【作者】 李博；

【导师】 张孝彬； 杨杭生；

【作者基本信息】 浙江大学 ， 材料物理与化学， 2016， 博士

【摘要】 随着工业经济的发展和社会的进步,人类生活水平的提高不可避免地带来了环境污染。其中,从日渐增长的燃煤电厂、柴油机、垃圾焚烧锅炉和汽车等和日常生活过程中排放出的氮氧化物(NOx)引发了酸雨、光化学烟雾等生态污染现象,并进一步演化为危害植物生长,甚至对人类健康构成威胁,已经成为大气的主要污染物之一。因而,严格减少NOx的排放量成为了各国迫在眉睫的研发课题。面对日益增长的NOx排放,选择性催化还原技术(SCR)已被证实是控制氮氧化物排放的最成熟有效的方法之一。选择性催化还原技术的重点是高效廉价的催化剂,其中钒基催化剂作为目前已经成熟并大量商业化应用的催化剂,存在工作温度偏高、引起SO2的氧化、易中毒失活,寿命偏短、以及具有高毒性等诸多缺点,这使得新型高效低温活性的环境友好催化剂研发成为全球性的研究热点。其中铜、铁和铈的氧化物作为催化剂的活性成分,因其优异的低温脱硝活性被广泛研究,而分子筛作为催化剂载体也得到广泛的关注。但是低温SCR面对的反应体系相对复杂,仍然亟需后续研究来进一步解析其脱硝机理,并克服催化剂的低温S02中毒的问题。针对上述问题,本文首先以ZSM-5(新型沸石)分子筛为载体,制备了一系列金属氧化物催化剂,并系统研究了其脱硝的反应路径和S02中毒机理。同时对CuO-CeO2复合金属氧化物催化剂上低温抗S02中毒机理进行了系统研究,取得了如下的研究成果：(1)揭示Fe-ZSM-5催化剂上脱硝机理和反应路径利用离子交换法制备一系列不同铁交换量的Fe-ZSM-5催化剂,在Fe的浓度较低时,Fe3+主要通过替换分子筛上的强酸性H+的位置,以离子形式存在,当Fe3+离子的浓度达到2.67 wt%时,全部的强酸性位H+都被替换,并且Fe3+离子成为NO2的主要吸附活性位。表观活化能计算显示,当反应温度低于250℃时,脱硝反应主要通过弱吸附的NH3和吸附态的N02之间的反应实现,遵循L-H机理；当反应温度超过250℃,吸附态的N02和气态的NH3之间成为脱硝的主要途径,遵循E-R机理。(2)揭示过负载Fen-ZSM-5催化剂上脱硝反应机理的温度依存性当进一步增加Fe3+离子浓度,Fe3+主要以FexOy团簇的形式负载在Fe-ZSM-5上,通过改变工艺,可以使一部分的强酸性位H+仍然保留在FeH-ZSM-5中,保留了催化剂吸附氨气的能力。FexOy团簇和分子筛之间的协同作用进一步提升了催化剂吸附NOx的性能。表观活化能计算显示,当反应温度在200℃以下时,反应主要在吸附的N02和未活化的吸附的NH3之间发生,遵循L-H机理；200-325℃时NO、NO2和NH3之间的快速SCR反应占据主导地位,遵循E-R机理；300℃以上时,催化剂对NH3的活化促进了NO和活化的NH3之间的反应,这有效补偿了因为高温下热力学限制引起的NO2浓度的下降对脱硝反应的抑制作用。同时高温下对NH3过氧化生成NO的抑制,保证了FeH-ZSM-5催化剂在400℃时达到91%的脱硝效率。(3)揭示FeH-ZSM-5和CuH-ZSM-5催化剂抗SO2中毒的机理在过负载的FeH-ZSM-5和CuH-ZSM-5催化剂上二氧化硫中毒的现象有显著差异。在FeH-ZSM-5上,SO2主要通过破坏催化剂中的FexOy团簇结构,使之发生团聚结晶,该过程不可逆,导致催化剂中毒后的性能不能恢复。催化剂脱硝性能下降的主要原因是其氧化NO和吸附NO2的性能的减弱。在CuH-ZSM-5催化剂上,S02并不引起催化剂结构的改变,相反提高了催化剂对N02和NH3的吸附能力。催化剂表面覆盖的硫酸盐和亚硫酸盐导致其在温度低于275℃时脱硝性能下降；当温度超过275℃时,大量的NO2的存在还可以促进硫酸盐的分解,反而使催化剂催化脱硝效率提升。因为催化剂结构没有改变,SO2引起的性能改变可以通过热处理恢复。(4)揭示低温下CuO-CeO2催化剂上O2对抑制SO2中毒的作用规律利用化学沉淀法制备了CuO-CeO2复合金属氧化物催化剂,调控催化环境中的O2浓度发现CuO-CeO2在低温环境下对SO2钝化后再生能力有不同表现。在240℃,1.0%的O2浓度下,只要SO2引起催化剂的表面的硫酸盐沉积达到1.34%的表面硫原子浓度,就引起催化剂的严重中毒。中毒后的催化剂可以在5%的02浓度下在线再生。研发出能在02浓度为10.0 vol%时,有效抑制S02对催化剂的钝化作用的新型催化剂,对催化剂的S02中毒过程中研究发现,N02可以和催化剂表面的硫酸铵中的铵根离子发生反应,生成具有强还原能力的NHX<3,可以在铜和铈离子的协同下,促进硫酸铵的低温分解,有效抑制低温下引起的SO2催化剂中毒,为工业环境下催化剂在低温领域的应用奠定了良好基础。更多还原

【Abstract】 The development of the industry, the progress of the society, and the improvement of people’s living standard inevitably cause the emission of various pollutants into the environment. Among them, nitrogen oxides (NOx) emitted from power plants, engines, waste incinerators and automobiles are one series of the most serious pollutants, which have caused not only the formation of acid rain but also photochemical smog. In fact, NOx have given rise to a variety of increasingly harmful impacts on plant growth and even human health. Strict reduction of NOx emission has become one of the hot research topics all over the world. Selective catalytic reduction (SCR) of NOx by NH3 is undoubtedly one of the most effective methods to decrease the NOx levels in gaseous emissions. The key point for SCR technique is the development of high-performance and cheap catalysts. As a