Growth and decomposition of the Pd5O4 surface oxide on Pd(111) were studied at sample temperatures between 573 and 683 K and O2 gas pressures between 10-7 and 6x10-5 mbar, by means of an effusive O2 beam from a capillary array doser, scanning tunnelling microscopy (STM) and thermal desorption spectrometry (TDS). Exposures beyond the p(2 × 2)O adlayer (saturation coverage 0.25 ML) at 683 K (near thermodynamic equilibrium with respect to Pd5O4 surface oxide formation) lead to incorporation of additional oxygen into the surface. To initiate the incorporation, a critical pressure beyond the thermodynamic stability limit of the surface oxide is required. This thermodynamic stability limit is near 8.9x10-6 mbar at 683 K, in good agreement with calculations by density functional theory. A controlled kinetic study was feasible by generating nuclei by only a short O2 pressure pulse and then following further growth kinetics in the lower (10-6 mbar) pressure range.
Growth of the surface oxide layer at a lower temperature (573 K) studied by STM is characterized by a high degree of heterogeneity. Among various metastable local structures, a seam of disordered oxide formed at the step edges is a common structural feature characteristic of initial oxide growth. Further oxide nucleation appears to be favoured along the interface between the p(2x2)-O structure and these disordered seams. Among the intermediate phases one specifically stable phase was detected both during growth and decomposition of the Pd5O4 layer. It is hexagonal with a distance of about 0.62 nm between the protrusions. Its well-ordered form is a (√67 × √67)R12.2° superstructure.
Isothermal decay of the Pd5O4 oxide layer at 693 K involves at first a rearrangement into the (√67 × √67)R12.2° structure, indicating its high-temperature stability. This structure can break up into small clusters of uniform size and leaves a free metal surface area covered by a p(2 × 2)-O adlayer. The rate of desorption increases autocatalytically with increasing phase boundary metal-oxide. We propose that at close-to-equilibrium conditions (693 K) surface oxide growth and decay occur via this intermediate structure.
Corresponding author: B. Klötzer. Reprints also available from M. Schmid (schmid).
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