摘" " " 要: NH3選擇性催化還原(NH3?SCR)是減少燃煤工業(yè)、柴油發(fā)動(dòng)機(jī)等氮氧化物(NOx)排放的最有效技術(shù),其核心是SCR脫硝催化劑。然而,煙氣中的金屬雜質(zhì)及硫化合物等會(huì)造成催化劑的失活,所以有必要對現(xiàn)有的SCR脫硝催化劑進(jìn)行改性。由于Ce具有獨(dú)特的氧化還原、儲(chǔ)氧和酸堿性能,被廣泛用作載體、啟動(dòng)劑和促進(jìn)劑,在SCR催化劑的研究中是一類常用助劑。本文介紹了SCR催化劑在脫硝反應(yīng)中的反應(yīng)機(jī)理,以及Ce對SCR催化劑在抗中毒方面和脫硝性能提升等方面所起到的重要作用。
關(guān)" 鍵" 詞:Ce;SCR脫硝催化劑;脫硝活性;抗中毒
中圖分類號(hào):TQ016文獻(xiàn)標(biāo)識(shí)碼: A" " "文章編號(hào): 1004-0935(2025)01-0137-05
氮氧化物(NOx)是我國空氣污染物主要成分之一,它能引起硝酸型酸雨、光化學(xué)煙霧及溫室效應(yīng)等全球性環(huán)境問題[1]。以NH3為還原劑的選擇性催化還原(selective catalytic reduction,SCR)技術(shù)在去除NOx的效率、穩(wěn)定性和成本等方面具有最佳的表現(xiàn),其核心是SCR脫硝催化劑[2-4]。
目前市面上的SCR脫硝催化劑多以V基催化劑為主,V基催化劑因其熱穩(wěn)定性好、脫硝效率高以及強(qiáng)抗水能力和抗二氧化硫能力而得到廣泛應(yīng)用,但V基催化劑也存在明顯的不足。第一,傳統(tǒng)的V基催化劑在煙道氣體中易受堿/堿土金屬、重金屬和一些非金屬雜質(zhì)的毒害而逐漸失去活性。第二,在低溫條件下,(NH4)2SO4/NH4HSO4和金屬硫酸鹽沉積更嚴(yán)重,阻塞和破壞活性位點(diǎn)。第三,V基催化劑體系的活性組分為V2O5,該物質(zhì)具有生物毒性,會(huì)對生態(tài)環(huán)境造成污染。
因此,面向工業(yè)應(yīng)用的催化劑,既要具有優(yōu)異的NOx去除率和低溫活性,又要具有良好的抗金屬中毒和抗硫性能,以及環(huán)境友好性。一些過渡金屬氧化物,如PtO[5]、NiO[6]、FeOx[7]、CuO[8]、MnOx[5-9]、CeO2[6-10]已被證明能有效地提高VWTi催化劑的脫硝性能。其中,CeO2因其獨(dú)特的氧化還原、儲(chǔ)氧和酸堿性能,被廣泛用作載體、啟動(dòng)劑和促進(jìn)各種催化劑NH3-SCR性能的主要活性成分[11]。
1 SCR脫硝反應(yīng)機(jī)理
常見的SCR反應(yīng)途徑可以描述如下:吸附的NHx與吸附的亞硝酸鹽/硝酸鹽反應(yīng)遵循Langmuir-Hinshelwood(L-H)機(jī)制或直接與氣態(tài)NO反應(yīng)遵循Eley-Rideal(E-R)機(jī)制生成NHx-NOx,隨后分解為N2和H2O[12]。TOPSOE等[13-14]給出了一種反應(yīng)機(jī)制,即在V2O5/TiO2催化劑上包含酸環(huán)和氧化還原環(huán)。在該機(jī)制中,NH3吸附在V5+-OH位點(diǎn)的Br?nsted酸位點(diǎn)上,NH3在V5+=O基團(tuán)上進(jìn)行活化。這個(gè)活化過程包括H從NH3分子轉(zhuǎn)移到V5+上,并導(dǎo)致V5+=O還原為V4+-OH。活化的NH3復(fù)合物可與氣態(tài)NO反應(yīng)生成中間產(chǎn)物,分解為N2和H2O?;钚缘腣5+=O基是通過還原的V4+-OH位點(diǎn)被O2氧化而再生的。釩氧化物催化劑上的NH3-SCR反應(yīng)歷程如圖1所示。
2 Ce對SCR催化劑中毒的影響
