Environmental Engineering Reference
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become smaller with increasing Pt content, accompanied by the appearance and
growth of new voltammetric features at higher potentials that we have labeled as
D/D
0
,E/E
0
, and F/F
0
. Integration from 0.3 to1.05 V yields about 0.5 and 0.4 electrons
per surface atom for x
Pt
¼ 0.57 and the full Pt monolayer, respectively, equivalent to at
most 0.25 and 0.2 ML O
ad
, respectively, at 1.05 V. Again, electrochemically this
cannot be distinguished from an OH
ad
or mixed OH
ad
/O
ad
adlayer with accordingly
higher coverages, and, for the mixed sites, the nature of the adsorbate at 1.05 V is
in fact not clear. The lower coverage of OH
ad
and/or O
ad
at the anodic potential
limit and the higher potentials necessary to stabilize these species agree well with
the expected smaller binding energies of OH
ad
on the mixed sites of the Pt
x
Ru
12x
surface alloys. One may speculate that the increasing symmetry between positive-
going and negative-going scan in the new features at E . 0.3 V, with increasing Pt sur-
face content, is due to an additional catalytic promotion of homolytic H
2
O formation
and dissociation by the mixed Pt
n
Ru
32n
sites, similar to the mechanism discussed
above for OH
ad
formation and reduction at E ¼ 0.1 V [Reactions (14.6) - (14.8)].
Overall, the comparative cyclic voltammetry analysis of bare Ru(0001), Pt adlayer-
modified Ru(0001), and Pt
x
Ru
12x
/Ru(0001) surface alloys has demonstrated that the
electrochemical adsorption properties of these surfaces can be largely described by a
local picture, including mainly electronic ligand and strain effects in the pseudo-
morphic surface layer, and ensemble effects induced by the composition of the three-
fold adsorption ensembles. Long-range effects, mediated, for example, by elastic
distortions of the surface layer, do not seem to be important. Pseudomorphic Pt mono-
layers on Ru(0001) adsorb H
upd
,OH
ad
, and O
ad
much more weakly than Pt(111) or
Ru(0001), in agreement with the findings in recent adsorption studies under UHV con-
ditions and theoretical predictions. Mixed M
3
adsorption ensembles containing fewer
than three Ru atoms or even Pt
3
ensembles provide new adsorption states that interact
less strongly with H
ad
,OH
ad
, and O
ad
than Ru
3
sites, and thus give rise to additional,
more reversible voltammetric features that first appear in addition to those of larger Ru
ensembles and replace them at increasing Pt content. The addition of Pt, either as Pt
monolayer islands or in a PtRu surface alloy, has a pronounced catalytic effect on
the kinetics of the H
upd
/OH
ad
exchange process, by providing an additional channel
for H
upd
formation (cathodic scan) and its oxidative replacement by OH
ad
(anodic
scan) on the OH
ad
/H
upd
blocked Ru(0001) surface areas or Ru
3
ensembles.
14.3.2 CO Electro-oxidation on Bare and Pt-Modified
Ru(0001) Electrodes
In this section, we focus on the Pt-induced modifications of the electrocatalytic
properties of Ru(0001), using the electro-oxidation of CO (CO bulk oxidation)
as example.
14.3.2.1 Pt-Free Ru(0001)
CO adsorption on Ru(0001) under UHV conditions
has been extensively studied [Buatier de Mongeot et al., 1998; Diemant et al., 2003;
Thomas and Weinberg, 1979; Williams and Weinberg, 1979; Pfn ¨ r et al., 1980, 1983;
Pfn ¨ r and Menzel, 1983; Michalk et al., 1983; Kostov et al., 1992]. A comprehensive
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