Silver Clusters in Molecular Sieves

Jarosław Sadło ,  Jacek Michalik ,  Marek Danilczuk ,  Janusz Turek 

Institute of Nuclear Chemistry and Technology (IChTJ), Dorodna 16, Warszawa 03-195, Poland

Abstract

Molecular sieves have unique abilities to stabilize small metal clusters which are the active sites for many catalytic processes. The most important question concerning metal clusters is their size and structure. For characterization of small paramagnetic clusters of magnetic nuclei electron paramagnetic resonance (EPR) spectroscopy is a method of choice.
For the last few years we had been characterizing by EPR cationic silver clusters in different zeolites reduced by gamma irradiation. Depending on Ag+ loadings we identified Ag32+ trimers and Ag6n+ hexamers in dehydrated AgNa-4A zeolites and Ag8n+ in AgNa-sodalites. In AgCs-rho zeolite at low temperature Ag0 atoms and Ag2+ dimers are observed. After annealing to RT Ag32+ trimers and Ag43+ tetramers are stabilized. When irradiated AgCs-rho containing Ag43+ clusters is exposed to ammonia the hyperfine splitting decreases due to shift of spin density from Ag nuclei to the ammonia ligands.
The EPR study on silver zeolites clearly showed two different mechanisms of silver agglomeration. In zeolite rho agglomeration process initiated by Ag0 formation involves the reaction of silver atoms, dimers and trimers with silver cations to form Ag43+. In zeolite A and sodalite pre-existing assembly of Ag+ cations traps electron during radiolysis forming Ag65+ or Ag87+ clusters, respectively.
In mesoporous molecular sieves like MCM-41 silver nanoparticles are formed inside channels showing characteristic singlet of conduction electron spin resonance (CESR). It was proved experimentally that the CESR linewidth depends on cluster nuclearity.

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Presentation: Poster at E-MRS Fall Meeting 2006, Symposium B, by Jarosław Sadło
See On-line Journal of E-MRS Fall Meeting 2006

Submitted: 2006-05-19 12:47
Revised:   2009-06-07 00:44
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