Investigations of surface structure and catalyst poisons, on electrochemical processes involving hydrogen discharge and evolution.

Title: Investigations of surface structure and catalyst poisons, on electrochemical processes involving hydrogen discharge and evolution.
Authors: Barber, John H.
Date: 1998
Abstract: HER at Pt single-crystal electrodes. It is well known that transition metals exhibit a wide range of catalytic activities for various processes. One of the best examples is the dependence of H$\sb2$ evolution kinetics on the nature of the cathode metal. This leads to an expectation that a similar specific dependence would also be observed at various single-crystal surfaces of Pt. In the present paper, it is shown that such conclusions arise from the actual surface-specific electrode kinetics being obscured by effects associated with diffusion of H$\sb2$ from a supersaturated region generated near the electrode. By means of Tafel polarization and impedance measurements at several Pt single-crystal surfaces rotated at 3500 rpm, it is shown that there are, in fact, clear specific dependences of the H$\sb2$ evolution kinetic parameters on the Pt surface lattice geometry which are related to the underpotential deposition (UPD) of H kinetics and to the well known surface specificity of the H UPD behaviour at Pt as seen in cyclic voltammetry. An increase of the pH, decreases the kinetic facility of the hydrogen electrode reaction, i.e. abstraction of H to form H$\sb2$ is more difficult from H$\sb2$O than from $\rm H\sb3O\sp{+}$; at such lowered rates, the H$\sb2$ diffusive effects can be totally eliminated by electrode rotation, unlike the situation in 0.5 mol dm$\sp{-3}$ $\rm H\sb2 SO\sb4$ where rotation is found to only decrease the effect of diffusion. Coadsoration of sulphur containing compounds and H at Pt, and its effect on the HER rates. The competitive adsorption of catalyst poisons and adsorbed H is of interest in relation to their effects in promoting H absorption into transition metals. The capacity of three sulphur-containing compounds, thiourea, L-cysteine, and 2,2$\sp\prime$ diethanolsulphide (DES), to block H adsorption at Pt electrodes in both the underpotential deposition (UPD) and overpotential deposition (OPD) regions was studied and is presented in chapter 6. Chapters 8 and 9 investigate the influence of poisons on the absorption of H into Pd and Ni electrodes. H adsorption into Pd and Ni cathodes and the effect of adsorbed thiourea. Pd and Ni electrodes both adsorb and absorb H electrochemically. Using these two materials as cathodes the effects of the adsorption of thiourea on the electrochemical reactions of H, adsorption, absorption and evolution were studied. The adsorption of thiourea dramatically inhibits, by over one order of magnitude of current, the rate of H entry into Pd. The adsorption of thiourea at a Ni surface on the other hand appears to have a relatively small effect on the rate of H entry, but it does have a significant influence on the rate of H$\sb2$ evolution, this manifests itself in a significant change in the chemical potential ($\mu\sb{H}$) of adsorbed H. (Abstract shortened by UMI.)
CollectionTh├Ęses, 1910 - 2010 // Theses, 1910 - 2010
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