Metal-N4-Functionalized Graphene as Highly Active Catalysts for C–N Bond Formation in Electrochemical Urea Synthesis

Abstract

Urea is a vital nitrogen-based fertilizer, traditionally produced through energy-intensive processes that consume significant resources and emit substantial CO₂. The electrochemical coreduction of CO₂ and NO₃^–/NO₂^– offers a sustainable alternative, but efficient catalysts are needed to drive this reaction. In this study, density functional theory calculations combined with a constant electrode potential model were employed to investigate the initial C–N bond formation in urea synthesis via the coreduction of CO₂ and NO₃^–. The reaction was driven by a CuN₄ moiety embedded in graphene (gCuN₄), a single-atom catalyst (SAC). Despite the common belief that SACs have a limited ability to facilitate C–N coupling due to the lack of adjacent active sites, our findings show that gCuN₄ can efficiently promote this reaction via coupling of surface-bound *N₁ intermediates, generated from NO₃^– reduction, with CO₂ through the Eley–Rideal mechanism. The calculated free energy barriers are near zero at the experimentally relevant potential of U = −1.0 V_SHE. The facile C–N coupling kinetics are retained when Cu is replaced with other first-row transition metals. Surprisingly, the electron dynamics analysis revealed that in most C–N coupling reactions, one of the two electrons forming the C–N bond originates from the graphene support, underscoring its critical role. Additionally, the high reactivity may be due to the use of high-energy electrons from the graphene support and/or nitrogen in the *N₁ intermediates rather than relying on the inert Cu–N bond electrons to form C–N bonds. Strategies to enhance C–N bond formation as well as methods to preserve the highly active single-atom state are discussed. These insights will contribute to the development of efficient and durable catalysts for sustainable urea synthesis.