姚小江,张 雷,孔婷婷,李红丽,杨复沫,,3*
(1.中国科学院重庆绿色智能技术研究院 水库水环境重点实验室,重庆 400714;2.重庆三峡学院 环境与化学工程学院,重庆 404100; 3.长江师范学院,重庆 408100)
综述与展望
低温氨-选择性催化还原氮氧化物催化剂的研究进展
姚小江1*,张雷2,孔婷婷1,3,李红丽1,杨复沫1,2,3*
(1.中国科学院重庆绿色智能技术研究院 水库水环境重点实验室,重庆 400714;2.重庆三峡学院 环境与化学工程学院,重庆 404100; 3.长江师范学院,重庆 408100)
摘要:氮氧化物(NOx)的排放对人类健康和植物生长造成了严重危害,对其进行净化治理刻不容缓。火电厂烟气是NOx的主要来源,氨-选择性催化还原(NH3-SCR)技术可对其排放进行有效控制。V2O5-WO3(MoO3)/TiO2脱硝催化剂的工作温度偏高,不能满足低温宽工作温度窗口等工况的需要,因此,开发具有宽工作温度窗口的低温脱硝催化剂成为研究热点,其中,钒基、锰基金属氧化物催化剂和金属离子交换分子筛催化剂的研究最为广泛。探讨催化剂脱硝性能的关键影响因素、抗水抗硫性能以及反应机理,有助于为高效、实用的低温脱硝催化剂的设计和开发提供科学依据。从钒基金属氧化物催化剂、锰基金属氧化物催化剂、金属离子交换的分子筛催化剂、低温脱硝催化剂的抗水抗硫性能以及低温NH3-SCR反应机理等方面对近年来国内外低温脱硝催化剂的研究进展进行综述。今后需要解决N2选择性不够理想、抗水抗硫性能差、低温工作温度窗口较窄和反应机理不统一等问题。
关键词:三废处理与综合利用;氮氧化物;低温氨-选择性催化还原;金属氧化物催化剂;分子筛催化剂;抗水抗硫性能
CLC number:X701;TQ426.99Document code: AArticle ID: 1008-1143(2016)06-0001-09
氮氧化物(NOx)作为主要的大气污染物,不仅导致酸雨和光化学烟雾的发生,还能参与PM2.5的形成,严重危害人类健康和植物生长[1-3]。火电厂是NOx的主要来源,对其烟气进行脱硝处理刻不容缓。氨-选择性催化还原(NH3-SCR)NOx是火电厂烟气脱硝的最实用技术。目前,V2O5-WO3(MoO3)/TiO2由于具有优异的脱硝性能和抗水抗硫性能而被选为工业化脱硝催化剂,广泛应用于NH3-SCR过程,催化剂的最佳工作温度为(300~400) ℃[4-6],因此,脱硝装置必须设置在除尘器和脱硫单元上游以满足工作温度的需要。在此工况下,脱硝催化剂容易因烟尘和二氧化硫堵塞和中毒,导致严重失活。
为满足各国政府愈加严格的排放法规,诸多火电厂必须进行改造,增加脱硝装置。然而,这些火电厂的各个结构单元通常布局非常紧凑,很难在除尘器和脱硫单元上游增加脱硝装置,而只能设置在下游。由于此处的烟气温度通常已降至200 ℃以下[2],传统的V2O5-WO3(MoO3)/TiO2脱硝催化剂已不能达到理想的催化效果,很难满足排放要求,开发低温脱硝催化剂势在必行。此外,采用低温脱硝催化剂还可避免因烟气加热带来的能源消耗,经过除尘器和脱硫单元之后,烟气中烟尘和SO2含量已降至较低,可有效减缓低温脱硝催化剂的堵塞和中毒。目前,低温脱硝催化剂的研究主要为金属氧化物催化剂[7-9]和分子筛催化剂[10-12]。
本文从钒基金属氧化物催化剂、锰基金属氧化物催化剂、金属离子交换的分子筛催化剂、低温脱硝催化剂的抗水抗硫性能以及低温NH3-SCR反应机理等方面对近年来低温脱硝催化剂的研究进展进行综述。
1钒基金属氧化物催化剂
工业化脱硝催化剂V2O5-WO3(MoO3)/TiO2具有生产工艺成熟、抗水抗硫性能优异等优点,在该催化剂体系基础上进行改进研究以提高其低温脱硝性能成为研究热点。通常,调整V2O5负载量、改变V物种前驱体、引入助剂、调变载体以及优化制备方法是常用提升钒基催化剂低温脱硝性能的有效途径。Cha W等[13]研究表明,对于V2O5/TiO2催化剂,其低温脱硝活性随着V2O5负载量的增加而提高,V2O5负载质量分数7%时,V2O5/TiO2催化剂活性最佳,200 ℃时脱硝率达到96%。原因归属于多聚态V物种的增加、孤立态V物种的减少以及表面酸性位的增多。Youn S等[14]采用不同价态的V前驱体(V3+,V4+,V5+)制备了一系列V2O5/TiO2催化剂,发现V3+前驱体制备的V2O5/TiO2催化剂具有最佳的低温脱硝活性和N2选择性,认为主要是由于低价态V前驱体在制备过程中更容易形成分散的高配位多聚态V物种。研究表明,活性组分的聚集状态显著影响催化剂的低温脱硝性能。
