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well with a (1 2 x
Pt
)
3
relation between Pt content and the experimentally determined
peak charges on the Pt
x
Ru
12x
/Ru(0001) surface alloy. Therefore, we conclude that
H
upd
and OH
ad
adsorption take place only on Ru
3
sites in this potential region, and
that therefore the abundance of these sites dominates the charge in the narrow and
sharp replacement signals.
Although the sharp signals for H
upd
/OH
ad
exchange dominate the CVs, the charges
outside this potential region are not negligible. Similar to the observations at Pt island-
modified surfaces (see Fig. 14.5), the peak pairs B/B
0
and C/C
0
decrease in ampli-
tude with increasing x
Pt
. They disappear, however, already long before approaching
x
Pt
¼ 1. They are essentially invisible already for x
Pt
¼ 0.25 (Fig. 14.5g) and x
Pt
¼
0.53 (Fig. 14.5h), respectively. Simultaneously, additional broad features develop at
0.4 and 0.6 V, which are labeled as D/D
0
and E/E
0
, respectively. For the full Pt
monolayer, the peaks F/F
0
at 1.0 V, which we assign to reversible OH
ad
formation,
mark the most pronounced feature. Although an unambiguous assignment of the
changes for E . 0.3 V is less straightforward than for the peaks A/A
0
, we will discuss
some basic trends at this point.
As stated above, our UHV data [Diemant et al., 2003, 2008] indicate that both CO
ad
and H
upd
bind more weakly to Pt
x
Ru
12x
/Ru(0001) surface alloys than to Ru(0001);
according to DFT calculations, a similar trend can be assumed also for OH and O
[Liu et al., 2003; Lischka et al., 2007; Hoster et al., 2009c]. In cyclic voltammograms,
weaker H
upd
adsorption gives rise to a negative shift of the corresponding peaks
[Karlberg et al., 2007], whereas OH
ad
and O
ad
formation will require higher potentials
[Climent et al., 2006]. Consequently, the new peaks D/D
0
,E/E
0
, and F/F
0
reflect
OH
ad
adsorption/replacement and H
upd
adsorption/replacement on these less strongly
binding adsorption sites. The binding energies of H
upd
and OH
ad
on mixed Pt
m
Ru
n
will be between the values of the Ru
3
site and the ( pseudomorphic) Pt
3
sites (see
also [Diemant et al., 2008]). Recent DFT calculations yielded values of 22.89 eV
(23.28 eV), 22.74 eV (22.73 eV), 22.56 eV (22.04 eV), and 22.44 eV (21.46
eV) for the binding energies of H
upd
(OH
ad
)onRu
3
,Ru
2
Pt, RuPt
2
, and Pt
3
sites in
Ru
32n
Pt
n
/Ru(0001) (n ¼ 0 - 3) surfaces, respectively [Hoster et al., 2009c].
Hence, the charge decrease in peak A
0
, which we showed to be proportional to
(1 2 x
Pt
)
3
, will be accompanied by the appearance of new features at lower or
higher potentials. Furthermore, on the mixed sites, the stability regions of H
upd
and
OH
ad
are likely to no longer overlap, in contrast to the Ru
3
sites. The peaks reflecting
H
upd
adsorption/desorption on/from Pt
3
sites are expected to be shifted to below 0.1
V, where they are masked by H
2
evolution [Hoster et al., 2004]. On the other hand, for
surface alloys with lower Pt content, the region between peaks B and C also exhibits
larger currents than for bare or Pt island-modified Ru(0001) surfaces. This is tenta-
tively associated with desorption of H
upd
/adsorption of OH
ad
on mixed Pt
x
Ru
32x
sites. They disappear only when approaching x
Pt
. 0.5. Therefore, we suggest that
on Ru
32n
Pt
n
sites with n
2, H
upd
and OH
ad
are formed only at E , 0.2 V and
E . 0.3 V, respectively.
The peak pairs B/B
0
and C/C
0
are proposed to reflect the reversible deprotonation
of 0.5 ML OH
ad
in peak B/B
0
(Reaction (14.5)) and completion of the O
ad
coverage up
to 1 ML in peak C/C
0
[Reactions (14.4) and (14.5)], respectively. Both peak pairs
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