眾所周知,硫和金屬雜質(zhì)對催化劑的影響很大,金屬雜質(zhì)包括堿金屬,堿土金屬和重金屬。這些物質(zhì)沉積在催化劑表面,覆蓋了催化劑的活性位點(diǎn)從而降低了SCR催化劑的活性[15]。本文將分類討論Ce對SCR催化劑抗各類金屬及硫中毒性能的影響。
2.1" 堿金屬中毒
堿金屬元素是毒害催化材料的有害元素,包括堿金屬氧化物、堿金屬硫酸鹽和堿金屬氯化物[16]。還原性降低、表面酸性喪失和催化劑孔道堵塞是催化劑堿中毒失活的主要因素[17],它對傳統(tǒng)的V基催化劑有嚴(yán)重的毒害作用,并且隨著其含量的增加,毒害作用會(huì)更加嚴(yán)重。JIANG等[18]研究發(fā)現(xiàn)隨著KCl加入量的增加,V2O5/TiO2催化劑的SCR活性逐漸降低。K+會(huì)優(yōu)先吸附在催化劑表面的Br?nsted酸位點(diǎn),導(dǎo)致表面酸性喪失。此外,K+還會(huì)引起催化劑的燒結(jié)、表面V原子的減少和V價(jià)態(tài)的改變。XIANG等[19]研究發(fā)現(xiàn)Na的存在阻礙了酸性V位點(diǎn)與氣態(tài)NH3分子的接觸,導(dǎo)致催化劑的脫硝活性下降。在堿金屬存在下,快速SCR反應(yīng)的活性中心由酸性中心變?yōu)椤皦A性中心”,活性中心上的活性物質(zhì)由吸附NH3變?yōu)槲絅O。目前,提高抗堿性能有兩種有效的策略,即提供更多的犧牲性酸性位點(diǎn)和建立捕堿位點(diǎn)以保護(hù)活性位點(diǎn)[20]。
提供額外的酸位點(diǎn)或使用強(qiáng)酸性載體是增強(qiáng)抗堿性的有效策略,因?yàn)閴A離子會(huì)優(yōu)先與酸性載體相互作用,從而保護(hù)活性位點(diǎn)。PENG等[21]研究發(fā)現(xiàn)在MnOx/TiO2催化劑中加入CeO2可以提高催化劑的耐堿性,因?yàn)镃eO2為NH3的吸附提供了新的Lewis酸位點(diǎn),并保留了NH3活化的還原性。形成的[?O?Ce?O?Mn?O?]單元使堿金屬聚集起來,為SCR反應(yīng)保留了足夠的活性位點(diǎn)。HU等[22]發(fā)現(xiàn)Ce摻雜可以增加Na中毒催化劑的表面吸附氧,促進(jìn)氧化還原循環(huán),并且促進(jìn)更多Br?nsted酸位生成,從而提高催化活性和抗Na中毒能力。
HU等[23]通過引入Ce和硫酸鹽(SO42-)來提高V基催化劑的脫硝活性和耐堿性能。引入Ce后,V-Ce0.5(SO4)2/Ti表面活性氧生成比例更高,H2還原速度更快,通過形成更豐富的表面NO+和NO3-物種,導(dǎo)致更多的NO氧化活化。其次,表面硫酸鹽(SO42-)可以提供強(qiáng)酸位點(diǎn),從而提高V基催化劑的表面酸度。Ce(SO4)2的加入提供了豐富的活性酸位點(diǎn),使吸附的NH3種類增加。
同樣,WANG等[24]研究制備了一系列Ce(SO4)2摻雜的VWTi催化劑,并測試了其對堿金屬的抗性。結(jié)果表明,負(fù)載7.5wt%Ce(SO4)2的催化劑對堿金屬具有最佳的抗中毒能力。SO42-引入后,VWTi-7.5Ce催化劑表面出現(xiàn)了大量的活性酸位點(diǎn),減輕了堿金屬對催化劑酸性位點(diǎn)的毒害作用。引入Ce4+/Ce3+氧化還原偶對也提高了催化劑的化學(xué)吸附氧,從而提高了催化劑的氧化還原能力。此外,Ce3+組分優(yōu)先與KCl相互作用,保護(hù)了活性的V5+組分。
2.2" 重金屬Hg中毒
