Environmental Engineering Reference
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Ru(0001) surface and the Pt monolayer island-modified Ru(0001) surface. On the
Pt
x
Ru
12x
/Ru(0001) surface alloys, the onset potential can be modified by the reduced
bond energy of the OH
ad
/O
ad
and CO
ad
species on the mixed Pt
x
Ru
32x
sites compared
with the Ru
3
sites. This increases their reactivity and thus allows the CO oxidation
reaction to start at lower potentials than on a bare or Pt island-modified Ru(0001)
surface. Compared with nonmodified Pt surfaces, on the other hand, the OH
ad
/O
ad
bonding is strong enough on the mixed Pt
x
Ru
12x
(x ΒΌ 1, 2) sites that H
2
O splitting
can occur at potentials significantly below that on nonmodified Pt surfaces (see
Figs. 14.4 and 14.8). This results in the observed downshift of the onset potential to
lower values compared with CO oxidation on these Pt surfaces.
Comparing PtRu monolayer surface alloys on the one hand and PtRu bulk alloy
surfaces on the other, these two differ in the slightly different electronic properties
of their respective surface atoms, owing to their interactions with different metal
atoms in the second layer (vertical ligand effects) and the different lattice constants
(strain effects) of these alloys. Both of these factors can result in further slight modi-
fications of the electronic surface properties, and thus can affect, for example, the onset
potential for CO oxidation. It is not expected, however, that this will change the major
result of the present measurements, and therefore we explain the higher CO oxidation
reactivity of PtRu bulk alloys compared with pure Pt or Ru electrodes in the same way
as for Pt
x
Ru
12x
/Ru(0001) surface alloys (see the previous paragraph).
For comparison with bimetallic electrode surfaces prepared by electrochemical or
electroless deposition of Ru on Pt(111) [Stimming and Vogel, 1998; Iwasita et al.,
2000; Crown et al., 2001, 2002; Herrero et al., 1999] or Pt on Ru(0001) [Brankovic
et al., 2001a, b, 2002a, b] substrates, respectively, one has to consider the rather differ-
ent morphology of these deposits. These bimetallic electrodes are characterized by a
large number of small deposit islands, which, depending on the amount of the respect-
ive material deposited, are mostly several layers high (multilayer islands) [Stimming
and Vogel, 1998; Herrero et al., 1999; Brankovic et al., 2001a]. Because of their differ-
ent electronic and geometric properties, comparison of Ru/Pt(111) surfaces with the
surfaces studied here is possible only on a rather qualitative scale. They will, of course,
also provide bifunctional sites at the Ru island edges, but the electronic and geometric
properties of these surfaces differ considerably from those of the present system,
owing to the different bulk substrate and to the different lattice constant of the deposit.
For Pt on Ru(0001) [Brankovic et al., 2001a, b; Zhou et al., 2007] and Pt/Ru(1010)
[Brankovic et al., 2002a, b; Pinheiro et al., 2005], the electronic properties should be
comparable to those of the present systems. Owing to the different morphology of the
electrochemically deposited islands (multilayer island formation; see above), how-
ever, we expect considerable differences in the electrochemical and electrocatalytic
properties. Base CVs of the Pt/Ru(0001) electrodes prepared by electrodeposition
resemble our data [Zhou et al., 2007]; CO bulk oxidation data are not available so far.
Finally, we want to compare the main mechanistic findings of our study with the
classic bifunctional mechanism, which is generally used to explain the improved
CO oxidation reactivity of PtRu surfaces and catalyst particles [Watanabe and
Motoo, 1975]. According to that mechanism, Ru acts as a promotor for the formation
of oxygenated adspecies on bimetallic PtRu surfaces, which can then react with CO
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