Ce摻雜改性Ni-Al-Ox催化劑CO-NO反應(yīng)性能

郭 磊,張 濤,?；?,李俊華

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Ce摻雜改性Ni-Al-O催化劑CO-NO反應(yīng)性能

郭 磊1,張 濤1,?；?*,李俊華2

(1.中國人民大學(xué)環(huán)境學(xué)院,北京 100872；2.清華大學(xué)環(huán)境學(xué)院,北京 100084)

通過尿素法制備了一系列Ce摻雜改性的Ni-Al-O復(fù)合氧化物催化劑,并研究了催化劑的CO-NO反應(yīng)性能.活性測(cè)試結(jié)果表明,Ni-Al-Ce-O的催化活性明顯優(yōu)于Ni-Al-O,且活性隨著Ce含量的增加而提高.當(dāng)Ce的摻雜量為20%時(shí),250℃條件下,NO轉(zhuǎn)化率高達(dá)95％以上,同時(shí)催化劑具有良好的抗H2O性能. XRD和Raman分析表明, Ce摻雜Ni-Al-O催化劑促進(jìn)了氧空位產(chǎn)生.而XPS結(jié)果顯示,氧空位隨著Ce含量的增加而增加.H2-TPR結(jié)果表明,Ce的摻雜使催化劑的氧化還原性能增強(qiáng).結(jié)合NO-TPD和FTIR的表征結(jié)果,進(jìn)一步發(fā)現(xiàn)Ce的加入使表面氧難以將NO氧化成硝酸鹽,有利于NO在Ce的氧空位上分解為N2或N2O,促進(jìn)CO-NO反應(yīng)的進(jìn)行.

Ni-Al-Ce-O；CO-NO反應(yīng)；氧空位；Ce摻雜

近年來,大氣污染物排放標(biāo)準(zhǔn)日趨嚴(yán)格,對(duì)機(jī)動(dòng)車尾氣凈化要求越來越高,尾氣中氮氧化物(NO)等的高效凈化成為一個(gè)亟待解決的問題[1-4].CO-NO反應(yīng)能同時(shí)去除尾氣中的CO和NO,是消除尾氣污染物的理想方法.目前,在CO-NO反應(yīng)中應(yīng)用較成熟的催化劑是貴金屬催化劑,但存在價(jià)格昂貴,高溫不穩(wěn)定等缺點(diǎn).因此,用更為經(jīng)濟(jì)的過渡/稀土金屬氧化物取代貴金屬在機(jī)動(dòng)車尾氣凈化領(lǐng)域一直是人們關(guān)注的方向.

類水滑石為前驅(qū)體制備的復(fù)合氧化物具有較大的比表面積和良好的氧化還原性能,近年來, 逐漸成為最具有開發(fā)前景的消除NO的催化劑之一.目前類水滑石結(jié)構(gòu)催化劑在脫硝領(lǐng)域的應(yīng)用主要體現(xiàn)在NH3-SCR,NO儲(chǔ)存以及CO-NO反應(yīng)中.在NH3-SCR的研究中發(fā)現(xiàn),過渡金屬Cu,Co摻入水滑石前驅(qū)體或堿土金屬M(fèi)g和兩性金屬元素Al作為水滑石前驅(qū)體可以顯著提高制得的催化劑的NH3- SCR催化活性.[5-6]在NO吸附存儲(chǔ)的應(yīng)用中, Co,Mg, Al元素也是很好的水滑石材料. Basile F等用水滑石焙燒得到的MgAl復(fù)合氧化物為載體,采用浸漬法得到了系列Pt和Pt-Cu負(fù)載的NO存儲(chǔ)還原催化劑,該系列催化劑低溫時(shí)的NO存儲(chǔ)活性大大提高,并且具有較強(qiáng)的抗SO2能力[7].Yu等[8]發(fā)現(xiàn)CoMgAl水滑石具有良好的NO吸附儲(chǔ)存性能.此外,研究發(fā)現(xiàn)堿金屬K可以顯著提高CoMgAl水滑石的NO儲(chǔ)存性能[9].類水滑石在CO-NO反應(yīng)中也展現(xiàn)出了良好的催化性能.陳英紅等將CoNiAl類水滑石復(fù)合氧化物催化劑用于CO-NO反應(yīng),發(fā)現(xiàn)當(dāng)Co:Ni: Al=5:1:1(物質(zhì)的量之比)時(shí),催化劑在低溫下有一定的反應(yīng)活性,200℃的NO轉(zhuǎn)化率達(dá)到95%,但溫度繼續(xù)升高,活性明顯下降[10].Dai等[11]發(fā)現(xiàn)向CoMgAl類水滑石復(fù)合氧化物催化劑中添加Ce能明顯提高催化劑的氧化還原能力,促進(jìn)其NO脫除性能.Palomares等[12]利用CoMgAl類水滑石復(fù)合氧化物催化劑處理催化裂化流化床單元條件下所產(chǎn)生的NO時(shí)發(fā)現(xiàn),CeO2的加入能明顯提高催化劑的抗SO2能力.此外,已有研究報(bào)道,將Ce加入NiO中可以產(chǎn)生新的氧化還原位點(diǎn),通過Ni和Ce之間的相互作用提升其CO-NO反應(yīng)性能[13-14].總體而言,Co,Mg,Al是很好的水滑石構(gòu)成元素,表現(xiàn)出在NH3-SCR、NO儲(chǔ)存以及CO-NO反應(yīng)中很好的性能.而Ce元素在CO-NO反應(yīng)中可能具有很好的應(yīng)用前景.