汞在煙氣中以3種基本形式存在:元素汞(Hg0)、氧化汞(Hg2+)和微粒結(jié)合汞(HgP)[25]。其中Hg0揮發(fā)性強(qiáng),不溶于水,難以去除。因此,近年來進(jìn)行了大量研究,以促進(jìn)煙氣中Hg0氧化為Hg2+[26-28]。研究發(fā)現(xiàn),由于SCR催化劑的作用,Hg0在SCR過程中被部分氧化[29]。最初使用SCR催化劑進(jìn)行脫硝,NOx與吸附的NH3反應(yīng)生成N2和H2O(g)[30]。Ce改性的SCR催化劑不僅不會(huì)抑制甚至改善NOx還原的情況下,而且還提高Hg0的氧化效率。
CHI等[31]合成了一系列Ce-Cu改性V基SCR催化劑,發(fā)現(xiàn)在250℃下,相對于7%Ce-1%Cu/SCR催化劑,Hg0的氧化效率為90.89%。Ce4++Cu1+?Ce3++Cu2+氧化還原循環(huán)的存在可以極大地促進(jìn)Hg0的氧化。萬奇等[32]制備了Ce摻雜的V2O5-WO3/TiO2催化劑,在模擬燃煤煙氣條件下開展Hg0的脫除實(shí)驗(yàn)。結(jié)果表明該催化劑在200~500℃的溫度范圍內(nèi)能脫除煙氣中95%的Hg0。催化劑表面的Ce是以Ce4+的形式存在,有利于Hg0的脫除反應(yīng)。此研究表明了Ce的加入的確具有改善催化劑脫硝性能和單質(zhì)汞氧化效果的作用。
YANG等[33]研究了CeO2改性V2O5/TiO2催化劑來同時(shí)脫除NO和Hg0。該課題組根據(jù)已有研究對SCR機(jī)理和Hg0氧化機(jī)理的理論分析,推測增強(qiáng)催化劑對NO的吸附同時(shí)也會(huì)改善SCR脫硝性能和V基催化劑上Hg0的氧化。并且提出了一種新穎的硝酸化途徑,即Hg0可以被氧化為Hg(NO3)2,而Hg0幾乎不被催化劑解吸。
MA等[34]研究了多壁碳納米管(MWCNTs)負(fù)載Fe-Ce復(fù)合氧化物催化劑的協(xié)同脫硝脫汞(Hg0)效果。Ce的加入通過增加氧空位來提升催化劑的儲(chǔ)釋氧能力,并通過捕獲O2轉(zhuǎn)化為活躍的化學(xué)吸附氧來參與SCR反應(yīng)以加速活性位點(diǎn)的復(fù)原,從而提升催化效率。與Fe(2)Ox/MWCNTs相比,Ce摻雜后的Fe(2)Ce(0.5)Ox/MWCNTs具有更高的催化活性,在240℃、30000 h-1空速下,F(xiàn)e(2)Ce/MWCNTs對NO和Hg0的脫除效率分別提高了6.4%和19.5%。Fe2Ce0.5Ox/MWCNTs在240℃時(shí)脫硝效率更是達(dá)到了99.1%。
2.3" 硫中毒
釩基催化劑的脫硝活性較高,但脫硝性能易受煙氣雜質(zhì)中硫的影響[35-37],導(dǎo)致催化劑直接失活。脫硝催化劑對SO2的耐蝕性是表征催化劑性能的一個(gè)重要指標(biāo)。一般來說,SO2對催化劑的影響主要表現(xiàn)在兩個(gè)方面:一是煙氣中的SO2會(huì)與NH3反應(yīng)生成硫酸鹽,如(NH4)2SO3和NH4HSO4,這些硫酸鹽在低溫下不分解,最終沉積在催化劑表面,導(dǎo)致催化劑活性位點(diǎn)被堵塞。二是SO2會(huì)與NH3爭奪表面活性物質(zhì),從而抑制催化劑的活性[38-39]。
由于CeO2與SO2之間具有強(qiáng)相互作用,生成CeSO4或Ce2(SO4)3。利用CeO2作為犧牲位點(diǎn)可以緩解主活性相的硫酸鹽化,從而以犧牲CeO2為代價(jià)提高催化劑的SO2耐受性。將CeO2引入催化劑后,在SO2存在的情況下,CeO2摻雜的催化劑優(yōu)先形成CeSO4,從而抑制了活性物質(zhì)在SCR過程中的硫酸鹽化[40-41]。這是通過構(gòu)建犧牲位點(diǎn)的方式來保護(hù)脫硝催化劑活性位點(diǎn),是一種提高SO2耐受性的有效策略。
