Structural and surface chemical properties of sol-gel derived TiO ₂-ZrO₂ oxides

Pure titania, zirconia, and binary TiO₂-ZrO₂ oxides with Ti/Zr molar ratios of 1:3, 1:1 and 3:1 were prepared by a modified sol-gel method. The samples were calcined at different temperatures and characterized by nitrogen adsorption porosimetry, XRD, DSC-TGA and FT-IR. After calcination at 450°C the pure titania and zirconia are in the crystalline phase (anatase and tetragonal, respectively) while the binary oxides remain in the amorphous phase. The binary oxides are crystallized after calcination at temperatures above 800°C, but only the TiO₂-ZrO₂ (1:1) sample is in the single phase of crystalline TiZrO₄ while the other two binary oxides contain TiZrO₄ and tetragonal (or monoclinc) zirconia and rutile phases, respectively. The crystallization temperature for the formation of ZrTiO₄ phase in the three binary oxides increases with titania content (respectively at 606, 716 and 754°C). Different from the pure oxides, which exhibit two types of surface OH group, the binary oxides after calcination at 450°C only have surface terminal OH group. The OH surface density decreases and, after passing through a minimum at about equimolar composition of Ti/Zr, increases with increasing titania content. The mixed oxides exhibit a large amount of surface acid sites.