Quasi In Situ 57Fe Mössbauer Spectroscopic Study: Quantitative

Apr 14, 2010 - In this study, Ir−Fe/SiO2 catalyst was prepared by coimpregnation and ... the effect of the air on the iron state after H2 or PROX tr...
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J. Phys. Chem. C 2010, 114, 8533–8541

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Quasi In Situ 57Fe Mo¨ssbauer Spectroscopic Study: Quantitative Correlation between Fe2+ and H2 Concentration for PROX over Ir-Fe/SiO2 Catalyst Kuo Liu,†,‡ Aiqin Wang,† Wansheng Zhang,† Junhu Wang,† Yanqiang Huang,† Jianyi Shen,*,§ and Tao Zhang*,† State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, P. R. China, Graduate UniVersity of Chinese Academy of Sciences, Beijing 100049, P. R. China, and Department of Chemistry, Nanjing UniVersity, Nanjing 210093, P. R. China ReceiVed: February 24, 2010; ReVised Manuscript ReceiVed: April 01, 2010

In this study, Ir-Fe/SiO2 catalyst was prepared by coimpregnation and investigated for preferential oxidation of CO under the presence of H2. It was found that the presence of H2, even in a slight excess, led to a large increase in the reaction rate for CO oxidation over the Ir-Fe/SiO2 catalyst, which was quite different from the case of Ir/SiO2. To reveal the promotional role of Fe associated with the presence of H2, quasi in situ Mo¨ssbauer spectroscopy, in combination with in situ DRIFTS and microcalorimetry, was employed. The results showed that the relative amount of Fe2+ increased with increasing H2 concentration in the reaction stream, well consistent with the trend of reaction rate for CO conversion, strongly suggesting that Fe2+ is the active site for oxygen activation. H2 promoted CO oxidation mainly via maintaining a substantial amount of Fe existing as Fe2+. 1. Introduction Preferential oxidation of CO in a large excess of H2 stream (PROX) has been intensively studied during the past decade, aiming for developing an efficient catalyst system that can selectively remove CO while minimizing H2 consumption in a wide operational temperature window (e.g., 80-180 °C). To this end, various catalyst formulations have been developed.1-3 Among others, noble metals combined with a reducible metal oxide (as a support or a promoter, and referred to as MOx hereafter) have shown promising performance, in particular at low temperatures. For example, Pt/CeO2,4 Pt/TiO2,5 Snpromoted Pt/Al2O3,6 Fe-promoted Pt/Al2O3,7,8 and Pt-Fe/ mordenite9 have been reported to be highly active and selective for PROX reaction, even at temperatures below 80 °C. These particular kinds of catalysts are distinctive from the conventional unpromoted Pt/Al2O3 catalyst, since the latter is active for the PROX reaction only at an elevated temperature (>150 °C). To explain the enhanced activity and selectivity of the MOxpromoted Pt catalysts, a noncompetitive dual site adsorption mechanism has been proposed where CO adsorbed on the Pt site reacts with O provided by the MOx at the Pt-MOx interface.9,10 Although it has now been well accepted that the MOx plays a key role in providing reactive oxygen, the oxidation states of the reducible metal in the MOx, in particular under the reaction conditions, are not clear yet. Sun et al.11 proposed that CO oxidation proceeded through the formation of an oxygenrich FeOx (1 < x < 2) film that reacted with CO on the FeO(111)/ Pt(111) film by employment of STM, LEED, AES, and TPD. Kotobuki et al.12 studied the state of Fe in Pt-Fe/mordenite catalyst by XANES, and they claimed that the Fe mainly existed as FeO after H2 pretreatment. Nevertheless, their studies were * To whom correspondence should be addressed. E-mail: [email protected] (T.Z.); [email protected] (J.S.). Phone: 86-41184379015. Fax: 86-411-84691570 . † Dalian Institute of Chemical Physics, Chinese Academy of Sciences. ‡ Graduate University of Chinese Academy of Sciences. § Nanjing University.

qualitative and did not give a clear picture for the distribution of different Fe species on the catalyst surface. Previously, we developed a bifunctional Ir-Fe catalyst which showed a remarkably improved low-temperature CO conversion in comparison with nonpromoted Ir catalyst.13-15 In those studies, we mainly focused on the effect of the impregnation sequence of Ir and Fe components on the adsorption and catalytic behavior of the resulting catalysts without consideration of the influence of H2 concentration. However, the effect of H2 is always an important issue to be addressed in the PROX reaction. It is generally accepted that the presence of H2 influences positively the rate of CO oxidation at low temperatures but negatively at high temperatures due to the competition reactions. The key question is how the promotional effect of H2 takes place at low temperatures. Some authors proposed that H adsorbed on Pt spilled over to the support surface, forming hydroxyl groups or adsorbed water that could contribute directly to the CO conversion via low-temperature water-gas shift reaction.4,16 On the other hand, some investigations pointed out that the adsorbed water enhanced the CO oxidation mainly by promoting the decomposition of carbonate or formate.17,18 In considering the promotional effect of H2 in the MOx-promoted catalyst system, one should bear in mind that the presence of H2 also influences the oxidation states of M in the MOx. As indicated by Sirijaruphan et al.,19 reoxidation of Fe during the reaction is likely a significant reason for the loss of activity of PtFe. In the present paper, we systematically studied the influence of H2 concentration on the oxidation state of Fe species and the CO oxidation rate in our Ir-Fe catalyst. Previous 57Fe Mo¨ssbauer studies on PROX reaction7,15 were not in situ and could not exclude the effect of the air on the iron state after H2 or PROX treatment. Nevertheless, our present study gives a more reliable result of iron state by employing quasi in situ 57Fe Mo ¨ ssbauer spectroscopy. In particular, by employing quasi in situ 57Fe Mo¨ssbauer spectroscopy, we give a quantitative analysis of different Fe species after Ir-Fe catalyst was

