任思達(dá),梁文俊,王昭藝,杜曉燕,李 堅(jiān),何 洪

Ce摻雜對(duì)Pd/γ-Al2O3催化燃燒甲苯性能的影響

任思達(dá),梁文俊*,王昭藝,杜曉燕,李 堅(jiān),何 洪

(北京工業(yè)大學(xué)區(qū)域大氣復(fù)合污染防治北京市重點(diǎn)實(shí)驗(yàn)室,北京 100124)

通過(guò)等體積浸漬法制備了單金屬Pd/γ-Al2O3催化劑和雙金屬Pd-Ce/γ-Al2O3催化劑,考察摻雜CeO2對(duì)Pd/γ-Al2O3催化劑催化氧化甲苯性能的影響.并通過(guò)N2吸脫附、SEM、H2-TPR表征催化劑比表面積、表面形貌及氧化還原性能.結(jié)果發(fā)現(xiàn),CeO2的摻雜一定程度上降低了Pd/γ-Al2O3催化劑的比表面積,但增加了10nm孔徑的孔密度,且催化劑仍保持介孔結(jié)構(gòu),當(dāng)添加4%CeO2時(shí)(質(zhì)量分?jǐn)?shù),下同),催化劑比表面積降至165m2/g,孔道存在一定程度的堵塞,阻礙污染物和反應(yīng)產(chǎn)物的擴(kuò)散,降低催化劑催化性能.H2-TPR結(jié)果表明,Pd和Ce之間存在較強(qiáng)的協(xié)同作用,與PdO相鄰的CeO2更容易打開(kāi)Ce—O鍵,相較于單金屬0.2%Pd/γ-Al2O3,摻雜了0.3%CeO2的催化劑具有更強(qiáng)的還原峰,表明CeO2的引入為催化劑提供了更多的表面氧空位,增強(qiáng)了催化劑的催化氧化能力,其10和90與單貴金屬催化劑相比分別降低10和40℃.

甲苯；催化氧化；CeO2；Pd；協(xié)同作用

VOCs是一種低沸點(diǎn)有機(jī)物,是霧霾、光化學(xué)煙霧的重要前驅(qū)體之一[1],已成為繼煙塵、二氧化硫(SO2)和氮氧化物(NO)之后,又一備受國(guó)民關(guān)注的污染物之一[2],同時(shí)嚴(yán)重危害人體健康,對(duì)呼吸道,心血管等有較強(qiáng)的毒害作用[3-4].由于在石油化工、印染、制藥、噴漆等生產(chǎn)活動(dòng)中甲苯的大量排放,因此以甲苯催化燃燒作為VOCs減排研究的典型模型反應(yīng)[5-6].與直接燃燒法、生物法、吸收吸附法等傳統(tǒng)方法相比,催化燃燒法具有能耗低、去除效率高,將污染物轉(zhuǎn)化成CO2和H2O,對(duì)環(huán)境無(wú)二次污染,使用范圍廣等特點(diǎn)[7],常見(jiàn)催化劑主要有貴金屬型和過(guò)渡金屬氧化物型兩種,盡管有報(bào)道稱少數(shù)過(guò)渡金屬氧化物(如Cu、Mn、Fe等)[8-10]對(duì)甲苯催化燃燒具有活性,但仍需要大量的工作來(lái)提高低溫活性和穩(wěn)定性.貴金屬型催化劑相比于過(guò)渡金屬氧化物型催化劑,具有更高的催化活性、穩(wěn)定性和耐抗性,尤其是Pt[11-12]和Pd[13-14],對(duì)甲苯和各種VOCs的燃燒均有較好的催化活性和穩(wěn)定性[15-16],且Pd基催化劑以其較低的起活溫度、高選擇性、高去除率等優(yōu)點(diǎn)被人們廣泛關(guān)注[17-20].在催化劑成本方面,重點(diǎn)是在盡可能減少貴金屬的情況下提高其低溫活性和穩(wěn)定性,又由于在高溫條件下其活性點(diǎn)位容易發(fā)生遷移和表面團(tuán)聚,影響催化效果[21],因此往往添加一些助劑來(lái)提高貴金屬催化劑性能,并降低貴金屬含量[22].

