Magnesium oxide nanoparticles on green activated

Magnesium oxide nanoparticles on green activated carbon as efficient CO 2 adsorbent Wan Nor Roslam Wan Isahak, Zatil Amali Che Ramli, Mohamed Wahab Mohamed Hisham, and Mohd Ambar Yarmo Citation: AIP Conference Proceedings 1571, 882 (2013); doi: 10.1063/1.4858766 View online: http://dx.doi.org/10.1063/1.4858766 View Table of Contents: http://scitation.aip.org/content/aip/proceeding/aipcp/1571?ver=pdfcov Published by the AIP Publishing

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Magnesium Oxide Nanoparticles on Green Activated Carbon as Efficient CO2 Adsorbent Wan Nor Roslam Wan Isahak, Zatil Amali Che Ramli, Mohamed Wahab Mohamed Hisham and Mohd Ambar Yarmo Low Carbon Economy (LCE) Research Group, School of Chemical Sciences and Food Technology, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor. Abstract. This study was focused on carbon dioxide (CO2) adsorption ability using Magnesium oxide (MgO) nanoparticles and MgO nanoparticles supported activated carbon based bamboo (BAC). The suitability of MgO as a good CO2 adsorbent was clarified using Thermodynamic considerations (Gibbs-Helmholtz relationship). The ΔH and ΔG of this reaction were - 117.5 kJ·mol-1 and - 65.4 kJ·mol-1, respectively, at standard condition (298 K and 1 atm). The complete characterization of these adsorbent were conducted by using BET, XRD, FTIR, TEM and TPD-CO2. The surface areas for MgO nanoparticles and MgO nanoparticles supported BAC were 297.1 m2/g and 702.5 m2/g, respectively. The MgO nanoparticles supported BAC shown better physical and chemical adsorption ability with 39.8 cm3/g and 6.5 mmol/g, respectively. The combination of MgO nanoparticle and BAC which previously prepared by chemical method can reduce CO2 emissions as well as better CO2 adsorption behavior. Overall, our results indicate that nanoparticles of MgO on BAC posses unique surface chemistry and their high surface reactivity coupled with high surface area allowed them to approach the goal as an efficient CO2 adsorbent. Keywords: Nanoparticles, Green activated carbon, CO2, Metal dispersion, Adsorption-desorption PACS: 68.43.-h; 81.16.Hc; 68.43.-h

INTRODUCTION Recently, carbon dioxide (CO2) abundance got great concerns worldwide. Very high CO2 concentration has been reported since years ago. The CO2 level was increasing by uncontrolled emission from industrial activities and transportation. Therefore, there is growing interest in developing technologies for CO2 capturing is necessary to achieve low energy and materials to reduce process cost. The commercial technologies exist today are very expensive and higher energy needed. Magnesium oxide (MgO) is one of the most promising solid base catalysts and has attracted much attention because of its superior performance. The extent of catalytic properties of MgO is highly controlled by its morphology, particle size, crystallinity and surface area. The MgO has shown an excellent CO2 ability at high temperature which is more than 300 °C. While, the activated carbon was contains very high micro pores which contribute higher gas adsorption [1]. The combination of highly porous supported materials and metal oxide can increase active sites and dispersion for better adsorption of weakly acidic gas especially CO 2. The reaction of metal oxides and CO2 was easily to form carbonates compound. The carbonate ligand, present almost everywhere in our natural environment, is one of the most important reactive anions. It is also true that CO2 molecules have a soft acidic nature [2]. This fact indicates the presence of strong base functional groups such as amine groups can enhance the carbon dioxide capacity. In our previous work, fresh copper oxide (CuO) was shown good chemical adsorption ability with 9.5 mmol of CO 2 per gram adsorbent after 24 hours, which was 52.4% of CuO surface covered by carbonate [3]. Park and Jung [4] were reported the presence of copper oxide (Cu2O) on carbon materials can increase the adsorption capacity of the acidic adsorbate which indicating that copper oxides serve as electron donor. The main objective of this research is to study the possibility of MgO as CO2 adsorbent based on thermodynamic consideration proposed by Gibbs-Helmholtz and to increase CO2 capture ability using MgO nanoparticles monodispersed on bamboo activated carbon (BAC). The high dispersion of MgO nanoparticles on BAC will increase the active sites and basicity to capture the weak acidic CO 2. The physical and chemical adsorptiondesorption was studied extensively through out this paper.