由以上可知,類水滑石催化劑在包括CO-NO反應(yīng)的催化凈化NO技術(shù)中有廣泛運(yùn)用,但也存在抗SO2性差,溫度窗口較窄等缺點(diǎn),而CeO2具有優(yōu)異的儲(chǔ)氧性能和氧化還原性,作為助劑可以有效提高催化劑的催化活性,更是能與Ni產(chǎn)生相互作用.因此,本文在以上研究的基礎(chǔ)上,考察了Ce摻雜改性的Ni-Al-O復(fù)合氧化物催化劑對(duì)CO-NO反應(yīng)的催化性能,并利用XRD、TPR、Raman等表征手段對(duì)Ce摻雜提升Ni-Al-O的CO-NO催化性能的原因進(jìn)行了探究.以期為Ce在水滑石催化CO-NO反應(yīng)中的作用機(jī)理的研究做出貢獻(xiàn).

1 實(shí)驗(yàn)部分

1.1 催化劑制備

按照Ni2++Al3++Ce3+= 1mol/L, (Ni2++Ce3+)/Al3+= 3配制鎳鹽和鋁鹽混合溶液A,尿素溶液B(尿素/(Ni2++Ce3++Al3+)=10).在室溫和劇烈攪拌條件下,A和B同時(shí)滴加到盛有一定去離子水的三頸燒瓶中.滴加完轉(zhuǎn)入燒杯中,放入95℃油浴中繼續(xù)攪拌24h.冷至室溫,5000r/min離心分離并用去離子水洗滌沉淀至混合液呈中性,80℃過夜烘干得前驅(qū)體.在500℃下煅燒4h得Ni-Al-Ce-O復(fù)合氧化物催化劑,以NiAlCeO表示,其中=Ce3+/(Ni2++Ce3+). Ni-Al-O催化劑的制備方法與此類似,制得的催化劑以NiAlO表示.

1.2 催化劑表征

XRD分析:本實(shí)驗(yàn)使用X射線衍射儀測(cè)定催化劑的晶相構(gòu)成,儀器型號(hào)為D8Advance.其中管電壓為40KV,Cu靶,掃描角度為10°~90°,8°/min連續(xù)掃描.依據(jù)謝樂公式(式1),計(jì)算催化劑晶粒尺寸.

式中:=0.89;為入射X射線波長(zhǎng),nm;為半峰寬,°;為衍射角,°.

H2-TPR:本實(shí)驗(yàn)使用AutoChem 2920進(jìn)行測(cè)定.首先稱取樣品50mg,在N2氛圍300℃下預(yù)處理1h.然后在10%H2中以10℃/min的速率從室溫升到1000℃. Raman光譜測(cè)定:本研究中采用英國的雷尼紹(inVia)儀器進(jìn)行Raman分析.

XPS分析:本研究中XPS分析是由PHI Quantera SXMTM(ULVAC-PHI Inc)儀器完成.采用Al/Mg雙陽極靶,結(jié)果中各個(gè)元素的結(jié)合能需要校正(284.8eV C1峰校正).

