Theoretical investigations of transition metal atom-doped MoSi₂N₄ monolayers as catalysts for electrochemical CO₂ reduction reactions

Following the principle of single-atom catalysts (SACs), the fourth-period transition metals (TM) were designed as active sites on a MoSi₂N₄ monolayer surface with N vacancy, and the catalytic mechanisms of these single-atom active sites for the conversion of CO₂ to CO were investigated by first-principles calculations. Our results showed that the doped TM atoms on the MoSi₂N₄ surface significantly enhanced the CO₂ reduction reaction (CO₂RR) activity compared with the pristine MoSi₂N₄ monolayer. Our findings after analyzing all the doped structures in our work were as follows: (1) the Sc-, Ti-, and Mn-doped structures exhibited very low limiting potentials; (2) out of Sc-, Ti- and Mn-doped structures, the Mn@MoSi₂N₄-N_v structure showed the best catalytic performance with a limiting potential of only -0.16 V, exhibiting an advantage over the hydrogen evolution reaction, which is a competitive reaction of CO₂RR. However, the positive binding free energy at 298.15 K of the intermediate reactant *COOH on the Mn@MoSi₂N₄-N_v surface indicated its unstable state, which hinders the CO generation process. This is contrary to the results derived from the adsorption/binding energy at 0 K, which indicated that the effect of temperature cannot be ignored when considering adsorption/binding energy. Our work provides insights into the effects of temperature on the catalytic mechanisms for CO₂RR through TM-doped MoSi₂N₄ monolayers.