyst, N2O require to accept a free electron. The presence of excess electron in N2O leads to the destabilization and disintegration of the molecule. Oxygen released in the process can then be used to neutralize CO. In practice, this process is carried out by use of Ceria-promoted rhodium catalyst. Then, the reaction looks as follows [274]: 𝐶𝑒𝑂/𝑅ℎ
𝑁2 𝑂(𝑔) + 𝑒 − ↔
𝐶𝑒𝑂/𝑅ℎ
𝑁2 𝑂∗ (𝑎𝑑𝑠) →
𝑁2 𝑂∗ (𝑎𝑑𝑠)
(18)
𝑁2 + 𝑂∗ (𝑎𝑑𝑠)
(19)
𝐶𝑒𝑂/𝑅ℎ
𝐶𝑂(𝑔) + 𝑒 − ↔
𝐶𝑂∗ (𝑎𝑑𝑠) 𝐶𝑒𝑂/𝑅ℎ
𝐶𝑂∗ (𝑎𝑑𝑠) + 𝑂∗ (𝑎𝑑𝑠) →
𝐶𝑂2(𝑔) + 2𝑒 −
(20) (21)
In summary: 𝐶𝑒𝑂/𝑅ℎ
𝑁2 𝑂(𝑔) + 𝐶𝑂(𝑔) →
𝑁2 (𝑔) + 𝐶𝑂2 (𝑔)
(22)
In CeO/Rh case, the key issue is the ability of the material for easy release of electron. For N2O, it means to brake N-O bond [257,267]. Reaction of NO with CO case is only slightly different. The reaction mechanism over Rh have to take into account that Rh is capable of chemisorption of both oxygen and nitrogen. As a result, just after NO is bonded with catalyst surface, the N=O double bond will be broken and on the surface only atomic nitrogen and oxygen will stay attached [255,257]. According to model proposed by Campbell and White [275], the reaction proceeds as follows: 𝑅ℎ
𝐶𝑂(𝑔) + 𝑒 − ↔ 𝐶𝑂∗ (𝑎𝑑𝑠) 𝑅ℎ
𝑁𝑂(𝑔) + 𝑒 − ↔ 𝑁𝑂∗ (𝑎𝑑𝑠) 𝑅ℎ
𝑁𝑂∗ (𝑎𝑑𝑠) + 𝑒 − → 𝑁 ∗ (𝑎𝑑𝑠) + 𝑂∗ (𝑎𝑑𝑠) 𝑅ℎ
2𝑁 ∗ (𝑎𝑑𝑠) → 𝑁2 (𝑔)
(24) (25) (26)
𝑅ℎ
𝑁𝑂∗ (𝑎𝑑𝑠) + 𝑁 ∗ (𝑎𝑑𝑠) → 𝑁2 (𝑔) + 2𝑒 − 𝑅ℎ
𝐶𝑂∗ (𝑎𝑑𝑠) + 𝑂∗ (𝑎𝑑𝑠) → 𝐶𝑂2 (𝑔) + 2𝑒 −
44
(23)
(27) (28)
Proposed later model by Cho [276] add additional steps of N2O release, re-adsorption and decomposition (Equations 29-31). 𝑅ℎ
𝑁𝑂∗ (𝑎𝑑𝑠) + 𝑁 ∗ (𝑎𝑑𝑠) → 𝑁2 𝑂(𝑔) + 𝑒 − 𝑅ℎ
𝑁2 𝑂(𝑔) + 𝑒 − → 𝑁2 𝑂∗ (𝑎𝑑𝑠) 𝑅ℎ
𝑁2 𝑂∗ (𝑎𝑑𝑠) + 𝑒 − → 𝑂∗ (𝑎𝑑𝑠) + 𝑁2 (𝑔)
(29) (30) (31)
45
6. CuO as catalytic material Catalytic properties of Copper oxides were known from the beginning of XX century. However, recognition as potential gas reducing material could be traced to 1969, when Scholten and Konvalinka described reaction of nitrous oxide with copper surfaces where CuO were step of N2O decomposition [2]. In 1981, G. Sengupta et al. reported reduction conditions upon CuO area [1]. The team observed intense increase of decomposition from above 70 [°C]. Following decades of research brought new findings indicating the significant potential of copper compounds as a catalyst. Nitrogen and Oxygen chemisorption, CuO surface crystallography and properties At this moment we know for sure that behind the catalytic properties of CuO corresponds to the ability of this material to capture O- or O2- or both [1]. However, the exact mechanism of the process is still under dispute. The key point is to answer how gas molecules behave in contact with the material surface. To understand this process it is worth to look at the materials that possess similar properties. The general basis and specific characteristics of each material type were described in the last three sections. Therefore, this part will refer to the grounds contained therein. In the first place, we should focus on the foundation of the process, the energy. In case of NO chemisorption on Pt (111) and Rh/Pt (111) the point is that Pt−Pt bond strength is -270.3 [kJ/mol] and for Pt−Rh -286.2 [kJ/mol]. An adsorption energy of NO at an FCC (111) site is for both materials respectively -202.6 [kJ/mol] and -201.9 [kJ/mol]. Due to the different bond strength between the two metals in each alloy system, different energy amount is concentrated on its surface. At the same time adsorption energy of NO on Pt (111) is 202.3 [kJ/mol] [151]. That relatively equal energy allows Pt alloys to be near perfect catalyst for nitrogen compounds. Since, as it was pointed before, Pt catalytic properties base on bonding with nitrogen. Meanwhile, CuO is good oxygen storage catalytic material. Similarly to REO, CuO base on its reductive potential over NOx. In mechanism proposed by Winter for NOx chemisorption involves adsorption of N2O molecules onto single anion vacancies, each of which either contains a trapped electron or acquires one via surface migration [277–280]. In the further course, the reaction over a catalyst surface follows model proposed by Cho [276] (Equations 23-31). Although the model was originally developed for REO. Nevertheless, even if specific
46
physicochemical properties of f-block elements 16 would be taken into account, simple extrapolation of that model allowed for early application of CuO as catalyst for chemical and fertilizer industry [281]. Equally important is where the reaction takes place. Depending on the atoms organization in those sites, material exhibits different behaviour. For example, perovskite-type oxides possess oxygen-deficient sites in their structure. Thereby, they exhibit relatively high catalytic activity in the high-temperature working conditions [250]. Already mentioned work of Scholten and Konvalinka pointed Cu (110) > (100) > (111) crystallographic planes as sites of highest adsorption for N2O. Accordingly, for CuO the order changes into (100) > (110) > (111) [2]. Related to the C-type cubic structure arrangement of atoms in 3D space is particularly important. By the presence of many oxygen-deficient sites may increase the probability of NO adsorption (Figure 13) [250].
(101)
(001)
(011)
(010)
(110)
(100)
Figure 13. Side views of relaxed (101), (001), (011), (010), (110), and (100) crystal planes of CuO (Nature Publishing Group, distributed under CC BY 2.0 license)[282].
During the NOx - CO remediation process we should pay particular attention to the fact of composition rotation. In the presence of highly reducing gas, which is CO, CuO is reduced to
16
f-block elements - The elements belonging to the block F on the periodic table. This group includes the lanthanides and by that Rare Earth Metals.
47
Cu2O, and consequently to the pure Cu. For CuO (111) plane, decrees of oxygen atoms presence lead to decrease of surface free energy γ. In consequence, surface oxygen vacancy formation speedup [283]. It is worth noting at this point that Cu (111), Cu2O (111) and CuO (110) coincide spatially throughout the remediation cycle [284]. We know that Cu2O (111) is high efficient CO oxidation catalyst [285]. Additionally, from work of Y. Duan et al. we know that chemisorption of NO is stronger on CuO (110) then for Co. However, chemisorption of CO on Cu2O (110) surface is definitely stronger then NO [286]. This means that in the case of gas mixtures of NO and CO, it may be a multi-stage process which results in the catalytic remediation. Literature research clearly distinguish CuO (110) as the most favoured plane for this task. It is supported by experimental data as well [287]. Theoretical studies conducted by J. Moreno visualize how NO recombination occurs on the
E [eV]
surface of both pure Cu and its oxides (Figure 14) [284].
Reaction coordinates [arbitrary units] Figure 14. Calculated potential energies for the reaction paths of NO dissociation over Cu (111) (blue lines), Cu-terminated Cu2O (111) (red lines) and Cu-terminated CuO (110) (green lines) surfaces. The top views of the corresponding geometric configurations were shown on insets over the reaction paths. Atoms are represented by: Cu as blue spheres, N as silver spheres, and O as red spheres (Copyright 2014 The Royal Society of Chemistry) [284].
The simulations conducted by Moreno assumed the perfect crystallographic planes. Nonetheless, his results can be extrapolated to real case where all three surfaces are present in the direct vicinity. A strong argument for that is that assumptions for the calculation model are consistent with currently available knowledge about Cu, Cu2O and CuO (Table 4) [3]. 48
Table 4. Summary of calculations and experimental parameters of the CuO bulk structure [3,288–305]. Angle β Lattice constant Magnetic moment Band Gap Ref. (h, k, l) [Å] (∠ h:l) [°] mB [µB] [eV] Anisimov [288]
–
–
0.66
1.9
Debbichi [289]
4.548, 3.305, 4.903
99.652
–
–
Ekuma [290]
4.68, 3.42, 5.13
90
0.68
1.25
Heinemann [291]
4.588, 3.354, 5.035
99.39
0.66
1.39
Heinemann [291]
4.513, 3.612, 5.141
97.06
0.54
2.74
Hu [292]
–
–
0.63
1.1
Jiang [293]
4.68, 3.42, 5.13
99.54
0.80
–
Lany [294]
–
–
–
1.19
Nolan [295]
4.395, 3.846, 5.176
–
0.53-0.7
0.17-2.11
Peng [296]
4.05, 4.06, 5.06
90.02
0.0
0.0
Peng [296]
4.56, 3.27, 4.96
100.2
0.63
1.32
Svane [297]
–
–
0.65
1.43
Szotek [298]
–
–
0.64
1.0
Wu [299]
4.55, 3.34, 4.99
99.507
0.6
1.0
Experiment
4.684, 3.423, 5.129
99.54
0.68
1.2–1.9
[300]
[300]
[3]
[301–305]
From the data presented in Table 4, emerges a picture of reaction where sites of CuO oxidizes CO and Cu reduces NOx. Overall reaction could be presented in the form of following equations [249,306]. 𝐶𝑢𝑂 + 𝐶𝑂 → 𝐶𝑢 + 𝐶𝑂2
(32)
𝐶𝑢2 𝑂 + 𝐶𝑂 → 2𝐶𝑢 + 𝐶𝑂2
(33)
𝐶𝑢 + 𝑁2 𝑂 → 𝐶𝑢𝑂 + 𝑁2
(34)
2𝐶𝑢2 𝑂 + 2𝑁𝑂 → 4𝐶𝑢𝑂 + 𝑁2
(35)
2𝐶𝑢 + 2𝑁𝑂 → 2𝐶𝑢𝑂 + 𝑁2
(36)
49
In this process, Cu2O most probably would form by oxygen atoms diffusion within the crystal structure and its surface. This may lead to local disequilibrium and formation of Cu2O. Of course, not only CO can remove the deposited oxygen from material surface. However, in general reaction would progress accordingly to that shown by the equations (32)-(36). By taking into account the necessity of the existence of defects (oxygen vacancies) on the surface of the CuO crystal we can clearly see why CuO posses only 45% efficiency in relation to pure Pt [307]. For the reaction to take place, it is necessary to satisfy the additional conditions. For instance, spatial displacements of all the reagents to sites where it can take place require additional time and energy. Moreover, according to the theory, proper organization of matter is a major obstacle to develop good replace catalyst out of CuO. Selectivity and specific surface area Selectivity is the biggest problem for all alternative catalysts. Mostly, because it cannot be predicted using simple plane expositions approach. For reaction to progress in preferred direction depends on whether all the factors necessary for its occurrence are in the right place and time. The more complex the process is, the less likely the occurrence of such conditions. Thus the reducibility and catalytic reactivity of CuO nanostructures highly depended on the shape and the exposed crystal planes [4]. A good example is CuO (001) plane. From density functional theory calculations with a Hubbard potential energy U (DFT+U) we know that surface energy of the CuO (001) is equal +1.70 [J/m2] and for CuO (100) +1.56 [J/m2], what translates directly into higher chemical activity of CuO (001) plane [260]. In nanoscale CuO (011) planes release oxygen from the surface lattice more easily then by CuO (001) planes [4]. The process is more energetically favourable in that way. As F. Auxilia et al. showed experimentally that oxygen atoms are removed from CuO (001) by incoming CO molecules. The result is formation of CO2 and an oxygen vacancy on CuO (001) surface. Subsequently, the CuO (001) plane recovers its original state when the oxygen vacancy removes oxygen atoms from incoming N2O (Figure 15) [260]. In this form, the reaction is much simpler and contains only two steps consistent with equations (32) and (36).
