On the mechanism of zirconia textural stabilization by siliceous species during digestion under basic conditions
Résumé
The mechanism of zirconia textural stabilization by means of basic reflux was studied. In agreement with earlier studies, zirconia was modified by silicon extracted from the glass material in the same manner as if silicon compounds (e.g., TEOS, SiO2) had been added to the reaction mixtures. The reflux in a Teflon bulb without the addition of silicon did not lead to improved textural properties. EXAFS study of both refluxed and nonrefluxed specimens showed their similarity and probably some dehydration of the precipitate. The FTIR, NMR, and XPS spectroscopic characterizations suggested that silicon was strongly segregated on the surface, probably in the form of silicate moieties anchored on zirconia. A reflux stage was necessary to achieve efficient spreading of the silicate over the surface of the precipitate. For the solids calcined at 1173 K, the surface stabilization corresponded to the formation of a “metastable monolayer” coverage, similar to that observed for zirconias doped by other oxoanions, such as molybdate or tungstate. Isooctane hydrocracking and o-xylene hydrogenation were studied in the presence of Pt supported on the oxides as prepared. Hydrodesulfurization of thiophene was carried out in the presence of Mo and NiMo sulphides using the same supports. In all three reactions studied, the digested zirconia support demonstrated properties similar to those of pure silica.