RESUMO
Ultraviolet (UV) light irradiation on CeO2 nanocrystals catalysts has been observed to largely increase the material's catalytic activity and reactive surface area. As revealed by x-ray absorption near edge structure (XANES) analysis, the concentration of subvalent Ce3+ ions in the irradiated ceria samples progressively increases with the UV-light exposure time. The increase of Ce3+ concentration as a result of UV irradiation was also confirmed by the UV-vis diffuse reflectance and photoluminescence spectra that indicate substantially increased concentration of oxygen vacancy defects in irradiated samples. First-principle formation-energy calculation for oxygen vacancy defects revealed a valence-hole-dominated mechanism for the irradiation-induced reduction of CeO2 consistent with the experimental results. Based on a Mars-van Krevelen mechanism for ceria catalyzed oxidation processes, as the Ce3+ concentration is increased by UV-light irradiation, an increased number of reactive oxygen atoms will be captured from gas-phase O2 by the surface Ce3+ ions, and therefore leads to the observed catalytic activity enhancement. The unique annealing-free defect engineering method using UV-light irradiation provides an ultraconvenient approach for activity improvement in nanocrystal ceria for a wide variety of catalytic applications.
