Exploring the photocatalytic conversion mechanism of gaseous formaldehyde degradation on TiO_2–x-OV surface

To understand the conversion mechanism of photocatalytic gaseous formaldehyde (HCHO) degradation, strontium (Sr)-doped TiO _2–x–OV catalysts was designed and synthesized in this study, with comparable HCHO removal performance. Our results proved that foreign-element doping reduced Ti ⁴⁺ to the lower oxidation state Ti ^{(4– x)+}, and that the internal charge kinetics was largely facilitated by the unbalanced electron distribution. Oxygen vacancies (OVs) were developed spontaneously to realize an electron-localized phenomenon in TiO _2–x–OV, thereby boosting O ₂ adsorption and activation for the enhanced generation of reactive oxygen species (ROS). At the chemisorption stage, in-situ DRIFTS spectra and density functional theory calculation results revealed that surface adsorbed O ₂ (O _ads) and lattice O (O _lat) engaged in the isomerisation of HCHO to dioxymethylene (DOM) on TiO _2–x–OV and TiO ₂, respectively. Time-resolved DRIFTS spectra under light irradiation revealed that the DOM was then converted to formate and thoroughly oxidized to CO ₂ and H ₂O in TiO _2–x–OV. While bicarbonate byproducts were detected from DOM hydroxylation or possible side conversion of CO ₂ in TiO ₂, owing to insufficient consumption of surface hydroxyl. Our study enhances the understanding on the photocatalytic oxidation of HCHO, thereby promoting the practical application in indoor air purification.