Unravelling the water oxidation mechanism on NaTaO₃-based photocatalysts

Understanding the detailed kinetics and mechanisms of water oxidation is crucial because water oxidation is the bottleneck reaction in overall water splitting. Here, we investigated photocatalytic water oxidation on NaTaO₃-based photocatalysts. With in situ FT-IR spectroscopy, two new water oxidation intermediates with IR absorption peaks centered at 1058 and 935 cm^-1 were observed and assigned to Ta(vi)O and Ta(O₂) peroxo species, respectively. With H₂¹⁸O as the reactant and NaTa¹⁶O₃ as the photocatalyst, both ¹⁶O¹⁸O and ¹⁸O₂ were evolved continuously from overall water splitting as analyzed by mass spectroscopy (MS), demonstrating that the lattice O atoms in NaTaO₃-based photocatalysts participate in the formation of O₂ during the reaction. DFT results are consistent with the formation of TaO, ¹⁶O¹⁸O and ¹⁸O¹⁸O intermediates and products, while predicting that several water oxidation pathways are thermodynamically accessible for water oxidation on NaTaO₃. Based on these observations, we propose a three-step two-site mechanism for photocatalytic water oxidation on NaTaO₃-based photocatalysts in which lattice oxygen plays a key role. The elucidation of the present mechanism should help in broadly understanding water oxidation on semiconductor-based photocatalysts.