SURFACE SPECIES FORMED UPON SUPPORTING MOLYBDENA ON ALUMINA BY MECHANICALLY MIXING BOTH OXIDES

MoO3/Al2O3 mechanical mixtures have been prepared with MoO3 loadings ranging from 0.1 to 2 monolayers (SBET of the support 105 m2/g), and calcination at 770 K for 8 and 24 h, in the presence and in the absence of water vapour, in order to assess the nature and distribution of the surface species formed. The interaction between the supported phase and the support and the nature of the species formed thereby have been followed through changes in the zero-point-charge (ZPC) of the solids. X-ray diffractograms, and temperature-programmed reduction profiles. The ZPC of the samples changes from 8.8 (parent alumina) to a final value of 3.0 in all samples, but this value is reached at different molybdenum loadings depending on the preparation method. For samples prepared by calcining both oxides in air for 24 h, a monolayer distribution of polymolybdate species is found for molybdenum loadings up to 0.7-0.8 monolayer. With samples calcined for 8 h in air or in dry oxygen, a complete coverage of the alumina surface is achieved only for 2-monolayer loadings. © 1993 Academic Press, Inc.