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

To afford high efficient catalytic oxidation alcohols activity, enhancement of the visible light photocatalytic activity of CeO2 have attracted great attention. On the photocatalytic oxidation of alcohols, the chemisorbed oxygen on the CeO2 nanostructure play an important promoter role. The deep understand on the formation and the detail roles of chemisorbed oxygen on the CeO2 nanostructure are therefore highly desirable. In this work, we successfully obtained the CeO2 photocatalysts that provide different concentrations of chemisorbed oxygen through hydrolyzing Ce(NO3)3 in the saturated atmosphere of Ar, air, and O2 (denoted as CeO2-Ar, CeO2-Air, and CeO2-O2, respectively). The visible light (≥400 nm) photocatalytic performance of 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 CeO2-Ar and CeO2-Air respectively, owing to their varying concentrations of chemisorbed oxygen on CeO2-Ar, CeO2-Air, and CeO2-O2 photocatalysts. Based on our detail characterizations relative to the chemisorbed oxygen, the enhancement mechanism of chemisorbed oxygen was proposed for CeO2 photocatalysts: First, the O2 molecules were chemisorbed on Ce(III) sites, forming a Ce peroxo structure, which was beneficial to the harvesting visible light for CeO2 photocatalysts. Whereafter, this Ce peroxo structure can easily capture the photoexcited electrons, resulting in the O2•− which collaboratively activate the photocatalytic oxidation reaction with holes. Furthermore, the Ce(III) defects sites will chemisorb oxygen involved in the reaction system to produce the ceria peroxo structure again. Such recycle procedure will undoubtedly increase the utilization efficiency of chemisorbed oxygen and thus enhanced the visible light photocatalytic activity of CeO2.

为实现高效的醇类催化氧化性能，提升二氧化铈(CeO2)的可见光光催化活性已受到广泛关注。在醇类光催化氧化过程中，二氧化铈纳米结构表面的化学吸附氧(chemisorbed oxygen)发挥着关键的促进作用。因此，深入阐明二氧化铈纳米结构表面化学吸附氧的形成机制与具体作用，极具研究必要性。

本研究通过在氩气(Ar)、空气与氧气(O2)的饱和气氛下水解硝酸铈(III)(Ce(NO3)3)，成功制备出具有不同化学吸附氧浓度的二氧化铈光催化剂，分别记为CeO2-Ar、CeO2-Air与CeO2-O2。在苯甲醇的选择性氧化反应中，CeO2-O2光催化剂的可见光（≥400 nm）光催化性能相较于CeO2-Ar与CeO2-Air分别提升了1.54倍与1.43倍，其对反应的选择性与转化率均接近100%，这一性能差异源于三类催化剂表面化学吸附氧浓度的差异。

基于针对化学吸附氧的详细表征结果，本研究提出了二氧化铈光催化剂的化学吸附氧增强机制：首先，氧气分子在三价铈(Ce(III))位点发生化学吸附，形成过氧铈结构，该结构可助力二氧化铈光催化剂捕获可见光；随后，该过氧铈结构可高效捕获光激发电子，生成超氧自由基(O2•−)，后者与空穴协同激活光催化氧化反应。此外，三价铈缺陷位点可吸附反应体系中的氧气，再次形成过氧二氧化铈结构。这一循环过程无疑提升了化学吸附氧的利用效率，进而增强了二氧化铈的可见光光催化活性。