Henny J. M. Bouwmeester

Importance of the surface exchange kinetics as rate limiting step in oxygen permeation through mixed-conducting oxides

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 La0.8Sr0.2CoO3−σ and SrCo0.8Fe0.2O3−σ are presented. The importance of the exchange reaction re to diffusion in limiting overall oxygen transport through (La,Sr)(Co,Fe)O3−σ perovskite-type oxides is emphasized.

Oxidative coupling of methane in a mixed-conducting perovskite membrane reactor

Ionic-electronic mixed-conducting perovskite-type oxide La0.6Sr0.4Co0.8Fe0.2O3 was applied as a dense membrane for oxygen supply in a reactor for methane coupling. The oxygen permeation properties were studied in the pO2-range of 10?3?1 bar at 1073?1273 K, using helium as a sweeping gas at the permeate side of the membrane. The oxygen semi-permeability has a value close to 1 mmol m?2 s?1 at 1173 K with a corresponding activation energy of 130?140 kJ/mol. The oxygen flux is limited by a surface process at the permeate side of the membrane. It was found that the oxygen flux is only slightly enhanced if methane is admixed with helium. Methane is converted to ethane and ethene with selectivities up to 70%, albeit that conversions are low, typically 1?3% at 1073?1173 K. When oxygen was admixed with methane rather than supplied through the membrane, selectivities obtained were found to be in the range 30?35%. Segregation of strontium was found at both sides of the membrane, being seriously affected by the presence of an oxygen pressure gradient across it. The importance of a surface limited oxygen flux for application of perovskite membranes for methane coupling is emphasized.

Oxygen exchange and diffusion coefficients of strontium-doped lanthanum ferrites by electrical conductivity relaxation

Electrical conductivity relaxation experiments were performed on thin specimens of La1–xSrxFeO3–delta (x = 0.1, 0.4) at oxygen partial pressures pO2 = 10–5 – 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 thicknesses of 350 to 460 µm indicated diffusion-controlled kinetics at pO2 > 0.03 bar. The chemical diffusion coefficients, D-tilde , were found invariant with oxygen pressure. At 1073 K the absolute values were D-tilde = 6.5 × 10–6 cm2 s–1 for x = 0.1 and D-tilde = 1.1 × 10–5 cm2 s–1 for x = 0.4, with activation energies of about 80 kJ/mol. The equilibration process was governed by surface exchange at pO2 O[sub 2] n , 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 kO was estimated at 110 to 135 kJ/mol.

Solid state aspects of oxidation catalysis

The main subject of this review is the consideration of catalytic oxidation reactions, which are greatly influenced by solid state effects in the catalyst material. Emphasis is laid upon the correlation between the presence of mobile ionic defects, together with the associated ionic conductivity, and the catalytic performance. Both total and selective oxidation reactions and oxidative conversion reactions are considered. Well-known examples of such behaviour include oxidative methane conversion with lanthanide oxides, carbon monoxide oxidation on fluorite type catalysts, selective olefin oxidation using vanadia based catalysts, etc. Furthermore, because oxygen exchange between gas and solid is always part of the oxidation process, this is considered too. The discussion of the application of the solid oxides under consideration to practically important oxidation processes, together with the influence thereon of their solid state properties, forms a major part of this review. Computational modelling and simulation of catalyst structure and behaviour is also considered. Special attention is given to the potentialities offered by using ionic and mixed conducting oxides either as the electrode material in a solid electrolyte fuel cell (SOFC) or as a separating, dense membrane in a ceramic membrane reactor. The use of porous membranes in such reactors is also taken into consideration. On the one hand these may be used to study the above relationship between catalytic behaviour and solid state properties, on the other hand to obtain a reactor configuration allowing better use of reactants and/or catalysts. Besides the controlled supply of (or removal) of oxygen to (or from) the side where the catalyst and the reactants are located, a promising feature of both experimental approaches is that the oxygen flux may alter the relative presence of different oxygen species (O2,O,O2−,O22−,O3−,O−, etc.) on the catalyst surface. In this way species are provided having a strong influence on the selectivity for partial oxidation reactions and oxidative conversion reactions.

Oxygen transport through La1-xSrxFeO3-(delta) membranes. I. Permeation in air/He gradients

Oxygen permeation measurements in air/He gradients were performed on dense La1 ? xSrxFeO3 ? ? membranes in the composition range x = 0.1?0.4 and temperature range 1123?1323 K. Pretreatment of the lower oxygen partial pressure side of the membranes in a CO-containing atmosphere for several hours at 1273 K led to higher oxygen fluxes, which were in the range of 0.1?4.5 mmol m?2 s?1. After treatment, the observed oxygen fluxes could be described in terms of bulk diffusion-limited permeation behaviour. Experimental evidence for a bulk-diffusion controlled flux was found from thickness dependence measurements on membranes with thicknesses between 0.5 mm and 2.0 mm. Model calculations, based on Wagner theory in conjunction with data of oxygen nonstoichiometry and vacancy diffusion coefficients from literature, were performed. The experimental flux values deviated from the model calculations with factors up to 2.5. Adjustment of the value of the vacancy diffusion coefficient led to good agreement between the experimental data and the model calculations. The calculated vacancy diffusion coefficients Dv0 were virtually independent of composition and were found to be in the range 5.3?9.3 × 10?6 cm2 s?1.

