A DFT study on the structural properties and CO2 electrocatalytic reduction activity of monolayer graphitic carbon nitride supported Ag/Au single atom catalysts

In this study, the DFT has been utilized to research the structural characteristics and CO₂ reduction performances of g-C₃N₄ supported Ag and Au single-atom electrocatalysts, namely Ag-C₃N₄ and Au-C₃N₄ respectively. The constructed structures have been optimized and both the electron density and charge density difference have been calculated, confirming the stability of the constructed single-atom catalysts (SACs). As demonstrated by charge density difference analysis, the CO₂ undergoes chemical adsorption on Ag-C₃N₄ and Au-C₃N₄, resulting in the activation of CO₂. Furthermore, the microscopic mechanisms for the formation of HCOOH, CO, CH₃OH and CH₄ from electrocatalytic CO₂ reduction reaction (CO₂RR) on these SACs have been fully investigated. Specifically, all structures of the reactants, products, and intermediates have been optimized, and their adsorption energy, zero-point energy, and free energy changes have been calculated. Subsequently, the product selectivity differences between Ag-C₃N₄ and Au-C₃N₄ catalysts are compared. Our findings show that Ag-C₃N₄ catalyzes CO₂ to CH₃OH and CH₄ with similar efficiency, while Au-C₃N₄ more effectively facilitates the conversion of CO₂ to CH₃OH over HCOOH and CH₄. Furthermore, by analyzing the free energy change values of rate-determining step for generating four types of C1 products, we find that Ag-C₃N₄ exhibits superior CO₂ catalytic performance over Au-C₃N₄. Moreover, the hydrogen evolution reaction (HER) on these catalysts has been investigated, revealing that HER is inhibited on both catalyst surfaces. This finding emphasizes the preferential electrocatalytic reduction of CO₂ on these catalysts. Through calculations of the energy band and density of state of the catalysts, it is found that the energy gap of Ag-C₃N₄ is smaller than that of Au-C₃N₄. This correlation suggests that a smaller energy gap is indicative of stronger catalytic activity, which is further evidenced by the higher activity of Ag-C₃N₄ in the electrocatalytic reduction of CO₂ compared to Au-C₃N₄. Our results reveal that both Ag-C₃N₄ and Au-C₃N₄ exhibit strong activity in the electrocatalytic CRR. These results establish a theoretical basis for the development of single-atom catalysts (SACs) that can efficiently reduce CO₂ through electrocatalysis.