Structure of ReOx Clusters Attached on the Ir Metal Surface in Ir–ReOx

Article pubs.acs.org/JPCC

Structure of ReOx Clusters Attached on the Ir Metal Surface in Ir− ReOx/SiO2 for the Hydrogenolysis Reaction Yasushi Amada,† Hideo Watanabe, and Keiichi Tomishige*,†

‡

Masazumi Tamura,† Yoshinao Nakagawa,† Kazu Okumura,§

†

Department of Applied Chemistry, School of Engineering, Tohoku University, 6-6-07, Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan ‡ Faculty of Pure and Applied Sciences, University of Tsukuba, 1-1-1, Tennodai, Tsukuba, Ibaraki 305-8573, Japan § Department of Chemistry and Biotechnology, Graduate School of Engineering, Tottori University, 4-101, Koyama-cho Minami, Tottori 680-8552, Japan S Supporting Information *

ABSTRACT: The structural change of Ir/SiO2 and Ir−ReOx/SiO2 during the temperature programmed reduction with H2 was investigated using in situ Ir L3-edge and Re L3-edge quick-scanning X-ray absorption fine structure in the combination of temperature programmed reduction with H2, XRD, the measurement of CO adsorption, Raman spectroscopy, and X-ray photoelectron spectroscopy. The metal oxide on Ir−ReOx/SiO2 was much more easily reduced than that on monometallic Ir/SiO2 and ReOx/SiO2, which is explained by the interaction between trioxo Re species and the IrO2 phase. The metal structure of Ir−ReOx/SiO2 constructed after reduction at 485−595 K is the cuboctahedron Ir metal particles covered with three-dimensional ReOx clusters.

1. INTRODUCTION The production of chemicals and fuels from biomass will be required because of the depletion of fossil fuel reservoirs and global warming issues, and this is regarded as a biomass refinery concept.1−3 One of the promising technologies in biomass refinery is hydrogenolysis, which consumes hydrogen to cleave the C−O bond. Recently, it has been reported that noble metal catalysts such as Ru, Pt, Rh, and Ir modified with metal oxide such as Re, Mo, and W species exhibit high catalytic performance in the C−O bond hydrogenolysis. These catalysts have been reported to be very effective in the hydrogenolysis of sugar alcohols such as glycerol4−18 and erythritol19 and cyclic ethers such as tetrahydrofurfuryl alcohol (THFA),11,12,20−25 tetrahydropyran-2-methanol (THPM),11,20,22,24,25 and 2,5-bis(hydroxymethyl)tetrahydrofuran.26 In particular, Rh−ReOx/ SiO2 and Ir−ReOx/SiO2 exhibited excellent regioselectivity. The reactions typically proceed on the interface between noble metal and partially reduced metal oxide,7,14,16,17,24,27 which is formed by reduction of fully oxidized metal species. Therefore, it is very important to clarify the formation mechanism of the interface. However, the characterization of the interface is difficult, and the determination of the formation mechanism is even more difficult. In the case of Rh−ReOx/SiO2, we have recently reported the model structure of the reduced state from various techniques and the formation mechanism from quickscanning X-ray absorption fine structure during temperature programmed reduction with H2 (TPR-QXAFS):28 First, the © 2012 American Chemical Society

reduction of Rh proceeds to give highly dispersed Rh metal particles. Next, Re species are reduced mildly. The reduced Re species interact with the Rh metal surface and are highly dispersed. Finally, Rh metal particles (ca. 3 nm) partially covered with two-dimensional Re oxide clusters are formed by the aggregation of remodified Rh metal particles and further reduction of Re species. On the other hand, in the case of Ir− ReOx/SiO2 which shows higher performance in glycerol hydrogenolysis than Rh−ReOx/SiO2, the structures of fully oxidized or reduced states were characterized.16,17 The small particles of IrO2 and highly dispersed Re species are observed before reduction, and Ir metal particles (ca. 2 nm) partially covered with three-dimensional Re oxide clusters are formed by the reduction of fully oxidized species. However, Ir−ReOx/SiO2 shows a curious behavior during reduction: the metal species on Ir−ReOx/SiO2 is more easily reduced than that on monometallic Ir/SiO2 and ReOx/SiO2 from TPR,16,17 while most modified noble metal catalysts including Rh−ReOx/SiO2 are less easily reduced than unmodified ones.14,29,30 Moreover, the optimum amount of Re is extraordinary large (Re/Ir: 1− 2),16,17 while in the cases of Rh−MOx/SiO2 (M = Re, Mo, and W) the optimum amount is ≤0.5. Therefore, the formation Received: August 28, 2012 Revised: October 10, 2012 Published: October 11, 2012 23503

