(526e) Electrochemical C-N Bond Formation: A Density Functional Theory Study of Urea Synthesis on Cu and Cu Alloys

The increasing demand for urea as a common commodity has motivated investigation of alternative synthesis strategies in recent years. Urea is industrially synthesized under conditions of high temperature and pressure, thus consuming substantial energy and leaving a significant carbon footprint. Electrocatalytic synthesis strategies, in contrast, could facilitate heterogeneous co-reduction of NO_x and CO_x to urea in voltage-driven reactors at ambient conditions with significantly lesser energy input. However, to realize the potential of this approach, fundamental knowledge of the electrocatalytic mechanism and catalytic structure-function relationships must be enhanced.

In this work, we combine Periodic Density Functional Theory (DFT) calculations with electrokinetic analyses to elucidate the mechanism of the urea synthesis in an aqueous electrochemical environment. Motivated by reports of electrocatalytic reduction of small molecules in the literature, we begin our analysis on single crystal Cu(111) and Cu(211) surfaces, as urea synthesis has been observed at ~-0.75V vs. SHE on Cu-loaded gas diffusion electrode. We propose a comprehensive mechanism involving adsorbate hydrogenation and coupling through multiple reaction pathways, ultimately leading to urea and non-selective products. The energetic parameters associated with corresponding elementary steps are obtained from DFT, and voltage dependence of the proton-coupled electron transfer steps is determined using Nernstian thermodynamics. Initial NO and CO hydrogenation and their coupling over catalytic surfaces are found likely to be rate-limiting reactions (Figure 1), as subsequent steps are thermodynamically spontaneous. Further, we analyze the changes in reactant binding energies, as well as associated changes in reaction mechanisms, that occur when copper surfaces are doped with various elements to form single atom alloys (Figure 2). We assess the impact of these alloys on product selectivity, and predict which such alloys will be stable under aqueous acidic conditions. We close by outlining future challenges that must be overcome to realize practical synthesis of urea under electrochemical conditions.