The Poisoning Effect of Sulfur-Containing Compounds on the Elelctro-Oxidation of Methanol and Ethanol on Ni and Cu Electrodes

Valadbeigi, Younes | 2010

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 40719 (03)
  4. University: Sharif University of Technology
  5. Department: Chemistry
  6. Advisor(s): Gobal, Fereydon
  7. Abstract:
  8. In this study a systematic effort was made to investigate the poisoning effect of sulfur-containing materials on the electro- oxidation of methanol and ethanol on Ni and Cu electrodes in alkaline solutions by the methods of cyclic voltammetry, chronoamperometry and electrochemical impedance spectroscopy. Activation energy of methanol electro-oxidation in the absence of Na2S was obtained 30.7 kJ/mol by cyclic voltammetric at different temperatures, and its values increased by increase in Na2S concentrations to 97kJ/mol. Na2S adsorption isotherms corresponded with Langmuire isotherm, equilibrium constants of adsorption – desorption and adsorption enthalpy (ΔHads= -62kJ/mol) were calculated for them. Electro-oxidation of methanol on Ni in NaOH solutions with the alkali concentration in the range of 0.05 to 0.5 M and in the presence of 0.5M Na2SO4 supporting electrolyte was studied. All the mentioned species compete for adsorption on Ni and OH- is essential for the electro-oxidation process. The electro-oxidation seems to proceed through the electro-chemical reactions of adsorbed methanol and hydroxide species and the removal of the resulting intermediate by adsorbed hydroxyl groups. The mechanism is consistent with the cyclic voltammetry and impedance spectroscopy studies and can semi-quantitatively account for the appearance of capacitive, inductive and negative resistance as loops well as Warburg lines in the Nyquist plots. No involvement of Ni2+/Ni3+ in the oxidation process at low and medium concentration of alkali was observed. It was observed that Na2S and Na2SO4 have the poisoning effect on the electro- oxidation of ethanol on Cu electrode, and the peak of the ethanol electro-oxidation diminished in presence of the mentioned materials. Chronoamperometry showed that ethanol and OH- have a determining effect on the current intensity and mechanism of copper oxide nucleation. Kinetics parameters of copper oxide nucleation were obtained through fitting the experimental amperograms with the theoretical models. Nucleation currents corresponded with I=IDL+I2DI + I2DP or I=IDL+I2DI + I2DI depending on OH- and ethanol concentrations.
  9. Keywords:
  10. Sulfur-Containning Compounds Poisoning Effect ; Methanol Electro-Oxidation ; Nickel Electrode ; Copper Electrode ; Alkaline Solution ; Sodium Sufate Sulforing Electrolyte

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