姚小江，張　雷，孔婷婷，李紅麗，楊復(fù)沫,,3*

(1.中國科學(xué)院重慶綠色智能技術(shù)研究院 水庫水環(huán)境重點(diǎn)實(shí)驗(yàn)室，重慶 400714；2.重慶三峽學(xué)院 環(huán)境與化學(xué)工程學(xué)院，重慶 404100； 3.長江師范學(xué)院，重慶 408100)

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綜述與展望

低溫氨-選擇性催化還原氮氧化物催化劑的研究進(jìn)展

姚小江1*，張雷2，孔婷婷1,3，李紅麗1，楊復(fù)沫1,2,3*

(1.中國科學(xué)院重慶綠色智能技術(shù)研究院 水庫水環(huán)境重點(diǎn)實(shí)驗(yàn)室，重慶 400714；2.重慶三峽學(xué)院 環(huán)境與化學(xué)工程學(xué)院，重慶 404100； 3.長江師范學(xué)院，重慶 408100)

摘要：氮氧化物(NOx)的排放對人類健康和植物生長造成了嚴(yán)重危害，對其進(jìn)行凈化治理刻不容緩?；痣姀S煙氣是NOx的主要來源，氨-選擇性催化還原(NH3-SCR)技術(shù)可對其排放進(jìn)行有效控制。V2O5-WO3(MoO3)/TiO2脫硝催化劑的工作溫度偏高，不能滿足低溫寬工作溫度窗口等工況的需要，因此，開發(fā)具有寬工作溫度窗口的低溫脫硝催化劑成為研究熱點(diǎn)，其中，釩基、錳基金屬氧化物催化劑和金屬離子交換分子篩催化劑的研究最為廣泛。探討催化劑脫硝性能的關(guān)鍵影響因素、抗水抗硫性能以及反應(yīng)機(jī)理，有助于為高效、實(shí)用的低溫脫硝催化劑的設(shè)計和開發(fā)提供科學(xué)依據(jù)。從釩基金屬氧化物催化劑、錳基金屬氧化物催化劑、金屬離子交換的分子篩催化劑、低溫脫硝催化劑的抗水抗硫性能以及低溫NH3-SCR反應(yīng)機(jī)理等方面對近年來國內(nèi)外低溫脫硝催化劑的研究進(jìn)展進(jìn)行綜述。今后需要解決N2選擇性不夠理想、抗水抗硫性能差、低溫工作溫度窗口較窄和反應(yīng)機(jī)理不統(tǒng)一等問題。

關(guān)鍵詞：三廢處理與綜合利用；氮氧化物；低溫氨-選擇性催化還原；金屬氧化物催化劑；分子篩催化劑；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)作為主要的大氣污染物，不僅導(dǎo)致酸雨和光化學(xué)煙霧的發(fā)生，還能參與PM2.5的形成，嚴(yán)重危害人類健康和植物生長[1-3]?；痣姀S是NOx的主要來源，對其煙氣進(jìn)行脫硝處理刻不容緩。氨-選擇性催化還原(NH3-SCR)NOx是火電廠煙氣脫硝的最實(shí)用技術(shù)。目前，V2O5-WO3(MoO3)/TiO2由于具有優(yōu)異的脫硝性能和抗水抗硫性能而被選為工業(yè)化脫硝催化劑，廣泛應(yīng)用于NH3-SCR過程，催化劑的最佳工作溫度為(300～400) ℃[4-6]，因此，脫硝裝置必須設(shè)置在除塵器和脫硫單元上游以滿足工作溫度的需要。在此工況下，脫硝催化劑容易因煙塵和二氧化硫堵塞和中毒，導(dǎo)致嚴(yán)重失活。

為滿足各國政府愈加嚴(yán)格的排放法規(guī)，諸多火電廠必須進(jìn)行改造，增加脫硝裝置。然而，這些火電廠的各個結(jié)構(gòu)單元通常布局非常緊湊，很難在除塵器和脫硫單元上游增加脫硝裝置，而只能設(shè)置在下游。由于此處的煙氣溫度通常已降至200 ℃以下[2]，傳統(tǒng)的V2O5-WO3(MoO3)/TiO2脫硝催化劑已不能達(dá)到理想的催化效果，很難滿足排放要求，開發(fā)低溫脫硝催化劑勢在必行。此外，采用低溫脫硝催化劑還可避免因煙氣加熱帶來的能源消耗，經(jīng)過除塵器和脫硫單元之后，煙氣中煙塵和SO2含量已降至較低，可有效減緩低溫脫硝催化劑的堵塞和中毒。目前，低溫脫硝催化劑的研究主要為金屬氧化物催化劑[7-9]和分子篩催化劑[10-12]。

