Study On The Influence Of Manganese Oxides On Photocatalitic Activity Of TiO₂

TiO₂ photocatalyst has been considered as one of the most promising techniques to control water pollution, but the deactivation of TiO₂ photocatalyst is a significant and practical problem. Mn is one of the major elements in the crust and it usually widely exists in wastwater and natural water with the form of dissolved, oxide phase, oxyhydroxide particles. Our studies showed that photocatalytic activity of TiO₂ could deactivate due to the presence of the manganese oxides particles.In this paper, the main content as follows:(1)Using methyl orange as a model pollutant, the influence of the presence of four kinds of manganese oxides (δ-MnO₂,α-MnO₂,γ-MnOOH and natural manganic ore) on photocatalytic activity of TiO₂ has been investigated. At pH6.0, methyl orange could be degraded completely in 110 minutes, however the presence of manganese oxides can inhibit the photocatalytic activity of TiO₂ significantly and in some cases deactivate the photocatalyst completely. This could be explained by the poisoning effect of manganese oxides on TiO₂ photocatalyst. When the initial pH value of methyl orange solution was adjusted at 4.0, 6.0, and 8.0, four kinds of manganese oxides could inhibit the photocatalylc activity of TiO₂. The effect ofγ-MnOOH and natural manganic ore is in the order pH8.0>pH6.0>pH4.0; That ofδ-MnO₂ andα-MnO₂ is pH4.0>pH6.0>pH8.0, Meanwhile, initial reaction rates has been compared at different pH value. The poisoning effect of is enhanced with the quality concentration (Cm) of manganese increased. Aerating oxygen to the reaction system, manganese oxides also inhibit the photocatalytic activity of TiO₂.(2)Using rhodamine B as a model pollutant, the influence of the presence of four kinds of manganese oxides on photocatalytic activity of TiO₂ has been investigated. At pH6.0, the presence of manganese oxides can inhibit the photocatalytic activity of TiO₂ significantly and in some cases deactivate the photocatalyst completely. The poisoning effect of manganese oxides is in the order pH8.0>pH6.0>pH4.0. The mean reaction rates ofα-MnO₂,γ-MnOOH and natural manganic ore are in accordance, but different fromδ-MnO₂. The order of the poisoning effect of manganese oxides is Cm=33.3%> Cm=20.00%> Cm=4.76%; the mean reaction rate is decreased with the Cm of manganese oxides.(3)Using phenol as a model pollutant, the influence of the presence of four kinds of manganese oxides on photocatalytic activity of TiO₂ has been investigated. At different initial pH value, The poisoning effect of manganese oxides is in the order pH8.0>pH6.0>pH4.0, and mean reaction rate is pH4.0>pH6.0>pH8.0. The effect of manganese oxides is enhanced with the Cm of that increased (Cm=33.3%> Cm=20.00%> Cm=4.76%). The mean reaction rate is decreased with the Cm of manganese oxides.(4)α-MnO₂ andδ-MnO₂, taken as samples of manganese oxides, were characterized by UV-visible diffuse reflectance spectroscopy (UV-vis DRS), photoluminescence (PL) and x-ray photoelectron spectroscopy (XPS). The poisoning effect was attributed to the formation of heterojunctions between MnO₂ and TiO₂ particles. The heterojunctions changed the chemical state of Ti⁴⁺ and O^2- sites in the crystalline phase of TiO₂. MnO₂ in contact with TiO₂ particles also broadens the band-gap of TiO₂, which decreases UV absorption of TiO₂. It can also create some deep impurity energy levels serving as photoelectron-photohole recombination center, which accelerates the electron-hole recombination.