Selective Catalytic Oxidation of CO in the Presence of H2 Over Gold Catalyst

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 ÿnd that the activity of Au catalyst depends very strongly upon the preparation method, with co-precipitation prepared

Keywords: CO oxidation; Gold catalyst; Ceria; Co-precipitation; Fuel cell

1. Introduction hydrogen is the ideal fuel for the fuel cell systemsince it simpliÿes system integration, maximizes system e2ciency, In the past, the world has widely relied on the energy and provides zero emission. However, since no e4ective and fromburning fossil fuels, which are expected to be de- potentially safe means of storing adequate amount of hy- pleted in the near future. Extensive uses of fossil fuels have drogen in a vehicle exists, on-board hydrogen generation by created air pollution, acid rain, and the greenhouse e4ect. steam reforming of methanol or partial oxidation of liquid Scientists have been searching for alternative means of hydrocarbons followed by water gas shift reaction are alter- powering vehicles more e2ciently without creating any pol- native ways of giving a fuel cell powered vehicle adequate lution. The proton exchange membrane (PEM) fuel cell has range. These technologies not only produce hydrogen as a been attracting much attention in improving fuel e2ciency main product but also has side products with a small amount and cleanliness of automobiles [1]. This is due to low tem- of 1% CO. Many studies have shown the negative e4ect of perature of operation, fast cold start, perfect CO2 tolerance CO on the performance of PEM fuel cell since the electrodes by the electrolyte and a combination of high power den- of the fuel cell are typically made of platinum which is very sity and high energy conversion e2ciency. Normally, pure sensitive to CO poisoning [2,3]. Selective catalytic CO oxidation is perhaps the most promising method of reducing CO in the feed to a 10 ppm or lower range with a minimal ∗ Corresponding author. Tel.: +662-2184-141; loss of hydrogen. fax: +662-611-7619. The preferential catalytic oxidation of CO was ÿrst E-mail address: [email protected] (S. Osuwan). 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 study the e4ect of preparation method on the catalytic ac-

