Chemistry and Photochemistry of Pyruvic Acid Adsorbed on Oxide Surfaces

Abstract

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).

Conformer-Specific Photolysis of Pyruvic Acid and the Effect of Water

“…Since the gas-phase photolysis of pyruvic acid is specific to the Tc conformer, which, unlike the Tt conformer, has an intramolecular hydrogen bond, its photochemistry can be modified by a hydrogen bonding environment such as water or dust. 108,109 We demonstrated that the presence of water decreased the relative population of the Tc conformer in the dark (Figures 7 and 8 and Tables S14 and S15) and dramatically decreased the photolysis rate constants of PA tot and Tc (Figures 5 and 6 and Figure S6 and Table 1 and Table S13). Our computational results explain these experimental observations by showing that water has the ability to change the relative abundances of the Tc and Tt conformers by decreasing the energy difference between them, to act as a donor and acceptor to hydrogen bonding, and to disrupt the intramolecular hydrogen bond of Tc.…”

Indoor Surface Chemistry: Developing a Molecular Picture of Reactions on Indoor Interfaces

“…50 These organic acids can undergo interesting reactions on surfaces, such as pyruvic acids on oxide surfaces. 70 Additionally, gases that partition into the aqueous phase on damp surfaces, such as glyoxal, can form different products (e.g., oxalic acid) than those that would form in the gas phase (e.g., formaldehyde). 50 Another set of important organic gases that displays an acid-base behavior indoors is obtained from environmental tobacco smoke (ETS).…”

On the Rise: Experimental and Computational Vibrational Sum Frequency Spectroscopy Studies of Pyruvic Acid and Its Surface-Active Oligomer Species at the Air–Water Interface

“…Aqueous PA favorably forms larger-molecular-weight oligomeric species, as demonstrated in Figure B, where the polar medium lessens the tendency to form intramolecular hydrogen bonds and enhances intermolecular hydrogen bonding . In fact, the formation of such oligomeric species is favorable enough to occur on metal oxide surfaces where unhydrated PA reacts with surface hydroxyl groups to produce pyruvate, which can go on to yield several higher-molar-mass oligomeric species …”

“…Aqueous secondary organic aerosols (aqSOA) are among the most ubiquitous environmental systems and have far-reaching impacts on the climate. Solvated organics are known to have significant roles in the formation, structure, and characteristics of aqSOA. − However, much of the aqueous phase processing of these organics remains poorly understood. ,,, Many of the small atmospheric carbonyls found in aqSOA can further react in the aqueous phase to form hydrated species and oligomers. ,− Due to the complexity of these systems, the molecular nature of many of these organics at aerosol interfaces is largely unknown. ,,− This is particularly true for pyruvic acid (PA). Beyond its biological significance, PA is also one of the most abundant atmospheric secondary organic ketoacids and is known to be involved in aqSOA formation. − ,− Because of this, PA has been studied extensively in this context and as a model atmospheric α-dicarbonyl.…”

“…Furthermore, PA has recently been established to favorably form oligomeric species. ,,,,, Vaida’s group has pioneered the characterization and mechanistic studies of PA and its oligomer species, demonstrating that, due to the variety of favorable pathways through which each monomeric hydration species can react, these higher-order oligomer products are numerous. ,, The abundance of these reaction products can vary greatly depending on solution and environmental conditions (such as pH , and photochemistry − ), and some of these oligomer species are also in equilibrium with favorable hydration species . Aqueous PA favorably forms larger-molecular-weight oligomeric species, as demonstrated in Figure B, where the polar medium lessens the tendency to form intramolecular hydrogen bonds and enhances intermolecular hydrogen bonding .…”