Chemisorption and catalysis by metal clusters. Hydrogenation of ethene and hydrogenolysis of ethane catalysed by supported ruthenium clusters derived from Ru3(CO)12 and from H4Ru4(CO)12

Abstract

Properties are described for catalysts containing active clusters derived from Ru₃(CO)₁₂ and H₄Ru₄(CO)₁₂ and supported on silica, alumina and titania. The clusters, protected against sintering by retained ligand–CO, ligand–C, and a support–cluster interaction, are stable under the range of reaction conditions used (0.101 MPa, 273–590 K) and provide highly reproducible activity for the hydrogenation of ethene and the hydrogenolysis of ethane. 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 with respect to activity continue to accumulate hydrocarbon and such retained hydrocarbons mediate hydrogen-atom transfer to reacting adsorbates. The concentration of the retained hydrocarbon species, which have been determined by material balance, are compared with the known site concentrations associated with fresh cluster-derived catalysts. Catalysts in the steady state exhibited low activity for ethene hydrogenation. This behaviour is interpreted in terms of the strength of bonding between ethene and the cluster. In ethane hydrogenolysis, the site and bonding limitations of the cluster allow C—C bond breaking before extensive dehydrogenation takes place. Ruthenium cluster catalysts show lower specific activity than corresponding osmium catalysts.