Previously, electrogenerated pyridinyl was implicated as a potent catalyst for the reduction of CO2 to methanol. This species was reported to form at -0.58 V vs. SCE in an aqueous electrolyte. However, recent quantum calculations by Keith and Carter suggest that pyridinyl formation via the reduction of dissolved aqueous pyridinium is expected to occur approximately 800 mV more negative than experimentally reported. However, a more complete consideration of the reaction including the realization that pyridinyl formation is tightly coupled to H2 evolution produces a calculated redox potential in agreement with the experimental findings.  Keith and Carter also report a pKa for pyridinyl of ~27, which precludes loss of the nitrogen bonded hydrogen, a key step if CO2 is to bind to pyrdinyl as we have previously suggested. We now report that aqueous solutions containing a near equimolar mixture of pyridine and pyridinium (i.e. solution pH near the pyridinium pKa = 5.2) contain a substantial concentration of a hydrogen bonded dimer formed by the generation of a N-H���N bond containing one strong NH bond and one elongated NH bond. This species has been identified by x-ray diffraction of crystals grown in aqueous media from pyridine/pyridinium mixtures and can be observed directly in solution using raman spectroscopy. DFT calculations indicate that the pKa for this species is ~22, a value that is consistent with a proton exchange capability. This suggests that this hydrogen bonded dimer may be the key pre-electrocatalyst for the observed activation of CO2.

Electrochemistry of aqueous pyridiniums: A key component in the electrocatalytic reduction of carbon dioxide to methanol

245th National Meeting (2013)

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