Growth and decay of the Pd(1 1 1)–Pd 5O 4 surface oxide: Pressure-dependent kinetics and structural aspects

Abstract

Growth and decomposition of the Pd 5O 4 surface oxide on Pd(1 1 1) were studied at sample temperatures between 573 and 683 K and O 2 gas pressures between 10 −7 and 6 × 10 −5 mbar, by means of an effusive O 2 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) at 683 K (near thermodynamic equilibrium with respect to Pd 5O 4 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 8.9 × 10 −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 O 2 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(2 × 2)O structure and these disordered seams. Among the intermediate phases one specifically stable phase was detected both during growth and decomposition of the Pd 5O 4 layer. It is hexagonal with a distance of about 0.62 nm between the protrusions. Its well-ordered form is a ( 67 × 67 ) R 12.2 ° superstructure. Isothermal decay of the Pd 5O 4 oxide layer at 693 K involves at first a rearrangement into the ( 67 × 67 ) R 12.2 ° 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.