Electronic Structure of LaBO 3 (B=Mn, Fe, Co, Ni, Cu) Perovskites and La 2 NiO 4
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ELECTRONIC STRUCTURE OF LaB0 3 (B=Mn,Fe,Co,Ni,Cu)PEROVSKITES AND La 2 NiO 4 P. DE LA MORA*,C. DE TERESA"', L. VICENTE** *Departamento de Fisica, Facultad de Ciencias, UNAM, 04510 Mdxico, D.F. "**Departamento de Fisica y Quimica Te6rica, Facultad de Quimica, UNAM, 04510 Mexico, D.F. ABSTRACT The electronic structure of LaB0 3 perovskites and La 2 NiQ4 perovskite- like materials (where B = Mn,Fe,Co,Ni,Cu) have been studied inside a tight binding scheme using the extended Hiickel Method. An analysis of the metal- oxygen bond is presented as well as the electronic structure in order to establish a correlation between electronic properties and catalytic activity (oxidation) of these compounds. INTRODUCTION The perovskite structure is one of the best known structures in oxide chemistry. It is adopted by compounds having the general formula ABO 3 where A is usually an alkaline earth and B is usualy a transition metal. These oxides are known to be important catalysts in practical processes such as oxidation [1-4] and hydrogenation
[5,6].
The oxides and halides crystallizing in a related structure, the so called K 2 NiF 4 type mixed oxides, consisting of alternating layers of AB0 3 perovskite and AO rock salt, have been of interest as intergrowth structures that exhibit strongly anisotropic magnetic interactions, but in contrast with simple perovskites, the catalytic action of K 2 NiF 4 -type oxides has been less studied. Due to the analogy with perovskites, materials with this structure are suitable for research of reactivity of surface oxygen and catalytic activity. The catalytic properties of the oxides are attributed to the active B cation by means of his oxidation state and coordination. Point defects are also of importance in catalysis, particularly cation and anion vacancies. In this work we are restricted to study the effect of the metal substitution on the oxidation of CO, methane and propylene, for which there are experimental results. In fact, the experimental results in actitivty for the oxidation of CO [7] (expressed in terms of the temperature at wich the rate of oxidation attains a certain level) show the trend LaCoO 3 >LaMn0 3 >LaFeO 3 >LaCr03. In the catalytic combustion of methane [4] the oxidation activity (evaluated as the temperature at which 50 percent conversion of methane oxidation is attained) follows LaCo0 3 >LaMn0 3 >LaFe0 3 >LaCuO 3 LaNiO3 >LaCr0 3 . Finally, in total oxidation of propylene [8] the trend is LaNiO3 >LaCo0 3 > LaMn0 3 >LaFeO3( expressed as percent conversion to C0 2 ). This last two results agrees if we use the same criterion, i.e. the activity of the Ni compound is greater than Co and so on. With respect to the La 2 NiO 4 system we have only data of the propylene oxidation wich show also a high activity and we compare it with the corresponding perovskite compound. The aim of this work was to search for a correlation between the electronic state of the transition metal B ion and the catalytic activity. THE METHOD A very widely used semiempirical quantum chemistry technique is the
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