The formation processes of oxide phases from polymer-salt complexes of ammonium molybdate and tungstate

The thermal decomposition processes of the polymer-salt solution of ammonium molybdate and tungstate are studied by small angle neutron scattering (SANS) and X-ray diffraction. The parameters of the crystal structure of the oxide phase particles WO3 and MoO3, a particle size distribution function, a specific surface and fractal dimension of particle to surface, a volume part and a mean particle size have been obtained. It is shown that the best catalytic properties are reached by heating the samples up to 400 degrees C. This stare is defined by the mean particle size similar to 15 nm, the specific surface similar to 10 m(2)/g, the particle volume concentration similar to 5 x 10(-2) and the fractal dimension of particles surface similar to 2.5. A general mechanism for the formation of the oxide phases from different polymer-salt solution is established. The processes of the phase formation as a first-order phase transition are also shown. (C) 2000 Elsevier Science B.V. All rights reserved.