widely used catalyst, the detrimental of vanadium-based catalyst is its high working temperature with a narrow temperature window, high activity for SO2 oxidation, short service life and high toxicity of Vanadia, which stimulated the continuing efforts to develop new effective catalysts which could be used at low reaction temperature. Recently, novel catalysts used Cu, Fe and Ce oxides as active components have attracted much interest because of their remarkable catalytic activity at low temperature. Meanwhile, Zeolites were recently well studied as potential catalyst supports. Up to now, however, Due to relatively complicated circumstance of low temperature SCR compared to the normal high-temperature SCR, the mechanisms for the low-temperature active SCR reaction is still not fully understood and deserve further study. Furthermore, the catalyst deactivation caused by SO2 at low temperature is still a big problem which restricted its application.With these as background, in this study we prepared a series of ZSM-5 zeolite supported metal oxides (CuOx and FeOx) catalysts, and the SCR of NO by NH3 was investigated systematically, mainly we focused on the temperature dependent of reaction mechanism and its SO2 induced catalyst deactivation. At the same time, we also clarified the mechanism of the resistance to SO2 induced deactivation over CuO-CeO2 catalyst at low temperature in NH3-SCR. The main achievements we obtained were listed as follows:(1) Clarification of SCR mechanism and reaction route over Fe-ZSM-5.Ion exchange method was used to prepare a series of Fe-ZSM-5 with different Fe loading. The Fe3+ mainly exchanged the strong acid H+ on zeolite, and existed as isolated ions at low Fe3+ concentration. All the strong acid H+ were exchanged by Fe3+ at a Fe3+ concentration of 2.67 wt%. NO2 were mainly adsorbed on Fe3+ ions, while NH3 was adsorbed on weak acid sites. According to the calculated apparent activation energy, we revealed that at temperature below 250℃, the NOx reduction followed the L-H mechanism through the reaction between adsorbed NO2 and adsorbed NH3. While at reaction above 250℃, the main reaction followed the E-R mechanism through the reaction between the adsorbed NO2 and gaseous NH3.(2) Clarification of temperature dependent SCR reaction mechanism over highly dispersed Fe2O3 loaded on ZSM-5 catalyst.For FexOy clusters heavy loaded on zeolite (FeH-ZSM-5), Fe3+ mainly existed as highly dispersed FexOy clusters, with further increasing of Fe3+ concentration. By changing the preparation process, part of strong acid H+ could be remained on FeH-ZSM-5, which changed its performance for NH3 adsorption. The synergy between Fe2O3 clusters and ZSM-5 also modified the NOx adsorptivity of FeH-ZSM-5, thus the SCR reaction routes changed accordingly. Based on the calculated apparent activation energy, at temperature below 200℃, the NOx reduction followed L-H mechanism