Electroreduction of CO ₂ Catalyzed by a Heterogenized Zn–Porphyrin Complex with a Redox-Innocent Metal Center

Abstract

Transition-metal-based molecular complexes are a class of catalyst materials for electrochemical CO ₂ reduction to CO that can be rationally designed to deliver high catalytic performance. One common mechanistic feature of these electrocatalysts developed thus far is an electrogenerated reduced metal center associated with catalytic CO ₂ reduction. Here we report a heterogenized zinc–porphyrin complex (zinc(II) 5,10,15,20-tetramesitylporphyrin) as an electrocatalyst that delivers a turnover frequency as high as 14.4 site ^–1 s ^–1 and a Faradaic efficiency as high as 95% for CO ₂ electroreduction to CO at −1.7 V vs the standard hydrogen electrode in an organic/water mixed electrolyte. While the Zn center is critical to the observed catalysis, in situ and operando X-ray absorption spectroscopic studies reveal that it is redox-innocent throughout the potential range. Cyclic voltammetry indicates that the porphyrin ligand may act as a redox mediator. Chemical reduction of the zinc–porphyrin complex further confirms that the reduction is ligand-based and the reduced species can react with CO ₂. This represents the first example of a transition-metal complex for CO ₂ electroreduction catalysis with its metal center being redox-innocent under working conditions.

Abstract

A zinc−porphyrin complex, with the redox-innocent Zn ion binding reaction intermediates and the ligand mediating electron transfer, catalyzes CO ₂ electroreduction to CO in high Faradaic efficiency.