Preferential CO Oxidation in a H2-Rich Stream ... - ACS Publications

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J. Phys. Chem. C 2010, 114, 6518–6526

Preferential CO Oxidation in a H2-Rich Stream on Pt-ReOx/SiO2: Catalyst Structure and Reaction Mechanism Tatsuya Ebashi,† Yoichi Ishida,† Yoshinao Nakagawa,†,‡ Shin-ichi Ito,† Takeshi Kubota,§ and Keiichi Tomishige*,†,‡ Graduate School of Pure and Applied Sciences, UniVersity of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573, Japan, International Center for Materials Nanoarchitectonics Satellite (MANA), UniVersity of Tsukuba, 1-1-1, Tennodai, Tsukuba, Ibaraki 305-8573, Japan, and Department of Material Science, Shimane UniVersity, Matsue 690-8504, Japan ReceiVed: December 16, 2009; ReVised Manuscript ReceiVed: March 9, 2010

Modification of Pt/SiO2 with ReOx species enhanced the catalytic activity of the CO oxidation with O2 in the presence of H2. The amount of Re modifier on Pt-ReOx/SiO2 was optimized to be Re/Pt ) 0.5 on the molar basis. The promoting effect of Re species appeared after the reduction above 673 K. Characterization of the reduced Pt-ReOx/SiO2 catalyst by means of EXAFS, XANES, temperature-programmed reduction, and CO adsorption measurements suggested the formation of the ReOx clusters on the surface of Pt metal particles with the average valence of Re of +2.7. The CO adsorption on Pt-ReOx/SiO2 gave a higher wavenumber then that on Pt/SiO2 at the same CO coverage, showing the weakened interaction between CO and the Pt surface, which is also supported by the desorption profile of the adsorbed CO. The CO coverage on Pt-ReOx/ SiO2 during the preferential CO oxidation was much smaller than that in the absence of H2, suggesting that an oxygen-containing species was coadsorbed on the reduced catalyst. The pulse reaction showed that the reduced Pt-ReOx/SiO2 can activate O2 even when the catalyst surface is saturated with CO. These tendencies can be explained by the mechanism where the reduced ReOx species activates O2 and the oxidizing species is spilled over from the ReOx species to the Pt surface. 1. Introduction Polymer electrolyte fuel cells (PEFCs) efficiently transform the chemical energy of H2 into electricity without the emission of pollutants and are promising technologies for the sustainable society. One of the problems in the PEFC technology is poisoning of the anode catalysts by CO contained in the hydrogen fuel in the case that the hydrogen fuel is produced by the steam-reforming reactions.1-3 Preferential oxidation (PROX) of CO contained in a H2-rich stream is an effective method for the decrease of the CO concentration. The PEFC technology demands that the PROX catalyst shows the acceptable CO conversion over a wide temperature range with a long catalyst life under realistic conditions, such as the presence of CO2 and H2O in addition to CO.4 Various catalysts for the PROX reaction, such as Pt,5-9 Cu,10-12 Au,13-16 and Rh,17,18 have been developed. However, effective catalysts are very limited under realistic conditions. Recently, our group has reported that Pt-ReOx/SiO2 catalysts are very effective in the PROX reaction of 1 vol % of CO in the presence of 10 vol % of H2O and 20 vol % of CO2, and a 20 ppm CO concentration in the effluent gas from the PROX reactor was achieved for 8 h at the reaction temperature of 383 K.19 In the present work, we investigated the additive effect of Re species on Pt-ReOx/SiO2 in the PROX reaction by means of extended X-ray absorption fine structure (EXAFS), X-ray absorption near-edge structure (XANES), in situ FTIR measurement, temperature-programmed reduction, and * To whom correspondence should be addressed. Tel: +81-29-853-5030. E-mail: [email protected]. † Graduate School of Pure and Applied Sciences, University of Tsukuba. ‡ International Center for Materials Nanoarchitectonics Satellite (MANA), University of Tsukuba. § Shimane University.

CO adsorption measurement. In particular, the promoting mechanism of the Re modifier on Pt/SiO2 in the PROX reaction is discussed. 2. Experimental Methods 2.1. Catalyst Preparation. The SiO2 support (CARiACT G-6 from Fuji Silysia Chemical Ltd.; BET surface area, 530 m2 · g-1; pore volume, 0.7 mL · g-1) was calcined for 1 h in air at 1123 K, and the BET surface area became 492 m2 · g-1. The Pt/SiO2 catalyst was prepared by impregnating the calcined SiO2 with a hot (353 K) aqueous solution of Pt(NO2)2(NH3)2 (Soekawa Chemicals; 0.033 M, pH ) 2.8). The impregnation was conducted at 373 K. The wet sample was dried at 383 K for 12 h and then calcined in air at 773 K for 3 h. The Pt-ReOx/ SiO2 catalysts were prepared by impregnating Pt/SiO2 after the drying procedure with a hot (353 K) aqueous solution of NH4ReO4 (Soekawa Chemicals; 0.033 M, pH ) 1.8). The impregnation was conducted at 373 K. The sample was dried again at 383 K for 12 h and calcined at 773 K for 3 h. The loading amount of Pt was 2 wt %. The loading amount of Re is denoted as the molar ratio of Re/Pt in parentheses, such as Pt-ReOx/SiO2 (Re/Pt ) 0.5). The BET surface area of both calcined Pt/SiO2 and Pt-ReOx/SiO2 was 438 m2 · g-1. The catalysts were reduced with hydrogen for 1 h in the reactor before the activity test. The standard reduction temperature was 773 K, and the reduction temperature was changed only when the effect of the reduction temperature was investigated. 2.2. Activity Test of the Preferential CO Oxidation and Related Reactions. The preferential CO oxidation in H2-rich gas was carried out in a fixed-bed flow reaction system at atmospheric pressure using 33 mg of the catalyst at the total flow rate of 50 mL · min-1, where the GHSV is estimated to be

