Enhanced nitrate reduction to ammonia using Cu-Ni catalyst: Synergistic mechanisms and reaction pathways

Abstract

• Cu 5 Ni@NC electrode shows excellent eNO 3 RR performance, with 99.19 % NO 3 - removal efficiency and 75.03 % NH 4 + -N selectivity. • Ni enhances aquatic active hydrogen generation, facilitating oxynitride hydrogenation during eNO 3 RR. • Ni shifts the catalyst's d-band center, improving nitrate and intermediate adsorption. • A detailed nitrate-to-ammonia conversion pathway is revealed through in-situ spectroscopic analysis. Accelerated industrialization combined with over-applied nitrogen fertilizers results in serious nitrate pollution in surface and ground water, disrupting the balance of the global nitrogen cycle. Electrochemical nitrate reduction (eNO 3 RR) emerges as an attractive strategy to simultaneously enable nitrate removal and decentralized ammonia fabrication, restoring the globally perturbed nitrogen cycle. However, complex deoxygenation-hydrogenation processes and sluggish proton-electron transfer kinetics significantly hinder practical application of eNO 3 RR. In this study, we developed carbon-coated Cu-Ni bimetallic catalysts derived from metal-organic frameworks (MOFs) to facilitate eNO 3 RR. The unique structural features of catalyst promote enhanced synergy between Cu and Ni, effectively addressing critical challenges in nitrate reduction. Comprehensive structural and electrochemical analysis demonstrate that electrochemical nitrate-to-nitrite conversion mainly takes place on active Cu sites, the introduction of Ni could efficiently accelerate the generation of aquatic active hydrogen, promoting the hydrogenation of oxynitrides during eNO 3 RR. In addition, Ni introduction could push up the d-band center of the catalyst, thus enhancing the adsorption and activation of nitrate and the corresponding intermediates. Detailed reaction pathways for nitrate-to-ammonia conversion are illuminated by rotating disk electrode (RDE), in-situ Fourier-transform infrared spectroscopy, in-situ Raman spectrum and electrochemical impedance spectroscopy (EIS). Benefiting from the synergistic effect of Cu and Ni, optimum catalyst exhibited excellent nitrate reduction performance. This work provides a new idea for elucidating the underlying eNO 3 RR reaction mechanisms and contributes a promising strategy for designing efficient bimetallic electrocatalysts.

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