摘要
By regulating the electronic environment of Fe active centers to modulate electrocatalytic CO 2 reduction behavior, an advanced dual Fe 2 -site catalyst (Fe 2 DAC) exhibiting CO current density (j CO ) of 10 mA cm −2 at an overpotential of 330 mV in a CO 2 -saturated 0.5 M KHCO 3 electrolyte was designed and characterized by HAADF-STEM microscopy and XAS/EPR spectroscopies. With regard to Fe 2 DAC displaying a higher charge transfer coefficient (α = 0.53), turnover frequency (TOF CO = 2.03 s −1 ), and CO faradaic efficiency (FE CO = 98.6%) at an overpotential of 400 mV compared to that of single iron atom catalyst (Fe SAC, α = 0.31, TOF CO = 0.25 s −1 , FE CO = 60.1%), the kinetic mechanism was investigated/elucidated by the cation effect and in operando spectroscopy. The higher DMPO-CO 2 EPR intensity (g = 2.0065, a N = 15.6 G, a H = 18.9 G) and the smaller separation of ATR-SEIRAS stretching frequencies (1554 (ν asym ), 1288 (ν sym ) cm −1 , Δν = 266 cm −1 ) suggest that the structural type of the [*COOCs]˙/[*COOCs] − intermediate is μ 2 -η3 CO 2 coordination (class II) for Fe 2 DAC-triggered electrocatalytic CO 2 reduction in CO 2 -saturated CsHCO 3 solution. The strong orbital interaction among the dual Fe 2 site, intermediate [CO 2 ]˙ − /[CO 2 ] 2− , and Cs + cation (6s orbital) is proposed to accelerate charge transfer kinetics and shift the rate-determining step from the electron transfer step (Li + , Na + ) to the protonation step (K + , Cs + ), as evidenced by Cs + -induced increase in the proton reaction order (0.86) and Cs + -induced decrease in the kinetic Tafel slope (57.2 mV dec −1 ) and electrochemical activation energy (23.7 kJ mol −1 ). In contrast, the structural transformation from the dual Fe II 2 motif to a single Fe II site revealed by the disappearance of the Fe-Fe distance (3.10 Å) in operando Fe K edge EXAFS lends support to the absence of stretching frequencies (1429 (ν asym ), 1380 (ν asym ), 1241 (ν asym ) cm −1 ) ascribed to μ 2 -η 2 CO 2 coordination in a CO 2 -saturated LiHCO 3 aqueous medium, demonstrating that the transformation of [*COOCs] − /[*COOK] − into the bridge [CO 2 ] 2− [Fe-μ-C( = O)O-Fe] is vital for electrocatalytic CO 2 -to-CO conversion. In addition to identifying the dinuclear Fe 2 II site as a catalytic center, this study demonstrates that the thermodynamic stabilization effect of both the cation size (large s orbital/soft hydration shell) and dual Fe 2 II motif toward [CO 2 ]˙ − /[CO 2 ] 2− intermediate is pivotal to the superior CO 2 RR kinetics (activity/selectivity). The proposed pathways (Cs + /K + /Na + vs. Li + and dual Fe 2 site vs. single Fe site) may provide insights into how the orbital interaction and the peculiar electronic structure of the dinuclear Fe 2 site impacts the molecular-level mechanism for efficient electrocatalytic CO 2 reduction.