Characterization of Al2O3-ZrO2 mixed oxide catalytic supports prepared by the sol-gel method

A series of Al₂O₃-ZrO₂ mixed oxides was prepared by the sol-gel method with variable amounts of ZrO₂ between pure alumina and pure zirconia. Textural, bulk and surface characterization of the samples was carried out by nitrogen physisorption (S_BET, porosity), surface acidity, zero point charge (ZPC), thermal analysis (DSC, TGA), X-ray diffraction (XRD) and FT-Raman spectroscopy. The textural results show that at low zirconia contents, higher surface areas than those of pure alumina are obtained, and that the mixed oxides samples show a bimodal pore size distribution different from that of a mechanical mixture of the pure oxides. Also, in the zirconia-rich samples, higher surface areas than for pure zirconia are stabilized. The ZPC results indicate the formation of a surface composition equivalent to the bulk composition of the two oxides. The acidity measurements show that, as the density of acid sites in the mixed oxides increases steadily with zirconia content, a sharp increase is observed between the zirconia-rich mixed oxides and the pure ZrO₂. It appears possible then to tune the acidity of the mixed oxide by changing its composition. The XRD and FT-Raman results show that the incorporation of alumina in the support stabilizes the metastable cubic and tetragonal zirconia phases, possibly by a matrix effect which constrains the size of the ZrO₂ particles below the critical size beyond which the crystallization and transformation process to a more stable zirconia phase occurs.