INTERACTION OF OXOMOLYBDENUM SPECIES WITH GAMMA-C-AL2O3 AND GAMMA-C-AL2O3 MODIFIED BY SILICON .2. THE MOO3/GAMMA-C-AL2O3 AND MOO3/SIO2/GAMMA-C-AL2O3 SYSTEMS

Several spectroscopic techniques, i.e., FTIR for characterizing OH groups, XPS for determining Mo repartition, Mo-95 NMR of the impregnating solution in interaction with the prepared supports, laser Raman spectroscopy for detecting MoO3, and chemical titration of basic hydroxyls, have been used to investigate the chemical processes occurring during adsorption and deposition of molybdate species on gamma(c)-Al2O3 and SiO2/gamma(c)-Al2O3 supports. Two different mechanisms of interaction between the oxomolybdenum species and Al2O3 are evident in the Mo monolayer-like coverage up to a limit of 3.7 Mo atom.nm-2 on the pure gamma(c)-Al2O3 surface. Basic OH groups of alumina are implied in the heptamer fixation up to 2 Mo atom-nm-2. These reactive basic OH groups can be progressively removed by Si grafting before Mo impregnation which, therefore, lowers the limit of Mo monolayer coverage on the SiO2/-gamma(c)-Al2O3 supports. The monolayer limit is approximately 1.5 Mo atom.nm-2 on SiO2/gamma(c)-Al2O3 supports with Si contents equal to or higher than 1.0 Si atom.nm-2. On pure alumina, between 2.0 and 3.7 Mo atom.nm-2, another deposition mechanism applies in which the residual basic OH groups, as well as the other OH groups, do not participate.