Electrochemistry at Well-Characterized Bimetallic Surfaces - Fuel Cell Catalysis: A Surface Science Approach - page 245

Figure 8.1 LEED pattern of Au 3 Pd(100), showing a sharp (1 1) unit cell. (Reprinted with
permission from Kuntze et al. [1999]. Copyright 1999. The American Physical Society.)
of the other metal Pd in the alloy stabilizes the unreconstructed surface. Obviously,
LEED provides an important insight into the arrangements of surface atoms of
bimetallic systems; therefore, surface structure, its stability, and surface adsorption
properties, as well as adsorption sites, can be revealed. Besides studies in UHV,
LEED has also been successfully employed to analyze adsorbates as an ex situ
probe for surfaces previously exposed to an electrochemical environment [Wagner
and Ross, 1983].
Furthermore, LEED is an important tool to study thin film growth, and can reveal
the mechanism of thin film formation as well as the final structure of mono/multi
topmost layers versus substrate [Attard et al., 1995].
The sharpness of the spots and the amount of diffuse background are good indi-
cators of the long-range order of the sample surface. By measuring the spot intensity
as a function of the incoming beam voltage (I - V curve) and comparing this with
simulations, models of the surface structure can be tested [Heinz et al., 1992]. Also
from an I-V analysis, the surface segregation profiles have been estimated for a
number of bimetallic alloys, and found to be in good agreement with theoretical
predictions [Gauthier et al., 1985; Vasiliev, 1997].
Overall, LEED in UHV provides the exclusive ability to study structure - function
relationships in heterogeneous catalysis, and for that reason it has become a routine
surface analysis tool.
Nevertheless, in applications relevant for electrocatalysis and reactions that occur
at solid - liquid interfaces, it has been essential to develop a methodology that can
provide detailed insight into the surface and near-surface structure during the course
of reaction. For that purpose, the in situ SXS diffraction technique, depicted in