Electrochemistry at Well-Characterized Bimetallic Surfaces - Fuel Cell Catalysis: A Surface Science Approach - page 251

are uniquely spread over the substrate by instantaneous evaporation of the aqueous
matrix, forming a thin and uniform film. With this approach, for instance, the com-
monly detected negative n CO absorbance is completely eliminated, so that the n CO
spectra can routinely be measured from nanoparticles and compared with the corre-
sponding spectra from single-crystal surfaces. The IRAS technique described here
has demonstrated its ability to provide invaluable details about the true morphology
of multimetallic well-defined extended surfaces as well as nanoparticle systems, as
will be discussed in the following sections.
8.3 RULES GOVERNING ELECTROCATALYSIS
AT BIMETALLIC SYSTEMS
A major focus in electrocatalysis is on the development of a fundamental understand-
ing of the catalytic activity of bi- and/or multimetallic systems in order to gain unpre-
cedented control of reactivity and stability during the transformation of the chemical
energy of hydrogen, hydrocarbons, and oxygen into electrical energy. The primary
interest is in controling the key parameters that could lead to a catalytic system with
advanced properties. It is well known that an enhancement of catalytic properties
could be achieved by the addition of a second metal [Appleby, 1970; Kinoshita,
1992], and the mechanism of improvement may occur through a change in the local
bonding geometry (structure effects), a bifunctional mechanism, the distribution of
active sites (ensemble effects), or directly by modifying the reactivity of Pt surface
atoms (electronic effects) [Ross, 1998]. In a real system, all of these factors will, in
general, operate simultaneously, and separating their effects and assessing their rela-
tive importance for catalytic activity and reaction mechanism is a very challenging pro-
blem [Kinoshita, 1992; Markovic et al., 2003]. This lack of fundamental knowledge
about the mechanisms responsible for the catalytic activity of bi/multimetallic sys-
tems, in turn, makes a further improvement of the materials in a rational, science-
based manner very difficult. In what follows, we will emphasize several illustrative
mechanisms for improved electrocatalysis on bimetallic systems.
8.3.1 Bimetallic Surfaces
Well-characterized Pt-bimetallic surfaces can be prepared by conventional metallurgy,
as bulk alloys. Pt x Ru y [Gasteiger et al., 1993], Pt 3 Mo [Grgur, ], and Pt 3 Sn(hkl)
[Gasteiger et al., 1995] alloys have been thoroughly characterized in our laboratory,
and we have been able to determine by UHV and in situ electrochemical techniques
that both metals are present and stable at the outermost surface layer under relevant
conditions. The Pt 3 Sn(111) system, for instance, is the most active catalyst for CO oxi-
dation [Stamenkovic et al., 2003; Gasteiger et al., 1996]. The structural properties of
the Pt 3 Sn(111) surface in an electrolyte have been thoroughly examined by in situ
SXS, including both the stability of the UHV-prepared p(2 2) structure in H 2 SO 4
solution and the potential-dependent relaxation of Pt and Sn atoms in the near-surface
region (Fig. 8.7).