Chen L等[15]通过引入助剂的方式考察了CeO2改性对V2O5-WO3/TiO2催化剂低温脱硝性能的影响,研究发现,CeO2的引入显著增强了NOx的吸附和NO氧化为NO2的能力,并增加了催化剂表面的B酸性位,从而促进反应通过快速NH3-SCR途径进行,低温脱硝性能优异。
制备方法的开发也为提升钒基催化剂的低温脱硝性能提供了新思路。Boningari T等[16]采用火焰辅助喷雾热分解法制备了一系列V2O5-WO3/ZrO2催化剂,研究发现,W与V物质的量比为0.66和(180~240) ℃时NO转化率约为98%,原因在于火焰辅助喷雾热分解法有利于得到分散态的V物种,而分散态V物种和WO3增加了活性晶格氧的含量,从而显著提高低温脱硝性能。进一步考察了不同载体对负载型钒基催化剂低温脱硝性能的影响,发现在(180~240) ℃的活性依次为:V2O5/TiO2>V2O5/CeO2>V2O5/Al2O3>V2O5/ZrO2,这主要与载体表面V物种的晶粒尺寸相关[17]。此外,碳材料由于具有大的比表面积、强的吸附能力以及独特的孔道结构,被广泛用作载体负载V2O5,并表现出优异的低温脱硝性能。Huang B C等[18]制备了系列碳纳米管(CNT)负载V2O5催化剂用于NH3-SCR反应,发现对于负载质量分数2.35%的V2O5/CNT催化剂,V2O5在CNT管壁上高度分散,190 ℃时NO转化率达92%。
2锰基金属氧化物催化剂
锰基催化剂由于具有优异的氧化还原性能和低温脱硝活性,而成为目前研究最为广泛的低温NH3-SCR催化剂。根据其化学组成,大致可分为单一锰氧化物(MnOx)催化剂、锰基复合氧化物催化剂和负载型锰基催化剂。对于单一MnOx催化剂,价态、晶体结构以及形貌均对其低温脱硝性能具有显著影响。研究表明,单位面积MnOx催化剂的NO转化率依次为:MnO2>Mn5O8>Mn2O3>Mn3O4>MnO,而Mn2O3表现出最佳的N2选择性[19]。戴韵等[7]采用水热法制备了隧道状α-MnO2纳米棒和层状δ-MnO2纳米棒,探讨MnO2晶体结构对其低温脱硝性能的影响,结果表明,δ-MnO2纳米棒主要暴露(001)晶面,该晶面上的Mn物种已达到配位饱和状态,L酸性位较少;而α-MnO2纳米棒主要暴露(110)晶面,该晶面存在大量配位不饱和Mn物种,形成较多的L酸性位,并且其较弱的Mn—O键和隧道结构有利于NH3和NOx的吸附与活化,从而表现出优于δ-MnO2纳米棒的低温脱硝性能。
对MnOx进行掺杂改性制备锰基复合氧化物催化剂,可显著改善其物化性质,并增强组分间相互作用,进而提升低温NH3-SCR反应性能,常见的掺杂元素有Ce、W、Sn、Cr、Fe、Zr、Ca、Cu、Co、Ni和Zn等[3,20-26]。Jiang H X等[27]将稀土金属氧化物CeO2与MnOx结合,采用超临界反溶剂法合成了一系列MnOx-CeO2纳米空心球催化剂用于低温NH3-SCR反应,研究表明,Mn与Mn+Ce物质的量比为0.32和焙烧温度500 ℃时,MnOx-CeO2纳米空心球催化剂表现出最佳的低温脱硝性能,并强调大的比表面积、良好的氧迁移能力和丰富的表面活性氧物种起至关重要的作用。Liu F D等[21]采用共沉淀法制备了一系列不同比例的MnWOx复合氧化物催化剂,发现Mn与W物质的量比为1∶1的样品在(70~250) ℃表现出优异的脱硝性能(如图1所示),其NO转化率在70 ℃时达到100%。认为W的掺杂导致活性相MnOx的晶粒尺寸变小和表面酸性位增加,有利于NO和NH3的活化,促进低温脱硝性能的提高。
图 1 不同物质的量比的MnWOx复合氧化物催化剂在NH3-SCR反应中的脱硝性能[21]Figure 1 NOx conversion over MnWOx composite oxide catalysts with different molar ratios in NH3-SCR reaction[21]
3金属离子交换的分子筛催化剂
