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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