Chemisorption and catalysis by metal clusters: III. Hydrogenation of ethene, carbon monoxide, and carbon dioxide, and hydrogenolysis of ethane catalyzed by supported osmium clusters derived from Os 3(CO) 12 and from Os 6(CO) 18

Abstract

Properties are described for catalysts containing high nuclearity metal clusters (nuclearity ~12) derived from Os 3(CO) 12 and Os 6(CO) 18 and supported on silica, alumina, titania, or ceria. Ethene hydrogenation (325–535 K), ethane hydrogenolysis (395–665 K), CO hydrogenation (455–665 K), and CO 2 hydrogenation (455–715 K) have been examined in pulsed-flow and static reactors. The high nuclearity osmium clusters, protected against sintering by retained ligand-CO, ligand-C, and a support-cluster interaction, are stable under these conditions and provide highly reproducible activity. Freshly prepared catalysts each exhibit an initial non-steady state, during which hydrocarbon is progressively retained and activity rises, passes through a maximum, and declines to a steady state value. Catalysts in the steady state continue to retain hydrocarbon which is probably branched in structure and unsaturated in character. Such retained hydrocarbon species mediate hydrogen atom transfer to reacting adsorbed species. Their concentrations, which have been determined by infrared spectroscopy, 14C-tracer studies, and material balances, are compared with the known site concentrations associated with fresh cluster-derived catalysts. Catalysts in the steady state exhibit activities the magnitudes of which diminish with increasing support-cluster interaction, viz., silica-supported clusters > titania-supported clusters > alumina-supported clusters. Preliminary measurements using a ceria-supported catalyst suggest that activity versus the strength of the support-cluster interaction exhibits a “volcano” relationship. Adsorption of ethene, ethane, and CO occurs at osmium atom sites on the high nuclearity osmium clusters, and the reaction intermediates are also adsorbed at these sites. CO 2, however, is adsorbed and reacts at ligand-C sites. Detailed mechanisms are presented for ethene, CO, and CO 2 hydrogenations, of which some aspects have been investigated by use of 14C as an isotopic tracer. Most cluster-derived catalysts show exceptional activity for ethane hydrogenolysis, some apparent turnover numbers being 2 orders of magnitude higher than for supported metallic osmium. The osmium clusters adsorb reactants less strongly than metallic osmium, because of their commitment to bonding with the protective CO-ligands, and this weaker reactant adsorption may provide superior catalytic properties.