Environmental Engineering Reference
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potentials around 0 V. This leads to a substantial overlap of OH
ad
adsorption and
hydrogen adsorption on Ru(0001) [Hoster et al., 2004; El-Aziz and Kibler, 2002],
in contrast to the electrochemical behavior of Pt(111), where the potential regions
of reversibly adsorbed H
upd
and OH
ad
are separated by the “double-layer region”
(see, e.g., Fig. 14.4b, where a Pt(111) CV is included as reference) [Garcia-Araez
et al., 2006]. The overlap of the H
upd
and OH
ad
potential regions on Ru(0001)
agrees with results of a recent first-principles study that predicted the equilibrium
potentials for the formation of H
upd
and OH
ad
adlayers on that surface to be so
close to each other that the double-layer region should virtually vanish in acidic elec-
trolytes [Taylor et al., 2007].
The overlap of the stability regions of the two adsorbates has two major conse-
quences. First, the potentials for H
upd
and OH
ad
formation are determined by the equi-
librium potential for OH
ad
$
H
upd
exchange rather than by the equilibria between
H
upd
and H
þ
and between OH
ad
and H
2
O, respectively. This is completely different
from Pt(111), where H
upd
and OH
ad
adsorption are determined independently by
the equilibria with H
þ
and H
2
O, respectively. Second, this exchange process is held
responsible for the pronounced hysteresis of about 0.2 V between OH
ad
!
H
upd
repla-
cement in the cathodic scan (peak A
0
) and the reverse reaction in the anodic scan ( peak
A). As will be discussed in more detail in the following sections, the equilibrium
potential for OH
ad
$
H
ad
exchange on Ru(0001) in 0.1 M HClO
4
can be estimated
to be around 0.1 V. The onset of OH
ad
!
H
upd
exchange on this surface at signifi-
cantly lower potentials, around 0.05 V (onset of peak A
0
with a maximum at 20.02
V), can be explained by the stability of the (metastable) OH
ad
layer, which will mea-
surably react with H
þ
only at potentials significantly below the equilibrium potential
for OH
ad
$
H
upd
exchange. Hence, the OH
ad
layer is kinetically stabilized. In a micro-
scopic picture, this can be tentatively explained by a mechanism where OH
ad
$
H
upd
replacement in peak A
0
proceeds via a two-step process:
OH
ad
þ
H
þ
þ
e
!
H
2
O
ad
!
H
2
O
þ
A
(14
:
1)
A
þ
H
þ
þ
e
!
H
upd
(14
:
2)
In this reaction scheme, hydrogen adsorption (on an empty site A) can occur only after
OH removal. Since the adsorption energy of hydrogen is gained only in the second
step, this may shift the onset of the reaction to more cathodic potentials. We will
see later that on a surface covered by Pt monolayer islands that allow an easy formation
of H
ad
(see below), a homolytic reaction according to
OH
ad
þ
H
upd
!
H
2
O
ad
þ
A
!
H
2
O
þ
2A
(14
:
1a)
is more probable than the heterolytic Reaction (14.1). For a pure Ru surface, however,
our data do not allow us to rule out one or the other possibility. Recently, Taylor et al.
(2007) suggested that a homolytic pathway is dominant for surfaces where the H
upd
and OH
ad
stability regions overlap.
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