Pathways of Photocatalytic Oxidation of Formic Acid on Dry and Hydrated Anatase TiO₂ Surfaces

The photocatalytic oxidation of formic acid (FA), which is one of the most abundant volatile organic compounds, is a promising air remediation technology inspired by nature. However, the detailed mechanism of this photocatalytic reaction on the surface of TiO₂, a typical photocatalyst, is not yet well-understood. In this work, we present a computational mechanistic study of the thermal vs photocatalytic oxidation of FA on dry and hydrated anatase TiO₂ (101) surfaces, based on periodic hybrid density functional theory (DFT) calculations, in which the photo-oxidation is treated as an excited-state process in a constrained triplet spin state. We first compare the adsorption modes of FA on the anatase (101) surface in the ground and excited states, followed by identification of the corresponding reaction intermediates that lead to the formation of CO₂. We unveil the pivotal role of photogenerated holes localized at surface under-coordinated oxygen sites in mediating the C-H bond cleavage, thereby promoting CO₂ formation through a highly stable intermediate and an exergonic reaction step. Further investigation of the effect of coadsorbed water molecules shows that hydrogen bonding with water stabilizes FA in a monodentate configuration. This is favored over the unreactive bidentate structure that is the most stable under dry conditions, thus providing insight into the experimentally observed increase of the reaction rate in the presence of water.