助劑添加對CuO/TiO2催化劑NH3-SCO性能的影響研究

陳浩輝,王亞洲,余 杰,邱 磊,尹一萌,王馳中,常化振*

助劑添加對CuO/TiO2催化劑NH3-SCO性能的影響研究

陳浩輝1,王亞洲2,余 杰1,邱 磊1,尹一萌1,王馳中1,?；?*

(1.中國人民大學環(huán)境學院,北京 100872；2.北京市生態(tài)環(huán)境保護科學研究院,北京 100037)

采用浸漬法制備了一系列CuO-MO/TiO2(M=W, Zr, La)催化劑用于NH3選擇性催化氧化(NH3-SCO),同時探究了SO2中毒對NH3氧化性能的影響.結果表明,過渡金屬氧化物的添加使Cu/TiO2催化劑NH3轉化率降低,但顯著提高N2選擇性.其中,WO3具有最好的促進效果,在300℃下催化劑N2選擇性提高了36%.通過H2-TPR和NH3-TPD表征發(fā)現,WO3的添加增加了Cu/TiO2催化劑表面酸性位點的數量,促進NH3的吸附,但降低催化劑氧化還原性能,抑制NH3氧化為NO.經SO2中毒處理后,CuO-MO/TiO2催化劑N2選擇性進一步提高,表征結果表明,酸性位點的增加和氧化氧化還原性的降低是提高催化劑N2選擇性的關鍵.

Cu/Ti基催化劑；NH3選擇性催化氧化；金屬氧化物添加；SO2中毒

氨(NH3)是重要的化工原材料,同時也是有毒有害的工業(yè)氣態(tài)污染物之一[1-2].NH3不僅會對人體健康產生危害,還會導致霧霾,光化學煙霧等各種環(huán)境問題[3-7].化肥的大量使用以及工業(yè)活動是NH3的主要排放來源.例如工業(yè)采用NH3或者尿素作為還原劑的選擇性催化還原(SCR)脫硝過程中NH3逃逸的發(fā)生[8-9].因此,有效地去除NH3具有非常重要的意義.

吸附?吸收?催化燃燒、催化分解等多種方法已被應用于NH3的去除,但存在成本高,適用范圍窄,容易產生二次污染物等問題.選擇性催化氧化(SCO)被視為一種高效的NH3處理技術[10-11].催化劑是NH3-SCO技術的核心.目前用于選擇性催化氧化NH3的催化劑可以分為三大類:(1)貴金屬催化劑,如Ag, Pt, Pd, Ru, Au等0[12-18];(2)過渡金屬氧化物催化劑,包括CuO, Fe3O4, Co3O4, MnO2, V2O5, CeO2等[19-25];(3)沸石分子篩催化劑,如Cu-ZSM-5, Pd-Y, Pt-ZSM-5和Fe-Beta等[26-30].雖然貴金屬催化劑在低溫(<300℃)表現出高的催化活性,但成本高,N2選擇性較低.相比之下,過渡金屬氧化物和分子篩催化劑因成本低,N2選擇性高等優(yōu)點得到廣泛研究[31].

在過渡金屬氧化物催化劑中,Cu基催化劑越來越受到關注.其低成本和較高的活性被證明是NH3-SCO反應的有前途的催化劑之一.

Il’chenko等[32]在1975年發(fā)現CuO可以將NH3催化氧化為N2和H2O.隨著研究的深入,發(fā)現Cu物種的種類、晶體結構、載體種類以及載體形貌等都會影響NH3的氧化性能[30-33].如有研究者比較了Cu-Mg-Al-O混合金屬氧化物與Cu/Al2O3催化劑,發(fā)現催化劑上形成的不同存在形式的氧化銅物種(高度分散的CuO物種和塊狀CuO物種)對NH3-SCO活性有較大影響[33].也有研究者將Cu負載在Al2O3上,發(fā)現類CuAl2O4相在NH3氧化反應中比CuO相更活躍[30]. CuO與載體Al2O3的相互作用也在其他研究中被提及[34-35]有研究者將CuO負載在TiO2上,發(fā)現CuO/TiO2催化劑N2選擇性與Cu/Al2O3催化劑相似,但是CuO/TiO2催化劑在低溫下活性更好[36-37].不同形貌CeO2負載的CuO催化劑上NH3-SCO的反應路徑和中間物種有很大差異[38]. Cu基催化劑在高溫下容易發(fā)生NH3的過氧化,所以其N2選擇性的提高也令人關注.研究者們發(fā)現將W和La等助劑摻雜到CuO基催化劑上后,催化劑NH3轉化率略微下降,但也使其N2選擇性顯著增高0[39-40].Zhang等通過研究不同比例的CuO-Fe2O3復合金屬氧化物,發(fā)現Cu/Fe比例越高,催化劑NH3氧化活性越高,Cu/Fe比例越低,催化劑N2選擇性越高[41].

