Study On Catalytic Kinetics Of Ammonia Selective Catalytic Reduction Over Metal-based MCM-22 Catalysts

Nitrogen oxides（NO_x）is one of the major pollutants in the atmosphere,which will cause serious damage to the environment and human health.Ammonia selective catalytic reduction（NH₃-SCR）has become the most widely used de NO_x technology due to its high efficiency,wide active temperature range and good application prospects.In this thesis,the NH₃-SCR catalytic performance and kinetic mechanism of Fe-and Cu-based MCM-22 catalysts were studied.The physicochemical property,structure,acid distribution,metal species distribution of each zeolite sample were analyzed.The catalytic structure-activity relationship of Fe-based zeolites in NH₃-SCR was obtained.Finally,the kinetic parameters of four zeolites were obtained through kinetic experiments.The main conclusions are as follows:（1）OP-Fe/MCM-22（x）catalysts with different Fe contents（x,1.86～4.88 wt.%）were prepared by one-pot method.IM-Fe/MCM-22 with Fe content of 4.40 wt.%and IM-Cu/MCM-22 with Cu content of 4.70 wt.%were prepared by impregnation method.All the samples are MWW structure with relative crystallinity ranging from 93 to 119%with similar crystal morphology,surface area and pore volume.With the increase of iron loadings,the particle size of OP-Fe/MCM-22（x）zeolite is slightly increased and aggregated iron oxides are gradually formed in zeolites.However,comparing with IM-Fe/MCM-22,the Fe species in OP-Fe/MCM-22（x）zeolites are more evenly dispersed.In contrast,a large amount ofα-Fe₂O₃ species was formed on IM-Fe/MCM-22.（2）According to UV-vis and ICP-AES characterizations,Fe species in OP-Fe/MCM-22（x）are mainly isolated Fe³⁺species and no Fe₂O₃ particles are formed.In addition,the concentration of isolated Fe³⁺species is positively correlated with iron loadings in Fe/MCM-22（x）.The low-temperature（≤300 ^oC）NH₃-SCR activity of Fe/MCM-22（x）gradually increases with the increase of iron loading.At high temperature（>500 ^oC）,the NH₃-SCR reaction activity gradually decreases with the increase of iron loading on Fe/MCM-22（x）due to the occurrence of ammonia oxidation side reaction catalyzed by Fe₂O₃ particles.IM-Fe/MCM-22 has fewer isolated Fe³⁺active sites than Fe/MCM-22（x）,which can explain its lower NH₃-SCR activity.In addition,by combining the NH₃-SCR activity of Fe/MCM-22（x）at low temperature（≤300 ^oC）,the isolated Fe³⁺concentration,and the calculated turnover frequency（TOF）in NH₃-SCR kinetic experiments at 150～200^oC,we can conclude that isolated Fe³⁺species are the main active sites of Fe/MCM-22（x）in NH₃-SCR at low temperatures（≤300 ^oC）.（3）The initial composition of feed gas and gas hourly space velocity（GHSV）are key kinetic parameters for reactor design.The concentration of oxygen in feed gas plays an important role in NH₃-SCR as oxygen can activate reactants and regenerate active metal sites.However,due to the limited concentration of active sites in catalysts,NO_xconversion of catalysts in NH₃-SCR can’t increase continuously by increasing the concentration of O₂,NH₃ and NO in feed gas.Therefore,the experimental conditions of NH₃/NO=1 and O₂=4.5 vol.%are selected to give full play to the potential of catalyst and to avoid the waste of active sites in catalysts.The influences of GHSV on catalytic performance of Fe/MCM-22 in NH₃-SCR decreases with the increase of reaction temperatures.Once reaction temperature exceeds 500°C,the NO_x conversion decreases due to the promotion of ammonia oxidation side reaction.（4）A reaction condition of 0.2947 m/s of feed gas linear velocity and 60～70 mesh of catalyst size is chosen to eliminate the external diffusion and internal diffusion limitations on the kinetic study of catalysts in NH₃-SCR.The reaction orders of NO,NH₃and O₂ in NH₃-SCR over OP-Fe/MCM-22（20）are approximately 1,0 and 0.5,respectively.Then,the first-order kinetic model of NH₃-SCR reaction over OP-Fe/MCM-22（20）is established,considering that oxygen in feed gas is excessive.Combining the first-order kinetic model and kinetic experiments,the kinetic parameters of four catalysts in NH₃-SCR at 150～225 ^oC are calculated as follows:for OP-Fe/MCM-22（20）,E_a=45.52 KJ/mol and A=1.85×10⁸mol/（g·min）;for OP-Fe/MCM-22（80）,E_a=44.58 KJ/mol and A=6.7×10⁷ mol/（g·min）;for IM-Fe/MCM-22,E_a=26.10KJ/mol and A=2.01×10⁵ mol/（g·min）;for IM-Cu/MCM-22,E_a=57.10 KJ/mol and A=3.38×10⁹ mol/（g·min）.（5）Apparent activated energy（E_a）only reflects the energy barrier of the redox reaction on active sites,which can’t represent the intrinsic catalytic activity of active sites in NH₃-SCR.As the electrostatic interactions between aggregated Fe_xO_y species and zeolite framework is relatively weak,IM-Fe/MCM-22 with large amounts of Fe_xO_yspecies had low E_a value,which means it is easy to activate those kind of Fe species.In contract,OP-Fe/MCM-22（20）with large amounts of isolated Fe³⁺species which have strong interactions with zeolite framework,shows a larger E_a value than IM-Fe/MCM-22due to a higher redox energy barrier of isolated Fe³⁺species in NH₃-SCR.According to our results,pre-exponential factor（A）mainly represents the number of active catalytic sites in NH₃-SCR over catalysts.According to Arrhenius equation,the apparent activation energy and the pre-exponential factor may have opposite effects on the reaction rate in NH₃-SCR over metal-based MCM-22 zeolites,i.e.,the kinetic compensation effect.