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Encapsulating cobalt phthalocyanine (CoPc) within the coordinating polymer poly-4-vinylpyridine (P4VP) results in a catalyst–polymer composite (CoPc–P4VP) that selectively reduces CO 2 to CO at fast rates at low overpotential. In previous studies, we postulated that the enhanced selectively for CO over H 2 production within CoPc–P4VP compared to the parent CoPc complex is due to a combination of primary, secondary, and outer-coordination sphere effects imbued by the encapsulating polymer. In this work, we perform in situ electrochemical X-ray absorption spectroscopy measurements to study the oxidation state and coordination environment of Co as a function of applied potential for CoPc, CoPc–P4VP, and CoPc with an axially-coordinated py, CoPc(py). Using in situ X-ray absorption near edge structure (XANES) we provide experimental support for our previous hypothesis that Co changes from a 4-coordinate square-planar geometry in CoPc to a mostly 5-coordinate species in CoPc(py) and CoPc–P4VP. The coordination environment of CoPc–P4VP is potential-independent but pH-dependent, suggesting that the axial coordination of pyridyl groups in P4VP to CoPc is modulated by the protonation of the polymer. Finally, we show that at low potential the oxidation state of Co in the 4-coordinate CoPc is different from that in the 5-coordinate CoPc(py), suggesting that the primary coordination sphere modulates the site of reduction (metal-centered vs. ligand centered) under catalytically-relevant conditions.
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Active Sites of Cobalt Phthalocyanine in Electrocatalytic CO 2 Reduction to Methanol
Abstract Many metal coordination compounds catalyze CO2electroreduction to CO, but cobalt phthalocyanine hybridized with conductive carbon such as carbon nanotubes is currently the only one that can generate methanol. The underlying structure–reactivity correlation and reaction mechanism desperately demand elucidation. Here we report the first in situ X‐ray absorption spectroscopy characterization, combined with ex situ spectroscopic and electrocatalytic measurements, to study CoPc‐catalyzed CO2reduction to methanol. Molecular dispersion of CoPc on CNT surfaces, as evidenced by the observed electronic interaction between the two, is crucial to fast electron transfer to the active sites and multi‐electron CO2reduction. CO, the key intermediate in the CO2‐to‐methanol pathway, is found to be labile on the active site, which necessitates a high local concentration in the microenvironment to compete with CO2for active sites and promote methanol production. A comparison of the electrocatalytic performance of structurally related porphyrins indicates that the bridging aza‐N atoms of the Pc macrocycle are critical components of the CoPc active site that produces methanol. In situ X‐ray absorption spectroscopy identifies the active site as Co(I) and supports an increasingly non‐centrosymmetric Co coordination environment at negative applied potential, likely due to the formation of a Co−CO adduct during the catalysis.
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- PAR ID:
- 10478217
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Angewandte Chemie International Edition
- Volume:
- 63
- Issue:
- 2
- ISSN:
- 1433-7851
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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