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3 nm. As can be seen, the dominant species at the surface of the speci-
men is oxygen (more than 50 at%). In addition to this, the ratio of metal-
lic elements deviates significantly from the bulk composition: Pt with an
initial bulk content of 90 at%, drops to
65 at% within the 2-3 first
atomic layers (ratio of the Pt/Ir metallic species, without consideration of
the oxygen concentration). Below these layers, a region of 2 nm marks the
transition to the bulk value of the Pt/Ir ratio. More precisely, the Pt/Ir de-
creases down to a minimum of
B
d
n
9
r
4
n
g
|
8
B
50% and then rises up to 90%. This
transition region is also characterized by the decrease of the oxygen con-
centration and a slight Ir depletion that indicates the diffusion of Ir from
the sub-surface layers towards the oxygen-rich surface. Annealing a similar
alloy under UHV conditions induces Pt segregation:
103
Ir enrichment is
thus a chemically induced process.
10.3.2.1.6 Pt-8.9at%Ru. Ruthenium is known to exhibit high selectivity
for conversion of NO into N
2
.
104,105
A Pt-8.9at%Ru alloy was oxidized
under 1 bar of oxygen gas for 5 h, over a range of temperatures.
106
A ra-
ther low level of oxidation is observed at 773 K, with the presence of both
PtO
x
and RuO
x
species observed. The oxide layer is enriched in Ru (the
Ru/(Pt
รพ
Ru) ratio increases to 0.22). Beneath the oxide layer, the Ru con-
tent is accordingly depleted for approximately 3 nm. At 873 K, the oxide
concentration remains low but in this case, only PtO
x
species were de-
tected. There is, however, a Ru-depleted region of approximately 4 nm be-
neath the oxide layer, suggesting that more Ru has diffused to the surface
oxide, but became volatile at this temperature once RuO
x
was
formed.
107,108
A further increase in temperature to 973 K shows that the
surface is Pt-rich, together with the absence of oxide species. A 5 nm Ru-
depleted region is also present which is in agreement with the formation
of volatile Ru-oxides.
.
10.3.2.1.7 Conclusion on Binary Alloys. The most important overall con-
clusion from these studies of binary alloys is that there is a marked vari-
ations in susceptibility to oxidation and in the atomic-scale structures
formed, depending on alloy composition and environmental exposure con-
ditions. Chemically induced diffusion leads to the segregation of one of
the metals in the presence of reactive gas. This may lead to an enrichment
of the surface in one species, for example in Pd-38at%Au and Pt-10.1at%Ir
alloys, or to the formation of a core-shell structure with a nearly single
element surface, such as in the Pt-22at%Rh alloy after oxidation and sub-
sequent reduction (Figure 10.15a). For Pt-17.4at%Rh, lower oxidation tem-
peratures highlight the presence of two different diffusion processes:
perpendicular to the surface (which corresponds to exchanges between the
bulk and the surface) and lateral (surface diffusion between different
crystallographic surfaces). In some cases, core-shell structures can be
finely controlled by varying the oxidation temperature. Pt-8.9at%Ru
forms a Ru-rich oxide at low temperatures and a Pt-rich shell at high
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