50
Figure 15. Schematic description of catalytic NO remediation cycle over Energy-minimized geometry of CuO (001). On CuO (001) it is more energetically favourable to remove oxygen atoms by incoming CO molecules. In effect, an oxygen vacancy and CO2 are formed. Crystal to mineralize energy, remove oxygen atoms from N2O on those vacancies, and the CuO (001) facet recover its original state. Atoms are represented: Cu as blue, O as red, C as black, and N as yellow dots. (Copyright 2014 WILEYVCH Verlag GmbH & Co.) [260].
Overall, in order to ensure a good selectivity the material has to reconcile both the exposure of the relevant crystallographic planes and their spatial orientation. To do so it has possess proper morphology with as highly developed specific surface area (SSA) as possible. There are many studies over CuO morphology issue [5,308,309]. Importantly, this material proved to be very customizable for creating new nanostructures (Figure 16). As far as new morphologies have been developed, there was none or little progress in predicting possible outcomes of synthesis. Therefore, the researches for new catalytic structures for CuO are done almost entirely experimentally. In order to obtain complex structures, the procedures used in the synthesis are complicated. They often require a multistage reaction at high temperatures involving toxic organic solvents [260,287,310–323]. Figure 17 under careful analysis shows that one reaction clearly stands out here. SSA of 190 [m2/g] were obtained by P. Pillewan et al. by using mesoporous alumina as supporting material for CuO nanoparticles [310]. It should also consider that even if the reaction took place at room temperature, final product required calcination at 450 [°C].
51
[319]
[324] (Copyright 2011 Indian Academy of Sciences)
(Copyright 2007 American Association of Nanoscience and Technology)
(Copyright 2007 American Chemical Society)
(Copyright 2007 American Chemical Society)
(Copyright 2009 Elsevier)
(Copyright 2011 Institute of Physics)
(Copyright 2007 Elsevier)
(Copyright 2010 Springer)
(Copyright 2010 Elsevier)
(Copyright 2009 Elsevier)
[325]
[325]
[326]
[327]
[328]
[329]
[330]
[331]
Figure 16. Examples of CuO particle morphologies available in the literature.
52
Figure 17. Summary of the literature data for CuO particles Specification surface area (SSA) and synthesis reaction temperature [260,287,310–323].
Another interesting observation from Figure 17 is lack of research in range between room temperature and 100 [°C]. It becomes important by fact of be a clear gap in knowledge of CuO morphologies synthesis. Significant argument for the lack of such a research are results of some studies which suggests that if temperature of the reaction will be lower than 100 [°C], the reaction will stay incomplete and some intermediates remains in the final product [332]. Thermal and chemical stability Under laboratory conditions tests are carried out with use of high purity reagents, very homogeneously mixed, under stable and well-known temperature and pressure. Material stability in the real operation conditions is probably the most important factor for any potential catalyst. Applicability of the material in the industry depends on it. For this reason, material should be tested under conditions resemble as closely as possible those under which the material could be exposed. This applies in particular for catalyst with nanoscale substructure where as a result of the surface energy and often quantum effects material shows much higher reactivity [333,334]. Particularly, in the case of CuO the prospects out from bulk material properties are encouraging. Under standard conditions CuO's melting point is 1326 [°C], but it decomposes at 1026 [°C] [186,335]. The exhaust gas temperature of typical car engine which leaving the combustion cylinder should be below 500 [°C] [252]. This means that CuO has a large reserve 53
of thermal stability. Nevertheless, melting temperature is inversely proportional to the particle diameter [336]. According to Ahmed et al., melting temperature of spherical nanoparticles (NP) with diameter of 10 [nm] should be approximately at 1200 [°C] [337]. Meanwhile, the prediction by the model of Zhang et al. puts that point at 770 [°C] [338]. Additionally, Cu–Cu first neighbour distances increase with decrease of particle size. The same time Cu–O bond length in nanoparticles is smaller in comparison to the bulk CuO. That negates temperature influence and indicates weakening of the structural stability of particle with decreasing size. In general, because of the lack of long-range interactions smaller NPs leads to an increase of local atomic arrangement deformation [337,339]. This means that thermal and chemical stability will depend on the morphology type and dimensions of it individual features. Interaction of CuO with known supporting materials The main features that support materials should posses already have been mentioned in the previous section. In case of CuO, from industrial standpoint, there are no problems with its application. Such issues like adhesion and uniform application are already solved in industrial practice. And existing technology can be easily adapted to CuO needs [340,341]. Very popular subject of in research of Ceria and Ceria–Zirconia oxides based solution well support chemical properties of CuO, but are expensive [342–346]. Since for industrial application the biggest issues are price and availability, very popular are natural materials like alumina, perovskite 17 , zeolite 18 or palygorskite clay 19 which are good candidates for supporting materials [267,342]. Carbon materials like activated carbon (AC) proved to improve adsorption of SO2 on CuO–AC catalysts [347]. This way, for catalyst that were not produced directly in support, carbon based adhesives could be used as well as glues and catalyst enhancers. There are non-known negative interactions of CuO with the most popular supporting materials used in industrial practice. Since almost all of those materials are simple or complex oxides, both direct deposition and on surface synthesis are methods that could be used for this purpose [341]. In summary, all of these factors give us a wide range of ready and tested solutions for direct contact interaction with CuO based materials [254,267].
17
Perovskite - a type of oxide mineral, CaTiO3 Zeolite - microporous mineral, commonly used as commercial adsorbent and catalyst, Na 2Al2Si3O10·2H2O 19 Palygorskite or attapulgite - a type of clay soil, (Mg,Al)2Si4O10(OH)·4(H2O) 18
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7. Motivation and the aim of the studies The impulse for the presented studies was industrial need for new class of catalytic materials mainly due to economical, political and environmental factors. Lasting from the middle of the last century researches resulted in enormous advances in the field of catalytic materials. In its present state, development of catalytic materials is primarily driven in the direction of the complex structures, exploits exotic effects and using rare and expensive elements. On the other hand, solutions used in the industry are rarely published. Details about their structures and exploited mechanism are trade secrets. Companies are very reluctant to share their knowledge about how cheaply solve complex problem. As a result, the path of science and industrial practice diverge on that filed from many years. For this reason, in this work an opposite approach was adopted. As justified in previous chapters Copper (II) oxide (CuO) was selected as target material to find cheap alternative for reduction catalytic material. Additionally, developed method has to have low energy requirements to ensure the highest possible chances of its competitiveness with respect to currently used catalytic materials. The consequence of this requirement is both simple reaction course and low-temperature condition. An additional objective of the study is to obtain the lowest temperature at which catalytic reaction would be possible. The currently used catalysts require the installation of heaters that provide the minimum operating temperature. The energy invested in this process is then released to atmosphere thus lowers the efficiency of the entire system. It also causes increased emissions during the so-called cold-start of the engine.
In short, the aim of this study is: The development of single-step and low-temperature method for the large scale synthesis of high specific surface area CuO nanopowder for better catalytic performance in the lower temperature range.
In order to achieve the aim of the work a proper selection of starting materials and optimization of synthesis process parameters had to be done. To verify of the results, the microstructure and composition analysis of synthesis products were carried out.
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Based on the results of experiments, the selection of material with potential catalytic features were chosen and tested in the performance for NO-CO remediation under controlled condition. The research plan developed on described requirements consisted of the set of milestones determined as following points:
selection of the starting materials,
determination of the optimal reaction stoichiometry,
optimisation of the reaction conditions,
development of the final product after-treatment and cleaning techniques,
characterization of the morphology, chemical and phase composition of the material,
final product catalytic properties characterization.
The research was carried out within collaboration with the National Institute for Materials Science (NIMS) in Japan as part of the Polish-Japanese International Joint Graduate Program (IJGP). Experimental studies were conducted at Supermolecules Group (NIMS), under supervision of Dr. Katsuhiko Ariga.
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8. Materials and methods
8.1. The synthesis of the catalyst The literature study at the initial stage of work aimed at understanding of role of various stages of chemical synthesis in obtaining material with desired feature. At the very beginning it was clear that in order to achieve all goals of this dissertation, the synthesis procedure have to be simple itself. The literature indicates that the key to controllable synthesis is a ligand. The ligand is the atom or molecule capable of producing or having in its structure specific chemical bond [239]. In effect ligand is capable of binding to a specific site of a molecule. This limits the number of possible spatial orientations in which the reaction can occur. By favoring specific orientations the presence of ligand coordinate particle formation [348]. 8.1.1. Raw materials The ligand of choice for this work is 2-piperidinemethanol (2PPM). The reason for this was a wide accessible literature and large experience of cooperated team to work with this reagent. All reagents used to perform this research were supplied by Wako Chemical Co. Ltd. Japan and were used as received. All used reagents are listed in Table 5. Table 5. List of reagents used. Reagent name 2-piperidinemethanol Copper (II) Acetate Copper (II) Nitrate Trihydrate Copper (II) Chloride Copper (II) Sulfate
Acronym 2PPM Cu Acet. -
Molecular weight [g/mol] 115.17 181.63 241.60 134.45 95.61
Molecular formula C6H13NO (CH3COO)2Cu Cu(NO3)2·3H2O CuCl2 CuSO4
CAS. No.20 3433-37-2 142-71-2 10031-43-3
7447-39-4 7758-98-7
* All chemicals used in the study had >99.9% purity.
20
CAS Registry Number - is a unique numerical identifier assigned by Chemical Abstracts Service (CAS) to every chemical substance described in the open scientific literature. The registry contains 66,600,513 organic and inorganic substances (state for 2016/03) [381].
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Preliminary research included several copper salts as source of copper (II) ion. As well, screening tests included solvents as distillated water, ethylene glycol (C2H6O2) and mixture of both. 8.1.2. Experimental procedure Initial screening tests were conducted in the purpose to determine the boundaries within which it was possible to obtain positive results. In first attempts the effect of solvent composition on reaction outcomes was surveyed. Set of trials covered mixtures of water and ethylene glycol as well as pure form of those solvents. Tests were carried out for all the salts listed in Table 5. Constant alternation in experiment resulted in high volatility of results. Reduction of reactants number involved trial allowed to limit the pool of variables and reduction of the noise. Based on screening tests negative results and data consistency, target reagents were selected. Copper (II) Acetate and 2-piperidinemethanol (2PPM) dissolved in pure distillated water have proven to be the most promising reactants. In order to maintain the consistency of data, adopted research methodology has been simplified to recursion with a variable temperature and reaction stoichiometry (Figure 18).
Figure 18. Schematic representation of research methodology.
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8.2. Characterisation methods In order to provide better outlook on characterisation methods applied in this work all of them were briefly described. Each description includes a physical principles explanation as well as typical settings used to collect data presented of this work following sections. 8.2.1. Scanning Electron Microscopy (SEM) Throughout the SEM was routine research method. SEM was used in order to collect information about material morphology, as well as evaluation of validity of actions taken to improve the quality of the produced material. In specific, SEM method was used to gather information as:
the degree of purification of material from the reaction waste products;
approximate particle size distribution and general size;
repeatability of syntheses.
Additionally, SEM method was used to evaluate quality of samples for TEM observations. Samples preparation Samples preparation procedure was conducted as presented below:
preparing particles suspension in distillate water with ultrasound as a mixer,
placing of one drop of suspension on piece of silicon monocrystal wafer or in case of sample prepared for TEM, on carbon grid,
drying in vacuum chamber with slowly decreasing pressure till full water evaporation.
