Mechanism and Reaction Kinetics of Formaldehyde Oxidation on TiO₂: Amorphous Hydroxylated Outperforms Anatase

Addressing low-occurrence problems from low concentrations, we demonstrate amorphous TiO_2–0.5x(OH)_x nanofilms (amT) that feature abundant hydroxyl groups (of bulk and surface) for formaldehyde oxidation under UVC irradiation. Such hydroxyl groups can significantly enrich bound water (of adsorbed and produced) and formaldehyde at a hydration layer on the surface. With this structure, mechanistic insights and reaction kinetics are correlated. Compared to the anatase one (anT), our amT exhibits a remarkable decrease in the recombination of charge carriers, complete formaldehyde oxidation even at dry conditions, a nearly zero-order reaction that elucidates the approximately independent kinetics, and an unprecedentedly 10⁴-fold increase in the reaction rate. Based on photoluminescence and Fourier transform infrared spectroscopy, light indispensability and moisture enhancement to formaldehyde oxidation are corroborated. Consequently, due to abundant hydroxyl groups, the mass transfer (of formaldehyde and water) at its hydration layer is strengthened, as evidenced by the mechanistic insights; hence, fascinating kinetics is achieved for environmental applications in practice.