EFFECT OF UNDERPOTENTIAL DEPOSITION (UPD) OF COPPER ON OXYGEN REDUCTION AT PT(111) SURFACES

The influence of underpotentially deposited Cu adlayers on the electro catalytic reduction of oxygen at Pt(lll) has been studied in 0.05 M H2 SO4 solutions using hanging meniscus rotating-disk (HMRD) voltammetry and electrochemical scanning tunneling microscopy (STM). Oxygen reduct ion at clean bare Pt(lll) proceeds by a direct four-electron transfer with the formation of H2O as the primary reaction product. After the f ormation of the first underpotentially deposited Cu adlayer with (root 3 X root 3)R30 degrees structure, the oxygen reduction current decrea ses to a steady-state value which is almost half that observed at bare Pt. This partial inhibition provided by the Cu adatoms can be explain ed by a change in the oxygen adsorption mechanism from the bridged ori entation, favoring a four-electron transfer, to the end on orientation , favoring a two-electron transfer. The effect of coadsorbed halides o n underpotential deposition (UPD) as well as the oxygen reduction reac tion, has also been examined. Oxygen reduction at Cu-modified Pt(lll) in the presence of chloride was completely inhibited after the first U PD process. Further, oxygen reduction in a pure H,SO, solution on bare Pt(lll) was carefully studied using HMRD voltammetry. The oxygen redu ction current at 0.02 V was almost half the constant limiting current observed in the potential region between 0.5 and 0.3 V. This result ca n also be explained by a change in the oxygen adsorption mechanism fro m the bridged to the end-on orientation.