Artificial photosynthesis - CO2 towards methanol

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Artificial photosynthesis - CO2 towards methanol

This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2011 IOP Conf. Ser.: Mater. Sci. Eng. 19 012010 (http://iopscience.iop.org/1757-899X/19/1/012010) View the table of contents for this issue, or go to the journal homepage for more

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Symposium A, E-MRS 2010 Fall Meeting IOP Conf. Series: Materials Science and Engineering 19 (2011) 012010

IOP Publishing doi:10.1088/1757-899X/19/1/012010

Artificial photosynthesis - CO2 towards methanol Dobieslaw Nazimek and Bozena Czech Department of Environmental Chemistry, Faculty of Chemistry, Maria Curie-Sklodowska University, 20-031 Lublin, Pl. M. Curie-Skłodowskiej 3 E-mail: [email protected] Abstract. The new insight into the problem of carbon dioxide utilization into valuable compound – methanol and then its transformation into fuel is presented. Because the highly endothermic requirements of the reaction of CO2 hydrogenation a photocatalytic route is applied. Combining of the two reactions: water splitting and CO2 hydrogenation using H2O as a source of hydrogen at the same time and place are proposed. The studies over modified TiO2 catalysts supported on Al2O3 were conducted in a self-designed circulated photocatalytic reaction system under at room temperature and constant pressure. Experimental results indicated that the highest yield of the photoreduction of CO2 with H2O were obtained using TiO2 with the active anatase phase modified by Ru and WO3 addition. The conversion was very high – almost 97% of CO2 was transformed mainly into methanol (14%vol.) and into small amount of formic and acetic acid and ester.

1. Introduction The intensification of the natural greenhouse effect e.g. the increase in the Earth's temperature at the range of 0.6 – 0.7°C compared to the year 1880 and the predictions for the further increase at about 0.57 – 0.6°C in the future, the raise of the sea level and the decrease of drinking water are believed to be caused by the enrichment of the atmosphere in the greenhouse gases (GHG) such as CO2, CH4, N2O, water vapour, O3 and CFC. Because the results of the higher concentration of GHG are unpredictable, different actions are performed in order to utilize them [1]. Also limited fossil fuel resources directed the interest of the studies for the seeking for the new efficient methods of their production. In recent years it is observed an intense progress in the field of photocatalytic studies. A great attention is paid to the application of photocatalysis in the air [2-4], water and wastewater treatment [5-7]. The studies dealing with the practical application of photocatalysis have started after the pioneer Japanese paper of Fujishima and Honda [8] about the the photo-electrolysis of water using a UV-irradiated titaniabased anode. More than 2000 recent publications were published at the beginning of the XXIst century in the field of photocatalysis. The scientific interest is directed into the water treatment but the industrial applications (observed in the patent literature) are opposite: the stronger emphasis on air treatment [9]. Heterogeneous photocatalysis can be described as to be efficient in Green Chemistry, in Fine Chemicals and in “Advanced Oxidation Processes” (AOP) [10]. All AOP base on the generation of the highly effective hydroxyl radicals *OH that can react with most of the compounds quickly and nonselectively [11]. Heterogeneous catalysis is a method that can be used efficiently in many processes. In heterogeneous photocatalysis ultraviolet (UV) band of the solar spectrum is used to photo-excite a catalyst – Published under licence by IOP Publishing Ltd

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semiconductor that is in contact with water and in the presence of oxygen [5]. Beside many tested semiconductors (ZnO, Fe2O3, CdS, ZnS) TiO2 turned to be close to the perfection: it is chemically and biologically stable, UV activated, effective, inert, affordable and cheap. The mechanism of the surface reactions over UV illuminated TiO2 are well known [12] and called HondaFujishima effect [13]. CO2 as a cheap, nontoxic and very abundant C1 feedstock can be a very useful substrate for further syntheses. Using heterogeneous photocatalysis CO2 can be utilized into a useful product, such as methanol or methane [14,15]. Solar energy can be harnessed to drive CO2 conversion by: – artificial photosynthesis using homogeneous and heterogeneous systems, – electrochemical reduction using solar electric power, – hydrogenation of CO2 using solar-produced hydrogen. CO2 is a thermodynamically stable and its reduction is highly endothermic [16]. The classical catalytic synthesis of methanol is based on the reaction of the synthesis gas (syngas) and it is well-know and widely tested and its side product is CH4 [17]. The problem is however the obtaining of high-purity syn-gas caused by the usage of mixture of gases: CO, CO2, H2, [18]. The successful studies of the catalytic synthesis of methane from the mixture containing CO, CO2 and H2 (or water steam as a source of H2) directed the area of the experiments towards methanol synthesis from CO2 and H2 [19]. Basing on photosynthesis processes that occur in plants which use the solar energy to synthesize organic compounds, the trials of dealing with CO2 using photocatalytic method were undertaken [20]. The earliest paper proposing dealing with CO2 photoreduction with H2O as a H2 carrier emerged in 1979 [21] and are in the area of interest mainly of Japanese and Chinese scientists [22-24]. It is proposed [25] to electrolyze the water with photoradiation of CO2; water is the source of hydrogen in this process: hv CO 2 + 2 H 2 O → CH 3OH +