Oxygen transport through La1−xSrxFeO3−δ membranes. II. Permeation in air/CO, CO2 gradients

Oxygen permeation measurements were performed on dense La1 − xSrxFeO3 − δ (x = 0.1-0.4) membranes in large oxygen activity gradients, i.e. air/CO, CO2, in the temperature range 1173–1323 K, yielding fluxes up to 25 mmol m−2 s−1 at 1273 K. The oxygen semipermeability was linearly proportional with the CO partial pressure and the strontium content. It was concluded that the fluxes are limited by the surface exchange kinetics. Two simple models are proposed for the oxygen exchange reaction in the presence of CO. In both models oxygen vacancies at the phase boundary play a definite role in the exchange process. XPS and SEM analysis of the perovskite/CO, CO2 interfaces indicated segregation of strontium, being present at the surface mostly in the form of SrCO3 and/or SrO. The enhanced oxygen fluxes in air/He gradients as they were observed after exposure of the membrane surface to CO are probably (indirectly) related to the level of strontium segregation. Based on SEM analysis and surface profile measurements, it is suggested that the segregation process is accompanied by an enlargement of the specific surface area, which may promote the speed of the overall permeation process under exchange-controlled conditions.

Chemical diffusion and oxygen exchange of La0.6Sr0.4Co0.6Fe0.4O3−δ

The transport parameters of La0.6Sr0.4Co0.6Fe0.4O3−δ were determined by electrical conductivity relaxation and high temperature coulometric titration experiments. The experimental response curves were analyzed in the frequency domain. The results obtained by both methods were in good agreement. The chemical diffusion coefficients measured at temperatures of 923–1255 K and oxygen partial pressures of 0.03–1 bar O2, vary between 10−6–5×10−5 cm2 s−1. The experimental activation energies are in the range 95–117 kJ mol−1. At oxygen partial pressures below 0.03 bar O2 the re-equilibration process is completely governed by the rate of oxygen exchange at the interface. The surface exchange coefficients were determined by conductivity relaxation experiments at temperatures of 1000–1285 K and oxygen pressures of 10−4–0.1 bar O2. The activation energy is about 60–70 kJ mol−1. The exchange coefficients are almost proportional to the oxygen pressure. Treatment of the surface in a nitric acid solution for several hours increases the surface exchange rates by factors of 3–5.

Microstructural development, electrical properties and oxygen permeation of zirconia-palladium composites☆

Yttria-stabilized cubic zirconia (YSZ)-palladium dual phase composites have been investigated. The percolative composite containing 40 vol% Pd (ZYPd40) showed a much larger oxygen permeability than that of the non-percolative composite containing 30 vol% Pd (ZYPd30). For a 2.0 mm thick percolative composite, an oxygen flux of 4.3 × 10−8 mol/cm2/s was measured at 1100 °C with oxygen partial pressures at the feed and permeate sides being 0.209 and 0.014 atm, respectively. This value is two orders of magnitude larger than that observed for a 2.0 mm thick non-percolative composite at the same temperature with the oxygen partial pressures at the feed and permeate sides being 0.209 and 1.5 × 10−4 atm, respectively. From the dependence of the oxygen permeation on the temperature and on the oxygen partial pressures, it was concluded that the transport of the oxygen ions through the YSZ phase in the percolative system was the rate limiting step.

Kinetic decomposition of La0.3Sr0.7CoO3−δ perovskite membranes during oxygen permeation

In this paper, a study is presented towards the stability of oxygen permeable membranes of perovskite La0.3Sr0.7CoO3−δ in an oxygen pressure gradient. It is shown that phase separation occurs at the oxygen-lean side of the membrane, at 900°C, when the membrane is exposed to streams of air and inert gas at opposite sides of the membrane. It is absent when the (homogenous) oxide is annealed in either flowing air or nitrogen for several days, indicating that the phase separation is induced by the dynamic forces acting during oxygen permeation.

Surface oxygen exchange of La0.3Sr0.7CoO3-d

The surface oxygen exchange of La0.3Sr0.7CoO3 − δ has been studied by 18O/16O exchange followed by dynamic SIMS analysis in the temperature region 700–;900 °C at a pO2 of 2.1 × 104 Pa. The activation energy for the surface exchange rate (k) is 28 ± 5 kJ mol−1 and for the oxygen tracer diffusion coefficient (D*) 56 ± 4 kJ mol−1. The absolute values for both k and D* are among the highest known in the literature. The characteristic thickness at a pO2 of 2.1 × 104 Pa is calculated to lie in the range 0.70 to 1.3 mm and is a function of temperature. The latter reflects the different activation energies for k and D*. The surface oxygen exchange coefficient is proportional to PO2n with N = 0.41 ± 0.02 at 800 °C, which has been explained by a rate determining step involving an adsorbed oxygen species and an oxygen vacancy. No oxygen pressure dependence for the tracer diffusion coefficient was observed at this temperature.