Preparation Of Composite Photocatalyst With Self-indicating Delayed Onset Of Performance In Polyethylene Degradation

Plastics have been excessively used in people’s daily life.Around 200 million tons of plastic wastes have been disposed after their usage each year which causes severe environmental problems.Photocatalyst can be used as additives for plastics to increase the degradability of plastic products.However,the photocatalytic performance of photocatalyst cannot be delayed,resulting in the unstable performance of the plastic product during use.Even if a delay can be achieved,the color change of plastics cannot be used to indicate whether photocatalytic degradation is started.This hinders the industrial application of photocatalytic technology in plastic degradation.In this thesis,a new type of composite photocatalyst,Ti O₂@amylose/polyiodide/hydroxyethyl cellulose,is designed with self-indicating delayed onset of performance.The composite photocatalyst is prepared via the wet chemistry method.It is blue before the photocatalytic performance starts（stable stage）,and the color changes to white when its photocatalytic performance starts（active stage）.The length for the stable stage and the reaction rate for the active stage are tunable.The stable stage for T@AIH is 110 min when the mass ratio of amylose to iodine to Ti O₂ to HEC is 1:1:0.5:2（T@AIH）.The color change point is 120 min and phenol degradation starts with a degradation rate of1.32 mg L^-1 h^-1.The property difference of T@AIH between different stages was characterized by using Raman spectrum,ESR,¹³C NMR,FTIR,UV-Vis absorption spectrum,TGA,and photoelectrochemical tests in order to further explore the mechanism of the synergistic control between the photocatalytic performance and color change of composite photocatalyst.The results suggest that efficient I₃^-/I₅^-interconversion of polyiodide in amylose helical pitch plays a key role in electron-hole recombination,resulting in the stable stage.The photocurrent is lower than 0.19μA/cm² at the stable stage,and no photogenerated active species is detected via the photogenerated charge transfer path.Electrons in T@AIH attack viaα-（1→4）glycosidic bond breaking of amylose to decompose the amylose/polyiodide with the increasing illumination,which destroys the helical pitch and causes the color of composite photocatalyst to change from blue to white.Meanwhile,the photogenerated electron-hole recombination efficiency is reduced.The high diffusivity of polyiodide in HEC leads to photogenerated electrons and holes transferred to the surrounding environment,which realizes the start of the active stage and the color change at the same time,and the self-indicating function is constructed.The photocurrent for the active stage is about 20μA/cm²,which is 100times that of the stable stage,and·O₂^-is detected via charge transfer.The LDPE-CP film was prepared by adding T@A_0.1IH to LDPE,and the degradation performance of LDPE-CP was tested in a xenon test chamber.The results show that T@A_0.1IH possesses self-indicating delayed onset of performance on LDPE degradation.When the mass ratio of T@A_0.1IH to LDPE is 0.02:1,the stable stage and the color change point of LDPE-CP film are 32 h,which corresponds to 30 d when the plastic is exposed to 20℃.The reaction rate for the active stage was 4.04 mg h^-1.The degraded gas product of LDPE by T@A_0.1IH was CO₂,and the solid product was a large molecular weight product with higher polydispersity.In this thesis,a new type of composite photocatalyst with self-indicating delayed onset of performance is proposed and constructed.A new type of degradable plastics with delayed onset of performance is fulfilled by adding composite photocatalyst to LDPE,which indicates its onset of photocatalytic performance via color change.The regulation between the photocatalytic performance and color of the composite photocatalyst was revealed.The mechanism of amylose,HEC,amylose and polyiodide in synergistic regulation of the photocatalytic performance and color of Ti O₂ were explored.The performance of the composite photocatalyst to degrade plastics was initially tested.It provides technical and theoretical support for promoting the application of photocatalytic technology in the field of plastic degradation,promoting the country’s sustainable development,and alleviating the contradiction between economic development and environmental burden.