Oxalic acid at the TiO2/water interface under UV(A) illumination: Surface reaction mechanisms

• Adsorption and surface photoreaction mechanisms of oxalic acid at rutile and anatase.
• Exploration of excited states of adsorbed species.
• OH radicals produced upon oxalic acid photocatalytic degradation.
• Photocatalytic interconversion of adsorbed species.
• Surface photocatalytic degradation into CO2 only from one specific surface species.

It is through the comparison of experimental results and theoretical calculations that the mechanistic details of several surface photoreactions initiated upon UV(A) illumination of adsorbed oxalic acid on rutile and anatase can be proposed. The absorption of light is found to be rather localized at surface Ti atoms and at the adsorbed species on both TiO2 polymorphs, respectively. Different surface complexes exhibit different photoreactivities, and consequently, each of them may follow a different reaction mechanism. Experimental data can be explained involving reactions such as the interconversion of monodentate into bidentate species which may further be oxidized to CO2 or may even produce OH radicals, while the reduction of monodentate species to the respective aldehyde results in combination with the oxidation of a neighbouring adsorbed OH group into the formation of an adsorbed OOH radical. On the basis of the results presented herein, it is concluded that the direct action of the photocatalytically produced electron–hole pairs on the adsorbed species is the primary step of the photocatalytic reaction, while the intermediate formation of free radical species followed by their reaction with an oxalate molecule can be regarded as a secondary process. Within the system described in this work, OH radicals only appear to be produced following the direct interaction of a hole with the adsorbed organic compound, but not with chemisorbed water molecules.

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