UHV AND ELECTROCHEMICAL TRANSFER STUDIES ON PT(110)-(1X2) - THE INFLUENCE OF BISMUTH ON HYDROGEN AND OXYGEN-ADSORPTION, AND THE ELECTROOXIDATION OF CARBON-MONOXIDE

We have investigated the anodic stripping of bismuth layers adsorbed o n Pt(110)-(1 x 2) by metal vapour deposition, and through under-potent ial deposition (upd) from solution. We have also studied the effect of the bismuth layers on hydrogen and oxygen adsorption in ultra-high va cuum (UHV), upd hydrogen adsorption-desorption, and the electro-oxidat ion 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 theta(Bf) = 0.75), with the lowest potential anodic peak as sociated with the formation of the bismuth adsorbed at coverages great er than 0.5 ML in the uniaxially compressed structure. An overall char ge of two electrons is associated with the oxidation process in the mo nolayer, 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 beta 2 hydro gen temperature programmed desorption (TPD) state without a shift in t he desorption temperature, and total blocking of hydrogen at 0.5 ML of bismuth at the completion of the c(2 x 2) overlayer on the unreconstr ucted surface. The blocking of the upd hydrogen layer by bismuth is no n-linear, and is fastest at lowest coverage of bismuth, At 0.5 ML cove rage 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 x 1) at 0.4 V produces on transfe r to UHV the TPD characteristic of 1 ML of CO in the Pt(110)-(2 x 1)pl gl overlayer. Electro-oxidation of this layer takes place in an anodic peak in the cyclic voltammetry at 0.69 V. CO adsorbed from solution a t 0 V saturates at a coverage of 0.8 ML, and undergoes electro-oxidati on at lower potentials, in a broader peak centred at 0.66 V and a smal l 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 sam e rate as CO adsorption is blocked in UHV. Bismuth also has the effect of increasing the electrochemical potential for CO oxidation from 0.6 9 V on the clean surface to 0.78 V when CO is co-adsorbed with bismuth in the Pt(110)-c(2 x 2) Bi overlayer, or at higher coverages. A shift to a higher CO oxidation potential is observed from the smallest cove rages studied, although at 0.1 ML the peak is broad, The electro-oxida tion of the CO also strongly influences the subsequent oxidation behav iour of the bismuth which takes place in a highly asymmetric anodic pe ak at 1.0 V. The potential of CO oxidation in the mixed bismuth CO lay er on Pt(110)-(1 x 2) is very similar to that on Pt(lll), and the resu lts are explained using a model involving substrate-mediated oxygen tr ansfer in CO electro-oxidation.