A comprehensive model for the supported vanadium oxide catalyst: The umbrella model

Abstract

Supported vanadium oxide catalysts are widely used in industry. However, the molecular structure of the active species, responsible for the actual catalysis, is for a large part still unknown. This thesis describes four years study on the elucidation of this molecular structure. It mainly focuses on the umbrella model (Figure 8.1). This model was introduced by Dr. Gijzeman as an alternative to models already present in the literature. In this model the vanadium atom is linked to the surface via only one Si-Os-V bond and further consists of a V=O and a perturbed O2 molecule linked to the central vanadium atom. Spectroscopic characteristics (Raman, IR) for the umbrella model have been calculated for two types of support, alumina and silica, and compare very well with experimental data found in the literature. From this comparison it could be concluded that the umbrella model is able to assign vibrations to the observed peaks at 900, 980 and 1040 cm?1 and can also explain the width of the peaks. A comparison between several model structures, the umbrella models and three alternative models from literature, has been made against experimental Raman and IR spectra. It could be concluded that the method of synthesis has an influence on the type of active species on the support. Regarding the conversion of methanol to formaldehyde, one of the processes for which this catalyst is used, a plausible reaction mechanism has been presented based on the umbrella model. Basically it involves three steps. In the first step a methanol molecule is converted into formaldehyde and water, with the removal of one oxygen atom from the -VO3 species. In the second step this process is repeated, which leaves a -VO species that can readily reactivated by the attachment of a whole oxygen molecule. For this last step to be possible spin orbit coupling, a relativistic property, is needed. A first study into the relativistic properties of the vibrations of molecules has been done. keywords: supported vanadium oxide catalyst model, theoretical chemistry, Raman, Ir