10.1021/jp911908c  2010 American Chemical Society Published on Web 03/23/2010

CO Oxidation in a H2-Rich Stream on Pt-ReOx/SiO2 90 000 h-1. The feed stream contained 1.0% CO, 1.25% O2, and 60% H2, and it was balanced with He. This reaction was denoted as PROX. The catalysts were also tested in the CO oxidation in the absence of hydrogen. The feed stream contained 1.0% CO and 1.25% O2, and it was balanced with He. This reaction was denoted as CO + O2. As another reference, we evaluated the activity of the water-gas shift reaction, where the feed contained 1.0% CO and 10% H2O, balanced with He. The steam was supplied to the reactor using a syringe pump through the thin tube in the preheater. The effluent gas was analyzed using an online gas chromatograph (GC) system equipped with a thermal conductivity detector (TCD). In addition, the CO concentration at the ppm level in the effluent gas was determined using GC equipped with a methanator and a flame ionization detector. The activity was evaluated by CO conversion, which can be calculated on the basis of CO concentration in the effluent gas. The turnover frequency (TOF) was calculated by (the reaction rate of CO)/ (amount of CO adsorption) because the amount of CO adsorption represents the number of Pt surface atoms. This is based on the tendency that CO is adsorbed on the Pt surface, not on the ReOx species.20,21 The selectivity of CO oxidation in the PROX reaction is defined as the ratio of O2 consumption for the CO oxidation (CO + 1/2O2 f CO2) to the total O2 consumption (CO + 1/2O2 f CO2, H2 + 1/2O2 f H2O). In all reactions, the activity was monitored for 30 min at each condition in order to verify that the results corresponded to the steady-state activity. 2.3. Catalyst Characterization. 2.3.1. Measurement of Adsorption Amount of CO. The catalysts were characterized by the amount of CO adsorption. The amount of the irreversible CO adsorption was measured at room temperature in a highvacuum system by the volumetric method. Before the measurements, the samples were pretreated in H2 at 773 K for 1 h in the cell. After evacuation at 773 K, the sample was cooled to room temperature, and CO was introduced. The pressure at the adsorption equilibrium was about 2.6 kPa. The dead volume of the apparatus was 64 cm3, and the sample weight was 150 mg. 2.3.2. Temperature-Programmed Reduction (TPR). Temperature-programmed reduction (TPR) profiles were measured in a fixed-bed quartz reactor. Before the TPR measurement, catalysts were treated in O2 at 773 K for 0.5 h and then in Ar at 773 K for 0.5 h. The sample weight was 50 mg. The heating rate was 10 K · min-1 from room temperature to 900 K, and 5% H2 diluted in Ar (30 mL · min-1) was used. The H2 consumption was monitored continuously with TCD-GC equipped with a frozen acetone trap for the removal of H2O from the effluent gas. The amount of H2 consumption was estimated from the integrated peak area of the profiles. 2.3.3. EXAFS and XANES Analyses. The Re L3-edge and Pt L3-edge extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) were measured at the BL01B1 station at SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI, Proposal No. 2008B1235). The storage ring was operated at 8.0 GeV. A Si(111) single crystal was used to obtain a monochromatic X-ray beam. The monochromator was detuned to 60% maximum intensity to avoid higher harmonics in the X-ray beam. Two ion chambers filled with N2 and 15% Ar + 85% with N2 were used as detectors of I0 and I, respectively. The samples for the measurement were prepared by pressing 50 mg of catalyst powder to the disks. The thickness of the samples was chosen to be 0.6-0.7 mm (7 mm diameter), to give an edge jump of 0.2 and 0.4 for the Re and Pt L3 edge,

J. Phys. Chem. C, Vol. 114, No. 14, 2010 6519 respectively. The samples were pretreated at 773 K with H2 for 1 h. After the pretreatment, we transferred the samples to the measurement cell without exposing the sample disk to air, using a glovebox filled with nitrogen. For EXAFS analyses, the oscillation was first extracted from EXAFS data using a spline smoothing method.22 The oscillation was normalized by the edge height around 50 eV. Fourier transformation of the k0-weighted EXAFS oscillation from k space to r space was performed to obtain a radial distribution function. The inversely Fourierfiltered data were analyzed using a curve-fitting method.23,24 The Fourier transform and Fourier filtering ranges are shown in each result. For the curve-fitting analysis, the empirical phase shift and the amplitude function for the Re-O, Re-Re, and Pt-Pt bonds were extracted from data of NH4ReO4, Re powder, and Pt foil, respectively. Theoretical functions for the Re-Pt and Pt-Re bonds were calculated using the FEFF 8.2 program.25 In the analysis of Re L3-edge XANES spectra, the normalized spectra were obtained by subtracting the pre-edge background from the raw data with a modified Victoreen equation and normalizing them by the edge height.26-29 Analyses of EXAFS and XANES data were performed using a computer program (REX2000 Ver. 2.5.9, Rigaku Corp.). Error bars for each parameter in the EXAFS curve fitting were estimated by stepping each parameter, while optimizing the other parameter, with the R factor becomes two times as its minimum value.30 2.3.4. FTIR Measurement. Fourier transform infrared (FTIR) spectra were recorded with a Nicolet Magna 550 spectrometer equipped with an MCT detector (resolution ) 4 cm-1) in a transmission mode, using an in situ IR quartz cell with CaF2 windows. All samples for the IR measurement were pressed into self-supporting wafers (20 mm diameter, ∼100 mg). The sample disk was put into the IR cell connected to the closed circulation vacuum system, and the samples were reduced with H2 at 773 K for 1 h at 13 kPa. In the experiments of the evacuation temperature effect on the CO adsorption, the temperature increased by 40 K step-by-step. In the experiments of the IR observation during the reaction, the sample was cooled to 313 K after the reduction pretreatment and the gases (CO + He, CO + O2, CO + O2 + H2) were introduced to the IR cell connected to the flowing and evacuating system and the temperature was raised to 433 K. The details of the gas flowing conditions are described in each result. The temperature dependence of FTIR spectra during the reactions was measured for 15 min at each temperature. FTIR spectra of adsorbed species were obtained by subtracting the spectra of the fresh catalysts before exposing gases at the same temperature. To evaluate the reactivity of adsorbed species with gaseous molecules, the FTIR observation was carried out after the introduction of CO and O2 pulses. Catalysts were reduced with H2 at 773 K for 1 h before the pulse experiments. Pulse gases containing CO or O2 were introduced with online six-way valves to the 100 mg Pt-ReOx/SiO2 (Re/Pt ) 0.5) catalyst in He carrier with a flow rate of 100 mL · min-1. The catalyst has 10 µmol of Pt atoms and 5 µmol of Re atoms. The amount of the single pulse of CO and O2 was 0.3 and 0.5 µmol, respectively. The pulses were fed every 0.5 min. The sample temperature was fixed at 303 K. The measurement of FTIR spectra was carried out in a similar way to that under the steady-state conditions. 3. Results and Discussion 3.1. Catalytic Performance in the Preferential Oxidation of CO and Related Reactions. Figure 1 shows the dependence of CO conversion and turnover frequency (TOF) in the preferential CO oxidation (PROX) at 373 K on the molar

