Candidate Product Identification Techniques

4.3.1

Nuclear Magnetic Resonance Spectroscopy (NMR)

NMR can provide information about structure, reaction state and dynamics of the analyte and the chemical environment in which the analyte is present.[4,24] NMR is non destructive and qualitative data can be obtained from samples weighing less than a milligram. [4,24,69] However, NMR is only suitable for liquid and solid samples.[4,24,69] The sample for analysis in this case is definitely not a solid, but is possibly a liquid or a gas. If the product is in the liquid state, the concentration of product within the electrolyte is significantly small that it would likely not be detected in an NMR spectrum. If the product is a gas, NMR is not a suitable technique for detection. There is the possibility of extracting the gas into a liquid alkane such as Hexane. However, the possible concentration of the product within the gas sample extracted will again be significantly small that only the Hexane would be detected. Therefore NMR was determined not to be a suitable method for product identification in this case.

4.3.2

Surface Enhanced Raman Spectroscopy (SERS)

Owing to its high sensitivity and specificity, SERS has a great potential for analytical chemistry and biological sensors.[4,24,70] However, SERS is sensitive to the surface on which the experiment is taking place, with good results only from specific metals such as Au, Ag, and Cu. [4,24,70,71] In this case the electrode material is lead, which is not one of the specific metals confirmed to be suitable for SERS. It could be possible to deposit a layer of lead on a gold or silver electrode. This would provide the lead for the electrochemical investigation and gold or silver for the SERS enhancement. However, this process adds extra electrochemical steps required to deposit the layer and extra care would need to be taken to obtain a suitable thickness layer so as not to inhibit the

Due to the lack of a suitable electrode material for SERS and the difficulty in the deposition of a suitable thin Pb layer on the Ag or Au, it was determined at this point that SERS is not the best technique for product identification.

4.3.3

Mass Spectrometry

In Mass Spectrometry it is possible to record molecules with a molar mass as small as 28 g mol1.[50] The molar mass of the smallest possible product being considered here (ethane Mr = 30.07 g mol1) is within the limits, albeit at the minimum limit, of the Bruker FTICR mass spectrometer used in this work. However, the amount of material being produced is very small. The collected gas sample, could contain a gaseous product from the electrochemical processes. However, it will likely also contain hydrogen gas produced at the cathodic limit of the CV, as seen by the background current due to H2 observed in Figs. 3.47 – 3.55 reported in chapter 3. Along with the

gaseous product and hydrogen gas there will possibly also be some ethanol and water vapor due to vaporization as gaseous product bubbles escape the aqueous buffer.

The volume of gaseous product collected in the Pb-ethanol system is approximately 6 ml in 5 hours, therefore, the maximum collected in any one day being < 10 ml. The possible amount of product assumed to be produced on the larger surface area electrode would be 7.88  109 moles for each scan as calculated based on eqn. 3.11 described in chapter 3, section 3.5.3. Up to 900 scans could be cycled in one day, therefore up to 7.09  106 moles of product could be produced. This amount of product could possibly be detected by a mass spectrometer.[68]

The possibility of running the system for several days to collect more gas was considered. However, as discussed in chapter 3, the electrode materials are not inert substances and there may be an effect on the surface of the electrode from prolonged cathodic potentials. This effect on the electrode surface is largely unknown. Any changes to the electrode surface due to prolonged exposure to cathodic potentials may have an effect on the products forming from the alcohol reduction of interest. Therefore the length of individual experiments was limited to 8 hours, then allowing time for mechanical and electrochemical cleaning of the electrode surface.

The spectra produced contain a certain amount of noise in the background. It is recommended that detection of a sample should be 10 times the noise of the instrument.[50,51] There is only a small volume, (5  10 mL), of gas potentially containing up to only 7.09  106 moles of product, a response from this small sample size may be difficult to detect over the instruments noise.

There are challenges to the use of mass spectrometry, however due to its sensitivity and detection limits this technique was considered a possible product identification technique for this work.

4.3.4

Infra Red Spectroscopy

Infrared spectroscopy is the measurement of the wavelength and intensity of the absorption of infrared light by a sample.[53,54] Infrared spectroscopy has high selectivity and the examination of the twisting, bending, rotating and vibrating motions of atoms is possible. IR spectroscopy can be used for gas, liquid or solid samples.[54] Every species examined has a unique fingerprint spectrum. Since no two chemical species have the same IR spectrum FTIR is a highly effective method for analysis. The product being considered here is either in liquid or gas phase, therefore, FTIR would be a possible product identification technique for this work.