Electrocatalytic Urea Synthesis via C-N Coupling from CO2 and Nitrogenous Species.

Abstract

ConspectusIndustrial urea synthesis consists of the Haber-Bosch process to produce ammonia and the subsequent Bosch-Meiser process to produce urea. Compared to the conventional energy-intensive urea synthetic protocol, electrocatalytic C-N coupling from CO2 and nitrogenous species emerges as a promising alternative to construct a C-N bond under ambient conditions and to realize the direct synthesis of high-value urea products via skipping the intermediate step of ammonia production. The main challenges for electrocatalytic C-N coupling lie in the intrinsic inertness of molecules and the competition with parallel side reactions. In this Account, we give an overview of our recent progress toward electrocatalytic C-N coupling from CO2 and nitrogenous species toward urea synthesis.To begin, we present the direct transformation of dinitrogen (N2) to the C-N bond by coelectrolysis, verifying the feasibility of direct urea synthesis from N2 and CO2 under ambient conditions. In contrast to the highly endothermic step of proton coupling in conventional N2 reduction, the N2 activation and construction of the C-N bond arise from a thermodynamic spontaneous reaction between CO (derived from CO2 reduction) and *N═N* (the asterisks represent the adsorption sites), and the crucial *NCON* species mediates the interconversion of N2, CO2, and urea. Based on theoretical guidance, the effect of N2 adsorption configurations on C-N coupling is investigated on the model catalysts with defined active site structure, revealing that the side-on adsorption rather than the end-on one favors C-N coupling and urea synthesis.Electrocatalytic C-N coupling of CO2 and nitrate (NO3-) is also an effective pathway to achieve direct urea synthesis. We summarize our progress in the C-N coupling of CO2 and NO3-, from the aspects of modulating intermediate species adsorption and reaction paths, monitoring irreversible and reversible reconstruction of active sites, and precisely constructing active sites to match activities and to boost the electrocatalytic urea synthesis. In each case, in situ electrochemical technologies and density functional theory (DFT) calculations are carried out to unveil the microscopic mechanisms for the promotion of C-N coupling and the enhancement of urea synthesis activity. In the last section, we put forward the limitations, challenges, and perspectives in these two coupling systems for further development of electrocatalytic urea synthesis.