Identifying the Origin of Ti³⁺ Activity toward Enhanced Electrocatalytic N₂ Reduction over TiO₂ Nanoparticles...

The ambient electrocatalytic N₂ reduction reaction (NRR) enabled by TiO₂ has attracted extensive recent attention. Previous studies suggest the formation of Ti³⁺ in TiO₂ can significantly improve the NRR activity, but it still remains unclear what kinds of Ti³⁺ are effective. Herein, it is demonstrated that mixed-valent Cu acts as an effective dopant to modulate the oxygen vacancy (V_O ) concentration and Ti³⁺ formation, which markedly improves the electrocatalytic NRR performance. In 0.5 m LiClO₄ , this electrocatalyst attains a high Faradic efficiency of 21.99% and a large NH₃ yield of 21.31 µg h^-1 mg_cat. ^-1 at -0.55 V vs reversible hydrogen electrode, which even surpasses most reported Ti-based NRR electrocatalysts. Using density function theory calculations, it is evidenced that mixed-valent Cu ions modulate the TiO₂ (101) surface with multiple oxygen vacancies, which is beneficial for generating different Ti³⁺ 3d¹ defect states localized below the Fermi energy. N₂ activation and adsorption are effectively strengthened when Ti³⁺ 3d¹ defect states present the splitting of e_g and t_2g orbitals, which can be modulated by its coordination structure. The synergistic roles of the three ion pairs formed by the V_O defect, including Cu¹⁺ -Ti⁴⁺ , Ti³⁺ -Ti⁴⁺ and Ti³⁺ -Ti³⁺ , are together responsible for the enhanced NRR performance.