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Paramagnetic silver clusters generated in activated Ag-NaA zeolite by y-irradiation at 77 K or by H2 reduction at 295 K have been studied by electron spin resonance (ESR) and diffuse reflectance spectroscopy. Depending on the silver content in the zeolite, neutral Ag30 or monovalent Ag6+ clusters were identified by ESR. They are formed during radiolysis by electron capture from diamagnetic Ag3+ or A g l + species, respectively. In Ag,-NaA zeolite the transformation of Ag30 to Ag6+ clusters has been followed by ESR and related to the reaction between Ag3+ and Ag30 in the same sodalite cage of the zeolite structure. Diffuse reflectance spectra of the same zeolite sample show some correlation between the optical bands and paramagnetic silver clusters identified by ESR. The mechanism of silver clustering in the sodalite cage of Ag-NaA zeolite based on the present ESR and optical data and earlier X-ray diffraction and far infrared studies is discussed.Small metal clusters of platinum group metals or copper, silver, and nickel supported on various solids appear to have significant potential in heterogeneous catalysis.' Small silver clusters in zeolites have been extensively studied in recent years with various experimental techniques such as X-ray electron spin resonance (ESR),+' and opticalIn activated silver A zeolite depending on the activation temperature and initial silver content the formation of trimeric and hexameric silver species has been proposed on the basis of X-ray diffraction data.4 The presence of Ag+-Ag-Ag+ linear clusters has been associated with a yellow color of the zeolite and an absorption band a t 445 nm. A band at 510 nm has been assigned to two Ag32+ clusters interacting in the sodalite cage of the zeolite structure.Applying Fourier transform far infrared spectroscopy to AgNaA zeolite Ozin et al.'* studied the reduction of Ag?+ species by H2 in the temperature range 293 to 413 K. They postulated a reversible redox processoccurring in the presence of H2 or 02. One limitation of these interpretations is related to the fact that no ESR spectra were observed for activated Ag-NaA samples although paramagnetic Ag3Z+ species were postulated. In a more recent paper Ozin et al.' proposed the interesting transformation of a trimeric cluster (AgSx+) to a hexamer pseudocluster (Ag3X+), without specifying specific numbers for the charges of the postulated silver species.The objective of this work was to use y-irradiation at 77 K to render the silver clusters, present in activated Ag-NaA zeolite, paramagnetic and identifiable by ESR. The ESR spectra of silver zeolites activated under various conditions as well as those subsequently exposed to H2 at room temperature were also studied.On leave from the Institute of Nuclear Chemistry and Technology, Department of Radiation Chemistry and Technology, 03-195 Warsaw, Poland.

Electron spin resonance studies on Ni+ in NiCa-X zeolite indicate the formation of two new Ni+ complexes, Ni+(H2) and Ni+(H2),, with n = 2 or 3 located in the a-cages in addition to a previously observed Ni+(H2) complex in the 8-cage. Both of the new cy-cage complexes are stable only in the presence of hydrogen and disappear after short outgassing at room temperature but are immediately restored after readsorption of hydrogen. Thus, the total number of Ni' spins is reversible with the desorption/readsorption of hydrogen. The formation of Ni+ ion pairs has been suggested to explain this behavior. After hydrogen desorption various molecules have been adsorbed on Ni+Ca-X zeolite. For Ni+ complexes with water and methanol a distorted octahedral symmetry is suggested which involves three lattice oxygens and three adsorbate molecules. The Ni+-ammonia complex has planar symmetry with two lattice oxygens and two adsorbate molecules. After adsorption of ethylene and propylene "reversed" g values in the ESR spectra (gl > g,,) are seen due most likely to compressed tetrahedral or compressed square-pyramidal geometry.

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