Impregnation of decamolybdocobaltate heteropolyanions over γ-alumina: detailed description of the physico-chemical phenomena.

In this work, the physicochemical phenomena occurring during equilibrium impregnation of Anderson-like decamolybdocobaltate H(4)Co(2)Mo(10)O(38)(6-) heteropolyanion aqueous solutions over γ-Al(2)O(3) were described in detail comprising chemical analysis, pH measurements, Raman, and UV-vis spectra. For a surface density lower than 2.5 Mo atoms nm(-2), the buffering effect of the support leads to decomposition of H(4)Co(2)Mo(10)O(38)(6-) into monomolybdates MoO(4)(2-) and Co(2+) cobalt cations that are then adsorbed by electrostatic and covalent interactions with γ-alumina. Between 2.5 and 3.8 Mo atoms nm(-2), MoO(4)(2-) monomers condense into heptamolybdates Mo(7)O(24)(6-) that are then adsorbed by electrostatic interactions and H(4)Co(2)Mo(10)O(38)(6-) becomes stable because of the lowering of the pH. Above 3.8 Mo atoms nm(-2), the quantities of adsorbed MoO(4)(2-) and Mo(7)O(24)(6-) become much smaller than that of electrostatically adsorbed H(4)Co(2)Mo(10)O(38)(6-). Adsorption of preserved H(4)Co(2)Mo(10)O(38)(6-) could be consecutive to the decomposition of the first molecules leading to prior adsorption of MoO(4)(2-) and Co(2+), and decrease in the buffering effect of γ-Al(2)O(3) and in the pH value. For dry impregnation, the same physicochemical phenomena occur considering a given Mo surface density. The methodology used in this work to rationalize the preparation of hydrotreatment catalysts from H(4)Co(2)Mo(10)O(38)(6-) heteropolyanions can be transposed to any supported catalyst.