NO-TPD:稱取催化劑100mg,然后在300℃,200mL/min N2條件下進(jìn)行預(yù)處理.之后將溫度降至室溫,通入500×10-6NO/N2吸附1h.然后用N2吹掃1h,最后以10℃/min從100℃升到500℃,記錄脫附出NO濃度.

In situ DRIFTS表征:在NICOLET6700型傅立葉紅外光譜儀上進(jìn)行實(shí)驗(yàn).將研細(xì)的樣品填滿樣品池,通冷卻水后,先在N2氣氛下400℃預(yù)處理1h.然后在N2吹掃下采集背景圖譜.切換至吸附氣體100℃下吸附至飽和,N2吹掃至穩(wěn)定,采集不同溫度下的DRIFTS圖譜.

1.3 CO-NO活性評(píng)價(jià)

活性評(píng)價(jià)在實(shí)驗(yàn)室自行設(shè)計(jì)的石英固定反應(yīng)床上測(cè)試.利用瑞士Eco Physics公司的氮氧化物分析儀(型號(hào):CLD 822Mh)檢測(cè)氣體濃度.反應(yīng)條件如下:0.15g樣品,500×10-6NO, 2000×10-6CO, 0或5%的H2O, N2平衡氣,總氣體流速為200mL/min.空速(GHSV)為90000h-1.NO轉(zhuǎn)化率計(jì)算公式如下:

式中:[NO]in和[NO]out分別代表穩(wěn)定狀態(tài)下NO的進(jìn)口和出口濃度.

2 結(jié)果與討論

2.1 CO-NO的反應(yīng)性能

圖1是不同Ce比例摻雜的NiAlCemO催化劑的CO-NO反應(yīng)活性曲線.結(jié)果表明,NiAlO的活性較低,摻雜5% Ce后,催化劑活性明顯提高.Ce摻雜量提高后,催化劑的低溫活性進(jìn)一步提高. NiAlCe30O催化劑的活性最佳,250℃時(shí)NO轉(zhuǎn)化率達(dá)到了95%.Ce的摻雜量再提高至40%后,反應(yīng)活性開始下降.可以看出,Ce的摻雜能夠提高催化劑的CO-NO反應(yīng)活性.

圖1 不同催化劑的CO-NO反應(yīng)活性

反應(yīng)條件為500×10-6NO,2000×10-6CO,N2平衡, GHSV = 9.0×104h?1(STP)

2.2 抗水性能探究

圖2 NiAlCemO催化劑在CO-NO反應(yīng)中的抗水性能

反應(yīng)條件為0.2g 樣品,500×10-6NO, 2000 ×10-6CO,5%H2O,N2平衡,GHSV = 9.0×104h?1(STP)

圖2顯示了制備的不同催化劑在H2O存在的情況下CO-NO反應(yīng)活性的變化.對(duì)于NiAlCe5O和NiAlCe10O,活性受水的影響很大,而對(duì)于Ce含量較高的樣品,僅低溫區(qū)間有所影響,以NiAlCe20O為例,200℃時(shí)活性由80.5%下降至47.3%,但250℃以后便幾乎不再受水的影響.總的來說,H2O在一定程度上會(huì)較大地抑制NiAlCeO的CO-NO氧化活性,但在一定范圍內(nèi)提高Ce的摻雜量確實(shí)能提高抗水性能.

2.3 NiCeO固溶體和氧空位的形成

2.3.1 XRD圖3是不同催化劑的XRD圖譜 NiAlO樣品上的峰均歸屬于NiO的特征峰,沒有Al的特征峰出現(xiàn),說明Al在該氧化物中具有良好的分散度.摻雜Ce后出現(xiàn)CeO2的特征峰,且隨著Ce的含量增加,CeO2的特征峰增強(qiáng),NiO的特征峰減弱.與純CeO2相比,NiAlCeO中的CeO2的特征峰都出現(xiàn)輕微偏移,這主要是由于Ce4+和Ni2+的離子半徑差異較大,Ni物種進(jìn)入CeO2后引起了后者的晶格畸變[15].這也說明Ni2+進(jìn)入CeO2晶格形成了固溶體.因此,摻雜的Ce可能以兩種物相存在:一部分與Ni2+相互作用形成固溶體,一部分以CeO2形式存在.

