Defect chemistry, transport properties and thermodynamic stability of acceptor doped and undeoped layered La 2 NiO 4
The market for oxygen has grown significantly because many industrial processes require oxygen. At present oxygen is produced on industrial scale mainly by cryogenic distillation of air. This is a very energy intensive process and economic operation is possible only on a large scale. An alternative method, relatively new development is dense oxide ceramic membranes which can exclusively separate oxygen from air at high temperature that has the advantage of direct supply of oxygen in membrane reactors for the (partial) oxidation of hydrocarbons. Two types of dense membranes can be distinguished, the solid oxide electrolytes and the mixed ionic electronic conductors. The latter type of membrane is the subject of research of this thesis. During of oxygen transport two basic processes occur in a single-phase ceramic membrane at steady state are discussed. In the bulk, coupled diffusion of oxygen ions and electrons takes place, while exchange reactions of oxygen from the lattice with molecular oxygen in the gas phase occurs at both interfaces of the membrane.Ceramic oxides materials with perovskite related structures, e.g. the K2NiF4 structure have been reported to exhibit substantial oxygen permeation flux, which makes these attractive candidates for High-temperature oxygen separation. In contrast to perovskite-type membranes, the oxygen flux in these materials supported by oxygen interstitial ions. In this work La2NiO4 based membranes was evaluated with respect to their transport properties, stability in different atmospheres and their aging. The structure of La2NiO4 may be described as an intergrowth of rocksalt La2O3 layers and perovskite LaNiO3 layers. The rocksalt layers accommodate interstitial oxygen, whereas the perovskite layers host oxygen vacancies, according to theoretical calculations on point defect formation and migration. The phasal composition of the ceramic powder prepared by a modified Pechini process, was analysed by X-ray diffraction. The prepared disc-shape membranes was analised by SEMM and EDX. Oxygen permeation measurements in air/Ar gradients were performed and the composition analysis of the permeate stream was performed by gas mass spectrometry. The observed oxygen fluxes was described in terms of bulk diffusion-limited permeation behavior. Model calculations were performed, based on Wagner theory in conjunction with data of oxygen nonstoichiometry and diffusion coefficients from literature. Defect modeling reveals that with decreasing p(O2) on the oxygen-lean side, the vacancy concentration in the membrane increases and the contribution of vacancy transport becomes more and more significant. The calculated mobilities of the vacancies and interstitials are approximately equal.Because mixed conduction is attained for single phase material their phase stability is required. Thermodynamically stability was studied in presence of several p(O2) for a number of compositions of Sr doped La2NiO4, at different temperatures. With decreasing p(O2) and increasing the temperature it was observed that the material become unstable.
info:eu-repo/classification/ddc/660, Technische Chemie, Defect chemistry, La2NiO4, ceramic membranes, oxygen permeation, oxygen transport, thermodynamic stability
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