pH effects on the potentiodynamic behavior of the Pt(111) electrode in acidified NaClO4 solutions

The pH increase was found to bring important changes in the potentiodynamic behavior of the Pt(111) electrode in perchlorate media: an additional couple of anodic-cathodic electrode processes is developed in the double layer region limited by the usual and unusual adsorption states, while all the peaks are shifted toward more negative potentials. The shift of the anomalous anodic wave (60 mV/pH unit) is present, however, only at pH < 4. The pH values around 4 proved to be critical for the other electrode processes too. The hydrogen evolution reaction ceased to occur around this pH value, being replaced by the water cathodic reduction. The surface H+ discharge step disappears also at a pH of 4.5 as a consequence of the site blocking with species formed in the high potential region. The results plead for the dissociative adsorption of the water dependent on the local changes of the pH within the solution close to the electrode induced by the surface reactions themselves. (C) 1998 Elsevier Science Ltd. All rights reserved.