CeO2 Catalysts for Preferential Oxidation of CO in

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Bimetallic Au-Ag/CeO Catalysts for Preferential Oxidation of CO in Hydrogen-rich Stream: Effect of Calcination Temperature Natarajan Sasirekha, Palanivelu Sangeetha, and Yu-Wen Chen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp500102g • Publication Date (Web): 02 Jul 2014 Downloaded from http://pubs.acs.org on July 7, 2014

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The Journal of Physical Chemistry

Bimetallic Au-Ag/CeO2 Catalysts for Preferential Oxidation of CO in Hydrogen-rich Stream: Effect of Calcination Temperature Natarajan Sasirekhaa, Palanivelu Sangeethab, Yu-Wen Chenc* a

Department of Chemistry, Bharathidasan Institute of Technology, Anna University, Tiruchirappalli 620024, India

b

Materials Chemistry Division, Centre for Nanomaterials, School of Advanced Sciences, VIT University, Vellore 632014, India

c

Department of Chemical and Materials Engineering, National Central University, Chung-Li 32054, Taiwan.

Corresponding author: *[email protected]; Tel: +886-34227151; Fax: +886-34252296

ACS Paragon Plus Environment

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ABSTRACT Au−Ag/CeO2 catalysts with various Au/Ag atomic ratios were prepared by deposition−precipitation method. These catalysts were tested for preferential oxidation of carbon monoxide (PROX). These catalysts have been characterized by XRD, TEM, TPR and XPS techniques to gain the structural information of the supported metal catalysts. Fine gold nanoparticles around 2−4 nm were formed and dispersed well on the support. Au−Ag/CeO2 with Au/Ag atomic ratio of 5:5 showed higher catalytic activity than monometallic and other bimetallic Au−Ag/CeO2 catalysts with Au/Ag ratios of 9:1 and 7:3. The CO selectivity increased with increasing silver amount. In Au−Ag/CeO2 catalysts, the higher calcination temperature resulted in gold sintering, which resulted in lower activity. Characterization by XPS and TPR revealed that the presence of different gold and silver species plays an important role towards the activity of the catalyst. The formation of bimetallic alloy in Au−Ag/CeO2 catalyst with Au/Ag ratio of 5: 5 which showed a lower reduction temperature, is the reason for its excellent performance towards PROX reaction. The bimetallic catalyst also exhibited higher stability than the monometallic catalysts. The electronic structures of both gold and silver can be crucial to CO bonding. CO adsorbs strongly on Au. Ag+ was the active species for CO oxidation. The presence of both Au0 and Ag+ synergistically facilitate CO oxidation by the reaction between CO and O2 to form CO2. The formation of the bimetallic nanoalloy enhanced the CO oxidation.

Keywords: CO oxidation, gold, silver, bimetals, ceria.


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INTRODUCTION Preferential oxidation (PROX) of carbon monoxide in H2−rich stream is an important topic of research because H2 is used as a feed in the most promising fuel cell technology, polymer electrolyte membrane fuel cell (PEMFC). The presence of carbon monoxide (1,000 to 10,000 ppm) in reformate H2 poisons the platinum electrodes and hence, it necessary to effectively oxidize CO without oxidizing H2 at the operating temperature of PEMFC (80–100 °C) to less than 10 ppm. Even though gold catalyst is supposed to be inert for catalytic applications, gold supported catalyst is found to be highly active for oxidation of CO at low temperature,1−3 without substantially oxidize H2. Although numerous works have been reported on PROX using supported gold catalysts,4−10 further improvement is still needed in catalytic activity, selectivity and durability of catalysts. The supports play a decisive role in modifying the catalytic activity of the gold catalysts. The metal oxide support can be classified into active or reducible support such as TiO2,11,12 Fe2O3, MnO2,13 and CeO2,14−17 and inert support such as SiO2,18 and Al2O3.19 The reducible supports are able to provide oxygen atoms during PROX reaction and improve oxidation of CO. Ceria is well−known for its abundant oxygen vacancy defects, oxygen storage capacity, and its better redox (Ce4+/Ce3+) property. The role of surface vacancies in activating superoxide molecular species has been clearly demonstrated in the CO oxidation reaction over supported gold catalysts.20, 21 The metal−support interaction and the size of CeO2 found to be significant in the catalytic performance of gold supported on CeO2 catalysts. It has been reported that nanoceria is more active than bulk CeO2 for CO oxidation. Laoufi et al.22 reported that catalytic activity is optimum for a gold nanoparticle with a diameter of 2.1 ± 0.3 nm and a height of about six atomic layers. The gold nanoparticles are