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between XAFS results and the TPR profile of Ir/SiO2. The spectrum of the sample before and after the in situ quick scanning XAFS also was measured at room temperature. In the analysis of the EXAFS spectra, the oscillation was first extracted from the EXAFS data using a spline smoothing method.34 Oscillation was normalized by the edge height around 50 eV. Fourier transformation of the k3-weighted EXAFS oscillation from the k space to the r space was performed to obtain a radial distribution function. The inversely Fourier filtered data were analyzed using a usual curve fitting method.35,36 For curve fitting analysis, the empirical phase shift and amplitude functions for Ir−O and Re−O bonds were extracted from data of IrO237 and NH4ReO4,38 respectively. The empirical phase shift and amplitude functions for all of the Ir−Ir, Ir−Re, Re−Re, and Re−Ir bonds were extracted from data of Ir powder. This is because it is very difficult to distinguish between Ir and Re as a backscattering atom. The Re−Re + Re− Ir bonds and Ir−Ir + Ir−Re bonds are represented by the Re− Ir (or −Re) and Ir−Ir (or −Re), respectively, in the curve fitting results. Analyses of EXAFS data were performed using a computer program (REX2000, ver. 2.5.9; Rigaku Corp.). In the analysis of XANES spectra, the binding energy of the sample was defined by the inflection point.39 The chemical shift of the binding energy from Ir and Re metal powder was evaluated on each spectrum. The chemical shift of the binding energy is almost proportional to the valence of the Ir and Re species. In order to estimate the valence of Ir and Re species, we used the data of Ir powder and IrO2 and Re powder, ReO2 and NH4ReO4, respectively. The relation between the valence and chemical shift of the binding energy is shown in Figure S1 (Supporting Information), and the Ir and Re valence on the samples was estimated on the basis of the relation. 2.3. Catalyst Characterization by Means of Other Methods. Temperature-programmed reduction (TPR) was carried out in a fix bed reactor equipped with a thermal conductivity detector using 5% H2 diluted with Ar (30 mL/ min). The amount of catalyst was 0.05 g, and the temperature was increased from room temperature to 895 K at a heating rate of 5 K/min. The H2 consumption amount was estimated from the integrated peak area of the reduction profiles. The sum of average valences of Ir and Re was calculated on the basis of the molar ratio of the H2 consumption amount to Ir and Re loading amount. The XRD patterns of the catalysts were recorded with a Rigaku Ultima IV instrument using Cu Kα (λ = 0.154 nm) generated at 40 kV and 20 mA. After the reduction pretreatment at a specific temperature for 1 h with H2 gas (30 mL/min), the sample was exposed to 2%O2/He gas (10 mL/min) for 0.5 h in order to passivate the samples. After that, the sample was exposed to air and put on the cell. The average particle size of Ir metal was estimated using the Scherrer equation.40 The amount of CO chemisorption was measured in a highvacuum system using a volumetric method. Before adsorption measurements, the catalysts were treated with H2 at specific temperature for 1 h. Subsequently, the adsorption was performed at room temperature. The gas pressure at adsorption equilibrium was about 1.1 kPa. The sample weight was about 0.1 g. The dead volume of the apparatus was about 60 cm3. The adsorption amount of CO is represented as the molar ratio to metals. The Raman spectroscopy was measured at 298 K by using a Laser Raman spectrometer (JASCO, NRS-5100) with the 532

mechanism of the interface between Ir and Re oxide should be elucidated clearly. In this article, the structural change of Ir−ReOx/SiO2 was analyzed by Ir L3-edge and Re L3-edge XANES and EXAFS in the combination of temperature programmed reduction with H2, XRD, the measurement of CO adsorption amount, Raman spectroscopy, and X-ray photoelectron spectroscopy. In particular, the comparison of the formation mechanism of particles between Ir/SiO2 and Ir−ReOx/SiO2 can explain the additive effect of Re species to Ir catalyst. On the basis of the characterization results, a model structure of Ir−ReOx/SiO2 is proposed, which can be the catalytically active site for the C−O bond hydrogenolysis reaction.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. An Ir/SiO2 catalyst was prepared by impregnating SiO2 with an aqueous solution of H2IrCl6 (Furuya Metals Co., Ltd.). The SiO2 (G-6, BET surface area 535 m2/g) was supplied by Fuji Silysia Chemical Ltd. After the impregnation procedure and drying at 383 K for 12 h, they were calcined in air at 773 K for 3 h. Ir−ReOx/SiO2 catalyst was prepared by impregnating Ir/SiO2 after the drying procedure with an aqueous solution of NH4ReO4 (Soekawa Chemical Co., Ltd.), and the loading amount of Re was 1 by the molar ratio to Ir which is the optimized amount of Re in terms of the catalytic activity of hydrogenolysis of glycerol17 as reported previously. These two samples were calcined in air at 773 K for 3 h after drying at 383 K for 12 h. The loading amount of Ir on Ir/SiO2 and Ir−ReOx/SiO2 catalysts was 4 wt %. All catalysts were in powdery form with granule size of 99%), sulfuric acid (Wako Pure Chemical Industries, Ltd.) diluted with water, and an appropriate amount of water. After sealing the reactor, the N2 content was purged by flushing thrice with 1 MPa hydrogen (99.99%; Takachiho Trading Co., Ltd.). The autoclave was then heated to 393 K,

nm line from a diode-pumped solid-state laser for excitation. The catalysts after the calcination were used in a powdery form. X-ray photoelectron spectroscopy (XPS) experiments were conducted with an AXIS-ULTRA DLD (Shimazu Co., Ltd.) using a monochromatic Al Kα X-ray radiation (hν = 1486.6 eV) operated at 20 mA and 15 kV at room temperature under 10−8 Pa. The binding energy was calibrated with C 1s (284.5 eV). The catalysts were reduced in flowing hydrogen at specific temperature for 1 h, and then, the reduced catalysts were transported to the analysis chamber in nitrogen atmosphere. 23505

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Table 1. Curve Fitting Results of in Situ Ir L3-Edge EXAFS of Ir/SiO2 during the Reduction reduction temp (K)

measurement temp (K)

shells

CNa

Rb (10−1 nm)

σc (10−1 nm)

ΔE0d (eV)

Rfe (%)

1 2 3 4

non-red. 395 435 475

r.t. 395 435 475

495

495

6

515

515

7

535

535

8 9 10 11 Ir powder IrO2

555 695 895 895

555 695 895 r.t. r.t. r.t.

