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smooth 1.08 ML Pt film on Ru(0001), which was prepared by Pt deposition at 300K
and subsequent annealing to 870K [Hoster et al., 2004]. Apart from a few vacancy
islands in the monolayer Pt film or second-layer Pt islands, the surface is covered
by a pseudomorphic Pt monolayer, and hence the base CV is characteristic of a Pt
monolayer-covered Ru(0001) surface. The absence of any distinct adsorption/
desorption features in the base CV over a wide potential range (from about 0.15 V
to about 0.80 V) points to rather low/high onset potentials for the adsorption of
H
upd
/OH
ad
species, respectively, compared with Pt(111) (dotted line in Fig. 14.4b).
This is indicative of a rather weak interaction with these species [Karlberg, 2007;
Climent et al., 2006], which agrees with the results of recent theoretical studies
[Koper et al., 2002; Hoster et al., 2009a] and fits to the data obtained for hydrogen
[Diemant et al., 2003, in preparation] and oxygen adsorption on Pt
ML
/Ru(0001)
[Lischka et al., 2007] under UHV conditions (see above). Consequently, base CVs
of Ru(0001) surfaces partly covered by pseudomorphic Pt monolayer islands are lar-
gely governed by features associated with adsorption on Ru sites [Hoster et al., 2004;
Zhou et al., 2007]. This is demonstrated in the base CVs in Fig. 14.5 g - j for Ru(0001)
electrodes covered by 0.03 - 0.8 ML Pt. STM images of the respective surface mor-
phologies are shown in Fig. 14.5b - e [Hoster et al., 2004]. In the anodic potential
region (E . 0.2 V), the CV basically resembles that obtained on bare Ru(0001)
(Fig. 14.5a, f ). However, the distinct signals A and A
0
that reflect oxidation of H
upd
and formation of OH
ad
from H
2
O, and vice versa, now set in at 0.1 V in either scan
direction, respectively, compared with 0.05 V (negative-going scan) and 0.15 V ( posi-
tive-going scan) on a Pt-free Ru(0001) surface. The charges in peaks A and A
0
decrease with increasing Pt coverage, while their general shape and position change
only slightly. The charge decay of the cathodic peak A
0
with increasing Pt content fol-
lows a linear relation (Fig. 14.6) and approaches zero for x
Pt
ΒΌ 1. These characteristics
in combination indicate that the peaks result from Pt-catalyzed adsorption/removal of
OH
ad
and H
upd
species on Ru sites (most likely Ru
3
adsorption ensembles; see below),
whose number decreases linearly with increasing Pt island coverage [Hoster et al.,
2004]. The maximum charge of 0.29 mC cm
22
(equivalent to 1.16 e
2
per surface
atom), which includes contributions from H
upd
adsorption and OH
ad
removal in the
cathodic scan, points to coverages in the range of 0.5 - 0.6 ML for either adsorbate
at potentials negative (0.08 V) and positive (0.12 V) of the A
0
peak, respectively
[Hoster et al., 2004]. This is identical to the respective coverage values at potentials
directly above and below the corresponding peak around 0 V reported for bare
Ru(0001) [El-Aziz and Kibler, 2002]. On the pseudomorphic Pt islands, the H
upd
cov-
erage is very low under these conditions, which can be rationalized by the weak bond-
ing power of these monolayer islands discussed above. By charge integration, we
estimated a H
upd
coverage of at most 0.12+0.1 ML at the onset of H
2
evolution on
these islands [Hoster et al., 2004].
The onset of H
upd
$
OH
ad
exchange at 0.1 V on the Pt monolayer island-modified
Ru(0001) surface in either scan direction is the basis for our estimate that the thermo-
dynamic equilibrium potential for H
upd
$
OH
ad
exchange on Ru(0001) is at about
this value (see the preceding section). Therefore, at a potential of 0.1 V, H
upd
and
OH
ad
are about equally stable. With increasing potential, they become more weakly
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