TEMPERATURE-PROGRAMMED DECOMPOSITION OF [MO(CO)(6)] - INDICATION OF SURFACE-REACTIONS AND CLUSTER FORMATION

The influence of the extent of hydroxylation on the surface-mediated d ecarbonylation of [Mo(CO)(6)] has been studied using temperature-progr ammed decomposition (TPDE). Different CO-evolution spectra were obtain ed on silica and gamma-alumina supports, which can be explained based on the density of the OH groups and the strength of Lewis-acid sites o n the surface of the supports. The TPDE spectra changed dramatically w ith the extent of surface dehydroxylation. The desorption signal can b e deconvoluted into individual signals which correspond to the stepwis e elimination of one CO group after the other from the carbonyl comple x. Intermediate subcarbonyl species are stable on hydroxylated surface s, whereas evidence for the formation of multinuclear clusters has bee n obtained on dehydroxylated surfaces. Increasing dehydroxylation of t he support lowered the temperature for the first elimination of CO, bu t the temperature for complete decarbonylation became higher. The reac tion mechanism changed from nucleophilic ligand exchange on hydroxylat ed surfaces to Lewis-acid-assisted decarbonylation on severely dehydro xylated surfaces. Owing to its surface sensitivity, the decomposition of [Mo(CO)(6)] can be used as a probe for surface acid-base properties . Besides evolution of CO, variable amounts of H-2 were also observed. Hydrogen is formed in a redox reaction between the metal and surface OH groups. The amount and temperature of hydrogen evolution depended o n the chemical nature and the pretreatment of the support. Even on tho roughly dehydroxylated supports, thermal decomposition of [Mo(CO)(6)] did not lead to zerovalent metal, but to a slightly oxidized species.