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Paramagnetic Silver Clusters in Molecular Sieves: Zeolite RHO

Jacek Michalik 

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


J. Michalik, J. Sadlo and M. Danilczuk
Institute of Nuclear Chemistry and Technology
Dorodna16, 03-195 Warsaw, POLAND

It is assumed that materials containing metal
nanoparticles will be playing important role in future electronics,
optics and heterogeneous catalysis. Nanoparticle properties depend
strongly on particle size, however usually metal clusters are produced
with broad distribution of nuclearity. Zeolite frameworks offer
excellent conditions for stabilization of nanoparticles with uniform
nuclearity and shape.
Although paramagnetic silver metal clusters have been
generated radiolytically in several types of molecular sieves Ag
agglomerates produced in zeolite rho and sodalites show unique
stability even above room temperature. In AgCs-rho zeolite silver
agglomeration process was followed by EPR in temperature range 77 -
370 K. In dehydrated samples the EPR doublet of silver atoms is
observed at 77 K. The Ag0 atoms decay during thermal annealing is
correlated with the formation of silver dimmers Ag[2]+ (A[iso]=29 mT,
g[iso]=1.983). At higher temperature the Ag[2]+ triplet is replaced
by Ag[3]2+ quartet. Finally, at room temperature the EPR quintet
(A[iso]=13.9 mT, g[iso]=1.970) of silver tetramer Ag[4]3+ is
recorded1. The results prove univocally that in gamma irradiated
AgCs-rho zeolites silver clusters are formed by the reactions between
Ag0 atoms or cationic Ag cluster and Ag+ cations.
Although Ag[4]3+ clusters in AgCs-rho are exceptionally
stable they are accessible for the small molecular adsorbates. When
irradiated AgCs-rho containing Ag[4]3+ clusters is exposed to ammonia
the hyperfine splitting is decreasing from 13.9 mT to 10.7 mT. This
indicates the significant shift of spin density from Ag nuclei to the
ammonia ligands. Additionally, in the presence of ammonia the outer
lines of Ag[4]3+ quintet show secondary structure. It was proved by
experiments with 15NH[3] that this structure is due to the
superhyperfine interaction with nitrogen nuclei. The simulation of the
spectra recorded in the presence of 14NH[3] and 15NH[3] indicated that
Ag[4]3+ coordinates two close and four more distant NH[3] molecules
forming multicore Ag[4]3+(NH[3])[2]'(NH[3])[4]" complex 2.

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Related papers

Presentation: poster at E-MRS Fall Meeting 2002, by Jacek Michalik
See On-line Journal of E-MRS Fall Meeting 2002

Submitted: 2003-02-16 17:33
Revised:   2009-06-08 12:55