本文從釩基金屬氧化物催化劑、錳基金屬氧化物催化劑、金屬離子交換的分子篩催化劑、低溫脫硝催化劑的抗水抗硫性能以及低溫NH3-SCR反應(yīng)機(jī)理等方面對近年來低溫脫硝催化劑的研究進(jìn)展進(jìn)行綜述。

1釩基金屬氧化物催化劑

工業(yè)化脫硝催化劑V2O5-WO3(MoO3)/TiO2具有生產(chǎn)工藝成熟、抗水抗硫性能優(yōu)異等優(yōu)點(diǎn)，在該催化劑體系基礎(chǔ)上進(jìn)行改進(jìn)研究以提高其低溫脫硝性能成為研究熱點(diǎn)。通常，調(diào)整V2O5負(fù)載量、改變V物種前驅(qū)體、引入助劑、調(diào)變載體以及優(yōu)化制備方法是常用提升釩基催化劑低溫脫硝性能的有效途徑。Cha W等[13]研究表明，對于V2O5/TiO2催化劑，其低溫脫硝活性隨著V2O5負(fù)載量的增加而提高，V2O5負(fù)載質(zhì)量分?jǐn)?shù)7%時，V2O5/TiO2催化劑活性最佳，200 ℃時脫硝率達(dá)到96%。原因歸屬于多聚態(tài)V物種的增加、孤立態(tài)V物種的減少以及表面酸性位的增多。Youn S等[14]采用不同價態(tài)的V前驅(qū)體(V3+,V4+,V5+)制備了一系列V2O5/TiO2催化劑，發(fā)現(xiàn)V3+前驅(qū)體制備的V2O5/TiO2催化劑具有最佳的低溫脫硝活性和N2選擇性，認(rèn)為主要是由于低價態(tài)V前驅(qū)體在制備過程中更容易形成分散的高配位多聚態(tài)V物種。研究表明，活性組分的聚集狀態(tài)顯著影響催化劑的低溫脫硝性能。

Chen L等[15]通過引入助劑的方式考察了CeO2改性對V2O5-WO3/TiO2催化劑低溫脫硝性能的影響，研究發(fā)現(xiàn)，CeO2的引入顯著增強(qiáng)了NOx的吸附和NO氧化為NO2的能力，并增加了催化劑表面的B酸性位，從而促進(jìn)反應(yīng)通過快速NH3-SCR途徑進(jìn)行，低溫脫硝性能優(yōu)異。

制備方法的開發(fā)也為提升釩基催化劑的低溫脫硝性能提供了新思路。Boningari T等[16]采用火焰輔助噴霧熱分解法制備了一系列V2O5-WO3/ZrO2催化劑，研究發(fā)現(xiàn)，W與V物質(zhì)的量比為0.66和(180～240) ℃時NO轉(zhuǎn)化率約為98%，原因在于火焰輔助噴霧熱分解法有利于得到分散態(tài)的V物種，而分散態(tài)V物種和WO3增加了活性晶格氧的含量，從而顯著提高低溫脫硝性能。進(jìn)一步考察了不同載體對負(fù)載型釩基催化劑低溫脫硝性能的影響，發(fā)現(xiàn)在(180～240) ℃的活性依次為：V2O5/TiO2>V2O5/CeO2>V2O5/Al2O3>V2O5/ZrO2，這主要與載體表面V物種的晶粒尺寸相關(guān)[17]。此外，碳材料由于具有大的比表面積、強(qiáng)的吸附能力以及獨(dú)特的孔道結(jié)構(gòu)，被廣泛用作載體負(fù)載V2O5，并表現(xiàn)出優(yōu)異的低溫脫硝性能。Huang B C等[18]制備了系列碳納米管(CNT)負(fù)載V2O5催化劑用于NH3-SCR反應(yīng)，發(fā)現(xiàn)對于負(fù)載質(zhì)量分?jǐn)?shù)2.35%的V2O5/CNT催化劑，V2O5在CNT管壁上高度分散，190 ℃時NO轉(zhuǎn)化率達(dá)92%。

