EVALUATION OF CL ADSORPTION IN ANODIC CL-2 EVOLUTION AT PT BY MEANS OF IMPEDANCE AND POTENTIAL-RELAXATION EXPERIMENTS - INFLUENCE OF THE STATE OF SURFACE OXIDATION OF THE PT

The behaviour of the adsorbed Cl. intermediate in anodic Cl2 evolution at Pt electrodes has been investigated by means of impedance spectroscopy. Because, in aqueous solution, surface oxide formation is competitive with Cl- ion and Cl. adsorption, experiments have been conducted (a) at Pt anodes pre-oxidized under controlled conditions and (b) at electrodes 'freshly reduced' before each polarization measurement or immediately prior to each a.c. frequency scan. At the freshly reduced electrodes, the complex-plane plots exhibit two semi-circles while at the pre-oxidized ones, only single semi-circles are observed. This difference indicates appreciable coverage and resulting adsorption pseudocapacitance of adsorbed Cl. at the 'reduced' surfaces while at the pre-oxidized surfaces, the presence of the oxide film greatly diminishes the pseudocapacitance, corresponding to much lower Cl. coverages. However, under both conditions of electrochemical surface pretreatment, the mechanism of anodic Cl2 formation appears to involve a rate-controlling Cl. recombination process leading to observable non-diffusion-controlled limiting currents. The impedance measurements are complemented by potential-relaxation studies which give corresponding major differences in the adsorption behaviour of Cl. at pre-oxidized and 'reduced' Pt electrodes. The presence of the oxide film modifies sensitively the Pt anode surface for chemisorption of the kinetically involved Cl. intermediate in anodic Cl2 evolution and, on that account, also inhibits electrocatalysis in that reaction.