Design and Characterization by EXAFS, FTIR, and TEM of Rh-Sn/SiO2 Catalysts Active for NO-H2 Reaction

Rh-Sn/SiO2 catalysts, prepared by the selective reaction between Sn (CH3)4 and small Rh metallic particles supported on SiO2, showed much higher catalytic activities for NO-H2 reaction and NO dissociation than Rh/SiO2 and coimpregnation Rh-Sn/SiO2. In order to examine the important factors for the efficient catalysis of the Rh-Sn/SiO2 catalysts, the samples were characterized by Sn K- and Rh K-edge EXAFS, FTIR, H2 and CO adsorption, and TEM. For the Rh-Sn/SiO2 catalysts (Sn/Rh ≥ 0.4), the surface concentration of Sn to Rh was estimated to be Sns/Rhs = 3, where a Rh atom is surrounded by six Sn atoms. According to the results of the Sn K-edge EXAFS analysis, the bond distance between a Sn atom and the nearest-neighbor atom in the first layer atoms was 0.270 nn, and the bond distance between a Sn atom and a metal atom in the second layer was 0.290 nm, suggesting a relaxation of the first bimetallic layer. A surface model structure of Rh-Sn particles on SiO2 as a catalytically active bimetallic ensemble is discussed.