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Figure 1. Dependence of catalytic performance in the PROX and CO adsorption amount on the molar ratio of Re to Pt (Re/Pt) over Pt-ReOx/ SiO2 catalysts: CO conversion (9), turnover frequency (0) at 373 K, and CO adsorption amount (4, CO/Pt, room temperature). Reaction conditions: 1.0% CO, 1.25% O2, and 60% H2, balanced with He; the total flow rate, 50 mL · min-1; catalyst weight, 33 mg; GHSV, 90 000 h-1; reaction temperature, 373 K; reduction pretreatment at 773 K.

ratio of Re/Pt over Pt-ReOx/SiO2 catalysts. Here, the TOF was calculated using the number of the active sites estimated from the amount of CO adsorption (CO/Pt), which is also shown in Figure 1. The CO adsorption amount (CO/Pt) decreased monotonously with increasing Re/Pt. On the other hand, the CO conversion and the TOF showed volcano-type dependence, and the maximum was obtained at Re/Pt ) 0.5. The ReOx/SiO2 catalyst with the same loading amount of Re as Pt-ReOx/SiO2 (Re/Pt ) 0.5) showed very little activity (0% CO conversion). Therefore, the modification of Pt/SiO2 with Re species remarkably promotes the catalytic activity of the PROX reaction. Figure 2 shows the reaction temperature dependence of the catalytic performance in the PROX, CO + O2, and water-gas shift (CO + H2O) reactions over Pt/SiO2 and the optimized Pt-ReOx/SiO2 (Re/Pt ) 0.5) catalysts. In the case of the PROX, the CO conversion on Pt-ReOx/SiO2 (Re/Pt ) 0.5) was much higher than that on Pt/SiO2 in the low reaction temperature range, and it approached 100% at 363 K (Figure 2a,b). In the

Ebashi et al. range of 363-413 K, the CO concentration in the effluent gas was maintained below 10 ppm over Pt-ReOx/SiO2 (Re/Pt ) 0.5). It should be mentioned that 40% selectivity of CO oxidation at this temperature range means that the O2 conversion is 100%. In contrast, a high reaction temperature is necessary for the decrease of CO concentration over Pt/SiO2 because of its lower catalytic activity. Another important point is that the CO conversion in the CO + O2 reaction was much lower than that in the PROX reaction over Pt-ReOx/SiO2 (Re/Pt ) 0.5), indicating that the presence of H2 promotes the oxidation of CO (Figure 2a,c). In contrast, the presence of H2 did not promote the CO oxidation in the case of Pt/SiO2 from the results that the CO conversion in the CO + O2 reaction was comparable to that in the PROX reaction (Figure 1b,d). One possible interpretation for the promoting effect of H2 on the CO oxidation in the PROX is due to the water-gas shift reaction, where water is formed by a side reaction of O2 with H2 in the PROX.31-36 To estimate the contribution of the water-gas shift reaction in the PROX, we also tested the catalysts in the water-gas shift reaction (CO + H2O). As shown in Figure 2c,d, the reaction rate of CO + H2O was much lower than that of PROX and CO + O2 reactions over both Pt-ReOx/SiO2 (Re/Pt ) 0.5) and Pt/ SiO2 catalysts. This indicates that the contribution of CO + H2O is very small in the PROX reaction, although it has been reported that Pt-Re catalysts are effective in the water-gas shift reaction.37-39 Another interesting point is that the activity of Pt-ReOx/SiO2 (Re/Pt ) 0.5) in the CO + O2 reaction was lower than that of Pt/SiO2 at a high reaction temperature, where Re addition can suppress the CO oxidation in the CO + O2 reaction. The presence of H2 is very essential for the high PROX activity of Pt-ReOx/SiO2, and this suggests that the reducing atmosphere and the reduced state of the catalysts are important. Figure 3 shows the effect of reduction temperature on CO conversion in the PROX reaction at 353 K. In this experiment, Pt-ReOx/SiO2 (Re/Pt ) 0.5) was reduced with hydrogen at 473-773 K or the catalyst was applied to the activity test

Figure 2. Reaction temperature dependence of CO conversion (9, 2, [), selectivity of CO oxidation (×), and CO concentration in the effluent gas (0). (a, c) Pt-ReOx/SiO2 (Re/Pt ) 0.5). (b, d) Pt/SiO2. (a, b) PROX. (c, d) CO + O2 and CO + H2O. Reaction conditions: 1.0% CO, 1.25% O2, and 60% H2, balanced with He for the PROX reaction; 1.0% CO and 1.25% O2, balanced with He for the CO + O2 reaction; 1.0% CO and 10% H2O, 50 mL/min for the CO + H2O reaction; catalyst weight, 33 mg; GHSV, 90 000 h-1; reduction pretreatment at 773 K.

CO Oxidation in a H2-Rich Stream on Pt-ReOx/SiO2

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Figure 3. Effect of reduction pretreatment temperature on CO conversion in the PROX at 353 K. Reaction conditions of the PROX are the same as those in Figure 1. Figure 5. Re L3-edge XANES spectra of Pt-ReOx/SiO2 catalysts and reference compounds (a) and the relation between the white line area and the valence of Re (b). TR, reduction temperature; NR, nonreduced.

