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Seminars
XAFS Spectroscopy in Catalysis Research: Application to Supported Au Catalysts
Jeffery Miller
BP
When: Thursday, September 1, 2005
Time: 2:30 PM to 3:30 PM
Where: 1064 Duncan Hall
Abstract:
In the last ten years, a number of new synchrotrons have been built throughout the world. With improved capabilities and access to more users, it is possible to solve problems only imagined just a few years ago. This is especially true for characterization of heterogeneous catalysts. While the capability and availability of synchrotrons have increased, many scientists have little experience with XAFS spectroscopy. Most often, EXAFS has been used for structural determination although information about the electronic properties of metal particles is also available from the near edge spectra. This seminar will give a few examples for the application of XAFS to understand the step of Au catalyst preparation, structural changes in bond distance in small particles, evidence for changes in the electronic properties in small nanoparticles and the poisoning of metallic Au by chloride ions.
Catalyst Preparation: Hydrogen tetrachloroaurate (III), HAuCl4 is the most frequently used compound for the preparation of supported gold catalysts. Deposition on supports, however, is only effective when the preparation solution is maintained at a pH of about 7. At these conditions, extensive hydrolysis of HAuCl4 is observed. In addition, upon adsorption of HAuCl4 solutions onto supports there are additional changes in the ligands bonded to Au+3. Pre-treatment of the adsorbed Au in air or hydrogen lead to additional changes also determined by XAFS.
Effect of Au particle size: Au catalysts with metallic particles less than about 30 Å display enhanced catalytic activity. For particles in this size range, as the size of the particles decrease, there is a large decrease in the Au-Au bond distance up to about 0.15 Å. In addition, there are systematic changes in the intensity of the XANES spectra suggesting that small particles are electron deficient compared to Au foil. These small Au particles also react with air at RT and above leading to oxidation of about 10% of the metallic atoms to Au+3. These results suggest that catalytic activity is due to the changes in the electronic properties in small Au particles.
Catalyst Poisons: Low levels of Cl remaining on the catalyst from the preparation lead to poorly active catalysts. Cl ions affect metallic Au particles in two ways. At higher temperature, Cl accelerates Au sintering leading to large inactive metallic particles. At low temperature, about 10% of small metallic particles oxidize to Au+3. Unlike Cl-free catalysts, which form Au-O bonds, in Cl-containing catalysts Au-Cl is formed with no Au-O. The Au-Cl bonds are inactive for oxidation.
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