An in situ FTIR spectroscopic study of the electrochemical oxidation of ethanol at a Pb-modified polycrystalline Pt electrode immersed in aqueous KOH.

A study of the mechanism of ethanol oxidation in alkaline solution on a platinum electrode modified with an irreversibly-deposited layer of lead has been carried out using in situ FTIR spectroscopy. This study provides support for the suggestion that the adsorption mechanism of ethanol is substantially modified in the presence of Pb, with a carbon-bonded intermediate being favoured leading to facile scission of the C-C bond in ethanol. We have found that the formation of carbonate takes place at potentials close to the thermodynamic value. At higher potentials, when Pb is lost to solution, the mechanism of oxidation of ethanol reverts to that found on a normal polycrystalline Pt surface, with the primary product being acetate.

The electro-oxidation of formate ions at a polycrystalline Pt electrode in alkaline solution: an in situ FTIR study.

This paper reports in situ FTIR studies on the oxidation of formate at polycrystalline Pt in aqueous KOH. Data are presented which show that hydroxyl species play a major role in the electro-oxidation of small organic molecules under alkaline conditions at polycrystalline Pt, and that a number of possible mechanistic pathways are possible. Small changes in experimental conditions appear to be able to cause the reaction to flick between these pathways; for example, the presence of oxygen has a marked effect upon the observed electrochemistry. In contrast to acid solution, our postulated model includes the formation of intermediates bonded through O atoms, rather than C, as being an important option in alkaline solution. Finally, the pH distribution across the reflective electrode in external reflectance IR is modelled and significant variations in pH across the electrode surface in FTIR cells predicted and confirmed experimentally.

Oxygen reduction and fuel oxidation in alkaline solution.

This paper reviews work carried out over a number of years to try to elucidate the mechanism of oxygen reduction and methanol oxidation in alkaline solution. We have sought to achieve this by combining electrochemical, spectroscopic and solid-state chemical approaches, bringing together as wide a variety of techniques as possible both to shed light on the mechanisms and to point the way to more effective and efficient fuel cells. This work has become considerably more topical in recent years with the development of anion-exchange electrolyte membranes that can operate in alkaline environments, an important advance since it permits both the use of non-noble-metal catalysts and organic fuels at the anode, the latter precluded in aqueous alkaline electrolyte due to precipitation of inorganic carbonates at the electrode surface.

Electrocatalytic reduction of hydrogen peroxide at a stationary pyrolytic graphite electrode surface in the presence of cytochrome c peroxidase: a description based on a microelectrode array model for adsorbed enzyme molecules.

Electrochemical reduction of H2O2 at pyrolytic graphite disc electrodes of radius 2.5 mm occurs at readily accessible potentials (600 mV versus the standard hydrogen electrode) in the presence of yeast cytochrome c peroxidase. Introduction of the enzyme into the electrolyte solution initiates large changes in the ellipsometric angles measured for the electrode-solution interface, consistent with time-dependent enzyme adsorption. This process may be correlated with changes in electrochemical activity. Over the same time course, linear-sweep voltammograms are characterized by a transition from a sigmoidal to a peak-type waveform. It is proposed that the time-dependent behaviour may be rationalized by use of a microscopic model for substrate mass transport, in which the two-electron reduction of peroxide occurs at electrocatalytic sites consisting of adsorbed enzyme molecules. A voltammetric theory based on treating the adsorbed redox enzymes as an expanding array of microelectrodes is in excellent agreement with experiment.

The absolute configuration of precocene I dihydrodiols produced by metabolism of precocene I by corpora allata of Locusta migratoria, in vitro.

The corpora allata from adult female Locusta migratoria metabolize precocene I (7-methoxy-2,2-dimethyl-2H-benzo [b]pyran to cis- & trans-precocene I dihydrodiols (3,4-dihydro-7-methoxy-2,2-dimethyl-2H-benzo [b]pyran-3,4-diol). Derivatization of the dihydrodiols with (-)menthoxy acetyl chloride allowed complete resolution of all four optical isomers. When [4-3H]-precocene I was incubated in vitro with Locusta migratoria corpora allata, it was metabolized stereospecifically to (-)trans-(3R,4S) and (+)cis-(3R,4R) dihydrodiols. Approximately half the precosyl residues bound to cellular macromolecules were discharged by heating to 95 degrees C at neutral pH, as dihydrodiols of the same stereochemistry.

Enzymic synthesis of juvenile hormone in locust corpora allata: evidence for a microsomal cytochrome P-450 linked methyl farnesoate epoxidase.

Homogenates of corpora allata from adult Locusta migratoria in phosphate-buffered EDTA have been analysed by sucrose-density-gradient centrifugation. Succinate-cytochrome c reductase activity (mitochondrial) bands between d20/4 1.13-1.15, whereas NADPH-cytochrome c reductase and NADPH-dependent methyl farnesoate 10.11-epoxidase activities band identically between d20/4 1.06-1.12. We conclude that the methyl farnesoate epoxidase is exclusively microsomal. Farnesoic acid O-methyltransferase is an exclusively soluble enzyme which stoichiometrically transfers the S-methyl group from S-adenosylmethionine to farnesoic acid. No carboxyl esterase activity was found. Isolated microsomes were used to obtain an apparent Km = 7.7 X 10-6 M for the epoxidase, although substrate solubility limits the rate to 0.5 V. As expected, the product (juvenile hormone III) is chiral (10 R). The epoxidase is inhibited by excess NADP+ and oxidised cytochrome c, but neither inhibited nor synergised by NADH. NADH supports less than 10% of the NADPH rate of epoxidation. The epoxidase is inhibited by a carbon monoxide/oxygen atmosphere, half-maximal inhibition occurring at a CO/O2 ratio of 4.0. This inhibition is reversed by white-light irradiation.