The strong effect of magnetic field on the electrochemical (EC) reduction of a diamagnetic species was monitored in situ in a 600 MHz (14 T) NMR spectrometer. Throughout EC-NMR experiments, the diamagnetic species is influenced by the Lorentz force (cross product of current density and magnetic field), which in turn acts on analyte transport and, as a result, enhances reaction rates. This phenomenon, known as magnetoelectrolysis, has not been considered in several in situ EC-NMR studies in solution, electron paramagnetic resonance (EC-EPR) spectroscopy, and magnetic resonance imaging (EC-MRI) involving the oxidation and reduction of organic compounds and lithium ion batteries. Recently, we have demonstrated the presence of this effect in the electroplating of a paramagnetic ion species by monitoring it in situ in a low-field (0.23 T) NMR spectrometer. In this report, a ca. five-fold enhancement in the electroreduction rate of benzoquinone was observed when the analyses were performed in situ in the NMR spectrometer. Therefore, this work has the objective of informing the scientific community that before every electrochemical reaction carried out in situ in NMR, EPR and MRI apparatuses, the influence of the magnetic field on the reactions must be evaluated, since it can alter the mechanism and kinetics of the reaction which, if not taken into account may lead to wrong interpretations of the data.
The in situ coupling between electrochemistry and spectrometric techniques can help in the identification and quantification of the compounds produced and consumed during electrochemical reactions. The combination of electrochemistry with nuclear magnetic resonance is quite attractive in this respect, but it has some challenges to be addressed, namely, the reduction in the quality of the NMR signal when the metallic electrodes are placed close to or in the detection region. Since NMR is not a passive technique, the convective effect of the magnetic force (magnetoelectrolysis), which acts by mixing the solution and increasing the mass transport, has to be considered. In seeking to solve the aforementioned problems, we developed a system of miniaturized electrodes inside a 5 mm NMR tube (outer diameter); the working and counter electrodes were prepared with a mixture of graphite powder and epoxy resin. To investigate the performance of the electrodes, the benzoquinone reduction to hydroquinone and the isopropanol oxidation to acetone were monitored. To monitor the alcohol oxidation reaction, the composite graphite–epoxy electrode (CGEE) surface was modified through platinization. The electrode was efficient for in situ monitoring of the aforementioned reactions, when positioned 1 mm above the detection region of the NMR spectrometer. The magnetoelectrolysis effect acts by stirring the solution and increases the reaction rate of the reduction of benzoquinone, because this reaction is limited by mass transport, while no effect on the reaction rate is observed for the isopropanol oxidation reaction.
In situ nuclear magnetic resonance (NMR) investigations of a Kolbe electrolysis reaction using a 43 MHz 1 H NMR spectrometer were performed in this work. The electrochemical oxidative decarboxylation of biomass-derived valeric acid into the value-added product n-octane has been monitored. All reactions were conducted in nondeuterated methanolic solution, using KOH as the supporting electrolyte. The working and counter electrodes consisted of Pt wire, and Ag wire was used as a pseudo-reference electrode. The influence of the magnetic field on the reaction kinetics, as well as on mass transfer, has been studied in detail. The findings show that the resulting mass transfer is highly dependent on the magnetic field. The significantly higher reaction velocity for in situ experiments is partly due to the strong Lorentz force, which agitates the solution and reduces the thickness of the electric double layer. The obtained results also suggest a strong influence of the magnetic field on the charge transfer from the electrode to the solution. The total resistance for the electrochemical reaction was significantly reduced by the presence of the magnetic field for all in situ experiments, at all points of the reaction. According to the reaction products, it was found that, at high applied potentials (>5 V) or currents (>15 mA), the reaction velocity can be increased but evaporation and overoxidation phenomena become more apparent. The results presented here show how NMR in situ electrochemistry can help to determine the optimal reaction conditions and improve quantitative analyses by example of a prominent green chemistry application.
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