10.1021/jp101697e  2010 American Chemical Society Published on Web 04/14/2010

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subjected to treatments with various feed gas compositions. Correlating the relative amount of various Fe species with the reaction rate of PROX, we provide strong evidence that the Fe2+ in close contact with Ir particles is responsible for providing reactive oxygen in PROX reaction. To the best of our knowledge, this is the first time that quantitative correlation between Fe2+ and the reaction rate has been established for the Fe-promoted catalysts in the PROX reaction. 2. Experimental Section 2.1. Catalyst Preparation. The catalyst used in this study is Ir-Fe/SiO2 which is similar to the Ir-Fe/Al2O3 in terms of catalytic behavior for PROX but is simpler for adsorption behavior, since SiO2 is more chemically inert than Al2O3 toward reactant molecules. The Ir-Fe/SiO2 was prepared by coimpregnation as reported previously.15 Briefly, SiO2 (SBET: 400 m2 g-1) was impregnated with an aqueous solution of chloroiridic acid and ferric nitrate, followed by drying at 80 °C for more than 8 h and calcination at 300 °C for 5 h in air. The loading amounts of Ir and Fe were 3 and 4.4 wt %, respectively, corresponding to an atomic Fe/Ir ratio of 5/1. For comparison, 3 wt % Ir/SiO2 and 4.4 wt % Fe/SiO2 were also prepared by impregnation. To get a better signal-to-noise ratio in a relatively shorter period in the 57Fe Mo¨ssbauer study, a desired amount of isotope 57Fe(NO ) (which was obtained by dissolving 57Fe powder into 3 3 concentrated nitric acid) was used to replace a portion of Fe(NO3)3 · 9H2O in the impregnation procedure so that 57Fe accounts for ∼40% of the total Fe amount in the Ir-Fe catalyst and 10% in the Fe/SiO2 catalyst. The activity tests for the 57Fecontaining catalysts showed that the incorporation of isotope 57Fe had no effect on the catalytic activity. 2.2. Catalytic Activity Tests. The catalytic activities were evaluated in a fixed-bed reaction system at atmospheric pressure. 100 mg of the Ir-Fe/SiO2 catalyst diluted with 100 mg of SiC was loaded into a quartz reactor with a catalyst bed length of 10 mm. A total gas flow rate of 66.7 mL min-1 was employed, corresponding to a space velocity of 40 000 mL h-1 gCat-1. For measurement of the reaction rate in a differential mode, 5-20 mg of the Ir-Fe/SiO2 catalyst diluted with 100-150 mg of SiC was loaded into a reactor with a catalyst bed length of 2 mm. In order to eliminate the diffusion effect, a total gas flow rate of 50 mL min-1 was used, which corresponded to a space velocity of 150 000-600 000 mL h-1 gCat-1. Under differential conditions, the CO conversion was less than 20% in the temperature range 50-140 °C. Prior to each test, the catalyst was reduced in situ with hydrogen at 300 °C (for Ir-Fe/SiO2) or 350 °C (for Ir/SiO2) for 2 h. The feed gas mixture was composed of 2 vol % CO, 1 vol % O2, x vol % H2 (x ) 0-40) and balance He. In some cases, 10 vol % water was also introduced to the feed gas. The effluent gas was analyzed using an online gas chromatograph (Angilent GC-6890) equipped with a thermal conductivity detector. CO2 and H2O were detected as the only products. No methane was found under our experimental conditions.

CO conversion (XCO) is calculated as XCO (%) ) {([CO]in - [CO]out)/[CO]in} × 100 CO oxidation rate (mol h-1 gCat-1) is calculated as rCO ) XCO × YCO,in × Vgas/mcat,, where mcat is the mass of the catalyst in g, Vgas is the total molar flow rate in mol h-1, XCO is the conversion of CO, and YCO,in is the molar fraction of CO in the feed gas mixture. 2.3. Characterization. In situ diffuse reflectance infrared spectroscopy (DRIFTS) was acquired with a BRUKER Equinox

Figure 1. CO conversions as a function of reaction temperature over Ir-Fe/SiO2 (a) and Ir/SiO2 (b) catalysts. The reaction feed gas contains various volume fractions of H2: (4) 0%, (9) 2%, (b) 10%, and (2) 40%; GHSV ) 40 000 mL h-1 gCat-1.

55 spectrometer equipped with a MCT detector and operated at a resolution of 4 cm-1. Before each experiment, a sample of 15 mg in a powder form was reduced in situ with H2 at 300 °C (for Ir-Fe/SiO2) or 350 °C (for Ir/SiO2) for 1 h. After cooling to 80 °C, CO gas was introduced into the reaction cell at a total flow rate of 100 mL min-1 for adsorption for 30 min, and then, the cell was purged with He for 30 min in order to remove gas phase or physically adsorbed CO. The spectra were recorded under steady-state conditions against a background of the sample at 80 °C under flowing He. Adsorption of CO, O2, and H2 was measured using a BT2.15 heat-flux microcalorimeter, as described in ref 20. Prior to adsorption, the sample was heated to 300 °C (for Ir-Fe/SiO2) or 350 °C (for Ir/SiO2) within 30 min and held at this temperature for 2 h in a special treatment cell under a highly pure H2 (99.99%) atmosphere, followed by evacuation at 350 °C for 1 h. The adsorption experiment was conducted at 40 °C. Quasi in situ 57Fe Mo¨ssbauer spectra were recorded at room temperature using a Topologic 500A spectrometer with a proportional counter. 57Co (Rh) moving in a constant acceleration mode was used as the radioactive source. The Doppler velocity of the spectrometer was calibrated with respect to R-Fe foil. The spectra were fitted with appropriate superpositions of Lorentzian lines using the MossWinn 3.0i computer program,21 and the free recoil fraction was assumed to be the same for all iron species. In this way, Mo¨ssbauer parameters such as the isomer shift (IS), the electric quadrupole splitting (QS), the full line width at half-maximum (LW), the magnetic hyperfine field (H), and the relative resonance areas of the different components of the absorption patterns (RI) were determined. Before each quasi in situ 57Fe Mo¨ssbauer measurement, a sample was loaded in a specially designed quartz reactor similar to that described in ref 22, which enabled the measurements of a sample under various atmospheres without exposing to air. The quartz reactor has two stopcocks at the ends and a side cell made of polymethyl methacrylate (PMMA). First, the sample placed in the quartz reactor was reduced under 300 °C for 2 h. After cooling to room temperature in He flow, both stopcocks were closed to seal the reactor filled with He gas. Then, the sample was moved into the side cell, and a Mo¨ssbauer spectrum of the sample was measured. Mo¨ssbauer spectra were also collected for the catalysts after they were treated in the mixture containing 2 vol % CO, 1 vol % O2, and 0-40 vol % H2 in He in simulation to the reaction conditions.

