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for both Ag+ and Cu2+ ions in zeolites A, X and Y, and has been shown to take place by two mechanism in two clearly defined temperature regions:11,12 (i) autoreduction in the presence of zeolite water (25 - 250 2(Ag+-Z-O-) + H2O ----> 2Ag0 + (1/2)O2 + 2Z-OH C) and stabilized by coordination to six silver ions ((Ag+)6(Ag)6) from structural determinations of a dehydrated silver-exchanged zeolite A.9,10 However, Jacobs et al. suggested that the formation of such large metal clusters is improbable since color changes are seen even at low temperatures and low silver loadings where extensive migration of neutral silver atoms and subsequent sintering into Ag6 metal clusters is highly unlikely.9,10 Alternatively, Jacobs et al. suggested, based on structural studies of Ag-A zeolites, the formation of linear (Ag3)2+ charged clusters (Ag+-Ag0-Ag+) upon thermal dehydration of the zeolite.11 Gellens et al. followed color changes and concomitant silver cluster formation in A, X, and Y zeolites using X-ray diffraction (XRD) techniques.13,14,15 They found that the number of clusters increases with framework Al content. It was also noted that in synthetic analogs of the faujasite zeolite (types X and Y), the dehydrated zeolites displayed a yellow color which increased in intensity with the number of clusters, while silver-exchanged A zeolites took a yellow color with dehydration at low temperatures, eventually becoming brick red after treatment at higher temperatures. Further (1) (2) (ii) autoreduction by oxygen from the zeolite lattice (127 - 380 2(Ag+-Z-O-) ----> 2Ag0 + (1/2)O2 + Z-O- + Z+ Kim and Seff proposed the formation of octahedral hexasilver metal clusters 11 C)PDF Image | PSA USING SUPERIOR ADSORBENTS
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