MgO-supported bimetallic catalysts consisting of segregated, essentially molecular rhodium and osmium species

Abstract

A family of supported bimetallic samples was prepared by the reactions of Rh(C₂H₄)(acac) (acac = acetylacetonate) and Os₃(CO)₁₂ with MgO. The samples were characterized by infrared and extended X-ray absorption fine structure spectroscopies, before and after various treatments with hydrogen at temperatures up to 393 K. The spectra identify the following combinations of supported species: (a) [Os₃(CO)₁₁]²⁻ + Rh(C₂H₄)₂, (b) [Os₃(CO)₁₁]²⁻ + Rh(CO)₂, and (c) [Os₃(CO)₁₁]²⁻ + rhodium clusters containing approximately 4 to 6 atoms each, on average. No bimetallic clusters formed. The triosmium frame remained intact while the rhodium surface species were reduced in the presence of H₂. The samples were tested as catalysts for ethylene hydrogenation at 298 K, and the catalytic activity matched that of the rhodium complexes alone; evidently the rhodium complexes were responsible for all the catalysis, with the osmium clusters remaining inactive because of the presence of the carbonyl ligands on them.