HETEROGENEOUS ELECTROCATALYSIS ON WELL-DEFINED PLATINUM SURFACES MODIFIED BY CONTROLLED AMOUNTS OF IRREVERSIBLY ADSORBED ADATOMS .4. FORMIC-ACID OXIDATION ON THE PT(111)-AS SYSTEM

The effect of arsenic irreversibly adsorbed on Pt(111) has been studied for the electrocatalytic oxidation of formic acid in the whole range of coverage. The maximum intrinsic oxidation rate on Pt(111) has been measured using pulsed voltammetry. The presence of adatoms leads to an increase in the activity for the direct reaction by a factor 1.5 in the range of potential where the adatoms are in their reduced state. When their oxidation takes place, the current of formic acid oxidation falls exactly in the potential range where the adatom oxidation occurs. Once the adatoms are in their oxidized state, the activity of the catalyst is lower than that of pure Pt(111). The presence of adatoms influences the selectivity inhibiting the poison formation even at low arsenic coverage suggesting an electronic effect at the surface induced by As adatoms. A model with two independent reactions taking place on two active surface domains (the Pt free sites and the As covered ones) for the active intermediate pathway has been proposed to account for the effect of adatoms in the potential range where they are in their reduced state.