Chemistry Reference
In-Depth Information
hydroxo ligands through hydrolysis at neutral or alkaline pH such that Au-hydroxo
complexes (Au(OH)
4
or AuCl(OH)
3
) are the primary precursors interacting with
the support, and by the washing step, which eliminates chlorine from the support
surface.
The DP approach used for the preparation of supported Au catalysts is particu-
larly well suited for studying the deposition of Au onto single-crystalline oxide
supports from an Au precursor solution because it merely requires the oxide surface
to be contacted with the precursor solution and does not rely on the presence of
pores such as in incipient wetness impregnation or the simultaneous precipitation of
the support and the active metallic phase as done in coprecipitation. We attempted
to recreate a situation similar to a DP procedure reported by Haruta by exposing the
surface of a 10 nm thin Fe
3
O
4
(111) film grown on Pt(111) to an aqueous solution of
HAuCl
4
adjusted to pH 10. Fe
3
O
4
(111)/Pt(111) was chosen because of its high
stability in aqueous solutions, as previously shown in our studies of Pd deposition
from both strongly acidic [
121
] and strongly alkaline precursor solutions [
122
], as
well as because of its conducting properties, which allows STM to be straightfor-
wardly applied for morphological characterization.
Figure
30a, b
display the Au 4f and Cl 2p photoemission spectra from an
Au-Fe
3
O
4
(111) sample obtained by exposure of Fe
3
O
4
(111) to 1 mM HAuCl
4
,
pH10, for 5 min. at room temperature, after individual stages of preparation. The
“as deposited” state represents the raw catalyst, which has been dried at room
temperature but not rinsed. The main Au component present at this stage of
preparation exhibits an Au 4f
7/2
binding energy of 87.1 eV, which is in line with
values reported for strongly oxidized Au and Au(OH)
3
. The hydroxo complex
[Au(OH)
4
]
is the main hydrolysis product of aqueous HAuCl
4
at pH 10 and the
occurrence of the high binding energy Au 4f component in XPS strongly supports
previous conclusions about the Au-hydroxo complex as the main adsorbing species
during DP of Au. In addition, two Au 4f
7/2
components at lower binding energy, at
85.5 and 84.6 eV, are present, which are attributed to oxidic and metallic Au
resulting from partial decomposition of the Au-hydroxo precursor on the surface.
As expected, a large amount of chloride is present on the raw catalyst.
Rinsing the surface with distilled water completely removed chloride from the
surface, but led in addition to reduction of the Au precursor to metallic Au, which
exhibits an Au 4f
7/2
binding energy of 84.2 eV at this stage of preparation (Fig. 30a,
b, “rinsed”). Subsequent heating to 600 K in UHV leads only to a small additional
0.1 eV shift of the Au 4f
7/2
component to 84.1 eV (Fig.
30a
, “annealed”).
The XPS results provide already a good indication for the successful realization
of Au deposition onto the Fe
3
O
4
(111) surface from aqueous precursors. In order to
obtain further proof, the annealed Au-Fe
3
O
4
(111) model catalyst was investigated
by STM. Figure
30c
displays a corresponding image taken in air, wherein the bright
features are assigned to Au nanoparticles. Closer inspection of this image reveals a
bimodal particle size distribution, with the larger particles exhibiting a diameter of
6-8 nm, and some smaller ones, which are more abundant on the left side of the
STM image, with a diameter of about 4 nm. For comparison, Fig.
30d
shows an
STM image of a Fe
3
O
4
(111) surface with Au particles obtained by physical vapor
