Extended X-ray absorption fine-structure spectroscopic study of α-zirconium phosphates fully exchanged by Cu2+

Abstract

EXAFS spectroscopy using synchrotron radiation has been utilised to investigate Cu—O bond-length changes in Cu²⁺-exchanged α-zirconium phosphates before and after dehydration and calcination. The bond distances found: Cu—O = 1.965 (10)Å[α-ZrCu(PO₄)₂·4H₂O], 1.923 (10)Å[α-ZrCu(PO₄)₂] and 1.958 (10)Å[CuZr(PO₄)₂], indicate that the site available to the Cu²⁺ is in all cases a tetragonally distorted octahedral one. The model compound used for extracting phase parameters was Cu(pyO)₄(ClO₄)₂. The coordination numbers extracted from relative amplitudes using this compound as a standard are in agreement with this geometry assignment, except for the case of the non-layer material, formed after calcination at 900 °C.

Considerable amplitude changes in a second shell lying between 2.38 and 2.78 Å(depending on the sample) are assigned to Cu⋯P distances. Combining the bond-distance information with inter-layer distances leads to formulation of a site for the layer compounds. It is concluded that, unlike the case of zeolites, the layers do not remain rigid after exchange and dehydration, but slide and twist to accommodate the Cu²⁺.