Enhancement of the visible-light photocatalytic activity of CeO2 by chemisorbed oxygen in the selective oxidation of benzyl alcohol

Abstract

To afford highly efficient catalytic oxidation of alcohols, the enhancement of the visible-light photocatalytic activity of CeO₂ has attracted significant attention. With regard to the photocatalytic oxidation of alcohols, chemisorbed oxygen on CeO₂ nanostructures plays an important promoter role. Therefore, a deeper understanding regarding the formation and detailed role of chemisorbed oxygen on the CeO₂ nanostructure is highly desirable. In this study, we successfully obtained CeO₂ photocatalysts that yielded chemisorbed oxygen at different concentrations via hydrolyzing Ce(NO₃)₃ under the saturated atmospheres of Ar, air, and O₂ (denoted as CeO₂–Ar, CeO₂–air, and CeO₂–O₂, respectively). The visible-light (≥400 nm) photocatalytic performance of the CeO₂–O₂ photocatalysts (∼100% selectivity and conversion) in the selective oxidation of benzyl alcohol was enhanced by a factor of 1.54 and 1.43 as compared to those of the CeO₂–Ar and CeO₂–air samples, respectively, owing to the varying concentrations of chemisorbed oxygen on the CeO₂–Ar, CeO₂–air and CeO₂–O₂ photocatalysts. Based on our detailed characterizations relative to chemisorbed oxygen, the enhancement mechanism of chemisorbed oxygen was proposed for CeO₂ photocatalysts. At first, the O₂ molecules were chemisorbed at the Ce(III) sites, forming a Ce peroxo structure, which facilitated the harvesting of visible light by CeO₂ photocatalysts. Thereafter, this Ce peroxo structure could easily capture photoexcited electrons, yielding O₂˙⁻ that collaboratively activated the photocatalytic oxidation reaction with holes. Furthermore, the Ce(III) defect sites chemisorbed the oxygen involved in the reaction system to produce the Ce peroxo structure again. This recycling procedure can undoubtedly increase the utilization efficiency of chemisorbed oxygen and thus enhance the visible-light photocatalytic activity of CeO₂.