Article pubs.acs.org/JPCC
Stable Low-Valence ReOx Cluster Attached on Rh Metal Particles Formed by Hydrogen Reduction and Its Formation Mechanism Shuichi Koso,† Hideo Watanabe,‡ Kazu Okumura,§ Yoshinao Nakagawa,† and Keiichi Tomishige†,* †
School of Engineering, Tohoku University 6-6-07 Aoba, Aramaki, Aoba-ku, Sendai, Miyagi 980-8579, Japan Graduate School 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, Koyama-cho Minami, Tottori 680-8552, Japan ‡
S Supporting Information *
ABSTRACT: The structural change of ReOx/SiO2, Rh/SiO2, and Rh−ReOx/SiO2 during the temperature programmed reduction with H2 was investigated using in situ Re L3-edge and Rh K-edge quick-scanning X-ray absorption fine structure. Monometallic ReOx/SiO2 was reduced at about 600 K to form Re metal particles covered with partially oxidized Re species. In contrast, in the case of Rh−ReOx/SiO2 (Re/Rh = 0.5), the reduction of Rh proceeded to give highly dispersed Rh metal particles at first (∼325 K), and then Re was reduced mildly (∼365 K). The reduced Re species interacted with the Rh metal surface and were highly dispersed. At higher reduction temperature (∼595 K), these Re-modified Rh metal particles aggregated and further reduction of Re to low-valence state proceeded on the Rh metal surface. The low-valence Re interacting with Rh metal surface comprises two-dimensional Re oxide clusters. No detection of Re metal or Rh−Re alloy formation even after reduction at high temperature (595 K) suggests that the two-dimensional ReOx clusters on the Rh metal surface are highly stable. to 1,3-propanediol.14 The added Re species are present both in Pt−Re bimetallic clusters and on the support surface. The average oxidation state determined by Re L3-edge XANES showed the incomplete reduction of the Re species by the treatment in H2, and this is explained by the presence of bimetallic particles and atomically dispersed oxidized Re species on the carbon. In addition, the formation of the OH group on Re in the bimetallic particles is suggested and Re−OH plays an important role on the glycerol hydrogenolysis. Our group has recently reported that Pt−ReOx/SiO2 was effective to the preferential CO oxidation in H2-rich stream.15,16 Characterization results showed that ReOx clusters are formed on Pt metal surface. The CO and O2 pulse reaction showed that Pt− ReOx/SiO2 can activate the O2 molecule when the surface is saturated with CO while Pt/SiO2 cannot. The ReOx clusters can play an important role in the activation of O2 to produce oxidizing species in the case of Pt−ReOx/SiO2, and the catalytically active sites in this reaction are the interface between the ReOx clusters and the surface of Pt metal particles.15,16 Recently, much attention has been paid to Re-modified noble metal catalysts other than Pt, especially related to the catalytic conversion of biomass-derived chemicals. We have recently found that silica- or carbon-supported Re-modified Rh catalysts
1. INTRODUCTION Supported noble metal particles modified with rhenium species have been known to have catalytic functions with high performance in various reactions. One of the most important catalysts is Pt−Re/Al2O3 for the naphtha reforming to aromatic hydrocarbons used in petroleum refineries.1−3 The additive effect of rhenium includes the improvement in the catalyst life and the selectivity to the aromatics. It has been reported that both Pt and Re are in the metallic state, and they form the alloy and bimetallic clusters on the basis of the characterization results by TPR,4,5 XPS,6,7 and XANES.8,9 It has been reported that Pt−Re/TiO2 was effective for selective hydrogenation of carboxylic acids to alcohols10 and amides to amines11 in the liquid phase under relatively mild conditions. Without the addition of Re, the activity of Pt/SiO2 is much lower. The characterization by in situ XANES analysis demonstrated that metallic Pt and metallic and oxidized Re were present on Pt− Re/TiO2 during the reaction.12 Dumesic et al. have reported that Pt−Re/C catalysts were more active for glycerol conversion to synthesis gas than Pt/C, and the addition effect of Re to Pt was mainly the promotion of the water−gas shift and alkane formation reactions.13 Characterization results suggest that Pt−Re catalysts consist of bimetallic alloy nanoparticles after the reduction and that Re surface atoms preferentially bond to oxygen species such as hydroxyl groups under the reaction conditions. In addition, Davis et al. have reported that Pt−Re/C catalysts were effective in the glycerol hydrogenolysis © 2011 American Chemical Society
Received: November 27, 2011 Revised: December 28, 2011 Published: December 29, 2011 3079
dx.doi.org/10.1021/jp2114225 | J. Phys. Chem. C 2012, 116, 3079−3090
The Journal of Physical Chemistry C
Article
interaction between two different components, and here, we focused on the bond formation of Re−Rh and Re−Re on Remodified Rh catalysts. The structure of the bimetallic Remodified Rh catalysts is compared with these of monometallic Rh and Re catalysts, and the catalytically active structure caused by the strong interaction between Re and Rh is discussed.
