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Title: Effects of aging on ScSZ/LSCrF dual-phase oxygen transport membrane for syngas production
Author: Wong, Chi Ho
ISNI:       0000 0004 8504 4466
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2017
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A dual-phase composite material comprised of scandia-stabilized zirconia (ScSZ) and doped lanthanum chromite (La,Sr)(Cr,Fe)O3-δ (LSCrF) was investigated in this work as a potential oxygen transport membrane material in the production of synthesis gas. The main focus of this work was to examine the effects of exposure to three aging environments (air, 4% H2-96% N2, 5% H2-95% CO2) for two durations (300 hours, 1000 hours) on the oxygen transport kinetics and surface chemistry of ScSZ/LSCrF. Determination of the oxygen ion conductivity via Electrochemical Impedance Spectroscopy of single-phase ScSZ showed a decrease in the activation energy above 600˚C corresponding to a change in crystal phase at this temperature. Determination of the oxygen transport kinetics in dry oxygen (18O2) atmosphere on single-phase ScSZ via Isotopic Exchange Depth Profiling combined with Secondary Ion Mass Spectrometry showed the material exhibited an oxygen self-diffusion coefficient on the order of 10-8 cm2 s-1 at 700˚C for 30 minutes but required the application of a catalytic layer to boost oxygen surface exchange to detectable levels. Examinations of the surface chemistry of polished single-phase ScSZ and LSCrF via Low Energy Ion Scattering and X-ray Photoelectron Spectroscopy showed minimal differences to the ideal bulk stoichiometry. Determination of the crystal structure of dual-phase ScSZ/LSCrF via X-ray diffraction showed secondary phase formation in the samples after 300 hours of aging in air and 5%H2-95%CO2, and in all samples after 1000 hours of aging. Oxygen transport kinetics of dual-phase ScSZ/LSCrF under dry oxygen showed minor differences in D* and k* after exposure to the aging environments for 300 hours when compared to the unaged material, but an increase in surface exchange kinetics was observed for the materials aged in reducing conditions for 1000 hours. D* and k* in dual-phase ScSZ/LSCrF were on the order of 10-8 cm2 s-1 and 10-8 cm s-1 respectively before and after aging up to 1000 hours. Oxygen surface exchange kinetics are increased by about one order of magnitude when performed in water vapor (H218O) over dry oxygen. Examinations of the surface chemistry of as-prepared dual-phase ScSZ/LSCrF showed Cr depletion on the outer atomic surface of all samples, in addition to elevated surface fraction of Zr within ~10 nm of the sample surface.
Supervisor: Skinner, Stephen ; Kilner, John Sponsor: Praxair Inc.
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral