Chemistry Reference
In-Depth Information
Fig. 29 (a) IRAS spectra showing the OD spectral detail of MgO
hydr
(
top
) and after subsequent
deposition 0.02 and 0.2 ML Au. (b) IRAS spectra showing the OD spectral detail of MgO
hydr
after
hydroxylation in water vapor at various pressures. (c) STM micrograph of the surface of a MgO
(001) thin film grown on Ag(001). (d) Model of a di-coordinated OH group located on a step site,
which is proposed to be the initial nucleation site of Au on MgO
hydr
2100 cm
1
, where CO vibrational signals from Au particles are expected. Two
bands at 2165 and 2125 cm
1
are observed, which are significantly blue-shifted
relative to CO on neutral gold and are indicative for the presence of cationic or
oxidized Au species on MgO
hydr
[
116
]. The cationic nature of Au is also inferred
from the corresponding XPS spectrum taken from this sample [
116
], which exhibits
two Au 4f
7/2
signals at 86.9 and 85.3 eV binding energy (Fig.
28a
) - significantly
shifted to higher binding energy relative to bulk gold (84.0 eV).
Since hydroxyl groups are obviously involved in the nucleation of Au on
MgO
hydr
, an impact of Au nucleation is also expected in the hydroxyl IR spectra.
The topmost spectrum in Fig.
29a
displays the OD spectral detail of the infrared
spectrum obtained from MgO
hydr
for the specific hydroxylation conditions used in
this experiment (10
3
mbar D
2
O at room temperature for 180 s). The broad band
can be deconvoluted into three signal components with
2737, 2745, and
2753 cm
1
, respectively, which are assigned to isolated and hydrogen-bond accep-
tor hydroxyls on the MgO surface (the corresponding OH bands are found at around
3750 cm
1
)[
119
]. Upon deposition of gold, the intensity of the OD signal at
2737 cm
1
is reduced (0.02 ML Au; Fig.
29a
, middle) and by increasing the Au
coverage further, this signal is almost completely depleted (0.2 ML Au; Fig.
29a
,
bottom). This result clearly indicates a very specific interaction of Au with hydrox-
ylated MgO that involves only one particular surface hydroxyl site [
116
].
To obtain more detailed structural information about the specific hydroxyl site
that is involved in the interaction with Au, it is instructive to inspect the OD-IR
signals from MgO
hydr
as obtained following hydroxylation of MgO(001) at increas-
ing D
2
O partial pressure (Fig.
29b
). The series of IR spectra shows, from top to
bottom, an increasing hydroxyl coverage, in agreement with the water vapor
pressure dependent hydroxylation activity of MgO, and corresponding shifts of
the OD-IR frequencies, which indicate that different hydroxyl states (coordination,
ʽ
(OD)
¼
