Behaviour of the adsorbed chloride intermediate in electrocatalysis of anodic chlorine evolution at oxide film surfaces at platinum and ruthenium.

Abstract

In this thesis, the investigation of the mechanism of chlorine evolution reaction at (i) both freshly reduced and pre-oxidized Pt electrodes; (ii) RuO$\sb2$-TiO$\sb2$ electrodes (DSA); (iii) other noble metals such as Pd, Ir, Ru and Rh by means of computer controlled anodic polarization, potential-relaxation transients and a.c. impedance measurements, will be presented. As described in Chapter V, the influence of co-deposited OH and O species in the surface oxidation process on the extent of chemisorption of the Cl$\sp-$ intermediate in the Cl$\sb2$ evolution reaction has been carefully examined. Experimental results reveal that the reaction rate at reduced Pt surfaces is much greater than that at pre-oxidized ones. This is because the oxide species have a significant effect in blocking the Cl$\sb2$ evolution kinetics. The detectable surface adsorption pseudocapacitance, which is associated with the "overpotential deposited" Cl$\sp-$ intermediate, at freshly reduced Pt and oxide free Pt surfaces in non-aqueous trifluoroacetic acid (TFA) medium supports this conclusion. In Chapter VI, some exploratory experiments at other noble metal (e.g., Pd, Ir, Ru, and Rh) electrodes in non-aqueous TFA solution have shown very interesting interfacial capacitance information, which must be interpreted probably in terms of more than one adsorbed reaction intermediate being present. In Chapter VII, a possible mechanism of recombination kinetically control has been proposed for the Cl$\sb2$ evolution reaction at RuO$\sb2$-TiO$\sb2$ electrodes, based on the observations of their steady-state behaviour, and electrochemical data from potential relaxation and a.c. impedance measurements. Local supersaturation of Cl$\sb2$ in the microstructure of the oxide film appears to make a contribution to the pseudocapacitance behaviour. Some of the results in this work have been published or in press, as listed below: (1) "Surface electrochemistry of the anodic chlorine evolution reaction at Pt influence of co-deposition of the Cl$\sp-$ intermediate", P. Gu and B. E. Conway, J. Chem. Soc. Farada Trans. 86(6) (1990) 923. (2) "Behaviour of the adsorbed Cl$\sp-$ intermediate in anodic chlorine evolution at thin-film RuO$\sb2$ surfaces", P. Gu and B. E. Conway, J. Appl. Electrochem., in press. (3) "Evaluation of Cl$\sp-$ adsorption in anodic chlorine evolution at Pt by means of a.c. impedance and potential relaxation experiments: role of the state of surface oxidation", P. Gu and B. E. Conway, J. Chem. Soc. Faraday Trans., in press.