The location of non-framework species in zeolite catalysts
The work presented in this thesis involves the study of a number of non-framework species in zeolites. In particular, nickel has been selected as an interesting non-framework species to study because it may be present either as charge-balancing cations interacting with the zeolite framework oxygens, or as metallic nickel clusters or particles, both in the zeolite cavities or on the outer surfaces of the zeolite particles. Temperature-programmed reduction studies have shown the influence of factors such as zeolite structure, Si/Al ratio, nature of the cocation, preparation and dehydration conditions and various modifying treatments on the ease of reduction of the nickel cations. Transmission electron microscope studies of these reduced samples have revealed the influence of such factors on the particle sizes, particle-size distributions and the sintering of the metallic nickel particles. The location of the nickel cations in the dehydrated zeolites have been determined using x-ray powder Rietveld refinement of data collected at temperatures of 400oC or higher in an environmental high-temperature cell. The sample preparation conditions and various modification procedures are found to significantly influence the distribution of cations amongst the available sites in the zeolite. In situ reduction studies have been carried out in the high-temperature environmental cell. X-ray diffraction data collected at high temperatures during reduction allows one to follow the changes in cation locations and the growth of metallic nickel particles, and to compare reducibility of nickel in samples subjected to different modification treatments. X-ray powder Rietveld refinement of data collected from the finally reduced samples has been used to locate cations after reduction; comparison with cation locations determined in samples prior to reduction help to confirm the assignment of scattering at a particular site to a given cation, and give information on the relative ease of reduction of nickel cations in different sites. Finally, some preliminary work has been carried out on a different type of non-framework species, namely sorbed organic molecules. Some infra-red studies, heat of adsorption measurements and neutron diffraction studies of zeolite-sorbate systems are discussed.