基于此,本文采用浸漬法制備了過渡金屬氧化物WO3, La2O3, ZrO2添加改性的CuO/TiO2基催化劑并將其用于選擇性催化氧化去除NH3,發(fā)現金屬氧化物的添加顯著提高了Cu/Ti基催化劑N2選擇性.結合XRD, NH3-TPD和H2-TPR等物化表征,對過渡金屬氧化物添加提高CuO/TiO2催化劑NH3- SCO反應N2選擇性的機制進行探究.此外,本研究還考察了SO2對CuO/TiO2基催化劑NH3-SCO活性和選擇性的影響.

1 材料與方法

1.1 催化劑制備

采用浸漬法制備了CuO/TiO2和CuO-MO/TiO2(M=W, Zr, La)催化劑,具體步驟如下:稱取4份一定質量的硝酸銅(Cu(NO3)2·3H2O),分別與相應質量的鎢酸銨(H40N10O41W12·H2O),硝酸鋯(Zr(NO3)4·5H2O),硝酸鑭(La2(NO3)3·6H2O)一起溶于60mL去離子水中,攪拌0.5h.稱取一定質量的TiO2(德固賽P25)粉末,倒入上述溶液中并攪拌2h.經80℃水浴加熱至糊狀,放入110℃的烘箱中烘干12h.取出研磨后放入馬弗爐中500℃下煅燒4h,升溫速率10℃/min.自然冷卻至室溫后取出,研磨壓片,篩分出40～60目的催化劑顆粒.其中CuO, La2O3, ZrO2和WO3質量分數均為5wt%.

SO2處理催化劑制備:在200℃下進行,中毒氣氛中各組分的體積分數分別為5×10-4NH3, 5×10-4SO2, 5×10-2H2O, 5×10-2O2, N2為平衡氣,氣體流量為200mL/min,中毒時間12h.催化劑再生處理為在400℃下熱處理2h.

為了便于區(qū)分,本文中新鮮的催化劑記為CuM/ Ti-f, SO2處理后的催化劑記為CuM/Ti-p,再生后的催化劑記為CuM/Ti-r.

1.2 催化劑表征

通過X射線衍射(XRD)對樣品的晶體結構進行分析,選用的X射線衍射儀型號為島津公司的XRD-700粉末衍射儀.實驗條件: Cu Kα(=1.5418?, 2=10°～90°,掃描速率=10°/min).

在化學吸附儀(Micromeritics, ChemiSorb 2720TPx)上進行H2-TPR(H2temperature programmed reduction)表征,以對樣品的氧化還原性進行分析.H2-TPR的測試步驟如下: (1)在Ar氣氛下對樣品進行350℃預處理,持續(xù)時間1h,以去除樣品表面的雜質; (2)待樣品冷卻至室溫后,切換氣體為5% H2/Ar,控制氣體流量為30mL/min,保持氣氛和流量不變,基線穩(wěn)定后,以10°C/min的升溫速率從室溫升至800℃,通過TCD檢測儀得到H2-TPR結果.

用氨分析儀(EAA-30r-EP)進行NH3-TPD(NH3temperature programmed desorption)實驗.樣品首先在N2氣氛中400℃預處理1h以去除表面雜質,自然降溫至100℃后,在100℃條件下通入NH3/N2.吸附飽和后,N2吹掃1h,去除弱吸附物質.后以10℃/min的速度升溫至400℃,記錄出口氣體中NH3的濃度,得到NH3的脫附溫度曲線.

單位質量催化劑樣品上NH3脫附量計算公式為:

式中:ads為NH3脫附量;des為TPD曲線的脫附峰面積;c為吹掃氣體流量(mL/min);T為升溫速率(℃/min);cat為催化劑質量(g).

1.3 催化劑活性測試

催化劑活性測試在固定床適應反應器(內徑6mm)進行.每次測試所用催化劑質量為0.15g,氣體流量200mL/min,體積空速(GHSV)為65000h-1.進氣中NH3的體積均為5×10-4, O2的體積分數為5×10-2,平衡氣為N2.出口氣中各氣體濃度由傅里葉紅外檢測器(Thermo Fisher Scientific iS50)檢測,記數時會在每個溫度點穩(wěn)定0.5h.

NH3轉化率計算公式為:

N2選擇性計算公式為:

式中:下標“in”和“out”分別表示進口和出口處的NH3, NO, NO2, N2O等氣體的濃度.