(Pt). Ru and Rh were very selective compared to Pt for tivity of Au=CeO2 catalysts. oxidizing CO in the presence of H2 diluted with N2 at 403 K. However, a gas composition of 900 ppm CO, 2. Experimental 800 ppmO 2 and 0.85% H2 in N2 background was very di4erent fromthe reformedgas that is generated by reforming a liquid hydrocarbon or methanol. The concept 2.1. Catalyst preparation of using a zeolite support on Pt catalysts was applied for this reaction by Watanabe et al. [5] and Igarashi Three methods of the catalyst preparation; impregnation, et al. [6]. co-precipitation and sol–gel were used in this work. Impregnation method: The Au catalysts were obtained Kinetic studies of selective CO oxidation on Pt=Al2O3 by impregnation of the commercial ceria support with an and Au=Fe2O3 catalysts in a simulated reformed gas (75% H2, the rest is N2) over a wide range of CO concentrations aqueous solution of HAuCl4:xH2O (Alfa AESAR) contain- (0.02–1.5%) were reported by Kahlich et al. [7,8]. In ad- ing the appropriate amount of Au. The catalysts were dried dition to the noble metal catalysts other catalysts have also overnight at 383 K and calcined at 773 K for 5 h. been investigated such as CoO catalyst [9], and Pt promoted Co-precipitation method: An aqueous solution of deposited on a monolith catalyst [10]. The supported Au cat- Na2CO3 (1 M) was added into an aqueous mixture of alysts have been found to be very active for CO oxidation HAuCl4:xH2O and Ce(NO3)3:6H2O (Fluka) and was kept reaction [11–13]. Recently, the metal oxide supports such at roomtemperatureand constant pH of 8.0. The precipitate as Al2O3 [14], MnOx [15], Fe2O3 [16], MgO–Al2O3 [17] was aged for an hour and then was washed several times were found to promote the activity of Au on selective CO with distilled water until there was no excess anions. Af- oxidation and the catalytic combustion of volatile organic ter washing with deionized water, the catalysts were dried compounds [18]. overnight at 383 K and calcined at 773 K for 5 h. Among the di4erent metal oxides used as the sup- Sol–gel method: The single step sol–gel catalysts were port of Au for CO oxidation, ceriumoxide or ce- prepared by hydrolyzing a solution of Ce acetate (Alfa ria (CeO2) is one of the interesting metal oxides. AESAR) and HAuCl4:xH2O with NH4OH. The reaction CeO2 is the oxide of the rare-earth metal cerium, mixture was kept at 353 K while the pH was maintained which may exist in several compositions, due to the between 9.0 and 9.5. Then, HNO3 was added until gelation, capacity of Ce to switch between the two oxida- the catalysts were dried overnight at 383 K and calcined at tion states of Ce3+ and Ce4+. A number of functions 773 K for 5 h. have been ascribed to ceria, including promoting water gas shift activity [19–21], maintaining the disper- 2.2. Catalyst characterization sion of the catalytic metals [2,22] and stabilizing the surface area of the support [23]. Extensive research, Powder X-ray di4raction (XRD) patterns were used to in particular during the past decade, has shown that obtain information about the structure and composition of ceria has beneÿcial e4ects on a number of catalytic crystalline materials. A Rigaku X-ray di4ractometer system reactions. equipped with a graphite monochromator and a Cu tube for Due to a facile redox reaction cycle, ceria exhibits oxy- generating a CuK radiation was used to obtain the XRD gen storage capacity and improves the CO and hydrocarbon patterns. oxidation. Summers and Ausen [24] claimed that ceria do- The Brunauer–Emmett–Teller (BET) method was used to nated oxygen to Pt in their study of the oxidation of CO on determine the surface area and pore size of the catalysts by N adsorption/desorption at 77 K. Speciÿc surface area was Pt=CeO2 catalyst. For CO oxidation, ceria has been found 2 to lower the activation energy, increase the reaction rate and determined using an Autosorb-1 surface area analyzer. suppress the usual CO inhibition e4ect [25]. The percent- The particle morphology of the catalysts was observed age loading of Ce has been shown to a4ect CO oxidation on by a Scanning Electron Microscope (SEM) using JEOL Pt [26]. Ceria promotes the activity of Pt in both lean and JSM -5410 LV scanning microscope operated at 15 kV. The rich reactant gases for CO oxidation [27]. Consequently, we Transmission Electron Microscopy (TEM) images were at- chose CeO2 as the support for studying selective catalytic tained using a JEM 2010 operating at 200 kV in bright and oxidation of CO in the presence of H2 based on the afore- dark ÿeld modes. Crystallinity and crystal structure of the mentioned properties of ceria. samples were evaluated from the selected area of electron di4raction patterns. Frompreliminaryresults, we found that Au =CeO2 catalysts also work very well for the selective CO oxidation. Therefore, the objectives of this work are to optimize the 2.3. Activity measurement operating conditions for preferential oxidation of CO by O2 in the presence of H2 over the temperature range of 323– Catalytic activity studies were conducted in a pyrex 463 K, which are suitable for fuel cell applications, and to glass U-tube reactor having an internal diameter of 6 mm A. Luengnaruemitchai et al. / International Journal of Hydrogen Energy 29 (2004) 429–435 431 at atmospheric pressure. One hundred mg of catalyst was no metal peaks indicating that the metallic particle size of packed between glass wool plugs in the middle of the re- the catalyst might be smaller than 5 nm. This observation actor. The reactant gas typically consisted of 1% CO, 0.5 was also conÿrmed by TEM investigations. The evidence of

–2% O2; 2% CO2; 2:6% H2 O; 40% H2 and helium. The Au in metallic form on impregnation and sol–gel catalysts space velocity was 30; 000 ml g−1 h−1. The reactant gas was observed at 2 =38:2◦ and the average Au crystallite was passed through ÿlters to remove particles and a check sizes were 29 and 30 nm, respectively. valve to prevent a reverse Now. The individual streamNow The TEM picture of 1% Au=CeO2 co-precipitation cata- rate was controlled by mass Now controllers to achieve the lyst shown in the Fig. 3(a) reveals the presence of the gold desired composition. particles as dark spots in the catalyst. The existence of Au

