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 C-2 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 Co-5 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 Fe-5 and Co-5 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 Fe-5-NC cluster has better activity than Co-5-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 Fe-5-NC cluster than that in Co-5-NC. However, Co-5-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.