Mechanistic Insights into Selective Acetaldehyde Formation from Ethanol Oxidation on Hematite Photoanodes by Operando Spectroelectrochemistry

This study employed operando spectroelectrochemical l and photoelectrochemical methods to investigate the charge carrier dynamics of photogenerated holes in hematite for ethanol oxidation and its possible over-oxidation. Ethanol oxidation was found to form acetaldehyde with around 100 % initial selectivity and faradaic efficiency. The overoxidation of acetaldehyde was suppressed by being unable to kinetically compete with ethanol oxidation in terms of turnover frequency by a factor of ten. Temperature-dependent rate law analyses were applied to determine the activation energies of these two oxidations. For the ethanol oxidation, the activation energy was 195 meV, compared to 398 meV for acetaldehyde oxidation. These results were correlated with the valence band potential to elucidate the advantage of using hematite for safer and sustainable value-added aldehyde synthesis compared to the industrial method. The dynamics of ethanol oxidation also addressed the challenges in broad-spectrum deep oxidation of organic compounds in water purification using metal oxides.