Unraveling the Electronic Structure and Dynamics of the Atomically Dispersed Iron Sites in Electrochemical CO 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 techniques that are sensitive to such sites and model single-atom systems. Herein, supported by state-of-the-art techniques, we provide an in-depth study of the dynamic structural and electronic evolution during the electrochemical CO reduction reaction (CORR) of a model catalyst comprising iron only as a high-spin (HS) Fe(III)N center in its resting state. Fe Mössbauer and X-ray absorption spectroscopies clearly evidence the change from a HS Fe(III)N to a HS Fe(II)N center with decreasing potential, CO- or Ar-saturation of the electrolyte, leading to different adsorbates and stability of the HS Fe(II)N center. With 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 CORR. Furthermore, theoretical calculations reveal that the electroreduction of the Pc ligand modifies the d-band center of the generated HS Fe(II)Pc species, resulting in an optimal binding strength to CO and thus boosting the catalytic performance of CORR. 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 CORR.