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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 8, Number 2
BKCSDE 8(2)
February 20, 1987 

 
Title
Silver Ions in Zeolite A are Reduced by H2 only at High Temperatures when 8-Rings are Blocked by Cs+. Crystal Structures of Dehydrated Ag9Cs3-A Treated with H2 at 23, 310 and 470℃
Author
Yang Kim*, Karl Seff
Keywords
Abstract
The structures of dehydrated Ag9Cs3-A treated with hydrogen gas at three different temperatures have been determined by single-crystal X-ray diffraction techniques. Their structures were solved and refined in the cubic space group Pm3m at 23(1) ℃. All crystals were ion exchanged in flowing streams of aqueous AgNO3/CsNO3 with a mole ratio 1:3.0 to achieve the desired crystal composition. The structures treated with hydrogen at 23℃ (a = 12.288(1) Å) and 310℃ (a = 12.291(2) Å) refined to the final error indices R1 = 0.091 and R2 = 0.079, and 0.065 and 0.073, respectively, using the 216 and 227 reflections, respectively, for which I > 3σ(I). In both of these structures, eight Ag+ ions are found nearly at 6-ring centers, and three Cs+ ions lie at the centers of the 8-rings at sites of D4h symmetry. One Ag°atom, presumably formed from the reduction of a Ag+ ion by an oxide ion of a residual water molecule or of the zeolite framework during the dehydration process, is retained within the zeolite, perhaps in a cluster. In these two structures hydrogen gas could not enter the zeolite to reduce the Ag+ ions because the large Cs+ ions blocked all the 8-windows. However, hydrogen could slowly diffuse into the zeolite and was able to reach and to reduce about half of the Ag+ ions in the structure only at high temperature (470℃). The silver atoms produced migrated out of the zeolite framework, and the protons generated led to substantial crystal damage.
Page
69 - 72
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