(434f) Alloy Catalysis Across Composition Space

Alloys
are known to possess superior catalytic properties than their pure components. Finding
the rational design of new alloy catalysts with optimal catalytic properties for
a given application is the major challenge in multicomponent catalyst design
due to the need to perform many catalyst preparation, characterization and
reactivity measurements across composition space. To accelerate this search Composition
Spread Alloy Films (CSAFs), thin multicomponent films that have composition
gradients parallel to their surfaces, A_xB_yC_1-x-y
with x = 0 → 1 and y = 0 → 1-x, are prepared to be able to span the
entire composition space. Many otherwise intractable fundamental scientific
problems in alloy science and catalysis that can be effectively addressed
through use of CSAFs as high throughput materials libraries.

Study
of alloy catalysis across composition space using a CSAF requires a
multichannel reactor system that can be used to run steady state catalytic
reactions at many different positions or alloy compositions on the CSAF. We
have developed a 100 channel microreactor array that can sample product
distributions from 100 different alloy catalysts of about 10 minutes. Cu_xAu_yPd_1-x-y
CSAFs spanning all of binary and ternary composition space have been prepared
using a rotating shadow mask CSAF deposition tool which is designed and developed
in our group. CSAF surface composition and valence electron energy
distributions are measured using X-ray Photoemission Spectroscopy (XPS).

The
relationship between alloy catalyst activity and electronic structure has been
investigated experimentally across a broad, continuous span of Cu_xAu_yPd_1-x-y
compositionspace. The CSAF was used as a catalyst library
with a multichannel microreactor to measure H₂-D₂
exchange kinetics at 100 discrete compositions on the CSAFs over a temperature
range of 333 – 597 K at atmospheric pressure. H₂ conversion was
chosen to be the indicator of activity. It was found that H₂-D₂
exchange activity varies across the CSAF and it tends to increase with
increasing Pd content. When the activities on AuPd and CuPd binary regions are
compared, it was found that more Pd is needed in CuPd than in AuPd to achieve
the same activity.  

A
microkinetic model that has been validated using a number of single component
Cu-Pd catalysts in a fixed bed reactor was used to estimate the energy barriers
to dissociative adsorption () and associative
desorption () of H₂
as functions of alloy composition, x and y. On the Cu_xAu_yPd_1-x-y
CSAF, increasing Pd content from 0 to 1 was found to decrease adsorption
barrier from 0.44 to 0.12 eV. Increasing Pd content from 0.25 to 1 was found to
increase desorption barrier from 0.4 to 0.74 eV which suggests H₂-D₂
exchange reaction is limited by H₂ desorption step within this Pd
content.

Spatially
resolved X-ray photoelectron spectra were obtained from the CSAFs and used to
estimate the energy of the valence-band center as a function of alloy
composition. The v-band center shifted monotonically from -3.4 to 5.6 eV across
the Cu_xAu_yPd_1-x-y CSAF. The barrier to
dissociative adsorption of H₂ was found to decrease as the v-band
energy increases. This data provides the first experimental correlation of
elementary reaction barriers with valence band energy across a continuous span
of alloy composition space.