Revealing electrocatalytic C[sbnd]N coupling for urea synthesis with metal–free electrocatalyst

Abstract

Urea is ubiquitous in agriculture and industry, but its production consumes a lot of energy. The conversion of nitrogen (N2) and carbon dioxide (CO2) into urea via an electrocatalytic CN coupling reaction under ambient conditions would be a major boon to sustainable development. However, designing a metal − free catalyst with high activity and selectivity for urea remains a major challenge. Herein, by means of density functional theory (DFT) and ab − initio molecular dynamics (AIMD) computations, the B12 cluster doped on nitrogenated graphene (C2N) substrate catalyst (B12@C2N) with superior stability was designed for electrocatalytic urea synthesis starting from the CO2 and N2 through four reaction mechanisms. The nature of the co-adsorption activation of CO2 and N2 on the B12@C2N catalyst was investigated, the electrochemical proton − electron transfer steps and the CN thermochemical coupling led to the synthesis of urea. The study showed that the B12@C2N catalyst exhibited high catalytic activity for urea synthesis with the lowest limiting potential of − 1.01 V following the *HNNH mechanism compared with other mechanisms. The potential − determining step (PDS) is the formation of the *CO+*NH2NH2 species. However, the two − step CN coupling barriers of *NCON species are 0.13 eV and 0.60 eV using AIMD and a “slow − growth” sampling approach in an explicit water molecules model. Calculations also showed that the byproducts of carbon monoxide (CO), methane (CH4), methanol (CH3OH), ammonia (NH3), and hydrogen (H2) can be inhibited on the B12@C2N catalyst. Therefore, the metal − free catalyst not only has a good performance for the hydrogenation of CO2 and N2 promoting the electrochemical reaction, but also facilitates CN thermochemical coupling for urea synthesis. This work provides new insights into the synthesis of urea via the CN coupling reaction on a metal − free electrocatalyst, a process that could contribute to greenhouse gas mitigation to help meet carbon neutrality targets.