Temperature-programmed decomposition of [Mo(CO)6]: indication of surface reactions and cluster formation

Abstract

The influence of the extent of hydroxylation on the surface-mediated decarbonylation of [Mo(CO) ₆ ] has been studied using temperature-programmed decomposition (TPDE). Different CO-evolution spectra were obtained on silica and γ-alumina supports, which can be explained based on the density of the OH groups and the strength of Lewis-acid sites on the surface of the supports. The TPDE spectra changed dramatically with the extent of surface dehydroxylation. The desorption signal can be deconvoluted into individual signals which correspond to the stepwise elimination of one CO group after the other from the carbonyl complex. Intermediate subcarbonyl species are stable on hydroxylated surfaces, whereas evidence for the formation of multinuclear clusters has been obtained on dehydroxylated surfaces. Increasing dehydroxylation of the support lowered the temperature for the first elimination of CO, but the temperature for complete decarbonylation became higher. The reaction mechanism changed from nucleophilic ligand exchange on hydroxylated surfaces to Lewis-acid-assisted decarbonylation on severely dehydroxylated 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 on the chemical nature and the pretreatment of the support. Even on thoroughly dehydroxylated supports, thermal decomposition of [Mo(CO) ₆ ] did not lead to zerovalent metal, but to a slightly oxidized species.