The Reactivity and Structural Dynamics of Supported Metal Nanoclusters Using Electron Microscopy, in situ X-Ray Spectroscopy, Electronic Structure Theories, and Molecular Dynamics Simulations [report]

Judith C. Yang, Duane Johnson, Anatoly Frenkel Ralph G. Nuzzo
2008 unpublished
Objectives Heterogeneous catalysis is surface chemistry over a supported metallic nanoparticle, where the critical first step in understanding structure-reactivity relationships is the accurate 3-D atomic arrangement of metal atoms on the support. Despite the relatively large number of available techniques to characterize nanoparticles, most of them obtain the overall, volume-average properties of nanoparticles, and give little, if any, insight into sometimes very elaborate actual arrangement
more » ... atoms within the particle. Our focus is the synthesis of stable nanoparticles (Nuzzo) on a variety of supports, and determination of the structure of metal clusters by utilizing existing techniques and developing new methods of synchrotron X-ray absorption spectroscopy (Frenkel) and transmission electron microscopy (Yang) with the validation/prediction of these shapes by theoretical simulations (Johnson). Technical Barriers Our coordinated experimental and theoretical program on nano-structural dynamics utilizing state-of-the-art synthesis, specialized nanoparticle characterization methods, and first-principles theory will provide a full 3-dimensional model of the structural habits at a non-precedented accuracy, both individually and as an ensemble, and the elucidation of the energy landscapes that lead to these structural habits. The development of in situ methods with these advanced methodologies will provide critical insights into the fundamental catalytic mechanisms. Abstract Our accomplishments to date are that ultra-small, monodisperse Au 13 clusters have been synthesized via a ligand-exchange and deposited on different supports, including C and TiO 2 , and the beneficial effect of ozone, and limited beneficial effects of atomic oxygen, post treatments to reduce sintering has been determined. Both XAFS and TEM revealed the structures of these Au nanoclusters, e.g icosahedral Au 13 clusters for ligand-protected Au on C, and XAFS determined the charge transfer between the Au atom and ligand. The theoretical effort focused on Pt-Ru systems because of the large amount of previous experimental data on this system. Theoretical analysis demonstrated that the experimentally observed bond disorders are due to changes in bond-lengths within a particle because of the metal-substrate interactions. Initial efforts into in situ experiments are being carried out. Our results demonstrate the strengths and necessity of a multi disciplinary approach for accurate determination of supported nanostructures utilized in heterogeneous catalysis. Progress Report Monodisperse sub-nanometer Au 13 particles provided a model system suitable for demonstrating the wealth of structural information obtainable using X-ray absorption spectroscopy (XAS) and transmission electron microscopy (TEM). Recent synthetic efforts have been focused on preparing supported Au nanoparticles using the ligand-protected Au 13 clusters as precursors. In order to remove the ligands from the deposited clusters, we compared annealing treatments versus reactive oxygen flow. The supports used in these studies included carbon black, g-alumina, titania (anatase form), and silica (mesoporous SBA-15). The structures of both mixed-ligand and fully-thiolated nanoparticles were investigated using X-ray absorption fine structure (XAFS), scanning TEM (STEM), and high resolution TEM (HREM). Full multiple-scattering EXAFS study of mixed ligand and fully-thiolated Au nanoclusters was performed. Visual observation of the EXAFS data reveals difference between the local order in the mixed ligand clusters and the fully thiolated nanocluster.
doi:10.2172/933137 fatcat:5b5okgi5kvc4nfi4gnlrvqlaz4