Surface characterization of sulfate, molybdate, and tungstate promoted TiO2-ZrO2 solid acid catalysts by XPS and other techniques Original Research Article

Titania–zirconia binary oxide supported sulfate, molybdate, and tungstate promoted solid acid catalysts were prepared by suspending the hydrous mixed oxide support in aqueous solutions of sulfuric acid, ammonium heptamolybdate, and ammonium metatungstate. The suspensions were refluxed at 383 K followed by evaporation of the water, drying and calcination at 1073 K. The resulting materials were characterized by means of X-ray powder diffraction, BET surface area, FT-Raman, and X-ray photoelectron spectroscopy methods. The calcined TiO2-ZrO2 binary oxide, X-ray amorphous at 773 K, forms a definite crystalline ZrTiO4 compound at 1073 K. The XRD measurements further reveal the presence of Ti2(SO4)3 and Zr(SO4)2 compounds in the case of SO42−/TiO2-ZrO2 sample and Zr(MoO4)2 and Zr(WO4)2, respectively, in the case of molybdenum- and tungsten-oxide promoted catalysts. The FT-Raman results also support these observations. The XPS measurements show peak broadening and shift in the binding energies of O 1s, Ti 2p, and Zr 3d lines in the case of sulfated catalyst. The characterization results indicate that the impregnated sulfate ion shows a relatively strong influence on the physicochemical properties of the TiO2-ZrO2 mixed oxide, which is followed by molybdate.