摘要：

The behavior of Rh6(CO)16 on the surface of inorganic oxides (alumina, magnesia, and silica-alumina) has been found to be strongly dependent on the mode of impregnation and on the water content of the support. On alumina it is possible to maintain the molecular nature of the cluster provided the water content of the alumina is low; otherwise the OH groups of the support are at the origin of a ready oxidation of the cluster resulting in a rhodium(I) carbonyl species. The rhodium(I) carbonyl species reacts with CO in the presence of excess water to give back the cluster Rh6(CO)16. (Rh(CO)2Cl)2 supported on alumina also reacts with CO in the presence of excess water to give Rh6(CO)16. A facile reduction of the rhodium(I) carbonyl species, obtained from oxidation of Rh6(CO)16, under hydrogen or excess water in the absence of CO leads to the formation of metallic rhodium together with an "intramolecular" rearrangement of a linear to a doubly bridged carbonyl ligand. It is possible to regenerate the rhodium(I) surface carbonyl species with oxygen followed by carbon monoxide. Infrared data for Rh6(CO)16 supported on alumina, magnesia, and silica-alumina indicate an electronic interaction between the support and the cluster, the rhodium(I) carbonyl species, or the metallic species obtained upon reduction of rhodium(I). A catalytic cycle is proposed which accounts for the water gas shift reaction carried out on the cluster frame and its oxidation products.

展开