Silver clusters in zeolites

Because in zeolites the silver clusters change their color when in contact with water, it has been suggested that these compounds may be used as water-vapor-sensing materials. Ozin et al. observed the vapor-pressure chromic, cathode-ray chromic, water chromic, photochromic, and thermochromic properties for silver-sodalite, and they proposed that these compounds hold promise for use as sensors.[30] In addition, the silver clusters in zeolites are also sensitive to other molecules, and therefore, they have great potential as chemical-sensing materials. 

Calculations were performed on a Digital VAXstation 3200. The preparation methods of the silver clusters in zeolite A and sodium clusters in zeolite X are described in reference [8] and [10] respectively, together with the experimental specifications on the scanning of the ESR spectra. 

Table 5. ESR parameters of alkali metal and silver clusters in zeolites...

The use of y-irradiation and controlled thermal annealing to produce silver clusters in zeolites and related materials was reviewed by Michalik [114]. Unusual stability was attributed to Ag + clusters formed in this way in dehydrated zeolite rho, which were found to persist for months at room temperature and could be observed up to 100°C. In the zeoHte A analogue, AgH-SAPO-42 (LTA), dehydrated at 300 °C in flowing oxygen, clusters such as AgJ and Agf" were detected in very low yield, but, in the presence of methanol, Agl" species were... 

Ozin GA, Flugues F, Mattar SM, McIntosh DF (1983) Low nuclearity silver clusters in faujasite-type zeolites optical spectroscopy, photochemistry and relationship to the photodimerization of alkanes. J Phys Chem 87 3445-3450... 

In an attempt to synthesize and capture silver clusters in this way, empty Ag, K-A was exposed to Cs(g) at 250 °C. The structure of the product showed that the potassium ions had all been reduced and were no longer present in the zeolite. The silver ions also had all been reduced to atoms, but most of these were found as hexasilver molecules at the very centers of the large cavities. Each hexasilver molecule was surroxmded by fourteen Cs" " cations. When this reaction was done with Ag, Ca-A and Rb(g) instead, hexasilver molecules each associated with thirteen Rb" " ions were found at the same position. The product calcium atoms had left the structure. 

D. Bruhwiler, R. Seifert, and G. Calzaferri, Quantum-sized Silver Sulfide Clusters in Zeolite A. J. Phys. Chem. B 1999, 103, 6397-6399. 

An iterative computer program for the optimization of ESR powder spectra is described. Its applicability to metal clusters is demonstrated. Spectra of silver and sodium clusters in zeolites are generally recognized to be composed of two isotropic signals metallic and ionic. An accurate iterative refinement requires introduction of a third underlying signal and dependence of the linewidth on the quantum number M,. 

Intermediates in high-temperature processes have been stabilized at low temperature after y irradiation of metal oxides and zeolites. Important early examples were oxygen anions. O, 02 and O. Some of their reactions with small molecules were also elucidated by EPR. Metal cluster ions have also been produced by radiolysis and stabilized in zeolites. Examples include alkali metal cation clusters in faujasites and silver cation clusters in zeolite A and in silicoaluminophosphate molecular sieves. Detailed information was obtained from EPR studies about their structure, thermal stability and formation of adducts. 

Metal clusters in zeolites an intriguing class of catalysts Zeolite-supported transition metal catalysts Stoichiometric and catalytic reactivity of organometallic fragments supported on inorganic oxides Silver clusters and chemistry in zeolites Structure and reactivity of surface species obtained by interaction of organometallic compounds with oxidic surfaces infra-red studies... 

Silver ions and quantum-sized silver sulfide clusters in zeolite A... 

A natural development of the ideas outlined above is to regard the dehydrated zeolite as a polar solid solvent and reactions (1) and (2) as dissolution processes. When Barrer and Whiteman [4] studied the reaction of mercury metal with a number of different ion-exchanged zeolites, they found that mercury uptake was limited in sodium-, calcium- and lead-exchanged forms and copious in silver- and mercury-exchanged forms, where reduction of the exchangeable cations Ag+ and Hg + (to Ag° or Hgl" ) by Hg° would be possible. The implication of these observations is that the ionization process outlined in reaction (1) is necessary for the reaction of elemental metals with zeolites - one of the main synthetic routes to ionic clusters in zeolites - to occur. Reactions (1) and (2), in fact, can usefully be regarded as models for the formation of ionic clusters in zeolites, and detailed calculations of solvation energies [1] can help... 

Ag-NaA zeolites Silver clusters Silver clusters, formed in X-ray irradiated samples at 77 K and annealed at 280 K, were characterized by electron spin resonance spectroscopy 556... 

The second example is the analysis of silver zeolite [7], in which it was shown that there is a substantial silver atom flux accompanying the relatively weak silver cation flux. There were no molecules or clusters containing silver in the gas phase. Pure silver metal heated to the sublimation point gives primarily neutral atoms and metallic clusters with no ions. Thus, sublimation of atomic silver ions from zeolite may be a pseudo-S-L type of process, although additional evidence concerning the species of silver in the solid state would be required prior to making this assertion. 

PL experiments characterizing Ag+-exchanged ZSM-5 zeolite (Ag(I)/ZSM-5) show that Ag(I) clusters (Agm +) are present in the cavities of ZSM-5, as shown by a band oc at 380 nm (species A) upon excitation at 332 nm (Ju et al., 2003 Kanan et al., 2000 Matsuoka and Anpo, 2001 Matsuoka et al., 2003). UV-irradiation at 285 nm of Ag(I)/ZSM-5 leads to the transformation of Agm + clusters into reduced silver clusters (Ag, (" l)l) which include a band [i at 465 nm (species B) upon excitation at 315 nm. Species B is stable at 77 K, except when the system is further UV-irradiated at 285 nm. However, once Ag+/ZSM-5 is heated to an ambient temperature, band ji disappears and whereas band a is again observed with its initial intensity, that is, that observed before UV-irradiation. Moreover, these UV-induced changes of Ag+/ZSM-5 are completely reversible under vacuum. 

In the past five years, the field of metal-zeolite chemistry has developed to the point where a number of metal atomic and small cluster guests have been synthesized and unequivocally characterized by spectroscopic and crystallographic techniques. The silver-zeolite A, X and Y systems in particular have yielded a wealth of valuable structural and site information pertaining to silver guests in the size range of one to six atoms (3). The spectroscopy of these silver-zeolites can therefore be probed in detail and compared with the corresponding data for silver atoms and silver clusters immobilized in weakly interacting rare gas solid supports (4). 

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