Electrochemical studies of two-dimensional processes at well-defined platinum single crystal electrodes.
Creators: name: Morin, Sylvie.
Publisher: name: Université d'Ottawa / University of Ottawa
Year: 1996
Subjects: subject: Chemistry, Physical.

description: The principal aims of this part of the thesis work were to examine the dependence of the kinetics of the charge-transfer leading to H chemisorption in various states on various crystal-plane geometries of single-crystals of Pt through evaluation of the charge-transfer resistance, R$\rm \sb{ct}$, and how this process depends substantially on the orientation and two-dimensional structure of the surface. Analysis of the ac results by means of equivalent-circuit fitting allows determination of the double-layer capacitance, C$\rm \sb{dl}$, and the H adsorption pseudocapacitance, C$\rm \sb{p}$, which is determined by H (and anion) coverages, as well as the kinetics of the charge-transfer itself as a function of the potential. Very good correlation between the results for total capacitance (C$\rm \sb{dl}$ + C$\rm \sb{p}$'s) determined by cyclic voltammetry and by impedance measurements was obtained for Pt(100), (111), (110) and (311) surfaces in 0.5 mol $\rm dm\sp{-3} H\sb2SO\sb4$ solution, with allowance being made, where appropriate, for HSO$\sb4\sp-$ ion co-adsorption. The variation of parameters with lowering of the temperature was also investigated for Pt(100), (111), (110) and (311) in 0.5 mol $\rm dm \sp{-3} H\sb2SO\sb4$. This allowed determination of the apparent activation energy for the H adsorption-desorption process at Pt(100) and (311). In another main part of the work, the effects of chemisorbed acetonitrile on the underpotential depositions (UPD) of H (and anion adsorption-desorption) in acidic solutions were evaluated. From studies of this process at polycrystalline Pt, acetonitrile was found to be reductively chemisorbed at Pt at the beginning of the "double-layer" potential region in the cyclic voltammogram for this electrode material, taken between 0.06 V and ca. 0.80 V vs: RHE. In this adsorbed state, it then undergoes an almost reversible stage of reoxidation or re-reduction, and slower further reduction over the potential range for UPD of H. In this respect, Pt, bearing a sub-monolayer of chemisorbed acetonitrile, was one of the first "chemically modified electrodes" to be studied. These processes were investigated by means of cyclic voltammetry at the (111), (110) and (100) planes, together with the (311) stepped plane, of well characterized single-crystal Pt surfaces which exhibit, in the absence of adsorbed acetonitrile, the characteristically distinguished cyclic voltammograms demonstrated by Clavilier and other research groups for clean, well ordered surfaces. The chemisorption and surface reactivity behaviour of acetonitrile at the above crystal planes is found to be very specific to the geometry of the surfaces and provides one of the clearest examples of such surface specificity in electrochemical surface science for an adsorbed organic molecule at Pt. Similarities to and contrasts with the surface-specific behaviour of the four crystal planes with respect to H UPD behaviour and HSO$\sb4\sp-$ or ClO$\sb4\sp-$ anion adsorption are observed and their significance is discussed. (Abstract shortened by UMI.)