Abstract:

The reactive sites on platinum metal catalysts for the hydrogenation of unsatd. hydrocarbons and related reactions were the most coordinatively unsatd. exposed metal atoms. The surface atoms were divided into 3 groups with one, two, or three units of coordinative unsatn. (corresponding to the no. of monodentate ligands which may become attached) and were symbolized by 1M, 2M, and 3M, then, when exposed to H the 1st structures formed were 2MmH2 and 3MH2, which were transformed, in part, to 2MH and 3MH by diffusion of H from the edge to other surface sites or to the interior of the crystallite. The proportions of 3M and 3MH present during a reaction were a function of the metal and the conditions of prepn. and use. These structures bore a formal relation to the structure of the complexes ClRh(PPh3)2 (I) and HRhCO(PPh3)2 (II) which are formed through the dissocn. of ClRh(PPh3)3 and HRh(CO)(PPh3)3, resp. The catalytic functions of both I and II involved the same kinds of elementary processes, but the presence of the hydrido group in II led to recognizably different phenomena (kinetics, stereochem., 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. [on SciFinder(R)]