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quite unstable and tend to sinter under reaction conditions, which lead to loss in catalytic activity. One way to prevent sintering of nanoparticles is by adding another active metal to form an alloy and to promote the catalytic activity. The presence of promoter significantly alters the properties of monometallic particles and the resultant bimetallic particles have unique catalytic, electronic and optical properties. Gold based bimetallic catalysts are used for various industrially important reactions.23 Dimitratos and Prati24 investigated liquid phase oxidation of D-Sorbitol using Au/Pd and Au/Pt on carbon and found that the addition of gold to Pd or Pt catalyst produced a system more resistant to oxygen poisoning and a significant increase in rate of the reaction than monometallic systems. Bimetallic Au−Pd nanoparticles supported on activated carbon was used for selective oxidation of glycerol to glyceric acid and found to be superior to the monometallic Pd or Au nanoparticles on the same support.25 Wang et al.25 found that the surface configuration of Pd monomers isolated by Au atoms and Au: Pd ratio on the surface of the particles has substantially affect the activity and stability of the catalyst. Pina et al.26 studied the effect of bimetallic Au−Cu/SiO2 for selective oxidation of benzyl alcohol to benzaldehyde and emphasized the key−role of gold highlighted by a synergistic effect with copper. Mozer et al.27 reported that the presence of Cu in Cu−Au/Al2O3 modified the catalytic properties of gold active sites and increased the selectivity of CO oxidation. However, Venezia et al.28 reported a drastic reduction in CO activity for Au−Pd/SiO2 (50/50) than Pd/SiO2. Yen et al.29 found that Ag present in Au−Ag@APTS-MCM predominantly resides on the surface of the bimetallic nanoparticle, which yields high CO activity. There is substantially less work reported on the use of bimetallic gold catalysts for PROX reaction. Au−Ag bimetallic particles receive significant attention because Au and Ag particles have FCC structure with almost identical lattice constants (aAu = 0.408 nm and aAg = 0.4089 nm)30 and


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this characteristics leads to a strong tendency towards formation of alloy, due to the difference in lattice constant that is smaller than the amplitude of thermal vibrations of atoms. Moreover, both Ag and Au have similar bond lengths in bulk materials (Ag−Ag: 2.889 Å; Au−Au: 2.884 Å), which will facilitate nanoalloy formation at all concentrations. According to Lim et al.,31 catalytically active oxygen species present in Ag supported on pyrolytic graphite is assigned to be Ag2O and/or AgO for CO oxidation. Density functional theory calculations revealed that the rate−limiting step of the CO oxidation was the reaction of CO with the Ag55O species32. In a catalytic CO oxidation over Au2− and AuAg− dimers, the Au site in AuAg− is more active than the Ag site. Moreover, Liu et al.33 reported that the calculated energy barrier values for the rate−determining step of the Au−site catalytic reaction are remarkably smaller than those for both the Ag−site catalytic reaction and the Au2− catalytic reaction. The higher catalytic activity of bimetallic AuAg− dimer was attributed to the synergistic effect between Au and Ag atom. Both Au and Ag supported catalysts are active for CO oxidation, however, Ag supported catalysts are catalytically active at reaction temperatures higher than that for Au catalysts.34 Alloying with Ag also improves oxygen activation as silver has strong affinity towards oxygen. Also, it can induce changes in the geometry of gold nanoparticles as well as electronic properties, because Ag has greater electron donating ability than Au. The objective of the present study was to achieve on understanding on the influence of bimetallic Au−Ag/CeO2 on preferential oxidation of carbon monoxide by varying Au/Ag loadings. The pretreatment of the catalysts is one of the key factors in determining the active gold and silver species and hence, the catalysts were calcined at different temperatures, which would reveal its influence on the morphology and catalytic activity of Au−Ag/CeO2 catalysts.