研究表明[23-24],催化劑摻雜一定量的稀土元素,可較為明顯的提高活性組分的分散性和抗燒結(jié)能力,增強(qiáng)載體與活性組分的結(jié)合強(qiáng)度,提高催化劑催化活性.稀土元素鈰(Ce)近年來(lái)在催化方面應(yīng)用較為廣泛,曹利等[25]采用浸漬法制備Cu1Mn2Ce0.2O2/γ-Al2O3催化劑,結(jié)果表明摻雜一定量Ce元素后,催化劑比表面積有所增大,且T(催化效率為90%時(shí)的溫度,下同)相比未摻雜Ce元素相比下降約70℃;不同形貌的CeO2對(duì)催化活性具有不同影響,何麗芳等[26]采用水熱合成法制備形貌規(guī)整的CeO2納米碳棒,測(cè)試表明CeO2具有高比表面積,較大的氧空位和高活性氧物種,且在250℃左右對(duì)甲苯的催化效率即達(dá)到90%;焦向東等[27]將不同含量的CeO2加入到Pd-Pt雙貴金屬催化劑中,其中1wt%含量的CeO2催化劑的50和90分別降低40℃和30℃,且在空速20000h-1、350℃條件下仍有90%的催化效率,表明不同含量的CeO2對(duì)催化劑活性有較大影響.在稀土金屬-貴金屬?gòu)?fù)合催化劑研究方面,貴金屬含量普遍在0.3~ 0.5wt%,仍有進(jìn)一步降低的空間,且針對(duì)雙金屬催化劑組成、貴金屬含量及稀土金屬對(duì)貴金屬催化劑的活性影響研究較少.

本文以等體積浸漬法制備Pd/γ-Al2O3催化劑,并摻雜不同含量CeO2,制備出低貴金屬含量、高活性的催化劑,并通過(guò)氮?dú)馕摳角€(BET)、掃描電鏡(SEM)、H2程序升溫還原(TPR)對(duì)考察稀土元素Ce的添加對(duì)催化劑形貌特征和氧化還原特性的影響,通過(guò)調(diào)配稀土元素與貴金屬的比重,降低貴金屬含量,并測(cè)試其對(duì)甲苯催化性能影響.

1 實(shí)驗(yàn)內(nèi)容

1.1 催化劑制備

采用等體積浸漬法制備催化劑:將0.1mol/L H2PdCl4溶液1.67mL和40mL去離子水于60℃下超聲攪拌1h,待溶液完全混合均勻后,再緩慢加入9.99g γ-Al2O3粉末,繼續(xù)超聲攪拌3h,110℃烘干4h,然后馬弗爐550℃焙燒3h,制得0.1%Pd/γ-Al2O3催化劑(質(zhì)量分?jǐn)?shù),下同),研磨并篩分至20~40目待用,其他%Pd/γ-Al2O3催化劑(=0.2,0.3,0.5,0.7)按計(jì)量比依上述方法制備.

將0.1mol/L H2PdCl4溶液3.33mL和0.1mol/L Ce(NO3)31.94mL按上述方法制得0.2%Pd-0.3%Ce/ γ-Al2O3催化劑,研磨并篩分至20~40目待用.其他0.2%Pd-%Ce/γ-Al2O3催化劑(=0.1,0.3,0.5,1,2,3, 4,5)按計(jì)量比依上述方法制備.

1.2 催化劑表征

采用美國(guó)Micromeritics ASAP2050型自動(dòng)吸附儀進(jìn)行催化劑的比表面積和孔結(jié)構(gòu)分析,吸脫附曲線以高純N2(N2純度>99.999%)為動(dòng)力源,采用低溫N2物理吸附法進(jìn)行測(cè)試,樣品在350℃下預(yù)處理1h.

