A DFT study of methanol dissociation on isolated vanadate groups

Molecular and dissociative adsorption processes of methanol on isolated vanadate groups have been studied by means of density functional theory calculation at the B3LYP computing level. The catalyst is represented by an isolated vanadia unit in two environments: hydrated and supported. First, a OV(OH)(3) model considers hydrated conditions and second, a OV(OTiO2H)(3) cluster accounts for the titania-supported site. The stationary points on the potential energy surface have been characterized and their geometries, relative energies and also the vibrational spectra have been obtained. In addition, the nature of chemical bonding has been highlighted by means of the analysis of the electron localization function. Our results show that methanol dissociation is a favourable process leading to methoxide groups. The molecular mechanism for dissociation on V-OH or supported V-O-Ti sites is preferred to that involving vanadyl V=O groups. Hydrogen bonding plays a key role stabilizing molecular methanol intermediates in hydrated conditions, while electrostatic acid/base interactions prevail in supported systems. Our calculated vibrational spectra confirm the experimental band assignment. An ELF bonding analysis identifies the ionic character of catalyst-methanol interactions. A moderate charge transfer of -0.34 electrons to the methoxy group would result in a blue-shift of the vanadyl band. (C) 2008 Elsevier B.V. All rights reserved.