Transcript The composition and structure of Pd

The composition and structure of Pd-Au surfaces
Journal of Physical Chemistry B, 2005, 109, 18535-18540
C. W. Yi, K. Luo, T. Wei, and D. W. Goodman
• Bimetallic catalysts
• Pd-Au alloys are used for CO oxidation, synthesis of vinyl acetate
monomer, hydrogenation of hydrocarbon,…
• The activity and selectivity of Pd catalysts can be significantly
enhanced by the addition of Au. Pd and Au are completely miscible
as a solid solution ,so overlayers of Pd on Au(111) and Au on Pd(111)
can be prepared.
• Surface of Pd-Au alloy differs from the corresponding bulk
concentration. The Pd-Au surface has significant enrichment of the
surface in Au compared to the bulk.
• Low energy ion scattering spectroscopy (LEISS) is the preferred
technique to analyze the surface composition as it detects only the
topmost surface layer. LEISS, XPS, IRAS, and TPD are the other
techniques that are used for to investigate the surface structure,
the Pd-Au surface ensembles (isolated Pd surrounded by Au), and
the corresponding variation in chemical properties.
• To facilitate the preparation of well-characterized mixed-metal
compositions, a Mo(110) was used to support multilayers of Au and
Pd deposited by physical vapor deposition.
• This study demonstrates a method for studying the surface versus
bulk composition of mixed-metal phases using multilayer metal
films on a refractory metal substrate.
XPS
• Upon annealing to 800 K, the
Au 4f 7/2 and Pd 3d3/2 peak
position shift to lower binding
energies (alloy formation)
• With a further increase in the
annealing temperature, the
core level of Pd 3d3/2 shifts to
higher binding energy because
of desorption of Au.
ION SCATTERING SPECTROSCOPY
When a beam of ions hits a solid
surface part of the projectiles will
be scattered back into the vacuum
after one or more collisions with
target atoms of the top few layers.
Measurement of the energy of the
backscattered particles can be
used to identify the mass of the
these atoms. The technique is
called Ion Scattering
Spectroscopy (ISS). The term
encompasses actually several
techniques depending on the
energy of the primary ion beam.
LEIS (Low Energy Ion Scattering)
spectroscopy is referred to
primary energies in the range of
100 eV to 10 keV.
LEISS
• Upon annealing to 600 K, the
Pd peak intensity gradually
decreases with a
corresponding increase in the
Au peak intensity.
• Au- Pd interdiffusion is clearly
apparent.
LEISS
• Between 700 and 100 K ,
Au0.8Pd0.2 remains
constant in order to
minimize the surface free
energy as the surface
free energy of Pd is
higher than of Au.
• Production of this stable
alloy is independent of
the metal deposition
sequence