Synthesis and Characterization of High-Surface-Area Silica–Titania Monoliths

Hybrid SiO₂–TiO₂ mesoporous monoliths were synthesized via sol–gel method. The water for hydrolysis of titanium precursor has been obtained in situ via esterification reaction between acid and alcohol without use of additional agents. The titania content in the samples has been varied from 2 to 40 M% molar content with respect to silica. The nitrogen adsorption isotherms showed that monoliths containing 2 M% of Ti retain a large surface area and micropores with size peaking in the 1 to 2 nm interval. For larger Ti content, a bimodal average micropores distribution is observed. The surface area decreases for titania content larger than 10 M%. The Raman spectra taken with excitation lines in the 244–785 nm interval and the UV–vis measurements demonstrate that in the 2 M% sample Ti is atomically dispersed and fully incorporated in the SiO₂ framework and that the local structure is similar to that of titanium silicalite. This conclusion is further confirmed by XAS measurements. Also, the reactivity toward hydrogen peroxide is similar. For larger Ti concentrations, the presence of segregated anatase particles grown in restricted space is observed, as demonstrated by XRD, TEM, Raman, and UV–vis measurements. The surface properties of the silica matrix and of the embedded anatase nanoparticles have been fully characterized by FTIR spectra of CO probe adsorbed at 100 K. Unlike the samples with 5–20 M% Ti, the 40 M% Ti system is fully amorphous, and only small traces of crystalline TiO₂ are present