Paramagnetic silver clusters in Ag-NaA zeolite: electron spin resonance and diffuse reflectance spectroscopic studies

Abstract

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.

Paramagnetic silver species in irradiated Ag−NaA zeolites. An EPR and ESEEM study

“…They recorded an EPR septet with a hyperfine splitting Aiso --72.6 G after hydrogen reduction of dehydrated Ag-A zeolite at 20~ and assigned it to an Ag6 n+ cluster. The same species was reported later in gamma irradiated Ag12-A zeolite [7], Ag-NaA zeolites [8] and in Ag-NaA and Ag-KA zeolite [9] reduced by hydrogen. Studying l~ zeolite Preston and Morton [10] observed the superhyperfine splittings on the outer lines of the septe t. They ascribed this to the interaction of the unpaired electron with eight Ag cations located in different hexagonal windows of the ~-cage thereby definitely proving the formaUon of the 14 atom clusters, Ag6 n+" 8Ag +, proposed earlier by Seff and Kim [11] on the basis of x-ray diffraction studies.…”

“…It was never stabilized in any other matrix. The formation of Ag6 n+ in the course of hydrogen reduction or gamma irradiation [8,13] is not a gradual process showing an increase of the nuclea¡ of the cluster during annealing as was the case for Ag32+ produced in Agl-NaA zeolite, of which the formation is correlated with the decay of its paramagnetic predecessors Ag ~ and Ag2 +.…”

“…The EPR spectra of smaller silver clusters Ag2 + and Ag32+ were recorded in A&2-A [7] and Ag-NaA [8] zeolites gamma irradiated at 77 K and annealed at speci¡ temperatures. The Ag43+ cluster was reported in gamma irradiated Ag-CsA zeolite [12].…”

Silver atoms and clusters in molecular sieves and clays

“…The recent review by Sun and Seff [11] summarizes for the first time both the XRD and ESR results for Ag clusters in A zeolites. It appears clearly from their review that, whereas XRD and ESR data support each other for hexameric silver [12][13][14][15][16], they are totally contradictory for trimeric silver in dehydrated A zeolites [10,17].…”

A Small Paramagnetic Platinum Cluster in an NaY Zeolite: Characterization and Hydrogen Adsorption and Desorption

“…[39][40][41][42] Information about cluster size, structure, charge, and local environment can be obtained. Besides those, exclusive paramagnetism of EPR active metal clusters can be separated from other magnetic behavior based on the study of temperature-dependent EPR spectra.…”