Chemistry Reference
In-Depth Information
10
7
mbar of oxygen, 50% of
the Au islands still adapt monolayer shapes, all Au deposits turn 3D grown in a
5
with increasing O
2
partial pressure. Whereas in 5
10
5
mbar O
2
background. The interplay between the observed growth mode
and the composition of the gas environment emphasizes the pivotal importance of
excess electrons from donor species for the reactivity of oxides towards adsorption
of metallic and gaseous species.
In summary, doped bulk oxides display in many aspects similar adsorption
properties as ultrathin oxide films. In both systems, excess electrons are transferred
into the ad-species and open up specific charge-mediated adsorption schemes.
Whereas for ultrathin oxide films, the extra electrons are provided by the metal
substrate below, doped oxides contain intrinsic charge sources in the form of
aliovalent impurity ions. Thin oxide films grown on metal single crystals are mainly
of academic interest, as they provide easy access to the properties of oxide materials
via conventional surface science techniques. Doped oxides, on the other hand, are
of practical relevance for many processes in heterogeneous catalysis. As shown in
this section, overvalent dopants are able to change the particles equilibrium shape
from 3D to 2D, which is expected to change the reactivity pattern of the metal-oxide
system. Moreover, charge transfer from electron-rich dopants might be a suitable
pathway for the activation of small molecules, such as oxygen or methane. A
possible activation mechanism would be the occupation of the antibonding orbitals,
leading to a destabilization or even dissociation of the molecules. In a recent
experiment, electron donation from Mo impurities was shown to produce super-
oxo (O
2
) species on a CaO surface, which might be a highly reactive precursor for
subsequent oxidation reactions. Although the chemical reactivity of doped oxides is
still subject of active research, it appears already that oxide doping forms a
promising route to fabricate highly reactive and selective catalysts for the future.
3 CO Adsorption on Supported Au Particles
The binding of molecules to specific sites of a metal nanoparticle is thought to
determine the reactivity of metal/oxide systems used in heterogeneous catalysis.
The model systems, described above, represent ideal objects to study the influence
of size, shape, and charge onto adsorbed molecules. Given the large body of
information on CO as an adsorbate, this molecule represents the probe molecule
of choice. Figure
20
shows the range of frequencies CO typically shows when
bound to Au in different oxidation states [
92
].
This knowledge, together with the possibility of imaging, allows us to investi-
gate, in detail, molecule-Au interactions. Before we discuss CO adsorption on
nanoparticles we briefly discuss the frequency of species not fitting the scheme of
Fig.
20
, i.e., the IR spectrum of CO interacting with a single Au atom [
93
]. When
small amounts of Au are deposited on a 10 monolayer thick MgO(100) film, the Au
atoms remain neutral, as conclusively shown via EPR measurements on the
system. CO adsorption leads to the appearance of a band at 1852 cm
1
in the IR
