Electrochemical Oxidation of Nitrogen towards Direct Nitrate Production on Spinel Oxides.

Nitrates are widely used as fertilizer and oxidizing agents. Commercial nitrate production from nitrogen is mainly through the Haber-Bosch (H-B) process and Ostwald process, both involving high-temperature-high-pressure processes. Therefore, establishing an alternative nitrate production method under ambient environment is of paramount importance. Herein, an electrochemical nitrogen oxidation reaction (NOR) approach is developed to produce nitrate catalyzed by ZnFexCo2-xO4 spinel oxides in an alkaline electrolyte. As revealed by theoretical and experimental results, Fe is found to facilitate the formation of the first N-O bond on the *N site, while high oxidation state Co assists in stabilizing the absorbed OH- for the successful generation of the second and third N-O bonds. Owing to the concerted catalysis, the ZnFe0.4Co1.6O4 oxide demonstrates the highest nitrate production rate of 130 +/- 12 mumol h-1 gMO-1 at an applied potential of 1.6 V vs. reversible hydrogen electrode (RHE). This work provides a choice for energy-efficient and environmentally friendly production of nitrate.