Selective Catalytic Oxidation of CO In the Presence of H2 Over Gold Catalyst | AT 200, Papers of Health sciences

Download Selective Catalytic Oxidation of CO In the Presence of H2 Over Gold Catalyst | AT 200 and more Papers Health sciences in PDF only on Docsity! Available online at www.sciencedirect.com International Journal of Hydrogen Energy 29 (2004) 429–435 www.elsevier.com/locate/ijhydene Selective catalytic oxidation of CO in the presence of H2 over gold catalyst Apanee Luengnaruemitchaia, Somchai Osuwana ;∗, Erdogan Gularib aThe Petroleum and Petrochemical College, Chulalongkorn University, Soi Chuia 12, Phyathai Road, Patumwan, Bangkok 10330, Thailand bDepartment of Chemical Engineering, The University of Michigan, Ann Arbor, MI 48109, USA Received 19 September 2003; received in revised form 25 September 2003 Abstract The proton exchange membrane fuel cells with potentially much higher e2ciencies and almost zero emissions o4er an attractive alternative to the internal combustion engines for automotive applications. A critical issue in fuel cell system is the small (∼ 0:5%) amount of CO present from output of fuel reformer. This amount of CO can deteriorate the performance of fuel cells. Consequently, an additional gas conditioning process is required to minimize CO content in reformed gas. The proposed research study focuses on preferential oxidation of CO in a simulated reformed gas to CO2 by using selective CO oxidation catalysts. In this work, the e4ects of preparation method, O2, water vapor, and CO2 concentration in feed stream on the selective CO oxidation over Au=CeO2 catalysts were investigated in the temperature range of 323–463 K. Catalytic stability test was also performed. We ;nd that the activity of Au catalyst depends very strongly upon the preparation method, with co-precipitation prepared Au=CeO2 catalyst exhibiting the highest activities. ? 2003 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. Keywords: CO oxidation; Gold catalyst; Ceria; Co-precipitation; Fuel cell 1. Introduction In the past, the world has widely relied on the energy from burning fossil fuels, which are expected to be de- pleted in the near future. Extensive uses of fossil fuels have created air pollution, acid rain, and the greenhouse e4ect. Scientists have been searching for alternative means of powering vehicles more e2ciently without creating any pol- lution. The proton exchange membrane (PEM) fuel cell has been attracting much attention in improving fuel e2ciency and cleanliness of automobiles [1]. This is due to low tem- perature of operation, fast cold start, perfect CO2 tolerance by the electrolyte and a combination of high power den- sity and high energy conversion e2ciency. Normally, pure ∗ Corresponding author. Tel.: +662-2184-141; fax: +662-611-7619. E-mail address: somchai.o@chula.ac.th (S. Osuwan). hydrogen is the ideal fuel for the fuel cell system since it simpli;es system integration, maximizes system e2ciency, and provides zero emission. However, since no e4ective and potentially safe means of storing adequate amount of hy- drogen in a vehicle exists, on-board hydrogen generation by steam reforming of methanol or partial oxidation of liquid hydrocarbons followed by water gas shift reaction are alter- native ways of giving a fuel cell powered vehicle adequate range. These technologies not only produce hydrogen as a main product but also has side products with a small amount of 1% CO. Many studies have shown the negative e4ect of CO on the performance of PEM fuel cell since the electrodes of the fuel cell are typically made of platinum which is very sensitive to CO poisoning [2,3]. Selective catalytic CO ox- idation is perhaps the most promising method of reducing CO in the feed to a 10 ppm or lower range with a minimal loss of hydrogen. The preferential catalytic oxidation of CO was ;rst studied by Oh and Sinkevitch [4] with alumina 0360-3199/$ 30.00 ? 