5.9 5.9 5.9 5.4 0.5 5.0 1.0 3.4 4.1 1.2 8.5 11.0 11.1 11.1 11.2 12 6

1.98 1.98 1.99 2.01 2.75 2.02 2.75 2.03 2.74 2.04 2.75 2.75 2.75 2.75 2.76 2.77 1.98

0.063 0.066 0.070 0.072 0.074 0.072 0.075 0.072 0.076 0.073 0.078 0.078 0.084 0.091 0.059 0.06 0.06

−0.5 −0.4 −0.5 0.4 1.2 −0.1 −0.4 0.0 −3.2 1.3 −3.3 −2.9 −2.7 −2.0 −1.9 0 0

1.2 1.3 1.5 2.9

5

Ir−O Ir−O Ir−O Ir−O Ir−Ir Ir−O Ir−Ir Ir−O Ir−Ir Ir−O Ir−Ir Ir−Ir Ir−Ir Ir−Ir Ir−Ir Ir−Ir Ir−O

entry

CN*Ir−Ir f

5.0 2.4 5.7 2.5 9.6 1.9 10.6 1.2 0.8 1.1 1.8

a

Coordination number. bBond distance. cDebye−Waller factor. dDifference in the origin of photoelectron energy between the reference and the sample. eResidual factor. Ir: 4 wt %. Fourier filtering range: 0.125−0.320 nm. fCNIr−Ir/((6 − CNIr−O)/6).

that of IrO2. This behavior is explained by the formation of IrO2 with high dispersion on the non-reduced Ir/SiO2. Above 495 K, the contribution of the Ir−Ir bond was clearly detected as well as that of the Ir−O bond in the IrO2 phase. The coordination number (CN) of the Ir−O bond decreased and that of the Ir−Ir bond increased with increasing reduction temperature. The EXAFS analysis suggests the structural change of Ir species in Ir/SiO2 from IrO2 to Ir metal during the reduction. The average valence of Ir species is also calculated from the CNIr−O bond. Here, it is assumed that six CNIr−O corresponds to the valence of 4+ based on IrO2, and the average Ir valence is calculated from 2/3·CNIr−O. The average valence estimated from the CNIr−O is also shown in Figure 1b. At the same time, the valence is also estimated from the TPR profile with H2 (Figure S2, Supporting Information) on the basis that the starting state of Ir/SiO2 is IrO2. The valence from the TPR profile is also shown in Figure 1b. The agreement of the average valence of Ir from the TPR profile, XANES, and EXAFS shows the validity of these methods for the determination of the valence of Ir. From the above results, IrO2 is reduced to Ir metal in the temperature range between 435 and 555 K over Ir/SiO2. When IrO2 and Ir coexist, the CNIr−Ir also includes the contribution of IrO2 and it does not directly connect to the size of Ir metal particles. However, if the contribution of IrO2 is excluded, it is possible to estimate the particle size of Ir metal from the coordination number. Assuming that only IrO2 and Ir metal species are present on Ir/SiO2 during this temperature range, the reduction degree (Ir0/(Ir0 + Ir4+)) is calculated by the average valence from CNIr−O ((6 − CNIr−O)/6). The CNIr−Ir limited to reduced and metallic Ir species (CN*Ir−Ir) excluding the oxidized Ir species can be obtained from the CNIr−Ir compensated by the reduction degree. Therefore, the CN*Ir−Ir was defined as CN*Ir−Ir = CNIr−Ir/((6 − CNIr−O)/6). The CN*Ir−Ir in the reduced Ir species is also listed in Table 1, indicating that the CN*Ir−Ir increased significantly with the reduction temperature (475−555 K). Initially, Ir metal clusters with very small CNIr−Ir (∼5) are formed, which corresponds to the size of ∼0.8 nm. Then, these are finally aggregated to larger

and the temperature was monitored using a thermocouple inserted in the autoclave. After the temperature reached 393 K, the H2 pressure was increased to 8 MPa. During the experiment, the stirring rate was fixed at 250 rpm (magnetic stirring). After an appropriate reaction time, the reactor was cooled down and the gases were collected in a gas bag. The autoclave contents were transferred to a vial, and the catalyst was separated by filtration. The standard conditions for the reaction were as follows: 4 g of glycerol, 2 g of water, 1.5 mg of H2SO4 (H+/Ir = 1), 150 mg of supported metal catalyst, 8 MPa initial hydrogen pressure, 393 K reaction temperature, 12 h reaction time. The products were analyzed using a gas chromatograph (Shimadzu GC-2014) equipped with FID. A TC-WAX capillary column (diameter 0.25 mm, 30 m) was used for the separation. Products were also identified using GC−MS (QP5050, Shimadzu). The products in the glycerol hydrogenolysis were 1,3-propanediol (1,3-PrD) and 1,2-propanediol (1,2-PrD). The overhydrogenolysis reaction of PrD's gave 1propanol (1-PrOH), 2-propanol (2-PrOH), and propane. The conversion and the selectivity were defined on the carbon basis in the similar way as reported previously.41−44 The mass balance was also confirmed in each result, and the difference in mass balance was always in the range of the experimental error (±5%). Selectivities were calculated in carbon basis.

3. RESULTS AND DISCUSSION 3.1. Structural Analysis of Ir/SiO2 during the Reduction by Ir L3-Edge X-ray Absorption Spectroscopy. Figure 1a shows the Ir L3-edge XANES spectra of Ir/SiO2 during the H2 reduction, and the average valences of Ir as a function of the reduction temperature are plotted in Figure 1b. The valence of the Ir metal species changed from Ir4+ to Ir0, and it much changed in the temperature range 435−555 K. The results of Ir L3-edge EXAFS spectra of Ir/SiO2 during the reduction are shown in Figure 1c and d, and the curve fitting results are listed in Table 1. The comparison between the experimental and calculated data is shown in Figure S3a (Supporting Information). The non-reduced Ir/SiO2 gave similar FT spectra to IrO2, although the intensity of the FT peak in the longer range than 0.25 nm of non-reduced Ir/SiO2 was weaker than 23506

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Figure 2. The results of Ir L3-edge XANES and EXAFS analysis of Ir−ReOx/SiO2. (a) Ir L3-edge XANES spectra. (b) Average valence of Ir as a function of reduction temperature. ◇, Coordination number of Ir−O in the Ir L3-edge EXAFS analysis (Table 1); ■, chemical shift of the binding energy in the Ir L3-edge XANES analysis (Figure S1a, Supporting Information). (c) k3-Weighted EXAFS oscillations. (d) Fourier transform of k3weighted Ir L3-edge EXAFS. FT range: 30−120 nm−1. The Fourier filtering range is listed in Table 1. The two temperatures in parentheses in parts a, c, and d represent reduction and measurement temperatures.