The 2013 UKM FST Postgraduate Colloquium AIP Conf. Proc. 1571, 882-889 (2014); doi: 10.1063/1.4858766 © 2014 AIP Publishing LLC 978-0-7354-1199-9/$30.00

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METHODS AND MATERIALS Thermodynamic Consideration Thermodynamic approach was carried out to study the suitability of MgO as a potential CO 2 sorbent at standard condition (298K and 1 atm). The route of MgO reacts with CO 2 to form carbonate was used. Thermodynamic data was provides to discuss phenomenologically the issue in terms of Gibbs-Helmholtz relationship as below. ΔG = ΔH – TΔS If, ΔG = 0 T = ΔH/ΔS

Synthesis of MgO Nanoparticles on BAC (MgO-BAC) 3 g of magnesium nitrate (Mg(NO3)2) was dissolved in 5 ml of distilled water. The solution was sonicated using Sonicator ultrasonic processor, Misonix, Inc. at amplitude (intensity) and power of 30 % and 15 watt, respectively, for 15 min. Then, 7 g of bamboo activated carbon (BAC) prepared using chemical method [5-7] was added to the solution. The mixture was stirred and heated at 70 °C for 2 hours. The mixture was then dried at 100 °C for overnight and calcined at 400 °C for 3 hours.

Physical and Chemical Characterization Nitrogen adsorption at 77 K (liquid nitrogen) was conducted using a Micromeritics ASAP 2010 instrument to obtain the adsorption isotherm of each sample. The Brunauer-Emmett-Teller (BET) surface area, micropores volume and micropores area were also calculated from the isotherms. Before analyses were done, samples were degassed at 350 °C for 6 hours. The crystallinity analyses were performed by using XRD’s Bruker AXS D8 Advance type with x-ray radiation source of Cu Kα (40 kV, 40 mA) to record the 2θ diffraction angle from 10 ° to 80 ° at wavelength (λ = 0.154 nm) of 1 g sample. The infrared spectra of the adsorbents sample were recorded on a spectrum 400, FTIR/FT-NIR Spectrometer (Perkin Elmer, UK) using the Attenuated Total Reflection (ATR) method for sample preparation technique. A mass of 0.5 mg of adsorbents sample was used for all tests. The surface micrograph of adsorbents was studied using TEM analysis was performed by using CM12 transmission electron microscope Philips type coupled with electron gun at operation volt 200 kV.

CO2 Adsorption Ability The physical adsorption and desorption of carbon dioxide (CO 2) was studied using CO2 adsorption isotherm plot using Physisorption instrument (Micromeritics 2010). The adsorption was determined at 25 °C and calculated by BET model. For chemical adsorption-desorption, Chemisorptions instrument (Micromeritics 2020) was used. The adsorption of 5% CO2 in He was carried out at 40 °C which the adsorbents were saturated for 1 hour.

RESULTS AND DISCUSSION Thermodynamic considerations The thermochemical considerations data of CO2 adsorption using MgO was included in TABLE (1). Considerations at various temperatures successfully identified the MgO has most favorable adsorption and desorption ability. By thermodynamic model, the stability of carbonates as a function of temperature have been calculated and compared to the experiment using TPD-CO2 as discussed in sub-topic: Chemical Adsorption Determination. The desorption temperature of CO2 from MgCO3 was favourable at 653 K and it was considerably low compared to CaO at higher than 1000 K.


TABLE (1). Thermodynamics of reactions involving adsorption-desorption of CO2 at various temperature. Reaction involved ΔH (kJ/mol) ΔS (J·mol−1·K−1) ΔG (kJ/mol) T (K) Reaction possibility MgO (s) + CO2 (g) --> MgCO3 (s)

- 117.5

- 175

- 62.7

313

(Adsorption)

Favourable (Reaction very easy to occur)

MgCO3 (s) --> MgO (s) + CO2 (g)

+117.5

+175

+ 3.2

653

Favourable at 653 K

(Desorption)

