Unlocking the Distance Effect on CO ₂ Electroreduction to Multi‐Carbon Products via Monolayer Model Catalysts

The interaction between adjacent active sites significantly influences electrocatalytic performance, for example, CO 2 electroreduction (CO 2 RR), 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–N 4 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 CO 2 RR. When Cu sites are spaced 0.98 nm apart,a Faradaic efficiency (FE) of 6.1% for C 2 H 4 is achieved. Increasing the inter‐distance of Cu sites to 1.50 nm dramatically reduces C 2 H 4 FE to 1.2%. At larger separations of 1.63 nm and 1.74 nm, C─C coupling is almost completely suppressed, and C 2 H 4 production becomes negligible. These results provide direct experimental evidence of the distance effect in CO 2 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.