Understanding surface properties and functions: the example of metal oxide
catalysts
V. Ganduglia-Pirovano
(Humboldt-Universität zu Berlin)
Mercredi, 21 novembre 2007
Heure: 11h00
Lieu: salle 1, Bâtiment St.Raphael, 3 rue Galilée, 94200 Ivry-sur-Seine
As the pressure and materials gaps between real catalysis and traditional
UHV single crystals
studies are narrowing, experiments and modelling that straddle both sides of
the gap
appear essential to understand surface properties and functions of
catalysts. Producing
well defined (experimental and theoretical) model systems of practical
catalysts such as
alumina supported vanadia is challenging because of the complexity of the
real systems
The aggregates' structure and the nature of the support in particular, are
important because
both relate to the (real) catalysts activity for partial oxidation
reactions; however, they are
not fully understood. Experimental model systems of VOx/Al2O3 catalysts had
been produced.
Their thorough experimental study yielded some puzzling results on the
particles'
surface termination, and pointedly left the VOx/Al2O3 vibrational properties
a matter of
great debate. I will discuss theoretical models which include the clean and
oxygen defective
V2O5(001) surface, as well as vanadia aggregates on alpha-Al2O3(0001),
kappa-Al2O3 (001) and an ultrathin alumina film on NiAl(110). These case
studies were investigated using
density functional theory-based calculations and statistical thermodynamics.
Highlights
of the results are the prediction of a missing-row vanadyl (O=V) oxygen
defect structure
at the V2O5(001) surface under reducing conditions and of the facile surface
reduction
along the [010] oriented rows; a phenomena that has been experimentally
confirmed. The
assignment of vibrational spectra will also be addressed. Both the
experimental and theoretical
model studies have resulted in relevance on a deep level \u2013 they challenge
the view
generally accepted in the real world of powder samples, in which vibrational
spectroscopy
is routinely used for catalyst characterization. Taking into account an
important aspect of
the complex systems functionality,i.e., their reducibility,
structure-reactivity relationships
will be discussed.