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Abstract

The PdCu(110) single crystal alloy, with Pd:Cu=1:1 in the bulk, was prepared by sputtering and annealing at low temperatures (<550 K, i.e. below that of Cu segregation) to produce Pd-rich surfaces that have a ratio of ≤0.4 as determined by Auger Electron Spectroscopy (AES) for a surface region of about four layers. Oxygen was adsorbed at 115 K and TDS were recorded for temperatures up to 500 K. The chemisorption of oxygen on these surfaces was studied using Low Energy Electron Diffraction (LEED), AES and Thermal Desorption Spectroscopy (TDS).
TD spectra of oxygen on the alloy surfaces showed one peak at 144 K and the shape of the spectrum denoted that at T_ad=115 K oxygen desorption was proceeding. The desorption rate increased with doses up to saturation at ~9 L. With further oxygen doses a decrease in intensity was recorded. By comparing such behaviour with that from hydrogen desorption, it follows that for doses higher than the saturation dose, the binding energies decrease. The influence of subsurface oxygen on this behaviour has been studied by LEED. The type of crystal lattice and the ratio of prior to adsorption determined the type of adsorbate structures obtained. Oxygen adsorption on the clean surface that had (1×1) structure produced c(2×4), (1×2) or (2×2) patterns. The original (1×1) structure could be restored only by leaving the alloy for few hours under vacuum at room temperature or by applying the CO cleaning procedure. Oxygen adsorption on the clean surface that had the (1×2) structure did not produce any changes on this structure. The influence of small Cu-concentrations on the adsorption properties of this Pd-rich surface, in the case of fcc (110) or when Cu together with Pd atoms builds the bcc(110), is discussed. These properties differ drastically from those for pure Pd(110) or from PdCu(111) planes.