A Theoretical Investigation of the Selective Oxidation of Methanol to Formaldehyde on Isolated Vanadate Species Supported on Titania

The selective oxidation of methanol to formaldehyde occurring on titania-supported vanadate species has been analyzed theoretically with the aim of understanding why the activity of VOx/TiO2 is ∼103 faster than that of VOx/SiO2. The active site was represented by a [(O)3V═O] group located at the corner of a cubic TiOx cluster, a model similar to that used successfully to describe the oxidation of methanol on isolated vanadate species supported on silica. Density functional theory was used to calculate the geometry, vibrational frequencies, and energy of all ground state and transition state structures. The equilibrium constants and rate coefficients for each elementary reaction step were calculated using statistical mechanics and absolute rate theory. Methanol oxidation to formaldehyde was taken to proceed via two key steps: the reversible adsorption of methanol across a V−O−Ti bond followed by the transfer of a hydrogen atom from an adsorbed methoxy group to a vanadyl O atom. The rate parameters and the apparent first-order rate coefficient determined for VOx/TiO2 were found to be very similar to those reported earlier in a theoretical analysis of VOx/SiO2 [J. Phys. Chem. C 2007, 111, 14753], indicating that the significantly higher rate of reaction seen experimentally for VOx/TiO2 is not due to an intrinsic electronic effect of the support on the catalytic properties of the active center. Introduction of an O-vacancy adjacent to the vanadate species results in a reduction in the activation barrier for the rate-limiting step and to close agreement between the rate parameters predicted and those found experimentally. The effect of O-vacancies in the support on the rate of methanol on metal oxide-supported vanadate species is further evidenced by a strong correlation between the turnover frequency for methanol oxidation and the energy required to form an O-atom defect on metal oxide supports.