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EXPERIMENTAL Catalyst Preparation A series of Au−Ag/CeO2 catalysts with Au/Ag atomic ratio of 9:1, 7:3 and 5:5 were prepared by deposition−precipitation method using an aqueous solution of HAuCl4, AgNO3 and powder CeO2 (Nikki). The Au/CeO2 and Ag/CeO2 catalysts were also similarly prepared for comparison purposes. Au/CeO2 was prepared by adding an aqueous solution of HAuCl4 at a rate of 10 mL/min into the solution containing suspended support under vigorous stirring and the precipitation temperature of the solution was maintained at 65 °C. Ammonia solution was used to adjust the pH value of the solution at 8. After aging for 2 h, the precipitate was filtered and the filtration cake was washed with hot water until no chloride ions were detected using 0.1M AgNO3 solution. Finally, the filtration cake was dried in air at 80 °C for 16 h. The cake was calcined at 180 °C for 4 h to obtain the gold catalyst. Au−Ag/CeO2 catalysts were prepared by simultaneous addition of both HAuCl4 and AgNO3 solution into suspended CeO2 solution and calcined at 180 °C or 350 °C for 4 h. The amount of gold loaded on the supported catalysts corresponds to 0.5 wt. %. Characterization The morphology and physico-chemical properties of the supported catalysts were analyzed by characterization techniques such as x-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and temperature programmed reduction (TPR) method. XRD (Burker KAPPA APEX II) patterns were collected using a Siemens D500 powder diffractometer using Cu Kα1 radiation (1.5405 Å) at a voltage and current of 40 kV and 40 mA,


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respectively. The sample was scanned over the range of 2θ = 20–70º at a rate of 0.05º/min to identify the crystalline structure. The morphologies and particle sizes of the samples were determined by TEM on a JEM2000 EX II operated at 120 kV. Initially, a small amount of sample was placed into the sample tube filled with a 95% ethanol solution and after agitating under ultrasonic environment for 3 h, one drop of the dispersed slurry was dipped onto a carbon−coated copper mesh (300#) (Ted Pella Inc., CA, USA), and dried in vacuum overnight. Images were recorded digitally with a Gatan slow scan camera (GIF). Based on the several images of TEM, more than 100 particles were counted and a size distribution graph was made. XPS spectra were recorded with a Thermo VG Scientific Sigma Prob spectrometer. The XPS spectra were collected using Al Kα1 radiation at a voltage and current of 20 kV and 30 mA, respectively. The base pressure in the analyzing chamber was maintained in the order of 10–9 torr. The spectrometer was operated at 23.5 eV pass energy and the binding energy was corrected by contaminant carbon (C 1s = 285.0 eV) in order to facilitate the comparisons of the values among the catalysts and the standard compounds. Peak fitting was done using XPSPEAK 4.1 with Shirley background and 30: 70 Lorentzian/Gaussian convolution product shapes. Temperature programmed reduction (TPR) was performed on the supported catalysts to understand the reducibility of the catalysts. First the sample was treated in 100 mL/min argon at 100 °C for 1 h to remove any adsorbed gases or water, followed by passing 25 mL/min 5% H2 in argon through the sample while the temperature was ramped from 25 to 720 °C at a rate of 10 °C/min. During each of these steps, the process was monitored using a thermal conductivity detector (TCD), and the resulting current was plotted to quantify the consumption of H2.


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Catalytic Activity The catalytic activity was measured in a downward, glass fixed−bed continuous−flow reactor loaded with 0.1 g catalyst. The mass flow controller was used to control the feed. The reactant gas containing 1.33% CO, 1.33% O2, 65.33% H2 and He for balance (vol. %) was fed into the reactor with a total flow rate of 50 mL/min. The reactor was heated in a temperature regulated furnace (heating rate: 2 °C/min) and the temperature was measured using a thermocouple placed inside the catalyst bed. After reaching an equilibrium temperature at 5 min, the product was analyzed by a gas chromatograph equipped with a TCD using MS−5A column. Calibration was done with a standard gas containing known compositions of the components. The catalytic performance of catalysts was determined as follows: CO conversion (XCO%) =

O2 conversion (XO2%) =

[CO]in − [CO]out *100% [CO]in

[O2 ]in − [O2 ]out *100% [O2 ]in

(1)

(2)

Selectivity (S%) = mol of O2 reacting with CO/mol of O2 consumed =

0.5 * X CO * 100% X O2

(3)

(The feed composition of CO/O2 was equal to 1.) O2 consumption by CO (Cco%) = 1 ∗ XCO

(4)

O2 consumption by H2 (CH2%) = X O 2 − CCO

(5)