除了金属氧化物催化剂外,金属离子交换的分子筛催化剂也被广泛用于低温NH3-SCR反应,并表现出优异的脱硝性能,其中,Cu2+和Fe3+是常见的用于交换的离子[10-12,43-45]。对于金属离子交换的分子筛催化剂,分子筛种类、金属离子交换量、交换金属离子的状态及迁移是影响其低温脱硝性能的关键因素[11,45-47]。杨海鹏等[45]制备了Cu/ZSM-5、Cu/β、Cu/USY和Cu/SAPO-34铜离子交换的分子筛催化剂,并系统考察了分子筛种类对其物化性能和低温脱硝性能的影响,研究指出,Cu/ZSM-5和Cu/β催化剂由于起始还原温度较低和Cu+物种含量较高而表现出优异的低温脱硝活性,150 ℃时,NO转化率约80%,170 ℃时可实现NO完全脱除。闫春迪等[47]通过调变铵盐种类和铜离子交换时间成功制备了不同铜离子交换量的Cu/SAPO-34分子筛催化剂,结果表明,Cu2+是低温NH3-SCR反应的主要活性中心,且随着铜离子交换量的增加,催化剂低温脱硝性能先增后降。铜离子交换质量分数为2.37%时,Cu/SAPO-34催化剂的低温脱硝活性最好,185 ℃时达到80%。铜离子交换量过高,大量的Cu2+取代桥式羟基Si-OH-Al中的H,抑制NH3在催化剂表面的吸附、储存与迁移,导致催化剂低温脱硝性能下降。Xue J J等[46]研究了Cu/SAPO-34催化剂中Cu物种状态与其低温脱硝性能的关系,发现Cu/SAPO-34催化剂中有4种Cu物种,分别为分子筛骨架外的团簇态Cu物种、晶相态CuO、分子筛骨架内的孤立态Cu2+和Cu+,分子筛骨架内的孤立态Cu2+是低温NH3-SCR反应的主要活性物种,并进一步指出分子筛骨架内的孤立态Cu2+有4种存在位置(见图2),即从六元环移位到椭圆腔(Site Ⅰ)、靠近椭圆腔中心(Site Ⅱ)、六方柱中心(Site Ⅲ)和靠近八元环(Site Ⅳ),其中,Site Ⅰ的孤立态Cu2+是真正的活性中心。此外,Vennestrøm P N R等[11]研究发现,对Cu/SAPO-34催化剂进行高温活化处理可使铜离子在催化剂表面发生迁移和分布更加均匀,从而使脱硝性能成倍增长。
图 2 Cu/SAPO-34催化剂的骨架结构示意图[46]Figure 2 Schematic diagram of skeleton structure of Cu/SAPO-34 catalyst[46]
4低温脱硝催化剂的抗水抗硫性能
反应气氛中的水蒸汽会破坏催化剂表面酸性位,导致催化剂失活。根据排除水蒸汽后催化性能是否恢复,可分为可逆性失活和不可逆性失活。可逆性失活主要是由于H2O与NO和NH3等反应物分子之间的竞争吸附所致,而不可逆性失活则是由于H2O在催化剂表面的化学吸附和解离产生羟基所致[2]。随着催化剂制备技术的发展,通过调变合成参数可有效减缓因水蒸汽所引起的催化剂失活。Wu S G等[48]采用十六烷基三甲基溴化铵辅助的共沉淀法制备的FeMnTiOx复合氧化物,在低温脱硝反应中表现出优异的抗水性能(如图3所示),分析原因发现主要是因为十六烷基三甲基溴化铵的引入有助于稳定TiO2的锐钛矿晶相和增加催化剂表面酸性位。
图 3 CTAB辅助合成的FeMnTiOx复合氧化物催化剂在150 ℃时NH3-SCR反应中的抗水性能[48]Figure 3 Anti-water performance of FeMnTiOx composite oxide catalyst synthesized with CTAB assistant in NH3-SCR reaction at 150 ℃[48]
对于离子交换的分子筛催化剂,水蒸汽主要通过分子筛载体坍塌脱铝以及活性组分迁移转化从而引起催化剂失活。近年来,开发的Cu/SAPO-34和Cu/SSZ-13催化剂在NH3-SCR反应中表现出卓越的水热稳定性[46,49]。
