Role of intermediate solid phase in the process of magnesium and lanthanum aluminates formation in sub- and supercritical water.

Torbin S.N. , Danchevskaya M.N. , Martynova L.F. , Muravieva G.P.

3-rd International Conference on Solvothermal Reactions ICSTR-3, Bordeaux, July 19-23, 1999, p. 66

ABSTRACT. Synthesis of inorganic complicated powder oxides is of significant interest owing to wide application of these materials in a modern engineering. In this paper the mechanism of fine crystalline magnesium and lanthanum aluminates formation in sub- and supercritical water was investigated. Mechanical mixtures of aluminium hydroxide and magnesia as well as lanthana were used as precursors. The synthesis was carried out in water steam atmosphere at P H2O - 20 MPa and temperature 300 - 400 C in the presence of small amounts of Cr (VI) ions. It was found, that the mechanism of this process cannot be represented in the frameworks of the traditional approach: dissolution of precursors in supercritical fluid and precipitation of the products. This approach assumes a complete homogenisation of reactionary system at the intermediate stage of the process. By means of X-ray analysis of products of the intermediate stage of synthesis the presence the crystalline phase was revealed by us. This phase will be derived in an outcome dehydration of precursors, which are flowing past in quasi equilibrium conditions. In the case of MgAl2O4 the similar phase is boehmite, in the case of LaAlO3 - lanthanum hydroxide. Using methods of X-ray analysis, thermal analysis; UF,VIS and IR -spectroscopy it was shown that diffusion and homogeneous distribution of Mg+2 ions in the structure of intermediate phase - boehmite are limited by disordering of boehmite crystalline lattice degree. Cr (VI) ions supplement in reaction system promotes the formation of additional defects in boehmite structure. As a result the processes of diffusion and alumina matrix rearrangement are relieved. Magnesium ions stabilise the cubic oxygen sublattice of spinel crystalline structure. It was revealed that the main part of alumina component at the intermediate stage of LaAlO3 synthesis is amorphous. This state is promoted by the presence of Cr (VI) ions in the reactionary system. The process of amorphous aluminous component introduction in rare-earth matrix occurs by diffusion on mosaic blocs boundaries of La(OH)3 structure and also on intrinsic defects in blocs. This process is facilitated due to mobility of La-O matrix in quasi equilibrium conditions of dehydration in water medium. Near certain temperature (350-370 C) the condition of two components most complete contact is achieved and the formation of the new phase - LaAlO3 begins. It happens a reorganisation of hexagonal oxygen sublattice La(OH)3 in cubic LaAlO3 at this stage. This research was made with financial support of RFBR pr. 98-03-32 645Ъ.

Laboratory of Catalysis and Gas Electrochemistry