Electrochemistry of Acids on Platinum. Application to the Reduction of Carbon Dioxide in the Presence of Pyridinium Ion in Water

Abstract

A detailed cyclic voltammetric investigation of the reduction of moderately weak acids on platinum reveals that they are reduced in two steps: one involving the hydrated protons initially present at equilibrium and the second the reduction of the acid through its prior conversion into hydrated protons. The reduction of pyridinium ions (protonated pyridine) follows this reaction scheme as does any other acid of similar pK (e.g., acetic acid). Rather than being catalytically reduced, CO2 plays a similar role through its prior conversion to carbonic acid. No trace of methanol or formate could be detected upon preparative-scale electrolysis of CO2 on the same electrode in the presence of pyridinium ions.

A Comparison of Pyridazine and Pyridine as Electrocatalysts for the Reduction of Carbon Dioxide to Methanol

“…The retention time for the methanol peak maximum was, in all measurements, typically 2.07 min. If this low faradaic efficiency is compared with the strong current enhancement in the cyclic voltammetry studies between N 2 -and CO 2 -saturated systems, it is clear that CO 2 reduction to methanol is only partly responsible for the observed current increase, as proposed by Bocarsly et al [7] The additional current increase is expected to come from a superposition of the contributions of the two acids present; namely, pyridinium and CO 2 in water or, more dominantly, pyridazinium and CO 2 in water, as concluded by SavØant et al [15] The difference in current densities can then be understood by the different pKa values of the two materials, with 5.14 for pyridine and 2.10 for pyridazine.…”

“…Furthermore, for the study of pyridazinium, several cyclic voltammetry experiments at different pH values were performed; however, at lower pH values, no electrochemical current enhancement upon CO 2 saturation was observed. This current enhancement may then either be attributed to the reduction of CO 2 to methanol through a chain mechanism over several pyridinum radicals, as proposed by Bocarsly et al, [7] or, as SavØant et al [15] concluded, to the superposition of the contributions of the two acids present; namely, the reduction of dissociated protons from pyridinium and CO 2 in water.…”

“…[13] These findings are in agreement with additional theoretical predictions by the group of Batista et al [14] Different to these findings, SavØant et al reported shortly after that no trace of methanol or formate could be detected upon preparative-scale electrolysis of CO 2 on the same system when using pyridinium ions as putative catalyst materials. [15] SavØant concluded that the reduction of pyridinium follows a reduction of the hydrated protons generated by rapid dissociation of the pyridinium ions, which does not lead to the formation of pyridinium radicals and catalytic CO 2 reduction. However, in cases where pyridinium radical intermediates could indeed be photochemically generated, [16] the detection of significant amounts of CO 2 reduction products failed.…”

Reduction of CO2 to Methanol Catalyzed by a Biomimetic Organo-Hydride Produced from Pyridine

“…We stress that we consider a Pt electrode to be a special case. On a Pt electrode, 1e -reduction of PyH + is favored to form adsorbed H-atoms (Pt-H*) [93][94][95][96] such that using it as a cathode introduces additional routes (e.g. H2 formation) which likely outcompete any processes catalyzed by Py.…”

Organic, Organometallic and Bioorganic Catalysts for Electrochemical Reduction of CO2

“…Indeed, the study from Bocarsly and co‐workers55 created more controversy in the field and drove several studies 58, 59, 60, 61. Another study was conducted by Savéant and co‐workers arguing the plausibility of this catalytic process 62. The authors argue that the process apparently works only on Pt and/or Pd, metals that are known to reduce hydrated protons 63.…”