Influence Of Iron Oxide On The Photocatalytic Activity Of Tungsten Oxide Through Bulk Doping And Surface Modification

Semiconductor photocatalysis and dye photosensitization are important for many current environmental and energy issues because, in addition to splitting water to supply clean and recyclable hydrogen energy, it can utilize solar energy to decompose harmful organic and inorganic pollutants present in air and aqueous systems. TiO2is the mostly studied photocatalyst, owing to its low cost, high reactivity and stability. Moreover, the UV light required for excitation of TiO2only accounts for about5%in the solar spectrum reaching on the Earth. So, an imperative and challenging issue is to develop new and efficient visible-light-sensitive photocatalysts.Iron oxide and tungsten oxide are colored semiconductors. It was observed that photocatalytic reaction could occur on both oxide under UV or visible light irradiation. In the presence of external bias potential, water can be splitted into oxygen. In this thesis, researches have been carried out on photodegradation of dye and (chloro)phenol in aqueous solutions by using iron-modified WO3, and a relatively systematic investigation was accomplished about the basic principle and factors of photocatalysis. Furthermore, the oxidation of chlorophenol sensitized by metal phthalocyanine (MPc) was partially studied for MPc was efficient visible light absorbing materials. There are six chapters in this thesis, the main contents and results are as follows:(1) The valence band holes of WO3have a similar reactivity to that of TiO2, but it is poor photocatalyst for organic degradation in water, due to its conduction band electrons difficultly reacting with O2. For the first time, we use H2O2as an electron scavenger of WO3, resulting into notable improvement in activity for dye and (chloro)phenol degradation either under UV or visible light. DMPO spin-trapping EPR showed that the production of hydroxyl radicals was greatly enhanced upon the addition of H2O2. The yield of phenol loss increased toward saturation with initial H2O2concentration.(2) Two kinds of iron-containing WO3(FeW) were synthesized through thermal decomposition of a ferrotungstenic acid, they displayed much higher activity than pure WO3(HW) prepared in parallel. As the sintering temperature increased, both FeW and HW showed an exponential increase in activity. The maximum rate constant of phenol degradation obtained with FeW at400℃was about two times larger than that with HW at600℃. Sample characterization with electron paramagnetic resonance spectroscope (EPR) and other techniques revealed that ferric species (0.3wt%Fe2O3) were mainly present as clusters on the oxide surface at120℃, and then they diffused toward the lattice sites of WO3at high temperature, which was detrimental to the photocatalytic reaction. It shown that surface modification of WO3with a trace amount of Fe2O3clusters can result into significant improvement in activity for organic degradation, while ferric species into the lattice sites of WO3is detrimental to the photocatalytic reaction. The catalysts after excited at350nm displayed a blue emission centered at469nm, the intensity of which varied with the catalyst activity nearly as expected. Possible mechanism for the improved photoactivity of WO3is proposed, involving the electron transfer from WO3to Fe2O3, and the reaction of the reduced oxide with H2O2to generate hydroxyl radicals.(3) We find that simple mixing of WO3and Fe2O3can result into significant enhancement in activity for phenol degradation in water in the presence of H2O2under either UV or visible light. The composite oxides were prepared by mixing a commercially available WO3with a synthetic Fe2O3in water, followed by sintering400℃in air. The enhanced activity greatly varied with the weight percent of Fe2O3in the mixed oxide. The best catalyst was1.0wt%Fe2O3/WO3, whose activity, in relative to bare WO3, increased to3.8-fold under UV light, and2.1-fold under visible light, respectively. A spin trapping electron paramagetic spectroscopy revealed that the production of hydroxyl radicals over Fe2O3/WO3was greatly enhanced. Solid characterization with several techniques indicated that Fe2O3particles were highly dispersed in the sample, with a small size and high surface area. However, organic degradation was mainly initiated by the excited WO3in Fe2O3/WO3. We propose that the observed synergistic effect between two oxides is due to the electron transfer from the conduction band of WO3to low energy trapping sites of Fe2O3. Such interfacial charge transfer between two oxides would facilitate the separation of WO3charge carriers, and consequently accelerate the surface reaction on both oxides for organic degradation.(4) Metal phthalocyanine (MPc) is widely used as the photosensitizer for singlet oxygen generation under visible light irradiation. There are many reports about the degradation of environmental pollutants using MPc, but a detailed study of the degradation products are rarely. The degradation products of2,4,6-TCP sensitized by Aluminum tetrasulfophthalocyanine (AlPcS4), was studied in aqueous solutions. The products of TCP degradation were observed to be2,6-dichloro-1,4-benzoquinone,2,6-dichloro-1,4-benzenediol, chloromaleic, chlorofumaric, maleic, fumaric acid and Cl-. The product amounts various with the pH of the solution. In base solution, TCP was more mineralized than that in acid solutions. The large-molecule products decreased with pH, and they can be degraded under visible light either with or without catalyst and oxygen. So we can conclude that TCP will be thoroughly decomposed into low molecular weight carboxylic acids and Cl-with enough light irradiation time.