圖3 NiAlO,NiAlCemO和CeO2催化劑的XRD圖譜

2.3.2 Raman光譜 如圖4所示, Ce摻雜量在10%及以上的樣品在461cm-1出現(xiàn)了明顯的尖峰,這是由[Ce-O8]的伸縮振動(dòng)(又稱F2g振動(dòng)模式)引起[16].結(jié)合文獻(xiàn)可知,CeO2原本位于465cm-1的特征峰向低波數(shù)移動(dòng),證明Ce-O鍵強(qiáng)度減弱,這極有可能是由于Ni2+進(jìn)入Ce-O晶格形成的.為了維持電荷平衡,當(dāng)Ni2+進(jìn)入Ce-O晶格后,必然伴隨氧空位的產(chǎn)生[17].位于227,634cm-1的拉曼特征峰則直接證明了氧空位和Ni-Ce-O固溶體的形成[18-19].位于500~ 600cm?1的對(duì)稱拉曼特征峰歸屬于Ni-O鍵的振動(dòng)[17].加入Ce后,該吸收峰向高波數(shù)移動(dòng),可能是CeO2上氧空位的峰(D)[19-21]與Ni-O鍵的振動(dòng)峰重合.Raman光譜的結(jié)果不僅明確證明了Ni-Ce-O固溶體的產(chǎn)生,也給出了氧空位存在的證據(jù).

圖4 NiAlO和NiAlCemO催化劑的拉曼光譜

2.3.3 XPS 為了進(jìn)一步探討材料表面原子價(jià)態(tài)變化,接下來利用XPS分析對(duì)催化劑進(jìn)行了表征.NiAlCeO催化劑樣品的O 1s譜圖如圖5(a)所示.經(jīng)分峰可知,結(jié)合能在531.4~531.8eV之間的峰可以歸屬于表面化學(xué)吸附氧,如O2-、O-,這些氧物種來自于氧空位或羥基類(記為O)[22];而結(jié)合能在529.8~530.1eV之間的峰可以歸屬于晶格氧(記為O)[23].通過積分計(jì)算可以得到O/(O+O)的比例(圖中列出).顯然,O/(O+O)的比例與Ce的摻雜量呈正相關(guān).更多的化學(xué)吸附氧意味著催化劑表面存在更多可經(jīng)CO還原產(chǎn)生的氧空位.同時(shí),O 1s結(jié)合能的大小(具體見表1)跟Ce的摻雜量呈現(xiàn)負(fù)相關(guān),這是由于Ce極強(qiáng)的電負(fù)性導(dǎo)致.根據(jù)以往的研究,同一體系內(nèi)O 1s的結(jié)合能單調(diào)減小意味著O的堿性增強(qiáng)[24],不難推測(cè)Ce的添加增加了催化劑內(nèi)O的堿性,使其在反應(yīng)中更易吸附CO.這有利于NiAlCeO催化劑CO-NO反應(yīng)活性的提高.

NiAlCeO催化劑 Ce 3d譜圖如圖5(b)所示.v和u分別代表3d5/2和3d3/2的自旋軌道耦合.v,v′′,v′′′,u,u′′和u′′′屬于Ce4+的特征峰.v0,v′和u′屬于Ce3+的特征峰[25]. Ce4+含量要高于Ce3+,說明在NiAlCeO催化劑中Ce4+和Ce3+共同存在,且Ce4+占據(jù)主導(dǎo)地位.這與已報(bào)道的結(jié)果一致[26].在該系列催化劑中,Ce3+/(Ce3++Ce4+)的比例與Ce的添加量成正相關(guān)(表1).相對(duì)較高的Ce3+比例意味著更多的氧空位[27].這可能是CO-NO活性隨Ce含量增加的原因之一.

表1 NiAlCemO的耗氫量和XPS信息

注:-為未檢驗(yàn).

