High loading TiO₂ and ZrO₂ nanocrystals ensembles inside the mesopores of SBA-15: Preparation, texture and stability

TiO₂ (30-80wt.%) and ZrO₂ (48-75wt.%) were inserted inside the pores of SBA-15 mesostructured silica host by chemical solution decomposition (CSD) or internal hydrolysis (IH) of the corresponding alkoxides. Both methods yielded composites with 85-94% TiO₂ crystallinity (anatase). In case of ZrO₂, CSD yielded >95% crystallinity (tetragonal phase), while IH gave an amorphous ZrO_x-phase that does not crystallize up to 1073 K. The guest Ti(Zr)-oxide phases did not block the SBA-15 pores, and their surface was fully accessible for nitrogen adsorption. Calcination in air of TiO₂/SBA-15 and ZrO₂/SBA-15 (CSD) composites up to 1073 K did not change the nanocrystals structure and slightly increased the domain size derived from XRD data from 5.0-8.5 to 6-10nm for TiO₂ (IH and CSD) and from 4.5 to 6.5 nm for ZrO₂ (CSD). After the same treatment the crystals domain size of bulk reference TiO ₂ increased to >100nm with full conversion to rutile polymorph and of reference bulk ZrO₂ - to 20-25 nm with partial conversion to monoclinic modification. Thorough characterization of the texture, structure, location and dispersion by HRTEM, SAXS, EDS, SEM, XRD, N₂-adsorption methods allowed evaluation of the assembling mode of TiO₂ and ZrO₂ inside SBA-15 nanotubes: amorphous layer, ensemble of small 4-5 nm crystals (TiO₂-IH and ZrO₂-CSD) or single large 8.5nm crystals (TiO₂-CSD).