Unraveling the Electronic Structure and Dynamics of the Atomically Dispersed Iron Sites in Electrochemical CO 2 Reduction.

Single-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 conditions is still limited due to the challenge of combining operando techniques that are sensitive to such sites and model single-atom systems. Herein, supported by state-of-the-art operando techniques, we provide an in-depth study of the dynamic structural and electronic evolution during the electrochemical CO 2 reduction reaction (CO 2 RR) of a model catalyst comprising iron only as a high-spin (HS) Fe(III)N 4 center in its resting state. Operando 57 Fe Mössbauer and X-ray absorption spectroscopies clearly evidence the change from a HS Fe(III)N 4 to a HS Fe(II)N 4 center with decreasing potential, CO 2 - or Ar-saturation of the electrolyte, leading to different adsorbates and stability of the HS Fe(II)N 4 center. With operando Raman 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 CO 2 RR. Furthermore, theoretical calculations reveal that the electroreduction of the Pc ligand modifies the d-band center of the in situ generated HS Fe(II)Pc - species, resulting in an optimal binding strength to CO 2 and thus boosting the catalytic performance of CO 2 RR. This work provides both experimental and theoretical evidence toward the electronic structural and dynamics of reactive sites in single-Fe-atom materials and shall guide the design of novel efficient catalysts for CO 2 RR.