Photochemical Activity, Structural Stability Of Perovskite-type Catalysts And Its Composite With SiO₂ Materials

With the rapid development of modern textile industry, more and more discharged dye sewage as a persistent organic pollutant has terribly jeopardized human health and ecological balance. Heterogeneous semiconductor photocatalytic oxidation technology is a promising method for sewage disposal, due to its unique feature of complete mineralization of pollutants without causing secondary pollution. In order to utilize the visible light (account for 43%of the solar spectrum), intensive efforts have been carried out to develop visible-light-responsive photocatalysts. To synthesize novel high active visible-light-responsive photocatalysts and research their photocatalytic activities have become a hot focus in this field.Recently, ABO3 perovskite-type oxides have exhibited many excellent properties, such as photocatalytic activity in dye degradation. However, varied transition metal ions are contained in these compound oxides at A-site or B-site, and the high valent B-site transition metal ion may cause the oxidizing property in some cases. Then this oxidizing property also may cause the degradation of organic dye. Additionally, for perovskite-type oxides, the octahedron structure is flexible and shows an allowable distortion or metastability with vacancies, which is sensitively affected by temperature and composition. The states of oxygen atom and the valence of B-site transition metal ion are crucial for the activity of perovskite-type oxides, which also sensitively depend on temperature and composition. So temperature and composition will finally affect the activity of perovskite-type oxides strongly. In this paper, the in-depth exploration on these influence mechanisms was investigated.Besides, in order to degrade organic pollutant in sewage effectively, just applying catalytic oxidation is not enough. Combining photocatalytic oxidation with other methods is an inevitable trend for catalytic technology. Modifying powder photocatalyst with porous materials is a promising effective method for improving their photocatalytic activity and stability, besides, this also benefits the recovery and reuse of powder photocatalyst. Many works reported that SiO2 material with large surface area, better stability and high adsorption activity is a superior practical support for photocatalysts. But the combination of SiO2 support with novel photocatalysts and the mechanism of action between SiO2 and photocatalyst are still rarely reported.In this thesis, the effect of oxygen vacancy variation on the photo-assisted degradation activity and structural transition of oxygen defective BaFeO_3-x in methyl orange degradation was investigated. XRD, FTIR, XPS, UV-vis analyses were adopted to characterize the oxygen vacancy variation caused by calcination temperature, and then the induced variation in crystal structure, crystallite size, component, specific surface area, adsorption edge of optical spectrum, photocatalytic activity and stability. The results showed that perovskite-type BaFeO_3-x can be obtained via citrate sol-gel method calcined at 700℃. Then with further increase in calcination temperature, oxygen vacancy content increases and perovskite structure phase transition occurred. In dye degradation process, CO2 generated from the continuous degradation of organic pollutant can improve the generation of BaCO3, and then A-site and O-site defects generated and B-site transition metal ion Fe4+ was reduced, finally the photocatalytic activity of perovskite BaFeO_3-x declined. In the dye photocatalytic degradation catalyzed by BaFeO_3-x, its photocatalytic activity and oxidizing property caused by B-site high valent Fe4+ took effects together.In order to synthetically investigate the effect of structure characteristic on the photocatalytic activity and stability for perovskite-type photocatalyst, another related oxygen defective perovskite BaCoO_3-x was also investigated. The results showed that BaCoO_3-x can be synthesized via citrate sol-gel method calcined at 700℃. Samples synthesized above 700℃show oxygen deficiency, and structure transition of perovskite occurred. But different from BaFeO_3-x, BaCoO_3-x improves the stability. The dye degradation catalyzed by BaCoO_3-x under visible light was caused by cooperation of its photocatalytic activity and oxidizing property. During the structure formation of peroskite BaCoO_3-x, the increase in oxygen vacancy induced B-site instable Co4+ was partly reduced to the more stable Co3+. This stabilization inhibited the structure transition of BaCoO_3-x in photocatalytic application. And then its photocatalytic activity was improved. Comparably, BaCoO_3-x perovskite is more stable than BaFeO_3-x.Besides, the photocatalytic activity and stability of another related perovskite-type photocatalyst LaCoO_3-x with a different A-site ion was also investigated. The results showed that perovskite LaCoO_3-x can be synthesized via citrate