The reactant and product were analyzed by an on-line gas particles on CeO2 support was veriÿed by using EDS fo- chromatograph, (Hewlett Packard 5890 Series II equipped cusing on the regions containing highly contrast spots under with a thermal conductivity detector). The column utilized in TEM. For 1% Au=CeO2 co-precipitation catalyst, the Au the chromatograph was a Carbosphere, 80/100 mesh, 10 ft× particles were seen as highly contrast spots both on and in- 1=8 in stainless-steel packed column. side the support and the average particle size of Au is about The CO conversion was calculated based on the carbon 4:5 nm. Fig. 3(b) shows the Au particle size distribution of dioxide formation. The selectivity of CO oxidation was de- co-precipitation catalyst. ÿned as the oxygen consumption for CO oxidation divided by the total oxygen consumption. There was no methane 3.2. Catalytic activity formation observed under reaction conditions performed in this study. 3.2.1. Temperature dependence Fig. 4 illustrates the e4ect of catalyst preparation method

3. Results and discussion on the catalytic activity of 1% Au=CeO2. The Au=CeO2 co-precipitation catalyst has the best performance among the 3.1. Catalyst characterization three methods of catalyst preparation. The CO conversion showed a peak on all of the catalysts. The temperature at

Table 1 lists the BET surface areas, ceria crystallite sizes which the activity of the Au=CeO2 co-precipitation catalyst and Au particle sizes of the Au=CeO2 catalysts studied. The was maximum was 383 K. The reaction at 383 K is the most surface area varies signiÿcantly with the catalyst prepara- suitable temperature for the fuel cell operating temperature tion method. The CeO2 crystallite sizes of the catalysts were level (353–393 K). At higher temperature, there is a con- determined from X-ray line-broadening using the Debye tinuous decrease in selectivity, indicating higher activation

Scherrer equation and the results are also shown in Table 1. energy for the H2 oxidation than for the CO oxidation. The The sol–gel method gave the largest CeO2 crystallite size low temperature behavior of the selectivity is in agreement while the co-precipitation method gave the smallest CeO2 with the studies of Igarashi et al. [6] and Kahlich et al. [7]. crystallite size on Au catalysts. The sizes of CeO2 crystallites The morphologies of Au=CeO2 co-precipitation cat- are inversely correlated with the surface areas of the cata- alyst by SEM and XRD are very di4erent fromthose lysts. The SEM pictures of Au=CeO2 catalysts for impreg- of impregnation and sol–gel catalysts. It was found nation, co-precipitation and sol–gel are given in Fig. 1. The that the co-precipitation catalyst has a crystallite size typical XRD patterns of the CeO2 in the Au=CeO2 samples of about 4:5 nm which is much smaller than those pre- prepared by three di4erent methods are shown in Fig. 2.Itis pared by impregnation and sol–gel methods which have apparent that the patterns consist of eight main reNections, Au crystallite sizes of 29 and 30 nm, respectively. The typical of a cubic, Nuorite structure of CeO2 corresponding results obtained with the Au=CeO2 catalyst prepared by to (1 1 1); (200); (220); (311); (2 2 2), and (4 0 0) planes. co-precipitation method were similar to those observed with

XRD patterns of Au=CeO2 co-precipitation catalyst showed the co-precipitation catalyst reported by Haruta et al. [12]

Table 1 Catalyst surface area, ceria crystallite size and Au particle size

Catalyst Preparation method BET surface area (m2=g) Ceria crystallite size (nm) Au particle size (nm)

XRD TEM

a 1% Au=CeO2 Impregnation 104.2 17.4 29 Co-precipitation 124.1 14.5 ¡ 5 2–8 Sol–gel 66.7 38.6 30 a

aNot measured. 432 A. Luengnaruemitchai et al. / International Journal of Hydrogen Energy 29 (2004) 429–435

