A Study of Ascorbic Acid Electro-Oxidation Process on Silver and Silver Selenide Electrodes

Majari Kasmaee, Laleh | 2011

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 42417 (03)
  4. University: Sharif University of Technology
  5. Department: Chemistry
  6. Advisor(s): Gobal, Fereydoon
  7. Abstract:
  8. Electrocatalytic oxidation of L-ascorbic acid (L-AA) on silver electrode in alkaline solutions was studied employing cyclic voltammetry, chronoamperometry and impedance spectroscopy techniques. In comparison to Pt, the onset potential for L-AA electro-oxidation process on Ag, in a solution containg 0.02 M L-AA and 0.5 M NaOH, was 0.1 V more negative. Also, L-AA oxidation current increased more than 900% at -0.26 V/SCE. According to voltammety and impedance spectropy findings, although hydroxide anion presence was vital for L-AA oxidation to occur, at potentials more positive than -0.2 V/SCE, hydroxide ion tends to inhibit the oxidation process. Studies on the L-AA oxidation process on Ag in presence of both hydroxide and poly sulfide species revealed poisoning of Ag caused by the competitive adsorption of hydroxide and poly sulfide anions. As a result of electrode poisoning, time constant of the electron transfer process increases by about 102 folds. In the course of a separate study, a 1 μm thick Ag2Se layer was electrodeposited on silver by means of repetitive cyclic voltammetry. SEM and EDX analysis showed the presence of 5% excess Ag as crystallite on Ag2Se film. The onset potential of L-AA oxidation on Ag2Se was 80 mV more negative than that on Ag electrode and the anodic peak current was 14% larger on Ag2Se than on Ag at -0.3 V/SCE. Also, on the basis of voltammetric findings, L-AA oxidation occured on Ag crystallite sites as well as Ag2Se layer. Kinetics parameters of ascorbate anion oxidation process such as its diffusion coefficient, standard heterogenous rate constant, exchange current density and transfer coefficient were calculated. A preliminary BOC-MP study on the mechanism of ascorbate anion oxidation on Ag in the presence of adsorbed hydroxide anion was carried out and some thermodynamic parameters such as reactions enthalpies, activation energies and free Gibbs energies of reactions were derived
  9. Keywords:
  10. Inhibitor ; Electro-Oxidation ; Ascorbic Acid ; Bond-Order Conservation-Potential Method (DBOC-MP) ; Poly Sulfide Anion ; Silver Selenide Electrodeposition ; Time Constant

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