TiO₂/Sepiolite Nanocomposites Doped With Ions: Preparation,Characterization And Photocatalytic Activity

Of the dyes available in major industries worldwide,about 70%are azo compounds.Approximately 15%of these dyes are being lost and released as effluent during the dyeing process,which are toxic and carcinogenic to human health and ecological systems.Degradation of these dyes is of paramount before discharge into the environment.The heterogeneous photocatalysis,as one of the advanced oxidation processes?AOP?has received a great deal of attention.Among these photocatalysts,TiO₂ have caused concern because of its biocompatibility,low cost,photochemical stability,non-toxicity,and efficient activity.However,some limitations of TiO₂ are still under concern:?1?in the practical applications,the high recombination rate of the charge carriers makes the quantum yield quite low;?2?demerits like agglomerates,low surface area and little absorbability restricts the environmental applications of pure TiO₂;?3?the light absorption for anatase is limited to only 4%–5%solar spectrum in the UV region,due to the wide band gap of 3.2 eV.Therefore,recently researchers have mainly focused on developing TiO₂ supported catalysts.Among the supports,natural clays are promising as the supporting materials because of their strong mechanical and chemical stability,high surface area and high adsorption capacity.TiO₂ pillared clay reduces the aggregation of nanoparticles,leading to enhanced catalytic performance,while recent research indicated that clay minerals with fibrous morphology can also be used to increase TiO₂ photocatalytic property.Sepiolite is a 2:1 type of layered and fibrous mineral with molecular sized channels.It is a nonswelling,lightweight,porous clay mineral with a high specific surface area.In addition,the acidic[SiO₄]and alkalescent[MgO₆]centers in sepiolite have the ability to change the adsorbed organics into activation complex compounds,leading to easier degradation process.Fibrous sepiolite with high surface area,has not been investigated to a great extent as a support for TiO₂ catalyst but could act as an effective support of TiO₂ as a photocatalyst.Doping with ions can extend the band edge to the visible light region by reducing carrier recombination and introducing energy levels in the band gap of TiO₂.Numerous efforts have been focused on doping of impurities?cationic or anionic?in TiO₂ substrate to extend the absorption edge to the visible light range.anionic impurities have been considered as better dopants than metal species because of their closer position to oxygen in the periodic table.Among them,N is being proven to be one of the most effective anionic dopants due to its low ionization energy and comparable atomic size with O(N^3-:0.146 nm,O^2-:0.140 nm).Because of the special incompletely filled 4f orbital structure,rare earth?RE?metals often serve as catalysts to promote catalysis.RE elements are noted for the capacity to form complexes through the interaction of the f-orbital with various functional groups present in pollutant and concentrate the pollutant molecules onto the TiO₂ surface.Therefore,doping with RE³⁺provides a potential mean to improve the photocatalytic activity.In the thesis,taking the sepiolite with fibrous morphology for research object,the purification and defibering were first conducted based on the compositions,phase components and structure.Sepiolite was purified using the sedimentation method followed by microwave assisted acid treatment in order to effectively remove non-silicate impurities.Also,physical method of lyophilization and chemical method of surface modification were analyzed comparatively to evaluate the defibering effect on high crystalline sepiolite.Purifying and defibering of sepiolite samples are the prerequisite for the composites preparation with sepiolite as the support.In addition,the TiO₂/Sep nanocomposites were synthesized by two approaches.The first approach was conventional calcination process and the second approach was by the novel M-H treatment.At the same time,OG was used as the degradation object,the effects of various physicochemical conditions,such as Ti/Sep ratio,photocatalyst amount,initial dye concentration,and solution pH on photocatalytic degradation of OG were analyze and,to determine the desirable parameters.A detailed L-H kinetic model was elucidated,and the apparent rate constants of different degradation processes were determined by fitting the model to experimental data.Finally,considering the limited absorption in UV light region of the TiO₂/Sep nanocomposites,N-TiO₂/Sep or RE-TiO₂/Sep nanocomposites were prepared to extend the absorption edge to the visible light range.The morphology,composition,microstructure,textural and optical properties of all the obtained nanocomposites were investigated and the photocatalytic performance of the prepared nanocomposites were assessed from the photodegradation