Fig. 2. XRD patterns of the Au=CeO2 catalysts.

exhibits much higher activity than other two catalysts due to the aforementioned characteristics. It has been reported that

the high activity of Au=CeO2 catalyst might be related to the capacity of Au nanoparticles to weaken the Ce–O bond. Thus, the mobility/reactivity of the surface lattice oxygen is increased [18]. The performance of Au catalysts in this work can

be compared with Au=MnOx and Au=Fe2O3 catalysts available in the literature. Kahlich et al. [8] and Tor- res Sanchez et al. [15] reported that the high activity and selectivity at the considerably lower temperature

of 393 and 353 K were observed over Au=MnOx and Au=Fe2O3 catalysts, respectively. However, their reactant gas mixture used containing CO, O2 and H2 was not

a typical simulated reformed gas. In addition, Au=CeO2 co-precipitation catalyst is most active at 383 K which is suitable for the systemconnected directly to PEM fuel cells. This catalyst shows much higher activity and selectivity when compared to the Ag-based composite oxide catalysts reported by GuldQ urQ and BalikRci [28]. How- ever, recent work by Avgouropoulos et al. [29] at similar

reaction conditions shows that CuO–CeO2 sol–gel catalyst exhibited very high activity with selectivity close to 90%. Fig. 1. SEM pictures of the Au=CeO2 catalysts: (a) impregnation; (b) co-precipitation; (c) sol–gel. 3.2.2. Eects of O2 and water vapor concentration The CO conversion and selectivity as a function of O2

concentration in the feed, for the Au=CeO2 co-precipitation and ScirÂe et al. [18]. Besides, fromSEM pictures, we ob- catalyst is shown in Fig. 5. As expected, the CO conver- served only crystalline ceria on Au=CeO2 co-precipitation sion increases with increasing O2 concentration while the catalyst which is also conÿrmed by the XRD results. It is CO selectivity decreases with increasing O2 concentration. then concluded that the Au=CeO2 co-precipitation catalyst With 0.5%, 1% and 2% O2 concentration the maxima in A. Luengnaruemitchai et al. / International Journal of Hydrogen Energy 29 (2004) 429–435 433

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. 3. (a) TEM picture of gold particles (dark spots) on the

Au=CeO2 co-precipitation surface; (b) Gold particle size distribution determined by TEM.

CO conversion are ∼ 70%, 92% and 98% at 383 K, respectively. 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 Fig. 5. E4ect of O2 concentration in reactant gas over 1% Au=CeO2 water vapor. The e4ect of water vapor in the feed stream co-precipitation catalyst. Reactant composition: 1% CO, 0.5–2% O , 2.6% H O, 2% CO , 40% H and helium: ( ) 0.5% O ;(c) on the CO oxidation activity and selectivity of Au=CeO 2 2 2 2 • 2 2 1% O ;()2%O;-----,selectivity; —, CO conversion. co-precipitation catalyst is shown in Fig. 6. The feed 2 2 gas mixture was humidiÿed by bubbling through a con- tainer of temperature controlled water, yielding 10% water vapor in the reactant gas. Under the humidiÿed condition, This is due to the strong adsorption of water on the active water lowered CO conversion in the region of temperature site [30]. However, at high temperatures water seemed to be lower than 373 K compared to the unhumidiÿed condition. 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% Fig. 7. E4ect of CO2 concentration in reactant gas over 1% Au=CeO2 co-precipitation catalyst. Reactant composition: 1% CO, Au=CeO2 co-precipitation catalyst. Reactant composition: 1% CO, 1% O2, 0–10% H2O, 2% CO2, 40% H2 and helium: ( ) w/o • 1% O2, 10% H2O, 2–20% CO2, 40% H2 and helium: (c)2% water; (c) 10% water;-----,selectivity; —, CO conversion. CO2;(•) 20% CO2;-----,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. Eect 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 from2% to 20% reduced the activity of the Au=CeO2 catalyst. The maximum in conversion dropped from ∼ 92% to ∼ 85% with a shift of 30 K to higher temperatures. The selectivity of the catalyst was not signiÿcantly 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- Fig. 8. Deactivation test over 1% Au=CeO2 co-precipitation catalyst lysts reported by Kahlich et al. [8], Avgouropoulos et al. in a feedstreamcontaining 1% CO, 1% O 2,2%CO2, 2.6% H2O, [29] and Lin et al. [13], respectively. 40% H2, and helium: (•) CO conversion; (c) selectivity.