Surface Properties Surface adsorptive properties of nanoscale MgO particles have been compared with MgO-BAC nanocomposites. From TABLE (2), there were shown that bamboo based activated carbon (BAC) has BET surface area of 1150.5 m2/g. The MgO nanoparticles and MgO nanoparticles supported BAC which was synthesized in this work gaves surface area of 297.1 m2/g and 702.5 m2/g. This showed the inherent non-porous or mesophorous nature of the materials with pore sizes in the range of 2 nm to 50 nm [8] where MgO nanoparticles supported BAC was micropores. It was shown that the MgO nanoparticles supported BAC shown a better surface area and high miropores area compared to MgO nanoparticles fresh. The N2 adsorption-desorption of these materials were shown in FIGURE 2. It was showed that higher N2 adsorption level at low pressure (P/P 0 < 0.5). The high composition of micro pores gave better gas adsorption at low pressure. Compounds

TABLE (2). Surface properties of selected adsorbents. BET surface area Micropore area (m2/g) Adsorption pore 2

(m /g)

Desorption pore

width (Å)

width (Å)

BAC

1150.5

270.4

21.5

22.1

MgO Nano

297.1

42.2

81.6

96.6

30%MgO/BAC

702.5

124.6

25.6

26.4

FIGURE 2. N2 adsorption-desorption isotherms for (a) MgO nanoparticles and (b) MgO nanoparticles supported BAC.

Carbonates Functionalized Group Determination From FIGURE 3, FTIR experiments show distinguishable carbonate species which adsorbed on different planes and defects, vibrating in different IR frequency ranges. For example, the non-coordinated carbonate was determined at 1410 cm-1. While, the monodentate carbonates were detected in range of 1130 to 770 cm-1 [9-11]. For MgO


nanoparticles after CO2 saturation, some Mg(OH)2 or moisture was clarified at 3700 cm-1. It may be affected by moisture from CO2 source was reacted with highly hygroscopic MgO nanoparticles. The higher intensity of new peaks for MgO nanoparticles supported BAC in FIGURE 3 (d) shown a better carbonates formation with a clear spectra compared to fresh MgO nanoparticles (FIGURE 3 (b)). It was indicated the highly dispersion of MgO nanoparticles on BAC surface can increase the mobility and CO2-metal surface interaction towards better adsorption.

FIGURE 3. FTIR spectra for (a) MgO nanoparticles fresh, (b) MgO nanoparticles saturated CO2, (c) MgO nanoparticles supported BAC fresh and (d) MgO nanoparticles supported BAC saturated CO 2.

Crystallinity Properties and New Phase Formation Determination The crystallinity properties of MgO and new phase fomation after CO2 saturation was clarified using XRD. There were four peaks observed at 36.9°, 42.5°, 62.2° and 78.6° (FIGURE 4 (a) (i)). All of the reflection peaks can be indexed to cubic MgO (JCPDS No. 075-0447). From FIGURE 4 (a) (ii), after saturated the adsorbent by CO2 for 1 hour, there were observed new phase of carbonates at 16.5°, 33.8°, 38.1°, 51.5°, 53.4° and 71.8° (JCPDS No. 0800042). Two peaks corresponded to Mg(OH)2 were detected at 19.1° and 58.7° (JCPDS No. 083-0114). This compounds was formed by chemical reaction between MgO and water or moisture from CO 2 source [3]. This hydroxide compounds would be an intermediates of MgO to form MgCO 3. The moisture also performed as coadsorbed which can increase chemical reactivity of CO2 [9]. The MgO supported BAC also showed a relatively same peaks which represented carbonate phases. The crystalline phase (graphite) of activated carbon was observed at 26.6° (JCPDS No. 026-1077) as shown in FIGURE4 (b). From Scherrer equation, the MgO nanoparticles supported BAC shown smaller crystallite size compared to fresh MgO nanoparticles with 10.9 nm and 21.1 nm, respectively.


a)

b) FIGURE 4. XRD patterns for (a) MgO nanoparticles (i) unsaturated CO2 and (ii) saturated CO2 (b) MgO nanoparticles suppported BAC (i) unsaturated CO2 and (ii) saturated CO2.

