Electro-oxidation of the Pd (100) surface

The Pd (100) surface, prepared and characterized in an ultra-high vacuum, was subjected to monolayer-level electro-oxidation treatments by increasing linearly the electrode potential in an aqueous HClO4 electrolyte. Subsequently, the potentiostatically emersed surface was recharacterized by XPS and LEED. This was made possible through utilization of apparatus coupling UHV and electrochemical practices. The first, chemisorption stage of electro-oxidation takes place over a narrow potential range around 0.6 V (vs. Ag/AgCl). In contrast to H2SO4 electrolyte, where the presence of surface palladium atoms with a rather large, 2.5 eV shift of the (3d) core levels was found after an equivalent electro-oxidation stage (T. Solomun, J. Electroanal. Chem., to be submitted), no such species were detected in the case of HClO4 electrolyte. This fact is significant in the light of the known difference with regard to the electrodissolution properties of palladium in the two electrolytes. The electroformation of the initial oxy-adlayer induces an irreversible modification of the surface by embedded oxygen atoms. Thus, repeated electro-oxidation/reduction at potentials below 0.65 V results in the formation of a c-(2 × 2) surface structure. At about only 300 mV higher potentials, a PdO phase, characterized by a 1.2 eV chemical shift of Pd-(3d) core levels, is incipient on the surface. Upon electroreduction of the oxide, a certain amount of oxygen species persists below the surface. The observed behaviour within the chemisorbed/embedded/oxide sequence, as a function of the electrode potential, correlates well with an effective-medium calculation (B. Chakraborty, S. Holloway and J.K. Norskov, Surf. Sci., 152/153 (1985) 660) concerning the interaction of oxygen with the (100) surface of Group VIII transition metals.