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 489

adsorbed on Pt sites. In this way, the reaction-limiting lack of oxygenated adspecies
characteristic of Pt surfaces at lower potentials is counteracted by the enhanced for-
mation of these species on the Ru sites, and the subsequent reaction between CO ad
(on Pt sites) and OH ad (on Ru sites) is considered to be facile (for Pt, see
[Lebedeva et al., 2000; Shubina et al., 2004; van Santen, 1991]). Based on the present
data, we propose, in contrast, that on mixed Pt- and Ru-containing surfaces (PtRu
surface alloys and PtRu bulk alloys), mixed Pt 2 Ru and PtRu 2 sites act as active centers
for the formation of OH ad /O ad species of optimized stability and reactivity for the CO
oxidation reaction. The reaction-enhancing effect of the mixed sites with their intermedi-
ate OH ad /O ad stability, which is between those on pure Ru or Pt electrodes, is a
classic example of the well-known Sabatier principle [Sabatier, 1913], which predicts
that there is an optimum stability of the reactants for a given catalytic reaction, with
lower activities resulting for a too high or too low stability of the adsorbed reactants,
or the variation of the catalytic activity with the position of transition metals in the per-
iodic table described by the volcano curves as first described by Balandin [1969].
Although closely related in its physical origin, the concept of a reaction-enhancing
effect of the mixed Pt 2 Ru and PtRu 2 sites for the CO oxidation reaction on PtRu
(surface) alloys is distinctly different from the classic bifunctional mechanism.
14.4 CONCLUSIONS
The influence of Pt modifications on the electrochemical and electrocatalytic properties
of Ru(0001) electrodes has been investigated on structurally well-defined bimetallic
PtRu surfaces. Two types of bimetallic surfaces were considered: Ru(0001) electrodes
covered by monolayer Pt islands and monolayer PtRu/Ru(0001) surface alloys with a
highly dispersed and almost random distribution of the respective surface atoms, with
different Pt surface contents for both types of structures. The morphology of these sur-
faces differs significantly from that of bimetallic PtRu surfaces prepared by electroche-
mical deposition of Pt on Ru(0001), where Pt predominantly exists in small multilayer
islands. The electrochemical and electrocatalytic measurements, base CVs, and CO
bulk oxidation under continuous electrolyte flow, led to the following conclusions:
1. Owing to their high affinity to H upd ,OH ad , and O ad , Ru(0001) surfaces are cov-
ered by strongly bound adlayers in the entire potential region between 0 and 1 V,
with overlapping stability ranges of the respective adsorbates. These adlayers
inhibit catalytic reactions and thus make this surface and Ru in general a
rather poor electrocatalyst, for example for CO oxidation.
2. Pseudomorphic Pt monolayers on Ru(0001) interact very weakly with H upd ,
OH ad ,orO ad , because of electronic ligand (vertical ligand effects) and strain
effects (tensile strain), in agreement with results obtained under UHV con-
ditions and in DFT calculations. Therefore, base CVs on these surfaces do
not show pronounced voltammetric features.
3. Ru(0001) surfaces partly covered by Pt monolayer islands exhibit very interest-
ing catalytic effects. In reactions such as H upd $ O ad /OH ad exchange or CO