(209c) Reaction-Relevant Gold Structures in Au-Feox Catalysts for the Water-Gas Shift Reaction
Gold-iron oxide catalysts have been found equally active to gold-ceria and more stable for the WGS reaction at temperatures higher than 250 °C.1 An important stabilization of gold activity by magnetite at temperatures as high as 375 °C has been observed.1 However, the evolution of gold species on the surface of iron oxide and structural stability under different reaction gas compositions and temperatures is not well understood and is the subject of investigation. Here, we report on studies of gold structure evolution in the WGS reaction on Au-FeOx catalysts with the aid of X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy, high resolution transmission electron microscopy (HREM) and scanning tunneling microscopy/spectroscopy (STM/STS). From the results of in-situ XANES/EXAFS, the fresh AuFeOx sample contained only fully dispersed gold ions. This was also confirmed by sample images collected in an aberration- corrected HREM at ORNL. The coordination number (CN) of Au-Au grew after the WGS reaction, together with a decrease in CN of Au-O. Redispersion of gold on the iron oxide surface was observed after subsequent reoxidation in O2/He. Similar reduction/redispersion phenomena were recently found on gold-ceria catalysts for the WGS reaction.2 Catalytic tests showed that the initial activity (fully dispersed gold in ceria) was maximum. STM/STS work focused on the interaction of Au with the Fe3O4 (111) surface. Current data show that gold atoms stabilized on a magnetite (111) surface after high-temperature annealing change the electronic properties of the iron oxide surface, consistent with the strong interaction found in powder samples.3 These results as well as data from catalytic activity tests, XRD, XPS and H2-TPR analyses of the gold-iron oxide samples will be discussed in the presentation.
1. W. Deng, C. Carpenter, N. Yi, M. Flytzani-Stephanopoulos, Top. Catal. 2007, 44, 199.
2. W. Deng, A. Frenkel, R. Si, M. Flytzani-Stephanopoulos, J. Phys. Chem. C, 2008, 112, 12834.
3. K. T. Rim, D. Eom, G.. W. Flynn, R. Si, M. Flytzani-Stephanopoulos, ?Quantum confinement effects in gold clusters adsorbed on an iron oxide surface: A scanning tunneling spectroscopy study?, in preparation.