Effect of sodium on the CoMo/γ-Al2O3 system. Part 2.—Influence of sodium content and preparation methods on the state of dispersion and nature of molybdenum supported on γ-Al2O3
The effect of preparation method, calcination temperature and sodium content on the dispersion of molybdenum on the surface of γ-Al2O3, as well as on the kind of molybdenum species formed thereon, has been studied using diffuse reflectance spectroscopy (d.r.s), X-ray powder analysis (X.r.), X-ray photoelectron spectroscopy (X.p.s.), analytical electron microscopy (a.e.m.) and electron paramagnetic resonance (e.p.r.).
Two series of specimens (each series containing 6 samples having various amounts of sodium) containing 12% MoO3 have been prepared by, respectively, wet and incipient wetness impregnations of sodium-doped γ-Al2O3 with ammonium paramolybdate aqueous solution.
The adsorption of MoVIvia polymeric species, the presence of MoV after calcination at 300°C and the formation of small amounts of Al2(MoO4)3 after calcination at 500°C and above have been observed for a weakly-doped specimen prepared by pore volume impregnation. The formation of Na2MoO4 is observed only for high Na content, irrespective of the preparation method used.
The dispersion of molybdenum, as measured by X.p.s., for calcination temperatures of 500°C, is virtually independent of the sodium content up to 3.6% Na; it decreases above this content. Calcining at 300°C leads to lower dispersion; sodium has no appreciable effect.
Some mechanisms explaining the results are proposed.