Unlocking the Distance Effect on CO₂ Electroreduction to Multi-Carbon Products via Monolayer Model Catalysts

The interaction between adjacent active sites significantly influences electrocatalytic performance, for example, CO₂ electroreduction (CO₂RR), yet quantifying this effect experimentally remains challenging due to the difficulty in precisely controlling inter-site distances. Here, we construct a monolayer model catalyst using Cu-coordinated porphyrins with well-defined Cu-N₄ sites on Au(111). The distance between Cu centers is regulated at the sub-nanometer level by modifying molecular ligands and aggregation states, and is directly measured by scanning tunneling microscopy (STM). This adjustable spacing critically determines the selectivity toward multi-carbon products in CO₂RR. When Cu sites are spaced 0.98 nm apart,a Faradaic efficiency (FE) of 6.1% for C₂H₄ is achieved. Increasing the inter-distance of Cu sites to 1.50 nm dramatically reduces C₂H₄ FE to 1.2%. At larger separations of 1.63 nm and 1.74 nm, C─C coupling is almost completely suppressed, and C₂H₄ production becomes negligible. These results provide direct experimental evidence of the distance effect in CO₂ electroreduction, and precisely identify, for the first time, the distance between Cu sites capable of facilitating C─C coupling process. This study establishes a molecular-level platform for probing fundamental mechanisms in electrocatalysis.