采用日本日立公司Hitachi S-4300型場(chǎng)發(fā)射掃描電子顯微鏡(SEM)對(duì)催化劑的表面形貌進(jìn)行觀察和分析,催化劑樣品研磨成粉末,通過(guò)導(dǎo)電膠固定在樣品臺(tái)上,進(jìn)行噴金處理,加速電壓為20kV.

采用 Auto Chem Ⅱ 2920化學(xué)吸附儀進(jìn)行催化劑程序升溫還原(H2-TPR)測(cè)試,測(cè)試樣品約100mg,以10℃/min在空氣氣氛下升溫至3000℃,保持1h后,用氦氣(He)吹掃冷卻至室溫,再以體積分?jǐn)?shù)為10% H2/He的混合氣對(duì)催化劑進(jìn)行還原,以10℃/min升溫至850℃,TCD信號(hào)檢測(cè).

1.3 催化劑活性評(píng)價(jià)

本實(shí)驗(yàn)以自制常壓固定床反應(yīng)器(內(nèi)徑18mm、恒溫區(qū)長(zhǎng)80mm)對(duì)催化劑進(jìn)行甲苯催化活性測(cè)評(píng),平行測(cè)量3次,取平均值,評(píng)價(jià)流程如圖1所示.實(shí)驗(yàn)條件如下:將4mL催化劑(約2g)不經(jīng)任何預(yù)處理至于反應(yīng)器恒溫區(qū)中,催化劑床層高約15~16mm,熱電偶置于催化劑床層上部,通過(guò)溫控儀程序升溫控制反應(yīng)器溫度,升溫速率5℃/min.常壓下,以潔凈空氣為載氣(O2含量21%、N2含量79%)通過(guò)吹掃甲苯發(fā)生瓶產(chǎn)生甲苯飽和蒸汽,獲得甲苯濃度為3000mg/ m3的甲苯廢氣,并以2L/min的總流量流經(jīng)催化劑床層,空速60000mL/(g·h).為消除催化劑吸附甲苯對(duì)催化效果的影響,先將催化劑床層溫度升到120℃,并保持1h后,保證反應(yīng)器進(jìn)出口甲苯濃度相等,再進(jìn)行活性測(cè)試.采用安捷倫GC7890氣相色譜對(duì)反應(yīng)器出口和入口甲苯濃度進(jìn)行測(cè)定,傅里葉紅外測(cè)試反應(yīng)物中CO2濃度,以10、50和90分別表示催化劑催化甲苯效率為10%、50%和90%所對(duì)應(yīng)的溫度,其中催化劑催化效率由式(1)來(lái)表示,CO2產(chǎn)率由式(2)來(lái)表示:

式中:表示催化劑催化效率,%;CO2表示CO2產(chǎn)率, %;in表示反應(yīng)器入口氣體甲苯濃度,mg/m3;out表示反應(yīng)器出口氣體甲苯濃度,mg/m3;CO2表示反應(yīng)器出口CO2濃度,mg/m3.

圖1 催化劑活性評(píng)價(jià)流程

1-空壓機(jī),2-干燥管,3-轉(zhuǎn)子流量計(jì),4質(zhì)量流量計(jì),5-甲苯發(fā)生瓶,6-恒溫水浴鍋,7-緩沖瓶,8-三通,9-反應(yīng)管,10-加熱爐,11-溫控儀,12-氣相色譜

2 結(jié)果與討論

2.1 催化劑表征測(cè)試分析

2.1.1 BET 對(duì)制備的0.2%Pd-%Ce/γ-Al2O3(=0,0.1,0.3,0.5,4)系列催化劑進(jìn)行孔道結(jié)構(gòu)參數(shù)測(cè)試,如表1所示.在相同的焙燒條件下,隨著Ce含量的增加,催化劑的比表面積、孔容和孔徑均呈現(xiàn)下降趨勢(shì),且Ce含量越多,數(shù)值下降越快.說(shuō)明在焙燒過(guò)程中,催化劑表面上的褶皺或孔道因CeO2的存在造成晶粒的增大或孔道的填充,引起催化劑比表面積的降低[28].但孔徑仍為10~13nm,說(shuō)明Ce的加入,未影響0.2%Pd/γ-Al2O3催化劑的孔道特性,依然保持介孔有序狀態(tài).

