Abstract
The relationship between the zeolite framework and the coordination number and electron-transfer properties of Eu 2+ and Eu 3+ have been investigated by time-resolved luminescence (TRL) EXAFS, EPR and Mössbauer spectroscopy. TRL-determined lifetimes are sensitive to non-framework oxygen atoms and can be used to help distinguish between coordination involving OH groups and the framework oxygen atoms. The suitability and some of the present limitations of this application of TRL, which is new to the study of zeolites, are described. The average coordination of hydrated Eu 3+ in the large-pore zeolite Y is identical to that in aqueous solution, while the coordination number in zeolites A and ZSM-5 is smaller, with framework oxygen atoms displacing water from the Eu 3+ coordination sites. Europium metal dissolved in liquid ammonia can be used to generate Eu(NH 3) 6 2+ and solvated electrons. With zeolite X, this solution results in the formation of Na 4 3+ and two kinds of defects. These are assigned to an electron trapped in an oxygen vacancy arising from a Lewis acid site and a V-type hole center in which the unpaired electron resides on a non-bridging oxygen atom bonded to silicon.
Published Version
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