Influence Of MnO₂ On Photoactivity Of TiO₂ In Photocatalytic Degradation Of Different Organic Pollutants

The photocatalytic oxidation technology is one of the methods in waste water treatment which is applied widely and sensitive to nocuous organic compound,has received much attention in recent years.However,it is a common phenomenon of photocatalyst deactivation in practical application;besides it is a major issue to avoid photocataly deactivation in the practical application needed to be solved pre-requisite.So the significance of studying the deactivation mechanism of photocatalysts and the effects of organics structure on photocatalytic degradation rates are important for the practical application and the development of the photocatalytic technology.Manganese is one of common elements in earth crust.Manganese element exists widely in earth crust and natural water with the form of combination.It was known that different organic structures caused different degree of inhibition effects with the presence of certain MnO₂.The results of this dissertation are as following:(1) According to the methods which were provided of literature,the MnO₂ of three crystal types(α-MnO₂,β-MnO₂ andδ-MnO₂) were prepared that exists widely in the earth crust.The crystal structures and form of MnO₂ were characterized by XRD and SEM.(2) With the light of UV365/70W and the initial pH 6.0,the influences of MnO₂ with different crystal about the photocatalytic activity of TiO₂ have been studied respectively by the experimentation of the aniline degraded.By the results,we know that the MnO₂ in a trace amount of different crystalline can depress the activity of TiO₂,especially the poisoning effect ofδ-MnO₂ was the most obvious when it was mixed with TiO₂ photocatalysts.The inhibitory effects with the different crystalline MnO₂ are various and it is in the order:δ-MnO₂＞α-MnO₂＞β-MnO₂.According to HPLC,it was found that although the degradation mechanism of aniline was unchanged,the elementary reactions of forming intermediate products were inhibited by the presence of MnO₂.(3) The samples have been studied with the DRS and PL to analyze the mechanism of MnO₂ inhibitory effect on the photocatalytic activity of TiO₂ according to the thermodynamic analysis of semiconductor oxide.At the contact interface of MnO₂/TiO₂,it is formed for heterojunction and changes the chemical state of Ti⁴⁺ and O^2- in the crystalline phase of TiO₂. On the side,the band gap of TiO₂ was increased and the capacity of UV absorption was decreased of TiO₂ with the presence of MnO₂.As the impurity of deep subband energy levels,it is accelerated that the photoinduced electron-hole pairs recombination.Because of these factors,the reasons were known that TiO₂ photoeatalytic activity was inhibited of MnO₂.(4) With the light of UV365/70W and initial pH 6.0,the relationship has been investigated between the photocatalytic biodegradation rate of aniline,benzoic acid and their derivatives and organics structure.The results showed that the degradation rate was related to the electronic density of benzene ring.Moreover,the electronic density of benzene ring was different with substituting groups and substituting position.The larger electronic density of benzene ring,the more favorable of the reaction processes,vice versa.(5) With the light of UV365/70W and the initial pH 6.0,the influences of MnO₂ with different crystal about the photocatalytic activity of TiO₂ have been studied respectively by the experimentation of the aniline,benzoic acid and their derivatives degraded.With the results,it was known that the MnO₂ with a trace amount of different crystalline can markedly depress the activity of TiO₂.The inhibitory effects with the different crystalline MnO₂ are various and it is in the order:δ-MnO₂＞α-MnO₂＞β-MnO₂.Inhibitory effect of homogeneous crystalline of MnO₂ was different with organics structure.Inhibitory effect of MnO₂ on the photocatalytic activity of TiO₂ is weak in the process of photocatalytic degradation of p-chloroaniline and m-chloroaniline.