2錳基金屬氧化物催化劑

錳基催化劑由于具有優(yōu)異的氧化還原性能和低溫脫硝活性，而成為目前研究最為廣泛的低溫NH3-SCR催化劑。根據(jù)其化學(xué)組成，大致可分為單一錳氧化物(MnOx)催化劑、錳基復(fù)合氧化物催化劑和負(fù)載型錳基催化劑。對于單一MnOx催化劑，價態(tài)、晶體結(jié)構(gòu)以及形貌均對其低溫脫硝性能具有顯著影響。研究表明，單位面積MnOx催化劑的NO轉(zhuǎn)化率依次為：MnO2>Mn5O8>Mn2O3>Mn3O4>MnO，而Mn2O3表現(xiàn)出最佳的N2選擇性[19]。戴韻等[7]采用水熱法制備了隧道狀α-MnO2納米棒和層狀δ-MnO2納米棒，探討MnO2晶體結(jié)構(gòu)對其低溫脫硝性能的影響，結(jié)果表明，δ-MnO2納米棒主要暴露(001)晶面，該晶面上的Mn物種已達(dá)到配位飽和狀態(tài)，L酸性位較少；而α-MnO2納米棒主要暴露(110)晶面，該晶面存在大量配位不飽和Mn物種，形成較多的L酸性位，并且其較弱的Mn—O鍵和隧道結(jié)構(gòu)有利于NH3和NOx的吸附與活化，從而表現(xiàn)出優(yōu)于δ-MnO2納米棒的低溫脫硝性能。

對MnOx進(jìn)行摻雜改性制備錳基復(fù)合氧化物催化劑，可顯著改善其物化性質(zhì)，并增強(qiáng)組分間相互作用，進(jìn)而提升低溫NH3-SCR反應(yīng)性能，常見的摻雜元素有Ce、W、Sn、Cr、Fe、Zr、Ca、Cu、Co、Ni和Zn等[3,20-26]。Jiang H X等[27]將稀土金屬氧化物CeO2與MnOx結(jié)合，采用超臨界反溶劑法合成了一系列MnOx-CeO2納米空心球催化劑用于低溫NH3-SCR反應(yīng)，研究表明，Mn與Mn+Ce物質(zhì)的量比為0.32和焙燒溫度500 ℃時，MnOx-CeO2納米空心球催化劑表現(xiàn)出最佳的低溫脫硝性能，并強(qiáng)調(diào)大的比表面積、良好的氧遷移能力和豐富的表面活性氧物種起至關(guān)重要的作用。Liu F D等[21]采用共沉淀法制備了一系列不同比例的MnWOx復(fù)合氧化物催化劑，發(fā)現(xiàn)Mn與W物質(zhì)的量比為1∶1的樣品在(70～250) ℃表現(xiàn)出優(yōu)異的脫硝性能(如圖1所示)，其NO轉(zhuǎn)化率在70 ℃時達(dá)到100%。認(rèn)為W的摻雜導(dǎo)致活性相MnOx的晶粒尺寸變小和表面酸性位增加，有利于NO和NH3的活化，促進(jìn)低溫脫硝性能的提高。

圖 1　不同物質(zhì)的量比的MnWOx復(fù)合氧化物催化劑在NH3-SCR反應(yīng)中的脫硝性能[21]Figure 1　NOx conversion over MnWOx composite oxide catalysts with different molar ratios in NH3-SCR reaction[21]

