ROLES OF THIOUREA AS AN INHIBITOR IN HYDROGEN ABSORPTION INTO PALLADIUM ELECTRODE

The effects of thiourea as a catalyst poison on hydrogen absorption re action (HAR) into Pd foil electrode in the under-potential deposition region have been investigated by using ac impedance spectroscopy combi ned with cyclic voltammetry and hydrogen permeation technique. The ac impedance measurements were carried out in the overpotential range of 0 to 0.22 V-RHE in 0.1 M NaOH solution containing various concentratio n of 0, 10(-8), 10(-7), and 10(-6) M thiourea. Measured impedance spec tra were analyzed by using complex nonlinear least squares fitting met hod on the basis of faradaic admittance equations for the HAR under Im permeable boundary conditions. Based upon the measured thiourea concen tration dependence of charge-transfer resistance, cyclic voltammogram, and steady-state permeation current density the transition of the HAR into the Pd foil electrode from diffusion controlled to inter-face co ntrolled has been discussed at the concentration of 10(-7) M thiourea. The experimental results indicate that the poison thiourea added into the electrolyte impedes the HAR into the Pd electrode and hence binde rs the beta-PdH formation in the Pd electrode, in contrast to such met als as Fe, Ti, and Ni.