STUDY OF THERMAL CO-DECOMPOSITION OF MANGANESE AND CERIUM OXALATES IN AIR AND IN INERT MEDIA

Abstract

Controlled thermal decomposition of oxalate salts is a promising method for the preparation of highly dispersed materials. In this work, co-decomposition of Mn and Ce salts is studied upon variations of the cation ratio and the gas atmosphere of the process (air and inert gas). To this aim, a series of Mn and Ce oxalates is prepared by co-precipitation from aqueous solutions of nitrates with Mn:Ce ratios varying from 0:1 to 1:0. It is shown by X-ray powder diffraction and by scanning electron microscopy that cerium Ce(C2O4)3·10H2O and manganese MnC2O4·2H2O oxalates are formed. Also, increasing manganese content increases the amount of the corresponding salts and affects the morphology of the particles. It was shown by a number of physicochemical methods such as thermal analysis, in situ X-ray diffraction, and mass spectrometry shows that the oxalate decomposition proceeds in two stages and depends on the cation ratio and the decomposition atmosphere. The first stage is a weight loss accompanied by removal of structural water; this process is accelerated in a flow of an inert gas. The second stage is the formation of oxides from the anhydrous salt accompanied by CO2 or CO/CO2 release. Due to exothermic oxidation reaction, the decomposition occurs at lower temperatures in air than in an inert gas. The introduction of manganese does not significantly affect the temperature ranges of the first and second stages of salt decomposition. However, the addition of the second cation affects the decomposition product: as the manganese content increases, the size of CeO2 particles decreases and simple manganese oxides Mn3O4 and MnO are formed in air and in the inert atmosphere, respectively. The catalytic properties in the oxidation of CO, Mn and Ce oxides obtained by the decomposition of oxalates in an inert gas and in air are studied. It is shown that the catalysts formed in the oxidizing environment are more active.