Theoretical study on the synthesis of urea by series electrocatalysis of lithium main group embedded in COF structure

Replacement of the Harber-Bosch process for urea production by electrochemical reduction is an attractive alternative that reduces chemical energy consumption, reduces environmental pollution and improves efficiency. Among them, the method of electrochemical coupling of N2 and CO2 to produce urea has been widely used, but the inertness of nitrogen and nitrogen bond and high dissociation energy make it difficult to activate N2. Meanwhile, the method of synthesis of urea by NO3RR came into being. Nitrate in sewage is more abundant and convenient than nitrogen raw material, and more importantly, the dissociation energy of NO bond is much lower than that of NN. In this study, the transition metal embedded lithium COF catalyst was taken as an example, and the optimal electrocatalyst was selected through screening and synthesis energy barrier comparison to study the mechanism of urea synthesis. We studied the NRR and NO3RR paths, respectively, and found that the NRR synthesis of urea was optimal on the LiTaS-Pc VPPs catalyst, with a limiting potential value of −0.57V, and the NO3RR synthesis of urea was optimal on the LiVS-Pc VPPs catalyst, with a limiting potential value of −0.39V. It is found that NO3RR on the lithium catalyst is relatively better, and the required limit potential value is lower. In addition, the advantage of choosing lithium metal doping as catalyst in our study is that HER can be well inhibited, and the desorption of urea is easier than that of transition metal doping (taking molybdenum as an example). This study points out the direction for the electrocatalytic synthesis mechanism of urea, and provides a new idea for the selection of catalysts, paving the way for the future production of urea. This paper investigated the mechanism of urea synthesis using NRR and NO3RR on lithium-based COF series catalysts (LiTMS-Pc VPPs), respectively. Among them, the adsorption of reactive nitrogen, C–N coupling, and urea synthesis are mainly carried out on LiTMS-Pc. The process of generating CO via CO2RR on VPPs overflows onto the phthalocyanine structure and undergoes C–N coupling with nitrogen-containing intermediates to synthesize urea. Comparing and organizing the two methods of urea synthesis, NO3RR is relatively superior. In addition, a comparison was made between the synthesis of lithium-based catalysts and transition metal molybdenum catalysts, and it was found that the lithium-based catalyst improved urea removal. [Display omitted] •The synthesis