Element-dependent effects of alkali cations on nitrate reduction to ammonia

Catalytic conversion of nitrate (NO₃^-) pollutants into ammonia (NH₃) offers a sustainable and promising route for both wastewater treatment and NH₃ synthesis. Alkali cations are prevalent in nitrate solutions, but their roles beyond charge balance in catalytic NO₃^- conversion have been generally ignored. Herein, we report the promotion effect of K⁺ cations in KNO₃ solution for NO₃^- reduction over a TiO₂-supported Ni single-atom catalyst (Ni₁/TiO₂). For photocatalytic NO₃^- reduction reaction, Ni₁/TiO₂ exhibited a 1.9-fold NH₃ yield rate with nearly 100% selectivity in KNO₃ solution relative to that in NaNO₃ solution. Mechanistic studies reveal that the K⁺ cations from KNO₃ gradually bonded with the surface of Ni₁/TiO₂, in situ forming a K-O-Ni moiety during reaction, whereas the Na⁺ ions were unable to interact with the catalyst in NaNO₃ solution. The charge accumulation on the Ni sites induced by the incorporation of K atom promoted the adsorption and activation of NO₃^-. Furthermore, the K-O-Ni moiety facilitated the multiple proton-electron coupling of NO₃^- into NH₃ by stabilizing the intermediates.