Figure 4. TPR profiles over Pt-ReOx/SiO2 and Pt/SiO2 catalysts: (a) Pt/SiO2, (b) Pt-ReOx/SiO2 (Re/Pt ) 0.2), (c) Pt-ReOx/SiO2 (Re/Pt ) 0.5), (d) Pt-ReOx/SiO2 (Re/Pt ) 1), and (e) ReOx/SiO2. TPR conditions: heating rate, 5 K/min; room temperature to 900 K; flow rate of 5% H2/Ar, 10 mL/min; catalyst weight, 150 mg. Pt: 100 µmol · g-cat-1. Re loading of the ReOx/SiO2 was the same as that of Pt-ReOx/SiO2 (Re/ Pt ) 0.5).

without the reduction pretreatment, which is denoted as Nonreduced. The PROX activity is strongly dependent on the reduction temperature; the catalyst activation can start at the reduction temperature above 473 K, and it is almost saturated above 673 K. A suitable reduction temperature is required for the high catalytic activity. To elucidate the promoting effect of Re addition, it is important to characterize the structural change during the reduction. 3.2. Characterization of Pt-ReOx/SiO2 Catalysts. Figure 4 shows the temperature-programmed reduction (TPR) profiles of the catalysts with H2. In the case of Pt/SiO2, the ratio of total H2 consumption amount to the Pt amount (H2/Pt) was calculated to be 0.25, which is much lower than 1. This means that the major part of Pt species is in the metallic state even after the calcination and/or it is reduced with H2 below room temperature. Regarding Pt-ReOx/SiO2, the H2 consumption peak at 400 K grew with increasing Re amount, and the increase of the peak intensity between 500 and 700 K was also observed. These two peaks can be assigned to the reduction of Re species, and it is suggested that the reduction of Re species can proceed in two steps. In the case of ReOx/SiO2, two peaks in the similar temperature range were observed and the two-step reduction can proceed. From the comparison between Pt-ReOx/SiO2 (Re/ Pt ) 0.5) and ReOx/SiO2 with the same Re loading (Figure 4c,e), the larger peak at the lower temperature and the smaller peak at the higher temperature on Pt-ReOx/SiO2 (Re/Pt ) 0.5) indicate that the reduction of Re species is promoted by the

presence of Pt, probably because of the spilled-over hydrogen species from Pt to Re species.40,41 It should be noted that the reduction at a higher temperature around 600 K can be connected to high PROX activity, considering the effect of reduction on the PROX activity (Figure 4). To characterize the reduced state of Re, Re L3-edge XANES analysis was carried out. In the spectrum of the L3-edge XANES, the first absorption peak is called a white line. The white line intensity of the L3 edge is known to be an informative indication of the electron state of the absorbing atoms.42 The larger white line is due to greater electron vacancy in the d orbital. In addition, it has been reported that a relative electron deficiency and ionic valence can be determined on the basis of the white line intensity.26-29 Figure 5 shows the results of Re L3-edge XANES spectra and the relation between the white line area and the valence of Re on the catalysts and reference compounds. The relation between the white line area and the valence of Re is almost linear on the reference compounds, as reported previously.43,44 Therefore, the average valence of Re species on the catalysts can be estimated from the relation. The white line intensity on nonreduced Pt-ReOx/SiO2 (Re/Pt ) 0.5) was comparable to that on NH4ReO4 and Re2O7, and this suggests that the Re species is in the state of +7 before the reduction, which is the same as the state of the Re precursor. The white line intensity decreased with increasing reduction temperature, and the reduction of Re was saturated above 673 K in the state of +2.6 on Pt-ReOx/SiO2 (Re/Pt ) 0.5). In addition, the valence of Re on Pt-ReOx/SiO2 (Re/Pt ) 0.2) is determined to be +2.8. From these results, the H2 consumption amount for the reduction of Re species up to 773 K is calculated to be 117 and 47 µmol · g-cat-1 on Pt-ReOx/SiO2 (Re/Pt ) 0.5) and Pt-ReOx/ SiO2 (Re/Pt ) 0.2), respectively, on the basis that the state of Re before the reduction is +7. On the other hand, the H2 consumption amount for the Re reduction was measured in the TPR results (Figure 4) and was 106 and 46 µmol · g-cat-1 on Pt-ReOx/SiO2 (Re/Pt ) 0.5) and Pt-ReOx/SiO2 (Re/Pt ) 0.2), respectively, in good agreement with the XANES analysis. Figure 6 shows results of the Re L3-edge extended X-ray absorption fine structure (EXAFS) of Pt-ReOx/SiO2 (Re/Pt ) 0.2 and 0.5) and model compounds, such as NH4ReO4 and Re powder. Curve-fitting results are listed in Table 1. Here, to distinguish between Re and Pt atoms as a scattering atom, the curve-fitting analysis based on k0-weighted EXAFS oscillations has been adopted according to the previous report.45 The curve-

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Figure 6. Result of the Re L3-edge EXAFS of Pt-ReOx/SiO2 (Re/Pt ) 0.5 and 0.2) and model compounds. (a) k0-weighted EXAFS oscillations. (b) Fourier transforms of k0-weighted EXAFS oscillations; FT range ) 26-120 nm-1. (c) Fourier-filtered EXAFS data (solid line) and calculated data (dots). TR, reduction temperature; NR, nonreduced.

