Zusammenfassung
Combining scanning tunneling microscopy (STM), IR reflection absorption
spectroscopy (IRAS) and
molecular beam (MB) techniques, we have investigated particle size
effects on a Pd/Fe3O4 model
catalyst. We focus on the particle size dependence of (i) CO adsorption,
(ii) oxygen adsorption and (iii)
Pd nanoparticle oxidation/reduction. The model system, which is based
on Pd nanoparticles supported
on an ordered Fe3O4 film on Pt(111), is characterized in detail with
respect to particle morphology,
nucleation, growth and coalescence behavior of the Pd particles. Morphological
changes upon
stabilization by thermal treatment in oxygen atmosphere are also considered.
The size of the Pd
particles can be varied roughly between 1 and 100 nm. The growth and
morphology of the Pd particles
on the Fe3O4/Pt(111) film were characterized by STM and IRAS of adsorbed
CO as a probe molecule.
It was found that very small Pd particles on Fe3O4 show a strongly
modified adsorption behavior,
characterized by atypically weak CO adsorption and a characteristic
CO stretching frequency around
2130 cm�1. This modification is attributed to a strong interaction
with the support. Additionally, the
kinetics of CO adsorption was studied by sticking coefficient experiments
as a function of particle size.
For small particles it is shown that the CO adsorption rate is significantly
enhanced by the capture zone
effect. The absolute size of the capture zone was quantified on the
basis of the STM and sticking
coefficient data. Finally, oxygen adsorption was studied by means
of MB CO titration experiments.
Pure chemisorption of oxygen is observed at 400 K, whereas at 500
K partial oxidation of the particles
occurs. The oxidation behavior reveals strong kinetic hindrances to
oxidation for larger particles,
whereas facile oxidation and reduction are observed for smaller particles.
For the latter, estimates point
to the formation of oxide layers which, on average, are thicker than
the surface oxides on
corresponding single crystal surfaces.
Nutzer