Interactions of O(2) with Pd nanoparticles on alpha-Al(2)O(3)(0001) at low and high O(2) pressures.

The interaction of O(2) with small Pd particles (2-10 nm) supported on an alpha-Al(2)O(3)(0001) single crystal under both ultrahigh vacuum (UHV) and high-pressure conditions has been studied by temperature-programmed desorption (TPD), temperature-programmed low-energy ion scattering (TP-LEIS), and X-ray photoelectron spectroscopy (XPS). A low O(2) exposure (30 L) at 500 K leads to surface oxygen adatoms on the Pd nanoparticles, which desorb in TPD as O(2) in a peak at approximately 880 K. Surface O adatoms on the smallest Pd particles move to subsurface sites starting at 400 K, and they almost all move subsurface by approximately 750 K, desorbing mainly at considerably higher temperature. The dominant oxygen species above 700 K is subsurface, implying that it is more stable than oxygen adatoms on Pd. Exposures of the Pd nanoparticles to 25 Torr O(2) at 373-473 K readily convert the Pd to a species whose Pd XPS peak shifts by the same amount as the binding energy difference between bulk Pd and bulk PdO. We attribute this to PdO nanoparticles (or a thin film of PdO on or under the Pd for the larger particles). The decomposition of the PdO on these nanoparticles to Pd in an equilibrium O(2) pressure of 10-7 Torr does not occur until approximately 750 K, or approximately 200 K higher than the equilibrium decomposition of bulk PdO. This is attributed to the higher energy of Pd nanoparticles compared to bulk Pd and, for the larger particles, to the adhesion energy of the PdO film to the Pd, both of which stabilize the PdO on these Pd nanoparticles relative to bulk PdO. This PdO-like film on the larger particles may be similar to the ordered oxide thin film previously reported to form on Pd(111) but may also reside at the alpha-Al(2)O(3) interface and be partially stabilized by adhesion to this interface.

Surface products and coverage dependence of dissociative ethane adsorption on Pt{110}-(1 x 2).

The dissociation of ethane on Pt{110}-(1 x 2) has been studied using supersonic molecular beam and temperature-programmed reaction techniques. The study unequivocally shows that the stable dissociation product of ethane on Pt{110}-(1 x 2) at all coverages is CCH2 at 350-400 K and CCH at 440 K. Temperature-programmed-reaction (TPR) experiments indicate that the CCH2 species decomposes to CCH with a reaction-limited peak temperature of 430 K. Above 450 K, the CCH species becomes unstable and decomposes with a peak temperature of 540 K. By 600 K, ethane dehydrogenates completely to form a surface carbon layer. The sticking probability is initially 0.02 at 370 K and 0.03 at 600 K and follows a linear (1-2theta) dependence for coverages of up to theta = 0.4 ML, where theta is defined as the number of C2Hx units per (1 x 2) unit cell. However, a much weaker coverage dependence at 800 K suggests that the carbon agglomerates into high-density islands.