Constructing Monolayer Fe Clusters as Model Catalysts for CO₂ Electroreduction

Constructing model catalysts to clarify the active sites and elucidate structure-activity relationships represents a bottleneck issue in electrocatalytic reactions. For instance, Fe-based materials have been widely investigated for various electrocatalytic reactions, e.g., CO₂ electroreduction (CO₂RR). However, the precise role of Fe in the catalytic mechanism remains debated and is under extensive investigation due to the complex properties of those catalysts. Herein, we successfully construct a series of monolayer Fe clusters (ML-Fe) on a single-crystal Au(111) substrate via vapor deposition under ultrahigh vacuum (UHV) conditions, which serve as model catalysts for CO₂RR. Notably, the size of Fe clusters can be tuned at the atomic scale (∼2 nm) as observed by scanning tunneling microscopy (STM). Experimental and theoretical calculations demonstrate that ML-Fe achieves >60% Faradaic efficiency (FE) for CO production during CO₂RR. When the Fe layer further increases to form nanoparticles, Fe sites exhibit stronger electron interaction and binding strength with *CO intermediates, consequently shifting the dominant reaction pathway from CO₂RR to hydrogen evolution (HER). This work could provide valuable insights into designing model catalysts with well-defined active sites to investigate the structure-activity relationships in diverse electrocatalytic systems.