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RESULTS AND DISCUSSION Influence of Ag Loading on Au−Ag/CeO2 Catalyst The size of gold particles affects the catalytic activity of the supported gold catalysts for oxidation of CO in H2−rich stream. TEM was used to determine the size of metals in the catalysts. Figure 1 shows the TEM micrographs and histograms of the corresponding gold particle size distribution of various Au−Ag/CeO2 catalysts, revealing that the gold particles were dispersed uniformly on the support. It can be observed from the histogram that the loading of silver controlled the particle size of gold and prevented aggregation of gold particles. The average sizes of gold particles in Au−Ag/CeO2 catalysts with different Au/Ag molar ratios are listed in Table 1, and found to be in the range of 2−3 nm. It should be noted that the average size of gold particles decreased from 2.67 nm to 2.43 nm with an increase in silver content, which might be due to the formation of bimetallic nanoparticles instead of a physical mixture of individual metallic nanoparticles at high silver content. It could be due to the seeding role of Ag or Au for the formation of Au−Ag bimetallic nanoparticles, depending on Au/Ag ratio.35 The powder XRD patterns of the prepared Au−Ag/CeO2 catalysts were compared with those of Au/CeO2, and Ag/CeO2, as depicted in Figure 2. In the XRD patterns of Au−Ag/CeO2 catalysts with different silver contents, the observed peaks at 2θ = 28.55° (111), 33.08° (200), 47.48° (220) can be indexed to the fluorite structure of CeO2, which could be detected in every catalyst. The crystallinity of CeO2 remains unaltered after the loading of Au and Ag. No distinct peaks related to Au (2θ = 38.18° (111), 44.39° (200) and 77.55° (311)) and Ag (2θ = 38.12° (111), 44.23° (200) and 64.43° (200)) were observed in the XRD patterns of supported catalysts. This indicates that either the gold and silver particles were highly dispersed and the particle sizes of


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gold and silver particles were smaller than the detectable limit of XRD (> 5 nm) due to low loadings, which is consistent with the results of TEM. The reducibility of the supported monometallic and bimetallic catalysts was studied by TPR. The H2−TPR profiles of bimetallic Au−Ag/CeO2 catalysts along with Au/CeO2 are shown in Figure 3. According to literature reports, the reduction peaks of the surface−capping oxygen and the bulk oxygen of CeO2 are centered at 500 and 800 °C, respectively,36 but the reduction peaks can shift to lower temperatures by the addition of a small amount of noble or transition metals. In the case of Au/CeO2, the reduction profile was characterized by a peak with a maximum at T = 141 °C, and the reduction in the sample began at 100 °C and ended at 166 °C. A broad reduction peak was observed, which might be due to hydrogen consumption either by the reduction of surface oxygen attached to a surface Ce4+ ion or partially due to the reduction of cationic Au species. The cationic gold species present in the catalyst could strongly bound to ceria, Aun−O−Ce and hence, the presence of gold nanoparticles substantially weakened the surface oxygen of CeO2, and shifted to lower temperature. It is worthy to note that the area of low temperature reduction peak at 145 °C for Au−Ag/CeO2 (9:1) catalyst was smaller than that of Au/CeO2, which is probably due to the presence of less cationic gold species than Au/CeO2. However, in Au−Ag/CeO2 (7:3) catalyst, the reduction peak area was larger than that of both Au−Ag/CeO2 (9:1) and Au/CeO2, with a slight shift to higher temperature. The large peak area can be ascribed to the reduction of lattice oxygen of both cationic gold species and cationic silver species along with surface oxygen of CeO2. The slight increase in the reduction temperature indicates that the support becomes much more difficult to be reduced due to the possible formation of stable species. It is quite obvious that the shift in the position of the reduction peaks depends on the particle size of active metal and support, or the interaction between active metal


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phase and the support. When the ratio of silver increased from 10% to 30%, the influence of Ag on the surface reduction of CeO2 was greatly enhanced as Ag nanoparticles could activate surface lattice oxygen in CeO2 and easily dissociated H2.37 In the case of Au−Ag/CeO2 (5:5) catalyst, the peak maximum was located at significantly lower temperature (102 °C) than that of Au/CeO2, Au−Ag/CeO2 (9:1) and Au−Ag/CeO2 (7:3) catalysts. Furthermore, the reduction peak was different from other catalysts, which implies that both Au and Ag interacted strongly and weakened the surface oxygen on CeO2, due to the formation of a new bimetallic phase that improved the reducibility of CeO2. Moreover, the average gold particle size of Au−Ag/CeO2 (5:5) was also relatively small compared to other catalysts, suggesting that highly dispersed Au cation was reduced at relatively lower temperature. It is interesting to note that the peak area was smaller than that of Au−Ag/CeO2 (7:3), and the increased H2 consumption in Au−Ag/CeO2 (7:3) could also be associated with lattice oxygen spill over from CeO2 lattice to the Ag surface. The presence of both silver and gold significantly improved the reducibility of surface oxygen of CeO2 in all the catalysts. In order to analyze the oxidation states of Au species in the prepared bimetallic catalysts, XPS measurements were conducted. The XPS spectra of Au 4f for all the catalysts are shown in Figure 4. The binding energies of Au 4f5/2 and Au 4f7/2, and surface atomic concentration of gold present in Au−Ag/CeO2 catalysts with different Au: Ag ratios are listed in Table 1. Gold displayed two peaks of Au 4f7/2 and Au 4f5/2 at around 84 eV and 88 eV. The metallic gold and oxidized gold displayed peaks at 87.57 eV and 88.2 eV in Au 4f5/2, while at 83.9 eV and 84.7 eV in Au 4f7/2.38 Basically gold is more stable in metallic state than in oxidized state. However, according to the literature39,40 the interaction between ceria and gold can stabilize the oxidized gold species efficiently, such as AuO−, which is in accordance with the results displayed in Table


