Chemistry Reference
In-Depth Information
Fig. 27 (a) STM micrographs of 0.2 ML Au deposited on MgO(001) at room temperature taken
immediately after Au deposition (
top
) and after subsequent heating to 600 K (
bottom
). (b) Same as
(a) but for 0.02 ML Au deposited onto hydroxylated MgO (MgO
hydr
). (c) Comparison of CO-TPD
spectra from 0.02 ML Au deposited on MgO(001) (
black traces
) and MgO
hydr
(
red traces
) for Au
particles grown at room temperature (
top
) and the same samples after heating to 600 K (
bottom
).
CO was dosed at 100 K
The enhanced stability of Au-MgO
hydr
towards sintering is also reflected in the CO
adsorption capacity of the Au particles. The comparison of CO-TPD spectra from
Au-MgO(001) and Au-MgO
hydr
taken for particles grown at room temperature
(Fig.
27c
, top) and after subsequent heating to 600 K (Fig.
27c
, bottom) clearly
shows the enhancement of CO adsorption on Au-MgO
hydr
, in line with the larger Au
surface area of the more dispersed Au particles on MgO
hydr
[
117
]. This is a
significant finding in light of the importance of small Au particle size for the
catalytic activity of oxide-supported Au catalysts. Indeed, chemical functiona-
lization of a TiO
2
support with hydroxyls prior to the deposition of gold particles
was found to have a strong enhancing effect on the CO oxidation activity [
118
].
What is the origin of the enhanced sinter resistance of Au particles on the
hydroxylated MgO surface? As mentioned above, hydroxylation of MgO(001) is
accompanied by microscopic roughening of the surface because of hydrolysis of
Mg-O surface bonds. The morphological changes of the MgO surface may lead to
increased barriers for Au atom diffusion and thus limited mobility, even at high
temperature. On the other hand, additional or new nucleation centers may be
created upon hydroxylation, which could enhance the adhesion of the Au particles
because of a distinct chemical
interaction with the substrate. While the first
