Photocatalytic Reaction in Aqueous Suspension: FTIR Spectroscopy with Attenuated Total Reflection in Diamonds

The photocatalytic conversion of an organic compound on rutile nanoparticles dispersed in aqueous solutions was characterized by infrared absorption spectroscopy. A diamond prism for total reflection of infrared light provided convenient and reliable access to the absorption spectrum of adsorbed chemical species photocatalytically converted under ultraviolet light irradiation. Pivalic acid, a reactant to be decarboxylated by hole capture, was dissolved in water at concentrations of 100-3 mmol L^-1 and exhibited vibrational bands of 0.01-0.001 absorbance in the 1500-1100 cm^-1 wave number region. When rutile particles were suspended in the solutions, dissociative adsorption leading to the formation of pivalate anions on the particles was detected in vibrational spectra. The adsorbed pivalate anions decomposed by ultraviolet light irradiation through the prism, releasing CO₂. In an anaerobic atmosphere, the excited electrons were accommodated in the particles as small polarons, resulting in an optical absorption centered at 7000 cm^-1. Conversely, in an aerobic atmosphere, the electrons were transferred to the surrounding atmosphere, eliminating the polaron-induced absorption. This study demonstrates the feasibility of infrared absorption spectroscopy for operando monitoring of vibrational and electronic transitions, enabling the tracking of photochemical reactions at liquid-solid interfaces.