燃煤烟气中的SO2是导致催化剂失活的另一重要因素,活性物种硫酸盐化和硫酸铵(硫酸氢铵)沉积覆盖活性位是催化剂因SO2中毒的两大主要途径。研究发现,在催化剂体系中添加助剂改性可有效提高其抗硫性能[22,38,50]。Jiang B Q等[38]考察了Zr改性对Fe-Mn/Ti低温脱硝催化剂抗硫性能的影响,表明Zr的添加有利于NO分子在催化剂表面形成更多的硝酸盐物种和NO2,从而减缓SO2对Langmuir-Hinshelwood机理的抑制,最终表现出优异的抗硫性能。Kim Y J等[12]发现,Mn-Fe/ZSM-5催化剂中添加Er可显著提高抗硫性能,并优于Cu2+交换的ZSM-5催化剂和工业化Cu基脱硝催化剂,还指出这类Mn基低温脱硝催化剂的SO2中毒途径主要是活性Mn物种被硫酸盐化形成MnSO4。在较多工况下,H2O和SO2共存,因此,同时考察H2O和SO2对催化剂低温脱硝性能的影响更合乎实际。添加助剂改性可显著提高低温脱硝催化剂的抗水抗硫性能。Qi G S等[51]制备的MnOx-CeO2低温脱硝催化剂在2.5%的H2O和100×10-6的SO2气氛中150 ℃反应3 h后,NO转化率下降15%,引入Fe或Pr后,其抗水抗硫性能进一步提升,NO转化率几乎不下降。
5低温NH3-SCR反应机理
图 4 Cu/SSZ-13催化剂表面的低温NH3-SCR反应机理[54]Figure 4 Low-temperature NH3-SCR reaction mechanism on the surface of Cu/SSZ-13 catalyst[54]
6结语与展望
钒基、锰基金属氧化物催化剂和金属离子交换的分子筛催化剂在低温NH3-SCR反应中表现出优异的脱硝性能,因而成为本领域的研究热点。通过调变催化剂中活性组分含量、优化制备方法以及引入助剂进行掺杂改性等,可进一步提高其脱硝效率和抗水抗硫性能。
借助于各种原位表征手段对催化剂表面的低温NH3-SCR反应机理及抗水抗硫机理有了初步的认识。但是,目前还存在N2选择性不够理想、抗水抗硫性能有待进一步提高、低温工作温度窗口较窄和反应机理不统一等问题。近年来,材料制备科学与技术、理论计算化学和仪器开发等方面取得了长足发展,可以通过催化剂的结构设计、引入各种物理场和借助于多种原位或准原位表征手段并结合理论计算等有望解决上述难题。此外,现有的低温脱硝催化剂性能考察多是在实验室模拟烟气条件下进行,与燃煤锅炉实际烟气状况存在较大差距,今后还有待于深入研究。
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收稿日期:2015-12-10;修回日期:2016-03-25
基金项目:国家自然科学基金(21507130);重庆市科学技术委员会项目(cstc2013jcsf20001,cstc2014pt-gc20002);北京分子科学国家实验室开放课题基金(20140142);重庆文理学院环境材料与修复技术重庆市重点实验室开放课题基金(CEK1405);重庆工商大学催化与功能有机分子重庆市重点实验室开放课题基金(1456029)
作者简介:姚小江,1986年生,男,重庆市人,博士,助理研究员,主要从事火电厂烟气脱硝、机动车尾气净化以及挥发性有机物污染治理方面的研究。
doi:10.3969/j.issn.1008-1143.2016.06.001 10.3969/j.issn.1008-1143.2016.06.001
中图分类号:X701;TQ426.99
文献标识码:A
文章编号:1008-1143(2016)06-0001-09
Research progress in selective catalytic reduction of nitrogen oxides by ammonia at low temperature
YaoXiaojiang1*,ZhangLei2,KongTingting1,3,LiHongli1,YangFumo1,2,3*
(1.Key Laboratory of Reservoir Aquatic Environment of CAS, Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences, Chongqing 400714, China; 2.College of Environmental and Chemical Engineering, Chongqing Three Gorges University, Chongqing 404100, China;3.Yangtze Normal University, Chongqing 408100, China)