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1 that Au−Ag/CeO2 has higher atomic concentration of Au0 than that of Au+. The addition of silver increased the content of Au0, which also infers that gold exists in the metallic state. Among all catalysts, Au−Ag/CeO2 (5:5) had the highest concentration of Au0. XPS indicated that Au0 and Au+ species coexisted in these catalysts. Moreover, the silver has a tendency to lose electrons, which may weaken oxygen molecule bond or transfer oxidized gold to metallic gold. Figure 5 shows the XPS spectra of Ag 3d, in which silver displayed two peaks of Ag 3d5/2 and Ag 3d3/2 at around 368.5 eV and 374.5 eV. The metallic silver and oxidized silver displayed peaks at 368 eV and 365 eV in Ag 3d5/2, while at 374 eV and 372 eV in Ag 3d3/2. It should be noted that there is a shift in the binding energy to lower value with increase in silver loading, which might be due to decrease in the particle size of nanoparticles with the interaction between the metal and the support. On the contrary to Au species, Ag+ increased with increase in silver loading. In the bimetallic catalysts, silver donates electron and reduce Au+ to Au0, which in turn oxidize Ag0 to Ag+.41 There is a possibility of alloy formation in Au−Ag/CeO2 (5:5) catalyst, in which maximum Au0 and Ag+ coexisted. XPS analysis revealed that the oxides of silver seems to be prevalent than the metallic silver in the sample richer in Ag. The catalytic activities of bimetallic Au−Ag/CeO2 catalysts along with monometallic catalysts for PROX reaction are displayed in Figure 6. At 80 °C, the Au−Ag/CeO2 catalysts showed superior activity than those of Au/CeO2 and Ag/CeO2 catalysts, and the conversion improved with the addition of silver amount. The enhancement on selectivity with silver loading to Au/CeO2 was observed in Figure 7. The increase in selectivity was consistent with the increase in silver loading. For the Au−Ag/CeO2 catalysts, CO conversion decreased with increasing temperature above 80 °C. It may be related to the differences between monometallic catalysts and the bimetallic catalysts. The increase in reaction temperature would be


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accompanied by dominance of oxygen desorption, which might lead to decrease in the reaction activity. Among all of the Au−Ag/CeO2 catalysts, the Au−Ag/CeO2 (5:5) catalyst could convert CO to CO2 totally at reaction temperature of 80 °C, and it also showed the highest selectivity. It can be observed that the conversion of CO was less than 10% for Ag/CeO2, whereas for Au/CeO2, the conversion increased from 50% at room temperature to reach a maximum at 50 °C, followed by a gradual decrease with temperature. Au−Ag/CeO2 (9:1) catalyst showed a decrease in the conversion of CO at room temperature, with a silver loading of 10% than Au/CeO2. However, the conversion increased with increase in temperature, and found to be higher than Au/CeO2 after a temperature of 50 °C. Furthermore, increase in silver loading from 10% to 30% in Au−Ag/CeO2 (7:3) decreased CO conversion at room temperature significantly, but showed higher CO conversion at high temperatures than the catalyst with low silver content. In the case of Au−Ag/CeO2 (5:5) catalyst, in which there is an equal contribution of gold and silver, the CO conversion at room temperature was almost the same as that of Au/CeO2, but with high selectivity. Increase in temperature, increased the negligible CO conversion from the temperature to almost 100% conversion at 80 °C. Similarly, Au−Ag/CeO2 (5:5) catalyst displayed the highest CO selectivity at all temperatures than the bimetallic and monometallic catalysts. The origin of high activity is due to the unique synergistic effect between gold and silver species on CeO2. As suggested in the earlier reports, the dispersion and particle size of gold nanoparticles can control the catalytic activity of gold catalysts. The increase in CO conversion at high temperatures for bimetallic catalysts than monometallic catalysts infers the high stability exerted by linked monometallic nanoparticles or bimetallic alloys, which is in accordance with the decrease in gold particle size with silver loading. The result in this study revealed that gold nanoparticles were in the shape of fine particles and hence displayed high activity.