fitting results of the Pt-ReOx/SiO2 (Re/Pt ) 0.5) without the reduction pretreatment is similar to that of NH4ReO4, which is also supported by the result of the Re L3-edge XANES. The coordination number of the RedO bond on the nonreduced

sample was as high as that of NH4ReO4, and the contribution of the Re-Pt and Re-Re bonds was not detected at all. The spectra of the reduced samples were much different from that of the nonreduced sample. By the H2 reduction at 473 K, the coordination number (CN) of the RedO bond decreased and the Re-O bond appeared; at the same time, the formation of the Re-Pt and Re-Re bonds was clearly detected. At a higher reduction temperature than 473 K, the RedO bond disappeared, and the CN of the Re-O bond decreased. In contrast, the CN of the Re-Pt and Re-Re bonds increased. These tendencies were almost saturated at 673 K. The saturation is also observed in the case of Re L3-edge XANES results, and these behaviors are explained by the TPR results that the H2 consumption was very small in the temperature range above 673 K. The bond length of the Re-Re bond (0.267-0.269 nm) on Pt-ReOx/SiO2 (Re/Pt ) 0.5) was shorter than that of the Re-Re bond in the Re metal (0.274 nm). The disagreement in the bond lengths indicates no formation of Re metal on the catalyst. In addition, it has been reported that low-valent Re clusters have a short Re-Re distance.46-48 The bond length of the Re-Pt bond (0.274-0.275 nm) agreed well with those reported on [Re2Pt(CO)12]-derived cluster catalysts supported on γ-Al2O3.45 Figure 7 shows the results of the Pt L3-edge EXAFS of the catalysts, and the curve-fitting results are summarized in Table 2. In the case of nonreduced Pt-ReOx/SiO2 (Re/Pt ) 0.5), the Pt-O bond as well as the Pt-Pt bond was detected. The much higher CN of the Pt-Pt bond than that of the Pt-O indicates that Pt metal particles are formed even on the nonreduced catalyst. The Pt-O bond can be due to the adsorbed oxygen on the Pt metal surface. On the Pt-ReOx/SiO2 catalysts after the H2 reduction, the Pt-Pt and Pt-Re bonds are detected, and the CN of the Pt-Pt bond is almost constant on the Pt-ReOx/ SiO2 (Re/Pt ) 0.5) reduced at various temperatures. The higher CN of the Pt-Pt bond than that of the Pt-Re bond indicates the formation of Pt metal particles. The CN of the Pt-Re bond

TABLE 1: Curve-Fitting Results of the Re L3-Edge EXAFS of Pt-ReOx/SiO2 Catalysts after the Reduction at Various Temperatures shells

CNa

R/10-1 nmb

σ/10-1 nmc

Pt-ReOx/SiO2 (Re/Pt ) 0.2), 773

Re-Pt Re-Re Re-O

3.8 ( 0.6 2.9 ( 1.3 1.1 ( 0.2

2.74 ( 0.01 2.69 ( 0.03 2.06 ( 0.04

0.069 ( 0.016 0.066 ( 0.020 0.067 ( 0.045

9.6 ( 1.2 3.7 ( 3.4 4.6 ( 4.1

1.5


Re-Pt Re-Re Re-O

3.4 ( 0.7 4.3 ( 1.2 1.0 ( 0.3

2.75 ( 0.02 2.68 ( 0.02 2.05 ( 0.03

0.067 ( 0.018 0.067 ( 0.019 0.070 ( 0.030

6.7 ( 1.0 1.7 ( 3.1 -3.3 ( 4.5

1.1


Re-Pt Re-Re Re-O

3.4 ( 0.5 4.3 ( 0.7 1.1 ( 0.3

2.75 ( 0.02 2.68 ( 0.02 2.05 ( 0.02

0.067 ( 0.012 0.067 ( 0.010 0.071 ( 0.025

3.8 ( 1.4 -3.6 ( 2.0 5.8 ( 2.5

1.4


Re-Pt Re-Re Re-O

2.6 ( 0.4 3.1 ( 0.6 1.8 ( 0.4

2.75 ( 0.02 2.68 ( 0.02 2.04 ( 0.01

0.068 ( 0.016 0.064 ( 0.015 0.062 ( 0.017

8.7 ( 1.4 -9.8 ( 1.4 -4.7 ( 2.1

1.7


Re-Pt Re-Re Re-O RedO

1.7 ( 0.2 2.5 ( 0.5 2.0 ( 0.3 0.7 ( 0.2

2.75 ( 0.02 2.67 ( 0.01 2.04 ( 0.01 1.74 ( 0.04

0.066 ( 0.017 0.068 ( 0.015 0.065 ( 0.010 0.063 ( 0.039

9.8 ( 2.5 5.1 ( 2.4 -0.7 ( 1.2 -4.4 ( 2.7

1.7

Pt-ReOx/SiO2 (Re/Pt ) 0.5), nonreduced Re powder NH4ReO4

RedO Re-Re Re-O

4.0 ( 0.3 12.0 4

1.75 ( 0.01 2.74 1.74

0.062 ( 0.011 0.060 0.060

-1.3 ( 1.2 0 0

catalyst, reduction temperature/K

∆E0/eVd

Rf/%e

0.5

a Coordination number. b Bond distance. c Debye-Waller factor. d Difference in the origin of the photoelectron energy between the reference and the sample. e Residual factor. Fourier transform range ) 26-120 nm-1 (Re powder, Pt-ReOx/SiO2), 30-120 nm-1 (NH4ReO4). Fourier filtering range ) 0.129-0.316 nm.



CNPt-Re/CNRe-Pt ) MRe/MPt

Figure 7. Result of the Pt L3-edge EXAFS of Pt-ReOx/SiO2 (Re/Pt ) 0.5, 0.2) and Pt foil. (a) k0-weighted EXAFS oscillations. (b) Fourier transforms of the k0-weighted Re L3-edge EXAFS; FT range ) 30-100 nm-1. Fourier filtered EXAFS data (solid line) and calculated data (dots). TR, reduction temperature; NR, nonreduced.

increased with increasing the reduction temperature. In addition, in the Re L3-edge and Pt L3-edge EXAFS analysis, it is verified that the CN ratio (CNPt-Re/CNRe-Pt) is equal to the molar ratio (MRe/MPt).