2003 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2003.10.005 430 A. Luengnaruemitchai et al. / International Journal of Hydrogen Energy 29 (2004) 429–435 supported ruthenium (Ru), rhodium (Rh) and platinum (Pt). Ru and Rh were very selective compared to Pt for oxidizing CO in the presence of H2 diluted with N2 at 403 K. However, a gas composition of 900 ppm CO, 800 ppm O2 and 0.85% H2 in N2 background was very di4erent from the reformed gas that is generated by re- forming a liquid hydrocarbon or methanol. The concept of using a zeolite support on Pt catalysts was applied for this reaction by Watanabe et al. [5] and Igarashi et al. [6]. Kinetic studies of selective CO oxidation on Pt=Al2O3 and Au=Fe2O3 catalysts in a simulated reformed gas (75% H2, the rest is N2) over a wide range of CO concentrations (0.02–1.5%) were reported by Kahlich et al. [7,8]. In ad- dition to the noble metal catalysts other catalysts have also been investigated such as CoO catalyst [9], and Pt promoted deposited on a monolith catalyst [10]. The supported Au cat- alysts have been found to be very active for CO oxidation reaction [11–13]. Recently, the metal oxide supports such as Al2O3 [14], MnOx [15], Fe2O3 [16], MgO–Al2O3 [17] were found to promote the activity of Au on selective CO oxidation and the catalytic combustion of volatile organic compounds [18]. Among the di4erent metal oxides used as the sup- port of Au for CO oxidation, cerium oxide or ce- ria (CeO2) is one of the interesting metal oxides. CeO2 is the oxide of the rare-earth metal cerium, which may exist in several compositions, due to the capacity of Ce to switch between the two oxida- tion states of Ce3+ and Ce4+. A number of functions have been ascribed to ceria, including promoting wa- ter gas shift activity [19–21], maintaining the disper- sion of the catalytic metals [2,22] and stabilizing the surface area of the support [23]. Extensive research, in particular during the past decade, has shown that ceria has bene;cial e4ects on a number of catalytic reactions. Due to a facile redox reaction cycle, ceria exhibits oxy- gen storage capacity and improves the CO and hydrocarbon oxidation. Summers and Ausen [24] claimed that ceria do- nated oxygen to Pt in their study of the oxidation of CO on Pt=CeO2 catalyst. For CO oxidation, ceria has been found to lower the activation energy, increase the reaction rate and suppress the usual CO inhibition e4ect [25]. The percent- age loading of Ce has been shown to a4ect CO oxidation on Pt [26]. Ceria promotes the activity of Pt in both lean and rich reactant gases for CO oxidation [27]. Consequently, we chose CeO2 as the support for studying selective catalytic oxidation of CO in the presence of H2 based on the afore- mentioned properties of ceria. From preliminary results, we found that Au=CeO2 cata- lysts also work very well for the selective CO oxidation. Therefore, the objectives of this work are to optimize the operating conditions for preferential oxidation of CO by O2 in the presence of H2 over the temperature range of 323– 463 K, which are suitable for fuel cell applications, and to study the e4ect of preparation method on the catalytic ac- tivity of Au=CeO2 catalysts. 