Iron porphyrin flanked by viologen redox units for persistent carbon dioxide reduction in the presence of oxygen

Abstract

The electrocatalytic reduction of CO₂ to energy-rich forms such as CO or hydrocarbons is typically realized with pure CO₂. This is primarily to exclude O₂, which is a far better oxidant and a major competitor upon reduction of the CO₂/O₂ feed gas. Furthermore, the presence of O₂ can deactivate the catalytic material and reduce its effectiveness for CO₂ reduction. To confront this major challenge, different strategies are being pursued. We utilize a molecular design approach by adjoining to a known catalyst a redox active module that can competitively divert the deleterious O₂ activity. We tailored an iron porphyrin, a prominent catalyst for CO₂ reduction, flanked by viologen units known for their efficient O₂ reduction. Electrochemical studies on the homogeneous phase of the pre-catalyst, the iron(III)-μ-oxo form, show the independent activity of both modules. When heterogenized with carbon nanotubes on a carbon paper electrode, we found that the catalyst could sustain the aerobic (5% O₂) reduction of CO₂ to CO with a faradaic efficiency of 62%, while the activity of the unmodified iron porphyrin fell to 18% under the same experimental conditions.