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

The mechanism for methanol oxidation on both TiO 2 and V/TiO 2 was investigated using temperature-programmed experiments with in-situ infrared spectroscopy. Infrared and Raman spectroscopy, along with XANES, show that the V/TiO 2 sample consists predominantly of isolated VO 4 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−OCH 3 /Ti−OH pairs, and (3) across V−O−Ti bonds to form V−OCH 3 /Ti−OH pairs. Upon heating, two desorption peaks for CH 3 OH and H 2 O were observed on all samples below 500 K. Although TiO 2 produced small amounts of CH 2 O, the addition of vanadium greatly enhanced the rate of formaldehyde formation. Also, on the V/TiO 2 samples, it was noticed that the Ti−OCH 3 groups disappear much more rapidly than on TiO 2 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, CO 2, and H 2 O. The apparent activation energy of V/TiO 2 for the formation of CH 2 O is 16 kcal/mol.