This reaction raise many questions, including the mechanism or the electrochemical steps as well as the role of the anatase electrode. We used Density Functional Tight Binding (DFTB ) method to study the interaction between organic acids and possible intermediates of the reduction reaction. For the simulation, models of the (101) surface of anatase as well as nanoparticle models were also used.
TiO2 nanoparticles, unlike surface slab models, always have a strong dipole moment, which interfere with absorption of organic acids as well as the solvent water molecules. The morphology of the nanoparticles has also a strong influence on the bandgap of TiO2: calculations showed an approx. 0.5 eV smaller gap for the nanoparticles. The decrease of the gap were mostly due to the significantly lower LUMO energy levels when they are compared to surface models (Figure 1). These low energy LUMO levels are due to edge effects and are expected to play an important role in the electrochemical reduction over these nanoparticles.
Calculations showed very strong absorption of the acids on the anatase models (in order of 150 kJ/mol). Such a strong absorption of the carboxyl group may actually protect it from the reduction. We hypothesized that one of the carboxyl group of oxalic acid acts as an anchor, while the reduction reaction is taking place mostly on the other side of the molecule. This would also explain why ordinary carboxylic acids like acetic acid cannot be reduced under the same conditions.