Accumulation Behavior of Intermediate Products on The Surface of TiO2/ACF Catalysts during The Photocatalytic Oxidation of Toluene: Study of The Influence of Environmental Relative Humidity

Photocatalytic oxidation (PCO) tests have been carried out for toluene adsorbed on theactivated carbon fibers (ACFs) supported TiO2 photocatalyst in an environmental conditioncontrolled chamber. Through exploring the remnant of toluene and the accumulation ofintermediates on the TiO2/ACF catalyst including species, amount and their change processesunder different relative humidity (RH), this study aimed to explore the influence of RH on thePCO of toluene and the role of water vapor in the PCO process: PCO reaction paths and theaccumulation of intermediates on the TiO2/ACF catalyst. Results showed that: (1) with theincrease of RH in the chamber (15%, 30%, 45% and 60%) the PCO conversion rate of toluenewas positive correlated and no catalyst deactivation was observed under all RH; (2) beside theintermediates of side chain oxidation including benzyl alcohol, benzaldehyde and benzoicacid, which had been reported, byproducts of aromatic ring oxidation including p-toluquinoneand o (m, p)-cresol were also observed on the TiO2/ACF catalyst during the gas-solid PCOprocess of toluene; (3) although benzaldehyde was the primary intermediate under all RHlevel, amounts of the byproducts of aromatic ring oxidation were increased with the increaseof RH; (4) elevated RH increased the accumulation of benzyl alcohol but assuredly decreasedthe accumulation of benzaldehyde. These results suggested that: (1) RH affects both the PCOrate and the PCO reaction path of toluene; (2) although methyl group oxidation is the majorpath, aromatic ring oxidation, which is not the expected path for the PCO of toluene, isenhanced when the RH increases; (3) apart from the role of hydroxyl radical (OH) arose fromwater by TiO2, water molecular also directly takes part in the PCO process. A hypothesis hasbeen proposed: transition species comprised of benzaldehyde, hydroxyl and water molecularexists in the PCO conversion process from benzaldehyde to benzoic acid, though thehypothesis has not been confirmed.