Important: Vacuum chamber must be emptied from the atmosphere slowly to avoid the appearance of steam bubbles inside the drop. It leads to sample boiling and splatter around. In effect, contamination of neighbouring samples. Silicon wafer were installed on observation platform inside microscope chamber with carbon tape. For preliminary control of TEM on-grid samples, the SEM was equipped with proper holder. Settings In this work, SEM images were taken using a HITACHI S-4800 field emission scanning electron microscope operating at an accelerating voltage of 10 or 30 [kV].
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8.2.2. High-Resolution Transmission Electron Microscopy (HRTEM) HRTEM method was used to obtain detailed information on the morphology of studied nanopowder. With its help following information were gathered: internal structure of the particles examined nano-powder; characteristics and the diameter of the dendrites; crystallographic orientations at the ends of the dendrites. Sample preparation HRTEM requires specimen which is both transparent and highly pure. For fine particles studied in this research, specimen preparation procedure included cleaning in order to remove environment contamination and application on supporting grid. Application was done by preparing a suspension in distillated water and applying a drop of suspension on the grid and then drying. Regular grid is made out of copper (Cu). However, since tested specimen contained that element it was required to use manganese (Mn) grid to avoid interference during EDX measurements. Applied specimen preparation methodology was developed on the basis of the information from literature. In particular, from Sample Preparation Handbook for Transmission Electron Microscopy by J. Ayache et al.[349,350]. Settings All High-Resolution TEM (HR-TEM) images were captured on a JEM-2100F transmission electron microscope operating at an accelerating voltage of 200 [kV]. This particular TEM set were equipped with an EDX module. Before HRTEM observation, each sample was subjected to a pre-screening with use of method SEM in order to verify the appropriate quality of prepared test material. 8.2.3. Energy-dispersive X-ray spectroscopy (EDX) EDX or EDS was carried out in order to collect information about elements distribution within nano-powder particles. Base on accumulated results it was possible to determine whether the material contained impurities; chemical composition was uniform throughout the particles or not. Sample preparation and settings Specimen was prepared accordingly to method described for TEM observations. All EDX measurements were performed on JEM-2100F transmission electron microscope with an EDX module, operating at an accelerating voltage of 200 [kV]. 60
8.2.4. X-Ray Diffraction (XRD) In current study, XRD technique was used for two purposes. The first was to identify the chemical composition of test material. The second was quality control and conformity of the composition between series of produced research material. In order to do it XRD patterns were recorded and compared with JCPDS21 card. Sample preparation First step to prepare a sample for measurement was proper drying. using a lyophilizer. This operation was carried out by freeze drying. Then the material was placed on an amorphous holder (in this case made out of glass) with a profiled cavity. The nano-powder was placed in the cavity and distributed in such way to obtain flat, smooth surface with cavity edges. Settings In this work, all measurements were conducted with use of Powder X-ray diffraction (XRD). XRD patterns were recorded on a Rigaku Ultima III X-Ray diffractometer using Cu Kα radiation (λ=0.154 [nm]) in the 2θ range from 5° to 90°. 8.2.5. Thermogravimetric Analysis (TGA) TGA method was used to record mass changes profiles of material reaction for constantly rising temperature. These results in correlation with the other methods were used to confirm material chemical composition, explore the possibility of contaminants presence. Sample preparation The procedure for TGA instruments equipped with differential balance requires preparing specimen and counter weight (Figure 19). For measurements up to 500 [ºC], aluminium containers were used. For hire temperatures, containers made out of platinum are required. As counter mass of choice was Al2O3 due to its thermal stability and low price. Both powders were prepared in approximately the same weight and installed into containers. For measurement, both specimen and counter weight were put into the furnace chamber. In case of TGA apparatus used in this study, an analytical balance situated inside an electric furnace. Outcome data of TGA tests was differential change of mas between specimen and thermally stable counter weight. Joint Committee on Powder Diffraction Standards (JCPDS) – since 1978 known as International Centre for Diffraction Data (ICDD), institution which maintains a database of powder diffraction patterns. 21
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Figure 19. Schematic representation of the TGA instrument with differential balance and protective atmosphere capabilities (base on construction of Hitachi HT-Seiko 6300 TG/DTA instrument).
Settings In this work, TGA was performed using a Hitachi HT-Seiko 6300 TG/DTA instrument at a heating rate of 5 [°C/min] in air and nitrogen atmosphere. Measurements results were collected in range between room temperature and 500 [°C]. 8.2.6. Brunauer, Emmett and Teller (BET) BET method was used to obtain quantitative information about properties of surface of tested material. In particular, specific surface area (SSA) as well as the presence of pores, their type and size distribution. Directly measured value is relative pressure (p/p0)22 used to determinate the saturation of material surface by adsorbed gas (Figure 20a) [351]. The complete adsorption/desorption analysis creates adsorption isotherm on the plot. In order to obtain complete information about pores, it was necessary to conduct full adsorption/desorption cycle. Obtained isotherms were then compered to six IUPAC23 standard isotherms profiles which represents different gas/solid interactions and strictly dependent on the surface morphology (Figure 20b) [351–354]. Base on that information the presence of micropores was verified. With use of proper software, analysis of profiles characteristics was used to determinate pores parameters.
Relative pressure (p/p0) – it is relation between partial vapour pressure of adsorbate gas in equilibrium with the surface at 77.4 K (p) and saturated pressure of adsorbate gas (p0). 23 IUPAC - International Union of Pure and Applied Chemistry. 22
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Figure 20. Surface adsorption of gas molecules under increasing pressure (a). IUPAC standard adsorption isotherms (b) [354]. Schematic representation of the BET instrument without degasser (c).
Sample preparation Prior to measurement, the mass of specimen was carefully measured. Next, specimen cell and then cell with specimen in order to have reference data for further weight measurements. Then, remove of physically adsorbed material from the sample surface is required. To avoid irreversible changes to the surface, the maximum temperature at which specimen was not affected had been identified with use of TGA. Next, specimen is degassed under vacuum conditions. The residual pressure of less than 1 [Pa] was used for this purpose. Degassing at 120 [°C] for 20 [h] allowed for acceleration of measurement and improve the quality of the results. After degassing specimen cell weight was be measured again. Possibly fast specimen was than installed in BET instrument to avoid environment back-contamination. Before starting the measurement leak test has been performed. For this purpose, specimen cell stayed under vacuum by 15 to 30 [min]. During that time, residual pressure stayed constant. Methodology adopted for conducting those measurements was based on the experience of the laboratory support team and good practices described in the literature [354].
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Settings In this work, nitrogen adsorption/desorption measurements were conducted on a Quantachrome autosorb iQ2 gas sorption analyzer at liquid nitrogen temperature. Prior to measurement all specimens were degassed. Specific surface area (SSA) was determined by a multipoint Brunau–Emmet–Teller (BET) method and pore volume was determined by using the Barrett–Joyner–Halenda (BJH) model. Calculations were carried out using dedicated software provided by the manufacturer of measurement instrument. 8.2.7. Dynamic Light Scattering (DLS) and Zeta-potential DLS and Zeta-potential methods were performed to provides information about particles properties and interactions. In specific, DLS allowed for determining the size distribution of specimen particles. While Zeta-potential was used to determinate electrokinetic potential of particles in colloidal dispersions. Depending on the zeta-potential value particles behave differently. Zeta-potential close to zero is related to rapid coagulation or flocculation. Meanwhile, high values of zeta-potential (more than 61 [mV]) effects in excellent stability [352]. Sample preparation To conduct measurement, 0.1 [mg] of purified specimen had been suspended in medium with known viscosity (distillated water in this case). Mixing was conducted with sonication assistance. The suspension was then placed in a plastic specimen cell and installed in machine to conduct measurement. The test material was prepared directory before the test to ensure adequate quality of suspension. Settings All DLS and zeta-potential measurements described in this work were conducted on Beckman Coulter Delsa™ Nano zeta potential and micron particle size analyser. Measuring procedure and all settings used pre-programmed measurement set delivered by device manufacturer. This applies to both the setting parameters of the cell and the suspension medium.
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8.2.8. Temperature-Programmed Reduction (TPR) TPR method is used to determine the physico-chemical behaviour of the surface of heterogeneous catalysts. In this work TPR was used to obtain reactivity and reduction rate profiles as a function of temperature. As the method is specific for the study of catalysts, a brief introduction may be needed. In TPR technique, a catalyst or catalyst precursor is exposed to constant flow of a reacting gas. The temperature of catalyst is linearly increased while measuring the composition of gas mixture that leaves the measuring system. The measurement is carried out under low partial pressure of the reactive gas. The reactive gas mixture typically consists of a few volume percent of hydrogen (H2) or carbon monoxide (CO) mixed with an inert gas [355]. By continuous measurement of reducing gas composition, experiment allows for determination of most efficient reduction conditions.
Figure 21. Diagram of a multipurpose apparatus for TPR, pulse chemisorption, TPD and TPO [355].
Sample preparation Approximately 15 [mg] of catalyst was placed between two pieces of glass wool in the Ushaped tube located in the reactor (Figure 21). In order to obtain a clean surface and to eliminate undesired contaminants, all samples had been pre-treated with high temperature under constant flow of He with a flow rate of 10 [cm3/min]. Physisorbed water and other pollutants should be completely removed from sample before analysis.
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Settings In this work typical TPR procedure were conducted on a Micrometrics Equipment model AutoChem II TPR/TPD-2910 fitted with a TCD detector. During TPR measurements specimen were subjected to 1 [%] CO/He stream at a heating rate of 10 [°C/min] and constant flow rate of 20 [cm3/min]. 8.2.9. NO-remediation catalysis test Final stage of catalytic test is to test material catalytic performance under close to real conditions. As there is no single universal catalyst, the methods used to conduct the tests are not standardized. Remediation is one of available catalytic tests methods allowing to easily fitting the required specification by proper test atmosphere and temperature selection. The NO-remediation catalytic test is a method in which the material is exposed to atmosphere consisting of a mixture of NO and CO. the atmosphere is designed to test the reducing catalyst, with general reaction should take place according to the Equation 9. Used in this study dedicated laboratory installation contains three main sections (Figure 22).
The atmosphere preparation chamber.
Gas circulation loop equipped with a by-pass to isolate specimen cell.
Sampling system with the ability to redirect it into the chromatograph.
During the test, a gas mixture was circulated through the catalyst at different temperatures. The amount and composition of circulating gas was well defined. Around 5 [min] after closing by-pass and exposing specimen to atmosphere the specimen was isolated again. At that time sample of atmosphere was extracted from the loop and transferred to gas chromatograph to monitor reaction products formation. Every time loop atmosphere was sampled, the pressure inside the loop slightly decreases. The atmosphere sampling was repeated as long as it was possible to obtain constant pressure in the sampling chamber. The design of the experiment ensured comparability of results. For this purpose, the measurements at each of temperatures was performed on a new sample. Sample preparation Specimen preparation required that catalyst was first vacuum-dried in the reactor by 30 [min] and pre-heated to 50 [°C], prior to the measurement. During the oven warm up, the test atmosphere was prepared. Each gas mixture component was introduced separately to prepreparation chamber. Amount of introduced gas was controlled by pressure measurement.
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After the introduction of the first gas component to the loop, the pre-preparation chamber was vacuumed.
Figure 22. Schematic representation of NO-CO remediation installation used for catalytic performance tests.
Settings During the experiment specimens were heated up to 100 and 150 [°C]. The gas mixture composition that tested material was exposed to consisted 5.00 [kPa] of NO and 5.00 [kPa] of CO (volume ratio CO:NO = 1:1). Circulating-gas reactor was equipped with a gas chromatograph Shimadzu GC-8A. The formation of N2O and CO2 was monitored in intervals of 5 to 7 [min].
