Decomposition pathways of glycolic acid on titanium dioxide

Fourier-transformed infrared spectroscopy has been employed to study the adsorption, thermal reactions and photodegradation of glycolic acid (HOCH₂COOH) on TiO₂ in a gas-solid system. The intriguing research focus is the reactivity and evolution of the two functional groups. Glycolic acid can exist on TiO₂ at 35 °C in two dissociative adsorption forms, OCH₂COOH and HOCH₂COO, which are derived from hydrogen loss of the COH and COOH groups, respectively. Heating the surface to a temperature higher than ∼100 °C causes a largely enhanced carbonyl stretching band at ∼1750 cm^-1, indicative of oxidation of the OCH₂ groups of the surface glycolic acid molecules. This chemical process is supported by the adsorption of glyoxylic acid (HCOCOOH) on TiO₂. As the surface temperature is further increased to 200 °C or higher, formate (HCOO) and methoxy (CH₃O) are produced. Their formation is proposed via dioxymethylene (OCH₂O) intermediate. CO and CO₂ are found to be the final thermal products. Photoirradiation of a TiO₂ surface covered with glycolic acid at ∼325 nm leads to its decomposition, generating CO₂, CH₃O, HCOO and carbonate species. O₂ is found to promote the photochemical reactions of glycolic acid on TiO₂ to form CO₂, HCOO and carbonates. O₂ may play a role hampering recombination of photogenerated electron-hole pairs and participating in the formation of CO₂ and HCOO.