Unravelling the Electronic Structure and Dynamics of the Atomically Dispersed Iron Sites in Electrochemical CO2 Reduction

Abstract

AbstractSingle-atom catalysts with a well-defined metal center open unique opportunities for exploring the catalytically active site and reaction mechanism of chemical reactions. However, understanding of the electronic and structural dynamics of single-atom catalytic centers under reaction condition is still limited due to the challenge of combiningoperandotechniques that are sensitive to such sites and model single-atom systems. Herein, supported by state-of-the-artoperandotechniques, we provide an in-depth study of the dynamic structural and electronic evolution during electrochemical CO2reduction reaction (CO2RR) of a model catalyst comprising iron only as a high-spin (HS) Fe(III)N4center in its resting state.Operando57Fe Mössbauer and X-ray absorption spectroscopies clearly evidence the change from a HS Fe(III)N4to a HS Fe(II)N4center with decreasing potential, CO2- or Ar-saturation of the electrolyte leading to different adsorbates and stability of the HS Fe(II)N4center. WithoperandoRaman spectroscopy and cyclic voltammetry, we identify that the phthalocyanine (Pc) ligand coordinating the iron cation center undergoes a redox process from Fe(II)Pc to Fe(II)Pc−. Altogether, the HS Fe(II)Pc−species is identified as the catalytic intermediate for CO2RR. Furthermore, theoretical calculations reveal that the electroreduction of the Pc ligand modifies the d-band center of thein situgenerated HS Fe(II)Pc−species, resulting in an optimal binding strength to CO2and thus boosting the catalytic performance of CO2RR. This work provides both experimental and theoretical evidence towards the electronic structural and dynamics of reactive sites in single-Fe-atom materials and shall guide the design of novel efficient catalysts for CO2RR.