The similar CN of the Pt-Pt bond on the catalysts indicates the formation of Pt metal particles with similar sizes.49 According to the TEM observation, the average metal particle size of Pt/SiO2 and Pt-ReOx/SiO2 (Re/Pt ) 0.5) reduced at 773 K was determined to be 4.0 ( 0.3 and 4.4 ( 0.3 nm, respectively.19 On the other hand, the presence of the Re-Re bond is interpreted by the formation of the ReOx cluster in a low positive oxidation state of Re, such as +2 and/or +3. The ReOx clusters can interact directly with the surface of Pt metal particles, suggested by the presence of the Re-Pt bond, and the core-shell structure containing the Pt metal core and ReOx shell is suggested on the Pt-ReOx/SiO2 catalyst. A similar core-shell structure has been reported in the case of Rh-ReOx/SiO2 catalysts.44 Furthermore, it is possible to estimate the number of surface Pt atoms from the CO adsorption measurement because CO is not adsorbed on ReOx species but on the Pt metal surface.20,21 The CO adsorption amount of Pt-ReOx/SiO2 (Re/ Pt ) 0.5) was CO/Pt ) 0.09, which was much smaller than that of Pt/SiO2 (CO/Pt ) 0.25) (Figure 1). On the basis of the results that the particle size of the Pt metal on Pt-ReOx/SiO2 (Re/Pt ) 0.5) was almost the same as that on Pt/SiO2, the decrease of the CO adsorption amount can be explained by the partial covering of the Pt metal surface with ReOx clusters. In addition, in the case of Pt-ReOx/SiO2 (Re/Pt ) 0.2), reduced at 773 K, the CN of the Re-Pt bond was almost the same as that on Pt-ReOx/SiO2 (Re/Pt ) 0.5), whereas the CN of the Re-Re bond on Re/Pt ) 0.2 was smaller than that on Re/Pt ) 0.5. This tendency indicates that ReOx clusters have a monolayer morphology on the surface of Pt metal particles, and the size of monolayer structure can increase with increasing the amount of added Re. 3.3. FTIR Observation. Figure 8 shows the effect of the evacuation temperature on IR spectra of adsorbed CO on the catalyst. On Pt/SiO2, the IR band due to linear CO on Pt was

TABLE 2: Curve-Fitting Results of the Pt L3-Edge EXAFS of Pt/SiO2 and Pt-ReOx/SiO2 Catalysts after the Reduction at Various Temperatures catalyst, reduction temperature/K

shells

CNa

R/10-1 nmb

σ/10-1 nmc

∆E0/eVd

Rf/%e

Pt/SiO2, 773

Pt-Pt

10.3 ( 0.8

2.77 ( 0.01

0.072 ( 0.012

-0.5 ( 1.1

1.2


Pt-Pt Pt-Re

10.5 ( 1.0 0.8 ( 0.5

2.76 ( 0.01 2.77 ( 0.04

0.069 ( 0.011 0.069 ( 0.010

2.5 ( 1.4 -8.1 ( 2.5

1.7


Pt-Pt Pt-Re

10.5 ( 0.8 1.8 ( 0.9

2.76 ( 0.01 2.76 ( 0.04

0.069 ( 0.017 0.066 ( 0.044

0.3 ( 1.2 -3.6 ( 4.4

1.5


Pt-Pt Pt-Re

10.5 ( 0.7 1.7 ( 0.9

2.76 ( 0.01 2.76 ( 0.04

0.068 ( 0.008 0.066 ( 0.025

1.4 ( 1.0 -4.0 ( 0.6

1.6


Pt-Pt Pt-Re

10.5 ( 1.0 1.3 ( 0.7

2.76 ( 0.02 2.77 ( 0.03

0.066 ( 0.023 0.066 ( 0.030

1.3 ( 1.4 -2.5 ( 4.4

1.5


Pt-Pt Pt-Re

10.5 ( 1.3 0.8 ( 0.5

2.76 ( 0.01 2.77 ( 0.03

0.066 ( 0.018 0.066 ( 0.025

0.4 ( 1.1 -3.8 ( 4.1

1.5

Pt-ReOx/SiO2 (Re/Pt ) 0.5), nonreduced

Pt-Pt Pt-O

10.4 ( 1.1 0.7 ( 0.4

2.77 ( 0.01 2.00 ( 0.03

0.069 ( 0.019 0.066 ( 0.025

1.6 ( 1.2 -1.0 ( 2.4

1.4

Pt foil Na2Pt(OH)6

Pt-Pt Pt-O

12.0 6

2.77 2.05

0.060 0.060

a

0 0

Coordination number. b Bond distance. c Debye-Waller factor. d Difference in the origin of the photoelectron energy between the reference and the sample. e Residual factor. Fourier transform range ) 30-100 nm-1. Fourier filtering range ) 0.144-0.328 nm.

6524


Ebashi et al.

Figure 8. Effect of evacuation temperature on IR spectra of adsorbed CO on (a) Pt/SiO2, (b) reduced Pt-ReOx/SiO2 (Re/Pt ) 0.5), and (c) nonreduced Pt-ReOx/SiO2 (Re/Pt ) 0.5) and (d) the peak area (2, b, 9) and position (4, O, 0) of adsorbed CO as a function of evacuation temperature. Reduction temperature ) 773 K. The samples were exposed to 0.3 kPa of CO at 313 K.

observed at 2071 cm-1 at 313 K and the peak position was shifted to a lower wavenumber when the evacuation temperature became higher and the CO coverage decreased. In the case of Pt-ReOx/SiO2 (Re/Pt ) 0.5), the peak at 2075 cm-1 can be assigned to linear CO on Pt because CO is not adsorbed on ReOx species.20,21 The peak area on Pt-ReOx/SiO2 (Re/Pt ) 0.5) was much smaller than that on Pt/SiO2, corresponding to the results of CO adsorption amounts (Figure 1). Figure 8d shows the peak area of the CO adsorption band as a function of evacuation temperature on the basis of the results in Figure 8a,b. When the evacuation temperature was increased, the peak area on Pt-ReOx/SiO2 (Re/Pt ) 0.5) decreased significantly even at a low evacuation temperature. The profiles indicate that the CO adsorption on Pt-ReOx/SiO2 (Re/Pt ) 0.5) was much weaker than that on Pt/SiO2. In addition, the peak area on Pt/ SiO2 evacuated at 473 K was almost the same as that on Pt-ReOx/SiO2 (Re/Pt ) 0.5) evacuated at 313 K, and in this case, the peak position on Pt/SiO2 and Pt-ReOx/SiO2 (Re/Pt ) 0.5) was 2044 and 2075 cm-1, respectively. The CO adsorption on Pt-ReOx/SiO2 (Re/Pt ) 0.5) gave a 31 cm-1 higher wavenumber, suggesting the weak CO adsorption. The shift to a higher wavenumber caused by the modification of Pt metal particles with ReOx can be explained by the electron transfer from Pt to Re ions directly interacted with the Pt surface. In addition, Figure 8c shows the FTIR spectra of CO adsorption on the nonreduced Pt-ReOx/SiO2 (Re/Pt ) 0.5), and the peak area and position are also plotted in Figure 8d. The peak area and position at 313 K on the nonreduced Pt-ReOx/SiO2 (Re/Pt ) 0.5) were almost the same as those of the reduced Pt-ReOx/ SiO2. This indicates that the CO adsorption on the nonreduced Pt-ReOx/SiO2 (Re/Pt ) 0.5) is as weak as that on the reduced Pt-ReOx/SiO2. It has been proposed that high PROX activity at low temperature is caused by the weak CO adsorption in the case of Pt/Al2O3 catalysts modified with alkali ions,50-53 Pt-Fe/ MOR54 and Au/CeTi.55 The PROX and CO + O2 reactions also started at a lower reaction temperature on the reduced Pt-ReOx/