Correlation between Fe2+ and H2 Concentration for PROX

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Figure 2. CO conversions as a function of reaction temperature over Ir-Fe/SiO2 in the absence (4) or presence of 2 vol % H2O (b) or 10 vol % H2O (9).

3. Results 3.1. Effect of H2 Concentration on CO Oxidation. The influence of H2 concentration on CO oxidation rate was reported earlier for gold-based catalysts.23-26 By contrast, for the MOxpromoted noble metal catalysts, the influence of H2 was mainly investigated under PROX conditions (i.e., H2 vol % > 50%) in the previous literature.27,28 To gain a deep insight into the effect of H2 on the CO oxidation, we made a systematic study by varying the H2 volume fractions spanning from 0 to 40% in the feed gas. Figure 1 illustrates CO conversions with the reaction temperature under different feed gas compositions over Ir-Fe/ SiO2 and Ir/SiO2 catalysts. For Ir-Fe/SiO2 (Figure 1a), CO conversion was low without the presence of H2. Even at an elevated temperature up to 220 °C, the CO conversion was still lower than 40%. When 2% H2 was present in the feed gas, however, the CO conversion was increased significantly, and the highest CO conversion of 85% was obtained at 140 °C. Increasing the H2 concentration from 2 to 10% led to a further increase in low-temperature CO conversions and the temperature for the maximum CO conversion shifting to 120 °C. When the H2 concentration was further increased to 40%, the CO conversions below 100 °C remained essentially the same as those in the case of 10% H2, while the CO conversions above 100 °C had a remarkable decrease, since the competitive effect of H2 oxidation became pronounced with increasing H2 concentration and reaction temperature.23 Our activity results on the Ir-Fe/SiO2 catalyst clearly showed that the presence of H2 indeed promoted greatly the low-temperature CO oxidation, and this promotional effect was closely related to the H2 concentration in the feed gas. For Ir/SiO2 catalyst (Figure 1b), the positive effect of H2 on the CO conversion was also observed. However, compared with Ir-Fe/SiO2, this effect was much less remarkable, in particular at temperatures lower than 140 °C. In addition, with an increase in the H2 concentration, the enhancement of CO conversion became smaller and smaller due to the competition of H2 oxidation. This trend was different from the case of Ir-Fe/SiO2. Kim and Lim28 studied CO oxidation in H2-rich mixture on Pt/ Al2O3 catalyst and observed that the CO conversion did not change with H2 concentration from 10 to 50%, which was different from our observation on the Ir/SiO2 catalyst but in agreement with our result obtained on the Ir-Fe/SiO2 below 100 °C. However, they did not consider the effect of H2 at lower H2 concentrations. According to the literature,4,16 H2 promotes the low-temperature CO oxidation by forming adsorbed water which partici-

Figure 3. Differential heat curves vs coverage of (a) H2, (b) CO, and (c) O2 adsorbed separately on (9) Ir/SiO2 and (b) Ir-Fe/SiO2 catalysts.

pates directly in the reaction via water-gas shift reaction. In order to know whether the same mechanism works on our Ir-Fe/ SiO2 catalyst, we investigated the effect of water vapor on CO oxidation. As shown in Figure 2, when 2 or 10% H2O was present in the CO oxidation stream, the CO conversion was indeed enhanced. To be noted, however, the promotional effect of H2O on CO oxidation was far less pronounced at temperatures lower than 160 °C as compared with the effect of H2. This result may imply that H2 promotes CO oxidation in a different way from what water does. Only at temperatures higher than 160 °C, the CO conversion was increased largely by the addition of

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TABLE 1: Saturation Uptakes and Initial Adsorption Heat of CO, O2, and H2 on Freshly Reduced Ir-Fe/SiO2 and Ir/SiO2 Catalysts H2 absorbate

Ir-Fe/SiO2 mol-1

differential heat (kJ coverage (µmol gCat-1)

TABLE 2:

57Fe

25

)

Ir/SiO2

Ir-Fe/SiO2

Ir/SiO2

90 55

120 20

140 108

400 240

389 55

ISa (mm s-1)

chemical state

after calcination at 300 °C for 5 h after 100% H2 reduction at 300 °C for 2 h

1% O2 at 80 °C for 1 h

Fe3+ Fe3+ Fen+ Fe0 Fe2+ FeIr Fe3+ Fe0

(spe) (mgf) (2 < n < 3)

QSb (mm s-1)

0.33 0.36 0.58 0.00 1.13

0.87 0.40 0.71 0.00 2.15

0.34 0.00

0.94 0.00

-0.11

(spe)

IS, isomer shift relative to R-Fe. QS, electric quadrupole splitting. H, magnetic field. mg, magnetic; uncertainty is (5% of the reported value. a

