Coverage-dependent adsorption and desorption of oxygen on Pd(100).

We have studied the adsorption and desorption of O2 on Pd(100) by supersonic molecular beam techniques and thermal desorption spectroscopy. Adsorption measurements on the bare surface confirm that O2 initially dissociates for all kinetic energies between 56 and 380 meV and surface temperatures between 100 and 600 K via a direct mechanism. At and below 150 K, continued adsorption leads to a combined O/O2 overlayer. Dissociation of molecularly bound O2 during a subsequent temperature ramp leads to unexpected high atomic oxygen coverages, which are also obtained at high incident energy and high surface temperature. At intermediate temperatures and energies, these high final coverages are not obtained. Our results show that kinetic energy of the gas phase reactant and reaction energy dissipated during O2 dissociation on the cold surface both enable activated nucleation of high-coverage surface structures. We suggest that excitation of local substrate phonons may play a crucial role in oxygen dissociation at any coverage.

The molecular dynamics of adsorption and dissociation of O2 on Pt(553).

Molecular adsorption and dissociation of O2 on the stepped Pt(553) surface have been investigated using supersonic molecular beam techniques and temperature programmed desorption. The initial and coverage-dependent sticking probability was determined with the King and Wells technique for various combinations of incident kinetic energy, surface temperature, incident angle, and surface coverage. A comparison with similar data for Pt(533) and Pt{110}(1 × 2) shows quantitatively the same high step-induced sticking at low incident energies compared to Pt(111). The enhancement is therefore insensitive to the exact arrangement of atoms forming surface corrugation. We consider energy transfer and electronic effects to explain the enhanced sticking. On the other hand, dissociation dynamics at higher incident kinetic energies are strongly dependent on step type. The Pt(553) and Pt(533) surfaces are more reactive than Pt(111), but the (100) step shows higher sticking than the (110) step. We relate this difference to a variation in the effective lowering of the barrier to dissociation from molecularly adsorbed states into atomic states. Our findings are in line with results from experimental desorption studies and theoretical studies of atomic binding energies. We discuss the influence of the different step types on sticking and dissociation dynamics with a one-dimensional potential energy surface.

Reaction dynamics of initial O2 sticking on Pd(100).

We have determined the initial sticking probability of O2 on Pd(100) using the King and Wells method for various kinetic energies, surface temperatures, and incident angles. The data suggest two different mechanisms to sticking and dissociation. Dissociation proceeds mostly through a direct process with indirect dissociation contributing only at low kinetic energies. We suggest a dynamical precursor state to account for the indirect dissociation channel, while steering causes the high absolute reactivity. A comparison of our results to those previously obtained for Pd(111) and Pd(110) highlights how similar results for different surfaces are interpreted to suggest widely varying dynamics.

Long-range influence of steps on water adsorption on clean and D-covered Pt surfaces.

We have examined water desorption from Pt(111) terraces of varying width and its dependence on precoverage by deuterium (D) with temperature programmed desorption studies. We observe distinct water desorption from (100) steps and (111) terraces, with steps providing adsorption sites with a higher binding energy than terraces. Preadsorption of D at the steps causes annihilation of water stabilization at the steps, while it also causes an initial stabilization of water on the (111) terraces. When the (111) terraces also become precovered with D, this water stabilization trend reverses on all surfaces. Destabilization continues for stepped surfaces containing up to 8-atom wide (111) terraces with a (100) step type and these become hydrophobic, in contrast to surfaces with a (110) step type and with the infinite (111) terrace. Our results illustrate how surface defects and a delicate balance between intermolecular forces and the adsorption energy govern hydrophobic vs. hydrophilic behavior, and that the influence of steps on the adsorption of water on nano-structured platinum surfaces has a very long-ranged character.

Tuning hydrophobicity of platinum by small changes in surface morphology.

Stepped platinum surfaces can become hydrophobic when they are hydrogenated. Even though the Pt(533) and Pt(553) surfaces have similar geometries, the hydrophobicity on the deuterated surface is surprisingly different: on Pt(533) the surface is hydrophobic with water clustering at steps, whereas the entire surface is wet on Pt(553).

A detailed TPD study of H2O and pre-adsorbed O on the stepped Pt(553) surface.

Water molecules desorbing from the bare Pt(553) surface desorb in a three peak structure, associated with, respectively, desorption from step and terrace sites and the water multilayer. Upon pre-covering the step sites with O(ad) we mainly observe OH formation on step sites. When terrace sites are also pre-covered with O(ad), OH(terrace) formation is favored over OH(step) formation, presumably because OH formed at terrace sites is more easily incorporated in a hydrogen bonded network of OH/H(2)O. This is a gradual process: with increasing θ(O) less OH(step) is formed. Thus, in spite of the fact that OH at step sites has a higher binding energy than OH at terrace sites, the possibility of the formation of OH at terrace sites actually inhibits the formation of OH at step sites, leaving O(step) as the most stable water dissociation product on the step.

The influence of step geometry on the desorption characteristics of O2, D2, and H2O from stepped Pt surfaces.

We have compared the desorption characteristics of O(2), D(2), and H(2)O from the Pt(533) surface to the Pt(553) surface using temperature programmed desorption. Both surfaces consist of four atom wide (111) terraces interrupted by monoatomic steps of the different step geometries: (100) versus (110), respectively. We find that desorption is influenced significantly by the presence of step sites and the geometry of those sites. In general, molecules and atoms are thought to be bound more strongly to step sites than to terrace sites. Our D(2) desorption data from Pt(553) provide an anomalous counterexample to this common belief since D atoms on this surface appear to be bound stronger by terrace sites. We also show that it is not possible to say a priori which step geometry will bind atoms or molecules stronger: recombinatively desorbing O atoms are bound stronger to (100) sites, whereas H(2)O molecules are bound stronger to (110) sites. Furthermore, the amount of adatoms or molecules that are affected by the presence of steps varies for the different species, as is evident from the various step: terrace ratios of approximately 1:1.3 for O(2) (O), approximately 1:3 for D(2) (D), and approximately 1:1 for H(2)O. This indicates that, in contrast to deuterium, more oxygen atoms and water molecules are affected by the presence of steps than would be expected on geometrical arguments alone.