Oxygen surface exchange and oxidative dehydrogenation on oxide ion conductors

Abstract

The research described in this thesis mainly aims at investigation of the rate of oxygen exchange at the surface of oxide ion conductors. The introduction is given in Chapter 1. A fast and simple method, referred to as pulse 18O-16O isotopic exchange (PIE), for measurement of the rate of surface exchange on oxide ion conductors has been developed, as described in Chapter 2. The method is used to measure the rate of oxygen exchange on yttria-stabilized zirconia (YSZ), La2NiO4+?, and Ba0.5Sr0.5Co0.8Fe0.2O3-? (BSCF). Analysis of the experimental data in terms of a two-step model for the isotopic exchange reaction shows that for the two mixed conductors, La2NiO4+? and BSCF, the exchange reaction is limited by the rate of dissociative adsorption of O2 molecules at the oxide surface, whilst for the solid electrolyte, YSZ, this reaction is competing with that of incorporation of adsorbed oxygen adatoms into the oxide lattice. In Chapter 3, the PIE method is used for measurement of the oxygen exchange rate of phases La1-xSrxCoO3-? (LSC). The observed power law dependence of the exchange rate on the concentration of oxygen vacancies in these phases, with exponent ~0.75, emphasizes the importance of the oxygen vacancies in the kinetics of oxygen exchange for these materials. In Chapter 4, the PIE method is used for in-situ assessment of the role of CO2 on the rate of oxygen exchange on BSCF. This rate is largely annihilated by the formation of a carbonate layer at the BSCF surface, but can be partially restored by thermal annealing of the oxide in a CO2-free atmosphere at 850 oC for 5 h. In Chapter 5, the oxidative dehydrogenation (ODH) reactions of propane and ethane are investigated in a catalytic membrane reactor, incorporating oxygen-permeable membranes based upon La2Ni0.9V0.1O4+? (LNV10) or Ba0.5Sr0.5Co0.8Fe0.2O3-? (BSCF). The results show the dominating role of the oxygen flux across the membrane and that of available sites at the membrane surface in primary activation of the alkane. Finally, a brief evaluation of the work described in this thesis and some recommendations for further research are provided in Chapter 6.