Metalloporphyrin Monolayers as Tunable Platforms for CO₂ Electroreduction

While molecular electrocatalysis for CO₂ reduction has advanced significantly, a key challenge persists in accurately defining the active site distribution, surface density, and structural organization of heterogenized catalysts at electrode interfaces. In this study, we address this gap by employing self-assembled monolayers of physisorbed metal porphyrins (Fe and Cu) on graphite electrodes as structurally well-defined model systems for the electrochemical CO₂ reduction reaction (CO₂RR). High-resolution scanning tunneling microscopy (STM) revealed crystalline monolayers with uniform active site distribution, offering periodicities of ∼1.5 nm. These monolayers exhibit moderate Faradaic efficiencies (FEs) for CO production, with Fe porphyrin outperforming its Cu analogue. Isotopic labeling experiments were employed to confirm that the product CO originates from added CO₂. Bicomponent monolayers comprising Fe porphyrin and catalytically inactive free-base porphyrin were fabricated to investigate composition-activity relationships. STM imaging enabled direct, quantitative assessment of surface composition, and subsequent CO₂RR measurements revealed a systematic decrease in FE with increasing coverage of the catalytically inactive component. These findings demonstrate the utility of molecularly ordered porphyrin monolayers as a powerful platform for probing structure-activity relationships in electrocatalytic CO₂ reduction with exceptional spatial and compositional precision.