Controlling electrocatalytic performance of cobalt phthalocyanine for carbon dioxide reduction by modulating the catalyst's primary and outer coordination sphere

Abstract

Selective electrocatalytic redn. of CO_2 over competitive hydrogen evolution remains a fundamental challenge in electrochem. CO_2 conversion. We have previously shown that encapsulating a mediocre CO_2 redn. catalyst, cobalt phthalocyanine (CoPc), within a coordinating polymer, poly-4-vinylpyridine (P4VP), results in a composite CoPc-P4VP material that enhances the electrocatalytic activity for the carbon dioxide redn. reaction while essentially shutting down competitive hydrogen evolution. Our recent work further elucidates the role of the primary and outer-coordination spheres in CoPc-P4VP on catalytic activity and selectivity. In particular, we have conducted a series of expts. including electrochem. kinetic isotope effect measurements to probe catalytic mechanism, proton inventory studies to understand proton delivery mechanisms through the encapsulating polymer, and systematic modifications of the polymer and catalyst to probe the role of axial coordination on catalytic activity. The results of these studies have confirmed 1) that proton delivery to the catalytic active sites in CoPc-P4VP occurs via a proton relay mechanism, and 2) that axial coordination to CoPc changes the rate detg. step in the catalytic mechanism from an initial CO_2-coordination step to a subsequent protonation event. This work suggests that polymer encapsulation may be a viable strategy for enhancing catalytic activity and selectivity of mol. catalysts for CO_2 redn.

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