ABSTRACT
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.
ABSTRACT
In the present study, aqueous solutions of decamolybdocobaltate H(4)Co(2)Mo(10)O(38)(6-) heteropolyanions were prepared from molybdenum oxide, cobalt carbonate precursors and hydrogen peroxide used as oxidizing agent. The preparation was optimized adding a consecutive hydrothermal treatment at 150 °C to obtain pure H(4)Co(2)Mo(10)O(38)(6-) aqueous solutions for Co/Mo atomic ratio of 0.5. Combining quantitative Raman and UV-visible measurements and chemometric methods, it was demonstrated that a mixture of H(4)Co(2)Mo(10)O(38)(6-) and octomolybdate Mo(8)O(26)(4-) species is obtained for Co/Mo ratios lower than 0.5, and the relative quantities of H(4)Co(2)Mo(10)O(38)(6-) are determined by the presence of Mo(8)O(26)(4-) species and by the quantity of Co(2+) countercations available in the solutions to ensure the electroneutrality. As these quantities can be predicted for each Co/Mo ratio, this finding allows rationalization of the preparation of heterogeneous catalysts using impregnation by H(4)Co(2)Mo(10)O(38)(6-) aqueous solutions. Parameters relevant of the impregnation step such as the pH, the Co/Mo ratio, and the molybdenum concentration were varied to determine the domains of stability of H(4)Co(2)Mo(10)O(38)(6-) heteropolyanions after formation. Stable from pH 1 to 4.5, this dimeric Anderson species is destabilized above pH 4.5; Co(2+), monomolybdate MoO(4)(2-) ions, and precipitates are then formed. For Co/Mo ratios lower than 0.5, the relative quantity of dimer does not vary with the pH and with a change of the Co/Mo ratio consecutive to the hydrothermal treatment. On the contrary, the coproduced Mo(8)O(26)(4-) species can be transformed into other isopolymolybdates varying the pH according to their domains of stability. For all of the ratios, H(4)Co(2)Mo(10)O(38)(6-) dimers were also shown to be stable in a wide range of molybdenum concentrations.
ABSTRACT
Ever stronger environmental concerns prompt the research in the area of heterogeneous catalysis to play an ever more crucial role to produce ever cleaner fuel from the refining of petroleum effluents. The catalytic active phase is often used in a dispersed state over a porous oxide material. This paper is a review of recent progress brought by periodic density functional theory (DFT) calculations in the area of two relevant industrial supported catalysts. We focus on two important supports used in the refining industry: anatase-TiO(2) and γ-alumina. According to the various reaction conditions, the presence of H(2)O, H(2) and H(2)S may change the surface states of the support. In particular, it is crucial to know and control the hydroxylation state depending on temperature and partial pressure of reactants (H(2)O, H(2), H(2)S). The support effects on the catalytic active phases are presented for MoS(2) particles, used in hydrodesulfurization catalysis, and for Pd particles, used in hydrogenation catalysis. It is shown how the wetting property and equilibrium morphology of the active phase depend on the support. A discussion on the impact for catalytic activities is provided.
