Revealing the Ligand Effect of Axially Coordinated Poly(iron phthalocyanine) Catalysts for Oxygen Reduction Reaction by an In Situ ATR-FTIR CO-Probe Method

Identifying the structure–activity relationship of transition metal–nitrogen–carbon (MNC) catalysts during the oxygen reduction reaction (ORR) remains a challenge due to the complexity of active sites and the lack of effective methodologies. In this study, an in situ attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) CO-probe method is developed to investigate the electronic effects and structural changes of MNC catalysts, using axial-ligand poly(iron phthalocyanine) (PFePc-X2, X = F, Cl, Br, I) model catalysts. These axial ligands modified the electronic structure of the central Fe atoms, leading to the blue–red shift in the in situ ATR-FTIR CO adsorption (COad) wavenumber. By comparing the COad wavenumbers of PFePc-X2 catalysts, the electronic effects of the axial ligand on the active sites were clarified as follows: F > Cl > Br > I. Moreover, structural changes involving the isoindole nitrogen (N), bridge carbon (C), and benzene-ring carbon (C), collectively assigned as the NCC group, indicated that weaker coordinating ligands were more susceptible to desorption during CO adsorption. These results demonstrate that the in situ ATR-FTIR CO-probe method is an effective strategy for investigating ligand effects in MNC catalysts and holds promise as a potential activity descriptor for ORR performance.