Preparation Of Bi-based Mixed Oxide Catalysts And Their Photocatalytic Performance For Organics Oxidation

The development of economic and activity of human beings result in the environmental pollution and energy crisis,which are the worldwide problems of influencing sustainable development of the human society.Due to inexhaustible and pollutant-free solar energy,photocatalysis has been considered as one of the most potential technologies to resolve the energy and environmental issues.The key to photocatalysis is developing high-performance photocatalysts.The mainly included Bi-based mixed oxide photocatalytic materials are Bi VO₄,Bi₂WO₆,Bi₂Mo O₆,Bi OCl,Bi OBr,and Bi OI,which are the hot research topics due to their strong visible-light responding ability and modifiable band structures.However,fast recombination of photogenerated carriers and low conduction band positions of the Bi-based photocatalysts limit their practicable applications.To overcome the above disadvantages and improve the photocatalytic performance,three kinds of the modified photocatalysts(including Bi VO₄,Bi₂WO₆,and Bi₂Mo O₆)were investigated in this dissertation by means of morphological preparation,surface modification,and heterojunction construction,and their catalytic performance for the complete oxidation of organic pollutants(e.g.,phenol and 4-nitrophenol(4-NP))in wastewater to CO₂ and H₂O and the selective oxidation of the low-value organics(e.g.,benzyl alcohol and toluene)to the value-added benzaldehyde.In addition,spectral and photoelectrical measurements were conducted to reveal the relationship between physicochemical properties and photocatalytic performance of these materials.Furthermore,the active species trapping and detecting experiments were carried out to clarify the involved photocatalytic reaction mechanisms.The main outcomes obtained in this thesis are as follows:(1)The ascorbic acid-assisted polymethyl methacrylate(PMMA)-templating and incipient wetness impregnation methods were adopted to prepare the three-dimensionally ordered macroporous(3DOM)Bi VO₄ and its supported iron oxide(x Fe₂O₃/3DOM Bi VO₄)photocatalysts.Monoclinic 3DOM Bi VO₄ possessed a good quality of 3DOM structure and Fe₂O₃ nanoparticles(NPs)were uniformly dispersed on its surface.The x Fe₂O₃/3DOM Bi VO₄ catalysts displayed much better photocatalytic activities than the 3DOM Bi VO₄ sample,with 0.97Fe₂O₃/3DOM Bi VO₄ performing the best for 4-NP degradation under visible light illumination(98%4-NP conversion was achieved in the presence of 0.6 m L H₂O₂ within 30 min of reaction).The reaction mechanism reveals that H₂O₂ addition was essential in promoting the photocatalytic process,in which the·OH generated via the reaction of photoinduced electrons and H₂O₂ was the main active species.It is concluded that the unique porous architecture,high surface area,Fe₂O₃-Bi VO₄ heterojunction,good light-harvesting capacity,high adsorbed oxygen species concentration,and excellent separation efficiency of photoinduced electrons and holes as well as the photo-Fenton degradation process were responsible for the enhanced photocatalytic performance of0.97Fe₂O₃/3DOM Bi VO₄.(2)The leaf-like monoclinic Bi VO₄ and its supported metal(Co,Pd or Co-Pd)NPs were prepared by the hydrothermal and polyvinyl alcohol(PVA)-protected chemical reduction methods,respectively.Co Pd NPs with a diameter of 4-6 nm were uniformly dispersed on the surface of Bi VO₄.The photocatalytic activities over the y Co_xPd/Bi VO₄ samples were much higher than those over the Bi VO₄,0.058Co/Bi VO₄,and 0.083Pd/Bi VO₄ samples,with the 0.062Co_1.70Pd/Bi VO₄ catalyst showing the highest degradation activity under visible light illumination(90%phenol conversion was achieved within 3 h of reaction at an initial phenol concentration of 0.2 mmol/L).The Co doping could effectively stabilize the metallic Pd⁰ species,giving rise to an increase in surface Pd⁰ species concentration and activation ability of O₂ molecules.The reactive species trapping experiments reveal that the superoxide anion radical and photogenerated holes played important roles in phenol degradation reaction.Partial deactivation of the 0.062Co_1.70Pd/Bi VO₄ sample after 15 h of three recycle tests was mainly due to the decrease in adsorbed oxygen species concentration.The excellent photocatalytic activity of 0.062Co_1.70Pd/Bi VO₄ was associated with the generation of highly active superoxide anion radicals and good separation efficiency of the photogenerated charge carriers.(3)The oxygen-deficient 3DOM Bi VO₄-supported Ru nanoparticle photocatalysts were fabricated using the PMMA-templating,ethylene glycol reduction,and post-thermal treatment methods.0.95Ru/3DOM Bi VO4-Ar-300 showed the best photocatalytic performance for the selective oxidation of benzyl alcohol under visible-light irradiation for 8 h:78%benzyl alcohol conversion and 100%benzaldehyde selectivity were achieved,benzaldehyde formation rate was 1380?mol/(g h).The 0.95Ru/3DOM Bi VO4-Ar-300 sample also exhibited good photocatalytic stability after four recycling tests.Based on the results of reactive species trapping and radicals detecting experiments,we conclude that the reactive O₂·^-species played a decisive role in the selective oxidation of benzyl alcohol.The enhanced photocatalytic activity was related to the combined effect of highly dispersed Ru NPs and oxygen vacancy formation.The supported Ru NPs could improve separation efficiency of the photogenerated electrons and holes,whereas presence of oxygen vacancies was favorable for the activation of O₂.(4)The cetyltriethyl ammnonium bromide(CTAB)-assisted hydrothermal method was adopted to prepare Bi₂WO₆,Bi₂Mo O₆,and Bi₂W_xMo₁-_xO₆ solid solution photocatalysts.Bi₂W_0.3Mo_0.7O₆ showed the best photocatalytic performance for the selective oxidation of toluene under visible-light irradiation for 5 h:1.46%toluene conversion and 91%benzaldehyde selectivity were achieved,benzaldehyde formation rate was 1663?mol/(g h).The Bi₂W_0.3Mo_0.7O₆ catalyst also showed good photocatalytic stability after four recycling tests.The results indicated that thin nanosheet was favorable for the separation of photogenerated charges,but the BET surcace area,bandgap energy,and reduction and oxidation potentials of the photogenerated electrons and holes were also important factors influencing the photocatalytic activity.The excellent photocatalytic activity of Bi₂W_0.3Mo_0.7O₆ was ascribed to balance of the photogenerated charge separation efficiency,visible-light response ability,and oxidation and reduction potentials.The result of the reactive species trapping experiments indicate that the photogenerated holes and carbon-centered radicals played a decisive role in the selective oxidation of toluene.