Property Study On Removal Of Nitrogen Oxides By MnFeCeO_x/Al₂O₃ Nano Catalyst

Nitrogen oxides(NOx)is one of the important factors affecting atmospheric pollutants.Large amounts of nitrogen oxides can cause acid rain,global warming and ozone depletion.The use of selective catalytic reduction of nitrogen oxides(SCR)on high-efficiency catalysts using NH3 or urea is the most economical and effective technique for eliminating nitrogen oxides.The main SCR reactions include standard SCR reactions,fast SCR reactions,and NO2 SCR reactions.Studies have shown that the rate of fast SCR reaction is significantly improved compared to the standard SCR reaction.NOx is usually a mixture of 5%NO2 and 95%NO.The NO2 ratio in NOx can be increased by pre-oxidation of NO,thereby increasing the efficiency of SCR reaction.The most widely used catalyst in the near future is V2O5/TiO2 with WO3 or MoO3 as an auxiliary agent,but the catalyst is strictly limited by the temperature window,and does not exhibit sufficient NOx removal activity below 250°C.Therefore,low temperature SCR catalysts have an urgent market demand.FeMnOx-based catalysts not only exhibit outstanding low-temperature SCR activity,but also promote NO oxidation to NO2.However,the use of such manganese-based catalysts is greatly limited due to low N2 selectivity and low catalytic activity at low temperature.It has recently been found that Ce-based catalysts have a high selectivity for the reduction of nitrogen oxides in the low temperature range,in addition,niobium is also beneficial for the oxidation of NO to NO2.On this basis,a series of MnFeCeOx/Al2O3 nanocatalysts with different molar ratios of ferromanganese and antimony doping were prepared.Scanning electron microscopy(SEM),transmission electron microscopy(TEM),X-ray diffraction(XRD),X-ray photoelectron spectroscopy(XPS),hydrogen temperature programmed reduction(H2-TPR),ammonia temperature programmed desorption(NH3-TPD),and nitric oxide temperature programmed desorption(NO-TPD)are used to analysis the physicochemical properties of the catalyst samples,therefore reveal the relationship between structure and properties.The catalytic activity of the catalyst sample in the oxidation of NO to NO2 and fast SCR reaction was studied,and the synergistic effect of lanthanum with manganese and iron was further explored.The possibility of preparing nanocatalysts with excellent catalytic performance for the oxidation of NO to NO2 and fast SCR reaction was discussed.We find following interesting phenomena:when molar ratio of manganese to iron reaches the optimum ratio 1.0,the catalyst reaches the optimum coexistence state of manganese,iron and antimony,the active material is widely distributed,and the crystallinity of the metal oxide is suppressed.At the same time,it is found that the ratio of Mn4+/(Mn2++Mn3+Mn4+),Fe2+/(Fe2++Fe3+)and Ce3+/(Ce3++Ce4+)is also the maximum value,which can promote the catalytic oxidation of NO to N02 to improve the fast SCR catalytic activity.The optimum cerium doping amount is 1.0%(mol/mol),and the NO.conversion rate is greater than 95.5%at 100-350 ?,reflecting a strong interaction between Ce,Mn and Fe.After introduction of Ce,the oxidation rate of NO and the NOx conversion rate of fast SCR reaction were significantly improved.On the other hand,cerium doping enhances the coexistence of iron and manganese,which improves the dispersion of active elements and reduces the crystallinity of metal oxides.Besides,the addition of cerium improves the ratio of Mn4+/(Mn2++Mn3++Mn4+)and Fe3+/(Fe2++Fe3+),and enriches the lattice oxygen and oxygen vacancies.This promotes the oxidation of NO to NO2 and further promotes a fast SCR reaction.In general,the MnFeCeOx/Al2O3 catalysts show the potential of an excellent catalyst for simultaneously catalyzing NO pre-oxidation and fast SCR reactions.