Dynamics of Interfacial Charge Transfer to Formic Acid, Formaldehyde, and Methanol on the Surface of TiO2 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 TiO2 with methanol, formaldehyde, and formic acid (compounds that, together with methane, have been observed in the photocatalytic reduction of CO2). The ultrafast dynamics of hole scavenging was found to be an order of magnitude faster on the surface of TiO2 than in the corresponding homogeneous systems. Additionally, the equilibrium constant for the reaction of photogenerated electrons in TiO2 with adsorbed CO2 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 ((OCOH)-O-center dot), and formyl radicals, respectively. For methanol and formaldehyde only photo-oxidation, 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.