不同晶相MnO_(2)催化剂的NH_(3)-SCO反应性能

NH_(3)-SCO Reaction Performance of Different Crystal Phase MnO_(2) Catalysts

以柴油车尾气净化过程中氨逃逸的治理为背景,采用水热合成法制备了不同晶相(α-,β-和δ-)MnO_(2)、并考察晶相结构对低温氨气选择性催化氧化(NH_(3)-SCO)反应性能的影响;并运用多种手段,如X射线衍射(XRD)、扫描电子显微镜(SEM)、氮气低温吸-脱附、X射线光电子能谱(XPS)、氢气程序升温还原(H2-TPR)、氧气程序升温脱附(O_(2)-TPD)、氨气程序升温脱附(NH_(3)-TPD)等对催化剂的组织结构、化学状态和氧化还原性能进行表征。研究结果发现MnO_(2)的晶相结构和隧道结构与氨气选择性催化氧化反应性能密切相关。具有独特的[2×2]隧道结构的α-MnO_(2)具有最佳的反应性能,NH3可在175℃实现全转化且N_(2)选择性为94%,其在120℃时的反应速率是具有层状结构δ-MnO_(2)的12倍。α-MnO_(2)表面具有较高的Mn^(3+)/Mn^(4+)比有利于催化剂氧化还原性能的提高和对氧的活化,丰富的酸中心和酸量有助于对NH3吸附和活化;二者的共同作用使得该催化剂具有良好的反应性能。

With the increase of automobiles,the emission of NO_(x)worsensed the atmosphere in the case of photochemical smog and acid rain.How to efficiently remove NO_(x)became urgent for environment protection.Although NH3 selective catalytic reduction of NO_(x)was a mature technology for reducing NO_(x)emission.To efficiently remove NOx,the over stoichiometric NH_(3)in application resulted in the NH_(3)slip causing serious environmental pollution and endangered human health.NH3 slip could be avoided by ammonia selective catalytic oxidation(NH_(3)-SCO)technology,which could convert NH3 into N_(2)and H_(2)O in an oxidizing atmosphere.MnO_(2)had been widely used in catalytic fields because of its good low-temperature catalytic activity,and its reaction performance was closely related with crystal phase structure.Mn-based catalysts with different crystallization phases,α-,β-andδ-MnO_(2)prepared by hydrothermal synthesis,were investigated for NH_(3)-SCO.The physical and chemical properties of the catalysts were characterized by X-ray diffraction(XRD),scanning electron microscope(SEM),N_(2)adsorption-desorption and X-ray photoelectron spectroscopy(XPS).Meanwhile,the redox property was tested by oxygen temperature programmed desorption(O_(2)-TPD)and H_(2)temperature programmed reduction(H_(2)-TPR),and acid by ammonia temperature programmed desorption(NH_(3)-TPD).According to the test results,the order of catalyst activity and N_(2)selectivity followed the sequences:α-MnO_(2)>β-MnO_(2)>δ-MnO_(2).Complete conversion of NH3 were achieved onα-MnO2 at175℃.with 94%N_(2)selectivity,and the by-products were N2 O and NO.The reaction rates(120℃)and low reaction energy also confirmedα-MnO_(2)was most active in NH3-SCO,whose reaction rate was 12 times higher than that ofδ-MnO_(2).Considering the byproducts,it was worth mentioning that except N2 O and NO,NO2 was also detected onβ-MnO2 andδ-MnO2.SEM results showed that MnO_(2)with different crystalline phase,α-,β-andδ-MnO2 were in the form of nanowire with(2×2)tunnel structure,nano rod with(1×1)tunnel structure and flowerlike with layered stack structure,respectively.The particle size ofδ-MnO_(2)was 12.4 nm,while it was up to23.2 nm onβ-MnO_(2).XPS displayed that the ratio of Mn^(3+)/Mn^(4+)onα-MnO_(2)was up to 2.24 with the lowest average of oxidation state(AOS)of Mn(3.49),and the ratio was only 0.79 onδ-MnO_(2)with the highest average of oxidation state(AOS)of Mn(3.83).The increase in the Mn oxidation state led to the decrease in the amount of adsorbed oxygen.In the H_(2)-TPR profiles,it was found thatα-MnO_(2)had the lowest onset reduction temperature indicating the easily reducible property ofα-MnO_(2).Compared with the O_(2)-TPD profiles ofβ-MnO_(2),it was found that the onset temperature for O2 desorption was much lower onα-MnO_(2)andδ-MnO_(2).Furthermore,a broad and intensive O_(2)desorption peak was found showing the high amount of adsorbed oxygen species onα-MnO_(2).Acidity and amounts played an important role in ammonia adsorption and activation,so NH3-TPD were conducted.It was shown in the NH_(3)-TPD results that the distribution of acid sites and the acid amount were closely related with the crystalline phases.Strong acid sites and the highest total acid amount were found onβ-MnO_(2),in contrast,the lowest total acid amount with even distribution of strong and weak acid sites were detected onδ-MnO_(2).The increase in the highest total acid amount were beneficial to ammonia adsorption,and the optimum acid strength would further favor ammonia activation.Based on the test and characterization results,nanowireβ-MnO_(2)with(2×2)tunnel structure had higher ratio of Mn^(3+)/Mn^(4+)which was beneficial to the improvement in redox and oxygen activation.Meanwhile,different kinds of acid center and abundant acid amount on the surface favored the adsorption and activation of NH_(3).So,the combination of the two properties madeα-MnO_(2)own the high performance in NH_(3)-SCO.