The positive effect of water on acetaldehyde oxidation depended on the reaction temperature and MnO2 structure

The effect of H2O on acetaldehyde oxidation was studied and α-, γ-, δ-MnO2 can be affected by the H2O vapor, while β-MnO2 is hardly affected by the H2O vapor during acetaldehyde oxidation. The H2O vapor can facilitate the catalytic oxidation of acetaldehyde at moderate temperatures (ca. 70∼100 °C), with a negative effect observed at high temperatures (>100 °C). The stretching mode of the Mn−O−Mn chains in α-, γ-, δ-MnO2 allows the associatively adsorbed water to absorb on the surface prior to acetaldehyde. Afterwards, acetaldehyde can form hydrogen bonds with the associatively adsorbed water molecules, thus, producing the intermediate carboxylic acid species, which are easily degraded to CO2. The negative effect of H2O on the acetaldehyde oxidation at high temperatures is attributed to the difficulty in adsorbing acetaldehyde on MnO2 via hydrogen bonding with the adsorbed H2O. These findings reveal the role of water in the catalytic oxidation of acetaldehyde. [Display omitted] •A positive effect of H2O on acetaldehyde oxidation over MnO2 at moderate temperature.•The positive effect of H2O is attributed to the Mn−O−Mn structure of MnO2.•The associatively adsorbed H2O on Mn−O−Mn structure hydrogen bonded with acetaldehyde.•A negative effect of H2O on acetaldehyde oxidation over MnO2 at high temperature.•The acetaldehyde cannot hydrogen bond with the adsorbed H2O at high temperature.