Surface characterization and electrochemical behavior of well-defined Pt-Pd{111} single-crystal surfaces: A comparative study using Pt{111} and palladium-modified Pt{111} electrodes

Two Pd-Pt{111} single-crystal alloy surfaces have been prepared with 6.25% and 25% Pd bulk composition. Their surface structure and composition have been characterized both in ultrahigh vacuum (UHV) using low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), and low-energy ion scattering (LEIS) and electrochemically using cyclic voltammetry (CV). Comparison has been made with the voltammetric response of Pt{111} and Pt{111} modified with a monolayer of palladium [Pd-Pt{111}] The alloy crystals when clean and thermally annealed in UHV gave rise to sharp (1 X 1) LEED patterns. AES and LEIS revealed that a surface enrichment in palladium took place in agreement with an earlier study using polycrystalline Pd-Pt alloy samples. Gentle ion etching of the thermally equilibrated surface (to remove the selvedge region) followed by AES and LEIS analysis established that the bulk composition was in precise agreement with the nominal bulk composition expected on the basis of the amount of palladium introduced into the melt for the 25% alloy, but a slightly higher value was determined for the 6.25% alloy. Nonetheless, the veracity of the Clavilier method in producing high-quality single-crystal Pt-Pd alloy bead electrodes is confirmed. Electrochemical studies of the alloy surfaces in perchloric acid, perchloric acid containing chloride ions, and aqueous acidic copper chloride revealed a systematic and smooth gradation of voltammetric response when compared with results for Pt{111} and Pd-Pt{111}. For example, CV features associated with "Pt-like" and "Pd-like" regions can be identified readily and a scaling in their intensity can be related rather straightforwardly to the concentration of palladium in the surface.