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 (C H3)(4) and small Rh metallic particles supported on SiO2, showed much higher catalytic activities for NO-H-2 reaction and NO dissociation th an Rh/SiO2 and coimpregnation Rh-Sn/SiO2. In order to examine the impo rtant 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 grea ter than or equal to 0.4), the surface concentration of Sn to Rh was e stimated to be Sn-s/Rh-s 3, where a Rh atom is surrounded by six Sn at oms. According to the results of the Sn K-edge EXAFS analysis, the bon d distance between a Sn atom and the nearest-neighbor atom in the firs t layer atoms was 0.270 nn, and the bond distance between a Sn atom an d a metal atom in the second layer was 0.290 nm, suggesting a relaxati on of the first bimetallic layer. A surface model structure of Rh-Sn p articles on SiO2 as a catalytically active bimetallic ensemble is disc ussed. (C) 1994 Academic Press, Inc.