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

Area of Science:

• Surface science
• Chemical kinetics
• Materials science

Background:

• Oxygen adsorption on platinum (Pt) surfaces is crucial for automotive exhaust purification and fuel cell electrodes.
• Previous studies on O2/Pt(111) show unexplained low sticking probability and unusual energy dependence.

Purpose of the Study:

• To elucidate the alignment dependence of the initial sticking probability (S0) of O2 on Pt surfaces.
• To clarify the origin of the low and energy-dependent O2 sticking probability.

Main Methods:

• Utilizing a single spin-rotational state-selected [(J,M)=(2,2)] O2 beam.
• Investigating the initial sticking probability (S0) as a function of translational energy (E0) and molecular alignment.

Main Results:

• Direct activated chemisorption occurs at low E0 only when the O2 molecule's axis is nearly parallel to the Pt surface.
• At high E0 (>0.5 eV), parallel O2 sticking probability decreases with increasing energy, while perpendicular O2 sticking probability increases.
• These alignment-dependent behaviors explain the previously observed energy-independent O2 sticking probability in non-state-resolved experiments.

Conclusions:

• Molecular alignment is a critical factor governing O2 adsorption dynamics on Pt surfaces.
• The findings provide a detailed understanding of the O2/Pt(111) system, resolving long-standing questions about sticking probability.
• This research has implications for optimizing catalytic processes involving O2 on metal surfaces.