Reserch On Modified Halloysite Gold-Loaded Core-Shell Composite Catalysts For Catalytic Degradation Of Dye Wastewaters

Organic dyes are widely used in consumer goods manufacturing industry,which brings people colorful life and huge economic benefits,but also produces a lot of dye wastewaters.Most of the organic pollutants in dye wastewaters are highly toxic,carcinogenic and mutagenic,and possess strong dispersibility in the aquatic environment,which is easy to cause damage to aquatic ecosystem.As an important intermediate of synthetic dyes,4-nitrophenol（4-NP）has high toxicity,water solubility and stability.The 4-NP contamination in wastewater has resulted in serious environmental problems that influence human health.Catalytic reduction process is an environmentally friendly and economically effective method,which can catalyze the conversion of organic pollutants from dye wastewater into low toxic,high value-added chemical substances.Supported nano Au composites have the important application potentials in the construction of high activity nanocatalysts for catalytic reduction process.In this thesis,through the interlayer reaction strategy,the core-shell composite structures are designed to support Au nanoparticles（NPs）between layers,improve the dispersion and stability of the supported Au NPs,and the gold-loaded core-shell composite catalysts are constructed.The bimetallic oxide shell structure or laminated structure are used to encapsulate and solidify the active site of Au NPs.The morphology,structure and physicochemical properties of the composite catalysts are studied,and the positive effects of the structure construction of the composite catalysts on improving their catalytic activity and stability as well as the structure-activity relationship between the composite catalysts and their catalytic reaction performance are discussed.The specific research contents are summarized as follows:（1）TiZr bimetallic oxide shells are constructed and encapsulated onto the gold-loaded halloysites to obtain the HNT-Au@Ti_0.5Zr_0.5O₂ composite catalyst with the hollow core-shell structure.The construction of Ti_0.5Zr_0.5O₂ complex shell improves the porosity of HNT surface,reduces the agglomeration of Au NPs,and promotes the electron transfer from Ti_0.5Zr_0.5O₂shell to Au NPs.Compared with catalysts with single oxide shells or other different Ti/Zr ratio shell structures,the HNT-Au@Ti_0.5Zr_0.5O₂ catalyst showed superior catalytic performance in the reduction reactions of 4-NP,methylene blue（MB）and methyl orange（MO）.After 5 cycles of uses,the composite catalyst achieves the conversion rate of above 90%.The core-shell composite structure encapsulated by Ti_0.5Zr_0.5O₂ shell effectively slows down the migration and loss of Au NPs,and generates the better synergic catalytic effect between composite metal oxides.The Au-H complexes on the surface of Au NPs and the electrons transferring behavior of the core-shell composite structure have the significant effect on the catalytic performance of the above pollutants.（2）Co₃O₄/m SiO₂ interlayers are constructed and encapsulated onto the gold-loaded halloysites to obtain the HNT@Co₃O₄-Au@m SiO₂ composite catalyst.The Co₃O₄-Au complexes in the core-shell composite structure form the large reactive surface,and provide sufficient active sites for the catalytic reaction.m SiO₂ shell can effectively encapsulate and solidify Co₃O₄-Au complexes,which would more effectively inhibit the aggregation and loss of Au NPs.Due to the synergistic catalysis between Co₃O₄/m SiO₂ interlayer composites and Au NPs,the composite catalyst has excellent catalytic reduction performance toward 4-NP,MB and MO,and even displays the good catalytic treatment effect on 4-NP and MB mixed wastewater.The amount of 4-NP,catalyst or Na BH₄,the reaction temperature and the anions(including Cl^-,SO₄^2-and CO₃^2-)have different effects on the catalytic efficiency of 4-NP reduction.The reaction rate of the catalyst is highly sensitive to the reaction temperature,and the surface catalytic reaction of Au NPs is primary,which effectively reduces the activation energy of the reduction reaction and is conducive to the catalytic reduction process.After 6consecutive cycles,HNT@Co₃O₄-Au@m SiO₂ catalyst still maintains the high 4-NP conversion rate and well structural integrity.The catalytic reduction mechanism of this composite catalyst is proposed in terms of valence state and valence bond.（3）CeO₂/CeO₂-MnO₂ interlayers are constructed and encapsulated onto the gold-loaded halloysites to obtain the HNT@CeO₂-Au@CeO₂-MnO₂ composite catalyst.Au NPs were uniformly dispersed on the surface of the mesoporous CeO₂ inner layer,and the CeO₂-MnO₂sheet array shells are grown in situ on the CeO₂-Au substrate materials.This unique structure provides more catalytic active sites for the efficient catalytic reduction of nitrophenols（including 2-NP,3-NP and 4-NP）,MB and MO contaminants.The differences of the catalytic reaction rates of the nitro positions between neighbors are related to the conjugation effect,induction effect and group molecular orientation.The composite structure,calcination temperature and KMnO₄/HNT dosage ratio have the important effect on the catalytic performance of HNT@CeO₂-Au@CeO₂-MnO₂catalyst.CeO₂-MnO₂ shell encapsulation effectively prevents the loss and agglomeration of Au NPs and improves the stability of the catalyst.After 6 cycles of uses,the composite catalyst maintained stable catalytic activity and complete morphology structure.The carrier effect of the composite oxides,the chemical effect between Au NPs and the carrier,the increase of electron migration rate at the reaction interface and the work function are the main factors affecting the catalytic performance of the catalyst.