O2

Ir-Fe/SiO2

Mo¨ssbauer Parameters of Ir-Fe/SiO2 Catalyst treatment

f

CO Ir/SiO2

b

water. At such high temperatures, we believe that the contribution from the water-gas shift reaction to the CO conversion might become significant.29 3.2. Adsorption of H2, CO, and O2. To address the promotional role of Fe on Ir/SiO2 catalyst, we first studied the adsorption behaviors of reactant molecules (CO, O2, and H2) by microcalorimetry on the Ir-Fe/SiO2 in comparison with the Ir/SiO2. The results are shown in Figure 3 and Table 1. The initial adsorption heat and the saturation uptake are 90 kJ mol-1 and 55 µmol gCat-1 for H2 and 140 kJ mol-1and 108 µmol gCat-1 for CO on Ir/SiO2 catalyst. This result indicates that CO is adsorbed more strongly than H2 on the Ir/SiO2 catalyst. In contrast with Ir/SiO2, the adsorption of either H2 or CO is weakened greatly on the Ir-Fe/SiO2 catalyst. Actually, H2 cannot be chemically adsorbed on the Ir-Fe/SiO2 catalyst at all, and the CO saturation uptake is only 20 µmol gCat-1, decreased by more than 80% compared to that on the Ir/SiO2. This result is consistent with PtFe catalyst where the addition of Fe caused a large decrease in CO adsorption.8 On the other hand, O2 adsorption on the Ir-Fe/SiO2 catalyst is significantly improved, with the saturation uptake increasing from 55 µmol gCat-1 on Ir/SiO2 to 240 µmol gCat-1 on Ir-Fe/SiO2. This result demonstrates that the addition of Fe facilitates the adsorption of oxygen, in good agreement with the results obtained by chemisorption under flow gas conditions.15

Figure 4. Typical DRIFT spectra of Ir-Fe/SiO2 and Ir/SiO2 catalysts obtained under steady-state CO adsorption conditions at 80 °C.

c

d

Hc (T) 52.0 32.9

33.0

RId (%) 87 13 20 43 26 11 76 24

e

RI, relative intensity. sp, superparamagnetic.

To further understand the difference of CO adsorption on promoted and unpromoted catalysts, we performed in situ DRIFT study for CO adsorption. As shown in Figure 4, CO adsorption on the Ir/SiO2 catalyst produces a strong band at 2070 cm-1 which can be assigned to the linear adsorption of CO on Ir0.30,31 In comparison with the CO adsorption on Ir/ SiO2, the CO adsorption band on the Ir-Fe/SiO2 catalyst red shifts by 3 cm-1, and the band intensity is at least 8-fold weaker than that on Ir/SiO2. The weak intensity of CO adsorption on Ir with a red shift after adding Fe indicates that the iron species blocks most of the Ir metal surface and modifies the electronic states of the Ir metal particles.32 In good agreement with the microcalorimetric result, the DRIFT study confirms again that the CO adsorption on Ir-Fe/SiO2 catalyst is weakened greatly. 3.3. Quasi In Situ 57Fe Mo¨ssbauer Spectra. To reveal the oxidation state of Fe species and correlate it with the catalytic performance, we performed 57Fe Mo¨ssbauer spectroscopic studies on the Ir-Fe/SiO2 catalyst which had been subjected to treatments in different atmospheres. Mo¨ssbauer spectroscopy is a powerful tool to identify the oxidation states of iron,33-35 and the relative amount of different Fe species can be quantitatively determined. In this study, all of the spectra were recorded at room temperature, and the spectra were well fitted into various Fe species (solid lines in the spectra). It should be pointed out that, in our spectral analyses, one iron component with IS ) 0.42-0.58 mm s-1 (see Tables 2-4) is tentatively assigned to the intermediate valent state, i.e., Fen+ (2 < n < 3). It is known that, among the 57Fe Mo¨ssbauer parameters, the IS value is the most important factor for diagnosing the valent states of iron.36 It can be seen from Tables 2-4 that the typical IS values are less than 0.40 mm s-1 for Fe3+ and more than 0.95 mm s-1 for Fe2+ obtained in the present study. In this case, the iron component with IS ) 0.42-0.58 mm s-1 should be reasonably ascribed to the intermediate valent state between Fe2+ and Fe3+. 3.3.1. Comparison of Ir-Fe/SiO2 with Fe/SiO2 Treated with H2 or O2. Figure 5 displays the 57Fe Mo¨ssbauer spectra of Ir-Fe/ SiO2 catalyst before and after H2 reduction, as well as reoxidation with O2. For comparison, the spectra of the Fe/SiO2 sample pretreated under similar conditions are presented in Figure 6. The corresponding Mo¨ssbauer parameters are listed in Tables 2 and 3. For the calcined Ir-Fe/SiO2 catalyst (Figure

Correlation between Fe2+ and H2 Concentration for PROX

Figure 5. Quasi in situ 57Fe Mo¨ssbauer spectra of Ir-Fe/SiO2 catalyst (a) before and (b) after 100% H2 reduction at 300 °C for 2 h, (c) followed by 1% O2 treatment at 80 °C for 1 h after (b).

Figure 6. Quasi in situ 57Fe Mo¨ssbauer spectra of Fe/SiO2 (a) after 100% H2 reduction at 300 °C for 2 h and (b) after 100% H2 reduction at 600 °C for 2 h, (c) followed by 1% O2 treatment at 80 °C for 1 h after (b).

5a), the spectrum consists of a doublet and a sextuplet, which can be assigned to superparamagnetic and relatively large particles of Fe3+ species, respectively. After reduction with H2 at 300 °C for 2 h, the spectrum (Figure 5b) is composed of a doublet (IS ) 0.58 mm s-1 and QS ) 0.71 mm s-1) for Fen+ (2 < n < 3), a doublet (IS ) 1.13 mm s-1 and QS ) 2.15 mm s-1) for Fe2+,37 a sextuplet for Fe0, and a singlet for FeIr alloy. Niemantsverdriet’s research group38,39 studied the structure of FeIr/SiO2 catalyst (atomic ratio Fe/Ir ) 1/1) during CO hydrogenation by Mo¨ssbauer spectroscopy and also detected TABLE 3:

