Selective Electrochemical Nitrate Reduction to Ammonia on Co-Based Catalysts

Abstract

About 150 million metric tons of ammonia (NH 3 ) were produced in 2023 mainly for agricultural purposes as a fertilizer [1]. This compound is industrially produced by the conventional Haber-Bosch process under high temperatures (350 – 550 °C) and high pressure (150 – 350 atm), thus, its production accounts for 1.8 tons of CO 2 emissions per ton and 2% of the energy consumed in the world [2]. These factors have led to an interest in developing green alternative methods that allow the production of large amounts of NH 3 . Therefore, it has been demonstrated that the electrocatalytic synthesis of NH 3 is a viable route that can be developed via the reduction of nitrate (NO 3 - ) to simultaneously mitigate its pollution in water sources, which has become a major human health concern when it is consumed [3]. Since the electrochemical reduction of NO 3 - involves a complex eight – electron process, it is vital to design a highly selective, active, and stable catalyst. In this work, the performance of Co-based catalysts in alkaline conditions was evaluated and our studies demonstrated that a three-dimensional (3D) Co substrate can achieve a faradaic efficiency (F.E.) of up to 86% at +0.023 V vs RHE (Reversible Hydrogen Electrode) with a partial current density of 23 mA cm -2 . However, in the presence of an excess of NH 3 , Co corrosion is promoted while creating a Co-NH 3 complex. In order to avoid this, the thermo-oxidation of Co foam at 400, 600, and 800 °C was performed, proving that a larger oxide layer improves the stability as well as increases the active surface area of the electrode. Furthermore, the samples could reach greater current densities when subjected to a pre-reduction treatment. The sample oxidized at 600 °C presented superior performance, achieving a F.E. of 66% at +0.023V vs RHE, a partial current density of 170 mA cm -2 , and approximately 8 h of stability. This work has provided an opportunity to explore the influence of the oxide layer on NH 3 synthesis, as well as the necessity of advancing novel strategies to enhance the selectivity and stability of the proposed catalyst. [1] Production of ammonia worldwide from 2010 to 2023. Statista (2024). Retrieved from: https://www.statista.com/statistics/1266378/global-ammonia-production/#:~:text=Ammonia%20production%20has%20remained%20fairly,approximately%2064.6%20million%20metric%20tons. [2] Wang, J., Cai, C., Wang, Y., Yang, X., Wu, D., Zhu, Y., Li, M., Gu, M., & Shao, M. (2021). Electrocatalytic Reduction of Nitrate to Ammonia on Low-Cost Ultrathin CoOx Nanosheets. ACS Catalysis, 11(24), 15135–15140. https://doi.org/10.1021/acscatal.1c03918 [3] Xu, H., Ma, Y., Chen, J., Zhang, W., & Yang, J. (2022). Electrocatalytic reduction of nitrate: a step towards a sustainable nitrogen cycle. Chemical Society Reviews, Issue 7. https://pubs.rsc.org/en/content/articlelanding/2022/cs/d1cs00857a