Ir metal particles with CNIr−Ir > 11, which corresponds to the size observed by XRD and TEM (3−4 nm).17 3.2. Structural Analysis of Ir−ReOx/SiO2 during the Reduction by Ir L3-Edge X-ray Absorption Spectroscopy. Figure 2 shows the result of the Ir L3-edge XANES and EXAFS analysis of Ir−ReOx/SiO2 during the reduction, and the curve fitting results are listed in Table 2. As shown in Figure 2b, the valence of Ir species from the chemical shift of the binding energy in the XANES spectra also agreed well with the CNIr−O in the EXAFS analysis in the case of Ir−ReOx/SiO2. The reduction temperature of the Ir species on Ir−ReOx/SiO2

(400−500 K) was lower than those on Ir/SiO2 (450−550 K) and ReOx/SiO2 (600−690 K), as reported previously with TPR measurements.16,17 In the Ir L3-edge EXAFS analysis, the Ir−Ir (or −Re) bond appeared above 445 K and the contribution increased with increasing reduction temperature. Unfortunately, it is very difficult to distinguish between Ir and Re as a backscattering atom in this system. This is why the direct interaction between Ir and Re cannot be detected. However, there is a suggestion of the interaction between Ir and Re. The CN*Ir−Ir (or −Re) for Ir−ReOx/SiO2 and CN*Ir−Ir for Ir/SiO2 are listed in Tables 1 and 2, and the CN* as a function of the 23507

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Table 2. Curve Fitting Results of in Situ Ir L3-Edge EXAFS of Ir−ReOx/SiO2 (Re/Ir = 1) during the Reduction entry 1 2 3 4 5 6 7 8 9 10 11 12 13 Ir powder IrO2

reduction temp (K)

measurement temp (K)

non-red. 415 435 445

r.t. 415 435 445

455 465 475 485 495 595 695 895 895

455 465 475 485 495 595 695 895 r.t. r.t. r.t.

shells Ir−O Ir−O Ir−O Ir−O Ir−Ir (or Ir−O Ir−Ir (or Ir−O Ir−Ir (or Ir−O Ir−Ir (or Ir−Ir (or Ir−Ir (or Ir−Ir (or Ir−Ir (or Ir−Ir (or Ir−Ir (or Ir−Ir Ir−O

−Re) −Re) −Re) −Re) −Re) −Re) −Re) −Re) −Re) −Re)

CNa

Rb (10−1 nm)

σc (10−1 nm)

ΔE0d (eV)

Rfe (%)

CN*Ir−Ir f

6.0 6.0 5.5 4.9 1.3 4.3 2.3 2.6 5.1 0.7 8.9 10.8 10.8 10.9 10.8 10.8 10.9 12 6

1.97 1.99 1.98 1.98 2.76 2.00 2.76 2.01 2.76 2.00 2.76 2.75 2.75 2.75 2.75 2.75 2.77 2.77 1.98

0.060 0.066 0.064 0.066 0.067 0.072 0.073 0.072 0.073 0.076 0.075 0.078 0.078 0.083 0.088 0.096 0.062 0.06 0.06

−0.2 0.1 −1.2 −1.4 2.2 −1.0 1.3 −0.8 1.3 −2.3 −0.6 −1.9 −2.4 −2.0 −2.4 −2.7 −0.4 0 0

1.1 0.7 1.7 2.6

7.4

2.3

8.4

2.5

8.9

1.6

10.2

1.1 0.7 0.7 1.0 1.6 0.9

a

Coordination number. bBond distance. cDebye−Waller factor. dDifference in the origin of photoelectron energy between the reference and the sample. eResidual factor. Ir, 4 wt %; Re, 3.9 wt %. Fourier filtering range: 0.126−0.322 nm. fCNIr−Ir/((6 − CNIr−O)/6).

reduction degree of Ir species (Ir0/(Ir4+ + Ir0)) from CNIr−O is shown in Figure 3. In the case of Ir/SiO2, the CN*Ir−Ir

Ir and Re can also be supported by the contribution of the Re− Ir (or −Re) bond on the sample reduced at 445 K (Table 2) because no interaction between Ir and Re species indicates that Re species are not reduced at all, indicating the Re−metal bond is not identified. The interaction between Rh and Re at low temperature starting at reduction of metal species and partially reduced state has also been observed on Rh−ReOx/SiO2.28 3.3. Structural Analysis of Ir−ReOx/SiO2 during the Reduction by Re L3-Edge X-ray Absorption Spectroscopy. Figure 4a shows the Re L3-edge XANES spectra of Ir ReOx/SiO2 during the H2 reduction, the average valence of the Re species was given by the XANES analysis, and the obtained valence as a function of the reduction temperature is plotted in Figure 4b. Figure 4c and d shows the Re L3-edge EXAFS results. The curve fitting results are listed in Table 3, and the comparison between the experimental and calculated data is shown in Figure S3c (Supporting Information). The Re L3edge EXAFS spectrum of the non-reduced IrReOx/SiO2 was almost the same as that of NH4ReO4, which is also supported by the XANES analysis. At 415 K reduction temperature, the ReO bond with 0.203 nm appeared together with the ReO bond with 0.173 nm. For curve fitting analysis of IrReOx/ SiO2 in the bond species between Re and O at 415−475 K reduction, the contribution of both ReO and ReO is necessary because these spectra cannot be fitted by only ReO or ReO at all. The CNReO decreased monotonously with increasing reduction temperature; on the other hand, the CNReO was maximum at 465 K reduction temperature. This behavior can be explained by the reduction of Re species. Therefore, the valence of Re species is calculated from 7/ 4·CNReO + CNReO, and the obtained values are also plotted in Figure 4b. The two valences based on the XANES and EXAFS results agreed well. It should be noted that the reduction temperature range of Re species on IrReOx/SiO2 was 415−495 K, which is almost the same as the reduction temperature range of Ir species on IrReOx/SiO2, indicating the simultaneous reduction of Ir and Re. As shown in Figure S2 (Supporting Information), the hydrogen consumption amount

