The structure of reactive sites on platinum metal catalysts for the hydrogenation of unsaturated hydrocarbons

The reactive sites, on platinum metal catalysts for the hydrogenation of unsaturated hydrocarbons and related reactions, are proposed to be the most coordinatively unsaturated exposed metal atoms. To simplify the discussion, the surface atoms are divided into three groups with one, two, or three units of coordinative unsaturation (corresponding to the number of monodentate ligands which may become attached) and are symbolized by ¹M, ²M, and ³M. When exposed to hydrogen the first structures formed are ²MH₂ and ³MH₂ which are transformed, in part, to ²MH and ³MH by diffusion of hydrogen from the edge to other surface sites or to the interior of the crystallite. The proportions of ³M and ³MH present during a reaction are a function of the metal and the conditions of preparation and use. These structures bear a formal relationship to the structure of the complexes ClRh(PPh₃)₂(1) and HRhCO(PPh₃)₂(2) which are formed through the dissociation of ClRh(PPh₃)₃ and HRh(CO)(PPh₃)₃, respectively. The catalytic functions of both 1 and 2 appear to involve the same kinds of elementary processes, but the presence of the hydrido group in 2 leads to recognizably different phenomena (kinetics, stereochemistry, deuterium exchange reactions). Accordingly, the characteristics of hydrogenations catalyzed by platinum metals may be rationalized by considering the elementary processes which may occur at each category of site.