Mechanistic Studies of Methanol Oxidation to Formaldehyde on Isolated Vanadate Sites Supported on High Surface Area Anatase

The mechanism for methanol oxidation on both TiO2 and V/TiO2 was investigated using temperature-programmed experiments with in-situ infrared spectroscopy. Infrared and Raman spectroscopy, along with XANES, show that the V/TiO2 sample consists predominantly of isolated VO4 units after calcination. Methanol was found to adsorb on the catalyst in three ways at 323 K: (1) molecularly, (2) across Ti−O−Ti bonds to form Ti−OCH3/Ti−OH pairs, and (3) across V−O−Ti bonds to form V−OCH3/Ti−OH pairs. Upon heating, two desorption peaks for CH3OH and H2O were observed on all samples below 500 K. Although TiO2 produced small amounts of CH2O, the addition of vanadium greatly enhanced the rate of formaldehyde formation. Also, on the V/TiO2 samples, it was noticed that the Ti−OCH3 groups disappear much more rapidly than on TiO2 alone. This is likely due to the reverse spillover of methoxide species from Ti to V, with the reaction occurring at lower temperatures at the vanadium center. Formate species were also detected during the experiments, and they are assumed to be intermediates in the decomposition of formaldehyde to CO, CO2, and H2O. The apparent activation energy of V/TiO2 for the formation of CH2O is 16 kcal/mol.