were effective in the hydrogenolysis of tetrahydrofurfuryl alcohol (THFA), 1 7 − 2 2 tetrahydropyran-2-methanol (THPM),17−19 glycerol,19,20,22−25 and 1,2-propanediol.19,20,25 Monometallic Rh catalysts showed much less activity and selectivity. Re catalysts without Rh showed almost no activity. The results showed that the synergy between Rh and Re causes high catalytic activity in the selective hydrogenolysis. Heeres et al. reported the hydrogenolysis of tetrahydrofuran-2,5dimethanol to 1,6-hexanediol using the Rh−ReOx/SiO2 catalyst in the presence of various solid acid catalysts such as zeolite, Nafion SAC-13, Amberlyst-16, and so on.26 A common feature of Re-modified Rh catalysts in the hydrogenolysis of these substrates is that Re addition remarkably promoted the hydrogenolysis of the C−O bond neighboring a −CH2OH group. The state of Re and Rh during the reaction was suggested to be the low-valent oxidized and the metallic state, respectively, by the results of H2-TPR and XANES. In addition, the formation of low-valent ReOx clusters directly bonded to the Rh metal surface is indicated by the results of XRD, TEM, CO adsorption and EXAFS. The kinetic analysis showed that substrate and hydrogen were activated at different sites. On the basis of those data, the catalytically active sites are thought to be the interface between the ReOx clusters and the Rh metal surface. Dumesic et al. have also suggested that the hydroxylated Re's near Rh on the bimetallic particles are the active sites of the hydrogenolysis on Rh−ReOx/C catalysts.27 The combination of Re with noble metals, other than Pt and Rh, has also given the characteristic catalysts. The Re-modified Ir catalysts were reported to be effective to the glycerol hydrogenolysis to 1,3-propanediol, and the catalyst structure where Ir metal particles were partially covered with the ReOx clusters was suggested.19,20,28,29 It has been reported that Ir−Re/ZrO2 was used as an effective catalyst in the liquid phase methanol reforming, and the high performance is thought to be due to the formation of the nanocomposites of Ir and Re.30 It has been reported that Re addition to Pd/TiO2 enhanced the activity of the water-gas-shift reaction on Pd−Re/TiO2. The Pd−Re bimetallic clusters were formed after the reduction and the Re species on the surface was oxidized by H2O.31,32 In the case of Ru, Whyman et al. used the Ru/Re catalysts prepared from Ru3(CO)12/Re2(CO)10 for hydrogenation of amides to amines and showed that the active catalysts contained bimetallic Ru/Re nanoclusters and Re oxides.33 He et al. have reported that Re addition to Ru catalysts enhanced the activity and improved the selectivity to propanediols by the suppression of the degradation reaction34−36 while addition of acids to Ru catalysts have been reported to be also effective to enhance the performance.37−40 They reported that the Re-modified Ru catalysts contain Ru metal and Re oxides.34−36 As shown above, the modification of noble metal catalysts with the Re species is very effective in many reactions involving H2; however, the characterization of the interface is difficult, and the formation mechanism has been much less investigated. Here, the structural change of Re-modified Rh catalysts during the H2 reduction was analyzed by in situ quick-scanning extended X-ray absorption fine structure (EXAFS) in order to elucidate the formation mechanism of ReOx species attached on the Rh metal surface. The reason why ReOx-modified Rh catalysts was used in the present work is that this case showed a very striking effect in the catalytic performance by Re modification, and it is easy to distinguish between Rh and Re as a back scattering atom, unlike the cases of Pt−Re and Ir−Re catalysts. EXAFS is a useful tool to analyze the direct
2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. A Rh/SiO2 catalyst was prepared by impregnating SiO2 with an aqueous solution of RhCl3·3H2O (Soekawa Chemical 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. Rh−ReOx/SiO2 catalyst was prepared by impregnating Rh/ SiO2 after the drying procedure with an aqueous solutions of NH4ReO4 (Soekawa Chemical Co., Ltd.), and the loading amount of Re was 0.5 by the molar ratio to Rh which is an optimized amount of Re in terms of the catalytic activity of hydrogenolysis of glycerol19,20,23,24 and tetrahydrofurfuryl alcohol17,21,22 as reported previously. ReOx/SiO2 was also prepared by impregnating SiO2 with an aqueous solution of the same precursor as in the case of Rh−ReOx/SiO2, and the loading amount of Re is also same as that of Rh−ReOx/SiO2. These three samples were calcined in air at 773 K for 3 h after drying at 383 K for 12 h. The loading amount of Rh on Rh/ SiO2 and Rh−ReOx/SiO2 catalysts was 4 wt %. All catalysts were in powdery form with granule size of
