Synthesis of mixed ZrO2–TiO2 oxides by sol–gel: Microstructural characterization and infrared spectroscopy studies of NOx

Single ZrO2, TiO2 and mixed ZrO2–TiO2 oxides with various Zr:Ti ratio (90:10, 50:50 and 10:90) were prepared by the sol–gel method. The samples were characterized by DSC, nitrogen adsorption, SEM, XRD, molecular simulation and DRIFT. According to the ZrO2/TiO2 ratio, the experimental results reveal that the TiO2 loading into ZrO2 had effects on shifts of exothermic peaks towards higher temperature, attributed to transition from amorphous to crystalline phase except in the ZT50 xerogel. The optimum concentration of titania into mixed ZrO2–TiO2 oxides to obtain high surface area (∼230 m2/g) and highly acidic surface was in the equimolar composition. An amorphous XRD pattern was observed in the ZT50 sample, this behaviour was related to the mixed Zr–O–Ti bond formed, which caused an atomic disorder in the structure of the sample and this result was confirmed by molecular simulation. All the oxides were constituted by microspheroid particles regardless of the Zr/Ti molar ratio and crystalline phase. Different kinds of surface nitrates (NOx, x = 2, 3) species were observed on the mixed ZrO2–TiO2 samples; these species are formed on Lewis sites and the amount of these species on the surface oxide is intimately related to the support acidity, which in turn depends on the specific area of the samples, as well as, on the Zr/Ti ratio and the Zr–O–Ti bond population. Bridged and bidentate NO3− species are adsorbed on the Zr-rich samples.

Graphical abstractSingle ZrO2, TiO2 and mixed ZrO2-TiO2 oxides with various Zr:Ti composition (90:10, 50:50 and 10:90) were prepared by sol-gel method. The samples were characterized by DSC, nitrogen adsorption, SEM, XRD, molecular simulation and DRIFT. An amorphous XRD pattern was observed in the ZT50 sample, this behavior was related to the Zr-O-Ti mixed bond formed which caused an atomic disorder in the structure of the sample and this result was confirmed by molecular simulation. Different kinds of surface nitrates (NOx, x = 2, 3) species were observed on the ZrO2-TiO2 samples; these species are formed on Lewis sites and the amount of these species on the oxide surfaces runs parallel with the surface area, Zr/Ti ratio and mixed Zr-O-Ti bond population. On the Zr-rich samples the bridged and bidentate NO3− species are the preferred coordination for these mixed oxides. Figure optionsDownload full-size imageDownload as PowerPoint slide