3 O2 2

(1)

H2O + 2 p+ → 1/2 O2 + 2H+

(2)

H2O → eaq-, H*, *OH, H+, H2, H2O2

(3)

The reaction is highly endothermic and the products have lover volume compared to reactants. Increasing the temperature and pressure in this system will shift equilibrium of this reaction to the right. Heat balance of this reaction explains that for producing of 1 kmole of methanol form CO2 and H2O, 586 MJ of energy and pressure of 300 bars at T = 873K are needed. This process conducted in the liquid phase is unsolved thermodynamically (low solubility of O2 in water). This is because the equilibrium cannot be reached (transfer of oxygen from liquid to gaseous phase): Kp = [ CH3OH]·[O2]3/2/[CO2]·[H2O]2

(4)

This is the reason why the reduction of CO2 with photocatalysis usage is so promising: CO2 may be transferred into valuable compounds during irradiation with UV light at ambient temperature [26-28]. The energy necessary for carrying the reaction is provided by photons, which energy is high enough (5 eV) thanks to high color temperature (λ < 250 nm). 2. Experimental Procedure Proposed by Department of Environmental Chemistry, Faculty of Chemistry of Marie CurieSklodowska University in Lublin, Poland a method of artificial photosynthesis relies on the application of the photocatalyst prepared according to the modified patent [29]. The first step was the preparation of pipes: they were mechanically dulled polish and treated with 0.5 n NaOH for 20 minutes and dried at 388K for 3 hours. Next pipe was fulfilled with 15% TiCl4 for 10 minutes and dried at 388K for 3 hours. The catalytic phase was prepared separately. The precipitated deposit was treated with 0.1M EDTA for half an hour and dried at 388K for 1.5 hour. The tungsten was incorporated from its 0.1 M ammonium salt for 15 minutes. The next stage was drying at 388K for 3 hours and calcination at 723K for 2 hours. Prearranged contact was treated again with 0.1 M solution of EDTA for 0.5 hour and dried at 388K for 1.5 hours. Ruthenium was 2



introduced from ruthenium red (1%wt. Ru/dm3) for 25 minutes. The obtained precursor of active phase was dried at 388K for 3 hours and warmed at 723K in the atmosphere of Helium for 3 hours. In the next stage active phase precursor was contacted with the inner walls of pipes creating a solution with 14%vol. of water glass. The pipe with the active phase was left for 24 hours in the air atmosphere at the room temperature till the binding. The last stage was calcination at 473K for 2 hours. Prepared in this way active phase was then tested by XRF, XRD, TPD and TPO and the argon adsorption in the liquid nitrogen temperature; precursors were examined by FT-IR. In Table 1 are shown the physicochemical properties of the catalysts. The system is designed as a set of bottles with water with the CO2 bubbled inside, and the mixture of CO2 dissolved in H2O is directed into the reactor with the set of the wall catalysts. The last step is the separation of methanol on the membrane. Designed bath system allows to transform 370 dm3 of CO2 per hour into methanol with yield about 97% (length of the one reactor 0,5 m, Φ = 4 cm). Figure 1 shows ideological scheme of methanol production using artificial photosynthesis.

Figure 1. The system for the photoreduction of CO2 with H2O for CH3OH production. Utilization of CO2 from coal combustion required the collecting procedure and the way of purifying sulfur and dust from reactants. Other gases and water vapor do not interfere during the process. 3. Results and Discussions Basing on XRF, XRD, TPD, TPO, argon adsorption and FT-IR studies the wall catalyst is in a form of anatase with the 4-7nm crystallite size modified with the WO3 and Ru (Table 1). Table 1. The physico-chemical properties of the studied catalyst [28]. Support Al (99.98%) Contents or layers

