Efficient C-N coupling in the direct synthesis of urea from CO₂ and N₂ by amorphous Sb_xBi_1-xO_y clusters.

Abstract

Although direct generation of high-value complex molecules and feedstock by coupling of ubiquitous small molecules such as CO₂ and N₂ holds great appeal as a potential alternative to current fossil-fuel technologies, suitable scalable and efficient catalysts to this end are not currently available as yet to be designed and developed. To this end, here we prepare and characterize Sb_xBi_1-xO_y clusters for direct urea synthesis from CO₂ and N₂ via C-N coupling. The introduction of Sb in the amorphous BiO_x clusters changes the adsorption geometry of CO₂ on the catalyst from O-connected to C-connected, creating the possibility for the formation of complex products such as urea. The modulated Bi(II) sites can effectively inject electrons into N₂, promoting C-N coupling by advantageous modification of the symmetry for the frontier orbitals of CO₂ and N₂ involved in the rate-determining catalytic step. Compared with BiO_x, Sb_xBi_1-xO_y clusters result in a lower reaction potential of only -0.3 V vs. RHE, an increased production yield of 307.97 μg h^-1 mg^-1_cat, and a higher Faraday efficiency (10.9%), pointing to the present system as one of the best catalysts for urea synthesis in aqueous systems among those reported so far. Beyond the urea synthesis, the present results introduce and demonstrate unique strategies to modulate the electronic states of main group <i>p</i>-metals toward their use as effective catalysts for multistep electroreduction reactions requiring C-N coupling.