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.