EFFECTS OF CATALYST POISONS ON UPD AND OPD H COVERAGE AT H-2-EVOLVINGCATHODES IN RELATION TO H SORPTION INTO THE METALS

In a previous paper we have directed attention to the apparently anoma lous situation that at H-2 evolving cathodes, e.g. Fe, steels, Ti, che misorption of catalyst poisons such as S or As-containing compounds mu ch enhance the rate or extent of H sorption into such metals yet they diminish the coverage by adsorbed H that is the intermediate in the H- 2-evolution reaction (HER) and the source of the sorbed H. In new work presented in this paper, the effects of competitive chemisorption of thiourea, cysteine, diethanolsulphide and As-species on the underpoten tial deposition (UPD) of H, i.e. below the H-2/H+ reversible potential , and on the extra overpotential-deposited (OPD) H associated with the mechanism of the HER, are quantitatively evaluated at Pt and Pd by me ans of cyclic voltammetry, steady-state polarization measurements and potential-step transients. H transfer rates through Fe-membrane bielec trodes are increased 10-fold by the presence of 31 p.p.m. As in the ch arging solution. Since H coverage is substantially reduced by the As, and H diffusion is a bull; process, this enhancement must be due to a kinetic effect in the interfacial of H transfer into the metal.