Electrosynthesis on non-metal electrodes

Cox, Philip (1989) Electrosynthesis on non-metal electrodes. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

The use of Ni/Co spinels has been investigated in aqueous base, as anodes prepared by spraying aqueous nitrate solutions onto nickel or titanium and followed by thermal decomposition. It has been shown that these materials are highly stable and have a good catalytic activity for a number of organic oxidations and the oxidation of manganate to permanganate. The I-E response for the oxidation of ethanol shows a well formed wave (E_1/2 = 280 mV vs SCE and I_L= 24 mA cm^-2 for the oxidation of a 0.1 M solution of ethanol). The limiting current is partially mass transport controlled and much larger than that found at nickel. The effect of various solution and electrode preparation parameters on the limiting current density were investigated. This reaction was shown to be general and the preparative scale oxidation of a range of alcohols to carboxylic acids and amines to nitriles was investigated. Excellent material yields with good current efficiencies were obtained at high current densities (30 and 120 mA cm^-2 for 0.1M solution or emulsions). The oxidation of manganate to permanganate was also investigated at both nickel and NiCo₂O₄ electrodes. The I-E response for nickel gave a well formed wave. The effect of various parameters on the oxidation were investigated and the reaction was found to be partially mass transport controlled. I-E curves for the oxidation of manganate at NiCo₂O₄ coated electrodes gave a less well formed wave but a current of 4.5 mA cm^-2 was observed. The preparative scale electrolysis at both nickel and NiCo₂O₄ electrode was examined, both gave good conversions and current efficiencies even at high current densities, the NiCo₂O₄ coated electrode performed slightly better. The use of various refractory non-oxide ceramics was also investigated. The negative and positive limits of these materials was investigated. The use of these materials seems to be limited to negative potentials, and many of the materials have high hydrogen overpotentials with no chemical change being observed. Several of these materials seem to exhibit good behaviour for electron transfer but their use for organic oxidations is rather disappointing and relatively irreproducible.