Silver species zeolites

Silver species have been studied in a variety of A-zeolites including Nai2-A, K -A, Li -A, CsyNas-A and Cag-A (11). Complete exchange of cesium ion for sodium ion cannot be achieved in the A-zeolites. Typically the major cation was exchanged by silver to an extent of about 0.7 ion per unit cell which is 6% of the exchangeable cations. After irradiation about 0.003 silver ions per unit cell were converted to atomic silver species. 

Irradiation at 4 K gives a silver species designated as Ag°(B) in all of the hydrated A-zeolites studied. This particular silver species is characterized by a silver isotropic hyperfine coupling of about 1480 MHz and a g-factor of 1.991. For comparison the isotropic coupling constant of the silver atom in the gas phase is 1979 MHz, so species Ag°(B) shows considerable interaction with its environment. If the A-zeolites are dehydrated by heating to 400° C no silver species are formed by irradiation. Presumably the silver atoms that are formed are more mobile in the absence of water and form dimers or multimers. In addition, more prominent spectra of Ag2+ are seen upon irradiation of the dehydrated A-zeolites. Thus it seems that species Ag°(B) is probably associated with some adjacent water molecules. 

N. Narayana and, L. Kevan, Characterization of Silver Species Locations and Environments in A, X, and Y Zeolites by Electron Spin Echo Spectroscopy. J. Chem. Phys., 1982, 76, 3999 1005. 

AgNaZSM-5 catalysts were investigated for the selectively catalytic reduction of NO by methane in the excess of oxygen. It was clearly depicted that the conversion rate of NO to N2 had a linear dependence on the silver loading (4.32 13.64%), which indicated that all silver species in the zeolite were active for the CH4-SCR reaction. The presence of excessive oxygen in the feed gas favored the CH4-SCR reaction. The temperature programmed desorption profiles in He and 2%CH4/He after the co-adsorption of NO and O2 revealed that surface nitrates were formed on silver catalyst, and could be effectively reduced by methane... 

In contrast, recent work (4-12) has shown that Raman spectroscopy can be used to study Ti) adsorption on oxides, oxide supported metals and on bulk metals [including an unusual effect sometimes termed "enhanced Raman scattering" wherein signals of the order of 10 - 106 more intense than anticipated have been reported for certain molecules adsorbed on silver], (ii) catalytic processes on zeolites, and (iii) the surface properties of supported molybdenum oxide desulfurization catalysts. Further, the technique is unique in its ability to obtain vibrational data for adsorbed species at the water-solid interface. It is to these topics that we will turn our attention. We will mainly confine our discussion to work since 1977 (including unpublished work from our laboratory) because two early reviews (13,14) have covered work before 1974 and two short recent reviews have discussed work up to 1977 (15,16). 

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. 

Additional evidence, useful in supporting the concept that silver zeolite and silver molten glass ion emitters are S-L processes, would be that both ions and neutrals arise from the same species in the solid state this issue is addressed in the following section, on high-temperature SIMS. 

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. 

Irradiation was therefore carried out on silver doped A-zeo-lites at 77 K and a new silver atom species was detected with a different hyperfine splitting than that of the one observed at 4K. This species is designated as Ag°(A) with an isotropic hyperfine splitting of about 1985 MHz which is very close to that of the free atom value. Species Ag°(A) is the dominant species formed by irradiation at 77 K in all of the A-zeolites studied. In addition, species Ag°(B) is also visible at 77 K. However, upon warming above 77 K species Ag°(B) decays to apparently yield Ag°(A). 

It the spent fuel is processed in a nuclear fuel reprocessing plant, the radioactive iodine species (elemental iodine and methyl iodide) trapped in the spent fuel elements are ultimately released into dissolver off gases. The radioactive iodine may then be captured by chemisorption on molecular sieve zeolites containing silver (89). 

The adsorption properties of silver- and cupper-loaded zeolites for C2 and C3 hydrocarbons were investigated to explore excellent materials for cold-start hydrocarbon trap. The adsorption property and the stability of the adsorbents depended significantly on the metal species and host zeolites. It has turned out that silver-loaded ferrierite zeolite is the promising material with excellent olefin selectivity, high adsorption capacity, desirable storage ability and hydrothermal stability. 

Irradiation by ionizing radiation able to penetrate into a zeolite material exchanged by metal ions allows to generate metal atoms and clusters in situ in the cavities.The observation of irradiated faujasite (Na-Y zeolite) by optical absorption spectroscopy at increasing doses and at low silver content demonstrates the formation of two bands at 265 and 305 nm which have been assigned to the charged trimer Agj ". Then, the ESR observation after a dehydration step indicates the reduction of this species into Agj . Irradiation of HjPtClg solution included in mesoporous channels of molecular sieves induces nanowires of platinum. ... 

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... 

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