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

The oxidation of methanol on both ZrO2 and V/ZrO2 was investigated using temperature-programmed experiments together with in situ infrared spectroscopy. Characterization of V/ZrO2 after calcination by Raman spectroscopy and XANES shows that the vanadium is present as isolated VO4 units in a distorted tetrahedral geometry. Methanol was found to adsorb dissociatively on both Zr−O−Zr and V−O−Zr species to create Zr−OCH3/Zr−OH and V−OCH3/Zr−OH pairs, respectively. Upon heating, CH3OH and H2O desorb initially from all samples. Above 423 K, surface formate species are detected while H2 and CO are the main products formed on ZrO2. Upon addition of vanadium, CH2O production increases dramatically during temperature-programmed reaction. The absence of CH2O during temperature-programmed desorption and oxidation experiments on V/ZrO2 is likely due to rapid readsorption of the product onto the ZrO2 support, leading to the formation of formate species and H2. The apparent activation energy for V/ZrO2 is 18 kcal/mol. The activities of isolated vanadate species supported on SiO2, TiO2, and ZrO2 are compared and discussed.