Titanium-based Catalytic Nanomaterials For Advanced Oxidation Process And Its Mechanisms

In recent years,water pollution and its resulting environmental health issues have posed severe challenges to the green development pathway under the carbon peaking and carbon neutrality goals.Special attention has been given to the emerging pollutants,including typical endocrine disruptors and antibiotics,due to their widespread detection,environmental persistency,and complexity of treatment.So far,the removal of such complex organic pollutants remains restrained using traditional water treatment technology.In contrast,the development of novel catalytic oxidation systems both efficient and reliable is a promising solution.This could be achieved using environmental functional materials（EFM）,featuring diverse structures and almost limitless functional design possibilities.The advanced oxidation systems using environmentally friendly titanium-based nanomaterials have demonstrated excellent application prospects in adsorption and deep catalytic oxidation of refractory organic pollutants.However,the mechanisms of titanium-based Fenton-like catalytic oxidation remain unclear,the maximization of catalytic active sites has not yet been realized,the performance degradation and deactivation of materials due to agglomeration need to be addressed,and the controllable preparation,storage,transportation as well as practical application potentials of nanocomposites are to be further explored.In this study,addressing the above-mentioned key scientific and technical problems,a photocatalysis and Fenton-like advanced oxidation system of titanium-based nanomaterials was constructed.A series of in-depth researches were carried out regarding the controllable synthesis,catalytic mechanisms and efficiency,and environmental applicability of novel titanium-based nanomaterials,in the hope of developing the synthesis technology of titanium-based nanomaterials,elucidating the catalytic mechanisms,and hence providing the theoretical basis and technical support for the development and application of efficient water treatment technology targeting endocrine disruptors and antibiotics in the real-world water environment.The main research contents and results of this dissertation are as follows:（1）A series of carbon and nitrogen-doped TiO₂（C,N-TiO₂-X）with sponge-like structure and surface-phase junctions were prepared via a sol-gel method,which effectively resolved the disadvantages of commercial titanium dioxide（P25）,such as low responsiveness towards visible-light,agglomeration induced inactivation,high carrier recombination rate.Based on the results,the C,N-TiO₂-1 displayed a unique lamellar and porous structure,which showed the highest specific surface area 77.6925m²·g^-1.Both typical TiO₂ crystalline phases,i.e.anatase（lattice fringe of 0.35nm corresponding to the 101 lattice plane）and rutile（lattice space of 0.325nm corresponding to the 110 lattice plane）were present in these thinner sheets with abundant surface-phase junctions（the ratio of anatase and rutile was 3:1）.To evaluate the utility of C,N-TiO₂-1 for pollutants removal,rhodamine B,tetracycline and Disperse Red 60 were used as model pollutant molecules for organic dyes and antibiotics.Under the dark condition,C,N-TiO₂-1 showed the highest adsorption capacity towards all three pollutant molecules,and the total organic carbon（TOC）removal rate for rhodamine B,tetracycline,and Disperse Red 60 were 53.51%,62.92%,and 34.77%,respectively.Hydroxyl radical（^·OH）played the most significant role in photocatalytic degradation while holes（h⁺）and superoxide anion(O₂^·-)also took part in the degradation processes.The photocatalytic activity of C,N-TiO₂-1remained stable after five recycles and neither the crystalline phase nor the morphology showed any changes,which proved the reusability and stability of the C,N-TiO₂-1.This study demonstrated the possibility of significantly enhancing the photocatalytic performance of TiO₂ through structural alteration and surface-phase junction formation.Such improvement was achieved by a one-pot sol-gel process that included the use of pore-making agent HCl O₄.The resulted modified TiO₂ showed effective photocatalytic degradation of organic pollutants upon visible light irradiation with excellent reusability and stability.The excellent pollutants removal rate was attributed to the relatively narrow bandgap resulting from C and N doping,the higher charge-carrier migration efficiency due to the presence of surface-phase junctions,and the large adsorptive capacity owing to the unique lamella structure.This strategy could also be applied to optimize other photo/electrocatalysts for environmental remediations.（2）Taking advantage of the Ti-deficit defects on T₃C₂ MXene after exfoliation,a mild oxidation by H₂O₂ resulted highly dispersed TiO_1.47 nanoclusters decorated on silk-like carbon substrate(TiO_1.47@C).Based on transmission electron microscopy（TEM）observation,increasing concentrations（0.5-10 M）and treatment duration（5-60 min）of H₂O₂ improved the uniformity of the nanoclusters decorated on the silk-like structure.When the concentration of H₂O₂ was 10 mol·L^-1 and the reaction time was 30 minutes,the distribution of nanoclusters was the most uniform and the catalytic activity was the highest.Moreover,the%yield of TiO_x@C quantified by the weight percentage of the final product vs.the exfoliated Ti₃C₂MXene used could reach～97%,showing an almost complete use of the source materials.The DFT calculations further demonstrated that the surface Ti-deficit vacancies in the exfoliated Ti₃C₂ were the key reactive sites for the formation of TiO_x nanoclusters.The analysis of bond formation changes by EXAFS also further proved the destruction of Ti-C-Ti bonds and the formation of Ti-O bonds during the process of in-situ oxidation of H₂O₂.The PALS and EPR indicated the highest amount of surface defects in TiO_1.47@C.The XPS and