Study On The Synthesis, Characterization, And Photocatalytic Performance Of Novel Nano-photocatalysts

As worldwide environment pollution and energy resource crisis are graving graduaily today,effective utilization of solar energy to solve pollution problems has become of great interest in many countries.The synthesis and applications of photocatalyst with high quantum-efficiency and high activity,in which solar energy can be utilized efficiently,have became hot research topics in the field of photocatalysis.In this dissertation,the basic principle about semiconductor photocatalysis was reviewed briefly at first.The recent research progress about improving the responsivity of photocatalysts to visible light and the pending problems in the area of photocatalysis were summarized in detail.On this base,two series of new nanosized photocatalysts were successfully synthesized using simple methods.One series is coupled semiconductor photocatalysts,the other is co-doped TiO₂ photocatalyst with rare earth and nonmetal elements.Subsequently,the effects of synthesis condition and reaction condition on photocatalytic activity of nanosized photocatalysts have been systematically studied.The degradation mechanism and dynamics of the as-prepared new type photocatalysts for organic targets under visible light and simulated sunlight irradiation were amply analyzed.Furthermore,the fastness technique of the as-prepared nanosized photocatalysts was carried out.In this thesis,a series of important conclusions and valuable achievements with practical application have been obtained,which have offered much experimental and theoretical guideline for the degradation of organic pollutants by using sunlight energy.This dissertation contains the following major parts: 1.Synthesis,characterization and photocatalytic activity of nanocomposite photocatalystsSnO₂,as a n-type semiconductor,which has excellent photoelectric properties and the similar crystal structure to rutile TiO₂.In view of these properties,we considered the coupling of SnO₂ and Fe₂O₃ or La₂O₃ to improve their photocatalytic activity.So,La₂O₃/SnO₂ and Fe₂O₃/SnO₂ coupled semiconductor photocatalysts were prepared by co-precipitation method.The preparation process displayed that calcining temperature,calcining time and the ingredient ratio affected the photocatalytic activity obviously.The characterization results of XRD,TEM,BET and UV-Vis found that the phase composition,crystallite size,BET surface area and optical absorption of the samples varied significantly with the calcining temperatures.Experiments indicated that the optimal heat-treatment temperatures of La₂O₃/SnO₂ and Fe₂O₃/SnO₂ photocatalysts are 400℃respectively.The samples calcined at 400℃has better crystallization,smaller crystal size,and shows stronger response to visible light(λ> 400nm).The average particle sizes of La₂O₃/SnO₂ and Fe₂O₃/SnO₂ photocatalysts calcined at 400℃are 8.5 nm and 15 nm,and the BET surface areas are 155.29 and 110.86 m²Â·g^-1,respectively.With the increase of calcination temperature,the crystallite sizes and apertures become larger,while the BET surface areas decreased.Photoreaction experiments showed that:â‘ the La₂O₃/SnO₂ photocatalyst calcined at 400℃for 2.5 h(the molar ratio of La to Sn is 1:4)exhibited maximum photocatalytic activity.Under the irradiation of simulated sunlight,the degradation rate of Acid Blue 62 reached 98.3%in 80min when the pH value was 5.6 or so and the concentration of La₂O₃/SnO₂ photocatalyst was 1.5g/l;â‘¡the Fe₂O₃/SnO₂ photocatalyst calcined at 400℃for 3 h(the molar ratio of Fe to Sn is 1:2)also exhibited maximal photocatalytic activity,the degradation rate of Acid Blue 62 reached 100%in 110min,and the removal rate of TOC is above 68%when the pH value was about 6.25 and the concentration of Fe₂O₃/SnO₂ photocatalyst was 1.25g/l. Under the irradiation of simulated sunlight,La₂O₃/SnO₂ and Fe₂O₃/SnO₂ coupled semiconductor photocatalysts still keep higher photocatalytic activity after many times of recycle use,which indicates that the as-prepared La₂O₃/SnO₂ and Fe₂O₃/SnO₂ coupled semiconductor photocatalysts are promising catalysts in industrialization area.When the concentration variety of Acid Blue 62 dye is in the range of investigation,the degradation kinetics of Acid Blue 62 follow pseudo first-order under simulated sunlight or visible light irradiation,and the degradation rates decrease as the initial Acid Blue 62 