Investigation On Efficient Catalysts And Their Performance For Selective Reduction Of NO_x With NH₃

Nitrogen oxides,the key contributor to atmospheric contaminations,are harmful to human health and environment.The elimination of NOx has been a hot topic of environmental catalysis in recent years.Selective catalytic reduction by ammonia(NH3-SCR)is one of the most commonly applied technologies for the abatement of nitrogen oxides(NOx)in the presence of excess oxygen for the stationary and mobile sources.As to the elimination of NOx for mobile sorces,employing Beta zeolites as substrate has been extensively investigated and Fe modified samples have exhibited excellent activity.However,application of Fe-Beta zeolites is limited to some extent by catalyst deactivation with hydrocarbon(e.g.C3H6),which is mainly attributed to the coke formation through the polymerization of C3H6 on the zeolite.Based on such background,a new strategy has been proposed to introduce additives in Fe-Beta by mechanical mixing or ion-exchange,for the purpose of promoting the C3H6 partial oxidation,consequently preventing C3H6 deposition on the zeolite and therefore improves the resistance to hydrocarbon of Beta zeolite.Moreover,the lamellar-like CoMn oxide catalysts aiming to utilize in the abatement of NOx for stationary sources have been designed,the effect of surface oxygen species and acid sites on activity has been clarified and the structure-activity relationships of this type of catalysts have also been established.The main results have been summarized as follows:Based on the superior oxidation property of MnOx/CeO2 oxides,a series of hybrid catalysts were prepared by mechanical mixing of zeolites and oxides,and their activity for the selective catalytic reduction of NOx with NH3 was investigated in the presence of propene.After systematically studied by XRD,H2-TPR and XPS,the results showed that the new phase being assigned to(Mn0.983Fe0.017)2O3 or Mn2O3 had formed by the interactions between Fe-Beta and MnOx/CeO2 during the mechanical mixing,which was beneficial for the improvement of oxidation property.The activity results demonstrated that the catalyst prepared by mixing FeBeta and MnOx/CeO2 in a mass ratio of 1:1 exhibited high SCR activity,NOx conversion in the temperature range 200-400℃ had exceeded 90%at a GHSV of 80,000 h-1,with low selectivity to N2O.By the combined DRIFTS and MS measurements,the function of MnOx/CeO2 in the hybrid catalysts was discussed.This hybrid catalyst was able to convert C3H6 to intermediates containing C=Oads and COO-ads functionalities,thereby decreasing carbon deposition on the zeolite and reducing the competitive adsorption between C3H6 and NOx.Moreover,these intermediates could react with NOx at lower temperatures than do the carbonaceous deposits on Fe-Beta,paving a new way to convert NOx at low temperature and enhancing the NOx reduction activity.Taking advantage of the abundant ion-exchange sites in Al-rich Beta zeolite,a series of copper modified Fe-Beta were prepared.Their activity for the selective catalytic reduction of NOx with NH3 was investigated in the presence of propene and the function of Cu had been revealed.The structural and redox property of as-prepared samples had been examined via XRD,H2-TPR and UV-Vis-NIR.The results showed that Cu and Fe were highly dispersed in the bimetallic zeolite while small CuOx particles(less than 5 nm)existed in the Cu6.8-Fe-Beta sample.After introduction of Cu,the redox property of catalyst had been enhanced.Consequently,Cu6.8-Fe-Beta had shown better C3H6 oxidation activity compared with Fe-Beta.Moreover,Cu6.8-Fe-Beta possessed NOx conversion higher than 80%in a wide temperature range(200-550℃)and preferable N2 selectivity for NH3-SCR in the presence of C3H6 at a GHSV of 80,000 h-1.The introduction of Cu could promote the partial oxidation of C3H6 and inhibit the polymerization of C3H6,thus alleviated the competitive adsorption between C3H6 and NOx.Furthermore,Cu6.8-Fe-Beta could maintain great NOx conversion after hydrothermal aging at 750℃.And the SO2 resistance could also be adjusted via changing the Cu content,while Cux-Fe-Beta with lower Cu content(1.2-2.3 wt%)possessed better SO2 resistance.In order to further improve the low-temperature activity of Mn-based oxides for NH3-SCR,a series of CoMn composite oxides with lamellar morphology had been prepared by coprecipitation.CoMn oxide calcined at 250℃(CoMn-LS-250)exhibited high activity,giving 91%NOx conversion at 60℃ and maintained 73.2%NOx conversion at 300℃.After 8 h on stream with H2O and SO2 at 300℃,the NOx conversion of CoMn-LS-250 was still higher than 90%.The excellent activity of CoMn-LS-250 was related to its abundance of Lewis acid sites as well as its superior redox ability which was advantageous to the adsorption and activation of NH3.By the combined DRIFTS and MS measurements,the reaction path can be inferred as:(i)the catalyst was able to trap NH3 and oxidized NH3(ads)to NH2(ads);(ii)NH2(ads)subsequently combined with NO to form the key intermediate,NH2NO(ads),which decomposed at low temperature(ca.85℃)with release of N2.The role of gaseous O2 was regenerating the surface oxygen species to complete the redox cycle.