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adsorbed hydrogen (note that a potential sweep to E , 0.1 V was necessary to form
this adlayer) [El-Aziz and Kibler, 2002]. In this way, a H
upd
coverage of u
H
¼ 0.5 -
0.6 ML was determined for E ¼ 0.08 V [El-Aziz and Kibler, 2002]. For E
0.11
V, negative charge transients were observed, which were attributed to the displacement
of roughly 0.5 - 0.6 ML of OH
ad
[Hoster et al., 2004; El-Aziz and Kibler, 2002].
Considering the additional positive charge accumulated in the positive-going scan
up to a positive potential limit of 1.05 V, which results from H
2
O dissociation, the
Ru(0001) surface must be covered by approximately 1 ML O
ad
at that potential [El-
Aziz and Kibler, 2002]. This also fits to the (1
1) LEED/RHEED pattern found
after emersing the electrode in this potential region [Zei and Ertl, 2000]. (It should
be noted, however, that a (1
1) pattern would be observed also for a disordered
adlayer, independent of the coverage.)
In the cathodic potential scan, the charge exchanged between 1.05 V and 0.3 V cor-
responds to about 1 e
2
per surface atom [El-Aziz and Kibler, 2002], equivalent to the
reduction of 1 ML O
ad
to 1 ML OH
ad
or to 0.5 ML O
ad
. Comparing with the (2
2)
LEED/RHEED diffraction pattern, which was observed after emersion at 0.3 V [Zei
and Ertl, 2000], we assign this to a (2
1)O phase with u¼ 0.5 ML O
ad
, similar to
that formed on Ru(0001) under UHV conditions [Pfn ¨ r et al., 1989]. In this case,
the cathodic peak B
0
in the range 0.1 - 0.3 V with an integrated charge of 0.133 mC
cm
22
(about 0.5 e
2
per Ru atom) corresponds to the reduction of 0.5 ML O
ad
to a
0.5 ML OH
ad
, adlayer. Extending the potential scan to a lower limit in the hydrogen
evolution region, a large cathodic peak A
0
(solid line in Fig. 14.2b) develops,
which, after subtraction of the hydrogen evolution current, contains a charge of
approximately 1.1 e
2
per surface atom [El-Aziz and Kibler, 2002]. Based on the
CO displacement experiments described above, this peak must reflect the replace-
ment of an OH adlayer by a more stable H adlayer [Hoster et al., 2004; El-Aziz and
Kibler, 2002].
In the anodic scan, the oxidation of the H adlayer formed below 0.1 V and the
re-formation of OH
ad
/O
ad
(both in peak A) are shifted to markedly higher potentials
compared with the O
ad
/OH
ad
removal and H
upd
formation ( peak A
0
) in the cathodic
scan (Fig. 14.2b). Furthermore, it overlaps with the peak B (OH
ad
oxidation) observed
for a cathodic scan limit of 0.1 V. At low scan rates, peak A starts at 0.1 - 0.15 V
and reaches up to 0.48 V. Hence, compared with a scan with a cathodic limit of
E . 0.1 V, the equilibration of the O
ad
/OH
ad
adlayer is shifted from 0.28 to 0.48 V.
The charge in peak A integrated in the range 0.1 - 0.48 V corresponds to 1.5 e
2
per
surface atom, which is equal to the sum of the charges in peaks B
0
and A
0
in the
negative-going scan.
The above discussion clearly illustrates that the electrochemistry of Ru(0001) is
largely influenced by its high affinity to adsorbed H, OH, and O species. Because
of the close relation between the metal - hydrogen bond strength and the potential
of electrochemical hydrogen adsorption [Jerkiewicz, 1998; Karlberg et al., 2007],
the anodic limit of the H
upd
potential range on Ru(0001) should be about 0.15 -
0.22 V higher than on Pt(111). This means that H
upd
should be stable up to about
0.5 V with respect to desorption as H
þ
. On the other hand, the strong bonding of
O
ad
and OH
ad
to Ru(0001) stabilizes adsorbed oxygen-containing species down to
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