3金屬離子交換的分子篩催化劑

除了金屬氧化物催化劑外，金屬離子交換的分子篩催化劑也被廣泛用于低溫NH3-SCR反應(yīng)，并表現(xiàn)出優(yōu)異的脫硝性能，其中，Cu2+和Fe3+是常見的用于交換的離子[10-12,43-45]。對于金屬離子交換的分子篩催化劑，分子篩種類、金屬離子交換量、交換金屬離子的狀態(tài)及遷移是影響其低溫脫硝性能的關(guān)鍵因素[11,45-47]。楊海鵬等[45]制備了Cu/ZSM-5、Cu/β、Cu/USY和Cu/SAPO-34銅離子交換的分子篩催化劑，并系統(tǒng)考察了分子篩種類對其物化性能和低溫脫硝性能的影響，研究指出，Cu/ZSM-5和Cu/β催化劑由于起始還原溫度較低和Cu+物種含量較高而表現(xiàn)出優(yōu)異的低溫脫硝活性，150 ℃時，NO轉(zhuǎn)化率約80%，170 ℃時可實(shí)現(xiàn)NO完全脫除。閆春迪等[47]通過調(diào)變銨鹽種類和銅離子交換時間成功制備了不同銅離子交換量的Cu/SAPO-34分子篩催化劑，結(jié)果表明，Cu2+是低溫NH3-SCR反應(yīng)的主要活性中心，且隨著銅離子交換量的增加，催化劑低溫脫硝性能先增后降。銅離子交換質(zhì)量分?jǐn)?shù)為2.37%時，Cu/SAPO-34催化劑的低溫脫硝活性最好，185 ℃時達(dá)到80%。銅離子交換量過高，大量的Cu2+取代橋式羥基Si-OH-Al中的H，抑制NH3在催化劑表面的吸附、儲存與遷移，導(dǎo)致催化劑低溫脫硝性能下降。Xue J J等[46]研究了Cu/SAPO-34催化劑中Cu物種狀態(tài)與其低溫脫硝性能的關(guān)系，發(fā)現(xiàn)Cu/SAPO-34催化劑中有4種Cu物種，分別為分子篩骨架外的團(tuán)簇態(tài)Cu物種、晶相態(tài)CuO、分子篩骨架內(nèi)的孤立態(tài)Cu2+和Cu+，分子篩骨架內(nèi)的孤立態(tài)Cu2+是低溫NH3-SCR反應(yīng)的主要活性物種，并進(jìn)一步指出分子篩骨架內(nèi)的孤立態(tài)Cu2+有4種存在位置(見圖2)，即從六元環(huán)移位到橢圓腔(Site Ⅰ)、靠近橢圓腔中心(Site Ⅱ)、六方柱中心(Site Ⅲ)和靠近八元環(huán)(Site Ⅳ)，其中，Site Ⅰ的孤立態(tài)Cu2+是真正的活性中心。此外，Vennestr?m P N R等[11]研究發(fā)現(xiàn)，對Cu/SAPO-34催化劑進(jìn)行高溫活化處理可使銅離子在催化劑表面發(fā)生遷移和分布更加均勻，從而使脫硝性能成倍增長。

圖 2　Cu/SAPO-34催化劑的骨架結(jié)構(gòu)示意圖[46]Figure 2　Schematic diagram of skeleton structure of Cu/SAPO-34 catalyst[46]

4低溫脫硝催化劑的抗水抗硫性能

反應(yīng)氣氛中的水蒸汽會破壞催化劑表面酸性位，導(dǎo)致催化劑失活。根據(jù)排除水蒸汽后催化性能是否恢復(fù)，可分為可逆性失活和不可逆性失活?？赡嫘允Щ钪饕怯捎贖2O與NO和NH3等反應(yīng)物分子之間的競爭吸附所致，而不可逆性失活則是由于H2O在催化劑表面的化學(xué)吸附和解離產(chǎn)生羥基所致[2]。隨著催化劑制備技術(shù)的發(fā)展，通過調(diào)變合成參數(shù)可有效減緩因水蒸汽所引起的催化劑失活。Wu S G等[48]采用十六烷基三甲基溴化銨輔助的共沉淀法制備的FeMnTiOx復(fù)合氧化物，在低溫脫硝反應(yīng)中表現(xiàn)出優(yōu)異的抗水性能(如圖3所示)，分析原因發(fā)現(xiàn)主要是因?yàn)槭榛谆寤@的引入有助于穩(wěn)定TiO2的銳鈦礦晶相和增加催化劑表面酸性位。

圖 3　CTAB輔助合成的FeMnTiOx復(fù)合氧化物催化劑在150 ℃時NH3-SCR反應(yīng)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]

對于離子交換的分子篩催化劑，水蒸汽主要通過分子篩載體坍塌脫鋁以及活性組分遷移轉(zhuǎn)化從而引起催化劑失活。近年來，開發(fā)的Cu/SAPO-34和Cu/SSZ-13催化劑在NH3-SCR反應(yīng)中表現(xiàn)出卓越的水熱穩(wěn)定性[46,49]。