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There is still no agreement regarding the relationship between electronic state of gold and catalytic activity. Some researchers suggested that the metallic Au is the active site,42 while the others emphasized the importance of the ionic Au.39,43 Qu et al.44 reported that Ag2O was the active species for CO oxidation. In this study, the activity of Au−Ag/CeO2 (5:5) was likely to be influenced by the presence of metallic gold nanoparticles, on which CO can adsorbs easily and the presence of silver nanoparticles, which can facilitate the adsorption of oxygen on silver. There might be a presence of randomly distributed alloy composed of gold and silver. It was reported that the existence of Au attribute to the molecular oxygen adsorption on Ag.23 Figures 8 and 9 show the oxygen consumption by reacting with H2 and CO in PROX reaction at 80 °C and 100 °C, respectively. Oxygen preferentially reacts with H2 at high temperatures, and the addition of silver content can inhibit H2 oxidation intensively. The O2/CO ratio in the feed gas was 1, and hence, CO was completely converted to CO2 at 80 °C. In that case, oxygen was consumed quickly by H2 oxidation when the temperature reached 100 °C. The Au/CeO2 catalyst exhibited a lower selectivity either for CO or H2 oxidation than all Au−Ag/CeO2 catalysts as shown in Figures 8 and 9. When the oxygen consumption approched 100%, the CO conversion started to decrease. A limited amount of oxygen was used to oxidize either CO or H2. The relative strengths of adsorption may also move in favor of H2 with increase in temperature. Calcination temperature also can influence the activity and selectivity of the Au-Ag/CeO2 catalysts. The results for the samples calcined at 350 oC are shown in supporting information. The increase in calcination temperature to 350 °C resulted in aggregation of gold and silver particles. The XPS results also confirmed the decrease in active gold and silver species due to high calcination temperature, which invariably affected the activity and selectivity of the


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catalysts. The presence of silver increased the stability of the catalysts by the formation of linked monometallic nanoparticles and alloys. The physico-chemical properties and the catalytic activities of the bimetallic catalysts reveal that the presence of Ag facilitates the stability of the gold supported catalysts. In accordance with the previous reports, the co-existence of metallic and oxidic gold species are responsible for the activity of supported gold catalysts for CO oxidation. The electronic structures of both gold and silver can be crucial to CO bonding. As given in Figure 10, CO adsorbs strongly on Au0 as the ability of Ag0 to adsorb CO is lower than that of Au0.45 On the other hand, Ag+ could be the active species for CO oxidation, because of the larger electron donating ability of silver species to adsorb oxygen. The presence of both Au0 and Ag+ synergistically facilitate CO oxidation by the reaction between CO and O2 to form CO2, even in the presence of H2−rich environment. The formation of a bimetallic alloy rather than linked monometallic nanoparticles enhanced the CO oxidation. There would be electron transfer from silver to gold nanoparticles as well as between silver nanoparticles and CeO2 support. The efficiency of CeO2 to provide more oxygen for CO oxidation even at high temperatures will be improved by the presence of silver nanoparticles.

CONCLUSIONS Gold nanoparticles deposited on CeO2 were modified by the addition of silver, and the influence of bimetallic properties on the catalytic activity for CO oxidation in H2−rich stream was investigated. The addition of silver reduced the average gold particle size to less than 3 nm and increases the surface active Au0 and Ag+ species at a low calcination temperature of 180 °C. Au−Ag/CeO2 (5:5) showed the highest CO conversion and selectivity at 80 °C, which could be


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due to the formation of bimetallic alloy with fine dispersion and particle size as confirmed from TPR and XPS techniques. The increase in calcination temperature to 350 °C resulted in aggregation of gold and silver particles. The XPS results also confirmed the decrease in active gold and silver species due to high calcination temperature, which invariably affected the activity and selectivity of the catalysts. The crystallinity of CeO2 was not affected by silver loading or calcination temperature. The presence of silver increased the stability of the catalysts by the formation of linked monometallic nanoparticles and alloys. Au−Ag/CeO2 (5:5) calcined at 180 °C showed the highest CO conversion and selectivity at the PEM fuel cells operating temperature compared with other catalysts, due to the presence of highly active Au0 and Ag+ species along with its synergistic effect with the counterpart Au+ and Ag0 species.

