DESIGN AND CHARACTERIZATION BY EXAFS, FTIR, AND TEM OF RH-SN/SIO2 CATALYSTS ACTIVE FOR NO-H-2 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-H-2 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, H-2 and CO adsorption, and TEM. For the Rh-Sn/SiO2 catalysts (Sn/Rh greater than or equal to 0.4), the surface concentration of Sn to Rh was estimated to be Sn-s/Rh-s 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. (C) 1994 Academic Press, Inc.