UHV and electrochemical transfer studies on Pt(110)-(1 × 2) : the influence of bismuth on hydrogen and oxygen adsorption, and the electro-oxidation of carbon monoxide

Abstract

We have investigated the anodic stripping of bismuth layers adsorbed on Pt(110)-(1 × 2) by metal vapour deposition, and through under-potential deposition (upd) from solution. We have also studied the effect of the bismuth layers on hydrogen and oxygen adsorption in ultra-high vacuum (UHV), upd hydrogen adsorption-desorption, and the electro-oxidation of CO in dilute sulphuric acid electrolyte. A bismuth monolayer (0.7 ML) undergoes electro-oxidation at 0.77 V and 0.88 V (vs. Pd | H 2) (saturation θ Bi = 0.75), with the lowest potential anodic peak associated with the formation of the bismuth adsorbed at coverages greater than 0.5 ML in the uniaxially compressed structure. An overall charge of two electrons is associated with the oxidation process in the monolayer, whereas three electrons are involved in the anodic stripping of the second layer at 0.59 V, and third and subsequent layers at 0.5 V. Bismuth adsorption results in a linear blocking of the β2 hydrogen temperature programmed desorption (TPD) state without a shift in the desorption temperature, and total blocking of hydrogen at 0.5 ML of bismuth at the completion of the c(2 × 2) overlayer on the unreconstructed surface. The blocking of the upd hydrogen layer by bismuth is non-linear, and is fastest at lowest coverage of bismuth. At 0.5 ML coverage of bismuth, ca. 0.5 ML of upd hydrogen can be adsorbed, and at 0.7 ML of bismuth 0.25 ML of upd hydrogen is adsorbed. The bismuth does not change the potential of the upd hydrogen adsorption-desorption. CO adsorbed from solution on Pt(110)-(2 × 1) at 0.4 V produces on transfer to UHV the TPD characteristic of 1 ML of CO in the Pt(110)-(2 × 1)plgl overlayer. Electro-oxidation of this layer takes place in an anodic peak in the cyclic voltammetry at 0.69 V. CO adsorbed from solution at 0 V saturates at a coverage of 0.8 ML, and undergoes electro-oxidation at lower potentials, in a broader peak centred at 0.66 V and a small peak at 0.38 V. The effect of bismuth adsorption is to decrease the amount of CO adsorbed, and available for electro-oxidation, at the same rate as CO adsorption is blocked in UHV. Bismuth also has the effect of increasing the electrochemical potential for CO oxidation from 0.69 V on the clean surface to 0.78 V when CO is co-adsorbed with bismuth in the Pt(110)-c(2 × 2) Bi overlayer, or at higher coverages. A shift to a higher CO oxidation potential is observed from the smallest coverages studied, although at 0.1 ML the peak is broad. The electro-oxidation of the CO also strongly influences the subsequent oxidation behaviour of the bismuth which takes place in a highly asymmetric anodic peak at 1.0 V. The potential of CO oxidation in the mixed bismuth CO layer on Pt(110)-(1 × 2) is very similar to that on Pt(111), and the results are explained using a model involving substrate-mediated oxygen transfer in CO electro-oxidation.