An experimental and theoretical investigation of the structure and reactivity of bilayered VO_x/TiO_x/SiO₂ catalysts for methanol oxidation

This study reports the results of a combined experimental and theoretical investigation of a bilayered VO_x/TiO_x/SiO₂ catalyst consisting of vanadia deposited onto silica containing a submonolayer of titania. Raman spectroscopy indicates that Ti atoms are bonded to the silica support via Ti{single bond}O{single bond}Si bonds, and Raman and EXAFS data indicate that the vandia is present as isolated vanadate groups bonded to the support through V{single bond}O{single bond}Si and V{single bond}O{single bond}Ti bonds. For a fixed vanadia surface density (0.7 V/nm²), the turnover frequency for methanol oxidation to formaldehyde increases with increasing Ti surface density (0.2-2.8 Ti/nm²) and the apparent activation energy decreases. These trends are well represented by a model of the active site and its association with Si and Ti atoms of the support. This model takes into account the distribution of Ti on the silica support, the fraction of active sites with 0, 2, and 3 V{single bond}O{single bond}Ti support bonds, and the rate parameters determined for each of these active sites determined from quantum chemical calculations and absolute rate theory.