4. Conclusions of 323–463 K. We found that 1% Au=CeO2 co-precipitation catalyst is the most active, exhibiting high activity and good

In this study, the catalytic activity of 1% Au=CeO2 cat- selectivity at 383 K. The presence of water vapor in the feed alysts prepared by three di4erent methods was investigated streamlowered the CO conversion only in the lower tem- for the selective CO oxidation over the temperature range perature region. Increasing CO2 concentration in the feed A. Luengnaruemitchai et al. / International Journal of Hydrogen Energy 29 (2004) 429–435 435 streamsigniÿcantly reduced the CO conversion. However, [14] Bethke GK, Kung HH. Selective CO oxidation in a both water vapor and CO2 showed no signiÿcant e4ect on hydrogen-rich streamover Au =–Al2O3 catalysts. Appl Catal the CO selectivity. The optimal amount of O2 required for A: Gen 2000;194:43–53. oxidizing 1% CO in the feed is 1%. The catalyst was found [15] Torres Sanchez RM, Ueda A, Tanaka K, Haruta M. Selective to be quite stable in the deactivation test. 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 Acknowledgements on gold nanoparticles, relation between morphology, electron structure and catalytic activity in CO oxidation. Catal Today 2002;72(1–2):101–5. The authors gratefully acknowledge both fellowship sup- [17] Grisel RJH, Weststrate CJ, Goossens A, Craje MWJ, port and research grant of Ms. Apanee Luengnaruemitchai van der Kraan AM, Nieuwenhuys BE. Oxidation of by the Royal Golden Jubilee Ph.D. Project, Thailand CO over Au=MnOx=Al2O3 multi-component catalysts in Research Fund (TRF), Thailand. a hydrogen-rich environment. Catal Today 2002;72(1–2): 123–32. [18] ScirÂe S, MinicÂo S, Crisafulli C, Satriano C, Pistone A. References Catalytic combustion of volatile organic compounds on gold/ceriumcatalysts. Appl Catal B: Environ 2003;40:43–9. [1] Gottesfeld S, Pa4ord J. A new approach to the problemof [19] Barbier Jr J, Duprez D. Steame4ects in 3-way catalysis. Appl carbon monoxide poisoning in fuel cells operating at low Catal B: Environ 1994;4(2–3):105–40. temperatures. J Electrochem Soc 1988;135(10):2651–2. [20] Bunluesin T, Gorte RJ, GrahamGW. Studies of the [2] GQotz M, Wendt H. Binary and ternary anode catalyst water-gas-shift reaction on ceria-supported Pt, Pd, and Rh: formulations including the elements W, Sn and Mo implications for oxygen-storage properties. Appl Catal B: for PEMFCs operated on methanol or reformate gas. Environ 1998;15:107–14. Electrochimica Acta 1998;43(24):3637–44. [21] Whittington BI, Jiang CJ, Trimm DL. Vehicle exhaust [3] Rohland B, Plzak V. The PEMFC-integrated CO oxidation— catalysis: I. The relative importance of catalytic oxidation, a novel method of simplifying the fuel cell plant. J Power steamreformingand water-gas shift reactions. Catal Today Sources 1999;84(2):183–6. 1995;26:41–5. [4] Oh SH, Sinkevitch RM. Carbon monoxide removal from [22] Diwell AF, RajaramRR, Shaw HA, Truex TJ. Role of ceria hydrogen-rich fuel cell feedstreams by selective catalytic in three-way catalysts. Stud Surf Sci Catal 1991;71:139–48. oxidation. J Catal 1993;142:254–62. [23] Ozawa O, Kimura M. E4ect of cerium addition on the [5] Watanabe M, Uchida H, Igarashi H, Suzuki M. Pt catalyst thermal stability of gamma alumina support. J Mater Sci Lett supported on zeolite for selective oxidation of CO in reformed 1990;9(3):291–3. gases. ChemLett 1995;1:21–2. [24] Summers JC, Ausen SA. Interaction of cerium oxide with [6] Igarashi H, Uchida H, Suzuki M, Sasaki Y, Watanabe M. noble metals. J Catal 1979;58:131–43. Removal of CO from H2-rich fuels by selective oxidation over [25] Shalabi MA, Harji BH, Kenney CN. Kinetic modeling of platinumcatalyst supported on zeolite. Appl Catal A: Gen CO oxidation on Pt=CeO2 in a gradientless reactor. J Chem 1997;159:159–69. Technol Biotechnol 1996;65:317–24. [7] Kahlich MJ, Gasteiger HA, BehmRJ. Kinetics of the [26] Tiernan MJ, Finlayson OE. E4ects of ceria on the combustion

