Electrochemistry at Well-Characterized Bimetallic Surfaces - Fuel Cell Catalysis: A Surface Science Approach - page 244

electrocatalysis, corrosion, and electrochemical synthesis. Although this field is still in
its early years, a great deal has already been learned, and trends are beginning to emerge
that provide some predictive ability with respect to the surface structure/composition
assumed by bimetallic surfaces and their corresponding chemisorption properties and
activities towards simple molecules. The aim of this chapter is to review some of the
recent advances in bimetallic surface electrochemistry and to give selected examples
that will demonstrate that it is indeed possible to prepare and characterize ordered bime-
tallic systems in an electrochemical environment, assess the adsorption and electroche-
mical reactivity of these systems, and use knowledge from well-defined surfaces in
nanoscale catalyst design. A brief summary of experimental techniques that have
been commonly used for the characterization of bimetallic systems is provided.
However, this overview covers techniques that have been employed at the authors' lab-
oratories, and the reader should be aware that this is not a complete summary.
8.2 SURFACE CHARACTERIZATION TECHNIQUES: EX SITU
VERSUS IN SITU
8.2.1 Surface Structure
In heterogeneous catalysis, chemical reactions are taking place at solid surfaces, and
therefore it is of paramount importance to obtain detailed insight into fundamental sur-
face properties such as structure, composition, and surface electronic profile. UHV
surface-sensitive tools have been extensively used for determination of these critical
parameters [Somorjai, 1992]. For instance, low energy electron diffraction (LEED)
is a widely adopted tool to study the surface crystallography of various materials
[Van Hove et al., 1986]. In LEED experiments, an incident low energy electron
beam that is in the range from about 20 to 500 eV is diffracted from the surface of
the sample. The emerging diffracted beams are usually detected on a phosphorescent
screen as a characteristic pattern, which represents the reciprocal space. From this pat-
tern, conclusions can be drawn about the symmetry of the surface structure and the size
of characteristic periodicities on the surface. LEED is usually applied to control the
surface quality of a sample after preparation in UHV by establishing the exact surface
structure, eventual presence of reconstruction, impurities, etc. Moreover, LEED
has been extensively used to study chemisorption and catalytic properties in UHV,
i.e., correlations between surface structure and adsorption/desorption processes
[Somorjai, 1993].
As an example, Fig. 8.1 shows a result from Bardi and co-workers obtained on a
bimetallic Au 3 Pd(100) single-crystal alloy [Kuntze et al., 1999]. The LEED pattern
indicates a sharp (1 1) unit cell that corresponds to the bulk-truncated structure
of the substitutionally disordered Au 3 Pd alloy. Additionally, the authors determined
the composition of the first outermost layer to be pure Au. These findings revealed
that the (100) oriented surface of Au over Au 3 Pd alloy is not reconstructed, which
is unique, since pure Au, Pt, and Ir (100) crystals are all known to be reconstructed
in similar ways [Van Hove et al., 1981; Ritz et al., 1997]. In this case, the presence