Chemistry Reference
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particular on the particle size and the structure of the Au-support interface, and it is
at the heart of catalyst preparation/synthesis to develop recipes that allow those
units that are responsible for catalytic activity to be stabilized on the catalysts'
surface. Indeed, much of the recent interest in gold surface chemistry arose from
Masatake Haruta's observation of the unusual catalytic activity of highly dispersed
Au supported by oxides, after finding a preparation method that resulted in stable,
nano-sized Au particles [
112
]. In most cases, catalyst synthesis involves at least one
wet-chemical preparation step, which adds some degree of chemical complexity to
the systems that is typically not covered by surface science experiments carried out
under ultrahigh vacuum conditions.
In the following we present two examples of surface science investigations into
gold nucleation and bonding on oxide surfaces, which attempt to include some of
the complexity of a real-world catalyst. The first one investigates the specific role of
hydroxyl groups present on an oxide support on the morphological and chemical
properties of Au nanoparticles. The second example presents an approach to study
the preparation of an oxide-supported gold model catalyst using a single-crystalline
oxide support in combination with a wet-chemical preparation procedure.
4.1 Gold Nucleation at Hydroxyl Groups
We have chosen to use MgO(001) as a model surface to investigate the effect of
hydroxyl groups on the properties of supported Au particles. Along the lines
described in the previous sections, MgO(001) films were grown on a Ag(001) or
Mo(001) substrate, but thicker films on the order of 10-15 ML thickness were used
to exclude any influence of the metallic substrate. As shown previously, hydroxyl-
ation of MgO(001) requires the surface to be exposed to elevated pressures of water
vapor, typically in the range of 10
4
to 1 mbar [
113
]. The formation of a stable
hydroxyl layer on MgO is suggested to proceed via hydrolysis of Mg-O surface
bonds and leads to microscopic roughening of the MgO surface [
114
,
115
]. The
results presented in this section were obtained from MgO thin-film samples that
were hydroxylated by exposure to 10
3
mbar D
2
O at room temperature in a
dedicated elevated pressure cell, which leads to a surface hydroxyl coverage of
~0.4 ML as estimated from quantitative XPS measurements [
115
].
A most obvious influence of hydroxyl groups on the properties of gold on the
MgO surface is seen in Fig.
27
, which compares STM images of Au-MgO(001)
(Fig.
27a
) and Au-MgO
hydr
(Fig.
27b
)[
116
]. The first set of images was taken
directly after Au deposition at room temperature (top), while the other set shows the
surface state after subsequent heating to 600 K (bottom).
The Au particles are moderately dispersed over the MgO(001) surface at room
temperature (Fig.
27
, top), but Ostwald ripening and particle coalescence lead to
particle growth and a strong reduction of particle density after the elevated tem-
perature treatment (Fig.
27a
, bottom), which is expected for this weakly interacting
system. By contrast, the Au particle size and the particle density remain unaffected
when the same thermal
treatment
is applied to Au-MgO
hydr
(Fig.
27b
).