LI等[42]研究發(fā)現(xiàn)Ce的引入有助于釩物種聚集到低聚物V-O-V結(jié)構(gòu)中,形成Ce4+-O-Ce3+-O-V5+-O-V5+-O-Ce3+氧化還原結(jié)構(gòu)。Ce4+-O-Ce3+-O-V5+-O-V5+-O-Ce3+-O-Ce4+的氧化還原結(jié)構(gòu)可以加強(qiáng)Ce物種與ABS的結(jié)合,從而削弱ABS的穩(wěn)定性,增強(qiáng)ABS衍生的NH4+物種的活性,進(jìn)一步抑制了硫酸鹽與氨物種之間的競爭反應(yīng)。由此可見,Ce摻雜能有效抑制ABS的形成,提高V/Ce/WTi的ABS耐受性。同樣,WANG等[43]制備了摻雜不同Ce含量的MnOx-CeO2/TiO2催化劑,發(fā)現(xiàn)Ce主要是通過抑制硫酸銨的積累和CeO2的優(yōu)先硫化來增強(qiáng)MnCe/Ti的抗SO2性能。
此外,促進(jìn)硫酸鹽的分解,降低硫酸鹽分解溫度可以提高其在較低溫度下的SO2耐受性,CeO2改性可以通過降低硫酸鹽的熱穩(wěn)定性來加速NH4HSO4的分解。NI等[44]通過在V2O5-WO3/TiO2催化劑中添加CeO2,激活硫酸氫氨(ABS)在低溫下NH3-SCR過程中的分解。原因是ABS優(yōu)先沉積在7Ce-VW/Ti催化劑表面的添加劑CeO2上,生成CeSO4,最終保護(hù)了活性V2O5位點(diǎn)和TiO2載體。此外,由于CeO2的保護(hù)作用,活性組分V2O5和載體TiO2之間能保持大量的V5+和電荷轉(zhuǎn)移,從而保持了高NH3-SCR性能,提高了SO2耐性。
3Ce對SCR催化劑脫硝活性的影響
Ce改性可以提高SCR催化劑的脫硝活性。V4++Ce4+?V5++Ce3+之間的氧化還原循環(huán)以及NO2和單齒硝酸鹽的形成是活性較高的原因[45]。CeO2摻雜的TiO2載體還提高了V2O5-WO3/TiO2催化劑的SCR活性,這是因?yàn)镃e和Ti之間的強(qiáng)相互作用、大的比表面積以及Ce3+的存在導(dǎo)致生成更多的化學(xué)吸附氧和/或弱結(jié)合氧[46]。SHEN等[47]制備了V2O5/CeO2/WO3-TiO2催化劑,在275~500℃范圍內(nèi)NOx的轉(zhuǎn)化率超過了90%,并且?guī)缀鯖]有N2O的生成。LEE等[48]利用10%Ce負(fù)載Sb-V2O5/TiO2,增強(qiáng)了催化劑的氧化還原性能,提高了吸附氧的流動(dòng)性和表面酸性,從而在220~500℃范圍內(nèi)表現(xiàn)出良好的SCR活性。
CHEN等[49]研究了添加Ce對低釩V2O5-WO3/TiO2在NH3選擇性催化還原(SCR)過程中的影響。Ce通過與V、W組分的協(xié)同作用增強(qiáng)了NOx的吸附,進(jìn)而加速了SCR反應(yīng)。在V0.1W6Ce10Ti催化劑中,Ce主要以Ce3+氧化物的形式存在,有利于NO氧化為NO2。此外,Ce添加劑在V0.1W6Ti上可以提供更強(qiáng)、更活躍的Br?nsted酸位點(diǎn),有利于SCR反應(yīng)進(jìn)程。
蔣旻等[50]采用溶膠-凝膠法制備了不同Ce含量的V2O5/CexTi1-xO2,考察了CeO2含量對催化劑的結(jié)構(gòu)、表面性質(zhì)和NH3-SCR反應(yīng)性能的影響,發(fā)現(xiàn)V和Ce物種之間的相互作用促進(jìn)了催化劑表面V5+物種的形成,增強(qiáng)了催化劑對NO的吸附和氧化能力,并提供較多的弱和中等強(qiáng)度的酸性位,導(dǎo)致其低溫SCR反應(yīng)性能的顯著提升。通過引入CeO2對其結(jié)構(gòu)及表面性質(zhì)進(jìn)行改性,顯著提高了V2O5/TiO2催化劑的NH3-SCR反應(yīng)性能。
4" 結(jié)束語
本文綜述了SCR反應(yīng)機(jī)理以及Ce在脫硝催化劑中的研究現(xiàn)狀,表明Ce在提高脫硝催化劑的抗中毒性能和催化活性等方面具有積極的影響,總結(jié)了Ce對各類金屬雜質(zhì)和硫中毒的作用機(jī)理以及對不同催化劑中毒的反應(yīng)原理等。
