Investigating Nano-structured Catalysts at the Atomic scale by Field Ion Microscopy and Atom Probe Tomography - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 265

remain totally inactive for the reaction. 8 The activity/inactivity of these facets
can be seen on the field emission pattern during surface explosions.
A snapshot taken during one of these surface explosions is presented in
Figure 10.7n.
The enlargement of major facets occurs to the detriment of surrounding
high Miller-index planes, as is the case for rhodium. However, the authors
suggest that the lack of brightness is due to the presence of chemisorbed
oxygen atoms which inhibit local field emission. Surface oxidation of plat-
inum with the occurrence of sub-surface diffusion of oxygen atoms and the
formation of an oxide layer is rather unlikely at a low temperature of 390 K.
The oxidation of platinum nanoparticles in the presence of NO x is only
observed at higher temperature (

d n 9 r 4 n g | 8

573 K). 65 During the reaction, dissociative
adsorption of NO 2 gas takes place leading to NO(ads) and O(ads) species.
Adsorbed oxygen atoms are active species which react with fast diffusing
H(ads) atoms to form water. From the reported field emission patterns, the
surface explosions indicate the location of an ongoing catalytic conversion.
This is supported by the fact that adsorbed oxygen is present and sub-
stantially increases the work function between surface explosions. 66 The
local current density of emitted electrons is lowered accordingly. The re-
action of O(ads) with H(ads) causes the local work function to decrease
within some 200 ms. This phenomenon is translated to a sudden peak of the
local brightness which images surface regions that are transiently metallic.
Subsequent adsorption of NO 2 and its dissociation over these areas causes
the field emission patterns to darken. Particular attention is paid to the {113}
facets which do not show brightness peaks. The imaged area of these regions
also extends with time. This can be attributed either to an important ex-
tension of these regions which are less reactive towards the NO 2 þ H 2 re-
action, or to the occurrence of a more complex adsorption process which
acts on the local field emission.
To further assess the morphological shape of the Pt tip after reaction, low
temperature FIM observations were carried out using helium imaging gas.
Images are presented in Figure 10.7o and p and lead to conclusions different
from those drawn for Rh. The Pt tip sample was first exposed to pure oxygen
(P O2 ¼ 2 10 3 Pa at 300 K during 180 s, in the absence of field), and then
imaged at 29 K. Under these conditions, crystal planes lying along the h 100 i
lines are resolved with atomic resolution. Facets along the [110] line appear
dark although they were visible at higher temperatures (not represented).
The experiment was conducted to avoid field evaporation as much as pos-
sible. In Figure 10.7p, the dark regions centered by the {113} facets are now
resolved by field ionization of helium atoms in the presence of a higher
imaging field. A careful analysis of the sample excludes the occurrence of
field evaporation between the two micrographs. This observation differs
from the rhodium case where field evaporation of several tens of atomic
layers—counted on the bright regions—are necessary to restore the original
field evaporation end form, i.e., a quasi-hemispherical shape. In the case of
Pt, the difference between the two field ion micrographs has its origin in the

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