Modulating the mechanism of electrocatalytic CO2 reduction by cobalt phthalocyanine through polymer coordination and encapsulation

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Jun 12, 2019
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Figure 1.
Figure 1. (left) A proposed mechanism for CO2 reduction by CoPc consistent with our data. (center) Schematic illustration showing the postulated primary-, secondary-, and outer-coordination effects for CoPc-P4VP with a coordinated CO2 intermediate. For comparison are shown the non-polymer encapsulated CoPc and CoPc(py) systems. (right) Electrochemical proton inventory measurements showing attenuation of catalytic rate as a function of partial solvent deuteration. The KIE ~ 3 and linear decrease in activity with increasing solvent deuteration for CoPc(py) indicates a rate-determining proton transfer event involving one hydrogenic site where proton delivery to the active center is likely controlled by diffusion. The KIE ~ 2.1 and non-linear decrease in activity with increasing solvent deuteration for CoPc-P4VP is indicative of a rate-determining proton transfer event where proton delivery to the active center involves numerous hydrogenic sites—e.g. a proton relay mechanism involving the pyridyl groups in the P4VP polymer. Adapted from Ref [1].

     The selective electrochemical CO2 reduction into useful fuels and chemical feedstocks in the CO2 reduction reaction (CO2RR) is an important strategy both as a means of renewable energy storage and for the recycling of industrial CO2 waste streams into value-added products.  Our research group’s approach to developing new catalysts for the CO2RR focuses on encapsulating transition metal catalysts within coordinating polymers that control the coordination environment of the catalysts to promote selective and active CO2 reduction.  Metalloenzymes serve as the inspiration for our approach where catalyst active sites are buried within hydrophobic cores that carefully control the primary, secondary, and outer coordination spheres of the enzyme’s active site resulting in fast catalytic activity and high product and reaction selectivity.  Our first studies of polymer-encapsulated catalysts for the CO­2RR have focused on cobalt phthalocyanine (CoPc) encapsulated within poly-4-vinylpyridine (P4VP) forming CoPc-P4VP composite films.  The CoPc-P4VP system has been shown by us [2] and others [3,4] to have increased CO2RR activity and selectivity for the CO2RR compared to the CoPc parent complex.  Three synergistic effects from P4VP are believed to contribute to the enhancement of CoPc-P4VP as a CO2RR catalyst: 1) axial-coordination of pyridyl moieties in P4VP to the Co center increases the catalyst’s nucleophilicity and facilitates coordination of the CO2 molecule to the Co center, 2) H-bonding interactions stabilize reactive CO2 intermediates, and 3) control of proton transport through the use of the pyridyl residues within the polymer as proton relays which suppress HER.  However, the exact mechanism of CO2 reduction by CoPc-P4VP has remained elusive.  Moreover, although extended proton-relays chains are often postulated in polymer-based electrocatalytic systems, until now there have been few studies that experimentally verify their existence and influence on catalytic activity.

     In our recent article published in Nature Communications [1], we have developed a strategy to determine the mechanistic implications of primary- and outer-coordination sphere effects on the CO2 reduction activity by P4VP encapsulated CoPc composites.  First, using electrochemical kinetic isotope effect (KIE) measurements, we have shown that axial-coordination to the CoPc changes the rate determining step from a CO2 coordination step in the case of CoPc (step i in the proposed mechanism) to a subsequent protonation event in the case of CoPc(py) or CoPc-P4VP (step iii in the proposed mechanism).  Additionally, using proton inventory studies—a technique used in enzymology to study the kinetics of proton delivery to enzymatic active sites—to our electrocatalytic system, we have confirmed that proton delivery to the Co active site in CoPc-P4VP is controlled by a proton-relay mechanism rather than proton diffusion through the film.  We believe this is one of the first experimentally-verified examples of a multi-site relay-based proton delivery mechanism in a synthetic electrocatalytic system.

     We believe these findings are both exciting and encouraging for the future use of polymer-encapsulated electrocatalysts for small-molecule transformations.  We have shown that we can modulate both the electrocatalytic mechanism at the catalyst active site and the mechanism of proton delivery to enhance selective and active CO2 reduction by encapsulating the parent complex within simple coordination polymers.  Moreover, the electrochemical tools we developed and adapted to study these effects based on KIE and proton inventory studies provide a strategy to elucidate fundamental catalytic mechanism of the CO2RR by other molecular electrocatalyst assemblies.  The electrocatalytic systems and experimental techniques developed in this study will facilitate the development of new, more-active electrocatalytic systems for selective CO2 reduction and other small-molecule transformations.

[1] Liu, Y.; McCrory, C. C. L. Modulating the mechanism of electrocatalytic CO2 reduction by cobalt phthalocyanine through polymer coordination and encapsulation. Nat. Commun. 10, 1683 (2019).

[2] Kramer, W. W. & McCrory, C. C. L. Polymer coordination promotes selective CO2 reduction by cobalt phthalocyanine. Chem. Sci. 7, 2506–2515 (2016).

[3] Yoshida, T.; Kamato, K.; Tsukamoto, M.; Iida, T.; Schlettwein, D.; Wöhrle, D.; Kaneko, M. Selective electrocatalysis for CO2 reduction in the aqueous phase using cobalt phthalocyanine/poly-4-vinylpyridine modified electrodes. J. Electroanal. Chem. 385, 209–225 (1995)

[4] Abe, T.; Yoshida, T.; Tokita, S.; Taguchi, S.; Taguchi, F.; Imaya, H.; Kaneko, M. Factors affecting selective electrocatalytic CO2 reduction with cobalt phthalocyanine incorporated in a polyvinylpyridine membrane coated on a graphite electrode. J. Electroanal. Chem. 412, 125–132 (1996).

 

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Charles C. L. McCrory

Assistant Professor of Chemistry and Macromolecular Science & Engineering, University of Michigan

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