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

TiO2 (30-80 wt.%) and ZrO2 (48-75 wt.%) 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 ZrOx-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 6-10 nm 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 TiO2 increased to >100 nm 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 8.5 nm crystals (TiO2-CSD).