Abstract

UV/vis spectra of hydrated and activated Ag⁺_xNa⁺_12-xA, Ag⁺_xCa²⁺_6-0.5xA, and Ag⁺_9.5ZK-4 materials were studied. Any absorption band or color observed in silver zeolite A materials are due to the presence of silver ions. The marked site preference of ions in Ag⁺_xCa²⁺_6-0.5xA was found to offer the unique possibility of investigating different coordination sites of Ag⁺ ions in zeolite A. Observations we made lead to the result that Ag⁺ coordinated to 6- and 8-ring oxygens gives rise to electronic transitions in the near UV region. Only Ag⁺ coordinated to 4-ring oxygens leads to the 22'000 cm^-1 absorption responsible for the typical deep yellow color. The red color which is caused by a strong absorption band at 19'000 cm^-1 is observed if a 4-ring coordinated Ag⁺ has a second Ag⁺ as a neighbour. We assume that the second Ag⁺ is at a 6-ring site. Yellow and red colored materials turn colorless again, when exposed to humidity. The reversibility is complete for the yellow samples while a broad but weak absorption in the near UV remains after rehydration of the red samples. Molecular orbital calculations were carried out on a unit structure cell consisting of 1296 atoms. The occuppied frontier orbital region consists mainly of two bunches of levels: the HOMO region which spreads from about -11 eV to - 12.6 eV and the next lower lying levels below -13.6 eV. The HOMO region consists of oxygen lone-pairs O(n). We find that 6-ring coordinated Ag⁺ gives rise to near UV electronic transitions and 4-ring coordinated Ag⁺ causes a band in the visible. These transitions can be interpreted as charge transfer from zeolite oxygen lone pairs to Ag⁺.

published in: J. Phys. Chem. B 2000, in press

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