of OG under visible^-light irradiation.The catalytic efficiency was also studied by the degradation rate of the real wastewater and the recycling experiments were conduct to investigate the stability of the catalyst.The innovations of this paper are shown as follow:?1?A method for purification and defibering of low-grade sepiolite was proposed.The sepiolite was upgraded by sedimentation to remove impurities like quartz,and the microwave assisted acid treatment allowed sepiolite purification and activation by removing carbonates.The physical lyophilization technology and chemical surface modification were proposed for defibering sepiolite bundles.?2?The microwave^-hydrothermal treatment was used to prepare TiO₂/Sep nanocomposites instead of conventional calcination process for the first time.The sample could be prepared by microwave^-hydrothermal treatment in a shorter time with intact structure of sepiolite and higher specific surface area.It eliminates the problems of structural collapse of the support and the particles aggregation by the calcination treatment of sample.?3?The N-Eu-TiO₂/Sep was obtained for the first time by doping N and Eu,which extended the absorption edge to the visible light range.The main research results of this paper are listed as below:?1?The purity of sepiolite used in this study is low with the main impurity of quartz,calcite and talc.The sepiolite exhibits high thermal stability and fibrous morphology with poor dispersibility.The sepiolite was up-graded by sedimentation to remove impurities like quartz,and the microwave assisted acid treatment allowed sepiolite purification by removing carbonates.The sepiolite was up-graded by sedimentation to remove impurities like quartz,and the microwave assisted acid treatment allowed sepiolite purification by removing carbonates.The natural sepiolite was processed by sedimentation and irradiated by microwave for 8 min yields the best results,the purity of sepiolite increased to more than 90 wt%from 42 wt%.The lyophilization process effectively dissociated the crystal bundles of sepiolite into individual nanorods without damaging the aspect ratio of sepiolite nanorods.The highly dispersed rod-like crystals without structural degradation were achieved after freezing at-50°C for 12 h.The added chemicals promote the dispersion of particles,restrains the RE-aggregation of dispersed nanorods by increasing the repulsion among rods.The sepiolite modified with HDTMA showed better disaggregate effect and a higher specific surface area.?2?TiO₂/Sep nanocomposites were synthesized by employing two different methods:the conventional calcination process,and the M-H treatment.The method of preparation was found to have an impact on the crystallite size,crystallinity,the porosity,the surface area,and the photocatalytic performance of the nanocomposites.The ordered structure of this clay mineral was damaged to a certain extent by the calcination treatment of sample.In addition,calcination process increased the size of the anatase nanoparticles,decreased the BET surface area of the nanocomposites and inevitably deteriorated the photocatalytic effect.Compared with the calcination method,Sepiolite-TiO₂ nanocomposites could be prepared by M-H treatment in a shorter time with smaller anatase crystallite size,better crystallization and higher specific surface area.Nanocomposites prepared by M-H treatment were superior to those prepared by conventional calcination process for significantly improving the photocatalytic performance.?3?Ti/Sep ratios have a significant impact on the photocatalytic performance of the TiO₂/Sep nanocomposites.Photodegradation efficiency of OG is negligible when photolysis was carried out without catalysts or in the presence of sepiolite only.Ti/Sep with the ratio of 40 mmol/g shows the highest photocatalytic activity.The modified L-H kinetic model provide a better perspective of influencing parameter such as Ti/Sep ratios,photocatalyst amount,initial OG concentration and pH.Kinetic studies depict the photodegradation of OG follows pseudo-first-order kinetics for most of the operational parameters other than in the condition of high OG concentration,which follows the zero-order kinetics.From the results of the apparent rate constant k_app,we also learn that the highest degradation rate is obtained at catalyst dosage 0.8 g/L,initial OG concentration10 mg/L and pH=3.In this situation,the removal efficiency can achieve 98.8%after 150min of irradiation.Observed photocatalytic degradation rate constant at this situation is30.74×10^-3 min^-1.?4?Different nitrogen sources,namely NH,UR,EDA and TEA,were used to dope N into TiO₂ using the microwave^-hydrothermal treatment to obtain photocatalysts with visible^-light absorption.XRD results indicate that the boiling point is closely related to the crystallinity.The higher boiling points of dopants like UR and EDA could suppress the growth of anatase