THE CONTROLLED DISPERSION OF SILICA SUPPORTED MOO3 - THE ROLE OF AMMONIA

The morphology of silica supported catalysts, prepared by impregnation of ammonium heptamolybdate and for various weight loadings up to 35 wt%, was studied using X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy. In addition to the orthorhombic phase, the behavior of the rarely studied hexagonal phase was fully characterized. All morphologies of silica supported Mo03 appear to be thermodynamically driven. At low loadings there appeared one stable dispersion, independent of initial precursor dispersion. For high loaded catalysts there appeared three states: a metastable sintered hexagonal state and a well dispersed hexagonal state at moderate temperature calcinations (300°C), and the sintered orthorhombic state at high temperature (500°C). Whereas the sintered orthorhombic phase is detected by XRD at loadings in excess of 1.1 atom Mo/nm2, the well dispersed hexagonal phase is not evident until 4.0 atoms Mo/nm2. It is possible to produce the well dispersed hexagonal phase from the sintered orthorhombic phase with an ammonia impregnation and subsequent calcination at 300°C. The apparently higher dispersion of the hexagonal phase may arise from some role of ammonia which results in a stronger MoO3-surface interaction. © 1992.