Lead Disc Summary

Using a rotating Pb disc electrode, the conditions for the reproduction of the reduction peak C2, the effect of bulk ethanol concentration, potential scan rate, and electrode rotation rate in pH 8.1, 0.1 M phosphate buffer were established. The initial hypothesis of this work was that the electrochemical reduction of alcohols would produce alkanes, i.e. ethanol reduces to ethane.

Using this working hypothesis of the production of alkanes, the amount of charge associated with this peak may suggest the formation of a monolayer of product on the surface of the electrode within the rotation rate, scan rate and concentration ranges considered in this work. Increasing the bulk alcohol concentration has no effect on the total charge produced; this is consistent with the reduction process being progressively stifled through the formation of an insulating layer of reaction product. Therefore, irrespective of the concentration of the ethanol in the system, it appears that the same amount of material is reducing and being deposited out on the surface. However, various other conditions of the experiments appeared to have some effect on the processes occurring at the electrode.

The anodic limit of the cyclic voltammogram had an effect on the reduction processes occurring at the electrode. There is a change in the cathodic wave of the cyclic voltammograms observed between the anodic limit of −0.65 V and −0.7 V where the reductive peak becomes substantially smaller and no longer reproducible. The absence of an accompanying oxidative wave with any if the anodic limits investigated suggests an irreversible reaction. Some inhibition of reduction on the electrode surface appears to be present with anodic limit E < −0.7 V. This inhibition is absent for those with anodic limit E > −0.65 V. Experiments with the potential hold technique suggested that the reproducibility of the peak at anodic limits E < −0.7 V had a time-dependent nature and by allowing enough time for the layer to be removed the reductive peak could be obtained reproducibly in subsequent scans. However, the size of the peak appears to

have a time-independent nature and the magnitude of the reductive current produced is less at the more negative potentials.

The potential scan rate also had an effect on the reduction processes. A decrease in the amount of material reducing was observed with increasing scan rate. The scan rate is the rate at which the potential is scanned through the potential range by the potentiostat. Small scan rates (i.e. 10 mV s−1) pass through the potential range over longer times than larger scan rates (i.e. 200 mV s−1), therefore a single scan takes a longer time, and more time is spent at each potential scanned, at a lower scan rate than at a larger scan rate. More reduction appears to be able to take place at slower scan rates indicating that the reduction process is relatively slow, allowing more time in the potential range where reduction occurs may allow for more reduction.

The rotation rate of the RDE also appears to have an effect on the reduction processes occurring. As noted in Section 3.5.5, there is a progressive decrease in observed peak size as the rotation rate is increased. An increase in rotation rate is expected to promote the loss of such insulating materials away from the electrode as they form due to hydrodynamic shear. However, in this case, increasing the rotation rate appears to promote an insulating reduction product at the electrode, therefore ruling out the facilitated removal of insulating products.

The facilitated replenishment of the alcohol at the surface of the electrode may promote a second non-electrochemical process that is responsible for formation of the insulating layer.

A two step process was suggested:

Step 1 ROH + 2H+ + 2e−→ intermediate (3.17)

Step 2 Intermediate + ROH → insulating product (3.18)

With the hypothesis being that increasing the rotation rate will not alter the rate of Step 1 but ensuring replenishment of ethanol (or propanol) at the electrode surface will promote Step 2.

The data in this section gives values of (7.0-4.1) × 1018 molecules m−2 of product at the surface which is less than the typical values for a monolayer of adsorbates on the metal

As discussed earlier, an insulating layer is not inconsistent with some of these results. The possible thickness of an insulating layer was therefore calculated, using the working hypothesis assuming the reduction products are alkanes (ethane and propane), providing values of approximately 0.78 nm, showing that only a thin layer is formed before the reduction can no longer continue. The calculated thickness is consistent with the possibility of a monolayer forming on the electrode surface. It is also suggested that the binding sites utilized in this process are isolated, spaced apart on the electrode. This is due to the number of molecules per area produced for the Pb electrode system being reported as approximately 4 × 1018 molecules m−2 whereas the typical number of active binding sites on an electrode is 1.3 × 1019 sites m−2. This indicates that not all the possible available binding sites on the electrode surface are occupied before the electrode is sufficiently covered and further reduction is inhibited. Assuming that each molecule of product occupies only one binding site on the electrode, the molecule therefore also effectively shields or occludes some of the electrode surface area surrounding the binding site.

The possibility of the formation of a longer chain alkane or an ether product was also considered. The possibility of the alcohol being required to adsorb to the surface of the electrode for reduction to occur could determine a two step process with an adsorbed intermediate leading to either of these possible products. The electrode rotation rate anomaly is also consistent with there being a 2 step process. Adsorption to the electrode would be thought to be through a carbon, most likely the carbon adjacent to the oxygen due to electronegativity, or through the oxygen itself as a result of its lone pairs.

Possible products considered at this point were butane, butene, and diethyl ether for the ethanol investigation, hexane, hexane and dipropyl ether for propanol and 2,3 dimethyl butane, and diisopropyl ether for propan-2-ol.

The Pb electrode systems exhibit some unexpected results and novel phenomena. These results suggest that there may not be a simple alkane product from a simple reduction process. The reduction product may be an intermediate in another process giving a non- reductive final product. These novel findings require further examination to be fully understood. Therefore product identification for these Pb electrode systems were considered. Chapter 4 discusses the product identification methods and suggested products of the Pb electrode systems.

Chapter 4