Electrical and Catalytic Properties of Some Oxides with the Fluorite or Pyrochlore Structure Part I: Synthesis, Characterization and Conductivity

In view of their possible use as electrocatalysts, several oxides with the fluorite or pyrochlore structure were investigated. Part I of this paper deals with the synthesis, characterization and electrical conductivity of these materials. All lanthanide titanates investigated (partly doped with Bi) show the pyrochlore structure and are poor conductors, except for the compounds containing Tb. For Nd-zirconates, partly substituted with Ce, the degree of order depends on the amount of Ce and temperature treatment. Ionic and in some cases mixed conductivity was observed. Part II reports about the catalytic activity for CO oxidation INTRODUCTION Aims Recent literature data indicate that several solid electrolytes have catalytic properties, which are of interest for applications in electrocatalytic reactors, fuel cells, oxygen sensors and pumps (e.g. [i-6]). In these investigations mainly stabilized zirconia is used as the oxygen ion conducting electrolyte and some noble metal as the electrode material. The electrocatalytic oxidation or reduction reaction takes place at the electrolyteelectrode-gas interphase, the electrode and/or the electrolyte surface providing the catalytically active sites. In the electrocatalytic reaction scheme the electrolyte serves as a reservoir for oxygen ions or oxygen vacancies, which can be pumped towards the interphase region by applying a proDer voltage across the electrolyte. Meanwhile the electrochemical potential of the oxygen species or vacancies at the reaction sites is strongly influenced, even for rather low oxygen ion mobilities within the electrolyte. This may strongly affect the reaction kinetics, provided that electrons or electron-holes are available locally for charge transfer. Therefore mixed conductivity of the electrolyte surface-layer may be advantageous. In this paper it is our aim to scan a number of materials that might be used as catalytically active substrates in particular for electrocatalytic applications. In part I the synthesis and characterization of these materials