MnO_x-CoO_x Catalysts For Selective Catalytic Reduction Of NO_x With NH₃at Low Temperature

NOx(NO, NO2and N2O) emmision in the flue gas of power plants can causeserious environmental problems, e.g. acid rain, photochemical somg, greenhouse effect.At present, the selective catalystic reduction (SCR) of NOxby NH3is the most effectiveway to remove NOxin the flue gas. The NH3-SCR process operated at medium to hightemperature (300-400oC) with vanadium catalysts has already be commercialized,however, in that process, the catalyts have to be set up at the upsteam of processesbefore desulfuration and electrostatic precipitation to meet the active temperature of thecatalysts, which leads to the attrition and poisoning of the catalysts. Therefore, thecatalysts with low active temperature that can be used in the low-temperature NH3-SCRprocess have been attracted more and more attention in recent years.NO in the flue gas can be removed via two routines: the standard SCR(NO+O2+NH3) or the fast SCR (NO+NO2+NH3). For the fast SCR has a much fasterraction rate than the standard SCR, the NH3-SCR reaction rate can be increased byimproving the activity of the catalysts to catalyze the oxidiation of NO to NO2. Cobaltoxides is reported to be quite effective for NO→NO2oxidation, manganese oxide hasexcellent low-temperature SCR activity. In this study, the performance of the mixedCoOx–MnOxcatalysts in the NH3-SCR reaction was studied to combine the advantagesof cobalt oxide and manganese oxide.In this study, the effect of the preparation methods was investigated. BET and TPRtests showed that the catalysts prepared by the co-precipitation method have larger BETsurface and low reduction temperatures than those prepared by the citric acid methodand solid reation method. Accordingly, the activity evalaution tests showed that thecatalysts prepared by the co-precipitation method exhibits a much higher NO conversion,Which demonstrated that co-precipitation is the best method to prepare he mixedCoOx–MnOxcatalysts.Subsquently, based on the catalysts prepared by co-precipitation method, the effectof the calcinations temperature and mole ratio of Mn/Co was investigated. Theexperimental results showed crystal form changes with calcination temperature. With arelative low calcination temperature(400℃), catalysts exist in the form of Co3O4whichfacilitates the oxidation of NO. As the calcinations temperature increases catalysts existin the form of spinel. However, the BET surface of the spinel catalysts drops sharply and reduction temperatures increase obviously. When the Mn/Co mole ratio greater than50%, catalysts tends to form a spinel structure and solid solution. When less than orequal to50%, catalysts exist in the form of Co3O4. And catalyst with a Mn/Co moleratio of5/5shows the best catalystic activity.Finally, CoOxwas proved to be more active than MnOxin oxidizing NO to NO2.So the removal of NO via both fast and standard SCR. The results of XPS show that theratio of Mn4+decreased after reaction and increased again after reactivation, the ratio ofCo3+shows a opposite trend, based on which redox cycle were deduced. The redoxcycle can be summarized as:Co2Mn4Co3Mn3.