The Study Of A Novel Mn-Ce/ZSM-5 Catalyst For Selective Catalytic Reduction（SCR） Of Nitric Oxide With NH₃ At Low Temperature

Selective catalytic reduction (SCR) method is one of the most effective ways to remove NOx in diesel exhaust emission aftertreatment technology. Because of higher catalytic activity, N2 product selectivity, SO2 resistance ability and broader operating temperature, TiO2-based metal oxide catalyst has been the hot research in NOx-SCR reaction at low temperature. But the shortcoming is that the anatase TiO2 can be easily transformed into the rutile phase at high temperature, resulted catalyst deactivation. HZSM-5 has been used as new catalytic materials in NOx-SCR reaction due to its high hydrothermal stability and shape-selecting selectivity. In this paper, after vast screening test, a novel Mn-Ce/ZSM-5 catalyst for selective catalytic reduction (SCR) of nitric oxide with NH3 was prepared using HZSM-5 as carrier, transition metal manganese as basal active component and rare earth elements Ce as promoter, in order to seek a catalyst with good performance. Research conclusion summarized as follows:(1) Mn-Ce/ZSM-5 catalyst exhibits the best SCR performance at low temperature when the mass fraction of Mn and Ce is 10% and 8% respectively, calcination at the temperature of 400℃.(2) The effect of preparation methods on catalytic activity of Mn-Ce/ZSM-5 catalyst for SCR of NO is obvious.The sample prepared by co-precipitation method shows higher SCR activity and broader temperature window. The NO conversion rate of over 92% was observed at 100℃, and the NO conversion rate can maintain an invariable value of 100% during a broad temperature window of 120℃ to 400℃.(3) When SO2 and H2O steam were both added in feed gas, the restraining effect of H2O steam and SO2 on catalytic activity was obvious below the temperature 200℃. When the temperature is higher than 300℃, the effect of SO2 and’H2O steam will completely disappear.(4) H2-TPR analysis showed that the redox ability and oxygen storage capacity of Mn/ZSM-5 are enhanced when Ce element is added, and with the increaseing of the Ce content, these abilities continuously increased.(5) NH3-TPD analysis showed that doping Ce component can promote the formation of active sites and enhance the strength of acid.And it can also promote the adsorption ability of NH3 on the surface of Mn-Ce/ZSM-5.The effect of calcination temperatures on catalytic activity mainly in change the acid quantity at some extent.(6) The structure and the phase of the Mn-Ce/ZSM-5 catalysts prepared by using different methods were investigated using XRD, XPS and ICP techniques. The results showed that Ce species existed mainly in Ce4+ in Mn-Ce/ZSM-5 catalyst, and many kinds of Mn oxidation state coexisted in Mn-Ce/ZSM-5 catalyst.The main form includes MnO2, Mn2O3, Mn3O4 and MnO, etc.The distinction among Mn-Ce/ZSM-5 catalysts prepared by different methods is MnOx phases. The sample prepared by impregnation method possesses much MnO2, the sample prepared by physical mixture method possesses much MnO2 and Mn2O3, but the sample prepared by co-precipitation method possesses much lower valence bond MnOx phases, such as Mn2O3 and Mn3O.