Synthesis And Characterization Of Micro-and Mesoporous Titania-Silica Composite Materials

With ordered meso-pore size range of pore structure and large specific surfacearea, mesoporous TiO₂ materials show a lot of excellent properties and its broadapplication prospect in photocatalysis, catalyst carrier, chemical sensors and so on.But thermal stability of mesoporous TiO₂ is not as good as mesoporous silica, whichlimits its application in high temperature field. Considering good thermal stability ofsilica materials, it may be one of the solutions to increase the thermal stability ofmesoporous TiO₂, that doping silicon into TiO₂ synthesizes a titania-silica composite.In this papers, using surfactant as a template, the porous precursors of titania-silica composite were synthesized by co-hydrolysis process when organic alkoxidesof titanium and silicon as raw materials (tetrabutyl orthotitanate TBOT and tetraethylorthosilicate TEOS) and sulfuric acid as a catalyst, or a process route of two-stephydrolysis when inorganic compounds of titanium and silicon as raw materials(titanium sulfate Ti (SO₄)₂ and sodium silicate Na₂SiO₃), respectively. After that theprecursors were calcined to remove the template. The titania-silica compositematerials with micro- and mesoporous multilevel structure, large specific surfacearea, high thermal stability and photocatalytic activity were obtained. Themechanism of stabilizing effect of doping silicon was explored preliminarily.The experimental results show that: (1) Using cetyl trimethyl ammonium bromide (CTAB) as template, the samplessynthesized from organic raw materials have a short-range ordered porous structureconstituted with more micropores. Using PEO-PPO-PEO (P123) as template, whichcan accelerate to form mesopore, the synthesized materials have more mesoporousstructure. The optimized synthesis conditions include hydrolyzing at pH value of2.5～3.0 and 60℃for 2h.(2)The samples synthesized from inorganic raw materials and cetyl trimethylammonium bromide (CTAB) as template have a porous structure with moremesopores. With the addition of octadecyl amine (ODA), the pore sizes and thespecific surface area of the samples are increased evidently. The optimized synthesisconditions of precursor are: the concentration of sodium silicate is 0.1 mol/L, theconcentration of titanium sulfate is 0.1mol/L, the hydrolyzing and condensing oftitanium sulfate carry out at pH value 1, at room temperature, then hydrothermaltreatment of the precursor is maintained for 24h at 125℃.(3)A heat treatment of the two-step calcination can remove the organic templateeffectively, and bring about less destruction of the pore structure. Because of siliconwith titanium together forming the skeleton, the porous structure is stabilized. Thesample calcined by the two-step calcination of 350℃C for 4h and 750℃fbr 4hremains a good porous structure and keeps a large specific surface area (369 m²/g).The heat treatment at higher temperature is profit to obtain product with moremesoporous structure.(4) Micro- and mesoporous titania-silica composite materials have a strongadsorption capacity and photocatalysis to degrade methylene blue. Doping Silica notonly increases the specific surface area which is good for photocatalytic reaction, butalso increases the thermal stability of mesoporous TiO₂, and prevents thetransformation of TiO₂ from anatase to rutile phase. The sample calcined at the hightemperature of 750℃for 4h, whose template was removed completely, has highphotocatalytic activity because of a high degree of crystallinity and keeping ofanatase phase. Micro- and mesoporous titania-silica composite materials are synthesized byusing inorganic compounds of titanium and silicon as raw materials instead oforganic materials. The process is simple and low cost. Silica in the compositematerials improves its thermal stability, and results in a titania-silica compositematerials with new structure, which is expected to have some new applications.