Identifying the Origin of Ti3+ Activity toward Enhanced Electrocatalytic N2 Reduction over TiO2 Nanoparticles Modulated by Mixed-Valent Copper.

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 (VO ) 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 mgcat. -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 eg and t2g orbitals, which can be modulated by its coordination structure. The synergistic roles of the three ion pairs formed by the VO defect, including Cu1+ -Ti4+ , Ti3+ -Ti4+ and Ti3+ -Ti3+ , are together responsible for the enhanced NRR performance.