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Silver Ions in Zeolite A are Reduced by H$_2$ only at High Temperatures when 8-Rings are Blocked by Cs$^+$. Crystal Structures of Dehydrated $Ag_9Cs_3$-A Treated with H$_2$ at 23, 310, and 470${^{\circ}C}$

  • KIm, Yang (Chemistry Department, Pusan National University) ;
  • Seff, Karl (Chemistry Department, University of Hawaii)
  • Published : 1987.04.20

Abstract

The structures of dehydrated $Ag_9Cs_3$-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) $^{\circ}C$. All crystals were ion exchanged in flowing streams of aqueous $AgNO_3$/$CsNO_3$ with a mole ratio 1:3.0 to achieve the desired crystal composition. The structures treated with hydrogen at $23^{\circ}C(a=12.288(1)\;{\AA})\;and\;310^{\circ}C(a=12.291(2)\;{\AA})$ 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 >3${\sigma}$(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 $D_{4h}$ symmetry. One $Ag^{\circ}atom$, 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 ($470^{\circ}C$). The silver atoms produced migrated out of the zeolite framework, and the protons generated led to substantial crystal damage.

Keywords

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  1. Zeolite-modified electrodes: Analytical applications and prospects vol.8, pp.11, 1987, https://doi.org/10.1002/elan.1140081102
  2. Framework-Type Determination for Zeolite Structures in the Inorganic Crystal Structure Database vol.39, pp.3, 1987, https://doi.org/10.1063/1.3432459