SOLID-STATE REACTION BETWEEN MOLYBDENA AND ALUMINA - EFFECT OF WATER-VAPOR PRESSURE ON THE DISPERSION AND NATURE OF THE SUPPORTED PHASES

The dispersion and nature of surface species formed upon calcination o f MoO3-gamma-Al2O3 mixtures at 770 K for 10 h and with different molyb dena loadings under different water vapour pressures [P(H2O)] have bee n studied by X-ray diffraction (XRD), zero-point charge (ZPC) measurem ents and temperature-programmed reduction (TPR); P(H2O) was in the ran ge 25-45 Torr with MoO3 loadings up to 2 monolayers (1 monolayer=0.168 1 g MoO3/g Al2O3). The intensity of the most intense XRD peak of MoO3 at 326 pm decreased as P(H2O) increased, indicating an increase in the concentration of surface species different from bulk MoO3. Up to 0.7 monolayer MoO3 loading, covering of the Al2O3 surface with Mo-containi ng species increases with increasing P(H2O); ZPC at P(H2O)=45 Torr was coincident with that of MoO3. P(H2O) has no effect on the dispersion of high loaded samples (above 1 monolayer), where all the support surf ace is covered even for P(H2O)=25 Torr. TPR results indicate the prese nce of species with different reducibilities, depending on P(H2O) and MoO3 loading; their nature has been assessed by comparison with previo us results for samples obtained by impregnation; bulk MoO3 (responsibl e for high-temperature reduction peaks) formation has been observed at high MoO3 loadings.