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

Catalytic conversion of nitrate (NO 3 − ) pollutants into ammonia (NH 3 ) offers a sustainable and promising route for both wastewater treatment and NH 3 synthesis. Alkali cations are prevalent in nitrate solutions, but their roles beyond charge balance in catalytic NO 3 − conversion have been generally ignored. Herein, we report the promotion effect of K + cations in KNO 3 solution for NO 3 − reduction over a TiO 2 -supported Ni single-atom catalyst (Ni 1 /TiO 2 ). For photocatalytic NO 3 − reduction reaction, Ni 1 /TiO 2 exhibited a 1.9-fold NH 3 yield rate with nearly 100% selectivity in KNO 3 solution relative to that in NaNO 3 solution. Mechanistic studies reveal that the K + cations from KNO 3 gradually bonded with the surface of Ni 1 /TiO 2 , in situ forming a K-O-Ni moiety during reaction, whereas the Na + ions were unable to interact with the catalyst in NaNO 3 solution. The charge accumulation on the Ni sites induced by the incorporation of K atom promoted the adsorption and activation of NO 3 − . Furthermore, the K-O-Ni moiety facilitated the multiple proton-electron coupling of NO 3 − into NH 3 by stabilizing the intermediates.