表1 催化劑孔道特性數(shù)據(jù)

從圖2a可以看出,催化劑均表現(xiàn)出Ⅳ型吸脫附曲線,且在相對(duì)壓力(/0)0.6~1.0范圍內(nèi)出現(xiàn)H3型滯后環(huán),表明催化劑存在介孔結(jié)構(gòu),與孔徑在10~ 13nm之間測(cè)試結(jié)果相吻合[29].采用BJH(Barrett- Joyner-Halenda)法測(cè)試催化劑孔徑分布結(jié)果如圖2b所示.從圖2b中可以看到,Ce的添加,使得催化劑喪失50nm以上的大孔,催化劑的最可幾孔徑(孔徑分布圖中峰值所對(duì)應(yīng)的孔徑)均在10~20nm,且0.2%Pd-0.3%Ce/γ-Al2O3在此范圍的孔密度最大,表明Ce元素對(duì)0.2%Pd/γ-Al2O3的介孔分布有促進(jìn)作用.較大的比表面積和孔容孔徑有利于反應(yīng)產(chǎn)物和污染物的擴(kuò)散,促進(jìn)甲苯分子的催化氧化,進(jìn)而提高催化劑的催化活性[30].

2.1.2 SEM 從圖3a中可以看出, 0.2%Pd催化劑表面凹凸和褶皺較為明顯,空隙較大;添加0.3%Ce后,如圖3b,由于CeO2的存在,引起晶粒變大,填充部分空隙,但仍有一定的褶皺,吸附位點(diǎn)和活性位點(diǎn)可充分暴露在催化劑表面[31];當(dāng)Ce含量進(jìn)一步增大到4%后,如圖3c,催化劑表面相比于0.2%Pd變得較為平滑,空隙和褶皺進(jìn)一步消失,說(shuō)明Ce的添加,會(huì)堵塞0.2%Pd/γ-Al2O3孔道,直觀的驗(yàn)證BET測(cè)試結(jié)果.

圖3 不同CeO2含量催化劑SEM圖

a. 0.2%Pd/γ-Al2O3; b. 0.2%Pd-0.3%Ce/γ-Al2O3; c. 0.2%Pd-4%Ce/γ-Al2O3; d. γ-Al2O3

圖4 不同CeO2含量0.2%Pd/γ-Al2O3催化劑H2-TPR圖

2.1.3 H2-TPR 從圖4可以看出,0.2%Pd還原曲線在50~200℃之間出現(xiàn)一個(gè)還原峰,對(duì)應(yīng)PdO被H2的還原[32].當(dāng)添加0.3wt%Ce時(shí),催化劑具有最大的還原峰面積,在30~220℃之間出現(xiàn)2個(gè)高而寬的還原峰,說(shuō)明Ce與Pd產(chǎn)生協(xié)同作用,強(qiáng)化了催化劑的氧化性[33],且在200℃以下催化劑的還原峰面積有較大提升,在300℃左右出現(xiàn)的還原峰歸屬于CeO2的表面氧及其次表層氧的還原[34-35];表面氧空位的多少影響著催化劑的催化氧化能力的強(qiáng)弱[36],將具有螢石結(jié)構(gòu)的CeO2引入Pd/γ-Al2O3催化劑體系中,不但由于Ce3+的存在產(chǎn)生大量表面氧空位,而且貴金屬與CeO2間存在相互作用,使得與貴金屬相鄰的Ce-—O鍵更容易斷裂[37],活化了CeO2表面氧及晶格氧,使得氧空位形成能降低,增強(qiáng)其表面氧的析出[38],進(jìn)而提高催化劑表面反應(yīng)速度,及其催化活性.此外,當(dāng)Ce含量增大到4%后,催化劑樣品的還原峰明顯向高溫區(qū)偏移,且峰面積相較于添加0.3wt%Ce也有所減少,同樣在300℃左右出現(xiàn)CeO2的還原峰[39],而此時(shí)比表面積和孔容孔也有一定程度的下降,表明過(guò)量引入CeO2不但會(huì)堵塞催化劑孔道,還會(huì)遮蓋PdO活性位點(diǎn),造成與貴金屬相鄰而被活化的CeO2表面氧及晶格氧不易析出,降低了催化劑氧化性.

