Nickel(II) Nitrate vs. Acetate: influence of the Precursor on the Structure and Reducibility of Ni/MCM-41 and Ni/Al-MCM-41 Catalysts
Résumé
Nickel(II) nitrate is the nickel precursor generally selected to prep. oxide-supported catalysts, due to its low cost, high soly. in water and decompn. at moderate temps. However, its decompn. produces NOx, and it is known to melt and redistribute over the support upon heating, leading to poorly dispersed metal particles after redn. and to their partial expulsion from the porous support system. The benefits of using NiII acetate as an alternative to NiII nitrate are explored by investigating a series of model Ni catalysts (1-5 wt %) prepd. from the mesoporous siliceous supports MCM-41 and Al-MCM-41. The problems encountered with NiII nitrate, in terms of particle size and redistribution of nickel, are not obsd. with NiII acetate. In the case of MCM-41-supported systems, metal particles are well dispersed and remain located within the porous support. The higher pH of the impregnation soln., favoring electrostatic interactions between nickel ions and the support, may account for the higher metal dispersion. The formation of silicate-like species in strong interaction with the support does not appear prominent on siliceous MCM-41 when nickel acetate is the precursor, but seems to be enhanced for higher Ni contents. Finally, Al-MCM-41 supports are more sensitive to the exposure to nickel acetate than their siliceous analogs because acetate ions are able to ext. Al ions from the support, resulting in a partial disruption of the support organization and to the formation of nickel silicate species. [on SciFinder(R)]