Gaseous CO2 Coupling with N-Containing Intermediates for Key C-N Bond Formation during Urea Production from Coelectrolysis over Cu

Abstract

Coelectrolysis of CO2 with simple nitrogen compounds can generate molecules containing C-N bonds, which makes it an appealing method for increasing the value and scope of products obtained from CO2 electrochemical reduction (CO2ER) alone. In this study, we used density functional theory (DFT) calculations combined with a constant electrode potential model to investigate C-N formation pathways in the coreduction of CO2 and NO3-/NO2- to produce urea on Cu(111). Strikingly, we found that the first C-N bond is formed through coupling of gaseous CO2, rather than an intermediate of CO2ER, with the surface-bound N1 intermediates (i.e., *NO2, *NOH, *N, *NH, and *NH2) generated during NO3-/NO2- reduction to NH3. The reaction follows the Eley-Rideal mechanism and requires only a single active site. This result is in contrast with the literature, where the carbon species for C-N coupling were assumed to be intermediates from CO2ER to CO (i.e., *COOH and *CO). Further barrier decomposition analysis indicated that the facile kinetics of C-N coupling involving CO2 are due to the lower energy cost to deform CO2 and the N1 intermediate to the transition-state structure as well as the attractive interaction between them. For these facile and hence important CO2 + N1 reactions, we determined that the kinetic barrier of C-N coupling correlates well with the deformation energy of the N-1 intermediate. Based on these insights, two strategies to improve C-N coupling have been proposed.