XAFS allowed the calculation of titanium element at approximately2.94⁺oxidation state in TiO_1.47@C.Fenton-like catalytic performance of TiO_1.47@C was evaluated towards the degradation of the typical agriculture herbicide ATZ via activation of H₂O₂.TiO_1.47@C([Ti]=0.1 g·L^-1)showed superior reactivity to activate H₂O₂（5 m M）to degrade ATZ(2.5 mg·L^-1)within 5 min,comparing to Ti₃Al C₂,Ti₃C₂and TiO₂@Ti₃C₂ under the same conditions.A total of seven transformation pathways and three main mechanisms（e.g.dealkylation,alkyl chain oxidation,dichlorination,and hydroxylation）were proposed.The degradation process was a surface catalytic reaction dominated by hydroxyl radical（^·OH）and superoxide anion(O₂^·-).And the electrochemically active surface area（ECSA）measurements suggested that the TiO_1.47@C had more active surface areas for catalytic reactions.Five repeated cycles of Fenton-like reactions showed little changes in the degradation efficiency.Neither the crystalline phase nor the morphology showed any changes after five Fenton-like reactions.The shelf-life of TiO_1.47@C is highly desirable as it demonstrated good structural stability beyond 6 and 15 months.And the solution p H had no significant effect.Compared to other Fenton-like catalysts reported in the literature,TiO_1.47@C showed clear advantages.Such an approach was also highly applicable to other MXenes family materials including V₂C and Nb₂C.This work demonstrated a strategic use of MXene materials as both substrate and source materials for the in-situ formation of nanoscale Fenton-like catalysts.（3）Titanium-containing/BC aerogels were synthesized by the cross-linking between bacterial cellulose（BC）and the above-mentioned titanium-containing nanomaterials,which promoted the potential for practical application of the materials.The influence of precursors on the structure of composites was explored.The catalytic degradation performance of titanium-containing/BC aerogel on typical tetracycline antibiotics（tetracycline（TC）,chlortetracycline（CTC）,oxytetracycline（OTC））and phenols（p-nitrophenol）pollutants was investigated.The stability and environmental suitability of the composite aerogel system were evaluated.Since the ice crystals formed by tert-butyl alcohol maintain the porosity in aerogels during the freeze-drying process,which was useful to impart a large specific surface area to the composite aerogels to enhance the mechanical property.A likely composition of the final product,i.e.titanium-containing nanostructure decorated on fiber skeletons since the cross-linking.According to the process of visible-light photocatalytic analysis,the removal rates for TC,CTC,and OTC were 97.79%,88.32%,and 88.14%,respectively.Visible light alone showed no degradation effects.These results were likely contributed by the generation of the reactive oxidative species to oxidize the antibiotics molecules.The aspect of reusability and stability of the titanium-containing/BC aerogel was addressed by conducting twenty repeated cycles of visible-light photocatalytic de-colorization of rhodamine B.Meanwhile,the active titanium species were monitored in real-time and almost no Ti ions were released during the twentieth degradation experiments,revealing the excellent stability of titanium-containing/BC aerogel.And neither the crystalline phase nor the morphology showed any changes after twenty cycles.Similarly,as high as 90%p-nitrophenol removal could be achieved in 180 min using C,N-TiO₂/BC aerogel under visible light irradiation,yet the rest of the as-prepared samples and pure light could hardly degrade the pollutant.Such an approach of titanium-containing nanostructure crosslinked with bacterial cellulose network backbone was also highly applicable to another 24different types of engineered nanomaterials and many typical applications（e.g.oil removal,heavy metal removal,and microwave absorption）were successfully discussed,which demonstrated the establishment of a universal platform for functional composite aerogels.A facile and robust synthesis strategy was proposed for the design and application of multifunctional nanocellulose-based aerogel.The inspiration to realize this change mainly came from the crafted design of origami（e.g.Miura-ori and accordion folding）and the variable elastic thickness of aerogel.Storage,transportation,and self-unfolding process were further demonstrated.To further explore the flexibility and foldability of the nano-functionalized aerogel,the multiple operations of curled,tied,folded,and spiraled were displayed.For the same quality aerogels,the volume was significantly reduced by 24 times（Accordion folding）and42 times（Miura-ori folding）,the area was significantly reduced by 25 times,which meant that the use of origami was conducive to the storage and transportation of products.The specific dynamic deformation processes of floating composite aerogels were recorded that the unfolding takes 14 s and 38 s for the Accordion folding and Miura-ori folding,respectively,without human intervention.This study has improved the adsorption and visible-light photocatalytic performance of commercial TiO₂,elucidated the mechanisms of titanium-based Fenton-like catalytic reactions,explored the application of MXene-based materials in Fenton-like catalysis,promoted the integration of origami technique into the preparation of functional composite aerogel materials,and has successfully constructed a modular library of functional nano-composite aerogel materials.This study has,to some extent,provided novel insights into the key scientific and technical problems,including the maximization of catalytic active sites,alleviation of performance degradation of materials due to agglomeration as well as material storage and transport,hence promoting the application of advanced oxidation systems based on environmental functional materials in water treatment and remediation targeting typical endocrine disruptors and antibiotics.