Why copper catalyzes electrochemical reduction of nitrate to ammonia

Abstract

Electrochemical reduction of nitrate (NO₃RR) has drawn significant attention in the scientific community as an attractive route for ammonia synthesis as well as alleviating environmental concerns for nitrate pollution. To improve the efficiency of this process, the development of catalyst materials that exhibit high activity and selectivity is of paramount importance. Copper and copper-based catalysts have been widely investigated as potential catalyst materials for this reaction both computationally and experimentally. However, less attention has been paid to understanding the reasons behind such high activity and selectivity. Herein, we use Density Functional Theory (DFT) to identify reactivity descriptors guiding the identification of active catalysts for the NO₃RR, establish trends in activity, and explain why copper is the most active and selective transition metal for the NO₃RR to ammonia among ten different transition metals, namely Au, Ag, Cu, Pt, Pd, Ni, Ir, Rh, Ru, and Co. Furthermore, we assess NO₃RR selectivity by taking into account the competition between the NO₃RR and the hydrogen evolution reaction. Finally, we propose various approaches for developing highly active catalyst materials for the NO₃RR.