selective CO oxidation in H2-rich gas on Pt=Al2O3. J Catal activity and surface properties of Pt=Al2O3 catalysts. Appl 1997;171(1):93–105. Catal B: Environ 1998;19:23–35. [8] Kahlich MJ, Gasteiger HA, BehmRJ. Kinetics of the selective [27] Holmgren A, Andersson B, Duprez D. Interaction of CO with low-temperature oxidation of CO in H2-rich gas over Au=– Pt/ceria catalysts. Appl Catal B: Environ 1999;22(2):215–30. Fe2O3. J Catal 1999;182(2):430–40. [28] GuldQ urQ C, BalikRci F. Selective carbon monoxide over [9] Teng Y, Sakurai H, Ueda A, Kobayashi T. Oxidative removal Ag-based composite oxides. Int J Hydrogen Energ of CO contained in hydrogen by using metal oxide catalysts. 2002;27:219–24. Int J Hydrogen Energ 1999;24:355–8. [29] Avgouropoulos G, Ioannides T, Papadopoulou Ch, Batista [10] Korotkikh O, Farrauto R. Selective catalytic oxidation of CO J, Hocevar S, Matralis HK. A comparative study of Pt=– in H2: fuel cell applications. Catal Today 2000;62:249–54. Al2O3; Au=–Al2O3 and CuO–CeO2 catalysts for the [11] Haruta M, Yamada N, Kobayashi T, Iijima S. Gold catalysts selective oxidation of carbon monoxide in excess hydrogen. prepared by coprecipitation for low-temperature oxidation Catal Today 2002;75:157–67. of hydrogen and of carbon monoxide. J Catal 1989;115: [30] Date M, Ichihashi Y, Yamashita T, Chiorino A, Boccuzzi F, 301–9. Haruta M. Performance of Au=TiO2 catalyst under ambient [12] Haruta M, Tsubota S, Kobayashi T, Kageyama H, Genet conditions. Catal Today 2002;72(1–2):89–94. MJ, Delmon B. Low-temperature oxidation of CO over [31] Haruta M. Size- and support-dependency in the catalysis of gold supported on TiO2;–Fe2O3 and Co3O4. J Catal gold. Catal Today 1997;36:153–66. 1993;144:175–92. [32] Wang GY, Lian HL, Zhang WX, Jiang DZ, Wu TH. Stability [13] Lin SD, Bollinger M, Vannice MA. Low-temperature CO and deactivation of Au=Fe2O3 catalysts for CO oxidation oxidation over Au=TiO2 and Au=SiO2 catalysts. Catal Lett at ambient temperature and moisture. Kinet Catal 2002;43: 1993;17(3–4):245–62. 433–42.