57Fe

J. Phys. Chem. C, Vol. 114, No. 18, 2010 8537 Fe0, Fe2+, and FeIr alloy in the 400 °C-reduced catalyst. In a similar study by Berry and Jobson,40 both Fe3O4 and Fe2+ were identified on the 270 °C-reduced IrFe/SiO2 catalyst with low concentration of Ir. With an increase in the Ir content in the IrFe catalyst, Fe0 or FeIr alloy was also detected on the reduced catalyst. These results were basically in agreement with our present study. Clearly, the treatment with H2 at 300 °C reduced the Fe3+ species to some low-valence iron species. The reduced catalyst was subsequently treated with 1% O2 at 80 °C for 1 h for investigation of the reactivities of different low-valence Fe species toward oxygen. The resulting spectrum (Figure 5c and Table 2) was an overlap of Fe3+ (76%) and Fe0 (24%). The disappearance of Fen+ (2 < n < 3), Fe2+, and FeIr alloy upon oxygen treatment strongly suggests that these low-valence Fe species are easily oxidized. It is interesting to note that there is still a significant amount of Fe0 retained in the Ir-Fe/SiO2 catalyst even after the oxygen treatment. It seems that only some outer layers of Fe0 particles were oxidized upon exposure to 1% O2 atmosphere at 80 °C for 1 h, while the cores of the particles remained as Fe0. Wang et al.41 pointed out that a critical size existed for which the iron could be fully oxidized, whereas, for particles larger than the critical size (∼8 nm), an iron/ironoxide structure could be found. Different from Ir-Fe/SiO2, Fe/SiO2 still showed a typical Fe3+ doublet spectrum (Figure 6a and Table 3) after the reduction with H2 at 300 °C. Only when reduced at 600 °C, low-valence Fe species including Fen+, Fe0, Fe2+ (1) (IS ) 1.10 mm s-1 and QS ) 2.25 mm s-1), and Fe2+ (2) (IS ) 1.06 mm s-1 and QS ) 1.69 mm s-1) could be detected (Figure 6b). Moreover, the relative amount of Fe0 in the Fe/SiO2 catalyst was significantly lower than that in the Ir-Fe/SiO2. This result indicates that the presence of Ir facilitates the reduction of Fe species, consistent with the previous H2-TPR results.15 When the 600 °C-reduced Fe/SiO2 sample was exposed to 1% O2 at 80 °C for 1 h, Fe0 disappeared, while the amount of Fe3+ was increased (Figure 6c) due to the oxidation of Fe0 and Fen+. The complete oxidation of Fe0 in the 600 °C-reduced Fe/SiO2 sample implies that the particle size of this part of Fe0 might be smaller than that in the reduced Ir-Fe/SiO2 catalyst.41 To our surprise, the total amount of Fe2+ remained almost unchanged, although Fe2+ (1) increased at the expense of Fe2+ (2). This result strongly suggests that the low-valence Fe species in the Fe/SiO2, in particular Fe2+, is more difficult to be reoxidized than those in the Ir-Fe/SiO2 catalyst. In other words, the Fe species in the Ir-Fe/SiO2 are highly reactive toward reduction by H2 and reoxidation by O2. 3.3.2. Mo¨ssbauer Spectra of Ir-Fe/SiO2 Catalyst Treated with Reactant Gas Mixtures. After reduction, the Ir-Fe catalyst was subsequently treated with various reactant gas mixtures at 80 °C for 1 h. The resulting 57Fe Mo¨ssbauer spectra are shown in Figures 7 and 8, and the corresponding parameters are

Mo¨ssbauer Parameters of Fe/SiO2 Catalyst

treatment after reduction at 300 °C for 2 h after reduction at 600 °C for 2 h

1% O2 at 80 °C for 1 h

chemical state Fe3+ Fen+ Fe0 Fe2+ Fe2+ Fe3+ Fe2+ Fe2+

(spe) (2 < n < 3) (1) (2) (spe) (1) (2)

ISa (mm s-1)

QSb (mm s-1)

0.34 0.53 0.00 1.10 1.06 0.35 0.98 0.95

0.87 1.38 0.00 2.25 1.69 0.92 2.40 1.73

IS, isomer shift relative to R-Fe. b QS, electric quadrupole splitting. c H, magnetic field. uncertainty is (5% of the reported value. a

d

Hc (T)

32.4

RId (%) 100 29 10 30 31 44 40 16

RI, relative intensity. e sp, superparamagnetic;

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Figure 7. Quasi in situ 57Fe Mo¨ssbauer spectra of reduced Ir-Fe/ SiO2 catalyst upon exposure to different atmospheres at 80 °C for 1 h.

Liu et al. with an iron atom, forming iron oxide, resulting in higher iron oxidation states. When the reduced catalyst was exposed to a mixture of CO, O2, and H2 (0-40%) at 80 °C for 1 h, i.e., the typical reaction condition for CO oxidation or PROX (Figure 8), both FeIr alloy and intermediate iron Fen+ disappeared completely. All of the spectra could be well fitted into Fe0, Fe2+, and Fe3+. It can be seen from Table 4 that the amount of Fe0 was kept steady around 28% for different H2 concentrations used in the feed gas mixtures. At the same time, the amount of Fe3+ decreased from 67 to 54%, whereas the amount of Fe2+ increased from 4 to 18% with an increase of H2 concentration from 0 to 40%. 57Fe Mo ¨ ssbauer spectroscopic study was also performed on the Ir-Fe catalyst after CO oxidation with the presence of 10% H2O, in order to make a comparison with that in the presence of H2. Different from that treated under PROX reaction conditions, the catalyst treated with a mixture of 10% H2O, 2% CO, and 1% O2 is composed of only two iron species, Fe3+ and Fe2+. The absence of Fe0 as well as the largely increased amount of Fe3+ suggest that Fe0 was oxidized to either Fe3+ or Fe2+ with the presence of 10% H2O. It is also interesting to note that the amount of Fe2+ in this case was identical to that after reaction with 2% H2, 2% CO, and 1% O2. This result indicates that Fe2+ species might be stabilized with the presence of H239 or H2O. In addition, it should be mentioned that the Fe3+ with IS ) 0.36 mm s-1 and QS ) 0.66 mm s-1 might be different from the superparamagnetic R-Fe2O3 with IS ) 0.32-0.36 mm s-1 and QS ) 0.87-0.94 mm s-1. According to the literature,43 the Fe3+ here was probably Fe3+ in ferrihydrite. 4. Discussion