2.4 H2-TPR

如圖6所示,所有催化劑在550℃附近均有一個(gè)明顯的還原峰(α峰).對(duì)于NiAlO,α峰歸屬于高度分散的NiO被還原過程[28-29].而對(duì)于NiAlCeO,除了代表催化劑表面NiO被還原, α峰還表明進(jìn)入CeO2晶格中的NiO的還原.顯然,隨著Ce摻雜量從10%上升到30%,還原峰逐漸向高溫區(qū)移動(dòng).一般來說,高度分散的NiO可以在相對(duì)較低的溫度下被還原,而進(jìn)入固溶體的Ni物種則相對(duì)來說更難被還原[29].因此,隨著Ce含量增加,越來越多的Ni進(jìn)入CeO2晶格形成固溶體(XRD和Raman的結(jié)果有所證明),最終導(dǎo)致α峰向高溫區(qū)移動(dòng).耗氫量的變化也說明了這一點(diǎn)(表1).此外,在該溫度區(qū)間僅有一部分CeO2能被還原.對(duì)于NiAlCe5O,α峰出現(xiàn)的溫度相對(duì)較高,可能是由于它擁有相對(duì)完整的水滑石結(jié)構(gòu),Ni2+在體系內(nèi)的分散度更高,所以更難被還原[28].對(duì)于NiAlCe20O和NiAlCe30O,在300℃左右出現(xiàn)了一個(gè)新的還原峰,可能歸屬于CeO2上表面Ce4+的還原,這從側(cè)面說明了Ni-Ce-O固溶體的形成[30].這也證明Ce的摻雜促進(jìn)了兩者的氧化還原性.而對(duì)于NiAlO在280℃出現(xiàn)的弱還原峰,可能屬于體相NiO的還原[31].

圖6 NiAlO和NiAlCemO催化劑的H2-TPR圖譜

2.5 NOx-TPD

為了評(píng)價(jià)NO在催化劑表面的吸附性能,進(jìn)行了不同催化劑的NO-TPD測(cè)試(圖7). NiAlO樣品在160~240℃(α峰)和350~500℃(β峰)各有一個(gè)脫附峰. NiAlCeO催化劑兩個(gè)脫附峰分別位于160~240℃和310~440℃.NO的脫附峰主要來自于亞硝酸鹽的分解,而NO2的脫附峰主要由硝酸鹽分解形成.值得注意的是低溫區(qū)間NO的脫附峰強(qiáng)度隨著Ce含量增加而增加,說明Ce的添加有助于催化劑表面亞硝酸鹽物種的形成.同時(shí),添加Ce后,高溫區(qū)間的脫附峰脫附溫度降低,這可能是由于部分硝酸鹽更容易脫附.

2.6 原位紅外吸附實(shí)驗(yàn)(In situ DRIFTS)

2.6.1 CO吸附 為了進(jìn)一步研究CO-NO反應(yīng)機(jī)理,進(jìn)行了CO的原位紅外吸附實(shí)驗(yàn).圖8是在不同溫度下, CO吸附在NiAlCe20O催化劑表面的DRIFTS譜圖.100℃在1650~1300cm-1區(qū)域的吸收峰主要是與碳酸鹽型物種的形成有關(guān)[32].一般將波長(zhǎng)大于2000cm-1的紅外吸收峰歸結(jié)于ν(C-O)的伸縮振動(dòng),即由線性吸附態(tài)的CO產(chǎn)生;當(dāng)波長(zhǎng)低于這個(gè)值時(shí),則歸屬于橋式或?qū)\生吸附態(tài)的CO[33].在NiAlCe20O上,并未出現(xiàn)這一系列吸收峰,表明CO在NiAlCe20O催化劑表面吸附較弱.而且該樣品中,碳酸鹽的紅外吸收峰所處位置很高(紅外波段在1643,1509和1400cm-1).碳酸鹽主要是通過CO和與催化劑表面氧物種的反應(yīng)形成,進(jìn)一步表明催化劑具備較高的氧釋放能力和流動(dòng)性,這與已報(bào)道的含Ce固溶體具有高儲(chǔ)氧能力一致.[34]當(dāng)溫度高于250℃時(shí),碳酸鹽開始分解,碳酸鹽的吸收峰幾乎完全消失,這是因?yàn)榇蟛糠諧O與氧結(jié)合,生成CO2并釋放.

