Enhancing Selective Nitrate Reduction to Ammonia by Oxygen Vacancies in Cerium-Modified Copper and Iron Oxides Derived from Metal Organic Frameworks

Abstract

Highly selective and efficient electricity-driven conversion of nitrate (NO3−) to ammonia (NH3) is both environmentally and economically desirable. However, this process remains a significant challenge due to the poor selectivity, low Faradaic efficiency, and catalyst instability and deactivation. In this work, we present a novel method to engineer the oxygen vacancy sites via incorporation of cerium into the copper and iron oxides derived from metal organic frameworks (MOFs). X-ray photoelectron spectroscopy shows that the Ce modification results in the formation of mixed metal oxides with different valence states as well as activated oxygen species originating from oxygen defects. The binding energy peak shifts also demonstrate modulated electron density within the matrix, contributing to the nitrate reduction reactions. It is shown that the heterogeneous catalysts exhibit outstanding electrocatalytic performance with more than 97% selectivity for nitrate reduction to ammonia. In the alkaline electrolyte (1 M KOH and 0.1 M KNO3), the ammonia yield rate is 2428.11 μg h-1 cm-2 (for CeCuOx) and 2928.23 μg h-1 cm-2 (for CeFeOx) at -0.4 V vs. RHE while the peak Faradaic efficiency is 70.0% and 81.9%, respectively, illustrating that the incorporation of Ce renders abundance active sites and defects, enhancing electrical conductivity and proton adsorption ability to gain a superior catalytic performance. This work offers valuable insights into the rational design of high-performance heterogeneous catalysts for biogeochemical cycles, including electrochemical nitrate reduction (e-NO3RR).

Keywords: Nitrogen cycle, heterogeneous catalysis, Defect engineering, Sustainable ammonia production