sol-gel method calcined at 500℃. With calcination temperature further increasing, the oxygen vacancy increased, its crystal structure changed, B-site slightly high valent Co3+ were partly reduced to stable Co2+. Then LaCoO_3-x improves the stability. LaCoO_3-x can degrade dye organic pollutant slowly in the dark, which is caused by the oxidizing property of slightly high valent Co3+. Under visible light irradiation, the dye degradation catalyzed by LaCoO_3-x was caused by the cooperation of its photocatalytic activity and oxidizing property. After dye degradation application, for LaCoO_3-x calcined below 800℃, a small amount of Co3+ transformed to Co2+ on LaCoO_3-x surface. After being used for many times, its perovskite bulk will partly transform to other phases. For the LaCoO_3-x calcined at 800℃, its perovskite skeleton structure is stable. Taken these three peroskite structure together, their stability regularity is BaFeO_3-x3-x3-x. The variation in stability caused by their structure is different, which is due to the different adjustment degrees between A-site and B-site ions.For the synthesis of perovskite-type photocatalysts, citrate sol-gel method is a common better method. For the perovskite structure BaFeO_3-x obtained from the adjustment of A-site and B-site ions and then B-site transition metal ion was made to rise to high valence, and another related BaFe2O4 spinel, the pH value will strongly affect the dissociation of complexing agent and then affect their formation when they are synthesized via citrate sol-gel method. The results showed that with the increase in pH value, citric acid dissociates step by step. Under pH<7, the baric citrate chelate compound is instable and difficult to chelate with Ba2+stably. And this will induce simple baric compound easy to segregate under lower temperature calcination, and then can not assure the sufficient Ba2+ source, finally takes negative effect on the formation of BaFeO_3-x during calcination. However, under pH>7, the formed much stable baric citrate chelate compound BaCit- will chelate with Ba2+stably, and then this can assure baric compound to transfer and combine under high calcination temperature, finally the pure BaFeO_3-x containing high valent instable Fe4+will form completely. Comparably, under pH>3, the formed instable baric citrate chelate compound will assure the complete formation of spinel BaFe2O4 with more stable Fe3+.Perovskite-like Bi₂WO₆ possess well stability and visible-light-responsive photocatalytic activity. So in the back part of this paper, Bi₂WO₆ was adopted to combine with SiO2 porous supports, and prepare loaded photocatalyst. Their combination status and the improvement in photocatalytic activity were investigated. Considering that synthetic silica gel possess high surface activity and is easy to be surface modified, it was first selected as the support in this paper. In this paper, adopting hydro-thermal method, wet silica gel as the raw support material, Bi₂WO₆ loaded on silica gel was prepared. The results showed that the optimal complex ratio is Bi₂WO₆ 60%and silica gel 40%. Then in this prepared Bi₂WO₆/silica gel sample, a large amount of Bi₂WO₆ grew uniformly on silica gel surface forming a close interface combination. Then the Bi₂WO₆/silica gel composite photocatalyst with flower-like particle coating was obtained. After loading Bi₂WO₆ on silica gel, the absorption edge of Bi₂WO₆ blue shift. Finally, this composite photocatalyst with only nearly half of Bi₂WO₆ can exhibit a similar photocatalytic activity to pure Bi₂WO₆ photocatalyst in RhB degradation.In order to further settle the suspension pollutant problem for powder photocatalyst, reduce the runoff of its active constituent as much as possible, diatomite as a high yielding natural mineral in our country with high adsorption activity was selected to modify Bi₂WO₆ in the final part of this paper. The results showed that adopting the combination of hydro-thermal method and in-situ synthesis, Bi₂WO₆/diatomite composite was prepared. The optimal complex ratio is Bi₂WO₆ 60%and diatomite 40%. Under this ratio, in the prepared Bi₂WO₆/diatomite sample, a large amount of Bi₂WO₆ with flower-like structure was loaded on diatomite surface. Then the synergistic effect of Bi₂WO₆ and diatomite functioned sufficiently. This loaded photocatalyst with only nearly half of Bi₂WO₆ exhibited a similar degradation activity to pure Bi₂WO₆ photocatalyst. This modification greatly solidified Bi₂WO₆, and greatly eased suspension pollutant problem.In this thesis, the influence factors for the photocatalytic activity and stability of perovskite-type oxides photocatalyst were further investigated, and the structure formation of novel perovskite-type photocatalyst and the influence mechanism on their activities were deeply studied. Additionally, a reference for future studies on novel modification with SiO2 materials for photocatalyst is provided. So this thesis provided a practical guidance for the preparation of high active photocatalyst.