Theoretical Study on Urea Synthesis from N2 and CO2 Catalyzed by Electrochemical Tandem Catalysis of CCFs Materials

Abstract

On account of the activation of N₂ and the high-energy barrier of the competitive hydrogen evolution reaction (HER), problems such as low Faraday efficiency, low urea yield, and slow synthesis speed are the bottlenecks of urea synthesis at present. The proper design of catalysts, especially electrocatalysts, is a challenge to improve the efficiency of urea production and to fully exploit its key properties. Because of its stronger electron holding capacity and wider π-electron system than that of mononuclear metal phthalocyanine, binuclear metal phthalocyanine has great application prospects in electrochemical catalytic reduction reactions. This paper anchors the two-dimensional conjugated covalent organic framework (2D c-CCFs) at the center of M–Nx–C as an electrocatalyst for urea synthesis, and these 2D c-CCFs (MoM–Pc–MnN₄, M = Cr, Fe, Mn, Tc, Re) are composed of metal phthalocyanine (MoM–Pc) and MnN₄ units. The activation of N₂ occurs at the bimetallic site of MoM₁–Pc. After the formation of CO on the M₂N₄ structural fragment, CO overflows onto the surface of MoM₁–Pc and is coupled with activated nitrogen to generate urea. The descriptors were screened in four steps to obtain five possible catalyst structures among 20 tandem catalysts: MoCr–Pc–MnN₄–CCFs, MoFe–Pc–MnN₄–CCFs, MoMn–Pc–MnN₄–CCFs, MoRe–Pc–MnN₄–CCFs, MoTc–Pc–MnN₄–CCFs. According to the calculation of DFT, the optimal catalyst and the optimal path were screened in the comparison of the urea path determination step. It was concluded that the optimal catalyst MoFe–Pc–MnN₄–CCFs has the lowest limiting potential (U_L = − 0.18 V) in the series catalytic synthesis of urea, and it could well inhibit HER. This indicates that the catalyst structure has high NRR selectivity and experimental feasibility. The adsorption mode of N₂ in this paper is mainly connected to the active site in the side-on mode. By comparing the calculated adsorption energy values, there is a strong adsorption energy of N₂ (− 1.32 eV) on the surface of MoFe–Pc–MnN₄–CCFs, and the length of the N≡N bond is extended to 1.22Å. It illustrated that N₂ adsorption and activation on the catalyst surface are enhanced. Comparing the C–N coupling barrier of the key step of urea synthesis, it is found that the kinetic barrier of *CO and *NH₂NH₂ coupling (E_a = 0.29 eV) is lower than that of *CO and *N₂ coupling (E_a = 0.85 eV), indicating that the C–N coupling mode is not limited to the coupling between *CO and *N₂, which provides a wider selectivity for urea synthesis. Our research offers a valid catalyst design strategy for improving the performance of Mo-based materials for the electrocatalytic synthesis of urea.

Graphical Abstract