ACKNOWLEDGMENT This research was supported by the Ministry of Economic Affairs, Taiwan, under contract no. 100-EC-17-A-09-S1-022.


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Suporting information. The results of the effect of calcination temperature on Au−Ag/CeO2 Catalysts are provided in supporting information. This material is available free of charge via the Internet at http://pubs.acs.org.

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35. Chen, D.-H.; Chen, C. J. Formation and Characterization of Au–Ag Bimetallic Nanoparticles in Water-in-oil Microemulsions. J. Mater. Chem. 2002, 12, 1557–1562 36. Yao, H. C.; Yao, Y. F. Y. Ceria in Automotive Exhaust Catalysts: I. Oxygen Storage. J. Catal. 1984, 86, 254–265 37. Chang, S.; Li, M.; Hua, Q.; Zhang, L.; Ma, Y.; Ye, B.; Huang, W. Shape-dependent Between Oxygen Vacancies and Ag Interaction in Ag/CeO2 Catalysts and Their Influence on the Catalytic Activity. J. Catal. 2012, 293, 195–204 38. Marino, F.; Descorme, C.; Duprez, D. Noble Metal Catalysts for the Preferential Oxidation of Carbon Monoxide in the Presence of Hydrogen (PROX). Appl. Catal. B: Environ. 2004, 54, 59–66 39. Delannoy, L.; Weiher, N.; Tsapatsaris, N.; Beesley, A. M.; Nchari, L.; Schroeder, S. L. M.; Louis, C. XPS Structural Studies of Nano-composite Non-platinum Electrocatalysts for Polymer Electrolyte Fuel Cells. Top. Catal. 2007, 44, 263–273 40. Venezia, A. M.; Pantaleo, G.; Longo, A.; Carlo, G. D.; Casaletto, M. P.; Liotta, F. L.; Deganello, G. Relationship Between Structure and CO Oxidation Activity of CeriaSupported Gold Catalysts. J. Phys. Chem. B 2005, 109, 2821–2827 41. Yamazoe, S.; Koyasu, K.; Tsukuda, T. Nonscalable Oxidation Catalysis of Gold Clusters. Acc. Chem. Res. 2014, 47, 816–824 42. Bond, G. C.; Thompson, D. T. Gold-catalysed Oxidation of Carbon Monoxide. Gold Bull. 2000, 33, 41–51


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43. Casaletto, M. P.; Longo, A.; Martorana, A.; Prestianni, A.; Venezia, A. M. XPS Study of Supported Gold Catalysts: The Role of Au0 and Au+δ Species as Active Sites. Surf. Interface Anal. 2006, 38, 215–218 44. Qu, Z.; Cheng, M.; Huang, W.; Bao, X. Formation of Subsurface Oxygen Species and Its High Activity Toward CO Oxidation Over Silver Catalysts. J. Catal. 2005, 229, 446–458 45. Rodriguez, J. A.; Truong, C. M.; Goodman, D. W. FT-IRAS Studies of CO Adsorbed on Ag/Pt(111): Anomalous Behavior of Vibrational Cross-sections. Surf. Sci. Lett. 1992, 271, L331–L337


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Table 1 Average gold particle size, binding energies and surface composition of catalysts, calcined at 180 °C

Catalysts

Au−Ag/CeO2 (9:1) Au−Ag/CeO2 (7:3) Au−Ag/CeO2 (5:5)

a

DAu,ave

Binding energy

Surface atomic

Binding energy

Surface atomic

(eV)b

concentration (%)

(eV)b

concentration (%)

a

(nm)