Figure 9. Effect of temperature on IR spectra of Pt/SiO2 (a) and Pt-ReOx/SiO2 (Re/Pt ) 0.5) (b) during the PROX, CO + O2, and CO + He. Reaction conditions: PROX, 1.0% CO, 1.25% O2, and 60% H2, balanced with He; CO + O2, 1.0% CO and 1.25% O2, balanced with He; CO + He, 1.0% CO, balanced with He. Reduction temperature ) 773 K.

SiO2 (Re/Pt ) 0.5) than Pt/SiO2 (Figure 2c,d); however, the activity of PROX on the nonreduced Pt-ReOx/SiO2 (Re/Pt ) 0.5) at low temperatures was very low, as shown in Figure 3. High PROX activity at low temperatures cannot be explained only by the weak CO adsorption. Figure 9 shows the results of in situ FTIR measurement of Pt/SiO2 and Pt-ReOx/SiO2 (Re/Pt ) 0.5) during PROX, CO + O2, and CO + He. In the case of Pt/SiO2, FTIR spectra in CO + O2 are similar to those in CO + He and the peak area is almost constant. This indicates that the CO adsorption is saturated in the presence of gas-phase CO at all the temperatures, which is supported by the strong CO adsorption on Pt/SiO2 shown in Figure 8. In the case of the PROX over Pt/SiO2, the CO adsorption is saturated at lower reaction temperatures, such as 313 and 373 K, but the CO adsorption during the reaction at 433 K became smaller. At 433 K, the CO conversion in the PROX reaction was almost as high as that in the CO + O2 reaction, as shown in Figure 2. Therefore, the difference in the CO coverage can be explained by the coadsorbed species formed during the PROX reaction, such as adsorbed oxygen and hydroxide.50,56-58 On the other hand, the CO coverage in the PROX over Pt-ReOx/SiO2 (Re/Pt ) 0.5) was much smaller than that in the CO + O2 and CO + He at all the reaction



Figure 10. FTIR measurement after the introduction of CO and O2 pulses at 303 K to Pt-ReOx/SiO2 (Re/Pt ) 0.5). (a) FTIR spectra. (b) Peak area and position. Pulse condition: CO, 0.3 µmol; O2, 0.5 µmol; flow rate of the He carrier, 100 mL/min. Catalyst weight: 100 mg (10 µmol of Pt atoms, 5 µmol of Re atoms, and 1 µmol of CO adsorption capacity). Reduction temperature ) 773 K.

temperatures. The low CO coverage even at low reaction temperatures can be related to high coverage of other adsorbed species, like the case of Pt/SiO2 at 433 K, and this tendency can be connected to high coverage of the oxidizing adsorbed species, resulting in the high PROX activity. To elucidate the mechanism of the O2 activation, the reaction between adsorbed CO with the oxygen pulse introduced from the gas phase was observed by FTIR. Figure 10 shows the FTIR results of the introduction of CO and O2 pulses at 303 K to Pt-ReOx/SiO2 (Re/Pt ) 0.5). Here, the 0.1 g of Pt-ReOx/SiO2 catalyst has 10 µmol of Pt, 5 µmol of Re, and 1 µmol of CO adsorption capacity, and the amount of CO and O2 pulses applied was 0.3 and 0.5 µmol, respectively. After the reduction pretreatment, seven pulses of CO were introduced. The FTIR peak due to CO adsorption grew gradually, and it was almost saturated after three CO pulses. Next, the O2 pulse was introduced to the sample with full coverage of adsorbed CO. The FTIR peak decreased gradually with increasing O2 pulse number. An important point is that the first O2 pulse decreases the peak intensity remarkably. The result indicates that the reduced Pt-ReOx/SiO2 activates oxygen molecules even when the CO adsorption is saturated. After the seven O2 pulses were fed, the O2 pulse was switched to the CO pulse as the second cycle of the pulse experiments. The CO peak grew again. When the CO adsorption was saturated, the peak intensity was larger than that in the first cycle, and the peak position was shifted to higher wavenumber. This is probably because the surface state is different from the fresh and reduced state and it is in the partially oxidized state. An important point is that the first O2 pulse in the second cycle did not decrease the CO peak intensity, which indicates that the O2 activation ability on the samples with CO saturation decreases after the first cycle of CO and O2 pulses. This tendency became more significant in the third cycle. The decrease in the O2 activation ability can be due to the oxidation of Re species. Although the details are not shown, in the case of the nonreduced Pt-ReOx/SiO2, a similar behavior to that of the third cycle on the reduced Pt-ReOx/SiO2 was observed even in the first cycle. In addition, in the second and third cycles on the reduced Pt-ReOx/SiO2, the second or third O2 pulse gave CO2 by the oxidation of CO. This can be

