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

Catalytic conversion of nitrate (NO) pollutants into ammonia (NH) offers a sustainable and promising route for both wastewater treatment and NH synthesis. Alkali cations are prevalent in nitrate solutions, but their roles beyond charge balance in catalytic NO conversion have been generally ignored. Herein, we report the promotion effect of K cations in KNO solution for NO reduction over a TiO-supported Ni single-atom catalyst (Ni/TiO). For photocatalytic NO reduction reaction, Ni/TiO exhibited a 1.9-fold NH yield rate with nearly 100% selectivity in KNO solution relative to that in NaNO solution. Mechanistic studies reveal that the K cations from KNO gradually bonded with the surface of Ni/TiO, in situ forming a K-O-Ni moiety during reaction, whereas the Na ions were unable to interact with the catalyst in NaNO solution. The charge accumulation on the Ni sites induced by the incorporation of K atom promoted the adsorption and activation of NO. Furthermore, the K-O-Ni moiety facilitated the multiple proton-electron coupling of NO into NH by stabilizing the intermediates.