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(iii) electronic strain effects reflecting changes in the chemical properties due to
modifications in the surface lattice geometry imposed by the substrate [Mavrikakis
et al., 1998; Sachtler, 1973, 1997; Soma-Noto and Sachtler, 1974; Sachtler and
Somorjai, 1983; Hammer et al., 1996; Gsell et al., 1998; Liu and Nørskov, 2001b].
In this chapter, we present and discuss results of a similar type of study on the fun-
damental electrochemical and electrocatalytic properties of structurally well-defined
bimetallic PtRu surfaces, focusing on the effect of Pt modification of Ru(0001) sub-
strates. Surface preparation and STM imaging were performed under UHV conditions.
The electrochemical and electrocatalytic characterization occurred under controlled
mass transport conditions in an electrochemical flow cell, which was attached to the
UHV - STM chamber via a sample transfer system. In analogy to the UHV exper-
iments, this study includes (i) Ru(0001) surfaces that are covered by pseudomorphic
Pt monolayer islands or a closed Pt monolayer [Hoster et al., 2004] or (ii) PtRu/
Ru(0001) monolayer surface alloys, where the Ru(0001) substrate is covered by a
two-dimensional (2D) Pt
x
Ru
12x
alloy layer, with different Pt coverages or different
amounts of Pt in the surface layer, respectively. For comparison, we include similar
measurements on an unmodified Ru(0001) electrode. In all three cases, the surface
layer rests on a Ru(0001) substrate, i.e., the neighbors in vertical direction are exclu-
sively Ru atoms, while the lateral neighbors can be either Ru or Pt, depending on the
respective type of model surface.
The chapter is organized as follows. After a brief description of the experimental
setup and procedures, we will first focus on the electrochemical properties of the
respective electrodes with regard to the adsorption/desorption of hydrogen and
oxygenated species (OH
ad
,O
ad
) and the characteristic modifications introduced by
the different types of Pt modification (Section 14.3.1). This includes a summary of
the structural characteristics of the bimetallic electrodes and of their structure-dependent
adsorption properties under UHV conditions determined previously. From a quantita-
tive evaluation of the Pt coverage-dependent H
upd
and OH
ad
adsorption capacities of
the bimetallic electrodes, we determine the nature of the stable adsorption sites on
these surfaces. In Section 14.3.2, we describe the electrochemical bulk oxidation of
CO on these model surfaces, focusing again on the effect of Pt modification on the
activity of Ru(0001) electrodes. These will be discussed in comparison with data
obtained under UHV conditions and with results from theoretical studies. We will
finally discuss the consequences of the structure-dependent surface properties for
the atomic-scale understanding of the activity of PtRu electrodes in general.
14.2 EXPERIMENTAL
14.2.1 Experimental Setup
The experiments were performed in a combined system for UHV and electrochemical
measurements. It consists of a UHV system equipped with standard facilities for sur-
face preparation and characterization and a pocket-size scanning tunneling microscope
(STM) [Kopatzki, 1994], a pre-chamber containing a flow cell for electrochemical
measurements, which was attached to the main UHV system via a gate valve, and
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