PdCu Surface Research Articles

Temperature-induced irreversible changes in the PdCu(110) surface microstructure

PdCu(110)single crystal alloy surfaces were studied after sputtering and annealing up to temperatures at which enhanced Cu segregation and partial surface disordering significantly influenced the surface structure and its composition. Pd-rich, Cu-rich and surfaces with mixed PdCu compositions were examined. Surfaces were investigated using video low energy electron diffraction, Auger electron spectroscopy for the surface region of ∼ 4 layers depth and thermal desorption spectroscopy of CO for outer surface layer analysis. The atomic concentration ratio 1:1 in the matrix and the misfit in building the bcc lattice of Pd and Cu atoms could contribute to considerable surface stress, which is released by the surface microstructure containing only between 28% and 60% atoms in ordered domains. The domains were some 10–20 lattice cells large and were limited by grain boundaries. Domain sizes and the crystallinity were measured at 115K, after annealing the samples at temperatures above the preparation temperature. The irreversible temperature-induced growth of the surface domains was influenced by volume (segregation) and by surface processes (roughening, partial disordering) and proceeded with the equilibrium shape of domains.

Properties of PdCu(110) single crystal alloy surfaces: Temperature-induced processes in the surface microstructure

The PdCu(110) alloy single crystal surfaces, with atomic concentration ratio Cu: Pd = 1:1 in the bulk, were prepared with various compositions. Cu/Pd ratios in the surface regions, measured ≈ 4 layers deep, ranged from 0.3 to 2. Different compositions of the top surface layers were obtained, from exclusively Pd atoms, over mixtures of Pd and Cu atoms, to exclusively Cu atoms. The behaviour of video LEED spot intensities and profiles during sample heating and cooling was measured. The PdCu(110) surface consists of domains containing ≈ 10–20 lattice cells depending on preparation procedure. The surface microstructure influences the temperature-induced processes at the surface: surface roughening and partial surface disordering, starting at ≈ 550 K and ≈ 700 K, respectively. The domain grain boundaries play an important role in the surface roughening process. At higher temperatures the domain size distribution, aside from grain boundaries, causes partial surface disordering. The two Cu segregation processes, starting at ≈ 550 K and at ≈ 700 K, are the volume counterparts to these surface processes.

Comparison between the adsorption properties of Pd(111) and PdCu(111) surfaces for carbon monoxide and hydrogen

Thermal desorption spectroscopy and LEED have been used to investigate the interaction of CO and hydrogen with a Pd 0.75Cu 0.25(111) single crystal surface with surface composition of about Pd 0.7Cu 0.3. The main objective was to make a comparison with the previously studied Pd 0.67Ag 0.33(111) (surface composition Pd 0.1Ag 0.9) and Pd(111) surfaces. In addition, the effect of preadsorbed H on subsequent CO dosage and the effect of adsorbed CO on postdosed hydrogen are described. Marked differences were found in the adsorption behaviour of the three surfaces towards CO and hydrogen. The maximum amount of H and CO that can be adsorbed at 250 K and pressures below 10 −9 mbar is much lower on the PdCu surface than expected on the basis of the surface composition. This effect appears to be caused by a low heat of adsorption of hydrogen and CO and Pd singlet sites. Arguments are presented that singlet Pd sites or isolated Pd atoms in a Cu or Ag matrix are able to trap and dissociate the hydrogen molecule at 250 K. The CO desorption spectra are not influenced by pre- or postexposed hydrogen. Adsorbed CO hampers the uptake of hydrogen upon subsequent exposure to hydrogen. Postdosed CO causes adsorbed H adatoms to move to the bulk (adsorbed H). CO exposure at 250 K results in a very broad desorption plateau between 310 and 425 K with hardly discernable maxima. The results can be explained in terms of the size and relative concentration of the various Pd sites present on the surface (triplet, doublet and singlet sites). It can be concluded that for Pd (111) the heat of adsorption of both CO and H differ appreciably for the triplet, doublet and singlet sites. The effect of site has a larger contribution to the decrease of the heat of adsorption with coverage than the effect of lateral interaction in the adlayer. For Pd(111), PdCu(111) and PdAg(111) the effect of the available Pd sites is the major effect that determines the heat of adsorption, followed by the effect of lateral interaction and for the alloy surfaces the electronic or ligand effect.