281385 Model Catalyst Study of Pt Nanoparticles Supported On g-Al2O3 Single Crystal

Tuesday, October 30, 2012: 9:30 AM
318 (Convention Center )
Zhongfan Zhang1, Long Li2, Dong Su3, Kim Kisslinger3, Eric A. Stach4 and Judith C. Yang5, (1)Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, PA, (2)Department of Chemical and Petroleum Engineering, Uinversity of Pittsburgh, Pittsburgh, PA, (3)Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, NY, (4)School of Materials Engineering and Birck Nanotechnology Center, Purdue University, West Lafayette, IN, (5)Chemical & Petroleum Engineering, University of Pittsburgh, Pittsburgh, PA

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 [5ĘC7].  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░Š 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.

References:

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