Ionic Size and Metal Uptake of Chromium(VI), Molybdenum(VI), and Tungsten(VI) Species on ZrO₂-Based Catalyst Precursors

Abstract

Zirconium-supported Me(VI) oxide (Me = Cr, Mo, W) was prepared by the equilibrium adsorption method at pH values 2 and 8 by using as a support either hydrous [ZrO2(383)] or thermally pretreated zirconium oxide [ZrO2(T)]. The prepared solids were characterized by X-ray photoelectron spectroscopy, surface area determinations, electron scanning microscopy and analytical methods. Samples of definite grain size ranges of ZrO2(383) were also compared. The results show that the W-containing samples based on ZrO2(383) differed from Cr- and Mo-samples in the uptake dependence on pH. Despite its higher specific surface area, ZrO2(383) had a smaller W uptake than ZrO2(T). This behavior depends on the polyoxometalate anion size and the microporous structure of hydrous zirconium oxide. The depth of penetration of the Cr(VI), Mo(VI), and W(VI) anionic species in the grains of the ZrO2(383) support was estimated by quantitative XPS measurements and by electron microscopy determinations. The nuclearity of the adsorbed species is also briefly discussed.

Spectroscopic characterization of tungstated zirconia prepared by equilibrium adsorption from hydrogen peroxide solutions of tungsten(VI) precursors

“…The W surface densities in the as prepared samples using solutions 1 and 2 as precursors have been determined in the range of 3.4-3.5 and 2.4-2.6 W/nm 2 , respectively. The latter value is higher than that reported in the literature (0.4 W/nm 2 ) for a system obtained by equilibrium adsorption from aqueous nonperoxide solutions at pH 2 using hydrated zirconia as a starting material [14,15].…”

“…The Raman spectra of the equilibrated solutions have shown that the adsorbing species in solution 1 [14] concluded that the size of the metatungstate anion is comparable with the main micropore radius of amorphous zirconia. Because the size of the metatungstate ion is larger than that of the peroxo complex, limited adsorption from solution 2 occurs which lowers the uptake.…”

“…In general, the equilibrium adsorption procedures allow better dispersion of the active phase and this method has been used by Valigi and coworkers [13][14][15][16] to prepare various zirconia-based catalysts starting from amorphous ZrO x (OH) 4-2x . These authors demonstrated the role of the solution pH in determining both the adsorption properties of the support and the predominant ionic form of the precursor.…”

“…The latter is particularly important for tungsten, which can exist as monomeric species, [WO 4 ] 2-, in solutions at pH higher than 8, whereas oligomeric species, [H x W y O z ] n-, are formed in acidic medium [17]. The adsorption of anionic species on zirconia is favored in solutions with a pH below 6.7 [18], the PZC of zirconia [14]. The large isopolyanions formed under these conditions suffer diffusion limitation into the micropores of the amorphous zirconium oxyhydroxide and can be adsorbed only on the external surface of the support grains [14].…”

“…The adsorption of anionic species on zirconia is favored in solutions with a pH below 6.7 [18], the PZC of zirconia [14]. The large isopolyanions formed under these conditions suffer diffusion limitation into the micropores of the amorphous zirconium oxyhydroxide and can be adsorbed only on the external surface of the support grains [14]. Therefore, the application of anionic precursors with reduced nuclearity for synthesis of zirconiasupported WO x species can increase the ion-exchange capacity of the support and improve the dispersion of the adsorbed species.…”