Silica-supported bismuth molybdate catalysts obtained by the sol-gel process via silicon alkoxides

Cauzzi, D;Deltratti, M;Devillers, Michel;Predieri, G;Tiripicchio, A;et.al.
(1998) 7th International Symposium on Scientific Bases for the Preparation of Heterogeneous Catalysts — Location: LOUVAIN LA NEUVE(Belgium) (1.September.1998)

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Authors
  • Cauzzi, D
    Author
  • Deltratti, M
    Author
  • Devillers, Michelorcid-logoUCLouvain
    Author
  • Predieri, G
    Author
  • Tirions, OlivierUCLouvain
    Author
  • Tiripicchio, A
    Author
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Abstract
The sol-gel route is applied to the preparation, in a one-step process, of bismuth molybdates highly dispersed into silica xerogels produced by hydrolysis/condensation reactions involving silicon alkoxides. The samples correspond to different Bi/Mo and (Bi+Mo)/Si stoichiometric ratios, according to the different bismuth molybdate phases (alpha-Bi2Mo3O12, beta-Bi2Mo2O9, gamma-Bi2MoO6) and for various metal (Mo+Bi) loadings (2, 5, 10, 25 or 50 mol.% with respect to silica). The samples were characterized by X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray microanalysis coupled with scanning electron microscopy (EDX-SEM). The xerogels containing low molybdate loadings (2, 5, 10 mol %) exhibited high S-BET values (300 to 700 m(2)/g), but the expected phase was never detected as pure in these materials. The XRD spectra of the xerogels characterized by high molybdate loadings (25, 50 mol %) clearly showed the formation of complex mixtures, the a-phase being predominant at 600 degrees C when the starting Bi/Mo ratio was 0.66, while phase mixtures were observed in all the other cases. Whereas the Bi/Mo ratios determined by XPS were systematically much lower than the expected ones, more satisfactory values were observed using EDX-SEM. This apparent discrepancy reflects the difference in surface depths explored by the two techniques and suggests Mo enrichment of the outer surface layers. This behaviour is thought to he related to the selective migration of ions throughout the incoming silica network, resulting in heterogeneous distributions of Bi-based and molybdate ions, that could actually lead to different phases and anomalous surface Bi/Mo ratios.
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Cauzzi, D., Deltratti, M., Devillers, M., Predieri, G., Tirions, O., & Tiripicchio, A. (1998). Silica-supported bismuth molybdate catalysts obtained by the sol-gel process via silicon alkoxides. Studies in Surface Science and Catalysis, 118, 699-706. https://hdl.handle.net/2078.5/143246 (Original work published 1998)