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Hydrogen Bonded Pyridine Dimer: A Possible Intermediate in the Electrocatalytic Reduction of Carbon Dioxide to Methanol

Category: Articles

Volume: 14 | Issue: 2 | Pages: 515-521
DOI: 10.4209/aaqr.2013.06.0227
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Yong Yan, Jing Gu, Andrew B. Bocarsly

  • Frick Laboratory, Department of Chemistry, Princeton University, Princeton, NJ 08544, USA

Abstract

Previously, electrogenerated pyridinyl was implicated as a catalyst for the reduction of CO2 to methanol. However, recent quantum mechanical calculations of both the homogeneous redox potential for the pyridinium/pyridinyl redox couple (900 mV more negative than experimentally reported) and the pKa of the reduced pyridinyl species (~27) have led to the proposal that the homogeneous reduction of pyridinium does not play a role in the observed catalytic reduction of CO2 to methanol. In contrast, a more complete consideration of the reaction including the realization that pyridinium reduction is tightly coupled to H2 evolution, produces a calculated redox potential in agreement with the experimental findings. In reexamining this system, it is found 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 (density functional theory) 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 pre-electrocatalyst for the observed activation of CO2.

Keywords

CO2 sequestration and conversion Catalyst Electrochemistry Pyridinium


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