Chemistry Reference
In-Depth Information
H-bonding) are populated as a function of hydroxyl coverage. Of particular impor-
tance for the present discussion is the fact that hydroxyl species exhibiting frequen-
cies of 2730-2740 cm
1
, i.e., in the range of the OD streching vibration that is
depleted upon deposition of gold (Fig.
29a
), appear exclusively after hydroxylation
at low (10
4
mbar) water vapor pressure (Fig.
29b
). It can be expected that under
these conditions the MgO(001) terraces are not affected by water and hydroxylation
occurs exclusively by water dissociation at defect sites. From STM investigations it
is clear that step sites are the most abundant defect sites on MgO thin films as used
in the present study (Fig.
29c
). This leads us to propose that hydroxyl groups
located at step sites such as the one sketched in Fig.
29d
are involved in the
nucleation of gold on MgO
hydr
[
116
].
In summary, the STM, XPS, and IRAS results presented in Figs.
27
,
28
, and
29
provide evidence for a selective chemical interaction between Au and hydroxyl
groups on MgO. Hydroxyl groups were found to act as strong anchoring sites for Au
and the spectroscopic results indicate that oxidized Au species are formed on
MgO
hydr
. The enhanced sinter stability of Au on MgO
hydr
may consequently be
explained by the stronger interaction of Au with the MgO surface due to the
formation of strong Au-O interfacial bonds. Though Au-oxide species are not
stable at elevated temperature and decompose (see Fig.
28b
; which shows that
after 600 K annealing the Au particles are essentially neutral), this strong interfacial
interaction is the main reason for the sinter resistance of Au on MgO
hydr
.
4.2 Surface Science Approach to Supported Au
Catalyst Preparation
The results presented in the previous section have shown that chemical modifica-
tion of an oxide surface with hydroxyl groups can have a strong stabilizing effect on
Au particles, which is an important criterion in Au-related catalysis. The strong
impact of Au particle size on the catalytic activity of oxide-supported Au has first
been demonstrated by Haruta. In the seminal work published in 1987 [
112
],
catalysts active in CO oxidation were synthesized by coprecipitation from aqueous
solutions containing HAuCl
4
and the nitrate of transition metals such as Fe, Co, or
Ni. This type of preparation has later on been replaced by another procedure, which
consists of suspending a metal oxide (the support) in a HAuCl
4
solution adjusted to
a fixed pH in the range pH 7-10, aging of the solution for 1 h at 343 K, followed by
washing with distilled water, drying, and calcination. This method is frequently
termed deposition-precipitation (DP), although several conditions that are typical
for DP, such as the gradual rise of pH and the preferential precipitation of the
precursor at the interface, are not met as outlined by Louis et al. [
120
]. A critical
factor for obtaining small Au nanoparticles is the removal of chlorine from the
supported phase, as chlorine enhances the mobility of Au species during drying and
calcination. This is achieved by replacement of the Cl ligands in AuCl
4
ions by