2. Experimental 2.1. Catalyst preparation Three methods of the catalyst preparation; impregnation, co-precipitation and sol–gel were used in this work. Impregnation method: The Au catalysts were obtained by impregnation of the commercial ceria support with an aqueous solution of HAuCl4:xH2O (Alfa AESAR) contain- ing the appropriate amount of Au. The catalysts were dried overnight at 383 K and calcined at 773 K for 5 h. Co-precipitation method: An aqueous solution of Na2CO3 (1 M) was added into an aqueous mixture of HAuCl4:xH2O and Ce(NO3)3:6H2O (Fluka) and was kept at room temperature and constant pH of 8.0. The precipitate was aged for an hour and then was washed several times with distilled water until there was no excess anions. Af- ter washing with deionized water, the catalysts were dried overnight at 383 K and calcined at 773 K for 5 h. Sol–gel method: The single step sol–gel catalysts were prepared by hydrolyzing a solution of Ce acetate (Alfa AESAR) and HAuCl4:xH2O with NH4OH. The reaction mixture was kept at 353 K while the pH was maintained between 9.0 and 9.5. Then, HNO3 was added until gelation, the catalysts were dried overnight at 383 K and calcined at 773 K for 5 h. 2.2. Catalyst characterization Powder X-ray di4raction (XRD) patterns were used to obtain information about the structure and composition of crystalline materials. A Rigaku X-ray di4ractometer system equipped with a graphite monochromator and a Cu tube for generating a CuK radiation was used to obtain the XRD patterns. The Brunauer–Emmett–Teller (BET) method was used to determine the surface area and pore size of the catalysts by N2 adsorption/desorption at 77 K. Speci;c surface area was determined using an Autosorb-1 surface area analyzer. The particle morphology of the catalysts was observed by a Scanning Electron Microscope (SEM) using JEOL JSM -5410 LV scanning microscope operated at 15 kV. The Transmission Electron Microscopy (TEM) images were at- tained using a JEM 2010 operating at 200 kV in bright and dark ;eld modes. Crystallinity and crystal structure of the samples were evaluated from the selected area of electron di4raction patterns. 2.3. Activity measurement Catalytic activity studies were conducted in a pyrex glass U-tube reactor having an internal diameter of 6 mm A. Luengnaruemitchai et al. / International Journal of Hydrogen Energy 29 (2004) 429–435 433 Fig. 3. (a) TEM picture of gold particles (dark spots) on the Au=CeO2 co-precipitation surface; (b) Gold particle size distribu- tion determined by TEM. CO conversion are ∼ 70%, 92% and 98% at 383 K, respec- tively. The selectivities at the point of maximum conversion for 0.5%, 1%, and 2% O2 concentration are 64%, 62% and 48%, respectively. The optimal O2 concentration needed for oxidizing 1% CO in the feed is 1% with high selectivity and minimal loss of hydrogen. In general, it is believed that the catalytic activity of the catalyst is liable to be suppressed in the presence of water vapor. The e4ect of water vapor in the feed stream on the CO oxidation activity and selectivity of Au=CeO2 co-precipitation catalyst is shown in Fig. 6. The feed gas mixture was humidi;ed by bubbling through a con- tainer of temperature controlled water, yielding 10% water vapor in the reactant gas. Under the humidi;ed condition, water lowered CO conversion in the region of temperature lower than 373 K compared to the unhumidi;ed condition. Fig. 4. Temperature dependence of the CO conversion of the 1% Au=CeO2 catalysts. Reactant composition: 1% CO, 1% O2, 2.6% H2O, 2% CO2, 40% H2 and helium: (c) co-precipitation; (•) impregnation; (
) sol–gel; - - - - -, selectivity; —, CO conversion. Fig. 5. E4ect of O2 concentration in reactant gas over 1% Au=CeO2 co-precipitation catalyst. Reactant composition: 1% CO, 0.5–2% O2, 2.6% H2O, 2% CO2, 40% H2 and helium: (•) 0.5% O2; (c) 1% O2; (
