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 N1 intermediate. Based on these insights, two strategies to improve C–N coupling have been proposed.

Copper with an atomic-scale spacing for efficient electrocatalytic co-reduction of carbon dioxide and nitrate to urea

“…S32 and Tables S3, S4 (ESI†). Consistent with the previous theoretical study by some of us, 39 C–N couplings of the five N 1 species with CO 2 are kinetically more facile than those with *COOH or *CO (Fig. 4b) for d s = 6, 7, and 8 Å and the bare surface.…”

“…38 Similarly, Cheng and co-workers also discovered the formation of urea on the Cu(111) surface through the C-N coupling mechanism from the co-reduction of gaseous CO 2 and NO 3 À /NO 2À . 39 According to the results of the DFT calculations, we assume that Cu is a unique catalyst that has a suitable binding energy for the formation of C-N bonds for urea synthesis.…”

Activity and Selectivity Roadmap for C–N Electro-Coupling on MXenes

“…This corresponds well with the selectivity problem and many byproducts generated in experiments. [11][12][13]18 On the other hand, we did find a green region that is thermodynamically more favored toward C−N coupling. For the materials in this area, the coupling between *NH 2 and *CO is more facile than the competing reaction, i.e., *NH 2 to *NH 3 or *CO to *CHO.…”

“…18 Due to the complexity of possible nitrogen precursors and unclear mechanism of C−N coupling reactions, the design and evaluation of new selective catalysts are quite challenging. Moreover, the process of discovering materials still relies heavily on trial-and-error methods, 13 which adds an additional layer of difficulty.…”

“…More importantly, such an electrochemical approach can reduce the dependency on fossil fuels in traditional industrial approaches, making it environmentally friendly and sustainable. It has been widely acknowledged that the initial C–N bond formation step is critical in determining C–N coupling activity and selectivity. − However, there still remain many debates about the C–N bond formation step, particularly in regard to the coupling precursor. The carbon precursor is widely recognized as *CO from in situ spectroscopy results and theoretical calculations, − while the nitrogen precursor remains uncertain.…”