Study Of The Hydrothermal Synthesis And Photocatalytic Properties Of Layered Niobates

In recent years, the layered niobate compounds, such as Ni-K₄Nb₆O₁7 Bi₂InNbO₇ etc, show outstanding catalytic property in the field of water splitting to H₂ and O₂, which has been paid more attention by many researchers. KNb₃O₈ and K₄Nb₆O₁7 are typical layered niobate compounds. The Photocatalytic activity of was studied less, the research on K₄Nb₆O₁7 was mainly focused on the Photocatalytic water-splitting property.In this thesis, K₄Nb₆O₁7 and KNb₃O₈ were synthesized by hydrothermal method. The Photocatalytic activity of photocatalysts for Photocatalytic degradation of acid red G was studied, and the mechanics of Photocatalytic degradation of acid red G by catalysts was discussed as well, which are as follow:a) The K₄Nb₆O₁7 can be obtained directly by the reaction of Nb₂O₅ and KOH within a wide range of reaction conditions, such as hydrothermal temperature, reaction time and concentration of KOH. SEM and XRD were used to characterize the morphologies and structure of the photocatalyst. The as-prepared K₄Nb₆O₁7 is assumed to orthorhombic structure and with the average size 13.9nm. SEM pattern showed the sheet morphology of K₄Nb₆O₁7 with the sheet-width less than 100nm. TG-DTA showed H₂O moleculars exist in the layers space of K₄Nb₆O₁7. The pure triniobate (KNb₃O₈) was obtained via tri-steps synthesis route: "hydrothermal dissolution - pH adjustment - hydrothermal crystallization". It was known that controlling the alkalinity of the reaction system is the crucial step during the hydrothermal synthesis process of KNb₃O₈. The pure KNb₃O₈ can be synthesized only under the appropriate pH (pH=5-6). The as-obtained KNb₃O₈ is assumed to orthorhombic structure and with nanometer leaf-like network morphologies. SEM confirmed the leaf-thickness is around 100nm which is about 70nm through calculating from the XRD results. The band gap and the Specific Surface Area (BET) of KNb₃O₈ crystals is estimated to be about 3.47 eV and 7.22 m²/g respectively.b) The Photocatalytic activity of the photocatalyst was evaluated by the Photocatalytic degradation of 100mg/l acid red G. The experimental results showed that the Photocatalytic activity of K₄Nb₆O₁7 prepared by hydrothermal method is better than K₄Nb₆O₁7 by solid-state synthesis and Degussa P25. After UV-light irradiation for 20min, 60% of the acid red G was decoloured, which was higher than 20% of the Degussa P25 and K₄Nb₆O₁7 obtained by solid-state reaction. KNb₃O₈ photocatalyst obtained by hydrothermal method also showed higher Photocatalytic activity than Degussa P25. After UV-light irradiation for 60min, the degradation rate of KNb₃O₈ could reach 78%, which was higher than 66% of P25. The results of the UV spectra changes during the Photocatalytic degradation of acid red G show that the structure of azo, condensed nucleus can be destructed by K₄Nb₆O₁7 and KNb₃O₈ under UV-light irradiation. Moreover, experiments showed that the degradation rate was influenced by some factors, such as the catalysis synthesis method, reaction temperature and time.Finally, the mechanics of Photocatalytic degradation of acid red G by catalysts was analyzed by infrared analysis (IR) and ultraviolet-visible light analysis (UV-Vis). In conclusion, the degradation of acid red G is not adsorption action, but the Photocatalytic oxidation under the UV irradiation.