Figure 8. Quasi in situ 57Fe Mo¨ssbauer spectra of reduced Ir-Fe/ SiO2 catalyst upon exposure to reaction atmosphere containing different concentrations of H2 at 80 °C for 1 h.

summarized in Table 4. Interestingly, all of the low-valence iron components were retained when 1% O2 and more than 2% H2 were fed simultaneously (Figure 7). Compared with the result obtained with 1% O2 treatment, the addition of H2 clearly stabilized Fen+, Fe2+, and FeIr alloy. Compared with the freshly reduced catalyst, the subsequent treatment with a mixture of H2 and O2 caused changes of the relative amounts of intermediate (Fen+), ferrous, metallic, and alloyed state iron components. Moreover, the IS values of Fen+ also deceased with decreasing H2 volume fractions, i.e., 0.58 mm s-1 (100%) > 0.48 mm s-1 (40%) > 0.42 mm s-1 (2%). This trend indicates that the intermediate iron component has the tendency toward a higher oxidation state (Fe3+) with decreasing H2 concentration in the treatment gas mixture. When the freshly reduced catalyst was exposed to 2% CO at 80 °C for 1 h, the Mo¨ssbauer spectrum was similar to that treated with a mixture of 2% H2 and 1% O2. In particular, the relative amounts of Fe0, Fe2+, and FeIr alloy were almost identical in both cases. The only difference was the identification of Fe3+ species, although its IS value of 0.39 mm s-1 was larger than the typical ferric component identified in the present study. It was reported that CO could be dissociated on Fe0 sites, forming carbon and oxygen atoms.42 The oxygen atoms would then react

It is widely accepted that CO oxidation on an unpromoted noblemetalcatalystfollowsacompetitiveLangmuir-Hinshelwood mechanism where CO, H2, and O2 are all adsorbed on the noble metal surface. In the case of Ir/SiO2 catalyst, the stronger adsorption of CO than H2 (Table 1) would be favorable to the preferential oxidation of CO even under the presence of rich H2. At the same time, however, the strong adsorption of CO makes the Ir sites almost fully covered by CO at low temperatures which leads to few sites available for oxygen adsorption.1 In such a case, the CO conversions on the Ir/SiO2 at low temperatures are very low, as shown in Figure 1b. To enhance the activity for low-temperature CO oxidation on Ir, the adsorption of CO on Ir sites must be weakened so that O2 has enough opportunities to adsorb on them. For this purpose, the second metal Fe is added. Both microcalorimetry and DRIFT studies have shown that the addition of Fe indeed weakens the adsorption of CO greatly. In this case, the possibility for the reaction between adsorbed CO and O on the neighboring Ir sites, i.e., competitive Langmuir-Hinshelwood reaction, should have been enhanced. However, our activity tests showed that the Ir-Fe/SiO2 catalyst was even less active than the Ir/SiO2 catalyst without the presence of H2. In other words, Fe did not impose any beneficial effect for CO oxidation without the presence of H2. This result clearly indicates that the promotional effect of Fe is associated with the presence of H2. Actually, the H2 concentration in the reaction stream strongly influenced the distribution of different Fe species in the Ir-Fe/SiO2 catalyst. Our 57Fe Mo¨ssbauer spectroscopic studies gave a quantitative analysis of the Fe species in the Ir-Fe/SiO2 catalyst treated with reaction gases containing different fractions of H2 (Table 4). To better correlate different Fe species with the catalytic activity, we plot the reaction rate and the concentration of Fe

Correlation between Fe2+ and H2 Concentration for PROX TABLE 4:

57Fe

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Mo¨ssbauer Parameters of Ir-Fe/SiO2 catalyst upon Exposure to Different Atmosphere treatment

40% H2 + 1% O2 at 80 °C for 1 h

2% H2 + 1% O2 at 80 °C for 1 h

2% CO at 80 °C for 1 h

40% H2 + 2% CO + 1% O2 at 80 °C for 1 h 10% H2 + 2% CO + 1% O2 at 80 °C for 1 h 2% H2 + 2% CO + 1% O2 at 80 °C for 1 h 2% CO + 1% O2 at 80 °C for 1 h 10% H2O + 2% CO + 1% O2 at 80 °C for 1 h

chemical state n+

Fe Fe0 Fe2+ FeIr Fen+ Fe0 Fe2+ FeIr Fe3+ Fe0 Fe2+ FeIr Fe3+ Fe0 Fe2+ Fe3+ Fe0 Fe2+ Fe3+ Fe0 Fe2+ Fe3+ Fe0 Fe2+ Fe3+ Fe2+

(2 < n < 3)

ISa (mm s-1)

QSb (mm s-1)

Hc (T)

0.48 0.00 1.13

0.73 0.00 2.16

33.0

0.42 0.00 1.10

0.73 0.00 2.19

33.1

0.39 0.00 1.10

0.72 0.00 2.14

32.8

0.35 0.00 1.12 0.35 0.00 1.10 0.36 0.00 1.10 0.34 0.00 1.11 0.36 1.04

0.92 0.00 2.16 0.92 0.00 2.16 0.92 0.00 2.15 0.95 0.00 2.13 0.66 2.34

-0.14

(2 < n < 3)

-0.12

(spe)

-0.15

(spe)

a IS, isomer shift relative to R-Fe. b QS, electric quadrupole splitting. c H, magnetic field. uncertainty is (5% of the reported value.

