Dynamics of O2 Chemisorption on a Flat Platinum Surface Probed by an Alignment-Controlled O2 Beam

Spin and Alignment effects in O2 Chemisorption on Fe(110), Ni(111) and Co(0001) films grown on W(110)

O2 has a spin triplet multiplicity in its ground state while the effect of its electron spin on O2-surface interaction is not well understood. In the present study, the spin and/or alignment effects in O2 chemisorption on surfaces of Fe(110), Ni(111) and Co(0001) films grown on W(110) have been investigated with the use of a single spin-rotational state-selected O2 beam. The results indicate that the spin effects for the Fe and Ni films are similar in that the initial sticking probability (S0) of O2 is higher when spins of O2 and the film are antiparallel, and the spin dependence in S0, which amounts to 30-40 % at thermal energy, decays with increasing the O2 kinetic energy (E0). In case of the Fe/O2 system, however, considerable spin dependence was found to remain even at E0 > 0.2 eV and on oxidized surface. It has been also shown that the barrier for O2 chemisorption increases in the order of Fe(110) < Ni(111) < Co(0001) while the difference in the observed alignment effect among the samples can be understood based the difference in the barrier.

Probing Copper and Copper-Gold Alloy Surfaces with Space-Quantized Oxygen Molecular Beam

The orientation and motion of reactants play important roles in reactions. The small rotational excitations involved render the reactants susceptible to dynamical steering, making direct comparison between experiments and theory rather challenging. Using space-quantized molecular beams, we directly probed the (polar and azimuthal) orientation dependence of O₂ chemisorption on Cu(110) and Cu₃Au(110). We observed polar and azimuthal anisotropies on both surfaces. Chemisorption proceeded rather favorably with the O-O bond axis oriented parallel (vs perpendicular) to the surface and rather favorably with the O-O bond axis oriented along [001] (vs along [1̅10]). The presence of Au hindered the surface from further oxidation, introducing a higher activation barrier to chemisorption and rendering an almost negligible azimuthal anisotropy. The presence of Au also prevented the cartwheel-like rotations of O₂.

Alignment and impact angular dependence to O2 sticking and dissociation on Pt(111) and close-packed steps

Oxygen's interaction with Pt surfaces serves as a model system in the development of an accurate theoretical description of reaction mechanisms that involve multiple precursor states. To benchmark the influence of surface structure on the dynamics of this interaction, we report absolute values of the initial sticking probability of O₂ onto Pt(111) and two vicinal surfaces for state-selected and rotationally-aligned O₂ molecules. Sticking probabilities vary significantly for helicoptering and cartwheeling molecules. Our data can be understood if normal energy scaling holds for all molecular orientations relative to the surface. Vicinal surfaces are much more reactive than Pt(111) with little to no dependence on the molecule's alignment and a more complex angular dependence. At low incident energies, sticking probabilities are highest for incidence into step facets. The weak alignment dependence points toward predominant scattering into a physisorbed state preceding chemisorbed states over a wide angular range.

Steps on Pt stereodynamically filter sticking of O2

Platinum is widely applied as an oxidation catalyst. The surfaces of metallic catalyst particles are generally composed of atomically flat planes connected by edges. In this paper, we unveil how Pt edges affect O₂ sticking. Opposing types of behaviors are observed for high and low incident energies. The dependencies identify the mechanisms by which O₂ molecules stick, which is the first step in the oxidation reactions.

Low coordinated sites on catalytic surfaces often enhance reactivity, but the underlying dynamical processes are poorly understood. Using two independent approaches, we investigate the reactivity of O₂ impinging onto platinum and resolve how step edges on (111) terraces enhance sticking. At low incident energy, the linear dependence on step density, independence of step type, and insensitivity to O₂’s molecular alignment show that trapping into a physisorbed state precedes molecular chemisorption and dissociation. At higher impact energies, direct molecular chemisorption occurs in parallel on steps and terraces. While terraces are insensitive to alignment of the molecule within the (111) plane, steps favor molecules impacting with their internuclear axis parallel to the edge. Stereodynamical filtering thus controls sticking and dissociation of O₂ on Pt with a twofold role of steps.

Steric effect in CO oxidation on Pt(111)

We present experimental evidence showing that the rate of CO oxidation on Pt(111) depends strongly on the geometry of an incident O₂ molecule. The O₂ sticking probability and the CO₂ production rate on CO-covered Pt(111) surfaces during the alignment-controlled O₂ beam irradiation were simultaneously monitored at a surface temperature of 330 K. The results indicate that, at the translational energy of 0.1-0.2 eV and at the CO coverage of <0.4 monolayers, both O₂ adsorption and CO oxidation proceed exclusively when the O₂ molecular axis is nearly parallel to the surface.