Figure 3. CN*Ir−Ir (or −Re) for Ir−ReOx/SiO2 (circles) and CN*Ir−Ir for Ir/SiO2 (squares) during the H2 reduction. CN*: Coordination number only for the reduced Ir species. CN* is obtained by dividing CN by the reduction degree of the Ir species (Tables 1 and 2).

increased gradually with increasing reduction degree, indicating the increase of the size of each Ir metal particle. It is characteristic that the CN*Ir−Ir (or −Re) for Ir−ReOx/SiO2 was clearly higher than the CN*Ir−Ir for Ir/SiO2 at low reduction degree, such as 445 K (18% reduction degree), and the tendency became opposite at higher reduction degree. The tendency of CN*Ir−Ir > CN*Ir−Ir (or −Re) at higher reduction degree can be interpreted by the effect of Re addition on the suppression of the aggregation of the Ir metal particles. In contrast, the tendency of CN*Ir−Ir (or −Re) > CN*Ir−Ir at low reduction degree suggested that the interaction between Ir and Re can increase the CN*Ir−Ir (or −Re). This interaction between 23508

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Figure 4. The results of Re L3-edge XANES and EXAFS analysis of Ir−ReOx/SiO2. (a) Re L3-edge XANES spectra. (b) Average valence of Re as a function of reduction temperature. ◇, Coordination number of Re−O in the Re L3-edge EXAFS analysis (Table 1); ■, chemical shift of the binding energy in the Re L3-edge XANES analysis (Figure S1b, Supporting Information). (c) k3-Weighted EXAFS oscillations. (d) Fourier transform of k3weighted Ir L3-edge EXAFS. FT range: 30−120 nm−1. The Fourier filtering range is listed in Table 1. The two temperatures in parentheses in parts a, c, and d represent reduction and measurement temperatures.

in the TPR profile is consistent with the valence change from XANES and EXAFS results (Figure S4, Supporting Information). In the previous report, the TPR profile of RhReOx/ SiO2 also gave the single peak; however, the reduction of Re followed the reduction of Rh.14 Compared to the case of Rh ReOx/SiO2, the interaction of Ir and Re on IrReOx/SiO2 can be intimate. In addition, these results indicate that the average valence of the Re species is maintained to be around +2 even when increasing the reduction temperature up to rather high temperature like 895 K. A similar behavior of the Re species has

been reported in the various catalyst systems including Re and noble metals.4−6,28−30 Another important point is the contribution of the ReIr (or Re) bonds, which appeared above 445 K. The CNReIr (or −Re) increased significantly in the range 445−485 K, it was almost constant in the range 485−595 K, and it again increased above 595 K. On the other hand, the CNIrIr (or Re) was almost constant above 595 K (Table 2). The different behavior of CNIrIr (or Re) and CNReIr (or Re) suggests that high temperature reduction influences the local structure of Re species more remarkably than that of Ir species. 23509

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Table 3. Curve Fitting Results of in Situ Re L3-Edge EXAFS of Ir−ReOx/SiO2 (Re/Ir = 1) during the Reduction reduction temp (K)

measurement temp (K)

1 2

non-red. 415

r.t. 415

3

435

435

4

445

445

5

455

455

6

465

465

7

475

475

8

485

485

Entry

9 10 11 12 13 Ir powder NH4ReO4

495 595 695 895 895

495 595 695 895 r.t r.t. r.t.

shells ReO ReO ReO ReO Re−O ReO ReO ReIr (or ReO ReO ReIr (or ReO ReO ReIr (or ReO ReO ReIr (or ReO ReIr (or ReO ReIr (or ReO ReIr (or ReO ReIr (or ReO ReIr (or ReO ReIr (or IrIr ReO

Re)

Re)

Re)

Re) Re) Re) Re) Re) Re) Re)

CNa

Rb (10−1 nm)

σc (10−1 nm)

ΔE0d (eV)

Rf e (%)

Fourier filtering range (nm)

4.0 3.3 0.5 2.8 1.0 1.7 1.5 0.6 1.0 2.5 1.4 0.7 2.7 2.6 0.4 2.0 4.6 1.6 6.0 1.6 6.5 1.6 6.5 1.7 6.9 1.7 7.7 1.4 7.9 12 4

1.74 1.73 2.03 1.73 2.03 1.73 2.03 2.72 1.74 2.02 2.72 1.74 2.02 2.72 1.75 2.04 2.72 2.04 2.73 2.05 2.73 2.05 2.72 2.07 2.74 2.09 2.74 2.13 2.76 2.77 1.73

0.078 0.088 0.088 0.088 0.088 0.088 0.090 0.087 0.092 0.099 0.088 0.094 0.099 0.088 0.097 0.100 0.090 0.107 0.090 0.118 0.091 0.126 0.093 0.129 0.095 0.133 0.105 0.087 0.090 0.06 0.06

−1.2 −1.0 −2.5 −5.9 −2.5 −9.2 0.1 10.7 −11.3 −0.7 4.6 −10.7 0.0 1.2 −10.9 1.9 −1.1 2.6 0.4 4.6 −0.8 8.7 −2.0 10.6 −0.5 11.0 −0.1 10.8 1.8 0 0

0.6 1.0

0.092−0.316 0.092−0.316

1.7

0.092−0.316

2.0

0.092−0.316

3.3

0.092−0.316

2.9

0.092−0.316

2.1

0.092−0.316

1.3

0.138−0.316

1.2

0.138−0.316

1.6

0.138−0.316

2.3

0.138−0.316

2.2

0.138−0.316

2.6

0.138−0.316

a

Coordination number. bBond distance. cDebye−Waller factor. dDifference in the origin of photoelectron energy between the reference and the sample. eResidual factor. Ir: 4 wt %.

