N2 Oxidation to Nitrate via a Phase Transition-Based Pathway

Abstract

Nitric acid is essential in fertilizer production, yet its current industrial synthesis via the Haber-Bosch and Ostwald process accounts for 1 % of the chemical sector’s energy consumption and produces significant greenhouse gas emissions. Despite considerable efforts, previous attempts at electrochemical nitrogen oxidation have proven unsuccessful due to competition with the oxygen evolution reaction (OER) and the inherent difficulty of activating the nitrogen (N2) bond under ambient conditions. In this work, we propose a phase transition-based approach inspired by electrochemical ammonia synthesis to circumvent these challenges. The strategy is centered around the formation of a metal nitride by direct nitridation of a suitable metal, followed by an oxidation step employing molecular oxygen (O2). Li and Ca are identified as particularly promising candidates to mediate such a pathway based on reaction thermodynamics, and we show experimentally that Li nitride can indeed be oxidized to yield nitrate in the presence of O2. The proposed concept demonstrates considerable potential for direct N2 oxidation, encouraging further computational and experimental investigation.