Investigation Of Tribocatalytic Performance And Mechanism Of Bi₁₂TiO₂₀

Catalytic degradation of pollutants through harvesting and converting sustainable energy from surroundings has been demonstrated to be an effective approach to address environmental pollution.For instance,the solar-driven photocatalysis and the cold-hot alternation-driven pyroelectric catalysis have been developed rapidly in recent years.However,constant illumination and temperature change are not achieved in all weather,which limits the application of these technologies.Mechanical energy as a widely distributed energy source has drawn increasing attention due to its low demands on light,temperature and other factors.Piezoelectric materials generate positive and negative charges on both sides of their surfaces in response to external mechanical forces,thus inducing a range of redox reactions.However,the high-frequent ultrasound required by piezocatalysis generally is rare in nature,which prevents piezocatalysis from achieving similar practical activity to that in laboratory.In contrast,friction is more common and the conditions it needs are mild.As rubbing two heterogeneous materials,triboelectrically positive and negative charges are respectively generated on surfaces of these two materials,and the tribocatalysis can use these triboelectric charges to trigger redox reactions.In this thesis,catalysts are chosen according to their physicochemical properties and synthesized by hydrothermal methods.Furthmore,we systematically investigated the friction-induced tribocatalysis of catalyst to degrade organic pollutants and its catalytic mechanism.Detailed works of this study are shown below:1.Bismuth titanate(Bi₁₂TiO₂₀)was prepared by hydrothermal treatment,and its tribocatalysis was evaluated through degrading Rhodamine B（Rh B）under magnetic stirring using a polytetrafluoroethylene（PTFE）-coated bar.It was demonstrated that Rh B can only be effectively degraded in the presence of catalyst and stirring（friction）,suggesting that both catalyst and friction play critical roles in tribocatalysis.The cyclic stability of the sheet-like Bi₁₂TiO₂₀ was verified by its good performance after five consecutive cycles.We also explored the effect of different factors,such as dye concentrations,stirring rates,magnet sizes,materials of beaker and magnet,on the reaction rate of sheet-like Bi₁₂TiO₂₀ for the catalytic degradation of Rh B.Based on the energy band structure of sheet-like Bi₁₂TiO₂₀ and the results of active species capture experiments,we have derived the following reaction mechanism:the PTFE-coated magnet exerted frictional forces on the catalyst through agitation,during which electrons（e^-）transferred from the sheet-like Bi₁₂TiO₂₀ to the surface of PTFE-coated magnet,while leaving oxidizing holes（h⁺）on the surface of Bi₁₂TiO₂₀.These holes could either oxidize Rh B directly or react with water to form hydroxyl radicals（·OH）,which in turn participated in the degradation process of Rh B.2.Four polymer particles,PTFE,polyvinylidene fluoride（PVDF）,polypropylene（PP）and polyamide 66（PA-66）,were used to form friction pairs with sheet-like Bi₁₂TiO₂₀,and then investigated the effect of their different triboelectric properties on the tribocatalytic activity for dye degradation.According to the results of the screening experiments on the type and amount of added polymer,the friction pair consisting of sheet-like Bi₁₂TiO₂₀（30 mg）and 5 mg PP was found to have the best catalytic performance,which was determined by both the tendency of the polymer to gain electrons during the friction process and the limited friction area.Since then,we continued to explore the catalytic performance of the optimized friction pairs with different dye concentrations,magnet sizes,etc.Finally,based on results of active species capture experiments,we proposed the following mechanism:during the magnetic stirring process,the friction between sheet-like Bi₁₂TiO₂₀ and the PTFE-coated magnet led to the generation of h⁺and·OH.Simultaneously,the friction also happened between sheet-like Bi₁₂TiO₂₀ and PP particles,which resulted in the transfer of e^-from Bi₁₂TiO₂₀ to PP.The holes generated on the surface of the sheet-like Bi₁₂TiO₂₀ were supposed to form·OH by reacting with water.The superoxide radical（·O₂^-）,which was generated by the reaction between dissolved oxygen and e^-accumulated on the surface of PP,also had the ability to oxidize Rh B.Quantitative experiments demonstrated that the introduction of PP increased the production of·OH and·O₂^-during the reaction,thus accelerating the rate of Rh B degradation.Under the combined effects of these active species,the conjugated structure of the Rh B molecule was continuously disrupted and would eventually be converted to H₂O and CO₂.3.Four different morphologies of Bi₁₂TiO₂₀,dandelion-like,flower-like,sheet-like and tetrahedral,were synthesised by hydrothermal methods.The relationship between the specific surface area of the catalyst and the tribocatalytic performance was investigated.The experimental results indicated that the dandelion-like Bi₁₂TiO₂₀ with the largest specific surface area showed the optimum catalytic performance.Subsequently,we formed friction pairs of PP,PTFE,PVDF and PA-66 with dandelion-like Bi₁₂TiO₂₀and then optimized the type and dosage of these polymers.It was found that the friction pair consisting of 3 mg PTFE and dandelion-like Bi₁₂TiO₂₀（30 mg）displayed the best performance of Rh B degradation with a kinetic constant of0.68 h^-1.The active species capture experiments of dandelion-like Bi₁₂TiO₂₀/PTFE catalyst system demonstrated that·O₂^-,·OH,h⁺and e^-all played the significant roles in the degradation reaction.The quantification of·O₂^-and·OH showed that the addition of PTFE promoted the production of·O₂^-and·OH during the same period of time,thus accelerating the degradation rate of RhB.