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9. Results & discussion
9.1. Selection of starting materials and synthesis conditions The work described in this section was a huge part of the research time, since it was purely empirical. The result was evaluated by elimination of unsuccessful attempts of carrying out the reactions and recursion base on collected experience. For this reason, information contained here relates mainly to the negative results of experiments. Methodical screening was aimed to develop starting composition with single product. A product having characteristic features of potential catalyst. CuCl2 was first attempt to give some product. With initial tested stoichiometry of 1:1 (Cu salt to ligand) it required temperature of 75 [°C]. Bellow that temperature non reaction product had been spotted. Effects such as solution separation, any kind of precipitates or other changes that may suggest the initiation of a chemical reaction were not been noticed as well. From this point stoichiometry of 1:1 (Cu salt to ligand) with solution molarity of 0.1 [M] was taken as standard for results comparison purposes and to reduce the number of necessary reactions. Further work with CuCl2 resulted in irregular product with a particle size of several tens to several thousands of nanometers. The particles do not have a specific shape or noticeable structural patterns. Along with increasing the reaction temperature above 100 [°C] particles size less than 500 [nm] had begun to dominate. It gave hope to filter fraction of interest. Unfortunately, these particles very quickly have melted under electron microscope beam. Cu(NO3)2 was another investigated salt. Subjected to the same test procedure showed no interaction under ambient condition. Solution subjected to higher temperature gave irregular product with non-specific shape or size. The use of higher reaction temperature did not affect features like size or shape. However, the appearance of the solution after passing reaction clearly indicated that below temperature of 75 [°C] the reaction was not completed. Up to temperature of 150 [°C], increase in the amount of reaction product was clearly visible. In order to verify whether change of stoichiometry will improve reaction performance, additionally stoichiometries of 1:2 and 2:1 were tested. The change did not bring noticeable changes in particles morphology, include temperature dependent behaviour of reaction. The only change was the color of the solution after the reaction. It is indicating that one of the reactant remained unreacted, as well as 1:1 stoichiometry is close to optimal reactant proportion. 68
CuSO4 was the third investigated copper salt. Below the temperatures of 75 [°C] the chemical reaction did not occur. Starting from a temperature of 75 [°C] the product of the reaction was formed, however heavily contaminated. The attempts to purify the product allowed to establish that contamination was the result of reaction of the by-products. Most likely after salt donates the copper ion remaining sulfate residue (SO42-) reacts with the ligand. This process can be compared to vulcanization and can explain why even the use of multiple purification stapes with organic solvents did not allowed for purification of the reaction product. Eventually, the CuSO4 was rejected because of failure to comply with the principles of green chemistry. Table 6. Short summary of the results obtained under the given temperature of synthesis proper for copper salts used in screening tests with 1:1 salt to ligand stoichiometry. Temperature [°C] CuCl2 Cu(NO3)2 CuSO4 Cu Acet. No response No response No response Weak response Room temp. Irregular Repeatable No response No response 50 structures The residual Irregular Product heavily Repeatable 75 product structures contaminated Irregular Irregular Product heavily Repeatable 100 structures structures contaminated The product Irregular Product heavily Repeatable unstable under 150 structures contaminated the SEM The product Irregular Product heavily Repeatable unstable under 200 structures contaminated the SEM
The lack of progress with inorganic acid salts caused shift of focus to organic salts. Selected for investigation Copper (II) Acetate proved to be good candidate. Solution prepared at a concentration of 0.1 [M] with 1:1 stoichiometry when left alone for 24 [h] initiate the reaction, leaving behind light blue sediment. Depending on the ambient temperature, sediment showed the presence of yellowish tone of color. From a temperature of 50 [°C] the reaction product was composed from dendritic particles. Together with particles, sludge was present in the product. Base on the previous experiences a series of tests was carried out. To safely separate particles from sludge required to re-disperse sediment after reaction completion. Originally performed on a vibration table, it proved to be safe with usage of sonication bath. The key to a successful separation is fast start of centrifugation. As well, time of centrifugation had to be limited to avoid re-deposition of sludge. Here, optimal time occur to be 5 [min] with centrifugal acceleration of 4800 [G]. In order to remove residual sludge from 69
particles surface, sediment with particles had to be re-dispersed in clean solvent and centrifuged again. After initial centrifugation to remove reaction by-products, optimal order of cleaning proved to be water, ethanol and water. The next stage of the work was the mastering of reproducible synthesizes of the material. The main effort was focused on the preparation of materials for the synthesis. The reaction was found to be sensitive to changes in stoichiometry. The greatest impact and source of problems on proper balancing of reaction, turned out to be caused by hygroscopic properties of 2PPM. 2PPM is provided in the form of crystals or chunks. Preparation of powder required for further work with reagent exposes it to the atmosphere. In effect, with time reagent gains water. Slowly, but it happens even when container is closed. Since it melting point is around 69 [°C], attempts have been made to measure 2PPM mass in melted form. However, they proved to be impractical and have been abandoned. The problem was solved by reducing the time of exposure to humidity, sealing the container with teflon tape and storage of reagent in the refrigerator. This approach allowed for accurate measuring of reactant weight which solved the problem of reproducibility for reactions starting from 50 [°C]. Since it was clear that the reaction underwent initiation at close to the room temperature, in order to test the feasibility of synthesis at this condition a decision was made to conduct a reaction aided by sonication. The reaction was coming to an end in each case, but a product was not fully repeatable. First, the influence of sonication caused temperature increase. After 24 [h] of reaction time, the temperature of the bath was between 35÷40 [°C]. Secondly, the temperature in the laboratory was found to affect the reaction outcome as well. After solving the problem of reproducibility, next step was to determine the effect of solvent composition, as well as salt to ligand ratio. The study included alternative stoichiometries of Cu Acet. to the ligand, as well as presence ligand itself. Ethylene glycol served as an organic solvent for solvent composition trials. The results of this study were summarized in Table 7. Table 7. Summary of the results obtained for specific solvent composition for tested Copper (II) Acetate to ligand stoichiometry at temperature of 75 [°C]. H2O / C2H6O2 H2O / C2H6O2 H2O / C2H6O2 Stoichiometry [Cu Acet. / ligand] 100/0 [%] 50/50 [%] 0/100 [%] 1:0 No response Known structure No response
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1:2
Weak response
Irregular structures
No response
1:1
Fully repeatable
Irregular structures
No response
2:1
Low repeatable
Low SSA
No response
This survey showed that presence of ligand is responsible for particles morphology. The best conditions for reaction to took place was close to 1:1 ratio. Importantly, from the cases described in literature it known that organic solvents are typically used to slowdown the synthesis [5]. In the case of those reaction trials, a slowdown caused the loss of its ability to achieve hierarchical morphology or completely prevented the reaction to occur. Adopted methodology gathered experience allowed to adjust reaction stoichiometry and conditions, as well as for elimination of conditions and obtained nanostructures known from the literature [320,330]. With iteration approach, it was possible to determine that the stoichiometry of 0.95 of 2PPM to 1 of Copper (II) Acetate and the solution molarity of 0.107 [M] permits to conduct reaction in which the morphology of the end product is controlled by reaction temperature. 9.1.1. Description of selected synthesis method Copper (II) Acetate and 2-piperidinemethanol (2PPM) with 0.1 [g] of Copper (II) Acetate and 0.06 [g] of 2PPM purity grater then 99.9% were dissolved in 10 [mL] of distilled water. Then, solution was loaded into a 20 [mL] Teflon-lined stainless steel autoclave and kept at specified temperature for 24 [h]. Subsequently, the reaction was allowed to cool to ambient temperature and mild sonication was applied. The resulting dark brown precipitates were sonicated, then centrifuged and washed twice with Milli-Q filtered water, ethanol and water then were freezedried. A schematic explanation of the synthesis preparation and product purification is shown in Figure 23. Conducted experiments covered a temperature range between room temperature and 200 [°C]. However, the reaction conducted under room temperature required sonication in order to avoid sedimentation of half-products.
Figure 23. Schematic representation of copper oxide (CuO) nanostructure particles synthesis preparation and product purification.
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The reactions where half-product sediment prior to its finish, re-dispersed sediment did not finished reaction. A side effect of sonication was gradually rising of the solution temperature. Depending on the ambient temperature, solution temperature could even reach 40 [°C]. This effect did not occur in solutions stirred intensely with magnetic pill stirrer. However, the reaction needed more than 24 [h] to complete that time.
9.2. Characterisation of temperature controlled synthesis products The subjects of characterisation were products of selected hydrothermal synthesis reaction. Conducted screening tests allowed to eliminate not promising compositions and stoichiometries of starting materials (as described in chapter 9.1). The reaction selected in section 9.1.1, permits to conduct reaction in which the morphology of the end product is controlled by reaction temperature only. Simultaneously, the products of this reaction have searched features described in the chapter 4. Their study and description was contained in this chapter. 9.2.1. The morphology of as prepared CuO The study conducted over the reaction products covered synthesis temperature range from room temperature up to 200 [ºC]. The most interesting are products synthesized below 100 [ºC]. According to prevailing opinion, it is only possible to obtain CuO nanostructures at temperatures higher than 100 [ºC] [5]. Regardless the fact that hydrothermal synthesis has been conducted below 100 [ºC], obtained materials exhibit interesting dendritic substructure. This indicates highly developed specific surface area. It had to be assumed that the space between the dendrites was accessible. In the case of material synthesized at room temperature, typical morphology was complexes of unidirectional dendrites. Observed particles were approximately 450 to 500 [nm] long with dendrites diameter of 10 [nm] each (Figure 24). Previous investigations of hierarchical CuO nanostructure indicated to call base unit of such morphology as “sheaf-like” (Figure 25) [5]. Closer look at bigger group of those particles point to distinct lack of such single units (Figure 24a). Numerous connections and offshoots between sheaf-like units indicate slow particles nucleation process [356]. Large heterogeneity between syntheses series were most likely related to arbitrarily determined temperature. Applied sonication period were constant. Nevertheless, depends from series product were almost only sheaf-like or sheaf-like complex (Figure 24b and c). In cases where the synthesis final temperature did not exceed 40 [°C],
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final product created multi-complexes without specific growth direction of dendrites (Figure 24d). a)
b)
c)
d)
Figure 24. Sample synthesized at room temperature with sonication assistance.
Particles synthesised at 50 [ºC] were characterised by highly uniform morphology (Figure 26a). In opposite to material obtained at room temperature, morphology of those particles was fully reproducible and homogeneous. Particles with almost unidirectional dendrites with size between 300 up to 500 [nm] in length and dendrites diameter of around 12 [nm] were observed. According to literature, similar morphologies use to be called shuttlelike nanocrystals [5,357]. Importantly, we can still observe both sheaf-like and shuttle-like structures sideFigure 25. Schematic representation
by-side (Figure 26c). It indicates that change between the of the “sheaf-like” morphology. dominant morphology occurs (Figure 27). By
comparison with literature there are reasons to think that synthesis conditions are on the verge between optimal nucleation and dendrites growth speed [356]. Interesting observation was single broken particle captured with uncovered internal features (Figure 26d). Internal structure of the particle highly resembles those observed on surface of
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particles what indicates that hierarchical nanostructure is present in entire volume of each particle. a)
b)
c)
d)
Figure 26. Sample synthesized at 50 [ºC].
Further analysis of literature data showed that nanostructures reported by Zhang [357] and Pan et al. [358] exhibited similar dendritic substructure. However, in both cases surfactantassisted hydrothermal routes were used at a temperature of 120 [ºC]. Vaseem et al. [359] reported lowtemperature
synthesis
of
flower-shaped
CuO
nanostructures by solution process using copper nitrate, sodium
hydroxide,
and
hexamethylenetetramine.
Figure 27. Schematic representation of transition between sheaf-like and and no discussion was made of CuO nanostructures and shuttle-like nanocrystals morphologies.
However, the experiments were conducted at 100 [ºC]
their textural properties for materials obtained from syntheses performed below 100 [ºC]. There exist only a few reports on the production of CuO nanostructure below 100 [ºC] [5]. Nanostructures with different morphologies, it generally requires temperatures above 100 [ºC] [357].
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a)
b)
c)
d)
Figure 28. Sample synthesized at 75 [ºC].
