Chemistry Reference
In-Depth Information
transformation to coarser and polyhedral structure. The missing-row type
reconstructions of the {011} and {113} facets are not visible due to a poorer
resolution under these conditions, but the extension of the {137} facets is
clearly visible as dark regions of both sides of the
h
100
i
zone lines. 1D-atom
probe studies indicate that the imaging species are H
2
O
1
ions resulting from
the ionization of the reaction product.
62
Reconstructions observed under
reaction conditions (Figure 10.7k) are undoubtedly assisted by oxygen.
A morphological change to such a polyhedral shape is not observed in ex-
periments with pure hydrogen, or in the presence of an electric field alone,
or in its absence under the same experimental conditions.
At higher temperatures (up to 600 K) the extension of {113} and {011}
facets occurs to the detriment of their vicinal orientations. The origin of the
contrast of around the dark extended areas is due to the extension and the
flattening of the facets leading to a polyhedral morphology: the displace-
ment of rhodium atoms—probably in the form of mobile Rh
x
O
y
species—to
sites of higher stability caused the appearance of flat areas where the electric
field decreases.
The onset of oxidation during the O
2
þ
H
2
reaction shows a strong sen-
sitivity to the surface structure at temperatures of 505 K and lower, and
occurs first along the
h
001
i
zone lines.
35,63
This effect vanishes with
increasing temperatures. A cross comparison of the respective influences
of the temperature and the tip morphologies indicates that reconstruct-
ions have only a limited influence on the reactivity at
d
n
9
r
4
n
g
|
8
temperatures
above
B
500 K.
.
10.3.1.3 O
2
/Pt and NO
2
þ
H
2
/Pt
This section reports on the occurrence of periodic oscillations during the
NO
2
þ
H
2
interaction over Pt tip samples,
64
focussing on the surface trans-
formation from a freshly developed quasi-hemispherical Pt tip to a sup-
posedly polyhedral shape. The transition from the first to the second is
observed through the evolution of the surface reactivity.
The Pt catalyst is imaged with high resolution by FIM before (Figure 10.7b)
and after the reaction. The reaction itself and related patterns during the
ongoing reaction are followed by FEM. The main observations during the
ongoing process are as follows: the reaction between NO
2
and H
2
on Pt
samples shows self-sustained kinetic instabilities that are expressed as peaks
of brightness around the {011} facets. Their ignition is synchronized within
the time resolution of the video recording device (40 ms), suggesting the
presence of a strong diffusional coupling. During the first minutes of the
interaction, while the tip apex is freshly developed and exhibits a quasi-
hemispherical shape, only {012} and {011} facets are active for the given
reaction. After a time delay of approximately 15 min, facets located along the
h
001
i
zone lines become active, as well as the topmost (001)-center. The
activity of the (001) facet results from the progressive reconstruction of the Pt
tip sample. Finally, during the whole process, the {113} and {111} facets
Search WWH ::
Custom Search