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Transition metal anchored on C9N4 as a single-atom catalyst for CO2 hydrogenation: A first-principles study

To alleviate the greenhouse effect and maintain the sustainable development, it is of great significance to find an efficient and low-cost catalyst to reduce carbon dioxide (CO2) and generate formic acid (FA). In this work, based on the first-principles calculation, the catalytic performance of a single transition metal (TM) (TM = Cr, Mn, Fe, Co, Ni, Cu, Zn, Ru, Rh, Pd, Ag, Cd, Ir, Pt, Au, or Hg) atom anchored on C9N4 monolayer (TM@C9N4) for the hydrogenation of CO2 to FA is calculated. The results show that single TM atom doping in C9N4 can form a stable TM@C9N4 structure, and Cu@C9N4 and Co@C9N4 show better catalytic performance in the process of CO2 hydrogenation to FA (the corresponding maximum energy barriers are 0.41 eV and 0.43 eV, respectively). The partial density of states (PDOS), projected crystal orbital Hamilton population (pCOHP), difference charge density analysis and Bader charge analysis demonstrate that the TM atom plays an important role in the reaction. The strong interaction between the 3d orbitals of the TM atom and the non-bonding orbitals (1π g) of CO2 allows the reaction to proceed under mild conditions. In general, our results show that Cu@C9N4 and Co@C9N4 are a promising single-atom catalyst and can be used as the non-precious metals electrocatalyst for CO2 hydrogenation to formic acid.