圖8 CO/N2吸附于NiAlCe20O催化劑上的DRIFTS譜圖

2.6.2 NO吸附 圖9(a)是在不同溫度下, NO吸附在NiAlO催化劑表面的DRIFTS譜圖.在1560, 1531,1280,1240cm-1出現(xiàn)的峰值歸屬于吸附在NiO上的雙齒硝酸鹽[35],1615,1311cm-1處的峰歸屬于吸附在Al2O3上的橋式硝酸鹽[35].以上結(jié)果表明對(duì)于NiAlO, NO吸附在催化劑表面主要形成硝酸鹽物種.圖9(b)是NO吸附在CeO2催化劑表面的原位紅外譜圖.在1199,1278cm-1處出現(xiàn)的強(qiáng)吸收峰說明存在吸附在Ce4+上的雙齒亞硝酸鹽物種(Ce4+=O2N)[36-37].隨著溫度的升高,該峰出現(xiàn)紅移,表明在氧空位或者是其他缺陷位置,形成了雙齒Ce+=O2N[38].在100~200℃時(shí), 1547cm-1處的吸收峰歸屬于吸附在Ce上的單齒硝酸鹽(Ce+=ONO2-)[39].當(dāng)吸附溫度在300~400℃時(shí),由于螯合雙齒硝酸鹽(Cex+= O2NO-)的形成,在1523~1556,1224,1031,1008cm-1處出現(xiàn)了一系列吸收峰[36-37].此外,低溫時(shí),1617, 1570cm-1處出現(xiàn)的吸收峰可歸屬于氣態(tài)或者弱吸附性態(tài)的NO2[40].這些數(shù)據(jù)表明,在CeO2上NO的吸附低溫時(shí)主要以亞硝酸形式存在,隨著吸附溫度升高形成穩(wěn)定的硝酸鹽物種.

圖9(c)是不同溫度下NO吸附在NiAlCe20O催化劑表面的DRIFTS譜圖.可以看出,在1240,1280, 1531,1560cm-1出現(xiàn)了雙齒硝酸鹽(非對(duì)稱NO振動(dòng),ν3)的吸收峰[41-42].與前面的結(jié)果進(jìn)行對(duì)比,在1199,1590cm-1的吸收峰可以分別歸屬于雙齒亞硝酸鹽物種和橋式硝酸鹽物種.而在1452cm-1出現(xiàn)的吸收峰則歸屬于螯合亞硝酸鹽物種[35,43].溫度從100 ℃升高到250℃時(shí),NO2和亞硝酸鹽物種的吸收峰強(qiáng)度逐漸減弱,這與NO-TPD中催化劑在160~240℃的脫附峰相對(duì)應(yīng).當(dāng)溫度從300℃升高至400℃,1560,1531,1280,1240cm-1處的吸收峰峰強(qiáng)減弱,說明硝酸鹽的吸附能力降低開始分解,這與NO-TPD中350~410℃的脫附峰相對(duì)應(yīng).

圖10 NiAlCe20O催化劑吸附CO/N2 30min,經(jīng)N2吹掃后通入NO/N2后表面的DRIFTS譜圖

圖11 NiAlCe20O催化劑吸附NO/N2 30min,經(jīng)N2吹掃后通入500′10-6 CO/N2后表面的DRIFTS譜圖

2.6.3 瞬態(tài)反應(yīng) 圖10是200℃下,NiAlCe20O先吸附CO后吸附NO的譜圖.結(jié)果與NO和CO共吸附類似,稍有不同的是,實(shí)驗(yàn)過程中出現(xiàn)了硝酸鹽(1274,1531,1560cm-1處)逐漸增加的現(xiàn)象,可能是由于沒有連續(xù)通入CO,NO與表面氧反應(yīng)生成硝酸鹽,顯然這不利于NO分解.

圖11是在200℃下,NiAlCe20O先吸附NO后吸附CO的譜圖,與200℃下NO-FTIR的結(jié)果相比(圖9(c)),兩者唯一的區(qū)別在于,該實(shí)驗(yàn)中出現(xiàn)了3個(gè)歸屬于碳酸鹽的特征峰.

