Browsing by Author "Conway, Brian E."

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Electric Modulated Reflectance at Mercury Electrodes: Transition between Electrostatic Adsorption and Chemisorption
(Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1974, 70 (10), 1974) Gottesfeld, Shimshon; Conway, Brian E.
Reflectance changes at polished electrode surfaces can arise from changes of optical properties of the metal surface and from changes of local ionic concentrations in the double-layer. The significance of these two effects in the interpretation of overall reflectance changes with potential (“electro-reflectance” effects), in the absence of film formation due to oxidation of the metal, is examined for the case of a mercury electrode. A liquid Hg surface is, however, unsatisfactory as a mirror electrode due to electro-mechanical oscillations which cause spurious reflectance changes. A thin Hg film on top of an electrolytically amalgamated Pt surface gives excellent results. The transition between electrostatic double-layer adsorption effects and more specific chemisorption can be demonstrated optically in the reflectance behaviour associated with Cl– and I– adsorption, by examination of reflectance of perpendicular and parallel polarized light in conjunction with phase-sensitive detection measurements. The behaviour of adsorbed thiourea is compared with that of the two halide ions.
Real Condition of Oxidized Platinum Electrodes: Part 2. —Resolution of Reversible and Irreversible Processes by Optical and Impedance Studies
(Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1973, 69 (6), 1973) Conway, Brian E.; Gottesfeld, Shimson
The reversible component in Pt surface oxidation (demonstrated in Part 1) has been characterised by means of electric modulated reflectance measurements and related to the electrochemical oxide formation and reduction processes studied by means of cyclic voltammetry. The reversible component gives a maximum response both in modulated reflectance and a.c. measurements at ca. 0.90 V and then decreases with increasing potential. The reversible component produced in the initial stages of oxidation is consumed at higher potentials or on holding the potential above 0.9 V. This is consistent with potentiodynamic results which show that a more stable oxide is produced at higher potentials or on holding the potential constant for some time and can be reduced only at relatively lower positive potentials. The change of properties of the surface oxide with time and/or with increasing positive potential and coverage, is consistent with a rearrangement of the initially formed ad-layer by a place-exchange mechanism. Such a process is absent at iridium.
Temperature and Pressure Effects on Surface Processes at Noble Metal Electrodes Part 2.—Volume of Adsorbed H and Oxygen Species at Pt and Au
(Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1978, 74 (06), 1978) Conway, Brian E.; Currie, John C.
The multiple states of electrosorbed H and oxygen species at Pt, which arise below monolayer coverage, are of current interest in electrochemical surface science. The results of experiments on the effects of high hydraulic pressure on H chemisorption and surface oxide formation at Pt electrodes are described. “Clean” electrochemical surface studies can be performed in a shrinkable Teflon vessel, under oil, up to several thousand bars. The equilibrium volume changes for electrodeposition of H and surface oxidation of Pt are evaluated allowing for the effect of pressure on the potential of the reversible reference electrode used. The volume of electrodeposited H is found to be ≈ 5.1 cm3 mol– 1 at Pt, a value similar to that for H in methylene groups. The pressure effects on surface oxidation arise from (a) the volume change in the surface oxidation reaction itself and (b) the volume change in desorption of specifically adsorbed anions which controls, in part, the potential for onset of surface oxidation of Pt and Au electrodes. Semi-quantitative interpretations of the observed behaviour are offered.
Temperature and pressure effects on surface processes at noble metal electrodes. Part 1.—Entropy of chemisorption of H at Pt surfaces
(Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1978, 74 (06), 1978) Conway, Brian E.; Angerstein-Kozlowska, Halina; Sharp, William B. A.
The temperature dependence of underpotential deposition of H at Pt electrodes has been investigated. The four main states of chemisorbed H which can be distinguished in dilute, highly purified aqueous acid solutions have temperature-dependent standard free energies of electrodeposition and ionization which enable the corresponding standard entropies of adsorption, ΔS0I, to be evaluated. Three of the states have a positive ΔS0I value consistent with discharge from H3O+ to 2-dimensionally mobile states in the Pt surface but with some restriction on mobility due to H2O and anion adsorption. An intermediate state “H3” has an almost zero standard entropy of adsorption which can be interpreted only in terms of deposition from water molecules in a special state, e.g. adsorbed and/or bound in the hydration shells of adsorbed anions. Under alkaline solution conditions, where complications due to anion adsorption are minimized, all states of electrodeposited H have the same entropy which is well accounted for by a 2-dimensional mobile monolayer deposited from water in its normal state. The significance of apparent decreases in total coverage by H attainable at Pt with increasing temperature is discussed and it is concluded that this effect arises simply because the onset of significant rates of H2 evolution occurs at more positive potentials (in N2) as the temperature is raised, thus allowing only a smaller and smaller coverage of adsorbed H to be experimentally measured before H evolution commences, i.e. some of the charge for H deposition must be passed above the H2 evolution potential for full coverage to be attained at elevated temperatures.