THE EFFECT OF SPECIFIC ADSORPTION OF IONS AND UNDERPOTENTIAL DEPOSITION OF COPPER ON THE ELECTROOXIDATION OF METHANOL ON PLATINUM SINGLE-CRYSTAL SURFACES

The specific adsorption of anions in acid electrolytes containing sulfuric and hydrochloric acids was found to have a strongly inhibiting effect on the rate of methanol electro-oxidation on both the (111) and (100) surfaces of platinum. The effect was much stronger in hydrochloric acid, requiring approximately three orders of magnitude higher concentration of sulfuric acid to achieve the same inhibiting effect (and surface coverage by adsorbed anion). Cl- adsorption isotherms were measured using the emersion technique and ex situ Auger electron spectroscopy analysis. The mechanism of inhibition appeared to be similar for both anions. On the (111) surface, anion adsorption was found to proceed in two stages, one coupled to hydrogen desorption producing a co-adsorbed state, and a second stage of adsorption at more positive potentials into the anion adlattice from which hydrogen has been desorbed. The second stage process on Pt(100) is different, occurring at more positive potentials than on Pt(111) and concurrent with OH formation. Completion of the second stage of adsorption appears to produce the inhibition that causes a current peak to be observed in the voltammetry of methanol in acids containing these anions. Underpotential deposition (UPD) of copper in acids containing these electrolytes caused strong additional inhibition of the methanol oxidation rate. The effect was attributed to induced anion adsorption on platinum atoms near to the UPD copper atoms due to a lowering of the local pzc by the copper adatoms.