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

Abstract

Previously, electrogenerated pyridinyl was implicated as a catalyst for the reduction of CO 2 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 pK a 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 CO 2 to methanol. In contrast, a more complete consideration of the reaction including the realization that pyridinium reduction is tightly coupled to H 2 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 pK a = 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 pK a 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 CO 2 .

Organically doped palladium: a highly efficient catalyst for electroreduction of CO2to methanol

“…They proposed a multistep mechanism that pyridinium ion works as a one-electron shuttle through its reduction to the pyridinium radical, that would then react with CO 2 , ultimately leading to methanol after a series of follow-up steps. 8,9 However, this mechanism was questioned by other workers with both experimental 10 and theoretical results [11][12][13][14][15] . We also did some research to explore the possible mechanism.…”

“…S6). [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30], which is almost identical to pure PYD (a). These are additional evidence that pyridine derivative was indeed entrapped within the composite.…”

Photoelectrocatalytic Reduction of Carbon Dioxide

“…Most interestingly, a formate dehydrogenase enzyme was reported to work with an illuminated p-type InP semiconductor electrode to produce formate from aqueous CO 2 at visible light irradiation with a maximum yield of 89%. 73,74 [Re(Bu-bpy)(-CO) 3 X] complexes have been extensively investigated by Kubiak et al 75 and have been used to photoreduce CO 2 to CO on a p-type silicon photoelectrode in which the photovoltage was reported to exceed 600 mV.…”

Challenges and Perspectives on Carbon Fixation and Utilization Technologies: An Overview

“…3, numerous chemicals, such as methane and ethanol and polymers, can be produced from CO 2 reduction reaction. The CO 2 reduction reaction can be achieved by several different approaches, such as catalytic hydrogenation (Kiss et al, 2016), complex metal hybrids (Yamazaki et al, 2015), electrochemically catalytic (Angamuthu et al, 2010;Yan et al, 2014), photocatalytic , and biological (Klinthong et al, 2015) methods. The principles and challenges of the above CO 2 Fig.…”