| Polychlorinated dibenzodioxins and polychlorinated dibenzofurans (PCDD/Fs, also defined as dioxins), which are mainly produced unintentionally in high temperature processes such as waste incineration and metallurgical industries, are very toxic, carcinogenic, and environmentally persistent. Therefore, the emission of dioxins has been one of the most focused topics in global environmental problems. Among the techniques which have been developed for dioxins removal, selective catalytic reduction with distinct energy and economic efficiencies has attracted great attentions. Moreover, several reports have shown that adding ozone into the catalytic oxidation process of organic pollutants can dramatically reduce the operation temperature and thus reduce the cost. Herein, effects of ozone on the catalytic oxidation of chlorobenzenes and dioxin were studied.1. Carbon nanotubes supported manganese oxide (MnOx/CNTs) and copper oxide (CuOx/CNTs) were prepared by impregnation method and their catalytic oxidation performance of chlorobenzene (CB) with the assistance of ozone was investigated. Experimental results indicated that ozone efficiently promoted the catalytic oxidation of CB. Even at temperature as low as 80 ℃, a CB conversion of 95% and CO2 selectivity of 100% could be achieved over MnOx/CNTs catalyst. This phenomenon could be attributed to the strong CB adsorptivity of CNTs and the highly active oxygen radical produced from ozone decomposition. It was also showed that at low temperature as 80 ℃, the performance of CuOx/CNTs was lower than that of MnOx/CNTs. However, with the increase of temperature, CuOx/CNTs gradually showed a better performance, which was contrary to MnOx/CNTs catalyst. And at 240 ℃, both CB conversion and CO2 selectivity of 100% were achieved over CuOx/CNTs. This phenomenon could be ascribed to the different effects of MnOx and CuOx on ozone decomposition.2. The number of chlorine substituents was also proved to have an influence on the ozone promoted catalytic oxidation over MnOx/CNTs catalyst. Experimental results indicated that chlorine would suppress the effects of ozone. For example, under 2300 ppm ozone and 50 ppm organic pollutants, the complete conversion of chlorobenzene,1,2-dichlorobenzene and 1,3,5-trichlorobenzene were 74.4%,56.5% and 46.0%, respectively. This phenomenon would be due to the eletrophilicity of chlorine.3. Carbon nanotubes and titanium dioxide supported vanadium, manganese and cerium oxide (V-Mn-Ce-Ox/TiO2-CNTs) catalyst was prepared by sol-gel method and impregnation method, and the temperature effects on its catalytic oxidation performance of dioxin with ozone was investigated. Experimental results showed that the performance at 120 and 170 ℃ was enhanced when 200 ppm ozone was introduced. However, at 220 ℃, the effects of ozone were not obvious, which might be attributed to the instability of oxygen radical at high temperature.4. V-Mn-Cu-Ce-Ox/TiO2-CNTs and V-Cu-Ce-Ox/Ti02-CNTs catalysts were also prepared and their catalytic oxidation performance of dioxin with ozone was studied at 220 ℃. Experimental results indicated that different ingredients had different effects. When there was no ozone, dioxin was synthesized over each catalyst. It was also found that CuOx promoted the synthesis of dioxin while MnOx suppressed the synthesis. When 200 ppm ozone was introduced, the synthesis of dioxin over all catalysts was suppressed and even dioxin destruction could be observed over V-Mn-Cu-Ce-Ox/TiO2-CNTs and V-Cu-Ce-Ox/TiO2-CNTs catalysts.5. A new method for dioxin abatement with ozone in industrial exhaust gas was designed, by which, the effects of ozone on dioxin catalytic oxidation could be fully developed. This method divided the process of dioxin abatement into dioxin adsorption online and dioxin destruction offline. In this way, the utilization efficiency of ozone could be remarkably increased and thus the cost was reduced. |
PDF Full Text Request