Thermal reactions of Mo(CO)6 on metal-oxide surfaces

Abstract

Temperature-programmed decomposition has been used to determine the energetics of metalcarbonyl bond dissociation in the surface-mediated decarbonylation of Mo(CO)₆ adsorbed on alumina, magnesia, silica, titania, zirconia and zinc oxide. To estimate the influence of the surface microstructure, the reaction was studied on a conventional silica gel and on MCM-41, a silica with uniform cylindrical mesopores. Activation energies for successive decarbonylation steps were calculated from the temperature of the desorption maxima using Redhead's equation. The activation energy for the elimination of the first CO from the complex is generally lower than in the gas phase and varies from below 100 kJ mol^–1 on the basic supports ZnO and MgO, to 126 kJ mol^–1 on SiO₂. The different microstructure of MCM-41 and a silica with a wide pore-size distribution has no influence on the desorption behaviour. It is proposed that the initial reaction step is the nucleophilic substitution of CO by the free electron pair of a surface O^2– or OH group. The electron density at the oxygen is strongly influenced by the adjacent metal cation. An empirical correlation between the activation energy and the field strength at a surface cation site was found to yield a linear relationship. Thus, the decomposition of Mo(CO)₆ may be a useful probe for the oxidation state of surface metal ions in simple oxides.