It should be noted that the observed length of the ReIr (or Re) bond was 0.272−0.273 nm. Considering the metal bonding radii of Ir (0.138 nm) and Re (0.137 nm), the observed bond length is a direct bond between Re and Ir (or Re), and oxide ions are not located between two metal atoms. In addition, the observed bond length (0.272−0.273 nm) was slightly shorter than the sum of the metal bonding radii of Ir and Re, which can be interpreted by a smaller radius of the low valent Re species. 3.4. Characterization of Ir−ReOx/SiO 2 by Other Methods and the Catalytic Performance in the Glycerol Hydrogenolysis. Figure 5 shows the XRD patterns of nonreduced Ir−ReOx/SiO2 and the sample after the reduction at 415−895 K. The peaks on the non-reduced Ir−ReOx/SiO2 agreed well with those of Ir/SiO2 (2θ = 26.0, 33.4, and 52.6°), which are due to the IrO2 phase17 (Figure 5a). The average particle size of IrO2 calculated from the line width of the peak is estimated to be 5.0 nm. The pattern of Ir−ReOx/SiO2 reduced at 415 K also showed peaks due to IrO2 (Figure 5b); however, the peak intensities were smaller than those on the non-reduced sample. This is due to the reduction of IrO2. The peak assigned to Ir metal was observed on Ir−ReOx/SiO2 reduced at 495 K at 2θ = 40.5 and 47.2°, and the peaks due to IrO2 disappered completely. This behavior agreed well with no contribution of the Ir−O bond in the L3-edge EXAFS analysis (Figure 2, Table

Figure 5. XRD patterns of Ir−ReOx/SiO2 (Re/Ir = 1). (a) Nonreduced; reduced at (b) 415 K, (c) 495 K, (d) 695 K, and (e) 895 K.

2). The XRD patterns of Ir−ReOx/SiO2 reduced at 495−895 K were almost the same. In this temperature range, no shift of the peak at 2θ = 40.5° was observed at all. This indicates that Ir 23510

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Table 4. Results of the Characterization and the Glycerol Hydrogenolysis of Ir−ReOx/SiO2 (Re/Ir = 1) Catalyst Reduced at Different Temperaturesa dispersion (DCO; CO/ Ir) (%)

Ir particle size (nm)

entry

reduction temp (K)

CO adsorption

XRD

1 2 3 4

415 495 695 895

7 16 14 11

2.0 1.9 2.0

dispersion (DXRD; Irs/ Ir) (%)

selectivity (%)

XRD

(DXRD− DCO)

conv (%)

1,3PrD

1,2PrD

1PrOH

2PrOH

propane

53 56 53

37 42 42

18.4 40.6 35.8 32.2

54.5 50.0 51.2 48.0

9.2 6.2 7.4 11.6

30.2 37.6 34.8 33.5

5.7 5.7 6.1 6.4

0.4 0.5 0.5 0.4

a

Reaction conditions: glycerol 4 g, water 2 g, catalyst 150 mg, H2SO4 (H+/Ir = 1), PH2 = 8 MPa, T = 393 K, t = 12 h. Reduction conditions: PH2 = 0.1 MPa, t = 1 h. PrD, propanediol; PrOH, propanol.

observed at 1048, 983, 968, and 810 cm−1, and in particular, the bands at 1048 and 983 cm−1 were located at higher wavenumbers than the bands of NH4ReO4 and Re2O7. At present, higher wavenumber shift can be due to the increase of the ReO oxygen double bond (ReO), and dioxo (Re( O)2) and trioxo (Re(O)3) species gave the bands at 1026 and 1013 cm−1,45 where the 1010 cm−1 band was assigned to SiORe(O)3 species.46 The band at 1048 cm−1 is a little higher than that of SiORe(O)3, and the reason for the high wavenumber shift is not clear at present and further investigation is necessary. The structural difference in Re species between non-reduced ReOx/SiO2 and IrReOx/SiO2 was not observed by the Re L3-edge XANES and EXAFS (Figure 3);28 this indicates that a part of Re has a trioxo-like structure, giving higher wavenumber shift, and this is caused by the presence of the IrO2 phase. One possible interpretation is that the trioxo Re species (ORe(O)3) is formed by the interaction with the IrO2 surface. On the basis of the TPR results, the H2 consumption on non-reduced IrReOx/SiO2 started at a lower temperature than that on Ir/SiO2 and ReOx/ SiO2 (Figure S2, Supporting Information). Combined with the results of Raman spectra, it is suggested that the trioxo Re species on IrO2 has higher reducibility. At the same time, the direct interaction between Ir and Re at low reduction temperature range is also suggested in the EXAFS analysis (Figure 3); this is connected to the formation of trioxo Re species on IrO2, where the IrORe bond is easily reduced to the IrRe bond. X-ray photoelectron spectra of Ir−ReOx/SiO2 reduced at 495 and 895 K are shown in Figure 7. In the case of Ir 4f7/2 core level spectra (Figure 7a), the peak top of both catalysts was 60.6 eV, which is assigned to Ir0 species.47 These agreed well with other characterization results such as Ir L3-edge XANES analysis (Figures 1 and 2). The spectra of the Re 4f7/2 core level are shown in Figure 7b. The peak tops (4f7/2, 4f5/2) of Ir− ReOx/SiO2 reduced at 495 and 895 K were (40.7 and 43.3 eV) and (40.6 and 43.2 eV), respectively. It has been known that the peak positions (4f7/2, 4f5/2) of Re metal and ReO2 are (39.7 and 42.3 eV) and (42.5 and 45.0 eV), respectively.48 The observed peak positions of the Re species on the reduced Ir− ReOx/SiO2 were located between Re0 and Re4+, which also agreed with Re L3-edge XANES analysis (Figure 3). These results indicate the Ir metal and low-valent ReOx species are formed on Ir−ReOx/SiO2 reduced at 495 and 895 K. The amount of CO adsorption on the reduced catalyst was measured to determine the number of surface metal atoms, and the results are listed in Table 4. The amount of CO adsorption on ReOx/SiO2 was zero, indicating that CO was not adsorbed on the ReOx.49 The adsorbed type of CO on Ir−ReOx/SiO2 is

