A More Realistic View of Gold Based Catalysts Using Aberration Corrected Analytical Electron Microscopy

2009 Spring Symposium

 
Dr Christopher J. Kiely
Center for Advanced Materials and Nanotechnology
Lehigh University
Bethlehem, PA


Abstract – Supported gold clusters and gold-palladium nanoparticles are intensely studied materials primarily because of their exciting potential applications in catalysis. The recent availability of aberration corrected analytical electron microscopes is revolutionizing our ability to characterize the morphology, crystallography and chemical composition of such nanoscopic volumes of materials and for the first time are giving us more realistic views of these catalyst systems. To illustrate the superior imaging performance of this new generation of electron microscopes, we will present a high angle annular dark field (HAADF) imaging study of a systematic set of gold on iron oxide CO oxidation catalysts, ranging from those with little or no activity, to others with very high activities. Using this approach, combined with XPS analysis, we will unambiguously demonstrate that the high catalytic activity for CO oxidation derives from the presence of bi-layer clusters which are ~0.5 nm in diameter. We will also demonstrate that core-shell structures in sub-5nm Au+Pd, Pd@Au and Au@Pd bimetallic nanoparticles can be directly visualized using the z-contrast sensitivity of the HAADF imaging technique. To illustrate the chemical analysis capabilities of aberration corrected analytical microscopes, we will describe the potential advantages of combining X-ray Energy Dispersive Spectroscopy (XEDS) spectrum imaging with multivariate statistical analysis (MSA) techniques. Through several case studies of the Au-Pd bimetallic catalyst systems, we will demonstrate that STEM-XEDS can provide invaluable high spatial resolution compositional information on (i) alloy homogeneity and phase segregation effects within individual nanoparticles, (ii) particle size – alloy composition correlations, and (iii) alloy composition changes that can occur as these catalysts are used.

Speaker’s Biography – Chris Kiely obtained his BSc in Chemical Physics (1983) and PhD in Microstructural Physics (1986) from Bristol University. From 1986-89 he was a visiting postdoctoral research associate in the Materials Research Laboratory at the University of Illinois at Urbana-Champaign. He joined the Materials Science and Engineering Department at Liverpool University as a Lecturer in 1989, where he worked his way through the ranks until eventually being awarded a Personal Chair in Materials Chemistry in 1999. Kiely joined Lehigh University (Pennsylvania, USA) as Professor of Materials Science and Engineering in 2002. He is currently the Director of the Center of the Nanocharacterization Laboratory at Lehigh University, which houses an array of twelve electron microscopes, including two aberration corrected instruments. He also serves as the Director of the Lehigh Microscopy Schools. His research expertise lies in the application and development of transmission electron microscopy techniques for the study of nanoscale features in materials. His areas of interest include catalyst materials, nanoparticle self-assembly, carbonaceous materials, and heteroepitaxial interface structures. He is also involved in microscopy technique development, and his current interests include X-Ray Ultramicroscopy (XuM) and aberration corrected Analytical Electron Microscopy (AEM).