Low-temperature Selective Catalytic Reduction Of MxOy/MWCNTs Catalysts And Reation Mechanism Study

Selective catalytic reduction （SCR） of NO_x with NH3is one of the most effectiveflue gas cleaning technologies for stationary sources. Catalysts are the crucial factorin the SCR technology, which determinate the efficiency of the SCR system. However,the commercial catalysts （V₂O₅–WO₃（MoO₃）/TiO₂） for this process are active withinthe narrow temperature window of300–400℃, and they would be deactivated by highconcentration of SO₂or dust. Since the temperature of the flue gas after thedesulfurizer and particulate removal device is generally below200℃, it is veryimportant to develop low-temperature SCR process and catalysts. Mn and Ce havehigh low-temperautre SCR activity due to their unique chemical characteristics.Additionally, Carbon nanotubes （CNTs） have been reported to be excellent catalystsupports based on their unique electronic properties and structure, such as highmechanical strength, thermol-chemical stability, field limitation, and wellconductivity, etc.A series of M_xO_y/MWCNTs catalysts （M=Mn or Ce） were prepared by the porevolume impregnation method. Before the preparation, the MWCNTs were pretreatedwith oxygen plasma. The SCR activity, structure, and chemical characteristics werestudied. And the results were shown below.Firstly, the SCR activity tests were carried out in a fixed-bed reactor. The effects ofMn loading, calcination temperature, and MWCNTs outer diameter on selectivecatalytic reduction （SCR） of NO with NH3were investigated. The results showed thatMnO_x dispersion on MWCNTs varied with the Mn loading, calcination temperature,and MWCNTs outer diameter. Three states of manganese species （MnO2, Mn3O4, andMnO） coexisted on MnO_x/MWCNTs catalysts, and most of the Mn existed as MnO₂and Mn₃O₄. Additionally, among the MnO_x/MWCNTs catalysts prepared, MWCNTs（60-100nm） supported10wt.%manganese calcined at400℃catalyst presented thebest SCR activity. Secondary, the SCR catalytic activity and characteristics of Mn-Ce/MWCNTscatalysts were compared with that of the Mn/MWCNTs catalyst. The results showedthat the addition of cerium oxides could improve the SCR activity of Mn/MWCNTscatalyst. The Mn-Ce/MWCNTs catalyst with Ce/Mn=0.6had the highest activity. Theaddition of cerium oxides could enhance the dispersion of metal oxides on theMWCNTs. It could also increase the specific surface area and total pore volume, anddecrease the average pore size. Ce would improve the concentration of oxygen andthe valence of manganese. The crystalline MnO_x disappeared by introducing Ce tothe catalyst.At last, the in situ DRIFTS results indicated that the main phases of NH3werecoordinated NH₃and NH₄⁺, both of them would react with the gas-phase NO andadsorptive NO. The active phases of adsorptive NO varied with the reactiontemperature increasing. Besides, the routine of the SCR reaction on theMnO_x/MWCNTs catalysts at low reaction temperatures belonged to the E-Rmechanism, while it belonged to E-R mechanism or L-H mechanism at higherreaction temperatures.