Study Of Cr-Mn Supported Catalysts For Selective Catalytic Reduction Of NO_x At Low-temperature

NO_xis one of the main air pollutants can cause photochemical smog, acid rain,ozone hole and the greenhouse effect and other environmental problems. Selectivecatalytic reduction （SCR） is an effective way to remove NO_xin the flue gas fromstationary sources. Recently, low-temperature SCR process has attracted more andmore interests for its low energy consumption, operating cost and efficient.Our previous work showed that Cr（0.4）-MnO_x（0.4） denotes the molar ratio of Crto the total metals) prepared by citric acid method exhibited the best activity. At120oC,98.5%NO conversion was achieved with a gas hourly space velocity of30000h^-1.The results found that the Cr-MnO_xmixed oxide formed a new phase CrMn_1.5O₄,which play a major role in the selective catalytic reduction of nitrogen oxides. If theCr（0.4）MnO_xmixed oxide want to achieve industrial applications, it needed toindicate the active component loaded on the porous carrier materials. So, the focus ofthis paper is use right the porous carriers to support the Cr（0.4）MnO_x, make the activecomponent dispersed into a more active grain, in order to get more activity center-CrMn1.5O4. Meanwhile improving the catalysts’ the sulfur resistance, stability andmechanical properties.In this thesis, the catalysts prepared by different methods and study their lowtemperature SCR activity. Results showed that catalysts prepared by the citric acidsolution impregnation method1（CA-IM） have better catalytic activity and the surfaceof the carrier generated CrMn_1.5O₄activity phase. On the basis of this preparationmethod, Cr（0.4）MnO_xwere supported on γ-Al₂O₃, HZSM-5, active carbon （AC）, TiO2and SiO2respectively, and investigated the low temperature SCR activity and sulfurresistance of these catalysts. From the comparison results, it was found thatCrMn/γ-Al₂O₃had higher low-temperature activity and better sulfur resistance thanother supported samples. X-ray diffraction （XRD） and X-ray PhotoelectronSpectroscopy （XPS） characterization found that the metal Cr and Mn on γ-Al₂O₃almost in the form of CrMn_1.5O₄, which further demonstrates the CrMn_1.5O₄was the active phase of the Cr-Mn catalysts. The article also examines the impact of theCr-Mn content on the catalysts’ activity and sulfur resistance. The results show thatthe CrMn/γ-Al₂O₃with Cr content is30%（massratio base on γ-Al₂O₃） yielded nearly99.6%NO conversion at180oC at a space velocity of30000h^-1. When100ppm SO₂was added to the reactants,4h later NO conversion decreased to about92.6%, atwhich it stabilized. Upon removal of the SO₂supply, the activity was rapidly restoredto nearly99.5%of its original level. The XPS studies have shown, the surface oxygenof Cr_30%Mn/γ-Al₂O₃is almost in the form of chemisorbed oxygen, which cause bygenerating a large number of CrMn_1.5O₄.