crystallites during the low-temperature M-H process.TEM-EDX images confirm the successfully doping of N.BET results indicate the introduction of N dopants increased the specific surface areas of the samples.XPS studies show that nitrogen is incorporated interstitially in the form of Ti-N-O or Ti-O-N in the lattice of TiO₂,and carbon is deposited on the nanocomposites and modified the surfaces.Interstitial nitrogen could narrow the band gap by forming the isolated N 2p state on the top of the O 2p VBM.The deposited carbon acted as the photosensitizer and reduced the chance of recombination of photoinduced charge carriers.Therefore,the photocatalytic activity of NTS-EDA was found to be higher than those of both undoped TiO₂/Sep and of single N doped TiO₂/Sep nanocomposites.Here,the enhanced visible^-light activity is mainly attributed to the low band gap energy?2.64 eV?.Moreover,the molar ratio of Ti to N has a significant impact on the photocatalytic performance.Ti/N with the molar ratio of 4 shows the highest photocatalytic activity.?5?A series of RE?La,Ce,Pr,Nd,Eu and Gd?doped TiO₂/Sep nanocomposites were successfully synthesized using the microwave^-hydrothermal treatment.All doped samples exhibit improved visible light photocatalytic activities.The formation of Ti-O-RE bonds effectively retard the aggregation and crystallite growth of titania.N₂adsorption-desorption analysis indicates that the textural properties are improved after doping with RE³⁺.XPS analysis confirms the coexistence of Ti⁴⁺ions and Ti³⁺ions.Furthermore,the intensity and range of visible light photo-absorption on RE-doped samples are stronger and larger,respectively than those of undoped sample.To sum up,the structures,chemical properties,and the degree of improvement of photocatalytic activities in RE-doped TiO₂/Sep varied with the type of doped RE³⁺.Among all the RE-doped samples studied,Eu-doped TiO₂/Sep shows the highest photocatalytic activity.The degradation efficiency of OG reaches to above 72%after 10 h visible light irradiation.During the photodegradation process,the unoccupied 4f level of Eu scavenges photogenerated electrons,while surface adsorbed OH^-traps hole.Therefore,the enhanced visible^-light activity is mainly attributed to the efficient interelectron transfer,the produced·OH and the suppression of electron-hole recombination.?6?The precursor EDA and Eu?NO₃?₃ were used to dope N and Eu into TiO₂ using the microwave^-hydrothermal treatment to obtain the N-Eu-TiO₂/Sep nanocomposites.XRD results indicate that the crystallization and size of anatase can be increased by the increase of Eu³⁺.The phase transformation from anatase to rutile also takes place when the amount of Eu³⁺is high enough.SEM-TEM images confirm the high crystallization and uniform distribution of TiO₂ on the surface of sepiolite by co-doping with N-Eu.N₂adsorption-desorption analysis indicates that the surface area and mesoporous characteristics are improved after co-doping.PL signals demonstrate that RE³⁺doping does not introduce the new light-emitting phenomenon but only effect the fluorescence intensity.The results suggest that a certain amount of Eu doping can inhibit the electron-hole pairs recombination and improve the lifetime of carriers.UV-vis DRS shows the band gap of the nanocomposites and the amount of Eu³⁺is a positive linear relationship.N doping plays a major role in the light absorption performance of the co-doping sample.The different existing states of N and Eu³⁺in nanocomposites lead to the different mechanism of visible light sensitization.Interstitial nitrogen could narrow the band gap by forming the isolated N 2p state on the top of the O 2p VBM.The Eu³⁺doping lead to the coexistence of Ti⁴⁺ions and Ti³⁺ions,the efficient interelectron transfer,the produced·OH and the suppression of electron-hole recombination.The synergistic effect of co-doping N-Eu-TiO₂/Sep nanocomposites contributes to higher visible^-light photocatalytic activity than the single^-doped nanocomposites.?7?The pretreatment of flocculation and precipitation is necessary before photocatalytic degradation of real wastewater.Among the flocculant used,FeCl₃ exhibits the best flocculation effect and the optimum dosage is 600 mg/L.N-Eu-TiO₂/Sep nanocomposites show rather high COD_Cr removal rate for real dye wastewater.The removal rate would be higher after the dilution with the COD_Cr removal rate above 80%.The reusability of photocatalyst indicates that the catalytic activity of the recycled catalyst has not changed substantially,it still possesses the high COD_Cr removal rate above 65%after five reaction cycles.SEM images validate that the anchored TiO₂ are well-attached to the sepiolite surface after five reaction cycles.The excellent photocatalytic activity,stability and good reusability suggest that the N-Eu-TiO₂/Sep nanocomposites could be used as potential candidate for the treatment of industrially discharged waste water.