Immobilization of porphyrin complexes into crystalline metal-organic frameworks (MOFs) enables high exposure of porphyrin active sites for CO 2 electroreduction. Herein, well-dispersed iron-porphyrin-based MOF (PCN-222(Fe)) on carbon-based electrodes revealed optimal turnover frequencies for CO 2 electroreduction to CO at 1 wt.% catalyst loading, beyond which the intrinsic catalyst activity declined due to CO 2 mass transport limitations. In situ Raman suggested that PCN-222(Fe) maintained its structure under electrochemical bias, permitting mechanistic investigations. These revealed a stepwise electron transfer-proton transfer mechanism for CO 2 electroreduction on PCN-222(Fe) electrodes, which followed a shift from a ratelimiting electron transfer to CO 2 mass transfer as the potential increased from À 0.6 V to À 1.0 V vs. RHE. Our results demonstrate how intrinsic catalytic investigations and in situ spectroscopy are needed to elucidate CO 2 electroreduction mechanisms on PCN-222(Fe) MOFs.

Immobilization of porphyrin complexes into crystalline metal-organic frameworks (MOFs) enables high exposure of porphyrin active sites for CO 2 electroreduction. Herein, well-dispersed iron-porphyrin-based MOF (PCN-222(Fe)) on carbon-based electrodes revealed optimal turnover frequencies for CO 2 electroreduction to CO at 1 wt.% catalyst loading, beyond which the intrinsic catalyst activity declined due to CO 2 mass transport limitations. In situ Raman suggested that PCN-222(Fe) maintained its structure under electrochemical bias, permitting mechanistic investigations. These revealed a stepwise electron transfer-proton transfer mechanism for CO 2 electroreduction on PCN-222(Fe) electrodes, which followed a shift from a ratelimiting electron transfer to CO 2 mass transfer as the potential increased from À 0.6 V to À 1.0 V vs. RHE. Our results demonstrate how intrinsic catalytic investigations and in situ spectroscopy are needed to elucidate CO 2 electroreduction mechanisms on PCN-222(Fe) MOFs.