Asymmetric iron–titanium pairs within ultrathin TiO2 nanosheets enable high-efficiency nitrate reduction to ammonia

Abstract

Electrochemical reduction of nitrate (NO₃RR) is emerging as an efficient alternative method for decentralized ammonia (NH₃) synthesis. However, the selectivity of nitrate reduction towards ammonia suffers from competitive hydrogen evolution owing to its preferred thermodynamics of hydrogen evolution. Generally, the localized electronic structure of active species is crucial for the adsorption of pivotal intermediates, thus altering catalytic efficiency and selectivity. Herein, monodispersed iron atoms were precisely anchored onto ultrathin TiO₂ nanosheets (Fe₁–TiO₂), bringing iron and titanium into close proximity and forming asymmetric Fe–Ti atom pairs. It yielded a maximal NH₃ faradaic efficiency of 90.1% and production rate of 2.2 mmol per hour per mg of catalyst (mmol h⁻¹ mg⁻¹), surpassing the performance observed over symmetric Ti–Ti pairs in bare TiO₂. Theoretical calculations elucidated the intrinsic charge transfer within the Fe–Ti atom pair, which significantly enhanced the selective adsorption of nitrate rather than hydrogen. Moreover, in situ infrared spectroscopy and theoretical calculations clarified that the delocalized electrons within the Fe–Ti pairs could effectively transfer to the antibonding orbital of nitrate and synergistically promote kinetic N–O activation. Consequently, the asymmetric Fe–Ti pairs within Fe₁–TiO₂ substantially promoted the optimal catalytic activity for nitrate reduction.