Figure 9. Plots of relative amounts of various Fe species determined by 57Fe Mo¨ssbauer spectra and CO oxidation reaction rate against H2 volume fractions in the reaction stream.

species in the reacted Ir-Fe/SiO2 catalyst as a function of the H2 concentration in the feed stream. As shown in Figure 9, Fe3+ continuously decreases while Fe2+ increases with increasing H2 concentration from 0 to 40%. Consistent with the change of Fe2+ amount, the reaction rate for CO oxidation increases linearly with the H2 concentration until 10% and then levels off at higher H2 concentrations. This result strongly suggests that Fe2+ in the Ir-Fe/SiO2 has a close relation with the catalytic activity and, furthermore, the amount of Fe2+ in the Ir-Fe/ SiO2 depends on the H2 concentration in the feed gas. Many researchers proposed that the middle valence metal oxide was responsible for O2 activation in the bimetallic catalysts. Kotobuki et al.12 suggested that Pt formed the metallic clusters after H2 pretreatment or PROX reaction, whereas a large part of Fe existed as FeO after PROX reaction according to XAFS results. For PtSn bimetallic catalyst, the suggested mechanism involved O2 adsorption predominantly on Sn/SnOx islands on or adjacent to the active Pt sites.10 Analogously, the redox changes between

d

32.9 32.9 32.9 32.9

RId (%) 29 40 19 12 41 29 23 7 49 23 21 7 54 28 18 56 27 17 60 26 14 67 29 4 86 14

RI, relative intensity. e sp, superparamagnetic;

Cu2+ and Cu+ have been considered as the main steps in the CO oxidation reaction mechanism for the Cu-based catalysts.44 In agreement with these reports, the positive correlation between the amount of Fe2+ determined by Mo¨ssbauer spectra and the reaction rate also suggests that Fe2+ in the Ir-Fe catalyst acts as the adsorption site for O2. Different from the changes of either Fe3+ or Fe2+, the amount of Fe0 keeps constant at roughly 28% in the range of H2 concentrations investigated. This result indicates that this part of Fe0 did not participate in the PROX reaction. Actually, although the freshly reduced Ir-Fe/SiO2 catalyst contains 43% Fe0, the subsequent treatment with either 1% O2 or a mixture of 2% H2 and 1% O2 oxidized only a part of Fe0. In other words, there is always ∼25% Fe0 remaining intact in the Ir-Fe/SiO2 catalyst. According to the literature41 and the magnetic field value we detected in the 57Fe Mo¨ssbauer spectra, we assign this part of Fe0 to that encapsulated by Fe3+ species. Since Fe2+ might be responsible for providing reactive oxygen for CO oxidation, we now need to understand the unique features of Fe2+ in the Ir-Fe/SiO2 catalyst. To address this issue, we performed a comparative Mo¨ssbauer spectroscopic study on the Ir-Fe/SiO2 and Fe/SiO2 catalysts. The results demonstrate that Fe3+ in the former catalyst can be reduced with H2 much more easily than that in the latter catalyst, and the reduced Fe species including Fen+, a portion of Fe0, Fe2+, and FeIr alloy are easily reoxidized with O2. In agreement with the previous H2-TPR results,15 the Mo¨ssbauer results also show that Ir facilitates the reduction of Fe3+ and the microcalorimetric study indicates that the addition of Fe greatly enhances O2 adsorption. These results strongly suggest that the reduced (low-valence) Fe species in the Ir-Fe/SiO2 catalyst could activate oxygen at low temperatures and be easily regenerated upon exposure to H2. Moreover, they must be in intimate contact with the Ir particles so that hydrogen spillover from Ir to Fe takes place easily. The disappearance of FeIr alloy upon exposure to CO oxidation (or

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PROX) atmosphere also implies that the active Fe2+ probably originates from the oxidation of FeIr alloy which forms intimately contacted Fe2+ oxide and Ir particles in the reaction atmosphere. Analogously, Margitfalvi et al.45 suggested that a reversible PtSn T Sn4+ + Pt interconversion took place in CO oxidation. Besides this part of Fe2+ arising from FeIr alloy, there may be another portion of Fe2+ contributing to PROX reaction as long as it is adjacent to the Ir particles. For example, the vanishing half amount of Fe0 upon exposure to the reaction atmosphere may be transformed into Fe2+ and then serves as the site for oxygen activation. However, Fen+ in the freshly reduced Ir-Fe/SiO2 catalyst is more probably transformed into Fe3+ rather than Fe2+ upon exposure to the reaction atmosphere, and therefore cannot be an active site for oxygen. Such a conclusion can be deduced from the decreasing tendency of IS values of Fen+ with decreasing H2 concentration in the treatment gas. In summary, when a reduced Ir-Fe/SiO2 catalyst is used for PROX reaction, the changes of Fe species in the catalyst may occur as follows: (1) the Fen+ is reoxidized to Fe3+ which is inactive for oxygen activation; (2) the FeIr alloy is oxidized and segregated into Fe2+ and metallic Ir which are active for CO oxidation; (3) the outer layers of large Fe0 particles are oxidized into Fe2+ or Fe3+, while the cores remain intact as Fe0 and therefore do not take part in the PROX reaction; (4) the Fe2+ in intimate contact with Ir particles is also active for providing reactive oxygen for PROX, whereas the left Fe2+ may be oxidized to Fe3+ and therefore has little to do with the PROX reaction. Thus, for PROX reaction over the Ir-Fe/SiO2 catalyst, Ir acts as the site for CO adsorption, while Fe2+ is responsible for providing the reactive oxygen; the reaction proceeds via a noncompetitive Langmuir-Hinshelwood reaction route. In the absence of H2, the lack of Fe2+ sites leads to a slow reaction rate for CO oxidation. The presence of H2, even in a slight excess, can provide enough Fe2+ sites for the reaction and promote greatly the reaction. In addition to stabilizing Fe2+ in the Ir-Fe/SiO2 catalyst, H2 may promote the formation of OH groups on the catalyst surface. In fact, a slight discrepancy existing between the activity and the Fe2+ content at the point of 2% H2 suggests that the enough amount of Fe2+ on the catalyst surface might not be the only factor determining the high activity of Ir-Fe/SiO2. In the absence of any promoters, a similar promotional effect of H2 was also observed on Au/ Al2O3 catalyst. In that case, hydrogen is proposed to be directly involved in the reaction.23,24 The positive effect of OH groups on the CO oxidation has also been claimed for several Pt-based catalysts4,46 where the hydroxyl groups might be consumed during CO oxidation through the formation of an active intermediate (-COOH) that decomposes to CO2.47 Therefore, we cannot exclude the possibility that OH groups formed by the reaction of H2 with O2 also act as an oxidant for CO. Thus, the increase of OH groups by the addition of H2 in the feed might be another factor in promoting the CO oxidation rate. 5. Conclusions The adsorptions of both CO and H2 are weakened significantly, whereas the adsorption of O2 is improved greatly by the introduction of Fe in the Ir/SiO2 catalyst. The resulting Ir-Fe/SiO2 has been found to be highly active and selective for PROX reaction, and the promotional role of Fe is always associated with the presence of H2 in the reaction stream. By employment of various techniques, in particular quasi in situ Mo¨ssbauer spectroscopy, we have given a clear picture about

