Single Mn atom modulated molecular oxygen activation over TiO₂ for photocatalytic formaldehyde oxidation

In single-atom catalysts, the atomically dispersed metal sites are pivotal for oxygen molecule activation. We hypothesize that dispersing single Mn atoms on TiO₂ nanosheets may improve the photocatalytic oxidation of formaldehyde (HCHO) in the gas phase under ambient conditions. Density function theory (DFT) and experimental experiments were carried out to single Mn atoms not only improved the transfer of localized electrons and photogenerated electrons but also enhanced the activation/dissociation of O₂ to generate monoatomic oxygen ions (O^-) as the final reactive oxygen species (ROS). In photocatalytic experiments, Mn/TiO₂ photocatalyst removed 100 % of HCHO at a low concentration of 7.6 ppm, and reaching excellent mineralization efficiency of over 99.6 %. According to the proposed reaction mechanism, O₂ spontaneously adsorbs onto the Mn/TiO₂ surface, forming two adsorbed O^- after electron donation into the π_2p* antibonding orbitals of O₂. The adsorbed O^- then reacts with gaseous HCHO to produce the key intermediate dioxymethylene (DOM), finally fulfilling a more favorable oxidation process on the Mn/TiO₂ surface. This research illustrates the key role of O^- in HCHO oxidation and paves the way for practical HCHO removal using TiO₂-based photocatalysts.