2.2 催化劑對(duì)甲苯的去除活性

2.2.1 Pd/γ-Al2O3催化劑 從圖5可以看出,隨著溫度的升高,催化劑的活性均隨之增大,且催化劑Pd含量越大,在相同溫度下催化劑對(duì)甲苯的催化活性也越大.但各催化劑10均高于150℃,且在200℃時(shí),催化劑的甲苯降解率均不足90%.由于除0.1%Pd外,其他Pd含量的催化劑在低溫區(qū)(<200℃)的活性相差較小,為檢驗(yàn)稀土元素Ce對(duì)Pd/γ-Al2O3催化劑催化活性的影響,同時(shí)進(jìn)一步降低貴金屬含量,選用0.2% Pd/γ-Al2O3催化劑作為本底進(jìn)行測(cè)試.

2.2.2 Pd-Ce/γ-Al2O3催化劑 貴金屬含量是決定催化劑催化活性高低的最主要因素,CeO2的加入可以提高催化劑的催化活性,但通過(guò)雙貴金屬?gòu)?fù)配、石墨烯為載體等途徑,可進(jìn)一步降低貴金屬含量(表2).

圖5 不同Pd含量下催化劑催化燃燒甲苯活性

由圖6可見(jiàn),隨著溫度的升高,催化劑的催化活性也隨之增大.添加少量稀土元素Ce后,催化劑的催化活性隨著Ce含量的增大而增大.當(dāng)CeO2含量為0.3%時(shí),其10<150℃,極大的加強(qiáng)了催化劑在低溫區(qū)(尤其是160~200℃)的催化活性,并在200℃時(shí)對(duì)甲苯的去除效率達(dá)到90%以上,且在220℃時(shí)將其完全轉(zhuǎn)化,催化效率相當(dāng)于0.7%Pd/γ-Al2O3催化劑;這主要是由于PdO與CeO2間存在協(xié)同作用,促進(jìn)Ce—O鍵的斷裂[37],產(chǎn)生更多的流動(dòng)氧和氧空位,增強(qiáng)了催化劑的催化氧化活性[38],且在220~240℃對(duì)應(yīng)著其H2-TPR的PdO-CeO2協(xié)同峰,此時(shí)催化劑對(duì)甲苯的去除效果最強(qiáng).進(jìn)一步添加稀土元素Ce,催化劑的活性開(kāi)始降低,但其活性仍高于未添加CeO2的0.2%Pd/γ-Al2O3催化劑;當(dāng)CeO2含量增大到3wt%后,催化劑的催化活性受到抑制,其對(duì)甲苯的催化活性低于未添加CeO2的催化劑,尤其是添加4wt%和5wt%CeO2的催化劑.表明CeO2的添加量在0.1%~2%時(shí),其與PdO間的相互作用是催化劑產(chǎn)生表面氧空位的主要影響因素,加強(qiáng)了催化劑的催化氧化性能,此時(shí)比表面積的降低并未影響到污染物和反應(yīng)產(chǎn)物的擴(kuò)散;當(dāng)CeO2的添加量大于2%以后,比表面積和孔容孔徑均下降20m2/g左右,通過(guò)圖3c可以看到添加4%的CeO2的催化劑表面褶皺減少,一些孔道被堵塞,影響著污染物和反應(yīng)產(chǎn)物的擴(kuò)散,同時(shí)催化活性位點(diǎn)PdO可能存在一定程度的遮蓋,不但阻礙污染物與PdO的接觸,還限制與活性位點(diǎn)相鄰CeO2表面活性氧的遷移與氧空位的產(chǎn)生,降低催化劑催化氧化性能.