through the reaction between adsorbed NO2 and non-activated but adsorbed NH3. At temperature between 200℃ and 325℃, fast SCR reaction between NO, NO2 and non-activated NH3 dominated the NOx removal, and the reaction followed E-R mechanism. The NH3 activation at temperature above 300℃ promoted the reaction between NO and activated NH3, which compensated thermodynamic limitation induced suppression of fast SCR. And the suppression of NH3 to NO over oxidation promised a high NO reduction efficiency of 91% at 400 ℃.(3) Clarification of SO2 deactivation mechanism over FeH-ZSM-5 and CuH-ZSM-5 catalyst.The performance of SO2 induced catalyst deactivation was significantly different over the heavy loaded FeH-ZSM-5 and CuH-ZSM-5 catalyst. Over the FeH-ZSM-5, SO2 induced the crystallization of FexOy clusters on the surface of ZSM-5. This catalyst structure change process is not reversible, as a result, the activity of FeH-ZSM-5 catalyst can not be recovered by post heat treatment. The inhibition of NO oxidation and NO2 adsorption was found to be the main reason for the suppression of de-NOx activity. The structure of CuH-ZSM-5 was not destroyed after the SO2 deactivation, on the contrary, the NO and NH3 adsorption were promoted compared to the fresh catalyst. Though the deposition of sulfate and sulfite blocking the active sites on the catalyst surface which suppressed its NH3-SCR performance at temperature below 275℃. At temperature above 275℃, however, the existence of sufficient NO2 promoted the decomposition of sulfate and thus improved its NOx removal efficiency. Because the catalyst structure was not destroyed during SO2 induced deactivation, the deactivation caused by SO2 could be recovered by post heat treatment.(4) Clarification of SO2 deactivation mechanism over CuO-CeO2 catalyst at low temperature.CuO-CeO2 catalyst was prepared by chemical deposition method, and its SO2 induced deactivation of selective catalytic reduction of NO over was studied at low temperature. In the case of reaction under low O2 concentration of 1.0 vol%, SO2 severely deactivated the catalyst at 240℃ with a surface S atomic concentration as low as 1.34%. However, the deactivated catalyst could be reactivated during online NO reduction under 5.0 vol% O2 without decreasing the surface S concentration of the catalyst, which could be attributed to the involvement of NO2 in the reactions. NO2 could promote the NO removal through three reaction routes:fast SCR reaction, reaction between NO2 and NH3, and reaction between NO2 and NH4+. Especially under conditions of 10.0% O2, the reaction between NO2 and NH3/NH4+ induced the formation of extra NHx<3 species which promoted the decomposition of surface-deposited sulfate to SO2 with the assistance of Ce2O3, further suppressed the accumulation of sulfate on the catalyst surface, and finally suppressed the SO2-induced catalyst deactivation at 240℃.更多还原