Ethanol Oxidation Reaction Catalyzed by Palladium Nanoparticles Supported on Hydrogen-Treated TiO₂ Nanobelts: Impact of Oxygen Vacancies

Nanocomposites based on palladium nanoparticles deposited onto TiO₂ nanobelts were prepared by chemical reduction of Pd(II) precursors and the catalytic activity towards ethanol oxidation reaction (EOR) was examined and compared within the context of TiO₂ oxygen vacancies formed by thermal annealing at controlled temperatures (400–600 °C) in a hydrogen atmosphere. Transmission electron microscopic measurements showed that the Pd nanoparticles (about 5 nm in diameter) were clustered somewhat on the surfaces of hydrogen-treated TiO₂ nanobelts (Pd/hTiO₂), but distributed rather evenly on the untreated ones (Pd/TiO₂). X-ray photoelectron spectroscopic studies suggested electron transfer from Ti to Pd in the Pd/hTiO₂ samples, as compared to untreated Pd/TiO₂, due to the formation of oxygen vacancies in TiO₂ nanobelts where the concentration increased with increasing thermal annealing temperature, as evidenced in electron paramagnetic resonance measurements. Significantly, electrochemical measurements showed markedly enhanced EOR activity of both Pd/TiO₂ and Pd/hTiO₂ in alkaline media, as compared to commercial Pd/C, and the activity increased drastically with the concentration of oxygen vacancies, most likely because oxygen vacancies facilitated the formation of hydroxyl species on the TiO₂ surface that played a critical role in the oxidation of ethanol to acetate.