Lecture

1
Lecture 21Surface Reaction Mechanisms
Langmuir-Hinshelwood Adsorbed species react with
each other. Very structure and coverage
dependent. Consider separately islanded
adsorbates vs. mixed surface phases. Most
common. Eley-Rideal Incident gas phase atom or
molecule reacts with adsorbate. Not particularly
structure dependent.

2
Lecture 21Surface Reaction Mechanisms, cont.
Parts of surface-catalyzed reaction Adsorption. Di
ffusion, collision and/or complex
formation. Chemical change. Desorption. Balance
interaction strengths. Reactants/intermediates
must stick (and be mobile). Products must be
able to desorb. Langmuir-Hinshelwood
Reactions Can be very site dependent. ?
Difficult to study by ensemble averaging
techniques. ? Difficult to reach relevant
conditions using local probes. As in many
reactions, can have steric requirements on
reactants.

3
Molecular Beams - Surface Scattering
Two main regimes Low energy light particles,
especially He atoms. Heavier reactive or
unreactive atoms or molecules. He atom beam
scattering Measure surface structures by
diffraction or surface phonon bands by inelastic
scattering. Energies to 100 meV with up to 0.3
meV resolution. No surface penetration. Minimal
multiple scattering. Very sensitive to
defects. Inelastic or reactive
scattering Measure scattered particle identity,
angle, energy, and quantum state
distribution. Analogous to field of gas phase
molecular beam scattering. Measure primary and
secondary reaction products. Determine reaction
steps by breaking apart reaction process. Good
Reference Atomic and Molecular Beam Methods,
Volume 2, Giacinto Scoles, editor (Oxford, 1992).