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

Abstract

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 Mo^VIvia polymeric species, the presence of Mo^V 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.