Environmental Engineering Reference
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mixed but nonreactive CO ad þ OH ad /O ad adlayer on the surface alloy and on the Pt
monolayer island-covered surface at lower potentials and for low Pt content. On the
PtRu/Ru(0001) surface alloys, however, the reduction current decreases much more
rapidly with increasing Pt content than on the Pt monolayer island-modified surface.
This agrees well with the lower OH ad /O ad coverage expected from the base voltam-
metry at E ¼ 0.55 V on mixed Pt n Ru m sites.
The CO oxidation behavior in the potential range around the reaction onset (see
Fig. 14.12a) is very similar to that of the Pt monolayer island-modified Ru(0001)
electrode for the low Pt content sample (x ¼ 0.07). For the higher Pt surface contents,
the onset of the CO oxidation reaction is shifted to lower potentials and the increase in
oxidation current with potential is much steeper than for the adlayer-modified sur-
faces. For the Pt 0.47 Ru 0.53 /Ru(0001) surface, the onset potential for CO oxidation
in the positive-going scan and the collapse of the CO oxidation current in the
negative-going scan are about 0.05 V more cathodic than for surfaces dominated by
truly “Ru-like” sites, as they appear on nonmodified Ru(0001) electrodes, Pt
monolayer-modified Ru(0001) electrodes, or PtRu (surface) alloys with low Pt
content. On the other hand, the onset potential and also the decay potential are still
more anodic than that on a sputtered, polycrystalline Pt 0.5 Ru 0.5 bulk alloy (see
Fig. 14.12c) [Gasteiger et al., 1995]. It should be noted, however, that measurements
on the latter sample were performed in a different electrolyte, in 0.5 M H 2 SO 4
[Gasteiger et al., 1995] rather than 0.1 M HClO 4 (our data), which can give rise to
slight differences in the potential-dependent behavior.
The significant downshift in the onset potential for CO oxidation on the PtRu/
Ru(0001) monolayer surface alloys with high Pt content compared with the Pt
island-modified Ru(0001) surface is explained by modifications of the local adsorp-
tion properties. Assuming that the onset of the CO oxidation Reaction (14.9) or
(14.12) is determined by the activity of the O ad /OH ad species, and considering that
the barrier for CO ad þ OH ad reaction is correlated with the stability of the initial
and/or final state (Brønsted - Polanyi - Evans relation [Brønsted, 1928; Evans and
Polanyi, 1938; Bondzie et al., 1999; Logadottir et al., 2001]), the downshift of the
reaction onset implies a destabilization of these adspecies with increasing Pt content.
As mentioned above, our studies of CO and deuterium adsorption on similar-type
PtRu/Ru(0001) surface alloys under UHV conditions revealed that these adsorbates
are increasingly destabilized with increasing Pt surface content owing to electronic
effects (lateral ligand and strain effects), i.e., with increasing number of Pt neighbors
[Rauscher et al., 2007, Diemant et al., 2008]. Similar trends can be expected also for
OH ad /O ad , and were in fact also determined in recent DFT calculations [Hoster et al.,
2009c]. These effects were made responsible also for the upshift of the peaks
associated with OH ad /O ad formation in the base CVs with increasing Pt content
(Fig. 14.8).
The much steeper increase of the current with potential in Fig. 14.12 after the initial
slow increase, as compared with bare or Pt monolayer island-modified Ru(0001), is
attributed to an increased abundance of OH ad /O ad species with increasing potential.
In agreement with our previous arguments, this can be rationalized by the weaker
adsorption of these species on the mixed Ru 32n Pt n sites, while on Ru(0001) the
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