A STUDY OF CATION ENVIRONMENT AND MOVEMENT DURING DEHYDRATION AND REDUCTION OF NICKEL-EXCHANGED ZEOLITE-Y BY X-RAY ABSORPTION AND DIFFRACTION

Using synchrotron radiation, we have monitored, by a combination of extended X-ray absorption fine structure (EXAFS) spectroscopy and high-resolution powder X-ray diffraction (XRD), (i) the location of Ni2+ ions in the hydrated form of nickel ion exchanged Y zeolite (Si/Al = 2.25) at room temperature, (ii) the environment and evidence of movement of Ni2+ ions upon heating the zeolite to 300-degrees-C in vacuo, and (iii) the changes in local environment of the nickel brought about by subsequent reduction in hydrogen. This combination of techniques gave both local (EXAFS) and long-range (XRD) structural information pertaining to the atomic environments of extra-framework species. We show that during dehydration approximately 70% of the Ni2+, which were present as solvated cations in the supercage, are transferred to the S(I), hexagonal prism site, where they are stabilized by inward relaxation of the surrounding oxygen ions. The remaining Ni2+ ions which are in a partially solvated state in the sodalite cage are readily reducible by hydrogen to yield small crystallites of metallic nickel.