Reaction mechanism of photooxidation of propane over alkali-metal-ion-modified silica-supported vanadium pentaoxide under irradiation by visible light

The active site and the intermediates in the photooxidation of propane over alkali-metal-ion-modified V2O5/SiO2 (VS-m) have been investigated and a possible mechanism has been proposed. Phosphorescent emission spectra and ab initio calculations suggest that the vanadium species in the catalyst is excited to the triplet state by irradiation by visible light, and the non-bonding orbitals of the oxygen atoms which interact with the alkali-metal ion in the vanadium species become unoccupied by the excitation. A stabilization energy has been observed for the interaction of the oxygen atom in the vanadium species in the triplet state with the hydrogen atom in propane, i.e. the oxygen atom in the vanadium species is the active site for the photooxidation. Adsorption experiments of propane and oxygen over the catalyst confirmed that the catalyst first activates propane under irradiation by visible light; when propane has been adsorbed on the catalyst, oxygen is photoadsorbed to form an intermediate comprised of one molecule of propane and one molecule of oxygen. In situ FTIR spectra showed that the intermediate formed by the photoadsorption of propane is isopropoxide-like; the intermediate is transformed to propanone immediately in the presence of gaseous oxygen under irradiation by visible light.