Solid-state reaction between molybdena and alumina: effect of water vapour pressure on the dispersion and nature of the supported phases

Abstract

The dispersion and nature of surface species formed upon calcination of Mo03-y-Al,0, mixtures at 770 K for 10 h and with different molybdena loadings under different water vapour pressures [P(H,O)] have been studied by X-ray diffraction (XRD), zero-point charge (ZPC) measurements and temperature-programmed reduction (TPR); P (H,O) was in the range 25-45 Torr with MOO, loadings up to 2 monolayers (1 monolayer=0.1681 g MoO,/g A1203). The intensity of the most intense XRD peak of MOO, at 326 pm decreased as P(H,O) increased, indicating an increase in the concentration of surface species different from bulk MOO,. Up to 0.7 monolayer MOO, loading, covering of the Al,03 surface with Mo-containing species increases with increasing P(H,O); ZPC at P(H20) = 45 Torr was coincident with that of MOO,. P(H20) has no effect on the dispersion of high loaded samples (above 1 monolayer), where all the support surface is covered even for P(H,O) = 25 Torr. TPR results indicate the presence of species with different reducibilities, depending on P(H,O) and MOO, loading; their nature has been assessed by comparison with previous results for samples obtained by impregnation; bulk MOO, (responsible for high-temperature reduction peaks) formation has been observed at high MOO, loadings.