Study On The Preparation,Characterization And Catalytic Performance Of Mn-based Catalysts For Low Temperature NH₃-SCR

Oxynitride is one of the main air pollutants.The technology of selective catalytic reduction?SCR?of NOx by NH₃ is one kind of widely used flue gas denitrification technology in the industry.On account of its low reaction temperature?<150 ^oC?,low energy consumption,low operating cost and the catalyst with innocuous to the environment and human health,recently,low-temperature SCR process has attracted more and more attention.Based on the review of the current situation of low-temperature SCR technology,it was pointed out that the following shortcomings are the main problem in low-temperature SCR technology:low activity at low temperature,the main factors affecting catalytic activity are disputed,poor H₂O resistance and SO₂ resistance of catalysts,and uncertainty of reaction and SO₂-poisoning mechanisms.In order to solve these problems,the low-temperature SCR catalytic performance was systematically investigated by using a series of manganese based catalysts in this dissertation.Firstly,the effect of Mn oxidation state?chemical valence?on the low-temperature SCR activity was deeply investigated.For this purpose,the pure MnOx?MnO,Mn₃O₄,Mn₂O₃,MnO₂?and physical mixed 30%MnOx/TiO₂ catalysts,as well as three Mn/TiO₂catalysts prepared by impregnation,were investigated for the low temperature NO-SCR with NH₃ in the presence of excess O₂.The catalysts were characterized by N₂adsorption,XRD,XAFS,and H₂-TPR to probe the physical and chemical properties of catalysts.By calculating the NO turnover rate?NO-TOR?on the surface of catalysts,it was found that the MnOx with different chemical valence exhibited distinct intrinsic activity.The Mn₃O₄ and MnO/TiO₂ showed the highest intrinsic activity?TOR?for the unsupported and supported catalysts,respectively.By analysising and comparing the data,we found that the MnOx species with low chemical valence?MnO,Mn₃O₄?exhibited overall high intrinsic SCR activity(larger than 0.012 NO h^-1 nm^-2_MNOx)for the first time.The MnOx-TiO₂ interaction played important role for both intrinsic and apparent activity,especially for catalyst synthesized by impregnation.And the MnOx with different valence showed some change?normally oxidation?after physically mixed with TiO₂.Secondly,we further researched that the effect of addition rare earth elements to manganese base catalyst for its low temperature SCR activity,water resistance and sulfur resistance.A series of rare-earth metal doped 30%Mn-3%RE/TiO₂?RE=Ce,Sm,Nd,Er,Y?catalysts were prepared by adopting two step incipient wetness impregnation method and investigated for the low-temperature SCR of NOx with NH₃.It was found that 3%of rare earth elements?Nd,Er,Y?was added to 30%Mn/TiO₂catalyst even could increase the catalytic performance at the low temperature,in addition to usual used Ce and Sm.And the promoting effect of Nd was among the most significant.According to the result of characteriation,we found that the addition of Nd made surface area and dispersion of MnOx species increase.It also led both the content of amorphous MnO₂ and the content of crystalline Mn₂O₃ to decrease,but the proportion of MnOx species with low oxidation state increased.H₂-TPR results showed that the low temperature reduction peak of 30%Mn-30%Nd/TiO₂?i?catalyst was shifted to much lower temperatures with the doping of Nd,which increased the redox property of the catalyst.NH₃-TPD results also showed that the Nd doping was able to make the capacity of NH₃-adsorption of catalysts increase at low temperature.The water and sulfur resistance was also modified by the introduction of Nd to Mn/TiO₂catalyst.Finally,the 30%Mn-3%Nd/TiO₂ catalyst was prepared by another two methods,Coprecipitation and Sol-gel,for a comparative study.The catalysts prepared by Coprecipitation and Sol-gel method both showed better low temperature SCR activity.Various techniques were applied to characterize the catalysts.It was found that catalysts prepared by Coprecipitation and Sol-gel method exhibited higher surface area and smaller average particle size in comparison with catalyst prepared by impregnation with TiO₂ support.