Surface Adsorption and Photochemistry of Gas-Phase Formic Acid on TiO₂ Nanoparticles: The Role of Adsorbed Water in Surface Coordination, Adsorption Kinetics, and Rate of Photoproduct Formation

Formic acid adsorption and photooxidation on TiO₂ nanoparticle surfaces at 296 K have been investigated using transmission FTIR spectroscopy. In particular, the role of adsorbed water in surface coordination, adsorption kinetics, and photoproduct formation is examined. Gas-phase formic acid adsorbs on the surface at low exposures to yield adsorbed bridged bidentate formate and, at higher exposures, molecularly adsorbed formic acid as well. Upon exposure to water vapor, adsorbed formate becomes solvated by coadsorbed water molecules, and the coordinatin mode changes as indicated by shifts in the vibrational frequencies. Adsorbed water also impacts the adsorption kinetics for formic acid on TiO₂ and increases the adsorption rate, potentially by providing a medium for facile ionic dissociation. Ultraviolet irradiation of adsorbed formate on TiO₂ in the presence of molecular oxygen results in the formation of gas-phase carbon dioxide, which increases in yield in the presence of adsorbed water on the surface. Additionally, the dispersion of TiO₂ nanoparticles in water suspensions is found to change if first exposed to gas-phase formic acid before dispersion. The environmental implications of these results are discussed