concentration increase.Furthermore,the degradation degree of Acid Blue 62 in simulated sunlight system is higher than that of in visible light system.The possible reason is that the ultraviolet part of simulated sunlight can effectively mineralize the intermediates adsorbed on the surface of photocatalysts, which keeps from poisoning of catalysts due to the gathering of intermediates.2.Supported Fe₂O₃/SnO₂/AC and La₂O₃/SnO₂/AC composite photocatalystsNowadays,exploiting some immobile techniques of fine photocatalyst particles is of great significance in photocatalytic area.In the present study,activated carbon supported Fe₂O₃/SnO₂ and La₂O₃/SnO₂ photocatalysts were prepared by dipping-calcining method,and their optimal preparation technical parameters were confirmed according to the characterization results of SEM and BET.Experimental results indicated that the Fe₂O₃/SnO₂/AC and La₂O₃/SnO₂/AC photocatalysts showed higher photocatalytic activity in photodegradation of industry dyes wastewater than Fe₂O₃/SnO₂ and La₂O₃/SnO₂ powder catalyst,which can degrade and mineralize the dyes wastewater.The improvement may be due to the synergetic effect of the stronger adsorption capacity of active carbon(AC)and the higher photocatalytic performance of Fe₂O₃/SnO₂ and La₂O₃/SnO₂ active ingredients.The large adsorption capacities of AC makes the organic pollutants concentrated around catalyst,which provided a higher concentration reaction environmental for photocatalytic reaction.Aiming at the invalidation of supported Fe₂O₃/SnO₂/AC and La₂O₃/SnO₂/AC photocatalysts after many times of recycle use,we make them regenerate using the method of 'washing-calcining-UV light irradiation'.The catalysts can keep higher activities after regenerating,which have potential practicality in intending industrialization. 3.Synthesis,characterization and photocatalytic activity of lanthanum and sulfur CO-doped TiO₂ photocatalyst under visible lightA novel lanthanum and sulfur co-doped TiO₂ photocatalyst was synthesized by precipitation-dipping method.The characterization results of XRD,TEM and BET indicated that the lanthanum and sulfur co-doped TiO₂ photocatalyst calcined at 450℃has smaller crystal size(about 5nm)and bigger BET surface area compared with the S-doped TiO₂ or La-doped TiO₂ photocatalysts.In addition,the XPS spectrums found that S atoms of the prepared photocatalysts were existent in the form of S⁴⁺state.UV-Visible reflectance spectrum indicated that the absorption edges of the modified TiO₂ photocatalysts shift towards the visible light region.The red shift degree of the samples is different because of the doping of different elements,it decreased in sequence of La-S/TiO₂＞S/TiO₂＞La/TiO₂.Characterization results also discovered that the co-doping of La and S significantly hindered the aggregation and growth of TiO₂ particles,and the inhibiting ability of La was stronger than that of S element.Experiments showed that the inhibiting ability is concerned with the doping concentration of lanthanum.In this dissertation,the optimal doping concentration of La is 1.5%.We also discovered that the doping of La and S accelerated slightly the anatase-to-rutile phase transition, which is significant in practice.Studies indicated that the lanthanum and sulfur co-doped TiO₂ photocatalyst calcined at 450℃could effectively mineralize phenol under visible light irradiation, which demonstrated that the sample has remarkable visible-light-activity. Photocatalytic degradation process of phenol under visible light irradiation with the aid of nanosized La-S/TiO₂ sample was investigated by UV-Vis spectrum and TOC measurement.Results showed that,under the attack of free hydroxyl radicals,at first the active intermediates formed,then formed carboxylic acid after opening benzene ring.Finally,the phenol was mineralized as the form of CO₂ and H₂O.Considering the application prospect of industrialization,activated carbon fibers (ACFs)supported La-S/TiO₂ photocatalyst were prepared successfully by coating-calcining method.Compared with the nanosized La-S/TiO₂ photocatalyst Powders,the supported photocatalyst showed stronger photocatalytic activity in photodegradation of industry wastewater under visible light irradiation.This method realized the loading of nanosized La-S/TiO₂ particles.These results all showed that present photocatalysts have excellent recycle use and stability,and possess important value in practical application.