4f5/2

4f7/2

Au0

Au+

3d5/2

3d3/2

Ag0

Ag+

2.67

87.7

83.4

52.2

47.8

367.7

373.7

63.5

36.5

2.52

87.5

83.4

54.7

45.3

367.7

373.6

51.2

48.8

2.43

87.5

83.8

60.7

39.3

367.0

373.5

48.3

51.7

TEM; b XPS


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Figure Captions Fig. 1 TEM images and histogram of particle size distributions of (a) Au−Ag/CeO2 (9:1), (b) Au−Ag/CeO2 (7:3), and (c) Au−Ag/CeO2 (5:5) catalysts calcined at 180 °C. Fig. 2 XRD pattern of (a) Au−Ag/CeO2 (9:1), (b) Au−Ag/CeO2 (7:3), (c) Au−Ag/CeO2 (5:5). (d) Ag/CeO2, and (e) Au/CeO2 catalysts calcined at 180 °C. Fig. 3 TPR profiles of (a) Au/CeO2, (b) Au−Ag/CeO2 (9:1), (c) Au−Ag/CeO2 (7:3), and (d) Au−Ag/CeO2 (5:5) catalysts calcined at 180 °C. Fig. 4 XPS Au 4f spectra of the catalysts calcined at 180 °C. Fig. 5 XPS Ag 3d spectra of the catalysts calcined at 180 °C. Fig. 6 CO conversion of catalysts calcined at 180 °C. Fig. 7 CO selectivity of catalysts calcined at 180 °C. Fig. 8 Oxygen consumption by H2 and CO in PROX reaction at 80 °C for catalysts calcined at 180 °C. Fig. 9 Oxygen consumption by H2 and CO in PROX reaction at 100 °C for catalysts calcined at 180 °C. Fig. 10 Mechanism for the oxidation of CO using Au−Ag/CeO2 (5:5) catalyst (calcined at 180 °C).


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Page 26 of 36

(a)

(b)

(c)

Fig. 1 TEM images and histogram of particle size distributions of (a) Au−Ag/CeO2 (9:1), (b) Au−Ag/CeO2 (7:3), and (c) Au−Ag/CeO2 (5:5) catalysts calcined at 180 °C.


26

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Fig. 2 XRD pattern of (a) Au−Ag/CeO2 (9:1), (b) Au−Ag/CeO2 (7:3), (c) Au−Ag/CeO2 (5:5). (d) Ag/CeO2, and (e) Au/CeO2 catalysts calcined at 180 °C.


27

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H2 consumption (a.u.)


Page 28 of 36

(d)

(c) (b) (a) 0

100

200

300 Temperature (oC )

400

500

Fig. 3 TPR profiles of (a) Au/CeO2, (b) Au−Ag/CeO2 (9:1), (c) Au−Ag/CeO2 (7:3), and (d) Au−Ag/CeO2 (5:5) catalysts calcined at 180 °C.


28

Page 29 of 36

Au-Ag/CeO2 (5:5)

Au-Ag/CeO2 (7:3) Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Au-Ag/CeO2 (9:1)

Au/CeO2 80

82

84

86

88

90

92

B.E.(eV)

Fig. 4 XPS Au 4f spectra of the catalysts calcined at 180 °C.


29


Au-Ag/CeO2 (5:5)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 36

Au-Ag/CeO2 (7:3)

Au-Ag/CeO2 (9:1)

362

364

366

368

370

372

374

376

378

B.E.(eV)

Fig. 5 XPS Ag 3d spectra of the catalysts calcined at 180 °C.


30

Page 31 of 36

100.00

80.00

Conversion (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60.00

Au/CeO2 40.00

Au-Ag/CeO2 (5:5) Au-Ag/CeO2 (7:3) Au-Ag/CeO2 (9:1)

20.00

Ag/CeO2

0.00 20

30

40

50

60

70

80

90

100

Temperature (°C)

Fig. 6 CO conversion of catalysts calcined at 180 °C.

.


31


100.00 Au/CeO2 Au-Ag/CeO2 (5:5) 80.00

Au-Ag/CeO2 (7:3) Au-Ag/CeO2 (9:1)

Selectivity (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 36

Ag/CeO2

60.00

40.00

20.00

0.00 20

30

40

50

60

70

80

90

100

Temperature (°C)

Fig. 7 CO selectivity of catalysts calcined at 180 °C.


32

Page 33 of 36

100%

80%

O2 Consumption (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60%

40%

20%

0% Au/CeO2

Au-Ag/CeO2(9:1)

Au-Ag/CeO2(7:3)

Au-Ag/CeO2(5:5)

Au/Ag ratio

Fig. 8 Oxygen consumption by H2 and CO in PROX reaction at 80 °C for catalysts calcined at 180 °C.


33


100%

80%

O2 Consumption (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 36

60%

40%

20%

0% Au/CeO2

Au-Ag/CeO2(9:1)

Au-Ag/CeO2(7:3)

Au-Ag/CeO2(5:5)

Au/Ag ratio

Fig. 9 Oxygen consumption by H2 and CO in PROX reaction at 100 °C for catalysts calcined at 180 °C.


34

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Fig. 10 Mechanism for the oxidation of CO using Au−Ag/CeO2 (5:5) catalyst (calcined at 180 °C). The yellow color is Au0 and the red color is Ag+.


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TOC


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CeO2 Catalysts for Preferential Oxidation of CO in

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