Effect of underpotential deposition (UPD) of copper on oxygen reduction at Pt(111) surfaces

The influence of underpotentially deposited Cu adlayers on the electrocatalytic reduction of oxygen at Pt(111) has been studied in 0.05 M H₂SO₄ solutions using hanging meniscus rotating-disk (HMRD) voltammetry and electrochemical scanning tunneling microscopy (STM). Oxygen reduction at clean bare Pt(111) proceeds by a direct four-electron transfer with the formation of H₂O as the primary reaction product. After the formation of the first underpotentially deposited Cu adlayer with (√3 × √3)R30° structure, the oxygen reduction current decreases to a steady-state value which is almost half that observed at bare Pt. This partial inhibition provided by the Cu adatoms can be explained by a change in the oxygen adsorption mechanism from the bridged orientation, favoring a four-electron transfer, to the end on orientation, favoring a two-electron transfer. The effect of coadsorbed halides on underpotential deposition (UPD) as well as the oxygen reduction reaction, has also been examined. Oxygen reduction at Cu-modified Pt(111) in the presence of chloride was completely inhibited after the first UPD process. Further, oxygen reduction in a pure H₂SO₄ solution on bare Pt(111) was carefully studied using HMRD voltammetry. The oxygen reduction 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 can also be explained by a change in the oxygen adsorption mechanism from the bridged to the end-on orientation.