Preparation, Characterization And Photocatalytic Activity Of ZnAlM (M=Ni/Ti) Layered Double Hydroxides (LDHs)

Layered Double Hydroxides （LDHs） not only exist in the natural environment,but also they are easy to synthetic. LDHs have many advantages, such as a widevariety of sources, simple synthesis process, high specific surface area, low cost,environmental friendliness, and so on. They are highly efficient and environmentallyfriendly clay material with tremendous development and application prospect. NiZnAllayered double hydroxides （NiZnAl-LDHs） with different Zn²⁺/Ni²⁺molar ratio wereprepared using co-precipitation, static urea hydrolysis and dynamic urea hydrolysismethods, and selecting the best synthesis method. The synthesized samples werecalcined in order to obtain mixed metal oxides with photocatalytic activity. TheZnAlTi layered double hydroxides （ZnAlTi-LDHs） at different Zn:Al:Ti ratios weresynthesized by dynamic urea hydrolysis method and further calcined to obtain themixed metal oxides with high photocatalytic activity. Structure of the obtainedmaterials were characterized by powder X-ray diffraction （XRD）, Fourier transforminfrared （FTIR） spectroscopy, Thermogravimetric and differential scanningcalorimetry （TG-DSC）, X-ray fluorescence （XRF）, BET analysis, X-ray photoelectronspectra （XPS）, Solid-state UV-vis diffuse reflectance spectra （UV-vis DRS）.For ZnO/NiO composites, their photocatalytic properties were evaluated bydegradation of orange G （OG） under solar radiation, before with the investigation ofisothermal adsorption behavior of OG. Several factors influencing the photocatalyticactivity like Zn²⁺/Ni²⁺molar ratio of the catalyst, catalyst dosage, initial dyeconcentration, and initial pH, have been studied. Exploring adsorption kinetics andcatalytic kinetic model about the catalytic material of orange G. For ZnO/TiO₂composites, their photocatalytic properties were evaluated by degradation ofmethylene blue （MB） dye under solar light irradiation. The experimental parameterssuch as the catalyst with different molar ratios, catalyst dose, initial MB concentration,calcination temperature and initial pH were studied in detail. Analyzing the UV-visabsorption spectra of methylene blue degradation process, and exploring thephotocatalytic reaction mechanism. The conclusions are as follows: （1） Urea dynamic method is the best method among the three preparationmethods, and the urea dynamic method has been applied successfully for synthesizingNiZnAl-LDHs with high degree of crystallinity. Furthermore, the obtainedNiZnAl-LDHs were calcined at873K in order to obtain mixed metal oxides withphotocatalytic activity.（2） The Langmuir and Freundlich adsorption isotherm model were used tosimulate the adsorption of OG on the ZnO/NiO composite surface, and the Langmuiradsorption isotherm model is more accurate. A maximum of99%photodegradation ofOG was achieved within100min solar light irradiation when the catalyst loading of1.0g/L and initial MB concentration of50mg/L, at the condition of neutral pH. Thekinetic study indicated that the photocatlytic process was well fitted to theLangmuir-Hinshelwood model, and photocatalytic degradation of OG was mainly dueto the strong oxidation of OH.（3） ZnAlTi layered double hydroxides （LDHs） with different Zn:Al:Ti ratio weresynthesized by dynamic urea hydrolysis method. Part of these samples were calcined at500,600,700and900℃, respectively, obtaining calcined mixed metal oxides with photocatalyticactivity.（4） The results showed that under the conditions of Ti:Al molar ratio of2:1,1.0g/L catalystloading,10mg/L initial MB concentration the calcination temperature of600℃within120min solar light irradiation,99%degradation rate of MB was obtained. Furthermore, afterthree cycles, the degradation of MB was still close to90%. The kinetic studyindicated that the photocatlytic process was well fitted to the Langmuir-Hinshelwoodmodel.