Preparation And Green Catalytic Characterisitics Of Highly Dispersed TiO₂@SiO₂ Catalysts

Highly isolated TiO₂@SiO₂ composite oxides were prepared by an improved sol-gel strategy using TBOT precursor and ethanol solvent. The prepared photocatalysts were characterized by FTIR, XRD, XPS, TEM, SAED and Raman techniques and the configuration characteristics of highly isolated TiO₂@SiO₂ nanoparticles were systematically investigated. The green catalytic characteristics of the highly isolated TiO₂@SiO₂ composite oxides were studied and some innovative research results were abtained.It is shown that the distribution of Ti species in SiO₂ matrix can be regulated and controlled at a molecular level to prepare highly isolated TiO₂@SiO₂ nanoparticles. In the XRD patterns of the catalyst samples, no peaks of rutile or anatase TiO₂ are found, which illustrates that TiO₂ does not exist in an aggregate state in SiO₂ matrix. TiO₂-SiO₂ phase prevents the highly isolated TiO₂ phase from aggregating and growing in SiO₂ matrix. Thus TiO₂ exists mainly in an amorphous state. The peak centering at 210 nm in the DRS of the catalyst samples corresponds to the LMCT of {Ti⁴⁺-O^2-}of tetrahedral titanium oxide species, the wide peak above 210 nm is attributed to {TiO_x, x=5,6}. Tetrahedral titanium species are active sites for selective oxidation while other Ti-0 species with high coordination numbers, {TiO_x, x=5,6}, are active sites for complete oxidation of cyclohexane. AFM images of the samples illustrate that the catalyst particles are approximatively round with tough surfaces and have a relatively unform average diameter of about 30 nm. No clear diffraction rings are found in the SEM image of the samples because TiO₂ exists in an amorphorous state. Meanwhile, in TEM images of certain samples, a small amount of silica nanotubes are discovered. The adsorption vibration peak of Ti-O-Si band centering at 940-980 cm^-1 is found and the result illustrates that Ti-0 species interweave with SiO₂ matrix with Ti-O-Si bands. In the XPS spectrum, Ti2_[2P(3/2)] exhibits an obvious increasing tendency indicating that electronegative Si atoms replace the Ti atoms in Ti-O-Ti networks. No characteristic peaks of crystal TiO₂ were found in the Raman spectrum, which corresponds well with XRD patterns.Photocatalytic oxofunctionalization properties of the prepared catalysts were investigated and the mechanism of activating molecular oxygen and photocatalytic reactions was initially discussed. The results illustrate that highly isolated TiO₂@SiO₂ catalysts efficiently activate molecular oxygen to promote the selective oxidation of light alkenes under mild conditions.The distribution state of Ti species, active sites, selective oxidation sites and physical and chemical properties of catalyst surfaces are important factors influencing green catalytic characters. Under optimum reaction conditions, the selectivity of cyclohexanol versus cyclohexanone could abtain 30%. The possible reaction mechanism is proposed based on the experiment results and catalysis theory. Lattice oxygen or absorbed oxygen on the surface of the catalyst are activitated and transformed into active oxygen species by capturing the carrier generated by tetrahedral titanium species under irradiation. O3" species that are formed through activation of absorped oxygen are active species for formation of cyclohexanol. The influence of solvent media was also investigated. The results suggest that the introducition of water solvent changes the reacting mechanism. In the water solvent reacting system, the selectivity of cyclohexanol versus cyclohexanone increases with the increasement of water volume and the maximum selectivity of 51.7% was abtained. In the acetic acid reacting system, the ionization of acetic acid influences the stability of radicals and the competition for absorption sites between solvent media and cyclohexane lead to the significant decline of the selectivity of cyclohexanol versus cyclohexanone.