Attention is drawn to the possible involvement of the surface exchange kinetics in limiting the rate of oxygen permeation through mixed-conducting oxide ceramics. A theoretical approach is provided with which it is possible to distinguish between surface exchange-and bulk diffusion controlled kinetics of oxygen permeation. New results on the oxygen permeability of perovskites Lao.aSro.2CoO3_~ and SrCoo.sFeo.203_6 are presented. The importance of the exchange reaction relative to diffusion in limiting overall oxygen transport through (La,Sr) (Co,Fe)O3_6 perovskite-type oxides is emphasized.

Power‐to‐gas is a storage technology aiming to convert surplus electricity from renewable energy sources like wind and solar power into gaseous fuels compatible with the current network infrastructure. Results of CO2 dissociation in a vortex‐stabilized microwave plasma reactor are presented. The microwave field, residence time, quenching, and vortex configuration were varied to investigate their influence on energy‐ and conversion efficiency of CO2 dissociation. Significant deterioration of the energy efficiency is observed at forward vortex plasmas upon increasing pressure in the range of 100 mbar towards atmospheric pressure, which is mitigated by using a reverse vortex flow configuration of the plasma reactor. Data from optical emission shows that under all conditions covered by the experiments the gas temperature is in excess of 4000 K, suggesting a predominant thermal dissociation. Different strategies are proposed to enhance energy and conversion efficiencies of plasma‐driven dissociation of CO2.

The structure, oxygen stoichiometry, and chemical and thermal expansion of Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3-δ (BSCF) between 873 and 1173 K and oxygen partial pressures of 1 × 10 -3 to 1 atm were determined by in situ neutron diffraction. BSCF has a cubic perovskite structure, space group Pm3 hm, across the whole T-pO 2 region investigated. The material is highly oxygen deficient with a maximum oxygen stoichiometry (3δ) of 2.339(12) at 873 K and a pO 2 of 1 atm and a minimum of 2.192(15) at 1173 K and a pO 2 of 10 -3 atm. Good agreement is obtained between oxygen stoichiometry data determined by neutron diffraction and thermogravimetry. In the range covered by the experiments, the thermal and chemical expansion coefficients are 19.0(5)-20.8(6) × 10 -6 K -1 and 0.016(2)-0.026(4), respectively.

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New results on the oxygen permeability of perovskite-type oxides SrCoo.sB~.203_ a (with B' =Cr, Fe, Co and Cu) and Lao.6Sro.4CoO3_6 are presented. The occurrence of order-disorder transitions at elevated temperatures (790-940°C) in these phases has been confirmed by DSC measurements and, in some cases, by X-ray powder diffraction of samples either slowly cooled or quenched from high temperature after annealing in different atmospheres. The oxygen permeability found upon exposing opposite sides of sealed disc specimens to a stream of air and of helium, respectively, increases sharply (between 5-6 orders of magnitude up to 0.3-3 × 10-7 mol cm -2 s-1 ) at the onset of the transition from a low-temperature vacancy-ordered state to defect perovskite, except for SrCoo.sFeo.203_a. In the latter case only a slight anomaly is found in the Arrhenius plot of the oxygen permeability at ~ 790 ° C. The comparatively high oxygen flux through SrCo0.aFeo.203_a observed at intermediate temperatures is interpreted in terms of a two-phase mixture of a vacancy-ordered state and disordered perovskite, while above ~ 790°C the sample is single-phase of defect perovskite structure.

Electrical conductivity relaxation experiments were performed on thin specimens of Lai_rSrFe0a_, (x =0.1, 0.4) at oxygen partial pressures Po, = 10 -1 bar in the temperature range 923 to 1223 K. The transient response of the electrical conductivity after a sudden change of the ambient oxygen partial pressure was analyzed in the frequency domain. The latter procedure allowed diffusion-limited and surface exchange-limited kinetics of re-equilibration to be distinguished. The response of specimens with thiçknesses of 350 to 460 p.m indicated diffusion-controlled kinetics at Po,> 0.03 bar. The chemical diffusion coefficients, D, were found invariant with oxygen pressure. At 1073 K the absolute values were D = 6.5 x 10-6 cm2 -i for x = 0.1 and D = 1.1 X 10 cm2 s1 for x = 0.4, with activation energies of about 80 kJ/mol. The equilibration process was governed by surface exchange at Po, < 0.01 bar. The surface exchange coefficient, k0, was proportional to pa,, where n = 0.65 to 0.85. This pressure dependency was interpreted in terms of a slow surface process involving an oxygen molecule and a surface oxygen vacancy, and causes the observed sharp transition from diffusion-to exchange-controlled kinetics. The activation energy of k0 was estimated at 110 to 135 kJ/mol.

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This paper gives a survey of the catalytic properties of solid oxides which display oxygen ion or mixed (i.e. ionic + electronic) conductivity. Particular consideration is given to the oxidation-reduction reactions of gas phase components, but attention is also devoted to oxygen exchange between gas and oxide. An attempt has been made to relate and explain the observed phenomena such as catalytic activity and selectivity in terms of the electrical conducting properties of the oxides, which depend on their crystal and defect structures.In a number of cases possible applications of these materials in (electro)catalytic reactors, sensors, fuel cells, oxygen pumps, etc. are indicated.