1st layer

2nd layer

Fundamental phase

Promoter

Activator

Other phases

Al2O3

TiO

TiO2

Ru

WO3

Sodium aluminosilicate

200 µm

250µm

78.30%

0.70%

11.00%

10.00%

3.5 nm

8 nm

Crystallite forms Crystallite size

anatase 8-10 nm

8-10 nm

4-7 nm 3



CO2 level at 12% is appropriate for carrying out the reaction. Separated methanol (through membrane process or thermal methanol – Figure 1) can be used as in MTG – methanol to gasoline process, for the obtaining of high quality synthetic fuels. It has been presumed that O2 (formed during the artificial photosynthesis process) will be removed into the collector. The collector may be modified and after modification then O2 may be used for combustion gasification of coal. The setup shown in Fig. 1 will put into practice the two technological elements: 1. preparation of one-phase mixture of water and CO2 (the excess of CO2 will be turned back to the process), 2. conversion the mixture in the photoreactor into methanol and its derivatives. The set of photoreactors will work continuously and the reagents preparation system will work periodically (switching over containers). That simple technological system guarantees work continuity and lack of CO2 re-emission in purge gases. This method allows to achieve methanol and the small amount of the other: formic and acetic acids, ester as presented in Table 2. Table 2. Products distribution in water phase. Methanol [%vol.]

CH3COOH [% vol.]

HCOOH [% vol.]

Ester volume [% vol.]

Acid volume [% vol.]

pH

14.86

0.39

0.2

3.68

3.1

2.8

The studied catalyst is efficient in the CO2 photoreduction and the conversion of CO2 is very high – over 96% (Table 3). Table 3. The changes of the conversion and selectivity for the studied catalyst. Conversion of CO2 [%]

Selectivity towards CH3OH [%]

96.78

96.18

Selectivity towards CH3COOH Selectivity towards HCOOH [%] [%] 2.52

1.3

The best results were obtained using the mixture of CO2 + H2O with the velocity at the range about above 8 dm3/h. With these velocities more than 0.8 kg of methanol per hour in the mixture was obtained (Figure 2).

4

1.2

0.012

0.8

0.008

0.4

0.004

0

0 0

4

8

12

16

0

Velocity of reagents mixture, [dm3/h]

a)


TOF, [s-1]

Velocity of methanol synthesis, [kg/h]


4

8

12

16

Velocity of reagents mixture, [dm3/h]

b)

Figure 2. The relationship between velocity reagents mixture (CO2 + H2O) towards CH3OH synthesis in reagents mixture: a) velocity of methanol synthesis depending on velocity of reagents, b) TOF as a function of velocity of reagents. With the velocity of reagents above 8 dm3/h TOF approximates to the value 0.01 s-1 and stands constant. Below 8 dm3/h the methanol production is straight-lined with the increasing reagents' velocities. The observed continuity of methanol production and lack of possibility to increase the efficiency is connected with the presence of active oxygen close to catalyst surface. The key problem in increasing the efficiency of photoreduction of CO2 with H2O and methanol production is how to remove the oxygen from the catalyst surface. Thermodynamics of the process (reaction is highly endothermic) causes that for synthesis of 1 mole of CH3OH (32g), 723.52 kJ of energy is needed i.e. on 1 kg of product – at 293K – 22.61 MJ/kg. In the one doze into the photoreactor the energy per hour on the mixture is (1.2 dm3 CH3OH ie. 0.95016 kg) 22.61 [MJ/kg] * 0.95016 = 21.483 MJ/mixture – 5.517 kWh on mixture.

Figure 3.The ideological scheme of synthetic hydrocarbons production based on MTG process [30]. 5



