Oxygen conducting membranes, catalytic membranes and oxygen reservoir materials.
Many oxides are excellent oxygen ionic conductors or mixed conductors, they may contain cations that easily may undergo redox reactions followed by uptake or release of oxygen. As such they are of interest for oxygen selective membranes, catalytic membranes and as metal supports for catalytic reactions involving reactive oxygen at elevated temperatures. Perovskite-type oxides represent one interesting class of materials with oxygen vacancies (either ordered as shown in the figure with octahedra and tetrahedra below; or in smaller concentrations induced by heterovalent substitutions) that facilitate oxygen transport from the bulk to the surface. A similar behaviour is investigated for layer-like oxides of Ruddelsden-Popper type for which mobile, redox active interstitial oxygen atoms exist in interlayer regions, see figure 1. These redox reactions are fast at high temperatures. We utilize e.g. in-situ X-ray diffraction (synchrotron radiation at Swiss Norwegian Beam Lines, ESRF) to investigate the reoxidation behaviour, see the two-dimensional diffraction pattern below, where the atmosphere for a reduced SrFeO3-x perovskite is suddenly switched from N2 to O2. Another category of interesting materials are of fluorite type (ZrO2, CeO2). In particular ceria has the ability to provide oxygen at high temperatures, implying Ce(IV)/Ce(III) conversions. The CeO8-cubes form building units in the fluorite structure, and vacancies at the O-sites along with possible interstitial sites in the empty cubes (see figure 3) provides routes for oxygen ion transport.
Published September 18, 2006