metal particles are formed, and no Ir−Re alloy was formed at all. The average metal particle size of Ir calculated from the line width of the peak is also shown in Table 4 and the average Ir metal particles estimated to be about 2 nm on these three samples. This particle size is almost the same as that obtained by the TEM observation of Ir−ReOx/SiO2 reduced at 473 K as reported previously. 16,17 From EXAFS, the smaller CNIr−Ir (or −Re) on Ir−ReOx/SiO2 than the CNIr−Ir on Ir/SiO2 is obtained at high reduction temperature (Figure 3, Tables 1 and 2). These results indicate that the aggregation of the Ir metal particle was suppressed at high reduction temperature. Another important point is that the peak assigned to Re species was not observed at all at each reduction temperature. This tendency can be connected to smaller CNRe−Ir (or −Re) than CNIr−Ir (or −Re). Figure 6 shows the Raman spectra of the non-reduced Ir ReOx/SiO2, Ir/SiO2, ReOx/SiO2, and reference compounds.

Figure 6. Raman spectra of calcined catalysts (a−c) and reference compounds (d−f): (a) Ir/SiO2, (b) Ir−ReOx/SiO2, (c) ReOx/SiO2, (d) IrO2, (e) NH4ReO4, and (f) Re2O7.

The spectrum of Ir/SiO2 was almost the same as that of IrO2. The ReOx/SiO2 gave the bands at 953, 902, and 878 cm−1, and the position and shape of the bands of ReOx/SiO2 are similar to NH4ReO4 rather than Re2O7. This suggests that the Re species on ReOx/SiO2 has a tetrahedral [ReO4]−. On the other hand, in the case of IrReOx/SiO2, the bands due to IrO2 were shifted slightly to lower wavenumber (6−8 cm−1), and the bands due to Re species in the range above 800 cm−1 were clearly different from those of ReOx/SiO2. The bands were 23511

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heterolytically to give a hydride species at the interface between the Ir metal surface and ReOx, and then the hydride attacks the 2-position of the 2,3-dihydroxypropoxide to produce 3hydroxypropoxide, explaining the high selectivity to 1,3PrD.16,17 When the non-reduced catalyst was used, the catalyst was reduced in situ at 393 K reaction temperature and 8 MPa H2. The catalyst gave low conversion (∼18%), and the activity was comparable to that of the catalyst reduced at 415 K (Table 4, entry 1). When the reduction temperature increased from 415 to 495 K, the glycerol conversion also increased. On the other hand, the glycerol conversion slightly decreased by the reduction above 495 K. The product selectivity was almost constant on all the catalysts, suggesting that the difference of catalytic performance is due to the number of the active site. The tendency of the CO adsorption amount can explain the glycerol conversion well. 3.5. Model Structure of the Reduced Ir−ReOx/SiO2 (Re/Ir = 1). We propose the structure of the Ir−ReOx/SiO2 catalyst on the basis of the characterization results. At first, the highest activity was achieved on the catalyst reduced at 495 K (Table 4). In addition, the catalyst structure formed by the reduction at 495 K is thought to be stable because the EXAFS spectra above this temperature were almost constant (Tables 2 and 3). Therefore, the model structure of Ir−ReOx/SiO2 reduced at 495 K is discussed. According to the XRD analysis, 2.0 nm Ir particles with 53% dispersion (DXRD) are formed (Table 4). Considering that the dispersion (DCO) from CO adsorption (CO/Ir) is 16%, the ReOx species covers 37% Ir atoms (DXRD − DCO) (Table 4). It is thought that one Re atom can suppress one CO adsorption on the Ir atom. Here, the molar ratio of Re to Ir is 1, and the position of 63% Re is important. If the 63% Re species are dispersed on the support surface, it is very difficult to give high CNRe−Ir (or −Re) such as 6.0−6.5 (Table 3). The single peak of the TPR profile is also inconsistent with Re species on the support. Furthermore, the results of the Ir−ReOx/SiO2 should be compared to those of Rh−ReO x /SiO 2 . 28 In the case of Ir−ReO x /SiO 2 , the CNRe−Ir (or −Re) is determined to be 6.5 (Table 3); on the other hand, the CNRe−Rh + CNRe−Re was about 4−5 on Rh− ReOx/SiO2.28 It has been proposed that Rh−ReOx/SiO2 gives two-dimensional ReOx clusters attached on the surface of Rh metal particles.28 One interpretation of the structural model of Ir−ReOx/SiO2 is the three-dimensional ReOx clusters attached on the surface of Ir metal particles. The model structure of the Ir−ReOx/SiO2 catalyst is discussed on the basis of the assumption that Ir metal particles have a cuboctahedron structure. Judging from the ∼2 nm particle size of the Ir metal atom radius (0.138 nm), a single side can consist of three Ir atoms. The surface of this cubooctahedorn has six faces of a regular square (1 0 0) and eight faces of a regular hexagon (1 1 1) (Figure 8a,b). This cuboctahedron particle has 122 atoms on the surface. This value is based on the number of the surface atoms of 4 1/3 (=1 + 4 × 1/2 + 4 × 1/3) for (1 0 0) and 12 (=7 + 6 × 1/2 + 6 × 1/3) for (1 1 1), where 1/2 is a factor of edge atoms and 1/3 is a factor of corner atoms. Six (1 0 0) faces and eight (1 1 1) faces give 4 1/3 × 6 + 12 × 8 = 122. In addition, this cuboctahedron particle has 201 atoms in total (details are shown in Figure S5, Supporting Information), and the dispersion calculated from the model structure is 61% (=122/201), a value which agreed well with XRD results (53%). Re atoms tend to be located at the 4-fold and 3-fold hollow sites on (1 0 0) and (1 1 1) surfaces, respectively. One possible