Particles synthesised at 75 [ºC] have morphology of shuttle-like nanocrystals (Figure 28a) [5,357]. Average length of particles decreased to approximately 250 [nm]. The same time observed diameter of dendrites doubled to around 20 [nm]. What is the most interesting, dendrites have not extended the entire length of the particles anymore, what can be visible in Figure 28d. Depending on the location of dendrite in the particle, the ratio between diameters to length is approximately 1:4 in particle body and 1:10 in side areas. Behaviour when hierarchical structure of particles was remained while the substructure grinds have already been reported. Shuttle-like nanocrystals synthesised by oriented attachment by Zhang [357] supports suggested by him mechanism of nanorods aggregation
and
diffusion-control
self-organized
growth. However, do not explain low temperature of Figure 29. Schematic representation of
synthesis. Some clue may be the fact of use shuttle-like nanocrystals morphology. cetyltrimethylammonium bromide (CTAB) which is
surfactant not ligand. Similar effect obtained by reported by Rao et al. [360] Sol-gel nucleation and annealed in air for 3h at 150 [ºC]. However, in this case dendrites are 75
shortened by decomposition. At comparable temperature of 80 [ºC], Tamaekong et al. [311] obtained agglomerated nanospheres with an average diameter of 10–20 [nm]. Tamaekong as well used Copper (II) Acetate. However, in his work sodium hydroxide (NaOH) were used as the precursors, and ethanediol (C2H6O2) as the solvent. Higher viscosity of the solvent decreases the rate at which particle growth occurs [356]. Partially, it allows to link both cases but it is not direct evidence of reduced the range of grain growth by reduction of resources available for nucleating particles.
a)
b)
c)
d)
Figure 30. Sample synthesized at 100 [ºC]
With further increase of synthesis temperature, observed effects manifest much stronger. At high level, shuttle-like morphology is still clearly distinguishable
(Figure
30a
and
b).
However,
substructure continues to comminute. Particles external dimensions decreased to approximately 300 [nm]. Previously dendritic structure was transformed into Figure 31. Schematic representation of
elongated subparticles with more or less uniform one transition between shuttle-like a cubic morphology. dimension of approximately 25-30 [nm]. 76
Most reported synthesis conducted at this temperature resulted in morphologies similar to those obtained at 50 [ºC] with method described in this dissertation. Zhang et al. [319] reported obtaining such result by using cupric chloride (CuCl2·2H2O) and sodium hydroxide (NaOH) with SDBS24 surfactant support (Figure 16a). Xu et al. [314] reported synthesis of Urchin-like particles prepared by thermal decomposition of copper hydroxide at 100 [ºC].
a)
b)
c)
d)
Figure 32. Sample synthesized at 150 [ºC]
Above 100 [ºC] previously seen hierarchical nanostructure were totally lost. Obtained, cubical morphology with average particle size of 40 [nm]. Produced material clearly showed a strong tendency to agglomerate (Figure 32). It was determined that it was a characteristic of the material itself and not the effects of contamination (Figure 36e). Despite prolonged sonication, material always remains agglomerated. However, this did not strongly affect its activity. Particles exposed to the beam of SEM have quickly melted. During high resolution imaging acquisition particles loosed their cubic structures and became rounded (Figure 32d).
24
SDBS – sodium dodecylbenzenesulfonate (C18H29NaO3S)
77
With the further raising of the reaction temperature particles morphology remained the same. For synthesis conducted at 200 [ºC] average particle size decreased to around 30 [nm] (Figure 33). Noticeable, more irregular shape imposes that the influence of ligand is somehow disturbed. Comparing syntheses products obtained at temperatures of 150 and 200 [°C], the relationship between the reaction temperature and the nucleation rate is clear.
a)
b)
c)
d)
Figure 33. Sample synthesized at 200 [ºC]
Nevertheless, it has to be noticed that at discussed level of temperatures literature describes most interesting morphologies. Yang et al. [321] described procedure of obtaining several morphologies at 150 [ºC] (flower-like, boat-like, plate-like and ellipsoid-like) with use of Cu(OH)2 as precursor in aqueous solution containing polyethylene glycol (PEG) and aqueous ammonia (NH3·H2O). In case of this work, controlling factor was time. Teng et al. [322] described flower-like CuO nanostructured synthesis in temperature range of 100 to 180 [ºC] by immersing copper threads (99.99 [%], 10×1 [mm]) in 2.0M HCl solution and then hydrothermal synthesis in solution of K2Cr2O7 and H2SO4. Yu et. al [320] obtained Templatefree CuO/Cu2O Composite Hollow Microspheres at 200 [ºC]. With use of copper (II) acetate as a precursor for hydrothermal synthesis, hollow microspheres were obtained. Controllable 78
diameter between 0.5 to 5 [µm] and composition of 23.4 to 80.6 [wt%] of Cu2O were controlled by precursor concentration and reaction time. Based on the literature, it should be assumed that the ligand is responsible for a significant reduction in synthesis temperature as well as, it interaction with the Copper (II) Acetate salt is driven force for the reaction.
Figure 34. Schematic representation of transition between morphologies with increasing temperature of synthesis.
Figure 35. The synthesis product morphology changes according to the reaction temperature. All main images are magnified 100k.
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Morphological changes as temperature rises. This process creates a predictable function between three morphologies (Figure 34). It becomes even more evident when images of individual products are presented together (Figure 35). Sheaf-like structure features that are very clear below 75 [ºC] slowly transit to shuttle-like morphology indicate the continuity in mechanism process of change. Because the only variable that remains is temperature, this change must be linked precisely with this parameter. A clear distinction between the synthesis below and above the water boiling point is obvious here. It has to be noted that the materials were synthesized in an autoclave. As is apparent from the basic theories about grain growth in micro and nanoscale, such parameters as pressure and temperature are directly related to growth speed [356]. This means that up to 100 [°C], synthesis occurred at a pressure only slightly higher than atmospheric (thermal expansion of the trapped air in the autoclave and a slight expansion of the water has been taken into account here). As established before, the presence of ligand significantly reduced the temperature range in which synthesis of complex morphologies occurred. However, to even approximately determine kind of mechanism of self-organization, it should be determine whether the ligand served as a scaffold or coordinator during synthesis. The answer to this question can be determined only by examining the chemical composition of material.
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9.2.2. Chemical composition of as prepared CuO Chemical composition analysis was performed using the XRD method. Measurements showed that up to 150 [°C] product of the synthesis reaction consists solely CuO. At reaction temperatures higher than 150 [ºC], XRD data showed coexistence of CuO and Cu2O. The fact that Cu (II) is readily reduced to Cu (I) somewhere 150 [ºC] implicates existence of another thermodynamic equilibrium. Similar observations have been made by other researchers [361,362]. But, it is hard to pinpoint the cause or mechanism that leads to change of oxidation state, nor is it the objective of these studies.
CuO + Cu2O
CuO
Figure 36. XRD patterns of as prepared material in function of hydrothermal reaction temperature. The vertical lines indicate the position of diffraction peaks from JCPDS25 cards of CuO (PDF file No.05-0661) and Cu2O (PDF file No.05-0667).
XRD results show a discrepancy between the mechanisms responsible for the formation of chemical composition and formation of particle morphology. As is was shown schematically (Figure 36), first qualities of cubic morphology start to appear at 100 [°C]. Particles synthesized at a temperature of 150 [°C] already have a clear cubic morphology, while Cu2O
25
JCPDS - Joint Committee on Powder Diffraction Standards. Old name of ICDD. Shortcut used for the old X-ray characterization pattern cards published before 1973.
81
becomes clearly visible starting from XRD results of the product synthesized at 180 [°C]. Base on those findings, it should be assumed that these mechanisms are completely independent from each other. In order to determine kind of mechanism of self-organization, possible presence of reaction by-products had been exanimated. If the ligand served as a scaffold in synthesis, it would leave impurities in the final product such as a ligand itself or reaction by-products. However, detailed analysis of XRD results showed no presence of any contaminants. Even the most expected by-product, Cu(OH)2 was not present in diffraction patterns (Figure 36). That is indicating completion of the reaction. More importantly, indicates that the presence of 2PPM leads to formation of CuO at reaction temperatures well below 100 [C] and refers to coordination as its role in the synthesis. 9.2.3. Specific surface area (SSA) of as prepared CuO In order to investigate the morphological dependence of surface area of these CuO nanostructures, nitrogen adsorption/desorption measurements (BET) were performed.
Figure 37. CuO particles Specification surface area (SSA) and synthesis reaction temperature with the context of summary of the literature data [260,287,310–323].
The largest specific surface area (SSA) were measured for products of synthesis conducted at room temperature (179.39 [m2/g]). However, it is characterized by almost twice more variation in the measured value, compare to results of 50 [ºC] synthesis (Figure 37). Product 82
of 50 [ºC] synthesis leads to SSA of 146.78 [m2/g] with much lower variation and fully reproducible morphology. As the temperature increases, clear decline in SSA is visible. By comparing Figure 36 with Figure 37 this dependence should be linked to an increase of the diameter of the dendrites. Their simultaneous shortening does not compensate this effect until around 80 [ºC] when dendrites length to diameter ratio dropped below 10. The plotted results support the theory that two different mechanisms are responsible for the formation of a nanostructure of material. Low-temperature mechanism responsible for formation of sheaf-like and shuttle-like hierarchical nanostructures and the high-temperature mechanism resulting in fragmentation of grains. It can be assumed that the fragmentation of dendrites observed at 100 [°C] is a result of second mechanism (see Figure 35d). As a result, a clear increase in SSA of material synthesized at 100 [°C] should be considered as a result of the simultaneous operation of both mechanisms.
Figure 38. Nitrogen adsorption/desorption isotherms and the corresponding pore size distributions of CuO nanostructures fabricated at different temperatures: ambient (a), 50 [ºC] (b), 100 [ºC] (c), and 200 [ºC] (d).
83
Additional analysis of the course of nitrogen adsorption/desorption isotherms and the corresponding pore size distributions showed that the material obtained at temperatures below 100 [°C] has significant porosity (Figure 38a and b). With decreasing reaction temperature, a strong increase of pores smaller than 2 [nm] were registered. This value is comparable with size of spaces between the dendrites. It should be assumed that the products of synthesis at lower temperatures have a much more compact structure (compare with Figure 35a to c). At the same time, it would support the assumption that the morphology obtained in this temperature range is a collection of interconnected dendrites. In the context of the results from the literature, presented method get much better or comparable results to those obtained by alternative methods at a given temperature (Figure 37). Distinctive out among other result of 189.25 [m2/g] obtained at room temperature by Pillewan et al. [310] is specific case. In this work CuO were introduced on mesoporous Al2O3 and then calcined at 450 [°C]. In all other cases, SSA results obtained in this work were better than the reported results of the direct synthesis under hydrothermal conditions. This means that the method presented here can be regarded as one of the most versatile methods of producing CuO. Should also be noted that Bottom-up approach is in the vast minority in acquiring this type of structures [5].
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9.3. Selection of potential catalyst Due to complexity of catalytic effect, material tailored for specific reaction have to be characterised by features that will fit to it purpose. In case of NO – CO remediation, basic feature is reducing character of such material. Reducing capabilities of CuO are well known and were described in few previews chapters. According to results obtained in first part of this research, products of synthesis produced at temperature up to 150 [°C] consists primarily of CuO (Figure 36). This material shows a substantially higher reducing capabilities then Cu2O, as well as much better selectivity in desired direction. Described in detail in the chapter 6 characteristics of CuO cause that all the reaction products synthesized above a temperature of 150 [°C] shall be automatically rejected. Second condition, which is highly expanded SSA further limits number of possibilities. Base on BET measurements, the best candidate is material synthesised at room temperature (RT). Highest SSA of RT is definitely strong advantage here. However, its morphology varied between specific series of synthesis. Assumed industrial requirements for future catalyst does not allow for such a wide range of fluctuations. Problem can be overcome by mixing together different series of particles, but it could not ensure scientifically valid results that properly could describe properties of the material. For this reason, this material has been rejected as well. Next in line, particles synthesised at 50 [°C] had much more consistent SSA between the synthesis series. SSA of 146.78 [m2/g] is still value significantly better the values obtained from the other test temperature synthesis (Figure 36). A more detailed analysis of nitrogen adsorption/desorption isotherms shows that porosity of particles synthesised at 50 [°C] is inferior to room temperature product only in the case of smallest pores (less than 2 [nm]). What is important, morphology is much easier to reproduce duo to full control over temperature during synthesis (Figure 26). Detailed analysis of results of initial experiments as well as wide assessment of the consequences for further research, for further work has been selected particles synthesised at 50 [°C].
