Unusual C-N coupling mechanism via NO dimerization for the electrochemical synthesis of urea on transition metal dimers supported by two-dimensional expanded phthalocyanine

Abstract

Electro-synthesis of urea is a powerful alternative to traditional demanding industrial engineering by selective reduction of N2 and CO2. Due to the existence of inert NN bond, usual C-N coupling has always hindered the design and performance of catalysts. Herein, we present an unusual C-N coupling mechanism for highly electrochemical catalytic synthesis of urea via NO dimerization on transition metal dimers supported by two-dimensional expanded phthalocyanine (TMTM-Pc) as a prototype through first principles calculations. The key is that the adsorbed *NO dimer can directly jump over challenging step of N-N bond breaking and associate with CO. Particularly, FeFe-Pc is singled out in terms of activity, selectivity, and stability via proposed reaction pathways. What's more, we discovered an easy descriptor, namely, the difference of the adsorption energies of *NO-CO-NO* and *NO-NO*, ∆E(*NO-CO-NO* - *NO-NO*), to consummately depict the thermal-kinetic relationships for the C-N coupling. This work may broaden the understanding of urea formation mechanism and provide more options for future research.