Synthesis, characterization, and photocatalytic activity of titania and niobia mesoporous molecular sieves

Mesoporous titania and niobia molecular sieves were prepared by a ligand-assisted templating method. For comparison, titania samples were also prepared by simple hydrolysis and condensation of Ti alkoxide in the absence of templating agents. All materials were thoroughly characterized by UV-vis reflectance spectroscopy, N-2 adsorption, and X-ray diffraction. The UV absorption thresholds, and thus the optical band gaps, of as-synthesized mesoporous titania and niobia were similar to those of bulk phase, crystalline reference materials. Adsorption of N-2 showed that extremely high surface area materials were synthesized by the templating method as long as thermal treatments were moderate. Higher temperature caused loss in surface area and partial crystallization of the sample. Elemental analysis revealed that neither high-temperature calcination nor solvent extraction removed the phosphorus component of the template used in the synthesis of mesoporous titania; However, solvent extraction of the template was effective for the mesoporous niobia sample. The transition metal oxides were tested as photocatalysts in the Liquid-phase oxidative dehydrogenation of 2-propanol to acetone. The observed quantum yield of the reaction was 0.45 over Degussa P25, a standard titania catalyst. However, mesoporous titania converted 2-propanol with a very low quantum yield of 0.0026. Amorphous titania synthesized by alkoxide hydrolysis also exhibited a very low photocatalytic activity for the reaction. The quantum yield increased as the amorphous titania was crystallized to greater extents. A very low quantum yield was also found for the mesoporous niobia sample compared to a crystalline standard. Apparently, the surface reactivities of the poorly crystallized samples were suppressed by defects that act as electron-hole traps.