Liu et al. the changes of different Fe species with increasing H2 concentrations from 0 to 40%. The Fe3+ in the Ir-Fe/SiO2 catalyst is easily reduced to Fen+ (2 < n < 3), Fe0, Fe2+, and FeIr alloy with the aid of Ir, and the reduced Fe species are also easily oxidized upon exposure to oxygen. When the reduced catalyst was exposed to reaction gas containing CO, O2, and 0-40% H2, FeIr alloy disappeared as a result of oxidation and formed intimately contacted Fe2+ oxide and Ir particles, which were active for PROX reaction. Meanwhile, Fen+ (2 < n < 3) was transformed into Fe3+ that was inactive for the reaction, and about half the amount of Fe0 remained intact probably because it was encapsulated by ferric oxide. The amount of Fe2+ increases with increasing H2 concentration in the reaction stream, well consistent with the change of CO oxidation rate, suggesting that Fe2+ acts as the active site for O2 activation. H2 promotes the reaction via maintaining a substantial amount of Fe existing as active Fe2+. Meanwhile, H2 may also participate directly in the reaction by forming -OH groups. Acknowledgment. Financial support from Chinese Academy of Sciences for “100 Talents” project and the National Science Foundation of China (20325620, 20773122, 20773124, and 20673055) is greatly acknowledged. References and Notes (1) Sirijaruphan, A.; Goodwin, J. G., Jr.; Rice, R. W. J. Catal. 2004, 221, 288. (2) Sedmak, G.; Hocˇevar, S.; Levec, J. J. Catal. 2003, 213, 135. (3) Kim, W. B.; Voitl, T.; Rodriguez-Rivera, G. J.; Evans, S. T.; Dumesic, J. A. Angew. Chem., Int. Ed. 2005, 44, 778. (4) Pozdnyakove-Tellinger, O.; Teschner, D.; Kro¨hnert, J.; Jentoft, F. C.; Knop-Gericke, A.; Schlo¨gl, R.; Wootsch, A. J. Phys. Chem. C 2007, 111, 5426. (5) Li, W.; Gracia, F. J.; Wolf, E. E. Catal. Today 2003, 81, 437. ¨ zkara, S¸.; Aksoylu, A. E.; O ¨ nsan, Z. I. Appl. Catal., (6) S¸ims¸ek, E.; O A 2007, 316, 169. (7) Yin, J.; Wang, J.; Zhang, T.; Wang, X. D. Catal. Lett. 2008, 125, 76. (8) Liu, X. S.; Korotkikh, O.; Farrauto, R. Appl. Catal., A 2002, 226, 293. (9) Kotobuki, M.; Watanabe, A.; Uchida, H.; Yamashita, H.; Watanabe, M. J. Catal. 2005, 236, 262. (10) Schubert, M. M.; Kahlich, M. J.; Feldmeyer, G.; Hu¨ttner, M.; Hackenberg, S.; Gasteiger, H. A.; Behm, R. J. Phys. Chem. Chem. Phys. 2001, 3, 1123. (11) Sun, Y.-N.; Qin, Z.-H.; Lewandowski, M.; Carrasco, E.; Sterrer, M.; Shaikhutdinov, S.; Freund, H.-J. J. Catal. 2009, 266, 359. (12) Kotobuki, M.; Watanabe, A.; Uchida, H.; Yamashita, H.; Watanabe, M. Catal. Lett. 2005, 103, 263. (13) Zhang, W. S.; Wang, A. Q.; Li, L.; Wang, X. D.; Zhang, T. Catal. Lett. 2008, 121, 319. (14) Zhang, W. S.; Wang, A. Q.; Li, L.; Wang, X. D.; Zhang, T. Catal. Today 2008, 131, 457. (15) Zhang, W. S.; Huang, Y. Q.; Wang, J.; Liu, K.; Wang, X. D.; Wang, A. Q.; Zhang, T. Int. J. Hydrogen Energy 2010, 35, 3065. (16) Kahlich, M. J.; Gasteiger, H. A.; Behm, R. J. J. Catal. 1997, 171, 93. (17) Jacobs, G.; Graham, U. M.; Chenu, E.; Patterson, P. M.; Dozier, A.; Davis, B. H. J. Catal. 2005, 229, 499. (18) Mhadeswar, A. B.; Vlachos, D. G. J. Phys. Chem. B 2004, 108, 15246. (19) Sirijaruphan, A.; Goodwin, J. G., Jr.; Rice, R. W. J. Catal. 2004, 224, 304. (20) Li, L.; Wang, X. D.; Shen, J. Y.; Zhou, L. X.; Zhang, T. Chin. J. Catal. 2003, 24, 872. (21) Nomura, K.; Kuzmann, E.; Barrero, C. A.; Stichleutner, S.; Homonnay, Z. Hyperfine Interact. 2008, 184, 57. (22) Tang, R. Y.; Zhang, S.; Wang, C. Y.; Liang, D. B.; Lin, L. W. J. Catal. 1987, 106, 440. (23) Quinet, E.; Morfin, F.; Diehl, F.; Avenier, P.; Caps, V.; Rousset, J.-L. Appl. Catal., B 2008, 80, 195. (24) Rossignol, C.; Arrii, S.; Morfin, F.; Piccolo, L.; Caps, V.; Rousset, J.-L. J. Catal. 2005, 230, 476.

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