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 (gCuN 4 ), 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 gCuN 4 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 VSHE. 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.