A2B′B″O6 perovskites: A review

The high expectations present-day society has for catalysis in solving the energy crisis, in coping with environmental pollution, and in providing fertilizers and food have made research in catalysis the focus of interest in many adjacent fields. As an example, the science of solid surfaces in vacuum is explored for its relevance to and experimentally, a link has indeed been e ~ t a b l i s h e d . ~ Perhaps of even more importance, new interest has been stimulated in the links between solid-state physics and chemistry and the characteristics of solid catalysts. In earlier years, this connection had of course been explored on the heels of the spectacular development of semiconductor physics,\" but the fact that the best-studied semiconductors were poor catalysts has impeded progress in the establishment of detailed relationships between collective electrical properties and catalysis. Even so, a general relation between catalytic activity and the width of the band gap of a semiconductor has been recognized,' notwithstanding the frequent exceptions. Not only collective electrical properties, but also collective magnetic properties and ferroelectric properties have in those early years been explored in relation to catalytic characteristics. The relation was indeed established in work on the ferromagnetic man anese perovskite La~,6sSro,3sMn039 and on the ferroelectric perovskite KNbO,,' but the effects were small, which is perhaps not surprising. I ' The study of collective electron effects in metallic catalysts, I2-I4 which was initially partly based on the now ~ u p e r s e d e d ~ ~ . ' ~ rigid-band model of alloy systems such as Cu-Ni and Ag-Pd, has recently taken an interesting turn in the proposed relation between spin fluctuations connected with ma netic interactions and the momentum transfer t o a desorbing or reacting adatom. Surveying the relations between solid-state chemistry and catalysis, it becomes evident that in particular the number and type of localized defects in the solid have been successfully correlated with catalytic activity, e.g., for dislocations and steps on metal surface^,^^.^^ for surface states split off from the conduction band in semiconductors,21 and for anion vacancies or cation vacancies in insulators22 and s e m i c ~ n d u c t o r s . ~ ~ ~ ~ ~ In the present series of lectures on solid-state chemistry and catalysis, further examples of the role of anion and cation vacancies are given by Sleight, Schuit, and Longo. The topic of the present paper, defect chemistry of perovskite-type catalysts for oxidation-reduction processes, has received some stimulus from the suggestions that LaCo03, LaMn03, and their derivatives might be interesting catalysts for application in fuel cell electrodes2' and in the conversion of N O and C O from automotive e x h a ~ s t . ~ ~ ~ ~ In the following, the reduction of N O with CO and H2, and the oxidation of CO with O2 will be used as test reactions. We will explore the relationships between the catalytic activity and the defect chemistry of perovskitelike cobaltites, manganites, chromites, and ruthenates. The crystal structure of perovskite (CaTi03) is shown in FIGURE 1. The 97.1,

Defect chemistry and catalysis in oxidation and reduction over perovskite-type oxides

Crystal growth, structures, and optical properties of the cubic double perovskites Ba2MgWO6 and Ba2ZnWO6

Preparation, Crystal and Magnetic Structure of the Double Perovskites Ba2CoBO6 (B = Mo, W)

Resume Des essais ont ete realises sur trois materiaux en tant qu'electrodes chaudes dans les installations MHD en regime generateur: la zircone stabilisee a l'oxyde de calcium coulable, l'oxyde de cerium-zircone a 50 pour cent mole, et le chromite de lanthane dope au strontium. Au bout de 106 minutes de marche, les electrodes en oxyde de cerium-zircone se sont trouvees fortement erodees; la zircone stabilisee a l'oxyde de calcium s'est revelee comme materiau d'electrode d'une certaine efficacite et les electrodes en chromite de lenthane ne sont guere alterees. Les auteurs concluent que l'emploi du chromite de lanthane dope au strontium comme electrode chaude jusqu'a une temperature a la face frontale de 1600–1700°C merite un examen beaucoup approfondi.

J.J. Hurst

Papers

Crystal growth, and electrical and magnetic properties of Ba2CoWO6