Chemistry and Photochemistry of Pyruvic Acid Adsorbed on Oxide Surfaces

The surface chemistry and photochemistry of gas-phase pyruvic acid (CH₃COCOOH) on two oxides, Al₂O₃ and TiO₂, have been investigated using transmission Fourier transform infrared spectroscopy and mass spectrometry. At 298 K, the carboxylic acid group within pyruvic acid is found to react with surface hydroxyl groups (M-OH, M = Al, Ti) to yield pyruvate as a predominant adsorbed organic species. Upon broad-band UV irradiation (λ > 280 nm), there is a loss of adsorbed pyruvate with the concomitant formation of new products. The photochemical loss of pyruvate is higher on TiO₂ than on Al₂O₃ indicating that the photochemistry is enhanced on the surface of a semiconductor oxide, TiO₂, compared with an insulator oxide, Al₂O₃. Analysis of products extracted from the surface with mass spectrometry shows the formation of several new compounds. This includes zymonic acid, which is found to be present under both dark and light conditions, and other higher-molar-mass oligomeric species such as parapyruvic acid, acetolactic acid, and 2,4-dihydroxy-2-methyl-5-oxohexanoic acid that form only under irradiation. Although this study shows that there are some parallels between the aqueous-phase photochemistry of pyruvic acid and the photochemistry of adsorbed pyruvic acid in terms of the products that form, there are also distinct differences, with several other new photoproducts observed on these oxide surfaces, including lactic acid dimers and trimers as well as significant amounts of even larger oligomeric species not seen in the aqueous phase. Because of the role of pyruvic acid, the simplest of the α-keto acids, in the atmosphere and in metabolic pathways, these results have implications for the chemistry that occurs in both indoor and outdoor environments and under prebiotic Earth conditions. Overall, this study provides insights into the surface chemistry and photochemistry of pyruvic acid on different oxides (Al₂O₃ and TiO₂).