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

This study reports the results of a combined experimental and theoretical investigation of a bilayered VOx/TiOx/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 Tisingle bondOsingle bondSi bonds, and Raman and EXAFS data indicate that the vandia is present as isolated vanadate groups bonded to the support through Vsingle bondOsingle bondSi and Vsingle bondOsingle bondTi 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 Vsingle bondOsingle bondTi support bonds, and the rate parameters determined for each of these active sites determined from quantum chemical calculations and absolute rate theory.