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Here we report the preparation of a model Pt/γ-Al2O3 catalyst and its characterization by a cross-sectional high-resolution electron microscopy (XHREM) method. Pt/γ-Al2O3 is the most important technologically-relevant heterogeneous catalyst in the fuel cell, oil refining and chemical industries. 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 (NPs) which determine the heterogeneous catalysts’ chemical properties. Experimental measurements [1] and theoretical simulations [2] were initiated to quantitatively study the interfacial atomic and electronic structure and adhesion energy between the NPs and their support. Previous 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 [2]. However, commercial γ-Al2O3 is polycrystalline and irregular in shape; hence, our aim is to create a single crystal γ-Al2O3 thin film in order to build up a model system to solve the structural and electronic relations between the Pt and γ-Al2O3 support, and compare our experimental results with theoretical predictions.

Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011.

Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010.

Extended abstract of a paper presented at Microscopy and Microanalysis 2009 in Richmond, Virginia, USA, July 26 – July 30, 2009

Extended abstract of a paper presented at Microscopy and Microanalysis 2009 in Richmond, Virginia, USA, July 26 – July 30, 2009

Platinum nanoparticles (NPs) on γ-Al2O3 support is a popular active catalytic system. Recently, in situ X-ray Absorption Spectroscopy (XAS) measurements of this system showed negative thermal expansion, where the nearest neighbor Pt-Pt bond distances decreased with increasing temperature. The unusual phenomena must involve the charge transfer interactions between the Pt particle and the γ-Al2O3 support. Preliminary theoretical simulations revealed that the energetically favorable Pt nanoparticle structures, including their bond-lengths, depend quite sensitively on orientation, surface structure and defects on the γ-Al2O3 surface. For example, O vacancies tend to pin the Pt nanoparticle and significantly change their structure, including a decrease of their bond-lengths with increasing temperatures. Therefore detailed knowledge of the atomic structure of both Pt particles and γ-Al2O3, as well as their structural correlations, is needed. Although high-angle annular dark-field (HAADF) technique is widely used in imaging heterogeneous catalytic materials because of the high contrast of the heavy metal relative to the low-Z support, the information from the support is limited, see Fig. 1. Phase contrast given by highresolution transmission electron microscopy (HRTEM) provides sub-nano information of both the Pt and γ-Al2O3 simultaneously. The samples were prepared by impregnating the Pt2+ precursor, Pt(NH3)4(OH)2·H2O, on γ-Al2O3, reducing in H2 gas at 573 K in order to remove the ligands to form metallic nanoparticles. The sizes of Pt particle were controlled by the loading amount, where 1 wt% produced an average Pt sizes of ∼1nm and heavy loading of 5 wt% produced sizes of ∼2.9 nm.

This project involves the characterization of metal nanoparticles with the goal of achieving a fundamental understanding of their structural habits and the corresponding energetic landscapes. To achieve this goal, we are pursuing a combined experimental effort, utilizing advanced electron microscopy methods and X-ray absorption spectroscopy, with theoretical simulations. The systems studied include ligand-protected metal nanoparticles. Specifically, small, monodisperse Au nanoclusters, e.g. Au13, have been prepared via a ligand exchange reaction of alkanethiols onto phosphine-halide gold clusters. In one synthetic protocol, two fractions separated with column chromatography exhibit very different UV-visible spectroscopy (Figure 1).

The success of future applications of metal nano-clusters in heterogeneous catalysis depends on the full understanding of their supported three-dimensional structure [1]. Supported gold nano-cluster catalysts exhibit catalytic activities that are radically different from bulk gold [2]. The detailed atomistic structural information on these small molecular gold clusters would be particularly important, but is practically missing. By performing experimental extended x-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) studies on Au13 nanoclusters [3], A. Frenkel, the third author of this paper, has gained structural results of the approximate size, the near neighbor Au-Au and Au-S distances, and the disorders in these distances. In this work, we present the applications of quantitative Z-contrast imaging and high-resolution electron microscopy (HREM) techniques in confirmation of aforementioned EXAFS results as well as further insights into structural habits and the dynamics of carbon-supported Au13 nano-clusters.

HUIPING XU*, RAY D. TWESTEN**, LAURENT MENARD***, ANATOLY FRENKEL****, RALPH NUZZO***, DUANE JOHNSON*****, JUDITH C. YANG * * Department of Materials Science and Engineering, University of Pittsburgh, Pittsburgh, PA 15261. ** Center for Microanalysis of Materials, University of Illinois at Urbana-Champaign, Urbana, IL 61801. *** Department of Chemistry, University of Illinois at Urbana-Champaign, Urbana, IL 61801. **** Department of Physics, Yeshiva University, New York, NY 10016. ***** Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801 Significant improvement on activity/selectivity of catalysts would be achieved, when their scale is reduced to small size, specifically, the subnanometer regime (