New understanding of the nature of OH adsorption on Pt(111) electrodes

The adsorption of hydroxyl on Pt(111) single crystal electrodes from aqueous acidic solutions is carefully reinvestigated. The effect of small additions (10(-8)-10(-5) M) of chloride and bisulphate anions on the OH adsorption region in perchloric acid solution has been studied. Two regions can be differentiated in the voltammetric profile, that behave differently after the addition of the foreign anion. The initial broad adsorption process is unaffected until the highest concentration is attained. However, the sharper peak at higher potentials is affected even at the lower anion concentration. Since mass transport limitations allow to discard the anion adsorption as the main process giving this peak, we propose that the two processes are due to the dissociative adsorption of two different kinds of water, that are affected by the anion in a different way. From this idea, a new model, based on the Frumkin adsorption isotherm, is proposed, which gives an excellent fit of the experimental results. (C) 2007 Elsevier B.V. All rights reserved.