Studies On The Photocatalytic Oxidation Reactions Of Heptane On Doped TiO₂ Nanoparticles

Semiconductor photocatalysts have attracted considerable attention of theworkers in environmental science due to their many significant advantages such aslower consumption, mild reaction condition, simple operation and decrease ofsecondary pollution in photodegradation of organic pollutants. When thesemiconductor particle is illuminated by light having higher energy than its bandgap energy, an electron is excited from the conduction band to valence band toreduce the molecule adsorbed on the surface of particles and the hole in valenceband can act as an oxidant. TiO2 is the most promising green catalyst due to itshigher photodegradation ability in decomposing of the organic pollutants into CO2,H2O and some mineral acid as well as its many advantages (i.e., safety, low costand no pollution). The photocatalytic characteristics of semiconductor particleshave been verified by many studies. As we have known, the photocatalytic activityof semiconductor particles is inversely related to the particle size. When theparticle size is decreased to nanosize level, its photocatalytic activity issignificantly enhanced due to volume effect and quantum size effect. Now theinterest is growing because of the potential application to contaminant control incontained air atmospheres as found in aircraft and spacecraft, office buildings andfactories. Under moderate conditions (room temperature, one atmosphere pressureand with molecular oxygen as the only oxidant), the semiconductors have beenproved to be effective photocatalysts for the thermodynamically favoredtransformations of many organics to CO2 and H2O. However, with respect to theutilization efficiency, there are some main shortcomings as follow: ①thesemiconductor can only utilize 1~3% sunlight energy because the light which canbe utilized mainly consists in the ultraviolet range and has very narrow range ofthe wavelength. ②The photocatalytic activity is lower due to the highercombination rate of photoinduced electron and hole.Metal-ion-doped TiO2 particles capture electron to inhibit the recombinationof electron and hole by electron trap. In the past years, the research focused muchon the transition metal ions having d electron, however, few reports were foundabout rare-earth-doped TiO2. As the fertility elements in our country, the rare earthelements that have f electron are tend to form multiple electron configuration andthe oxides have many features such as polymorph, strong adsorption selectivity,good thermal stability and electronic conductivity etc., so it is speculated that rareearth doped TiO2 is a kind of cheap and efficient photocatalysts. In the firstchapter of this paper, Eu3+/TiO2 rare-earth-ion doped TiO2 nanoparticles wereprepared by the method of sol-gel using tetraebutylorthotitante as precursor andeuropium as doping ion. The structure, crystallite parameter and grain size werecharacterized by XRD, SEM and TG-DTA, and the photocatalytic activity ofEu3+/TiO2 was investigated for heptane oxidation. The results indicate that Eu3+ion enters into the lattice of TiO2. And the doping of Eu3+ leads to the latticeexpansion, enhances of the distortion of lattice, inhibits the crystal phasetransformation and an increase of grain size, and improves the photocatalyticactivity. The optimal doping content of Eu3+ is 0.3mol%, and the photocatalyticactivity of Eu3+-doped TiO2 decreases with the increase of calcination temperature.Moreover, Eu3+/TiO2 prepared at higher hydrolytic activity exhibits higherphtocatalytic activity. In the second chapter of this dissertation, solid superacid particles SO4 /TiO2 2-nanoparticles were prepared by peptization method with Ti(SO4)2 solution asprecursor. The structure and surface properties of SO4 /TiO2 were characterized 2-by the methods of BET, TG-DTA, IR, XRD, XPS, SEM, UV-Vis, SPS, ESR andFTIR pyridine adsorption spectra. The peptizing step with HNO3 produced rutileTiO2, and allowed sulfate species to homogeneously disperse throughout the bulkof catalysts, which lets sulfate species have more chances to incorporate into thelattice of TiO2 and modify its properties in contrast to traditional impregnationmethod. The presence of sulfate species can not only effectively retard the growthof crystalline size, but also be helpful for obtaining higher content of surfacehydroxyl groups. It was found that sulfate species are sensitive to calcinationtemperature. Upon calcination at 600oC the active sulfate species obtained at lowertemperature (300oC) were decomposed. This means that the strength of surfaceacidity becomes weaker due to the decomposition of active sulfate species andsurface dehydroxylation as the calcination temperature was increased to 600oC. Itis concluded that there should exist at least three kinds of sulfate species withdifferent binding forces, decomposition of which is the reason of the two fastgrowths of crystalline size and the prompt decrease of surface area. Mostimportantly, the band gap of SO4 /TiO2 can be shifted to visible light region in 2-that sulfate species were incorporated into TiO2 network and occupied oxygensites to form Ti-S bonds. As a result, the band gap is narrowed because mixing S3p states with valence band (VB) contributes to the increased width of VB. Incomparison with traditional impregnation method, our method is more effective inmodifying the optical absorption properties of TiO2, and hence should be moresuitable for practical utilization. As the calcination temperature was increased,Ti-S bonds were broken, and the sulfate species in the lattice of TiO2 wereexpelled out and left oxygen behind. Thus, Ti3+ defects were produced. Comparedwith P25 and pure TiO2, SO4 /TiO2 calcined at 300oC exhibits much higher 2-activity toward heptane under either UV light or visible light. Two rapid decreaseof the UV photocatalytic activity of SO4 /TiO2 with the increase of calcination 2-temperature is attributied to the decomposition of sulfate species. Heptane is one of the most important vaporous organic contaminants. In thisdissertation, the reaction mechanism, adsorption kinetics, reaction kinetics and thestability of the photocatalyst in C7H16-O2-Eu3+/TiO2 and C7H16-O2-SO4 /TiO2 2-systems were investigated. Both the adsorption rate of C7H16-O2-Eu3+/TiO2, and C7H16 -O2-SO4 /TiO2 2-systems can be expressed by the Lagergren first-order kinetics equation: ln(Qeq-Qt)=lnQt-kt where Qeq and Qt are equation and t-time adsorption quantity (μmol/g),respectively, t is adsorption time (min) and k is adsorption rate content (min-1). For C7H16-O2-Eu3+/TiO2, or C7H16-O2-SO4 /TiO2 system, the oxidation of 2-...