Interaction of oxygen with a polycrystalline palladium surface over a wide temperature range

Abstract

Using thermal desorption spectroscopy (TDS) the interaction of gaseous oxygen with a polycrystalline palladium foil was studied for the first time over a very wide temperature (110 < T < 1400 K) and exposure (10 000 L) range. Since studies with polycrystalline and defect surfaces are generally hampered by a serious lack of structural information, the Pd(foil) surface was first characterized by means of the PAX technique (photoemission of adsorbed xenon). Hereafter the surface was found to consist of ≈ 60% (Pd(110)-like) defect sites and ~ 40% Pd(100)-like sites. This structural information proved very helpful for the interpretation of the complex high-coverage spectra of oxygen from this surface. Despite the general belief we find desorption of some amount of molecularly adsorbed oxygen from this highly defective surface before the detection of an atomically adsorbed oxygen species. This molecular oxygen desorbs at 160 K. Exposures greater than 0.7 L are required to first detect atomically adsorbed oxygen. Oxygen adsorption at T ad > 150 K results in atomic oxygen chemisorption which manifests itself as a single peak of second-order desorption kinetics near 900 K desorption temperature and which grows with increasing exposure. Its width, however, is not significantly affected by the presence of surface defects as revealed most clearly by a so-called Y-parameter (FWHM/ T p) analysis. Beyond an initial coverage of θ 0 = 0.25 this peak develops several shoulders at lower temperatures (≈ 700 K) which did not saturate even after 10 000 L and which are assigned to oxygen chemisorbed on the Pd(100) surface patches and the Pd(110)-hke surface defects, respectively. This assignment was possible on the basis of the PAX results and is consistent with literature data for the oxygen desorption from Pd(100) and Pd(110) single-crystal surfaces. At an adsorption temperature of T ad = 510 K the highest exposures lead to the formation of a surface oxide species which decomposes above 700 K with zeroth-order kinetics. A small fraction of each new exposure penetrates deeper into the bulk and is given off only at temperatures≳ 1300 K.