interpreted by the activation of O2 on the Pt surface on the Pt-ReOx/SiO2 catalyst. The activation of O2 on the Pt surface covered fully with adsorbed CO is rather slow. However, the vacant site can be formed after the reaction proceeds (CO(a) + O(a) f CO2 + 2v), and the vacant sites are the adsorption sites of oxygen for the further formation of CO2. Therefore, the oxidation of CO can be accelerated. It is concluded that the reduced Pt-ReOx/SiO2 has the high O2 activation ability even when the CO adsorption on the surface Pt atoms is saturated. High O2 activation ability is caused by the reduced Re species. On the basis of these results, the promotion of CO oxidation by the presence of H2 in the gas phase in the PROX can be caused by the O2 activation on the reduced Re species, where the presence of H2 can maintain the Re species in a reduced state, such as +2 to +3. In particular, the high O2 activation ability on the reduced Re species interacted with the surface Pt metal particles can enhance the coverage of oxidizing species via the oxygen spillover59-62 from Re species to the Pt surface, simultaneously decreasing the coverage of CO adsorption during the PROX reaction, as observed in Figure 9. 4. Conclusions Modification of Pt/SiO2 with Re species enhanced the activity of the preferential CO oxidation in a H2-rich stream and decreased the reaction temperature remarkably. The promoting effect of Re modification was influenced by the reduced state of Re species. The EXAFS analysis of reduced Pt-ReOx/SiO2 catalysts indicates the formation of Pt metal particles and the presence of the Re-Pt and Re-Re bonds. The Re L3-edge XANES analysis suggests that the average valence of Re species can be +2.8 to +3.5. Combined with XANES and EXAFS results, small ReOx clusters can be attached on the surface of Pt metal particles. Modification of Pt with Re species decreased the desorption temperature of adsorbed CO on both the reduced and the nonreduced Pt-ReOx/SiO2 catalysts, which means that it weakens the interaction between CO and Pt. FTIR observation during the PROX on the reduced Pt-ReOx/SiO2 (Re/Pt ) 0.5)

6526


indicates that the CO coverage was much smaller than that on the Pt-ReOx/SiO2 catalyst during CO + O2 and CO + He conditions and also that on Pt/SiO2 during all the conditions. This behavior suggests that the coverage of oxidizing species on the Pt surface site is rather high on the reduced Pt-ReOx/ SiO2 (Re/Pt ) 0.5) during the PROX reaction, which can be related to high catalytic activity. The reduced Pt-ReOx/SiO2 (Re/Pt ) 0.5) has high O2 activation ability even when the CO adsorption on the Pt surface atoms is saturated, and this property is caused by the reduced Re species. Under the PROX condition, the reduced state of Re species can be maintained by the presence of H2. The reduced ReOx species can activate O2, and the activated oxygen and oxidizing species are supplied from Re species to the Pt metal surface; as a result, the coverage of oxidizing species is increased and that of adsorbed CO is decreased. Acknowledgment. This work was, in part, supported by the World Premier International Research Center (WPI) Initiative on Materials Nanoarchitectonics, MEXT, Japan. References and Notes (1) Rohland, B.; Plzak, V. J. Power Sources 1999, 84, 183. (2) Divisek, J.; Oetjen, H.-F.; Peinecke, V.; Schmidt, V. M.; Stimming, U. Electrochim. Acta 1998, 43, 3811. (3) Song, C. Catal. Today 2002, 77, 17. (4) Park, E. D.; Lee, D.; Lee, H. C. Catal. Today 2009, 139, 280–290. (5) Ko, E. Y.; Park, E. D.; Lee, H. C.; Lee, D.; Kim, S. Angew. Chem., Int. Ed. 2007, 46, 734. (6) Pozdnyakova, O.; Teschner, D.; Wootsch, A.; Kro¨hnert, J.; Steinhauer, B.; Sauer, H.; Toth, L.; Jentoft, F. C.; Knop-Gericke, A.; Paa´l, Z.; Schlo¨gl, R. J. Catal. 2006, 237, 1. (7) Korotkikh, O.; Farrauto, R. Catal. Today 2000, 62, 249. (8) Kahlich, M. J.; Gasteiger, H. A.; Behm, R. J. J. Catal. 1997, 171, 93. (9) Igarashi, H.; Uchida, H.; Suzuki, M.; Sasaki, Y.; Watanabe, M. Appl. Catal., A 1997, 159, 159. (10) Tada, M.; Bal, R.; Mu, X.; Coquet, R.; Namda, S.; Iwasawa, Y. Chem. Commun. 2007, 4689. (11) Marin˜o, F.; Descorme, C.; Duprez, D. Appl. Catal., B 2005, 58, 175. (12) Avgouropoulos, G.; Ioannides, T. Appl. Catal., A 2003, 244, 155. (13) Deng, W.; Jesus, D. J.; Saltsburg, H.; Flytzani-Stephanopoulos, M. Appl. Catal., A 2005, 291, 126. (14) Guzman, J.; Carrettin, S.; Corma, A. J. Am. Chem. Soc. 2005, 127, 3286. (15) Grisel, R. J. H.; Nieuwenhuys, B. E. J. Catal. 2001, 199, 48. (16) Sanchez, R. M. T.; Ueda, A.; Tanaka, K.; Haruta, M. J. Catal. 1997, 168, 125. (17) Tanaka, H.; Ito, S.; Kameoka, S.; Tomishige, K.; Kunimori, K. Appl. Catal., A 2003, 250, 255. (18) Ito, S.; Fujimori, T.; Nagashima, K.; Yuzaki, K.; Kunimori, K. Catal. Today 2000, 57, 247. (19) Ishida, Y.; Ebashi, T.; Ito, S.; Kubota, T.; Kunimori, K.; Tomishige, K. Chem. Commun. 2009, 5308. (20) Daniell, W.; Weingand, T.; Kno¨zinger, H. J. Mol. Catal. A: Chem. 2003, 204-205, 519. (21) Pieck, C. L.; Vera, C. R.; Parera, J. M.; Gimenez, G. N.; Serra, L. R.; Carvalho, L. S.; Rangel, M. C. Catal. Today 2005, 107-108, 637. (22) Cook, J. W.; Sayers, D. E. J. Appl. Phys. 1981, 52, 5024. (23) Okumura, K.; Amano, J.; Yasunobu, N.; Niwa, M. J. Phys. Chem. B 2000, 104, 1050.

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