Photocatalytic Formaldehyde Oxidation over Plasmonic Au/TiO2 under Visible Light: Moisture Indispensability and Light Enhancement

With an extension of the absorption band toward visible light, plasmonic photocatalysts directly harvest energy from solar light without compromising the activity, offering a desirable way to address energy and environmental issues. Here we demonstrate photocatalytic oxidation of formaldehyde in air over plasmonic Au/TiO2 catalyst under visible light in a single-pass continuous flow reactor. Compared to that under dark, a significant enhancement of up to 5 times the reaction rate at 13% RH under visible light is achieved. Au/TiO2 catalyst exhibits very high activity, a complete conversion of formaldehyde of 83.3% under visible light at 44% RH, but is completely inactive in dry air even under visible light. Also, the plasmonic Au/TiO2 is efficient for photocatalytic oxidation of formaldehyde under visible light, which is evidenced by a slight difference of conversion between UV light and visible light. To disclose the underlying mechanism, in situ diffuse reflectance infrared Fourier transform (DRIFT) spectra studies are conducted. The contributions of TiO2 and Au (supported on TiO2), moisture, and visible light are identified. It is ascertained that moisture is indispensable to carbonate decomposition and also accelerates dioxymethylene (DOM) oxidation to formate. Visible light enhances the rate-determining steps of formate oxidation to carbonate and carbonate decomposition. It appropriately illustrates the remarkable difference in activities. On the basis of the spectra experiments, such a pathway of formaldehyde oxidation is proposed, which undergoes four sequential reaction steps (k1, k2, k3, k4) with reaction conditions dependent on moisture and visible light over Au/TiO2 catalyst. On the basis of the approximately identical spectra, which indicate the same reaction pathways under visible light or under dark, the insights into the mechanism for photocatalytic oxidation of formaldehyde in air under visible light over Au/TiO2, are obtained.