Dynamics of interfacial charge transfer to formic acid, formaldehyde, and methanol on the surface of TiO ₂ nanoparticles and its role in methane production

Transient absorption and electron paramagnetic resonance (EPR) spectroscopies were used to study reactions of photogenerated electrons and holes on TiO ₂ with methanol, formaldehyde, and formic acid (compounds that, together with methane, have been observed in the photocatalytic reduction of CO ₂). The ultrafast dynamics of hole scavenging was found to be an order of magnitude faster on the surface of TiO ₂ than in the corresponding homogeneous systems. Additionally, the equilibrium constant for the reaction of photogenerated electrons in TiO ₂ with adsorbed CO ₂ was estimated to be less than 3.2 M ^-1, regardless of the presence of hole scavengers and product molecules. Formic acid serves as both the hole and the electron acceptor, yielding the protonated radical anions (OC ^•OH), and formyl radicals, respectively. For methanol and formaldehyde only photooxidation, but no one-electron photoreduction, was observed by EPR spectroscopy; these molecules are either reduced in a two-electron process or act only as hole scavengers.