Ionic size and metal uptake of chromium(VI), molybdenum(VI), and tungsten(VI) species on ZrO2-based catalyst precursors

Zirconium-supported Me(VI) oxide (Me = Cr, Mo, W) was prepared by the equil ibrium 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, sur face area determinations, electron scanning microscopy and analytical metho ds. 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 stru cture of hydrous zirconium oxide. The depth of penetration of the Cr(VI), M o(VI), and W(VI) anionic species in the grains of the ZrO2(383) support was estimated by quantitative XPS measurements and by electron microscopy dete rminations. The nuclearity of the adsorbed species is also briefly discusse d.