Figure 7. X-ray photoelectron spectra of Ir−ReOx/SiO2 reduced at 495 K (a) and 895 K (b): (A) Ir 4f; (B) Re 4f.

mainly linear,17 which indicates that the amount of adsorbed CO represents the number of Ir metal atoms on the surface. The amount of CO adsorption relative to the number of total Ir atoms (DCO) was only 0.07 on the catalyst reduced at 415 K, increased by the reduction at 495 K, and it slightly decreased with increasing reduction temperature above 495 K. The metal dispersion calculated from the particle size determined from XRD is also listed in Table 4 (DXRD). An important point is that the dispersion calculated from CO adsorption (DCO) was clearly lower than DXRD. This suggests that CO adsorption is suppressed probably by the interaction between ReOx and the Ir metal surface because the oxidized Re species does not adsorb CO molecule.49 In particular, the difference between DCO and DXRD became larger with increasing the reduction temperature (≥495 K). This suggests that the interaction between ReOx and the Ir surface is stronger at higher reduction temperature, and the interaction is interpreted by the coverage of the Ir metal surface with ReOx species. Table 4 also lists the reduction temperature dependence of the catalytic performance in the glycerol hydrogenolysis over Ir−ReOx/SiO2. It was previously reported that Ir/SiO2 has no activity and Re addition to Ir/SiO2 drastically enhanced the activity and selectivity to 1,3-PrD.16,17 It has been proposed that ReOx adsorbs glycerol at the terminal position to form 2,3dihydroxypropoxide, and a hydrogen molecule is activated 23512

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CNRe−Ir (or −Re) is calculated to be 6.2 (=(4 × 9 + 4 × 3 + 1 × 8)/9) and 5.6 (=(3 × 8 + 3 × 3 + 1 × 6)/7) on the clusters on (1 0 0) and (1 1 1) surfaces, respectively. These values are close to the experimental CNRe−Ir (or −Re) (6.0−6.5), as listed in Table 3 on the sample reduced at 485−595 K (Table 3). Next, the number of Re atoms in the second layer is calculated to be 126 (=5 × 6 + 12 × 8), and the total number of Re atoms is 222 (=96 + 126). On the other hand, the total number of Ir atoms is estimated to be 201, as shown in Figure S5 (Supporting Information), which is almost comparable to the total number of Re atoms (=222). This agreement is connected to the molar ratio of Re to Ir (=1). Furthermore, we mention the Re−O bond. The length of the Re−O bond on the catalyst reduced at 485−595 K is determined to be 0.204−0.205 nm corresponding to the single Re−O bond. Therefore, Re−O−Re and/or Re−OH are thought to be present on the proposed structure (Figure 8). In addition, the structure of Ir−ReOx/SiO2 reduced at higher temperature is discussed. When the catalyst was reduced at 695 K, the particle size of Ir metal was maintained; however, the CO adsorption was decreased and the CNRe−Ir (or −Re) was increased. One possible interpretation is that the unoccupied 3-fold hollow site of Ir (1 1 1) (Figure 8d) is occupied by Re atom.

4. CONCLUSIONS (1) The reduction temperature of Ir−ReOx/SiO2 was lower than that on monometallic Ir/SiO2 and ReOx/SiO2. The more reducible species on the non-reduced Ir−ReOx/ SiO2 may be trioxo Re species interacted with IrO2 phase. (2) The interaction between Re and Ir species exists even at low reduction temperature range (445 K) and continued to high reduction temperature range. This behavior is connected to maintaining Ir metal particles with high dispersion. (3) The structure of Ir−ReOx/SiO2 formed by the reduction was stable at 485−595 K. The structure has small Ir metal particles (ca. 2 nm) and low-valent ReOx species. The surface of Ir metal particles can be partially covered with three-dimensional ReOx clusters. Here, a model structure considering of ReOx clusters attached on Ir cuboctahedron particles is proposed. (4) The performance of Ir−ReOx/SiO2 in the hydrogenolysis is explained by the number of the active sites from the CO adsorption amount. The active site is expected to interface between Ir metal and low-valent ReOx clusters based on the proposed structure where most surface Ir atoms are bonded with ReOx species via metallic bond.

Figure 8. Model structure of Ir−ReOx/SiO2 reduced at 495 K. (a) Ir metal surface having a regular square (1 0 0). (b) Ir metal surface having a regular hexagon (1 1 1). (c) Re first layer on the Ir (1 0 0) surface. (d) Re first layer on the Ir (1 1 1) surface. (e) Re second layer on the Re (1 0 0) first layer. (f) Re second layer on the Re (1 1 1) first layer. White circles, Ir metal atoms; light gray circles, Re atom located at first layer; dark gray circles, Re atom located at second layer.

model of the Re first layer on both surfaces is illustrated in Figure 8c and d. Here, the total number of Re atoms is calculated to be 96 (=4 × 6 + 9 × 8). If one Re atom suppresses one CO adsorption, the amount of CO adsorption is determined to be 26 (=122 − 96). Here, DCO/DXRD is calculated to be 0.21, which is similar to 0.30 (16%/53%) in Table 4, and the difference in the CO adsorption can be expected by the effect of edge and corner atoms, where the suppressing effect by Re atoms on the CO adsorption on the edge or corner sites is not so significant as that on the face sites. Considering that Re atoms are not dispersed on the SiO2 support surface, the residual Re atoms are put on the first Re layer as the second Re layer. Figure 8e and f illustrates the skeltal structure of the clusters including both first and second layers. The structure in Figure 8e has a pyramidal one with four Re atoms attached on four triangle faces. On the other hand, Figure 8f represents a tetrahedral structure with three Re atoms attached on three triangle faces. The CNRe−Ir and CNRe−Re on each atom are also described in Figure 8e and f. The average

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ASSOCIATED CONTENT

S Supporting Information *

The details of XAFS, TPR, and model structure are shown in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 23513

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ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI 23760737, and a part of this research is funded by the Cabinet Office, Government of Japan, through its “Funding Program for Next Generation World-Leading Researchers”.

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