FEATURES OF THE SOLID PHASE INTERACTION IN THE Mn₂O₃ - g-Al₂O₃ SYSTEM

S.V.Tsybulya, G.N.Kryukova, O.N.Kovalenko

Boreskov Institute of Catalysis, pr. Lavrentieva 5, Novosibirsk, 630090 Russia E-mail: tsybulya@catalysis.nsk.su

Keywords: alumina, manganese oxides, microstructure.

An effect of the partial interaction between manganese and aluminum oxides in MnO_x/gamma-Al₂O₃ catalyst calcinated at 900-950^oC is found. Compounds formed on the base of d-Al₂O₃ are cored solid solutions that appears to lead to their partial desintegration followed by the formation of a-Al2O3 . In turn, solid solution of Al ions in Mn₃O₄ (Mn : Al ~ 10 : 1) is in the state of metastable equilibrium due to the specific micrograined structure characterized by a developed coherent boundaries. According to TEM analysis size of the aggregates of this phase is over 100 nm, whereas X-ray particle size calculated with broadening of the X-ray diffraction peaks, is equal to 10 nm. This microstructure is highly thermostable. Calcination of samples at 950^oC for tens hours results in the complete transformation of d- to a-alumina. At the same time, micrograin boundaries in the structure of aluminium doped Mn₃O₄ is still well-developed. Calcination of this system at elevated (1100^oC) temperature does not change its phase composition and microstructure.

After the sample calcination at 950^oC for a short period of time phase of d-Al₂O₃ still preserves, among with a-Al₂O₃ and aluminium doped Mn₃O₄ . During calcination of such a system at 1100^oC delta-alumina interacts with Mn₃O₄ forming cubic spinel of Mn₂AlO₄ - MnAl₂O₄ type by conventional way.

It seems likely that partial solubility of Al ions in the structure of tetragonal Mn₃O₄ spinel at 900-950 ^oC is caused by thermodynamic restrictions for morphotropous phase transition from tetragonal structure to cubic one at these temperatures .

This work was made possible by Grant N97-03-33497 from the RFBR.