The energy requirements for methanol separation are about 0.4 kWh for 1 dm3. In the case of membrane process the consumption of energy and in consequence the cost of the separation of mixture formed after the reaction are lower. In the Figure 3 is presented an ideological scheme of synthetic hydrocarbons production. It based on previously mentioned MTG process. The idea of MTG method, also known as Mobil synthesis, is synthesis of gasoline with neglecting the direct reaction of CO or CO2 with H2. Instead, conversion into methane (or its homologue) is proposed. In that way the equations of synthesis are simplified (because we are dealing with intermediate product). The key of the synthesis is the catalyst based on the ZSM-5 zeolite. At present, a lot of catalysts more active than its prototype (which ZSM-5 matrix) is know. However, in of Mobil synthesis there is a possibility of water production as a by-product. Taking control over the artificial photosynthesis of CO2 gives the possibility of cheap motor spirit production. CO2 mission in Poland is about 340 million tons per year. The process of CO2 photoreduction in self designed photoreactor will be tested at a larger scale – in a plant which produces CO2. The last stage of the process would be initiating the MTG technology based at the catalyst made in Poland, for ETG (ethanol to gasoline) [31]. In the MTG processes has been found the access of 12.4375 MJ pr mixture i.e. 3.4548 kWh. In the summary: on the artificial photosynthesis of CO2 only 2.5127 kWh is needed. 4. Conclusions The new effective photocatalyst, described in the project [29] allows to carry out the photocatalytic process with the efficiency enough for commercial usage. The calculations show that implementation of the technology (period 3 years) will allow to reduce CO2 emission during the first year of the technology remarkably (~ 25% with further progression). It will also allow to raise modern secondary industry and modern scientific – research base. Methanol is the fundamental reactant in chemical production and it was important before mentioned MTG process. When we will be able to produce cheap methanol then the fast development of chemical production in Poland is only the matter of time. References [1] Huang L, Shen Y, Dong W, Zhang R, Zhang J, Hou H, 2008 Journal of Hazardous Materials 2-3 – 151 323 [2] Hager S, Bauer R, Kudielka G, 2000 Chemosphere 8-41 1219 [3] Hernández-Fernández J, Aguilar-Elguezabal A, Castillo S, Ceron-Ceron B, Arizabalo R D, Moran-Pineda M, 2009 Catalysis Today 1/2-148 115 [4] Wang K-H, Jehng J-M, Hsieh Y-H, Chang Ch-Y, 2002 Journal of Hazardous Materials 1-90 63 [5] Malato S, Fernandez-Ibanez P, Maldonado M I, Blanco J, Gernjak W, 2009 Catalysis Today 1-147 59 [6] LinW-Ch, Chen Ch-N, Tseng T-T, Wei M-H, Hsieh J H, Tseng W J, 2010 Journal of the European Ceramic Society 14-30 2849 [7] Mills A, Lee S K, 2002 Journal of Photochemistry and Photobiology A: Chemistry 1/3 - 152 233 [8] Fujishima A, Honda H, 1972 Nature 37/38 -238 37 [9] Paz Y, 2010 Applied Catalysis B: Environmental 3/4 - 99 448 [10] Herrmann J M, 1999 Catalysis Today 1-53 115 [11] Robert D, Malato s, 2002 The Science of The Total Environment 1/3-291 85 [12] Di Paola A, Garcia-Lopez E, Ikeda S, Marci G, Ohtani B, Palmisano L, 2002 Catalysis Today 1/3-75 87 [13] Watanabe T, Nakajima A, Wang R, Minabe M, Koizumi S, Fujishima A, Hashimoto K, 1999 Thin Solid Films 1/2- 351 260 [14] Lo Ch-Ch, Hung Ch-H Hung, Yuan Ch-Sh, Wu J-F, 2007 Solar Energy Materials and Solar Cells 19-91 1765 [15] Tseng I-H, Wu J C-S, 2004 Catalysis Today 2/3-97 113 [16] Ma J, Sun N, Zhang X, Zhao N, Xiao F, Wei W, Sun Y, 2009 Catalysis Today 3/4 - 148 221 [17] Lange J-P, 2001 Catalysis Today 1/2-64 3 [18] Zhang J, Wang Y, Chang L, 1995 Applied Catalysis A:General 2-129 L205 [19] Toyir J, Ramírez de la Piscina P, Fierro J L G, Homs N, 2001 Applied Catalysis B: Environmental 3-29 207 [20] Nguyen T-V, Wu J C S, 2008 Applied Catalysis A: General 1-335 112 6



[21] Inoue T, Fujishima A, Konishi S, Honda K,1979 Nature 227 637 [22] Wu J C S, Lin H-M, Lai Ch-L, 2005 Applied Catalysis A:General 2-296 194. [23] Tseng I-H, Wu J C S, Chou H-Y, 2004 Journal of Catalysis 2-221 432 [24] Tseng I-H, Wu J C S. 2004 Catalysis Today2/3- 97 113 [25] Grodkowski J, Dhanasekaran T, Neta P, Hambright P, Brunschwig B S, Shinozaki K, Fujita E, 2000 Journal of Physical Chemistry A 48-104 11332 [26] Takeuchi M, Sakamoto Y, Niwa S, 2001 The Science of The Total Environment 1/3-277 15 [27] Weatherbee G D, Bartolomew C H, 1981 Journal of Catalysis 1-68 67 [28] Wu J, Luo S, Toyir J, Saito M, Takeuchi M, Watanabe T,1998 Catalysis Today, 1/4-45 215 [29] patent PCT/PL2009/050029 Catalyst for the synthesis of methanol and its derivatives and method for its preparation [30] nzic.org.nz/ChemProcesses/energy/7D.pdf [31] patent PCT/PL2009/000037 Gasoline synthesis catalyst comprising copper ions on an aluminosilicate

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