The chemistry of Bi-Mo oxide catalysts I. Phase composition of catalysts and its relation to the structure of precursors
1972; Elsevier BV; Volume: 25; Issue: 1 Linguagem: Inglês
10.1016/0021-9517(72)90198-4
ISSN1090-2694
Autores Tópico(s)Catalysis and Hydrodesulfurization Studies
ResumoThe effect of the conditions of preparation and thermal treatment on the phase composition of Bi-Mo oxide catalysts at the atomic ratio BiMo = 11 has been studied. It has been shown that bismuth molybdate of the composition Bi2O3·2MoO3 (β-phase) is formed on crystallization of the precipitate prepared from appropriate salts at pH = 2 without additional reactions in the solid state. In the case of precipitation at pH = 7 and subsequent filtering, the calcined catalysts contained Bi2O3·MoO3 (γ-phase). The precursor formed in these conditions was identical to the precipitate obtained by treating the precursor of the β-phase with concentrated aqueous ammonia, which led to dissolution of half of the molybdenum present in the sample. Comparison of ir spectra leads to the conclusion that the β-phase precursor is composed of layers of [MoO6] octahedra and [MoO4] tetrahedra, the latter being dissolved on ammonia treatment leaving [MoO6] octahedra as the precursor of the γ-phase (koechlinite). Catalysts obtained by precipitation at pH = 7 and subsequent evaporation are composed of a mixture of γ- and α-phase (Bi2O3·3MoO3). The β-phase is unstable in the temperature range 400–550 °C, disproportionating on prolonged heating into α- and γ-molybdates. The region of stability of the β-molybdate is from 550–670 °C.
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