2 結果與討論

2.1 NH3-SCO活性測試

4種新鮮催化劑NH3-SCO活性和選擇性分別如圖1(a)和圖1(b)所示.Cu/Ti催化劑表現出高的NH3氧化活性,在250℃時,NH3的轉化率接近90%,比已經報道的其他Cu基負載型催化劑活性高20～60%[35-36].經助劑添加后,催化劑的活性明顯發(fā)生改變,NH3的轉化率依次為:Cu/Ti>CuZr/Ti>CuLa/ Ti>CuW/Ti.表明3種助劑的添加均導致催化劑活性的降低,其中W對活性的影響最大.如圖1(b)所示, Cu/Ti催化劑雖然具有良好的NH3氧化活性,但是表現出低的N2選擇性,這可能與NH3氧化的副反應進行有關.值得注意的是3種助劑的添加雖然降低了NH3氧化活性,但顯著提高了催化劑N2選擇性.其中WO3添加的催化劑表現出最高的促進效果,在300℃時,催化劑N2選擇性由50%提高到90%,與已經報道的Cu基催化劑300℃下的最高N2選擇性基本持平[39].以上表明W, Zr, La添加可以顯著提高Cu基催化劑N2選擇性,抑制副反應的進行.

新鮮催化劑在300℃時各種產物的選擇性分布如圖1(c)所示.4種催化劑催化NH3氧化的產物除了N2外,均含有N2O, NO和NO2,且各催化劑3種氮氧化物的選擇性差別較大.300℃時,CuW/Ti催化劑NO選擇性最低,而其N2選擇性最高;Cu/Ti催化劑NO選擇性最高,其N2選擇性最低.CuM/Ti催化劑NO選擇性與N2選擇性有一定的相關性.這可以用內部SCR(i-SCR)機理來解釋[42].i-SCR機理認為NH3- SCO反應可分為兩步: (1)NH3首先在催化劑表面轉化為NO;(2)NH3與NO發(fā)生SCR反應將NO還原為N2和H2O. NO選擇性低,可能是因為與NH3反應消耗的NO更多,產生的N2也更多,N2選擇性提高.

反應條件:5×10-4NH3, 5×10-2O2, N2平衡氣, GHSV=65000h-1

2.2 催化劑的物化表征

2.2.1 XRD 新鮮CuM/Ti催化劑的XRD結果如圖2(a)所示.在Cu/Ti催化劑上只存在銳鈦礦相TiO2和金紅石相TiO2的衍射峰,沒有觀察到CuO的特征峰[43],表明Cu物種高度分散在催化劑表面.同時出現銳鈦礦型TiO2和金紅石型TiO2的衍射峰是因為本研究所選用的德固賽 P25TiO2藥品屬于混晶型,其銳鈦礦和金紅石的質量比重大致為79:21.在W, Zr, La添加后,催化劑的晶體結構無明顯變化,且ZrO2, WO3, La2O3晶相均未檢測到[44-46],表明添加的元素不改變催化劑的晶體結構.

2.2.2 H2-TPR 催化劑的氧化還原性能是影響NH3-SCO活性的重要因素[47].新鮮CuW/Ti, CuZr/Ti, CuLa/Ti, Cu/Ti催化劑的H2-TPR表征結果如圖2(b)所示.

觀察Cu/Ti催化劑的H2-TPR曲線,在190和282℃出現2個H2還原峰,分別歸屬于Cu2+和Cu+的還原[43].在助劑添加后,2個還原峰的位置發(fā)生了不同程度地改變.在CuZr/Ti催化劑上,Zr的加入促使Cu2+和Cu+的還原峰向低溫偏移,表明Zr的添加可以促進Cu的還原,提高催化劑的氧化還原性.在CuW/Ti催化劑上,W的加入沒有影響Cu+的還原,但是Cu2+的還原峰由190℃偏移至209℃,表明Cu2+的還原受到抑制.此外,在725℃出現一個H2還原峰,這歸屬于W6+的還原[48].在CuLa/Ti催化劑上,La的添加沒有影響Cu2+的還原,但是顯著影響了Cu+的還原.WO3和La2O3的添加導致催化劑的氧化性能降低,這可能是CuW/Ti和CuLa/Ti催化劑的NH3-SCO活性不如Cu/Ti催化劑的主要原因.而催化劑氧化能力的降低,有助于提高催化劑的N2選擇性[39].之前的研究發(fā)現La2O3, ZrO2, WO3的添加可能有助于提高Cu基催化劑的NH3-SCR性能[49,51],即促進i-SCR機理中第二步NH3還原NO轉化為N2和H2O的過程,進而提高催化劑的N2選擇性.