圖7 220℃下不同催化劑對(duì)甲苯的催化效率及CO2產(chǎn)率的影響

此外,在最佳工況條件,即空速60000mL/(g·h),甲苯濃度3000mg/m3,反應(yīng)溫度220℃下,考察0.2%Pd/-Al2O3和0.2%Pd-0.3%Ce/-Al2O3催化劑對(duì)甲苯催化氧化時(shí)CO2選擇性及產(chǎn)率,其結(jié)果如圖7所示.表明在最佳工況條件下,添加0.3%CeO2的催化劑不但擁有更高的甲苯催化效率,實(shí)現(xiàn)甲苯完全轉(zhuǎn)化的同時(shí),CO2選擇性為99%,且尾氣中未檢測(cè)到副產(chǎn)物CO.

表2 各類催化劑對(duì)甲苯催化氧化性能

3 結(jié)論

3.1 Pd/γ-Al2O3催化劑中Ce的添加,使得催化劑比表面積降低,但仍保持介孔結(jié)構(gòu),且10nm孔的孔密度增大;添加4%CeO2后催化劑比表面積降至165m2/g,阻礙污染物和反應(yīng)產(chǎn)物擴(kuò)散,造成催化劑催化效率降低.

3.2 0.3%CeO2的加入增大了Pd /γ-Al2O3催化劑的還原峰面積,表明螢石結(jié)構(gòu)的CeO2為催化劑提供了更多的表面氧空位,且與PdO相鄰的CeO產(chǎn)生協(xié)同作用,Ce—O鍵更容易打開(kāi),更容易形成氧空位,增強(qiáng)催化劑催化氧化能力.

3.3 在甲苯催化反應(yīng)中,催化劑Pd-Ce/-Al2O3與Pd/-Al2O3相比,10、90分別下降10℃和40℃,具有更好的催化氧化能力,在220℃即可將甲苯完全氧化,并將甲苯催化氧化成CO2,其產(chǎn)率在99%以上,不存在二次污染.

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Effect of Ce doping on the performance of Pd/γ-Al2O3catalytic combustion of toluene.

REN Si-da, LIANG Wen-jun*, WANG Zhao-yi, DU Xiao-yan, LI Jian, HE Hong

(Key Laboratory of Beijing on Regional Air Pollution Control, Beijing University of Technology, Beijing 100124, China)., 2019,39(7)：2774~2780

A single metal Pd/γ-Al2O3catalyst and a bimetallic Pd-Ce/γ-Al2O3catalyst were prepared by equal volume impregnation method to investigate the effect of CeO2doping on the catalytic oxidation of toluene. The specific surface area, surface morphology and redox properties of the catalyst were characterized with N2desorption, SEM and H2-TPR. It was found that the doping of CeO2reduced the specific surface area of Pd/γ-Al2O3catalyst to a certain extent, but increased the pore density of 10nm, and the catalyst still maintained the mesoporous structure.When 4% CeO2was added (mass fraction, the same below), the specific surface area of ??the catalyst was reduced to 165m2/g, and there was a certain degree of clogging in the pores, which hindered the diffusion of pollutants and reaction products, and reduced the catalytic performance of the catalyst. The H2-TPR results showed that there is a strong synergistic effect between Pd and Ce. CeO2adjacent to PdO was more likely to open Ce-O bond, which was 0.3compared with single metal 0.2% Pd/γ-Al2O3catalyst. The catalyst of %CeO2had a stronger reduction peak, indicating that the introduction of CeO2provided more surface oxygen vacancies for the catalyst and enhances the catalytic oxidation ability of the catalyst. in which the10and90were reduced by 10℃ and 40℃ respectively.

toluene；catalysis；CeO2；Pd；synergistic effect

X511

A

1000-6923(2019)07-2774-07

任思達(dá)(1991-),男,北京工業(yè)大學(xué)博士研究生,河北唐山人,主要研究大氣污染控制方向.發(fā)表論文4篇.

2018-12-19

國(guó)家重點(diǎn)研發(fā)計(jì)劃(2016YFC0204300);北京市科學(xué)技術(shù)委員會(huì)科技計(jì)劃資助項(xiàng)目(Z161100004516013)

* 責(zé)任作者, 教授, liangwenj@bjut.edu.cn