Unveiling the role of oxygen vacancies on different crystal planes of ceria in catalytic toluene oxidation: Evidence from molecular dynamics and in situ DRIFTS

Oxygen vacancy defect engineering is an efficient tactic in metal oxide catalysts for the oxidation of volatile organic compounds (VOCs). Herein, a series of CeO2 catalysts with different dominant crystal planes ((111), (220), (311), and (200)) were successfully synthesized via a facile one-step hydrothermal method. The catalytic oxidation performance for toluene followed the order: (111) > (220) > (311) > (200). Molecular dynamics simulation directly demonstrated that toluene molecule was more readily adsorbed on the (111) crystal planes of CeO2 at actual reaction temperature. The characterization results revealed the local microenvironment of oxygen vacancy could be tailored by different exposed crystal planes, in which (111) crystal planes could induce more coordinative unsaturated sites and then generate more Ce3+-V-O-Ce4+ sites. The in-situ DRIFT spectra further elucidated the catalytic toluene oxidation pathways over these CeO2 catalysts. For the NC-CeO2 (200) catalyst, the insufficient oxygen vacancies made it difficult for gaseous oxygen to backfill onto the surface of catalyst, which induced dehydrogenation of methyl group was the rate-determining step. While the toluene on the surface of NP-CeO2 (111) could be immediately transformed into benzyl alcohol. With the assistance of electrophilic oxygen species, the benzyl alcohol was further oxidized into benzaldehyde, benzoic acid, maleic anhydride, H2O, and CO2. This work provides valuable insights into the design of efficient CeO2 catalysts for VOCs degradation and advances the understanding of their catalytic mechanisms.