Competitive effects of S-containing catalyst poisons on the UPD of H in relation of H2 evolution kinetics and OPD of H at Pt

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Abstract
The competitive adsorption of catalyst poisons and adsorbed H is of interest in relation to their effects in promoting H absorption into transition metals. The capacity of three sulfur-containing compounds, thiourea, L-cysteine and 2,2′-diethanolsulfide (DES), to block H adsorption at Pt electrodes in both the underpotential deposition (UPD) and overpotential deposition (OPD) regions is studied. Differences in the rates of adsorption (cysteine and DES < thiourea) and the reactivity of these molecules (thiourea > cysteine and DES) are shown to play important roles in the extent to which the poison, P, can block the UPD of H and modify the Tafel relationship for the hydrogen evolution reaction (HER). The relative H blocking, due to poison coverage, θp, is measured by means of cyclic voltammetry and potential-relaxation transient experiments. Measurements of rates of the HER as a function of overpotential are also made utilizing steady-state techniques. It is shown that, for θp of cysteine and DES < 0.9, inclusion of a constant site-blockage term is sufficient to account for the observed kinetic behaviour. In the presence of adsorbed thiourea, and virtually complete initial coverages of cysteine and DES, i.e. before any H2 evolution takes place, θp varies with potential according to an isotherm determined by the kinetics of hydrogenation and/or desorption of the poison.

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