) 2% O2; - - - - -, selectivity; —, CO conversion. This is due to the strong adsorption of water on the active site [30]. However, at high temperatures water seemed to be slightly favorable to the catalyst activity since it provides 434 A. Luengnaruemitchai et al. / International Journal of Hydrogen Energy 29 (2004) 429–435 Fig. 6. E4ect of water vapor addition in reactant gas over 1% Au=CeO2 co-precipitation catalyst. Reactant composition: 1% CO, 1% O2, 0–10% H2O, 2% CO2, 40% H2 and helium: (•) w/o water; (c) 10% water; - - - - -, selectivity; —, CO conversion. a hydroxyl group which is necessary for reaction to take place [12,31,32]. The selectivity of the catalyst was slightly a4ected by the presence of water vapor which is similar to CuO–CeO2 catalyst [29]. 3.2.3. E8ect of CO2 concentration Fig. 7 shows the e4ect of CO2 addition to the feed stream on the activity of the Au=CeO2 co-precipitation catalyst. In- creasing CO2 concentration from 2% to 20% reduced the activity of the Au=CeO2 catalyst. The maximum in conver- sion dropped from ∼ 92% to ∼ 85% with a shift of 30 K to higher temperatures. The selectivity of the catalyst was not signi;cantly impacted by the presence of CO2 in the feed. 3.2.4. Deactivation test Catalytic stability of Au=CeO2 co-precipitation catalyst was tested at the temperature of 383 K. As can be seen from Fig. 8, the CO conversion and selectivity were maintained for 2 days with very slight loss of activity. The result shows good stability of the Au=CeO2 co-precipitation catalyst as compared with Au=Fe2O3; CuO–CeO2 and Au=TiO2 cata- lysts reported by Kahlich et al. [8], Avgouropoulos et al. [29] and Lin et al. [13], respectively. 4. Conclusions In this study, the catalytic activity of 1% Au=CeO2 cat- alysts prepared by three di4erent methods was investigated for the selective CO oxidation over the temperature range Fig. 7. E4ect of CO2 concentration in reactant gas over 1% Au=CeO2 co-precipitation catalyst. Reactant composition: 1% CO, 1% O2, 10% H2O, 2–20% CO2, 40% H2 and helium: (c) 2% CO2; (•) 20% CO2; - - - - -, selectivity; —, CO conversion. Fig. 8. Deactivation test over 1% Au=CeO2 co-precipitation catalyst in a feedstream containing 1% CO, 1% O2, 2% CO2, 2.6% H2O, 40% H2, and helium: (•) CO conversion; (c) selectivity. of 323–463 K. We found that 1% Au=CeO2 co-precipitation catalyst is the most active, exhibiting high activity and good selectivity at 383 K. The presence of water vapor in the feed stream lowered the CO conversion only in the lower tem- perature region. Increasing CO2 concentration in the feed A. Luengnaruemitchai et al. / International Journal of Hydrogen Energy 29 (2004) 429–435 435 stream signi;cantly reduced the CO conversion. However, both water vapor and CO2 showed no signi;cant e4ect on the CO selectivity. The optimal amount of O2 required for oxidizing 1% CO in the feed is 1%. The catalyst was found to be quite stable in the deactivation test. Acknowledgements The authors gratefully acknowledge both fellowship sup- port and research grant of Ms. Apanee Luengnaruemitchai by the Royal Golden Jubilee Ph.D. Project, Thailand Research Fund (TRF), Thailand. References [1] Gottesfeld S, Pa4ord J. A new approach to the problem of carbon monoxide poisoning in fuel cells operating at low temperatures. J Electrochem Soc 1988;135(10):2651–2. [2] GQotz M, Wendt H. Binary and ternary anode catalyst formulations including the elements W, Sn and Mo for PEMFCs operated on methanol or reformate gas. Electrochimica Acta 1998;43(24):3637–44. [3] Rohland B, Plzak V. The PEMFC-integrated CO oxidation— a novel method of simplifying the fuel cell plant. J Power Sources 1999;84(2):183–6. [4] Oh SH, Sinkevitch RM. Carbon monoxide removal from hydrogen-rich fuel cell feedstreams by selective catalytic oxidation. J Catal 1993;142:254–62. [5] Watanabe M, Uchida H, Igarashi H, Suzuki M. Pt catalyst supported on zeolite for selective oxidation of CO in reformed gases. Chem Lett 1995;1:21–2. [6] Igarashi H, Uchida H, Suzuki M, Sasaki Y, Watanabe M. Removal of CO from H2-rich fuels by selective oxidation over platinum catalyst supported on zeolite. Appl Catal A: Gen 1997;159:159–69. [7] Kahlich MJ, Gasteiger HA, Behm RJ. Kinetics of the selective CO oxidation in H2-rich gas on Pt=Al2O3. J Catal 1997;171(1):93–105. [8] Kahlich MJ, Gasteiger HA, Behm RJ. Kinetics of the selective