85
9.4. Characterisation of selected material before catalytic test To be able to fully understand catalyst interactions, prior catalytic test material has been detail characterised. Investigation allowed to fully seeing hierarchical structure of CuO particles.
Figure 39. High resolution SEM images of as prepared nano-structured CuO particles.
High-resolution imaging revealed that individual dendrites are not precisely parallel (Figure 39c). Additionally, TEM revelled that internal structure of particles have a non-uniform density (Figure 40a). Lack of alignment is good explanation of a large SSA of 146.78 [m2/g]. At the same time, high magnification of individual dendrites revealed that these structures do not branch (Figure 40d). Bundles of nanorods have in average diameters of about 10 [nm]. Selected area electron diffraction patterns (Figure 40c) confirm that these nanostructures are polycrystalline in nature and shows almost uniform crystallographic orientation of all structures the field of view (Figure 40d). By that, previously described as a dendritic structure could be more precisely described as fibrous nanorod morphology. It also means that the grain growth takes place starting from the germ along the one axis only [356,363].
Figure 40. TEM images of nano-structured CuO particles obtained at 50 [°C].
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Further investigation by TEM/EDS analyses revealed the homogeneous distribution of copper and oxygen in the nanostructures (Figure 41). HRTEM images of CuO nanostructures (Figure 42) provided further structural information about individual nanorods surface characteristics. Evidence shows that this nanostructured CuO consists of bundles of polycrystalline nanorods or dendrites. The interlayer spacing distances (d) of 0.22 and 0.25 [nm] respectively correspond to the (130) and (111) planes of the monoclinic phase of CuO. The corresponding fast Fourier transform (FFT) patterns (inset in Figure 42c and d) also indicate that this nanostructured CuO is polycrystalline in nature. Irregular surface promotes the presence of surface defects and by that repeatedly increases the chance of occurring of catalytic reactions [364,365]. Planes of crystallographic direction orientation change (Figure 42c and d) simultaneously explains the lack of alignment and uniform global orientation of the crystal structure of the particles (Figure 40d and inset in Figure 42c and d).
a)
b)
c)
d)
Figure 41. HRTEM & EDX with X-ray spectroscopy (EDS) of nano-structured CuO particles obtained at 50 [°C] bright field TEM image (a), elemental mapping of Cu (b), and oxygen (c), and their distribution (d).
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a)
b)
c)
d)
Figure 42. HR-TEM observations of CuO nanostructures fabricated at 50 [C]. The insets of panel c and d represent the fast Fourier Transform (FFT) patterns.
Figure 43. Particles and agglomerates size distribution of nano-structured CuO particles obtained in 50 [°C]. DLS results.
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Conducted DLS provided information about particles size distributions with highest share of 353 [nm] by number (Figure 43). Values below that value can be assigned to width. However, first two measured fractions are responsible for approximately 60 [%] of readings combined. Measured Zeta Potential of the material gave value of 14.82 ± 7.5 [mV]. According to data available in literature, that value indicate incipient instability of suspension [352,366]. It is mean that material has a moderate tendency to coagulate which means that agglomerates are responsible for larger size fractions. To determine the thermal stability of CuO particles, thermogravimetric analyses (TGA) have been conducted. Test (Figure 44a) indicated that there is very little difference in the mass losses of the nanostructured CuO observed during heating in air or in nitrogen atmosphere.
Figure 44. TGA results of material obtained at 50 [°C]. Results for measurements up to 500 [°C] under air (red) and nitrogen (green) atmosphere and TGA of ligand 2PPM (blue) under nitrogen atmosphere.
89
According to those results, the oxide state of Cu (II) is stable in both air and protective N2. For comparison, the TG curve of 2PPM recorded in nitrogen atmosphere is also shown (Figure 44b). 2PPM decomposes almost completely by 150 [C]. TGA profiles of the nanostructured CuO heated in air (red curve in Figure 44a) and in nitrogen (black curve in Figure 44a) indicate that the CuO nanostructures are stable in the temperature range studied. Previous reports of nanostructured CuO obtained by a hydrothermal decomposition route [332] have suggested that if reaction temperature during synthesis is lower than 100 [C], the reaction is incomplete and some Cu(OH)2 remains in the final product. XRD study of as prepared material did not show its presence. Presence of Cu(OH)2 could reveal at high temperatures. Nevertheless, XRD conducted on samples after TGA examination din not prove it presence as well (Figure 51a to d). This means that even the reaction had been conducted at 50 [C], both TGA curves and XRD did not reveal such contamination in final product supports ligand coordination as possible mechanism of formation. Additionally, lack of diffraction peaks corresponding to the Cu2O phase in the heat-treated samples demonstrates that once the CuO phase is formed, it is thermally stable even upon heating in air.
Figure 45. SEM images of material obtained in 50 [°C] after TGA at 150 [°C], 200 [°C] and 500 [°C] under air and nitrogen atmosphere.
SEM images of the material after TGA provided certain information about the behaviour of the material at elevated temperature (Figure 45). A smooth surface of particles after TGA indicates increased mobility of surface atoms well below melting point of CuO (1326 [°C]). Despite this, particles preserved similar shape and size but no fine structure was observed (Figure 45a and c). 90
Figure 46. STEM images of material obtained at 50 [°C], after TGA measurement at 500 [°C] under air atmosphere.
Under closer look, material treated by 500 [°C] confirmed the presence of empty space between the rods. Scanning transmission electron microscopy (STEM) image reveals that the CuO has a rather mesoporous structure with average pore size in the range of 15 to 35 [nm] (Figure 46). Similar behavior of materials with nanosized substructure at elevated temperatures could be expected, but never had been reported. Therefore, it cannot be interpreted in a broader context.
Figure 47. Temperature-programmed reduction (TPR) profiles of CO gas for CuO nanostructure (as prepared) and CuO-Sigma Aldrich. Aldrich (203130-5G).
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In order to gain more information about the behaviour of CuO particles surface, Temperatureprogrammed reduction (TPR) has been conducted. TPR profiles revealed three reduction peaks at 124, 200 and 253 [°C] (Figure 47). This indicates the presence of three independent reduction sites most commonly attributed to highly reducible capping oxygen. The energy required to reduce oxygen and release it from a particle’s structure is strongly related with the crystallographic orientation at the material’s surface suggesting, in this case, that there are three different crystallographic planes at which the reduction reaction occurs. For TPR test conducted on commercially available, results are fully consistent with literature data [367,368]. Focusing on the findings of X. Wang et al. [367], CuO reduction under CO flow takes place in range between ~200 and 236 [°C]. In his work, Wang also used commercially available, spherical CuO powder. He pointed that CO oxidation to CO2 starts at approximately 210 [°C]. Gas flow he studied consisted 5 [%] CO and 95 [%] He (5 times CO content of used in this work), the only source of oxygen was the CuO itself. The necessary condition for the NO-CO remediation occurrence is formation of surface defects on the CuO surface. A similar TPR profile was described by L. Wang et al. [369] for CuO containing mesoporous silica spheres. What is important, tested material required an initial passivation to reduce copper ions from Cu2+ to Cu0. The procedure was conducted under N2O flow at 60 [°C]. Following TPR showed peaks at approximately 150 and 235 [°C] with some shifts depending on CuO particles size (diameters between 1.35 to 2.61 [nm]). In all reported cases 150 [°C] peak was present and consistent. However, it should be mentioned that those particles were synthesized in three stage process that included 18 [h] long calcination in 540 [°C]. The comparison of this work original results with literature indicates that developed morphology revelled not seen together before increased reduction of CuO at temperature of 124, 200 and 253 [°C].
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9.5. Characterisation of the material after catalytic test
9.5.1. The results of catalytic performance of the material TPR study prior the catalytic test revealed points of interest that NO – CO remediation catalysis test should be conducted. Additionally, SSA comparison with other reported CuO catalyst performance indicated that at temperature of highest activity could not be optimal [260,342,344,370,371]. According to the theory determination of action mechanism and the stability of the reaction may be questionable [140,249,372]. In order to avoid unnecessary problems, it was decided to skip this range of temperature. Additionally, since one of this research aims is designing catalyst to operate at the lowest possible temperature, two values in the range ± 25 [°C] from 124 [°C] peak has been selected, 100 and 150 [°C] respectively.
Figure 48. (a) Total catalytic conversion of NO to N2 and N2O at 100 and 150 [°C]. (b) Relative production of N2 to remediated gases at 100 and 150 [°C].
Figure 48 shows performance of nanostructured CuO for the catalytic conversion of a stoichiometric mixture of NO and CO gases (volume ratio of CO:NO = 1:1) to N2 (or N2O) and CO2 gases. Nanostructured CuO exhibited 15 [%] conversion of NO gas in the period of 85 [min] after exposure to the gas mixture at 100 or 150 [°C] (Figure 48a). NO gas was converted mostly to N2O at 100 [°C] with almost no conversion to N2 (Figure 48b). N2O is much less toxic than NO but can still contribute to global warming. Should be mentioned that it is one of the most prominent greenhouse gases. In contrast, at 150 [°C] the nanostructured
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CuO selectively promoted the NO-to-N2 conversion. This means that it leads reaction in the desired direction. In comparison with the data available in the literature, obtained results are interesting in the context of selectivity. For example, extensive research conducted by Auxilia et al. [260] showed possibility of obtaining 70 [%] of relative conversion after material activation after 50 [min] of constant exposition of material to commercial engine exhaust in work temperature of 170 [°C]. Tests conducted on commercial catalysts gave relative conversion of 97 [%] for Rh and 40 [%] for Pt base catalyst. However, after 20 [min] of exposition Auxilia reported approximately 100, 90 and 30 [%] gas volume remediation for CuO, Rh and Pt respectively. According to that study, crucial for that result was strong exposition of unstable edges of plates with (001) facets during the catalytic process. Interestingly, experiment included Temperature-programmed desorption (TPD) of NO on commercially available CuO nanopowder with a particle size less than 50 [nm]. Auxilia reported that in the range of 441– 446 [K] (167.85 to 172.85 [°C]) TPD showed peak of NO capacity for desorption from commercial CuO particles surface. In the same work, developed by her CuO based catalyst experience NO adsorption occurs as low as 300 [K] (26.85 [°C]) and desorption at 379 [K] (105 [°C]). As for remediation both, NO absorption and desorption have to occur and the fact that for (001) plane, NO desorption occurs at 105 [°C]. Undesired reaction path observed for NO-CO remediation experiment conducted at a temperature of 100 [°C] should be interpreted as effect of insufficient NO desorption speed. Considering all things, we can determine the boundaries in which desirable NO-CO remediation outcome could happen, where bottom border is marked by 105 [°C] and top by CuO reduction by CO gas. Of course adopted temperature refers to the (001) crystallographic plane. As HRTEM shown, at the ends of rods only (130) and (111) could be observed (back to Figure 42). However, for obvious reasons, it was not possible to determine the crystallographic orientation of the interface between the dendrites which is responsible for the majority of the particle surface.
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9.5.2. The changes in morphology of the material After NO – CO remediation catalysis test, surface morphology and structural characterization of materials were performed as well. SEM images (Figure 49) revealed changes of the surface morphology of particles although their overall morphology and dimensions are essentially unchanged (Figure 49a). Images compared to the original structure of the surface show a clear signs of changes in CuO substructure surfaces (high resolution SEM image in Figure 49c) and suggest the active participation of particulates in catalysis even at such low temperature as 150 [ºC]. Captured image structure indicates that during the catalytic reaction intensive movements of atoms occurs on the rods surface. Rods locally connected to each other on their tips suggest that in these areas the most intense reaction takes place. a)
c)
b)
Figure 49. SEM images of CuO particles after NO-CO catalytic remediation test at 150 [°C].
This indicates that during remediation test surface of the material passes very intense reaction connected with short range material transportation. It could be deduced from the fact that in scale of all particle, material retains its initial shape. In order to clarify the scope of changes the HRTEM imaging has been carried out. Broad view of CuO particles reviled strong fragmentation of particles substructure (Figure 50a). This is direct evidence of the participation of all the available surface of the rods in the catalytic reactions. Additionally, image shows the scope of chemical changes caused by interaction with NO-CO atmosphere. TEN is very sensitive to the density of the tested material. In turn, the density of pure Cu is much higher then it’s oxides. The density of Cu is 8.96 [g/cm3] when Cu2O and CuO are 6.00 and 6.31 [g/cm3] respectively. 95
This means that the zone closest to the outer layers of the particle the most extensively interacts with the atmosphere. Strong darkening observable in that zone occurs simultaneously with the strongest fragmentation of rods. Strong correlation between the variable in depth range and rods orientation indicates that gas exchange between particles core and surrounding atmosphere is most efficient along the rods. Alternatively, it may be related to fast oxygen diffusion along rods due to high potential between strongly oxygen depleted tips and core. The most probably, we are dealing with the combined effects of these two mechanisms.
