(233c) Model Catalyst Study of Pt Nanoparticles Supported On g-Al2O3 Single Crystal

Model Catalyst Study of Pt Nanoparticles
Supported on g-Al₂O₃
Single Crystal

Zhongfan
Zhang,¹ Long Li,² Dong Su,³ Kim Kisslinger,³
Eric Stach,³ Judith C. Yang²

¹Department of Mechanical Engineering and
Materials Science, University of Pittsburgh, Pittsburgh, PA 15261, USA.

²Department of Chemical and Petroleum
Engineering, Department of Physics, University of Pittsburgh, Pittsburgh, PA
15261, USA.

³Center for
Functional Nanomaterials, Brookhaven National
Laboratory, Upton, NY 11973, USA.

Tremendous progress in
recent years in nanoparticle synthesis, nano-characterization
and materials computation has brought us tantalizingly close to predictive
catalyst design.  However,
computational catalysis assume a specific heterogeneous catalyst system where
the nanoparticle shape, relative orientation to the single crystal support, defects,
and adsorbates are exceptionally well-defined,
whereas technologically-relevant heterogeneous catalyst materials are
polycrystalline and irregular, contain impurities and defects, and operate in a
complex environment may contain multiple gaseous species.  To bridge the gap between theory and
experiment requires the comparison of model catalyst systems to theory.  Recently, investigators have used single crystal oxide
supports to resolve the nanoparticle/support structure
and shape relations that can be directly compared to the theoretical
simulations and thus provide essential
insights into catalysis science [2]. Yet, a model Pt/g-Al₂O₃ system
has not been truly achieved due to challenges in producing single crystal g-Al₂O₃ thin film though Pt/g-Al₂O₃ is
arguably the most important technologically-relevant heterogeneous catalyst due
to its ubiquitous utilization in many critical energy and transportation
industries including oil refining, fuel cells, and catalytic converters. Surface
science studies have reported the formation of an ultra-thin gamma-like alumina
by oxidation of NiAl(110) [1,2], where
other studies revealed this structure to kappa alumina or Al₁₀O₁₃
[3,4].  Yang et. al and Doychak reported the
formation of metastable alumina during oxidation of (100)NiAl
in air in the temperature range of 800-950°C [5¨C7].  Here
we report the preparation of a model Pt/g-Al₂O₃ catalyst via oxidation of
NiAl(110)
followed by deposition of Pt nanoparticles and its
characterization by a cross-sectional high-resolution electron microscopy
(HREM) method.

The nanoparticle/support interactions, particularly the
role of their interface, play a key role in the 3-dimensional (3D) particle
shape, surface morphology and sintering behaviors of catalyst nanoparticles
which determine the heterogeneous catalysts' chemical properties. Experimental
measurements [8,9] and theoretical
simulations [10,11] were initiated to
quantitatively study the interfacial atomic and electronic structure and
adhesion energy between the NPs and their support. Our aim is to create a
single crystal g-Al₂O₃ thin
film in order to create a model system, characterize the structural and
electronic relations between the Pt and g-Al₂O₃ support, and compare our experimental results
with theoretical predictions.

The g-Al₂O₃(111) thin film was prepared via oxidization
of b-NiAl(110) at T=850°æ and dry air
for 1 hour, while the primary
orientation relation (O.R.) was determined to be NiAl(011)[110]||g-Al₂O₃(111)[211] and NiAl(011)[100]||g-Al₂O₃(111)[110] which is the
classical Nishiyama-Wasserman orientation
relationship. Pt nanoparticles were deposited onto g-Al₂O₃(111)
surface by electron-beam evaporation under ultrahigh vacuum. High-angle annular
dark-field (HAADF) imaging and HREM imaging (Figure.
2a and b) shows a significant fraction of the Pt NPs covering
the support with intimate contact to the g-Al₂O₃ support. The shape of the Pt
nanoparticles is a truncated octahedron which correlates with a dewetting shape. The dominant surface facets are the {111} and
{100} low- index planes which are the favorable facets for catalytic reactions [3].
The O.R. of the Pt NPs was found to be Pt(111)[211]||g-Al₂O₃(111)[211]
and Pt(100)[011]||g-Al₂O₃(111)[211],where the interface
planes Pt(110)|| g-Al₂O₃(110) followed the classical
lattice matching epitaxy. Considering
that the g-Al₂O₃ support
has a strong impact on the structural shape and electron density of the
catalytic NPs, as an extension of the Wulff
construction for 3D facets, Kaishew theorem, was used
to analyze the support effects on the Pt particle
shape, where the adhesion energy could be quantitatively obtained to interpret
the dewetting behavior and physical bonding of the Pt NPs.  Electron
energy-loss spectrum (EELS) will performed for insights on the electronic structure
of Pt/g-Al₂O₃ interface that will
affect its catalytic performance. We gratefully acknowledge DOE-BES funding:
DE-FG02-03ER15476 and DE-AC02-98CH10886. 
The structural characterizations were performed at the Center for
Functional Nanomaterials at Brookhaven National
Laboratory and the Nanofabrication and Characterization Facility at the University
of Pittsburgh.

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