Morphological Studies TEM micrograph in FIGURE 5 (a) shows the formation of MgO nanoparticles consist of a large number of adges and corners, step edges and top corners and numerous basic sites of various strenght (surface hydroxyl, such as Mg(OH)2, low coordinated O2 sites) which are recognized as active sites in heterogeneous catalysis phenomena [12]. It was shown a good agreement with XRD results. The random nanoflakes particles size ranging of 25 to 50 nm. From FIGURE 5 (b), there were MgO nanoporous with uniformly sphere structure formed after supported by BAC. The nanostructures of MgO-BAC nanocomposite are highly porous and made of MgO nanoparticles building blocks of the size 10 to 15 nm. MgO nanoparticles was chemically reacted with activated carbon surface to form higher surface area of nanocomposite. The surface area of the MgO-BAC nanocomposite was found to be 702.5 m2/g as discussed in N2 adsorption-desorption method.


FIGURE 5. TEM micrograph for (a) MgO nanoparticles and (b) MgO nanoparticles supported BAC, at magnification of 75,000X.

Physical Adsorption Studies Using BET Model From FIGURE 6, the MgO nanoparticles supported BAC showed a better adsorption of CO2 with 39.8 cm3/g compared to single BAC and MgO nanoparticles with 18.8 cm3/g and 12.8 cm3/g, respectively. The fresh BAC gaves relatively high CO2 adsorption. It was noted that physical adsorption of CO2 was depends on the type of pores contains of materials. By the combination of highly dispersed MgO nanoparticles on BAC results higher adsorption ability with the increment of 111.7%. The quantitative determinations of CO2 loading can be corresponded to multilayered physisorption on MgO nanoparticles which its flatter, extended planes, can apparently form more ordered multilayered structures and thus physically adsorb more CO2 [13]. Moreover, the highly dispersion of MgO nanoparticles on BAC surface was attributed better properties as adsorbent such as high surface area (porosity) and better basicity to capture CO2. By analyzing the nature of basic sites, the results show that the most active sites versus CO 2, which is a Lewis acid, is not the same as the strongest sites for the deprotonating adsorption of Bronsted acids.

FIGURE 6. Physisorption of CO2 using (a) Activated carbon, (b) MgO nanoparticles and (c) MgO nanoparticles supported BAC.

Chemical Adsorption Determination


From TPD-CO2 profile, there were significantly different peaks for fresh MgO nanoparticles (FIGURE 7 (a)) and the other one MgO nanoparticles supported by BAC (FIGURE 7 (b)). The MgO nanoparticles showed two peaks of CO2 desorption at temperature of 320 °C and 350 °C. It was may be corresponded to adsorbates desorption (CO2) based on monodentate and bidentate carbonates [14].

FIGURE 7. TPD-CO2 peaks for (a) MgO nanoparticles and (b) MgO nanoparticles supported BAC.

Highly dispersion of MgO nanoparticles on BAC gave more capability to adsorb CO 2. There were three peaks for CO2 desorption using MgO nanoparticles supported BAC observed at relatively higher temperature of 320 °C, 372 °C and 460 °C. The total desorption of CO2 was 6.5 mmol/g compared to unsupported MgO nanoparticles with 3.1 mmol/g. These three peaks were due to CO2 adsorbates desorption of carbonates interaction on new composite phase (MgO-BAC).

CONCLUSION MgO nanoparticle was shown high CO2 adsorption at particular condition. By highly dispersion of MgO nanoparticles on BAC (MgO-BAC) surface, a better physical and chemical adsorption was observed regarding the higher surface reactivity and active sites. The physical adsorption was increased almost 112% to 39.8 cm3/g compared to fresh activated carbon (BAC) with 18.8 cm3/g. However, MgO nanoparticles adsorbed 12.8 cm3/g of CO2. The CO2 adsorbates desorption from MgCO3 interaction was clarified at three different temperatures (320 °C, 372 °C and 460 °C). However, the chemical reaction of MgO and CO2 to form a strong chemical attractions of MgO--O-C=O (MgCO3) was typically slow than weakly physical attractions on MgO surfaces. The results showed that MgO nanoparticles supported BAC is much better CO2 adsorbent compared to fresh MgO nanoparticles with CO2 physisorbed of 39.8 cm3/g and chemisorbed of 6.5 mmol/g).

ACKNOWLEDGMENTS The authors wish to thank Universiti Kebangsaan Malaysia (UKM) for funding this project under research grant Dana Pembangunan Penyelidikan (DPP-2013-056), Long Term Research Grant (LRGS/BU/2011/USMUKM/PG/02) from Ministry of Higher Education (MOHE) Malaysia and Centre of Research and Innovation Management (CRIM) UKM for the instruments.


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