基于本文分析,改性脫硝催化劑的研究不應(yīng)僅僅局限于一種物質(zhì)的摻雜,應(yīng)考慮多種稀土元素的加入,再結(jié)合不同的高活性催化劑,如Mn、Fe、V等,耦合各組分在氧化還原性能、表面酸性及抗中毒性能等優(yōu)勢特性,并構(gòu)筑功能薄膜等特性結(jié)構(gòu)阻礙有害物質(zhì)的毒化作用等,研制出高活性、高N2選擇性及優(yōu)良穩(wěn)定性的寬溫脫硝催化劑,并進(jìn)一步加強(qiáng)脫硝催化劑的成型與應(yīng)用研究,實(shí)現(xiàn)對現(xiàn)有V基催化劑的替換或V的減量化。通過本文的綜述,希望可以在新開發(fā)的脫硝催化劑與實(shí)際需求之間架起一座橋梁,并在不久的將來實(shí)現(xiàn)其商業(yè)應(yīng)用。
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Research Progress of Ce Modification on Denitrification Performance and Anti-Poisoning of SCR Catalyst
YOU Mengyun1,2,WU Shufeng1,ZHANG Zhang2,WANG Yubin1
(1. Petrochina Petrochemical Research Center Louzhou Chemical Research Center, Lanzhou Gansu 730060,China;
2. College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou Gansu 730060,China)
Abstract: NH3 selective catalytic reduction (NH3-SCR) is the most effective technology to reduce nitrogen oxide (NOx) emissions from coal-fired industries and diesel engines, the core of which is SCR denitrification catalyst. However, metal impurities and sulfur compounds in flue gas can cause the deactivation of catalysts, so it is necessary for the existing SCR denitration catalyst for modification. Because of its unique REDOX, oxygen storage and acid-base properties, Ce is widely used as a carrier, initiator and accelerator, and is a common additive in the research of SCR catalysts. This paper introduces the mechanism of SCR catalyst in denitration reaction, and the important role of Ce on the anti-poisoning and denitration performance of SCR catalyst.
Key words: Ce; SCR denitration catalyst; Denitrification activity; Poisoning