Supported Mn-Fe Catalysts For Low-temperature Selective Catalytic Reduction Of NO_x With NH₃

The power plant exhaust gases have been one of major air pollutant resources inour country, because of this exhaust gases containing NO_xcould result in a series ofenvironmental issues to break the eco-environment balance, such as acid rain,photochemical smog, greenhouse effect, etc. Thus, more attention was paid on theproblem of NO_xemission. The NO_xemission legislation has strictly demanded itsoutput amounts. At present, Selective catalytic reduction（SCR）, the most widely-usedand efficient technology, has become the best choice for reducing NO_xfrom powerstation. The operating temperature for the industrial denitration catalysts used by mostpower stations was300⁴00℃. This made it necessary to lacate the SCR reactor unitupstream of the particulate and desulfurizer control device, but deactivation did easilyoccur by the high concentration of ash and sulphide.The catalyst by importedtechnologies also required high cost. The SCR reactor unit could be located thedownstream of the particulate and desulfurizer control device to avoid the toxicity ofcatalysts, but this place of fuel gases’ temperature got lower，so the research on bestperformance of low-temperature denitration catalysts （objective temperature≤200℃）became the topic of this paper.With the mole ratio Mn/Fe=1, Mn-Fe mixed oxides catalysts respetively supportedon two types of carriers，namely nano TiO₂and ZSM-5，were prepared byprecipitation methods. The reaction gas composition was as follows:1000ppmNO,1000ppmNH₃,5%O₂, and balance N₂, the effects of active components loadings andreaction temperature on the catalytic properties were investigated. With different activecompotent loadings, for various Mn-Fe/TiO₂catalyts and various Mn-Fe/ZSM-5catalyts, active tests results displayed that the optiamal active compotent loading was0.3（wt%）, the NO conversation could respectively reasch72.33%and73.59%at150℃,over supported on TiO₂and ZSM-5, respectively. And the best reaction temperaturewindow were the range of100³00℃and150³00℃，over supported on TiO₂andZSM-5, respectively. The catalysts were characterized by BET, XRD, TPR, and resultsproved that active components loadings could effect the disperse state lead to differentoxide speices and changes of catalyst structure. Fe₂O₃and MnO₂were benefited to thehigh catalytic activity, Mn₂O₃could improve the selectivity for N₂, the catalysts samplescontaining lots of amorphous Mn₃O₄were the main reason for undesired activities. The stability and H₂O resistance ability for0.3（Mn-Fe）/TiO₂catalyst and0.3（Mn-Fe）/ZSM-5catalyst were also inspected. Results indicated that both of theiractivities reduced after continuously reacting sererval hours. In contrast,0.3（Mn-Fe）/ZSM-5catalyst had a better stability and H₂O resistance ability than0.3（Mn-Fe）/TiO₂catalyst, the main reason were explained that ZSM-5had stabilizedstructrue unit and good pore size distribution. In addition, the addtion of H₂O could leadto competitive adsorption, but this resistance to catalytic activtity was reversible.In all, activities of the catalysts were determined by the oxide speices andcrystallinity of active components, especially amorphous MnO₂production were thevital factor for catalytic activity. In addition, active components loadings could makedifferent disperse states and a good disperse state could promote amorphous MnO₂production.