Redox mechanism for selective oxidation of ethanol over monolayer V2O5 TiO2 catalysts

The selective oxidation of ethanol to acetaldehyde and acetic acid over a monolayer V₂O₅/TiO₂ catalyst has been studied in situ using Fourier transform infrared spectroscopy and near-ambient-pressure X-ray photoelectron spectroscopy (XPS) at temperatures ranging from 100 to 300 °C. The data were complemented with temperature-programmed reaction spectroscopy and kinetic measurements. It was found that under atmospheric pressure at low temperatures acetaldehyde is the major product formed with the selectivity of almost 100%. At higher temperatures, the reaction shifts toward acetic acid, and at 200 °C, its selectivity reaches 60%. Above 250 °C, unselective oxidation to CO and CO₂ becomes the dominant reaction. Infrared spectroscopy indicated that during the reaction at 100 °C, nondissociatively adsorbed molecules of ethanol, ethoxide species, and adsorbed acetaldehyde are on the catalyst surface, while at higher temperatures the surface is mainly covered with acetate species. According to the XPS data, titanium cations remain in the Ti⁴⁺ state, whereas V⁵⁺ cations undergo reversible reduction under reaction conditions. The presented data agree with the assumption that the selective oxidation of ethanol over vanadium oxide catalysts occurs at the redox Vⁿ⁺ sites via a redox mechanism involving the surface lattice oxygen species. A reaction scheme for the oxidation of ethanol over monolayer V₂O₅/TiO₂ catalysts is suggested