Anomalous electrochemical behaviour of palladium in aqueous solution

Palladium electrode surfaces in aqueous sulphuric acid solution were activa ted or disrupted by repeated growth and reduction of multilayer hydrous oxi de films. A very low percentage (<1.0%) of the metal deposited on reduction of such films existed initially in an unusually active state. This active palladium exhibited a reversible redox transition at ca. 0.24 V (RHE) at 25 degrees C and the charge involved in the latter process was directly propo rtional to the much larger charge value associated with the reduction of th e HO2 component in the multilayer hydrous oxide deposit. The charge associa ted with the active state of the metal decreased gradually, in a first-orde r maimer, subsequent to the generation of the active state. The nature of t he active state of metal surfaces, which is assumed to be of considerable i mportance in heterogeneous catalysis and electrocatalysis, is discussed bri efly. (C) 1999 Elsevier Science S.A. All rights reserved.