Figure 50. HRTEM images of CuO particles after NO-CO catalytic remediation test at 150 [°C].
Further investigation reviled scale of short range transportation at most exposed sites of particles (Figure 50b). Is apparent that strong reduction of the rods diameter advancing from the tips towards the particle core where the individual rods are in contact with each other. As visible at point B1, in the case of shorter rods this leads to the formation of the walls and bridges between them. As for longer rods, more extended out of particles body, bridges are more scarce and are formed between rods parallel to each other, similarly to point B2. It is better to look at the single rod (Figure 50c), which will help to determine the distance between crystallographic planes. The most exposed crystallographic plane visible on the individual picture is Cu (110), known for its highest adsorption of N2O [2]. 9.5.3. The changes in chemical composition of CuO XRD carried out after catalytic test showed that in result of exposure to NO – CO atmosphere the material was partially reduced to pure copper (Figure 51e) with additional signal from Cu2O. Analysis did not reveal the presence of CuO in the sample. Depth resolution of XRD method is between ~20 [Å] and ~30 [µm] what is meaning that even if deepest possible range will be assumed, most exposed to the environment sample surfaces have been strongly
96
deprived of oxygen. The dominant signal of pure Cu most probably comes from very edge of particles. In correlation with SEM images, XRD data provides strong evidence for very intensive reaction between CuO and CO molecules inside particles body. Especially, since XRD measurements carried out on post TGA samples showed that once the CuO phase is formed, it is thermally stable (Figure 51a to d), the only explanation is the particles interaction with the surrounding NO-CO atmosphere.
Figure 51. XRD patterns of material (a) after TGA under 500 [°C] and nitrogen atmosphere; (b) after TGA under 500 [°C] and Air atmosphere; (c) after TGA under 150 [°C] and nitrogen atmosphere; (d) after TGA under 150 [°C] and Air atmosphere; (e) after catalytic test; (f) as prepared. The vertical lines indicate the position of diffraction peaks from Crystallography open database (JCPDC) card files.
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9.6. The results in the context of the physical chemistry of CuO surfaces For further analysis, CO and NO molecules interaction with CuO surfaces was considered from perspective of a single, flat crystal surface. This assumption was adopted on the basis of the fact that both NO and CO are strongly polarized dipoles. Even assuming that the molecules will interact in a space between two CuO surfaces, the driving force behind their behaviour will be Brownian motion and local availability of free electrons with resulting Coulomb forces. The key to determine the mechanism by which designed material performs a catalytic reaction is material behaviour during the first minutes of its interaction with NO – CO mixture. In case of both NO-CO remediation experiments the amount of the converted gas changed almost linearly at 100 [°C] but for 150 [°C] process was much more intense at the beginning and weakened with every minute (Figure 48a). Similar effect can be seen for relative production of remediated gases at 150 [°C] where an initial phase catalysis gave much better results than after stabilization. This effect is associated with the first defects formation on the material surface. According to N.J. Lawrence et al. [236] their presence allows for reduction the energy required to initiate dissociation of diatomic linear molecules on the material surface. This effect utilizes reduced distance between the absorbed molecule and catalyst (Figure 52a). In the case of CO over ceria, the presence of point defects at the surface can lower catalytic conversion to CO2 as much as 100 [°C]. The introduction of surface defects allowed to minimalize energy required for reaction to occur and stabilize. Nevertheless, proper functioning NO-CO remediation consists several steps. The first one is adsorption of CO. After adsorption COads stays in so called reactant state. At the moment when COads is capable to react with oxygen at the CuO surface it forms CO2 and leaves the oxygen vacancy behind (Figure 52b). It is a crucial step for entire process since it occurs spontaneously due to all system energy reduction. However, amount of system energy reduction varies depending on the crystallographic orientation of the surface on which CO is adsorbed. According to calculation conducted by Z.Zuo et al. [373] system gains −1.13
[eV]; −0.98 [eV]; −0.90 [eV] respectively for Cu (110) > Cu (111) > Cu (100) in vacuum condition. These values increase significantly when the medium is taken into account. And becomes even greater when we consider a single molecule interaction with direct surrounding of molecule. For specific planes, such a case has been calculated by M.A. van Daelen et al. [374]. In his work system gains -37.7 [kcal/mol] by adsorbing CO on Cu (111) surface (-1.635 [eV] per single adsorption) and -31.9 [kcal/mol] on Cu (100) surface (-1.383 [eV] per 98
site). In the case of CO, the undesirable effect is dissociation. It can occur as a result of surface energy fluctuation, but it spontaneous course occurring stopped by energy barrier of approximately 4.68 [eV] for Cu (111) and 3.95 [eV] for Cu (100). Another required step is adsorption followed by dissociation of NO (Figure 52c). For NO, the dissociation phenomenon is most relevant here. Only dissociated reagents take place in subsequent reactions. In particular, atomic oxygen diffuses into the catalyst which neutralizes oxygen vacancy. In turn, properly dissociated atomic nitrogen reacts with a neighbouring nitrogen atom to form free N2. For the catalyst, it is done by donation of redundant two electrons. Alternative, undesirable path occurs when the NO dissociation occurs only partially. If in direct adjacent of nitrogen is present adsorbed molecule of NO, catalyst forms N2O.
Figure 52. Reduction of the displacement path associated with overlapping energy reactions in and without presence of surface defect (a); reaction path of CO adsorption and its oxidation to CO2 on the catalyst surface (b); possible reaction paths of NO on the catalyst surface (c); schematic representation of energy profile of CO interaction with catalyst surface (d) (adapted from [374]); schematic representation of energy profile of NO interaction with catalyst surface (e) (data from [284,375]).
As experimental results show, catalyst surface dominant crystallographic orientation is Cu (110). One of characteristic of this plane is the presence of natural groove between two rows of atoms (schematic view in Figure 53b). NO behaviour over that plain was studied by 99
A.X. Brión-Ríos et al. [376] by density functional theory calculations (DFT). His work shows that NO molecule adsorption between two rows of Cu atoms results in −0.484 [eV] and only 0.355[eV] when it takes place on the top of them. However, NO adsorbed on the top position are more stable and require much less energy to move along the [1-10] direction, which is parallel to the Cu atoms rows. Additionally, such adsorption has been found to be marginally more stable than configurations tilted along the [001] direction, perpendicular to the Cu rows. From the perspective of catalysis this is undesirable effect since along the [001] NO rows on Cu (110) are relaxing structure. This means that the NO molecules adsorbed on Cu (110) are forced to stay in one of two states, where it is possible to fast dissociate or allows for fast movement along the surface of the material. Because in the second case is much harder to give up oxygen, it must be assumed that this path will promote N2O formation. The experiment conducted at 150 [°C] has shown that approximately 20 [%] of reaction product was N2O. This means that after the stabilization complete dissociation of NO did not occur at all or this process is too slowly compared to the CO adsorption. This indicates that additional thermal movements allows NO molecules to follow second path. However, the results presented by Scholten and Konvalinka show that Cu (110) is the best adsorbing plane for N2O [2]. In that context, the third option should be consider where readsorbed N2O blocks potential NO adsorption sites thus slowed the process. There is no direct evidence to support this assumption. Nevertheless, experimental data collected during NO-CO catalytic remediation test conducted at 100 [°C] provides partial supports it. Since catalytic process took place bellow NO desorption temperature (105 [°C] [260]), 100 [%]of process output was N2O.
Figure 53. Schematic visualization of phenomena associated with the NO-CO remediation on the CuO (130) surface (a); NO-CO remediation on the Cu (110) surface (b); simplistic model of progressive loss of oxygen by the particle and its impact on changes in the chemical compositions of rods (c).
To sum up, in the initial phase the catalyst behaves as was intended. The reactions are following desired direction on the CuO (130) and CuO (111) surfaces (Figure 53a). However, due to its high capacity to react with CO, material reaches the progressive loss of oxygen. 100
Variability in oxygen short range transportation causes local excess of oxygen depletion and conversion to the Cu (110). At this stage, a Cu (110) takes over as catalyst for the reaction and the course of it stabilize. Despite stabilization of the reaction, progressive oxygen loss still occurs. XRD of material after remediation did not detected the presence of CuO phase. Found using HRTEM centralization of Cu2O in the interior of the particles suggests that the catalytic process is still supported by oxygen stored within the material (Figure 53c). Without additional testing it is impossible to determine whether the reason is decrease in the ability of the surface to interact with NO, strong reactivity toward CO or perhaps Cu (110) tendency to N2O readsorption. Because determination of exact reaction mechanism was not the purpose of this dissertation, the problem is left unanswered. Presented results, as well as the discussion of the literature survey suggests that in order to improve the selectivity of the reaction, the temperature of the process could be reduced. However, TPR test results indicate that up to approximately 125 [°C] the ability of a material to react with the CO increases. So it would cause further deterioration of the catalyst oxygen loss problem. In turn, raising the temperature is associated with the loss of selectivity due to reduction of NO dissociation process. From a temperature of 175 [°C] to 200 [°C] TPR results for developed here substructural particles and commercial spherical CuO are overlaped. However, it is known that in this temperature range, the commercial material supports reaction in the direction of N2O. What is important in this case that the low-temperature reactivity of CuO with CO is rarely reported phenomenon. The results reported in the literature are derived from materials with different synthesis processes histories, with variable properties and morphologies. They were also tested for various purposes or under custom experiments settings which makes it more difficult to find the right reference. The equipment limitations suppressed TPD test for NO over studied material. Probably, TPD test could be useful to get an answer for question of surface chemistry in this process.
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10.Conclusions and final remarks The aim of the work was to develop single-step and low-temperature method for the large scale synthesis of high specific surface area CuO nanopowder for better catalytic performance in the lower temperature range. In order to achive the aim of the work, broad literature research and case study were conducted and advanced investigation techniques were applied, i.e.: Scanning Electron Microscope (SEM); High-Resolution Transmission Electron Microscopy (HRTEM); Energy-dispersive X-ray spectroscopy (EDX); X-Ray Diffraction (XRD); Thermogravimetric Analysis (TGA); Brunauer, Emmett and Teller (BET); Dynamic Light Scattering (DLS); Zeta-potential; Temperature-Programmed Reduction (TPR) and NOremediation catalysis test. The development of the new CuO base catalyst included the following tasks:
The development of methodology for chemical reaction capable of producing a wide spectrum of morphology.
Characterization of structure and morphology of CuO nanopowder in order to determine the relation between reaction parameters and characteristics of the product.
Optimization of reaction parameters in order to obtain best fitted material for NO-CO catalytic remediation purpose.
Detailed characterization of the structure and properties of CuO nanopowder in order to obtain a complete view of the material before catalytic tests.
A series of catalytic test to determine the catalytic performance of selected CuO nanopowder.
Detailed characterization of the material after the catalytic tests with analysis of the results.
The reaction developed in this study was found to fulfil stated objectives. The original CuO base catalyst synthesis by using hydrothermal method without adding structure guiding agents to synthesise nanostructured material was conducted. Final product was a crystalline with a dendritic morphology containing short nanorod-like substructures resulting in high surface area. Study of the effect of temperature on the morphology of the resulting CuO nanostructure allowed for determination of conditions under which it is possible to provide the most reproducible material with the largest specific surface area. Selected CuO nanostructure with fibrous nanorod morphology was synthesised at 50 [°C] and displayed a large surface area of 147 [m2/g], a value much larger than CuO nanostructures prepared by other methods.
