DFT Study on the CO₂ Reduction to C₂ Chemicals Catalyzed by Fe and Co Clusters Supported on N-Doped Carbon

The catalytic conversion of CO₂ to C₂ products through the CO₂ reduction reaction (CO₂RR) offers the possibility of preparing carbon-based fuels and valuable chemicals in a sustainable way. Herein, various Fe_n and Co₅ clusters are designed to screen out the good catalysts with reasonable stability, as well as high activity and selectivity for either C₂H₄ or CH₃CH₂OH generation through density functional theory (DFT) calculations. The binding energy and cohesive energy calculations show that both Fe₅ and Co₅ clusters can adsorb stably on the N-doped carbon (NC) with one metal atom anchored at the center of the defected hole via a classical MN₄ structure. The proposed reaction pathway demonstrates that the Fe₅-NC cluster has better activity than Co₅-NC, since the carbon-carbon coupling reaction is the potential determining step (PDS), and the free energy change is 0.22 eV lower in the Fe₅-NC cluster than that in Co₅-NC. However, Co₅-NC shows a better selectivity towards C₂H₄ since the hydrogenation of CH₂CHO to CH₃CHO becomes the PDS, and the free energy change is 1.08 eV, which is 0.07 eV higher than that in the C-C coupling step. The larger discrepancy of d band center and density of states (DOS) between the topmost Fe and sub-layer Fe may account for the lower free energy change in the C-C coupling reaction. Our theoretical insights propose an explicit indication for designing new catalysts based on the transition metal (TM) clusters supported on N-doped carbon for multi-hydrocarbon synthesis through systematically analyzing the stability of the metal clusters, the electronic structure of the critical intermediates and the energy profiles during the CO₂RR.