The Effects And Mechanism Of Manganese Oxides On Photocatalytic Activity Of TiO₂ And ZnO

The photocatalytic oxidation technology, which is a method of water treatment with wide potential applications, has received much attention in recent decades. However, photocatalyst deactivation is a common phenomenon in practical application; moreover it is an important issue need to be solved urgently. TiO₂ and ZnO are most extensively studied and used photocatalysts in recent years. The main research is how to improve the activity of photocatalysts (TiO₂ and ZnO), but the research on the factor and mechanism of photocatalyst deactivation has been seldom reported. In fact, it is significantly important for the practical application and the development of the photocatalytic technology to investigate the deactivation mechanism of photocatalysts.Manganese is a constant element in earth's crust. Manganese element exists in the form of soluble ions or oxide/hydroxide as suspended particles, which widely distributed in natural water and wastewater. It was found that the presence of manganese oxide particles deactivated TiO₂ photocatalysts, but its effect on the activity of ZnO was negligible. The following are the main research contents and results of this dissertation.(1) Four crystal types of manganese oxides (α-MnO₂,β-MnO₂,δ-MnO₂ andγ-MnOOH) which are ubiquitous in the environment were prepared according to Parida's methods. The crystal structures of manganese oxides were verified by XRD parameters and the morphologies of them were analyzed by SEM.(2) At initial pH 6.0, the influences of different crystalline manganese oxides on the photocatalytic activity of TiO₂ and ZnO have been investigated respectively through the photocatalytic degradation of methyl orange. The results showed that the different crystalline manganese oxides had markedly inhibitory effect on the photocatalytic activity of TiO₂ or even completely deactivated TiO₂ photocatalyst, especially the poisoning effect ofδ-MnO₂ on TiO₂ photocatalysts was the most obvious. For different crystalline manganese oxides, the inhibitory effects are various and the poisoning effect is in the order ofδ-MnO₂ >α-MnO₂ >β-MnO₂. However, under the same conditions, the effect of different crystalline manganese oxides on the activity of ZnO was negligible. (3) At initial pH values of 4.0, 6.0 and 8.0, the influences of different crystalline manganese oxides on the photocatalytic activity of TiO₂ and ZnO have been studied respectively through the photocatalytic degradation of methyl orange. The results indicated that the poisoning effect ofα-MnO₂,γ-MnOOH andβ-MnO₂ on TiO₂ photocatalysts was in the order of pH8.0 >pH4.0 >pH6.0, forδ-MnO₂, the order was pH4.0 >pH6.0≈pH8.0. It is suggested that the poisoning effect of manganese oxides cannot be avoided by pH adjustment if manganese oxides exist in the TiO₂ photocatalysis reaction system. Nevertheless, at the condition of different initial pH, manganese oxides have no evident influence on the photocatalytic activity of ZnO.(4) At initial pH 6.0, the influences of different concentration of manganese oxides on the photocatalytic activity of TiO₂ and ZnO have been evaluated respectively through the photocatalytic degradation of methyl orange. The results implied that the higher the concentration of manganese oxides was, the greater the poisoning effect on TiO₂ was, whereas the photocatalytic activity of ZnO changed slightly with the increase of the concentration of manganese oxides.(5) At initial pH 6.0, the influences of different crystalline of manganese oxides on the photocatalytic activity of TiO₂ and ZnO have been studied respectively through the photocatalytic degradation of phenol. The results showed that the three crystal types of manganese oxides could also markedly inhibit the photocatalytic activity of TiO₂, and the poisoning effect was in the order ofδ-MnO₂ >α-MnO₂ >γ-MnOOH >β-MnO₂ which is in accordance with results of photocatalytic degradation of methyl orange by TiO₂ in the presence of manganese oxides. At the same condition, the influence of manganese oxides on ZnO was negligible, which is similar to that of photocatalytic degradation of methyl orange by ZnO.(6) The samples have been characterized by the techniques of DRS, SPS, PL and XPS to analyze the effect mechanism and difference of manganese oxides on TiO₂ and ZnO photocatalysts. The characterizations indicated that the core-shell structures between manganese oxides and TiO₂ particles have been formed in suspension. At the manganese oxides/TiO₂ contact interface, heterojunction is formed and leads to the change in the chemical state of Ti4+ and O₂- in the crystalline phase of TiO₂. In addition, the presence of MnO₂ increases the band gap and causes the decrease in UV absorption of TiO₂, and also MnO₂ as impurity incurs deep subband energy levels can accelerate the recombination of photoinduced electron-hole pairs. These factors should be the main reasons that manganese oxides have evident poisoning effect on TiO₂ photocatalysts. However, manganese oxides only cause UV absorption of ZnO decrease slightly, but its absorption band edge remained unchanged. Furthermore, manganese oxides can eliminate the surface state of ZnO, but enhance the separation rate of photoexited carriers. The collective result is that the photocatalytic activity of ZnO remains relatively unchanged. This means that TiO₂ photocatalysts can be poisoned easily by manganese oxides while ZnO photocatalysts have capabilities to resist manganese poisoning effect.