102
Selected catalyst was found to be selective during NO-CO remediation test and promoted the NO-to-N2 conversion at a relatively low temperature of 150 [°C]. After stabilization of the reaction, the catalyst showed the efficiency of 80 [%]. For CuO, it is the highest efficiency in such low temperature between data available in the literature. Nevertheless, process was associated with gradual degradation of the material due to progressive loss of oxygen from the catalyst structure. It was also found that the material has rarely seen reactivity with CO at temperatures below 225 [°C]. Ability of formation of oxygen free radicals at low temperatures suggests that this material could be used for application beyond the purpose for which it was originally designed. It can be expected that this material would exhibit strong anti-bacterial properties. However, to confirm this, it would be necessary to draw away from the originally intended purpose of research. The results of present work pointed to the following conclusions:
The investigation on the synthesis of CuO particles showed that process of synthesis of particle with nanoscale features is very sensitive to the chemical composition of the starting materials and reaction temperature. It is necessary to strictly control the stoichiometry of the reaction and the stability of the synthesis conditions to obtain reproducible reaction product.
It is possible to produce a nanopowder of CuO with highly developed specific surface area in a single stage, low temperature reaction. The method demonstrated here was able to produce CuO nanopowder in energetically efficient and environmentally friendly way.
CuO nanostructures design, that involve synthesis at room temperature remaining a challenging task. However, it may be economically unjustified due to necessity of ensuring adequate mobility of chemicals during the reaction.
TPR examination showed peak of CO reactivity at 124 [°C]. This is one of the lowest reported value for CuO and indicates creation sites with highly active and easily accessible oxygen radical on the surface of nanorods.
During catalytic test results, the temperature of 150 was found to give most selective catalytic conversion. On the other hand, a temperature below 124 [°C] leads to change dynamic of the reaction in undesired direction.
Accumulating evidence showed that catalytic conversion is most active on the tips of nanorods. Additionally, those sites were pointed as most deployed of oxygen. Since these sites are located at the end of 1D structures (rods), they have most limited access 103
to oxygen that could diffuse along rods. By that, those are the sites in which degradation of particles was initiated.
Findings analysis showed that to develop a functional CuO base catalyst with stable chemical composition requires to ensure balance between the reactions of NO, CO and the catalyst. Likely path is to use the developed method as a platform for further research with more complex structures.
The studies were focused on ensuring the simplest method of potential catalytic material preparation while maintaining the principles of green chemistry. The results obtained during the research indicate that such catalyst could be developed. Omit slow reaction rate and lack of chemical stability due progressive degradation, the material investigated possess all the features that perfect catalyst should have. The results presented here have been collected and published [377,378]. The results are an important step forward in the search of a cheap substitute for PGM. Nevertheless, it will require farther extend research to reach that goal.
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List of Figures Figure 1. NO2 tropospheric column over Europe and Southeast Asia. Satellite data obtained by Ozone Monitoring Instrument on Aura Satellite (Aura - OMI) by KNMI/NASA. Visualization with ESA TEMIS v2 [21,22]. .................................................................................................................................................................. 13 Figure 2. Sector share of nitrogen oxides emissions (EEA member countries) (2011) [26]. ................................ 14 Figure 3. Selected national NOx emission from 1970 to 2008 [25]....................................................................... 15 Figure 4. Average annual NO2 changes 1996-2002 (ESA) [31]. .......................................................................... 15 Figure 5. NO2 tropospheric column over Ocean, averaged over 2005 - 2012. Satellite data obtained by Ozone Monitoring Instrument on Aura Satellite (Aura - OMI) by KNMI/NASA [33]. ................................................... 16 Figure 6. Selected international NOx emission from 1970 to 2008 [25]. .............................................................. 16 Figure 7. Prediction of baseline emissions NOx by region for 2010-2050 [35]. ................................................... 17 Figure 8. Nitrogen dioxide 2010 - Annual limit values for the protection of human health, European Environment Agency (EEA), 2012 [87]................................................................................................................ 24 Figure 9. Nitrogen dioxide 2012 - Annual limit values for the protection of human health, European Environment Agency (EEA), 2014 [88]................................................................................................................ 24 Figure 10. Relative abundance of elements in the Earth's upper crust [160]. ....................................................... 32 Figure 11. World production of PGM and Cu between 1900 and 2013 [155,163]. .............................................. 33 Figure 12. Historical market prices of Platinum(Pt), Rhodium (Rh), Palladium (Pd) and Copper (Cu) between 1960 and 2015 [163,171–173]. ............................................................................................................................. 34 Figure 13. Side views of relaxed (101), (001), (011), (010), (110), and (100) crystal planes of CuO (Nature Publishing Group, distributed under CC BY 2.0 license)[282]. ............................................................................ 47 Figure 14. Calculated potential energies for the reaction paths of NO dissociation over Cu (111) (blue lines), Cu-terminated Cu2O (111) (red lines) and Cu-terminated CuO (110) (green lines) surfaces. The top views of the corresponding geometric configurations were shown on insets over the reaction paths. Atoms are represented by: Cu as blue spheres, N as silver spheres, and O as red spheres (Copyright 2014 The Royal Society of Chemistry) [284]. ..................................................................................................................................................................... 48 Figure 15. Schematic description of catalytic NO remediation cycle over Energy-minimized geometry of CuO (001). On CuO (001) it is more energetically favourable to remove oxygen atoms by incoming CO molecules. In effect, an oxygen vacancy and CO2 are formed. Crystal to mineralize energy, remove oxygen atoms from N 2O on those vacancies, and the CuO (001) facet recover its original state. Atoms are represented: Cu as blue, O as red, C as black, and N as yellow dots. (Copyright 2014 WILEY-VCH Verlag GmbH & Co.) [260].......................... 51 Figure 16. Examples of CuO particle morphologies available in the literature. ................................................... 52 Figure 17. Summary of the literature data for CuO particles Specification surface area (SSA) and synthesis reaction temperature [260,287,310–323]. ............................................................................................................. 53 Figure 18. Schematic representation of research methodology. ............................................................................ 58 Figure 19. Schematic representation of the TGA instrument with differential balance and protective atmosphere capabilities (base on construction of Hitachi HT-Seiko 6300 TG/DTA instrument). ........................................... 62 Figure 20. Surface adsorption of gas molecules under increasing pressure (a). IUPAC standard adsorption isotherms (b) [354]. Schematic representation of the BET instrument without degasser (c). ............................... 63 Figure 21. Diagram of a multipurpose apparatus for TPR, pulse chemisorption, TPD and TPO [355]. .............. 65
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Figure 22. Schematic representation of NO-CO remediation installation used for catalytic performance tests. .. 67 Figure 23. Schematic representation of copper oxide (CuO) nanostructure particles synthesis preparation and product purification. .............................................................................................................................................. 71 Figure 24. Sample synthesized at room temperature with sonication assistance. ................................................. 73 Figure 25. Schematic representation of the “sheaf-like” morphology. ................................................................. 73 Figure 26. Sample synthesized at 50 [ºC]. ............................................................................................................ 74 Figure 27. Schematic representation of transition between sheaf-like and shuttle-like nanocrystals morphologies. .............................................................................................................................................................................. 74 Figure 28. Sample synthesized at 75 [ºC]. ............................................................................................................ 75 Figure 29. Schematic representation of shuttle-like nanocrystals morphology. .................................................... 75 Figure 30. Sample synthesized at 100 [ºC] ........................................................................................................... 76 Figure 31. Schematic representation of transition between shuttle-like a cubic morphology. ............................. 76 Figure 32. Sample synthesized at 150 [ºC] ........................................................................................................... 77 Figure 33. Sample synthesized at 200 [ºC] ........................................................................................................... 78 Figure 34. Schematic representation of transition between morphologies with increasing temperature of synthesis. ............................................................................................................................................................... 79 Figure 35. The synthesis product morphology changes according to the reaction temperature. All main images are magnified 100k................................................................................................................................................ 79 Figure 36. XRD patterns of as prepared material in function of hydrothermal reaction temperature. The vertical lines indicate the position of diffraction peaks from JCPDS cards of CuO (PDF file No.05-0661) and Cu2O (PDF file No.05-0667). ................................................................................................................................................... 81 Figure 37. CuO particles Specification surface area (SSA) and synthesis reaction temperature with the context of summary of the literature data [260,287,310–323]. .............................................................................................. 82 Figure 38. Nitrogen adsorption/desorption isotherms and the corresponding pore size distributions of CuO nanostructures fabricated at different temperatures: ambient (a), 50 [ºC] (b), 100 [ºC] (c), and 200 [ºC] (d). ..... 83 Figure 39. High resolution SEM images of as prepared nano-structured CuO particles. ...................................... 86 Figure 40. TEM images of nano-structured CuO particles obtained at 50 [°C]. ................................................... 86 Figure 41. HRTEM & EDX with X-ray spectroscopy (EDS) of nano-structured CuO particles obtained at 50 [°C] bright field TEM image (a), elemental mapping of Cu (b), and oxygen (c), and their distribution (d). ........ 87 Figure 42. HR-TEM observations of CuO nanostructures fabricated at 50 [C]. The insets of panel c and d represent the fast Fourier Transform (FFT) patterns. ............................................................................................ 88 Figure 43. Particles and agglomerates size distribution of nano-structured CuO particles obtained in 50 [°C]. DLS results............................................................................................................................................................ 88 Figure 44. TGA results of material obtained at 50 [°C]. Results for measurements up to 500 [°C] under air (red) and nitrogen (green) atmosphere and TGA of ligand 2PPM (blue) under nitrogen atmosphere. .......................... 89 Figure 45. SEM images of material obtained in 50 [°C] after TGA at 150 [°C], 200 [°C] and 500 [°C] under air and nitrogen atmosphere. ...................................................................................................................................... 90 Figure 46. STEM images of material obtained at 50 [°C], after TGA measurement at 500 [°C] under air atmosphere. ........................................................................................................................................................... 91 Figure 47. Temperature-programmed reduction (TPR) profiles of CO gas for CuO nanostructure (as prepared) and CuO-Sigma Aldrich. Aldrich (203130-5G). ................................................................................................... 91
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Figure 48. (a) Total catalytic conversion of NO to N 2 and N2O at 100 and 150 [°C]. (b) Relative production of N 2 to remediated gases at 100 and 150 [°C]. .............................................................................................................. 93 Figure 49. SEM images of CuO particles after NO-CO catalytic remediation test at 150 [°C]. ........................... 95 Figure 50. HRTEM images of CuO particles after NO-CO catalytic remediation test at 150 [°C]. ..................... 96 Figure 51. XRD patterns of material (a) after TGA under 500 [°C] and nitrogen atmosphere; (b) after TGA under 500 [°C] and Air atmosphere; (c) after TGA under 150 [°C] and nitrogen atmosphere; (d) after TGA under 150 [°C] and Air atmosphere; (e) after catalytic test; (f) as prepared. The vertical lines indicate the position of diffraction peaks from Crystallography open database (JCPDC) card files. ......................................................... 97 Figure 52. Reduction of the displacement path associated with overlapping energy reactions in and without presence of surface defect (a); reaction path of CO adsorption and its oxidation to CO 2 on the catalyst surface (b); possible reaction paths of NO on the catalyst surface (c); schematic representation of energy profile of CO interaction with catalyst surface (d) (adapted from [374]); schematic representation of energy profile of NO interaction with catalyst surface (e) (data from [284,375]). .................................................................................. 99 Figure 53. Schematic visualization of phenomena associated with the NO-CO remediation on the CuO (130) surface (a); NO-CO remediation on the Cu (110) surface (b); simplistic model of progressive loss of oxygen by the particle and its impact on changes in the chemical compositions of rods (c). ............................................... 100
List of Tables Table 1. Selected properties of COx and NOx chemicals [13–17]. ...................................................................... 11 Table 2. Ambient air quality recommendation according to WHO [93]. .............................................................. 25 Table 3. Ambient air quality standards in EU [95]. .............................................................................................. 26 Table 4. Summary of calculations and experimental parameters of the CuO bulk structure [3,288–305]. ........... 49 Table 5. List of reagents used. .............................................................................................................................. 57 Table 6. Short summary of the results obtained under the given temperature of synthesis proper for copper salts used in screening tests with 1:1 salt to ligand stoichiometry. ............................................................................... 69 Table 7. Summary of the results obtained for specific solvent composition for tested Copper (II) Acetate to ligand stoichiometry at temperature of 75 [°C]. .................................................................................................... 70
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