Isotope tracing study to directly reveal the roles of H2O’s oxygen and hydrogen atoms in photocatalytic oxidation of gaseous aromatic alcohols to aromatic products

H 2 O shows a significant contribution to form oxygen- and hydrogen-containing products during photocatalytic oxidation of aromatic hydrocarbons. However, it was unclear what the roles of atoms in H 2 O in subsequent oxidation of these products. In this study, the photocatalytic oxidation of two aromatic alcohols, benzyl alcohol and phenethyl alcohol, on TiO 2 (79 % anatase and 21 % rutile) were investigated in mixed H 2 O (or H 2 18 O, D 2 O) and O 2 system. Aromatic aldehydes, acids and esters were main aromatic products, while modified TiO 2 and isotope-containing H 2 O did not change the product species. Aromatic aldehydes showed ≤ 0.19 % and ≤ 1.58 % of 18 O- and D-labeling rates, suggesting hardly involvement of H 2 O in their formation. Aromatic acids exhibited a 18 O-labeling rate range of 28.12 %-40.33 % and the highest D-labeling rate of 0.76 %, while a maximum enhancement of 18 O-labeling rate was 0.11 % in the –C=O group of the acids, indicating that H 2 O selectively provided O atom in the formation of –OH group of aromatic acids. The 18 O- and D-labeling rates of aromatic esters were in the range of 9.01 % to 15.95 % and 1.73 % to 8.08 %, suggesting that both oxygen and hydrogen atoms from H 2 O contributed to aromatic ester formation. Further analyzing mass fragmentation confirmed that H 2 O separately contributed its hydrogen and oxygen atoms to the –C=O and R–O groups of the esters. Based on the isotope tracing results, the contribution roles of H 2 O’s hydrogen and oxygen atoms in oxidation of aromatic alcohols to corresponding aromatic products were proposed. The findings in this study help to clarify the role of H 2 O in the oxidation process of aromatic alcohols at the atomic level and comprehensively reveal the oxidation mechanism of the aromatics.