Oxygen Isotope Tracing Study to Directly Reveal the Role of O<sub>2</sub> and H<sub>2</sub>O in the Photocatalytic Oxidation Mechanism of Gaseous Monoaromatics
Jiangyao Chen, Jiajing Yi, Weikun Zhu, Weina Zhang, Taicheng An
Abstract
O 2 and H 2 O influence the photocatalytic oxidation mechanism of gaseous monoaromatics, but still in an unclear manner, due to the lack of direct evidence. Tracing an oxygen atom from 16 O 2 and H 2 18 O to intermediates can clarify their roles. The low H 2 18 O content suppressed the formation of benzenedicarboxaldehydes during the oxidation of xylenes and 16 O 2 greatly affected the yield of total intermediates, while neither of them altered the percentage order of the products. Methylbenzaldehydes, methylbenzyl alcohols, and benzenedicarboxaldehydes possessed greater 16 O percentage (≥69.49%), while higher 18 O distribution was observed in methylbenzoic acids and phthalide (≥59.51%). Together with the interconversion results of the products revealed, 16 O 2 determined the transformation of xylenes initially to methylbenzaldehydes and then to methylbenzyl alcohols or benzenedicarboxaldehydes, while H 2 18 O mainly contributed to conversion of methylbenzaldehydes to methylbenzoic acids or phthalide. Further interaction sites of xylene and its products with H 2 O and O 2 were confirmed by molecular dynamics calculations. The same roles of 16 O 2 and H 2 18 O in the degradation of toluene, ethylbenzene, 1,2,4-trimethylbenzene, and 1,3,5-trimethylbenzene were also verified. This is the first report that provides direct evidence for the roles of O 2 and H 2 O in the photocatalytic oxidation mechanism of gaseous monoaromatics. These findings are helpful to achieve controllable product formation from the oxidation of monoaromatics and predict their migration process in the atmospheric environment.