Environmental Engineering Reference
In-Depth Information
oxidation, they provide local adsorption channels for the respective second reac-
tant, whose adsorption is inhibited on local Ru(0001) areas, and can thus cata-
lyze the reactions. This gives rise to an accelerated exchange between OH ad and
H upd layers with onset potentials around 0.1 V, while on an unmodified
Ru(0001) electrode, the exchange reaction exhibits a much more pronounced
hysteresis, with much broader peaks and less well-defined onset potentials of
0 - 0.05 V in the negative-going and 0.15 V in the positive-going scan.
4. For the same reason, Ru(0001) modification by Pt monolayer islands results in a
pronounced promotion of the CO oxidation reaction at potentials above 0.55 V,
which on unmodified Ru(0001) electrodes proceeds only with very low reaction
rates. The onset potential for the CO oxidation reaction, however, is not measur-
ably affected by the presence of the Pt islands, indicating that they do not modify
the inherent reactivity of the O/OH adlayer on the Ru sites adjacent to the Pt
islands. At potentials between the onset potential and a bending point in the
j - E curves, CO ad oxidation proceeds mainly by dissociative H 2 O formation/
OH ad formation at the interface between the Ru(0001) substrate and Pt islands,
and subsequent reaction between OH ad and CO ad . The Pt islands promote homo-
lytic H 2 O dissociation, and thus accelerate the reaction. At potentials anodic of
the bending point, where the current increases steeply, H 2 O adsorption/OH ad
formation and CO ad oxidation are proposed to proceed on the Pt monolayer
islands. The lower onset potential for CO oxidation in the presence of second-
layer Pt islands compared with monolayer island-modified Ru(0001) is assigned
to the stronger bonding of a double-layer Pt film (more facile OH ad formation).
5. Pt x Ru 12x monolayer surface alloys with an atomically disperse distribution of
the two surface species offer adsorption sites (“mixed adsorption ensembles”),
which are not present on pure or Pt monolayer island-modified Ru(0001) sur-
faces. The adsorption strength of these mixed sites, for example for H, O, or
OH adsorption, decreases with increasing Pt content of the adsorption ensemble,
in this case from Ru 3 via PtRu 2 and Pt 2 Ru to Pt 3 . The lower binding power of the
Pt surface atoms in the mixed adsorption ensembles results from the same effects
as described for Pt monolayer islands. It is further reduced by Ru neighbors,
owing to the stronger interactions between Pt and Ru surface atoms compared
with Pt surface atoms (“lateral electronic ligand effects”). On the other hand,
compact Ru 3 ensembles maintain an adsorption behavior rather similar to that
of Pt-free Ru(0001). This is illustrated by their ability to adsorb hydrogen in a
sharp peak starting at 0.1 V in the negative-going scan, via reactive replacement
of OH ad . This proceeds in a similar mechanism as for Pt monolayer island-
modified Ru(0001). Accordingly, the charge in the replacement peak scales
with (1 2 u Pt ) 3 , as expected for a random distribution of the respective surface
atoms and OH/H adsorption on threefold sites. For mixed sites, OH adsorption
and H adsorption are shifted to higher and lower potentials, respectively.
6. The same energetic modifications also affect the CO bulk oxidation. Because of
the lower binding energy of the adsorbed reactants (CO ad and OH ad )onthe
intermixed surface, the barrier for reaction of these species and hence for
CO 2 formation is significantly reduced compared with reaction on Ru(0001)
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