Berlin 2008 – wissenschaftliches Programm
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O: Fachverband Oberflächenphysik
O 18: Poster Session I - MA 141/144 (Atomic Wires; Size-Selected Clusters; Nanostructures; Metal Substrates: Clean Surfaces+Adsorption of Organic / Bio Molecules+Solid-Liquid Interfaces+Adsorption of O and/or H; Surface or Interface Magnetism; Oxides and Insulators: Clean Surfaces)
O 18.12: Poster
Montag, 25. Februar 2008, 18:30–19:30, Poster F
Reaction Properties of Size-selected Silver Clusters Supported on LTA Zeolite — •Amgalanbaatar Baldansuren and Emil Roduner — Institute of Physical Chemistry, University of Stuttgart, Stuttgart, Germany
Size-selected clusters are important to fundamental investigations. Silver containing catalysts were prepared by aqueous ion exchange of Ag+ against Na+ cations in an LTA zeolite. A recent project deals with continuous wave and pulse EPR investigations into adsorption interactions of ethylene, oxygen, and nitrogen monoxide with supported silver clusters. EPR is quite straightforward to analyze structures and electronic properties of paramagnetic clusters. The present work is restricted to monitoring the activity of the Ag6+ cluster; however, depending on the metal loading, Ag0, Ag30 and Ag4+ clusters have been formed in the LTA pores as well. The aim of the project is to discern size-dependent effects by directly looking at the centre of potentially catalytic activity.
A well-defined cluster composed of 6 equivalent silver nuclei was obtained after oxidation and hydrogen reduction. As the unpaired spin density distribution is strongly affected by reactions with gas molecules, the symmetric structure of Ag6+ turns into less-compact or smaller clusters. After ethylene adsorption a weak proton hyperfine coupling was resolved by ENDOR. HYSCORE experiments determined that the unpaired spin density is mostly on the adsorbed ethylene and no longer on the cluster. Products of the catalytic conversion, e.g. the NO2 formed on the cluster surface after NO adsorption, exhibits molecular motion at temperatures between 30 K and 125 K.