Abstract:Nitrogen oxides are very harmful to human’s health and the growth of plants, so it is an urgent task to control the emission of NOx.NH3-selective catalytic reduction (NH3-SCR) has been proved to be the most efficient technique to remove NOx from flue gas of coal-fired power plant, which is one of the main sources of NOx. However, the operating temperature window of commercial V2O5-WO3 (MoO3)/TiO2 denitration catalysts is too high to satisfy the requirements of low temperature and wide operating temperature window,and therefore the development of low-temperature denitration catalysts with wide operating temperature window becomes a hot spot of this research field in recent years. Especially, vanadium-based and manganese-based metal-oxide catalysts as well as metal-ion exchanged zeolite catalysts are widely investigated. Discussing the key influence factors of catalytic performance, H2O and SO2 resistance, and reaction mechanism is conducive to provide some scientific basis for the design and development of efficient and practical low-temperature denitration catalysts. The research progress in low-temperature denitration catalysts,including vanadium-based metal-oxide catalysts, manganese-based metal-oxide catalysts, metal-ion exchanged zeolite catalysts, H2O and SO2 resistance of low-temperature denitration catalysts and low-temperature NH3-SCR reaction mechanism was reviewed.Some problems such as the unsatisfactory selectivity to N2,poor performance of water resistance and sulfur resistance, narrow low-temperature operating window,and not uniform mechanisms of the reaction need to be solved in future.
Key words:waste treatment and comprehensive utilization; nitrogen oxides; low-temperature NH3-SCR; metal-oxide catalyst; zeolite catalyst; H2O and SO2 resistance
通讯联系人:姚小江;杨复沫,1967年生,男,湖北省仙桃市人,博士,研究员,主要从事区域性大气复合污染特征、来源、成因及控制研究,出版专著1本,发表论文90余篇,其中SCI收录40余篇。