Structure–Activity Relationship in Structurally Diverse Cobalt Oxide Catalysts for the Electrochemical Reduction of Nitrate to Ammonia

Abstract

The electrochemical nitrate reduction reaction (NO3–RR) offers a sustainable method for converting NO3– waste into indispensable ammonia to rebalance the nitrogen cycle, which is considered more severely disturbed than the carbon cycle. Strenuous efforts have already yielded highly active Cu-based catalysts for the NO3–RR; however, experimental and computational studies focusing solely on Cu active sites are no longer sufficient to elucidate and control the complicated NO3– RR pathways. In this study, we selected several simple and complex cobalt oxides as model NO3–RR catalysts to exploit their rich structural diversity and clarify the fundamental structure–activity relationship. These compounds were systematically selected according to the CoO6 octahedral linkages: edge-sharing CoO, Co3O4, LiCoO2, NaxCoO2; face-sharing Ca3Co2O6, Sr6Co5O15, and BaCoO3; and corner-sharing LaCoO3. X-ray diffraction and Co K-edge X-ray absorption spectroscopy revealed that the Co–Co distances (dCo–Co) were distinct among face-, edge-, and corner-sharing oxides, i.e., approximately 2.5, 2.9, and 3.8 Å, respectively. This order was inversely reflected in the NO3–RR activity, namely, the electrode-area-normalized ammonia formation rate (face > edge > corner). Consequently, despite the different Co valence states, a clear linear correlation was identified. Density functional theory calculations on representative CoO, BaCoO3, and LaCoO3 unveiled that the initial NO3– adsorption step in the NO3–RR differed in the adsorption energies and geometries, thereby rationalizing a simple structural descriptor, dCo–Co, governing the NO3–RR activity. This study demonstrates a solid methodology for developing more robust catalysts and advancing next-generation CO2 and NO3– coreduction technologies.