Kinetic interpretation of promoting/inhibiting effects of chemisorbed species on hydrogen sorption at/in iron, mild-steel and palladium.

摘要

The aim of the studies described in this thesis was to examine the effects of chemisorbed catalyst poisons on the behaviour of the hydrogen evolution reaction (HER) and especially the enhancement by poisons of the coupled absorption and permeation of H that occurs at various transition metals. Such effects were studied comparatively at Fe, mild-steel and Pd electrodes in aqueous NaOH solutions using a Devanathan-Stachurski bi-electrode cell to follow H permeation currents in relation to simultaneously measured polarization behaviour on the H2-evolution side of the electrodes.; The poisoning effects associated with competitive chemisorption of As-containing compounds on the behaviour of the HER, and hydrogen permeation at Fe and mild-steel, have been investigated. By means of an equation-fitting procedure, applied to the kinetics of all the constituent partial reactions involved, including cathodic reduction of added AsO2- to As(0) and AsH3, and also considering the factors which represent modification of the rate constants due to induced heterogeneity caused by communal interactions between the adsorbed H and poison species on the surfaces of Fe and mild steel, a good account of the mechanism of the effects of As-species was able to be achieved. It was found that the presence of As species, initially as AsO 2-, significantly enhances H permeation into Fe and mild-steel but only at appreciable cathodic potentials; however, it reduces H permeation currents at less cathodic potentials. The effect involves reductive desorption of As-containing species, eventually to AsH3 (gas), coupled with increased H coverage at high cathodic overpotentials, leading to enhancement of H absorption and permeation.; The effect of chemisorption of S-containing species on H transfer into and through Fe and mild-steel was studied comparatively and complementarily in relation to poisoning by species derived from AsO2- . The S-containing compound, HS-, has a different effect on the H surface coverage, thetaH, (thetaH becomes decreased, compared to the poison-free case) at high cathodic overpotentials, but its poisoning effects are similar to those of arsenic species on H absorption and permeation. The H absorption and permeation are enhanced at high and inhibited at less cathodic overpotentials in both situations of decreased or increased limiting H coverage values, thetaH.; The effect of chemisorption of S species (as HS-) on H transfer into and through Pd has also been studied. The relations of surface coverage by H on the Pd surface in UPD region (also extended to the OPD region) to the subsurface concentration of H, and the effect of chemisorption of sulphide species on H transfer into and through a Pd membrane bielectrode, has also been examined. The thetaH over the UPD region is diminished by the chemisorption of the S species. The H adsorption and the H permeation rates are related in a 1:1 ratio in the UPD region in both the absence and presence of poison solutions, thus a 100% permeation efficiency, epsilon, in this potential region. (Abstract shortened by UMI.)