THERMAL-REACTIONS OF MO(CO)(6) ON METAL-OXIDE SURFACES

Temperature-programmed decomposition has been used to determine the en ergetics of metal-carbonyl bond dissociation in the surface-mediated d ecarbonylation of Mo(CO)(6) adsorbed on alumina. magnesia, silica, tit ania, zirconia and zinc oxide. To estimate the influence of the surfac e microstructure, the reaction was studied on a conventional silica ge l and on MCM-41, a silica with uniform cylindrical mesopores. Activati on 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 comple x is generally lower than in the gas phase and varies from below 100 k J 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-s ize distribution has no influence on the desorption behaviour. It is p roposed that the initial reaction step is the nucleophilic substitutio n of CO by the free electron pair of a surface O2- or OH group. The el ectron density at the oxygen is strongly influenced by the adjacent me tal cation. An empirical correlation between the activation energy and the field strength at a surface cation site was found to yield a line ar relationship. Thus, the decomposition of Mo(CO)(6) may be a useful probe for the oxidation state of surface metal ions in simple oxides.