圖12 CO-NO/N2在NiAlCe20O催化劑上吸附的DRIFTS譜

2.6.4 CO-NO共吸附 圖12是溫度為200℃時(shí), NO和CO 在NiAlCe20O上的共吸附譜圖.1390,1501, 1630cm-1處的吸收峰與碳酸鹽的形成有關(guān),這與CO吸附在NiAlCe20O的結(jié)果一致.結(jié)合CeO2和NiAlCe20O上的NO-DRIFTS結(jié)果可知,1199cm-1處的吸收峰與CeO2上的雙齒亞硝酸鹽有關(guān).1213cm-1處的吸收峰可能是由吸附在NiO上的橋式亞硝酸鹽物種造成[35].值得注意的是,在1840cm-1處出現(xiàn)了一個(gè)新的弱吸收峰,歸屬于吸附在NiO上的NO.這表明隨著NO氧化程度降低,吸附態(tài)的NO開始出現(xiàn),無疑有利于NO的分解反應(yīng).總之,與NO-FTIR結(jié)果進(jìn)行對(duì)比,主要有兩點(diǎn)不同:(1)吸附在NiO和CeO2上的亞硝酸鹽吸收峰峰強(qiáng)顯著增加.同時(shí),出現(xiàn)了吸附態(tài)的NO,說明了NO氧化程度顯著降低;(2)隨著反應(yīng)時(shí)間的增加,碳酸鹽的吸收峰逐漸增強(qiáng)(1390, 1501,1630cm-1),而亞硝酸鹽峰強(qiáng)逐漸減弱(1199, 1213cm-1),說明表面氧更傾向于與CO反應(yīng)生成碳酸鹽,而大多數(shù)通入的NO可能被還原為N2O或者N2.

結(jié)合以上的FTIR結(jié)果與已報(bào)導(dǎo)的結(jié)果[44],我們可以推斷出NiAlCe20O上CO-NO的反應(yīng)路徑:(1)CO消耗表面氧來形成氧空位;(2)NO在氧空位上分解生成N2O或N2;(3)NO解離產(chǎn)生的氧留在氧空位上,隨后被CO去除.

3 結(jié)論

3.1 NiAlCeO表現(xiàn)出比NiAlO更高的CO-NO反應(yīng)活性

3.2 Ni進(jìn)入Ce-O晶格形成Ni-Ce-O固溶體,由此產(chǎn)生的氧空位是反應(yīng)的活性位點(diǎn).

3.3 NiAlCeO上的CO-NO反應(yīng),同時(shí)存在2個(gè)反應(yīng):催化劑表面被CO還原并產(chǎn)生空位;NO被表面氧化成NO2,亞硝酸鹽或硝酸鹽.前者有利于CO-NO反應(yīng)發(fā)生,而后者則相反.

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Study on Ce-doped Ni-Al-Ocatalysts for NO reduction by CO.

GUO Lei1, ZHANG Tao1, CHANG Hua-zhen1*, LI Jun-hua2

(1.School of Environment and Natural Resources, Renmin University of China, Beijing 100872, China；2.School of Environment, Tsinghua University, Beijing 100084, China)., 2018,38(9)：3313~3321

A series of Ce-doped Ni-Al-Ocatalysts were prepared by a urea-hydrolysis method, and were applied for CO-NO reaction. The results showed that Ce-doped Ni-Al-Oexhibited much higher NO conversions than Ni-Al-O. Furthermore, the activity increased with the raising of Ce doping ratio. More than 95% NO could be converted at temperature as low as 250°C over NiAlCe30O and NiAlCe20O. These catalysts also showed excellent resistance to H2O. Ce-doped Ni-Al-Ocatalysts were characterized by XRD, N2physisorption, Raman, XPS, H2-TPR, NO-TPD, and in situ DRIFTs. XRD and Raman results showed that Ni-Ce-O solid solution was formed in Ni-Al-Ce-Ocatalyst. Meanwhile, oxygen vacancies increased with the increasing of Ce doping ratio. H2-TPR results indicated that the redox ability was improved by Ce doping. NO-TPD and in situ DRIFTs results revealed that the increasing number of oxygen vacancies resulted from Ce doping were beneficial to the dissociation of NO to N2or N2O rather than the oxidation of NO to nitrates.

Ni-Al-Ce-O；CO-NO；oxygen vacancy；Ce-doped

X505

A

1000-6923(2018)09-3313-09

郭 磊(1992-),男,湖南益陽人,中國人民大學(xué)碩士研究生,研究方向?yàn)榇髿馕廴究刂乒こ?發(fā)表論文1篇.

2018-03-05

國家重點(diǎn)研發(fā)計(jì)劃項(xiàng)目(2016YFC0203900,2016YFC0203901);國家自然科學(xué)基金項(xiàng)目(51778619,21577173)

* 責(zé)任作者, 副教授, chz@ruc.edu.cn