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through a Langmuir - Hinshelwood (L - H)-type reaction and follows a reaction
scheme in which oxygenated species can be adsorbed on Sn or Pt at low potentials:
Pt-CO
ad
þ
Sn-OH
ad
!
CO
2
þ
H
þ
þ
e
(8
:
1)
It should be mentioned here that Sn sites are not considered to be the solitary source for
OH
ad
, which could be adsorbed on Pt sites owing to the influence of adjunct Sn atoms
[Stamenkovic et al., 2005]. The promotional effect of Sn was later confirmed on a
PtSn/C nanocatalyst [Arenz et al., 2005], which exhibits similar behavior that was
assigned primarily to the formation of reactive OH species at much lower potential
than on pure Pt catalysts. Based on these findings, the bifunctional effect was unam-
biguously confirmed for Pt-Sn surfaces, where Sn sites serve as a source of oxygenated
species that boost CO oxidation at low potentials and allow these surfaces to be
employed as CO-tolerant catalysts.
8.3.2 Bimetallic Systems with Nanosegregated Profile
In our early work with bimetallic systems, we noticed that, depending on the prep-
aration procedure in UHV, different surface compositions could be produced
over the same bulk material owing to the phenomenon of surface segregation
[Stamenkovic et al., 2002]. It was essential, then, to establish a methodology for trans-
ferring a well-defined bimetallic system into an electrochemical environment for
further electrochemical characterization.
The first results reporting a relation between electrocatalytic activity and surface
composition determined by LEIS were reported by Gasteiger et al. (1993), for a
series of PtRu alloys. After annealing in UHV, these systems showed evidence of
Pt enrichment on the surface, and a clear link between the surface composition and
activity for electrooxidation of methanol was established. More recently, our initial
work on polycrystalline Pt
3
M alloys (M ¼ Ni, Co) [Stamenkovic et al., 2002, 2003]
shown in Fig. 8.10, revealed a complete segregation of Pt after annealing in UHV at
1000K, i.e., the outermost surface layer contains 100 at% of Pt. A pure topmost
atomic layer of Pt has been designated as the Pt-skin surface. The same results have
been found for all Pt
3
M(M¼ Co, Ni, Fe, V, Ti) alloys studied by our group, i.e., for-
mation of a Pt-skin composition upon annealing, and a surface composition after mild
sputtering that corresponds to the bulk ratio of alloying components (75 at% of Pt and
25 at% of Co) [Stamenkovic et al., 2007b]. However, in contrast to Pt
3
M systems, no
enrichment in Pt has been found for the annealed PtCo alloy surface, suggesting that Pt
segregation thermodynamics depends strongly on the bulk ratio of alloying com-
ponents [Stamenkovic et al., 2006a]. In addition, we have challenged the chemical
stability of well-defined surfaces that were exposed to electrochemical environment
utilizing AES and LEIS spectroscopy. Following (electro)chemical pretreatment,
each alloy surface was rinsed with ultrapure triply distilled water, dried in an argon
stream, and then transferred back to the UHV environment.
Comparison between data obtained on surfaces treated in water and acid unambigu-
ously confirms a dramatic change in surface composition; in fact, complete dissolution
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