Polyoxometalate And Its Innovative Application In Photocatalytic Degradation Of Explosive Wastewater

Polyoxometallate (POM) has advantages of high acidity, hydrothermalstability, redox property and excellent catalytic activity. These features makeit highly desirable in catalysis, especially in photoca talytic degradationprocess. However, the composite catalysts with POM are generally synthesizedbased on an impregnation method recently. Though this impregnation route isvery effective, it is industrially unfavorable because of an essential complexprocess and a long period for preparation. Moreover, the stability of thecomposite catalys ts prepared by the impregnation route needs improved further.This paper, focused on the synthesis, characterization and application ofPOM/Titania prepared by the sol-gel-solvothermal route. The above mentionedproblems in synthesis were overcame, a low-cost and effective synthesismethod developed, and the properties of composite catalysts in catalyticreaction improved. The photocatalytic activities of the prepared catalysts wereinvestigated via the degradation of typical nitro-compound dinitrotoluene(DNT) and trinitrotoluene (TNT). Photocatalytic oxidation mechanism andrecycling performance of the catalysts, coupled with reaction kinetics andmineralization degree o f the degradation for DNT and TNT were discussed indepth. The main results were concluded as follows.Synthesis and modification of the titled catalyst:(1) Composite catalystsH3PW12O40/TiO2and H4SiW12O40/TiO2were prepared by a modifiedsol-gel-hydrothermal route, which achieved the load and modification ofH3PW12O40or H4SiW12O40at the same time.(2) Rare earth elements have theadvantages for the photocatalyst modification. Ln-H3PW12O40/TiO2andLn-H4SiW12O40/TiO2were prepared using Y or La for modification.(3)Mesoporous H3PW12O40/TiO2(P123) and H4SiW12O40/TiO2(P123) weresynthesized using nonionic surfactant P123as a structure directing agent. Thispaper presented a new method for preparation of catalyst with high specificsurface area and uniform pore size distribution.(4) To enhance the surfaceacidity of catalyst is a new way to improve the photocatalytic efficiency.SO42--H3PW12O40/TiO2(P123) and SO42--H4SiW12O40/TiO2(P123) were prepared by acid treatment using1mol/L H2SO4. The surface acidity of catalystincreased substantially, and fast facilitated the charge migration from the bulkto the surface as well as the separation of electron and hole. In other word,surface modification using SO42-can improve the structure of the catalyst, andefficiently inhibit the crystal transition.The structure, morphology, optical absorption propert y, specific surfacearea and porosity of the catalysts were well characterized by X-ray diffraction(XRD), Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy,Scanning electron microscopy (SEM), UV-vis diffused reflection spectrum(UV-Vis DRS) and Nitrogen adsorption-desorption analysis. X-ray diffraction,Fourier transform infrared spectroscopy, and Raman spectroscopy resultsindicated that the catalysts have uniform anatase phase, and the primaryKeggin structures of H3PW12O40and H4SiW12O40remained intact. After themodification, t he average crystal sizes of H3PW12O40/TiO2andH4SiW12O40/TiO2decreased to9.54and7.75nm, respectively. Ultravioletvisible absorption spectroscopy showed that, the absorption wavelength ofcomposite film happened redshift and enhanced the uti lization ratio of thelight after doped H3PW12O40or H4SiW12O40in comparison with the pure TiO2.N2adsorption-desorption analysis and Scanning electron microscope showedthat the orderliness of the aperture was greatly improved and the aggregationof particles was effectively inhibited compared with TiO2.The specific surfacearea and pore volume were increased by the addition of P123. The specificsurface area of H3PW12O40/TiO2（2g P123） and H4SiW12O40/TiO2（1g P123）increased to252.16and250.0m2/g from177.9and179.3m2/g, respectively.The large surface area offered more active sites for heterogeneousphotocatalysis, which gave higher photocatalytic activity.The photocatalytic degradation of DNT and TNT wastewaters werestudied in the presence of UV light under different conditions, such as pHvalue, catalyst dosage, H3PW12O40/H4SiW12O40loadings, P123dosage and theconcentration of H2SO4. The reasons for the photocatalytic activity differencebetween different catalysts were revealed. The as-prepared catalysts were characterized by various analysis tests to show the relation between materialstructure, surface morpholog y, photoabsorption performance, etc andphotocatalytic activity. SO42--H3PW12O40/TiO2(P123) andSO42--H4SiW12O40/TiO2(P123) were chosen to evaluate the recyclability of thephotocatalysts by three tests of the DNT wastewater degradation reaction.Finally, photocatalytic degradation pathway and mineralization degree of DNTand TNT wastewaters were also discussed by identif ying and analyzing thetypes of reactants and products using Gas chromatography-mass spectrometry(GC-MS) and liquid chromatography-mass spectrometry (LC-MS) instruments.Based on the above research, this paper presented an economic andefficient method for the preparation of heterogeneous photocatalytic materials.Meanwhile, it supplied the basic research data for photocatalytic degradationof DNT and TNT wastewaters.