HYDROGEN PRESSURE EQUIVALENT TO OVERPOTENTIAL ON PD+AG ALLOY ELECTRODES IN ACIDIC SOLUTIONS IN THE PRESENCE OF THIOUREA

The hydrogen-electrode reaction on Pd + Ag alloy electrodes (x(Pd) > 0.60) in 0.5 M H2SO4 was studied by means of a transient polarization technique, with particular emphasis on the augmentation of the hydrogen pressure equivalent to overpotential by using the catalytic poison thiourea. No systematic dependence of the electrocatalytic activity on alloy composition was discernible. The reaction kinetics are consistent with the mixed controlled Volmer-Tafel mechanism, and the slowly decaying overpotential component, which is attributed to the Tafel step, increased with the addition of thiourea. At a thiourea concentration of 30 muM, the overpotential component was raised to almost -200 mV cathodic on all the electrodes studied at 0.25 A cm-2 and 30-degrees-C. That this large negative value indeed originates from the existence of hydrogen at high pressure/concentration and not from other trivial causes was demonstrated by observing the recovery of the value after application of an anodic stripping current pulse. The highest equivalent hydrogen pressure evaluated was ca. 10(6) atm.