The Effect of Structurally Well-Defined Pt Modification on the Electrochemical and Electrocatalytic Properties of Ru(0001) Electrodes - Fuel Cell Catalysis: A Surface Science Approach - page 487

“active” adsorbates required for reaction can be generated only by strongly repulsive
adsorbate - adsorbate interactions. (The CO ad species are also less strongly bound on
the mixed surface than on Ru(0001), but this does not change directly with potential.)
Owing to the lower steady-state adsorbate coverage on the (less strongly bonding) sur-
face alloy, the formation of OH ad and therefore also the following reactive removal of
CO ad increase more strongly with potential than on Ru(0001). This is equivalent to the
general concepts based on the Tafel slope [Trasatti, 2003] described above for reaction
on a Pt monolayer island-modified Ru(0001).
14.3.2.4 CO Oxidation on Bimetallic PtRu Electrodes: General
Aspects The general characteristics of the j - E curves for CO bulk oxidation on
bimetallic PtRu electrodes in the region E . 0.55 V largely resemble those reported
for CO bulk oxidation on Pt electrodes [Markovic et al., 1999]/supported Pt catalysts
[Schmidt et al., 1999; Jusys et al., 2001] and on PtRu bulk alloys [Gasteiger et al.,
1995] (see also Fig. 14.12c). In all cases, there is a pronounced hysteresis between
the onset of CO oxidation in the positive-going scan and the current decay in the
negative-going scan. Such hysteresis is generally attributed to a bistable behavior of
the surface, which “switches” between two states [Ertl et al., 1982; Cox et al.,
1983; Behm et al., 1983; Imbihl et al., 1984]. For CO oxidation at the gas/solid inter-
face, the two regimes are characterized by a CO ad -covered surface (“low rate branch”
[Engel and Ertl, 1982]) and an essentially clean surface (“high rate branch”). For CO
electro-oxidation, similar ideas apply, but with some differences. At lower potentials,
the barrier for CO 2 formation rather than surface blocking by CO ad is rate-limiting
[Santos et al., 1991; Shubina et al., 2004; Levia and S´nchez, 2003]. At higher poten-
tials, the surface is covered by an OH ad /O ad adlayer rather than being essentially
adsorbate-free as for CO oxidation in the high rate branch in the gas phase (see also
the simulations of CO electro-oxidation on a Pt(111) electrode [Markovic et al.,
1999; Koper et al., 2001; Saravanan et al., 2002]). These assignments and expla-
nations are largely applicable also for the Ru(0001)-based model systems. The main
difference between Pt(111) and Ru(0001) is that in the latter case the removal of
the strongly bound OH ad /O ad species is rather slow at all potentials above the onset
of the reaction. This results in a high steady-state OH ad /O ad coverage at these poten-
tials. In this respect, the reaction behavior of the surface alloys is much closer to that of
Pt electrodes, since the dominance of strongly bound OH ad /O ad decreases rapidly with
increasing Pt content (see also the base voltammetry data in Fig. 14.8).
Our results clearly demonstrate that the physical origin of the enhanced CO oxi-
dation activity of bimetallic PtRu surfaces or nanoparticles depends markedly on
the respective surface structure and composition. Adding Pt atoms to the Ru(0001)
surface layer generally promotes the dissociation of H 2 O by providing a fast pathway
for adsorption and desorption of H þ . This increases the reaction rate considerably at
potentials E . 0.55 V, where the OH ad /O ad species become sufficiently reactive. In
the potential range E , 0.55 V, in contrast, the reaction of coadsorbed CO ad and
OH ad is inhibited by the high stability of the OH ad /O ad species on the Ru(0001) sur-
face areas, and this is not changed by the presence of Pt monolayer islands. Therefore,
the onset potential of the CO oxidation reaction is identical for the unmodified