low-temperature oxidation of CO in H2-rich gas over Au=– Fe2O3. J Catal 1999;182(2):430–40. [9] Teng Y, Sakurai H, Ueda A, Kobayashi T. Oxidative removal of CO contained in hydrogen by using metal oxide catalysts. Int J Hydrogen Energ 1999;24:355–8. [10] Korotkikh O, Farrauto R. Selective catalytic oxidation of CO in H2: fuel cell applications. Catal Today 2000;62:249–54. [11] Haruta M, Yamada N, Kobayashi T, Iijima S. Gold catalysts prepared by coprecipitation for low-temperature oxidation of hydrogen and of carbon monoxide. J Catal 1989;115: 301–9. [12] Haruta M, Tsubota S, Kobayashi T, Kageyama H, Genet MJ, Delmon B. Low-temperature oxidation of CO over gold supported on TiO2; –Fe2O3 and Co3O4. J Catal 1993;144:175–92. [13] Lin SD, Bollinger M, Vannice MA. Low-temperature CO oxidation over Au=TiO2 and Au=SiO2 catalysts. Catal Lett 1993;17(3–4):245–62. [14] Bethke GK, Kung HH. Selective CO oxidation in a hydrogen-rich stream over Au=–Al2O3 catalysts. Appl Catal A: Gen 2000;194:43–53. [15] Torres Sanchez RM, Ueda A, Tanaka K, Haruta M. Selective oxidation of CO in hydrogen over gold supported on manganese oxides source. J Catal 1997;168(1):125–9. [16] Guczi L, Horvath D, Paszti Z, Peto G. E4ect of treatments on gold nanoparticles, relation between morphology, electron structure and catalytic activity in CO oxidation. Catal Today 2002;72(1–2):101–5. [17] Grisel RJH, Weststrate CJ, Goossens A, Craje MWJ, van der Kraan AM, Nieuwenhuys BE. Oxidation of CO over Au=MnOx=Al2O3 multi-component catalysts in a hydrogen-rich environment. Catal Today 2002;72(1–2): 123–32. [18] ScirPe S, MinicPo S, Crisafulli C, Satriano C, Pistone A. Catalytic combustion of volatile organic compounds on gold/cerium catalysts. Appl Catal B: Environ 2003;40:43–9. [19] Barbier Jr J, Duprez D. Steam e4ects in 3-way catalysis. Appl Catal B: Environ 1994;4(2–3):105–40. [20] Bunluesin T, Gorte RJ, Graham GW. Studies of the water-gas-shift reaction on ceria-supported Pt, Pd, and Rh: implications for oxygen-storage properties. Appl Catal B: Environ 1998;15:107–14. [21] Whittington BI, Jiang CJ, Trimm DL. Vehicle exhaust catalysis: I. The relative importance of catalytic oxidation, steam reforming and water-gas shift reactions. Catal Today 1995;26:41–5. [22] Diwell AF, Rajaram RR, Shaw HA, Truex TJ. Role of ceria in three-way catalysts. Stud Surf Sci Catal 1991;71:139–48. [23] Ozawa O, Kimura M. E4ect of cerium addition on the thermal stability of gamma alumina support. J Mater Sci Lett 1990;9(3):291–3. [24] Summers JC, Ausen SA. Interaction of cerium oxide with noble metals. J Catal 1979;58:131–43. [25] Shalabi MA, Harji BH, Kenney CN. Kinetic modeling of CO oxidation on Pt=CeO2 in a gradientless reactor. J Chem Technol Biotechnol 1996;65:317–24. [26] Tiernan MJ, Finlayson OE. E4ects of ceria on the combustion activity and surface properties of Pt=Al2O3 catalysts. Appl Catal B: Environ 1998;19:23–35. [27] Holmgren A, Andersson B, Duprez D. Interaction of CO with Pt/ceria catalysts. Appl Catal B: Environ 1999;22(2):215–30. [28] GQuldQur C, BalikRci F. Selective carbon monoxide over Ag-based composite oxides. Int J Hydrogen Energ 2002;27:219–24. [29] Avgouropoulos G, Ioannides T, Papadopoulou Ch, Batista J, Hocevar S, Matralis HK. A comparative study of Pt=– Al2O3; Au=–Al2O3 and CuO–CeO2 catalysts for the selective oxidation of carbon monoxide in excess hydrogen. Catal Today 2002;75:157–67. [30] Date M, Ichihashi Y, Yamashita T, Chiorino A, Boccuzzi F, Haruta M. Performance of Au=TiO2 catalyst under ambient conditions. Catal Today 2002;72(1–2):89–94. [31] Haruta M. Size- and support-dependency in the catalysis of gold. Catal Today 1997;36:153–66. [32] Wang GY, Lian HL, Zhang WX, Jiang DZ, Wu TH. Stability and deactivation of Au=Fe2O3 catalysts for CO oxidation at ambient temperature and moisture. Kinet Catal 2002;43: 433–42.