Preparation And Study Of Photocatalytic Properties On POM-based TiO₂ Or Ta₂O₅ Mesoporous And Microporous Composite

Two types of the solid materials containing polyoxometalates (POMs) including mesoporous Na₄W₁₀O₃₂/TiO₂ (anatase phase) and microporous H₃PW₁₂O₄₀/Ta₂O₅ (amorphous) were prepared by the sol-gel and programmed temperature hydrothermal treatment. UV-vis diffuse reflectance spectra (UV/DRS), infrared (IR) spectra, Raman spectra, ICP-AES, powder X-ray diffraction (XRD), were used to characterize the composition, structure and the integrity of the starting POMs during the process of the preparation. The morphology and surface physicochemical properties of as-prepared solid photocatalytic materials were analyzed by transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM) and nitrogen adsorption analysis. The results indicated that as-prepared materials exhibited porous structures. The average pore size of the hybrid catalyst Na₄W₁₀O₃₂/TiO₂ is about 3.74 nm and the particles of the composite are well-distributed although aggregation among particles occurred, in addition, the estimated particle size was in the range of 8 to 10 nm. The porous structures, morphology and the surface physicochemical properties of H₃PW₁₂O₄₀/Ta₂O₅ are different according to the increase of the amount of H3PW12O40 in the composition. The pore sizes are in the range of 1.15-1.91 nm although there are parts of pores with a size of ca. 2.51 nm existing in the composition with a higher loading of H3PW12O40. This mesoporous structure is attributed to the conglomeration of particles.Photocatalytic properties of the particles obtained here were tested by the degradation of polyacrylamide (PAM), salicylic acid (SA), Rhodamoine B (RB), and 4-nitrophenol (4-NP) under UV and visible-light irradiation. Obvious decreases of the viscosities were determined after irradiating the suspension of PAM and Na₄W₁₀O₃₂/TiO₂ under visible-light. On the other hand, the degradation of SA and RB are also be achieved by visible-light irradiation with H₃PW₁₂O₄₀/Ta₂O₅ in the reaction system, respectively. The H₃PW₁₂O₄₀/Ta₂O₅ composite can decompose 4-NP more efficiently than Ta2O5 and H3PW12O40/SiO2 under UV irradiation.Phase of Na₄W₁₀O₃₂/TiO₂ changed from amorphous into anatase after the programmed temperature treatment. This treatment can enhance the photocatalytic activity although phase transfer did not occur during the preparation of H₃PW₁₂O₄₀/Ta₂O₅. The photocatalytic activity of the solid photocatalytic materials is attributed to the structure of POMs, unique physicochemical properties of the porous materials, sensitized effect of the dye molecular adsorbed on the surface of the catalyst, and the synergistic effect yielded by the combination of POM and TiO₂ and Ta₂O₅, respectively.