燃煤煙氣中的SO2是導(dǎo)致催化劑失活的另一重要因素，活性物種硫酸鹽化和硫酸銨(硫酸氫銨)沉積覆蓋活性位是催化劑因SO2中毒的兩大主要途徑。研究發(fā)現(xiàn)，在催化劑體系中添加助劑改性可有效提高其抗硫性能[22,38,50]。Jiang B Q等[38]考察了Zr改性對Fe-Mn/Ti低溫脫硝催化劑抗硫性能的影響，表明Zr的添加有利于NO分子在催化劑表面形成更多的硝酸鹽物種和NO2，從而減緩SO2對Langmuir-Hinshelwood機(jī)理的抑制，最終表現(xiàn)出優(yōu)異的抗硫性能。Kim Y J等[12]發(fā)現(xiàn)，Mn-Fe/ZSM-5催化劑中添加Er可顯著提高抗硫性能，并優(yōu)于Cu2+交換的ZSM-5催化劑和工業(yè)化Cu基脫硝催化劑，還指出這類Mn基低溫脫硝催化劑的SO2中毒途徑主要是活性Mn物種被硫酸鹽化形成MnSO4。在較多工況下，H2O和SO2共存，因此，同時考察H2O和SO2對催化劑低溫脫硝性能的影響更合乎實(shí)際。添加助劑改性可顯著提高低溫脫硝催化劑的抗水抗硫性能。Qi G S等[51]制備的MnOx-CeO2低溫脫硝催化劑在2.5%的H2O和100×10-6的SO2氣氛中150 ℃反應(yīng)3 h后，NO轉(zhuǎn)化率下降15%，引入Fe或Pr后，其抗水抗硫性能進(jìn)一步提升，NO轉(zhuǎn)化率幾乎不下降。

5低溫NH3-SCR反應(yīng)機(jī)理

圖 4　Cu/SSZ-13催化劑表面的低溫NH3-SCR反應(yīng)機(jī)理[54]Figure 4　Low-temperature NH3-SCR reaction mechanism on the surface of Cu/SSZ-13 catalyst[54]

6結(jié)語與展望

釩基、錳基金屬氧化物催化劑和金屬離子交換的分子篩催化劑在低溫NH3-SCR反應(yīng)中表現(xiàn)出優(yōu)異的脫硝性能，因而成為本領(lǐng)域的研究熱點(diǎn)。通過調(diào)變催化劑中活性組分含量、優(yōu)化制備方法以及引入助劑進(jìn)行摻雜改性等，可進(jìn)一步提高其脫硝效率和抗水抗硫性能。

借助于各種原位表征手段對催化劑表面的低溫NH3-SCR反應(yīng)機(jī)理及抗水抗硫機(jī)理有了初步的認(rèn)識。但是，目前還存在N2選擇性不夠理想、抗水抗硫性能有待進(jìn)一步提高、低溫工作溫度窗口較窄和反應(yīng)機(jī)理不統(tǒng)一等問題。近年來，材料制備科學(xué)與技術(shù)、理論計算化學(xué)和儀器開發(fā)等方面取得了長足發(fā)展，可以通過催化劑的結(jié)構(gòu)設(shè)計、引入各種物理場和借助于多種原位或準(zhǔn)原位表征手段并結(jié)合理論計算等有望解決上述難題。此外，現(xiàn)有的低溫脫硝催化劑性能考察多是在實(shí)驗(yàn)室模擬煙氣條件下進(jìn)行，與燃煤鍋爐實(shí)際煙氣狀況存在較大差距，今后還有待于深入研究。

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收稿日期：2015-12-10；修回日期：2016-03-25

基金項(xiàng)目：國家自然科學(xué)基金(21507130)；重慶市科學(xué)技術(shù)委員會項(xiàng)目(cstc2013jcsf20001，cstc2014pt-gc20002)；北京分子科學(xué)國家實(shí)驗(yàn)室開放課題基金(20140142)；重慶文理學(xué)院環(huán)境材料與修復(fù)技術(shù)重慶市重點(diǎn)實(shí)驗(yàn)室開放課題基金(CEK1405)；重慶工商大學(xué)催化與功能有機(jī)分子重慶市重點(diǎn)實(shí)驗(yàn)室開放課題基金(1456029)

作者簡介：姚小江，1986年生，男，重慶市人，博士，助理研究員，主要從事火電廠煙氣脫硝、機(jī)動車尾氣凈化以及揮發(fā)性有機(jī)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

中圖分類號：X701;TQ426.99

文獻(xiàn)標(biāo)識碼：A

文章編號：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

通訊聯(lián)系人：姚小江；楊復(fù)沫，1967年生，男，湖北省仙桃市人，博士，研究員，主要從事區(qū)域性大氣復(fù)合污染特征、來源、成因及控制研究，出版專著1本，發(fā)表論文90余篇，其中SCI收錄40余篇。