Synthesis And Photocatalytic Properties Of Polymetallic Oxide / Mesoporous TiO <2> Hybrid Materials

Polyoxometalatates(POM) and titanium dioxide (TiO2) are two kinds of green photocatalysts, which were widely used in photocatalysis filed and show much higher UV-photocatalytic activity. However, most of POM catalysis suffer from their inability to be recycled due to high solubility in polar solvent, which restricts their technological applications. At the same time, high photocatalytic activity TiO2 has deficiencies of the recombination of photoinduced carriers. It is reported that there is an synergistic effect between POM and TiO2, which is to say that the recombination of h+ and e- was inhibited by the POM due to interfacial electron transfer from photoexcited TiO2 to POM. The pohotocatalytic activities of systems are improved greatly. Therefor POM loading on TiO2 is not only benefit recycling and utilizing of POM, but also restricting the recombination of photoinduced carriers inTiO2, which may be obtain the high photocatalytical efficiency.At present, the conventional load method are wet impregnation and Sol-Gel method which lack of the strong mutual-interaction between POM and TiO2. Thus, POM is easy to be lost in using process. The combination of POM and TiO2 through the covalent bond would make the firm combination between POM and TiO2, obtaining the composits photocatalysts with the high practical application value. Based on this point, Mesoporous TiO2 with higher anatase crystallinity was firstly synthesized vis Evaporation-Induced Self-Assembly by using acetonitrile as solvent. And then we introduce both direct synthesis and post-synthesis methods to synthesize POM/TiO2 mesoporous hybrid materials, realizing POM support on TiO2. The supported POM/TiO2 were characterization by IR, XRD, BET, TEM, ICP and finally their photocatalytic activities were measured by the photodegradation experiments of methyl orange (MO).1. The synthesis of mesoporous TiO2 and the direct synthesis of TBAPW11/mesoporous TiO2 hybrid materialIn order to bring POM(TBAPW11), firstly we discuss the synthesis of mesoporous TiO2 by using acetonitrile as solvent. The poor ordered mesoporous TiO2 materials were prepared by using TiCl4 and Ti(OBu)4 as titanium precursors in the presence of triblock copolymer P123 vis EISA method. Effects of the different titanium source ratio and the different calcination temperature on the mesostructure and the crystallization degree of sample were investigated. The results indicate that the mesostructure obtained in acetonitrile is similar to that obtained in ethanol. The uniform and cylinder-like channels and anatase TiO2 with higher crystallinity all can be obtained under such conditions, but the crystallinity of sample is higher in acetonitrile. The optimun molar ratio of TiCl4/Ti(OBu)4=1:1. With the increase of calcination temperature, the crystallinity of sample was enganced. The calcination at 350℃are the optimal conditions for the sample synthesized in acetonitrile.Furthermore, we bring TBAPW11 as POM precursor species in the synthesis of mesoporous TiO2 by using acetonitrile as solvent to link mesoporous TiO2 skeleton。The samples were investigated by IR, N2 adsorption-desorption. The results show that: the loading of POM in TBAPW11/mesoporous TiO2 hybrid material is extremely low and it can’t change as increasing TBAPW11 dosage. The result demonstrates that it is difficult to covalently bond between TiO2 and POM.2. Post-synthesis of POM/mesoporous TiO2 Hybrid MaterialsPOM was introduced to the parent mesoporous TiO2 of dealed with in various ways by wet impregnation, and then we make POM and Ti species or Ti-OH on surface of TiO2 react at a definit temperature, thereby POM will graft onto the surface of TiO2.In all kinds of parents, calcination stripped TiO2 is difficult to graft NaPW11 or TBAPW11 Si2 due to its high degree of crystallization and polycondensation as well as low Ti-OH on its surface.Ethanol refluxing stripped TiO2 with lower crystallization and lower skeleton condensation degree can link NaPW11 and TBAPW11Si2, then result in NaPW11/TiO2(e) and TBAPW11Si2/TiO2(e) hybrid material. Monovacant NaPW11 reacts with Ti-OH which is on the surface of TiO2, Ti atomics enter into the vacancy of NaPW11 to link it, but the loadings of TBAPW11Si2 much lower. Grafting TBAPW11Si2 into the channels of TiO2 via the condensation of Ti-OH groups and Si-OH groups on both surfaces. Calcination at a difinite temperature is essential for the condensation. Compared to the parent, the crystallinity of Hybrid Materials is higher while pore volume, pore size and surface area decend.The unstripped TiO2, whose skeleton is not crystallization and structure is loose, is easily generated hydrolysis Ti species in the process of impregnation NaPW11. Under acidic conditions these species react with NaPW11 to generate the protonated monomer or class of dimer of PW11Ti. The latter then with Ti-OH on the surface of TiO2 occured condensation and bond. After demoulding template at 350℃, we received NaPW11/TiO2(uc) hybrid samples with higher degree of crystallinity. The loading of POM in it is higher than that obtained by refluxing stripped NaPW11/TiO2(e). However, during the preparation, the dosage of NaPW11 cannot too large, otherwise it will cause TiO2 hydrolysis and can’t obtain the hybrid samples.3. The photocatalytic activity of POM/mesoporous TiO2 Hybrid MaterialsThe UV photocatalytic performance of the obtanied smples were examined by Azo dyes MO as target degragation products.Anatase TiO2 made in acetonitrile as solvent via EISA method has good photocatalytic activities. Higher crystallinity provides better photocatalytic efficiency. Although its crystallinity is siginificantly less than that of commercial photocatalyst P25, the activity of the former which own high surface area and tunnel structure approch the latter.NaPW11/TiO2(uc) hybrid material which gripped NaPW11 by the unstripped TiO2 as the parent and TBAPW11Si2/TiO2(e) hybrid material which gripped TBAPW11Si2 by ethanol refluxing unstripped TiO2 all show higher photocatalytic activities than their parent, especially the latter even exceed commercial photocatalyst P25. It exhibits the superiority of introduced POM. POM can not be found lost afer the Hybrid Materials is used, and can be recovered and reused.The photocatalytic activities of POM/mesoporous TiO2 Hybrid Materials possesses much high photocatalytic activity than pure TiO2, which attributed to the synergistic effect resulting from the combination of TiO2 and POM. In the POM-TiO2 system, the recombination of hole(h+) and electron(e-) was inhibited due to interfacial electron transfer from TiO2 to POM. Therefore, the photocatalytic activities of POM/TiO2 are improved greatly.