2.2.3 NH3-TPD 如圖3所示,助劑添加明顯影響了NH3的吸附性能.在W和Zr添加后,NH3的脫附曲線向低溫方向偏移,且脫附峰較Cu/Ti催化劑出現了寬化,表明添加的W和Zr能促進NH3的低溫脫附,同時提高NH3的吸附能力.對于La添加的催化劑而言,NH3的脫附曲線向高溫側偏移,且脫附峰的面積出現降低,表明La的添加抑制了NH3在催化劑表面的吸附與脫附.

計算了4種催化劑的NH3的吸附量,結果如表1所示,明顯地發(fā)現助劑添加可以改變NH3的吸附能力,尤其是CuW/Ti催化劑,表現出最高的NH3吸附能力.在i-SCR機理中,NH3首先先被氧化為NO, NO再與未轉化的NH3反應轉化為N2和H2O.催化劑的NH3吸附能力越強,越有利于這兩步反應的進行.4種催化劑的NH3吸附能力順序為CuW/Ti> CuZr/ Ti>Cu/Ti>CuLa/Ti,但N2選擇性順序為CuW/Ti> CuZr/Ti>CuLa/Ti>Cu/Ti,可能的原因如下:除CuW/Ti催化劑外,另外3種催化劑的NH3吸附量差別不大.根據圖2(b)中H2-TPR的結果,Cu/Ti催化劑的氧化還原性能比CuLa/Ti催化劑更強,可能會促進NH3轉化為NO.所以Cu/Ti催化劑上未轉化的NH3的濃度比CuLa/Ti更低,導致與NO反應生產N2的NH3不足,降低了Cu/Ti催化劑的N2選擇性. CuZr/Ti催化劑的氧化還原能力比Cu/Ti更強,但其NH3轉化率卻比Cu/Ti催化劑低.可能是催化劑的形貌等其他因素,抑制了CuZr/Ti催化劑的NH3轉化能力,同時也提高了CuZr/Ti催化劑的N2選擇性.

圖3 CuM/Ti催化劑的NH3-TPD譜圖

2.3 SO2的影響

2.3.1 SO2中毒后催化劑的NH3-SCO性能 經過SO2處理后的CuM/Ti催化劑的NH3-SCO活性和各種產物的選擇性如圖4(a)和圖4(b)所示.之前的研究指出,催化劑經SO2中毒處理后,催化劑上的活性組分會被硫化形成金屬硫酸鹽.同時當處理氣氛中存在NH3和H2O時,催化劑表面還會伴隨硫酸氫銨/硫酸銨物種的沉積.如圖4(a)所示,在SO2中毒處理后,4個催化劑的NH3氧化活性顯著降低.在300℃下,催化劑基本無催化活性,這主要是由于金屬硫酸鹽以及硫酸氫銨/硫酸銨的形成導致.值得注意的是,通過觀察中毒前后4種催化劑的N2選擇性發(fā)現,相較于中毒前,中毒后的催化劑的N2選擇性都出現了提高.其中CuLa/Ti-f催化劑的N2選擇性的提高最為明顯.表明SO2中毒處理能有效提高Cu基催化劑的N2選擇性.

而從中毒后催化劑在400℃時的產物選擇性上來看,4種中毒后催化劑的N2選擇性均大于新鮮催化劑.4種中毒后的催化劑的NO?NO2和N2O的總選擇性與NO選擇性高低順序保持一致.400℃下,NO選擇性按從低到高排列,順序為CuLa/Ti-p

反應條件:5×10-4NH3, 5×10-2O2, N2平衡氣, GHSV=65000h-1

2.3.2 H2-TPR和NH3-TPD 中毒后CuM/Ti催化劑的H2-TPR譜圖如圖5(a)所示.觀察到Cu/Ti-p催化劑,經SO2中毒處理后,歸屬于Cu2+和Cu+的特征峰消失,同時在350℃出現新的特征峰,這歸屬于硫酸銅物種[43].表明經SO2中毒處理后,形成的金屬硫酸鹽降低了催化劑的氧化還原性.對于助劑添加的催化劑而言,經過SO2的處理后,歸屬于Cu2+和Cu+的特征峰均消失,并伴隨著硫酸銅物種的形成.此外,在CuZr/Ti-p和CuLa/Ti-p催化劑上除了觀察到硫酸銅的特征峰(371和336℃)之外,還形成新的特征峰(409,410,530,603℃),這可能是由于其他金屬硫酸鹽的形成導致的,如硫酸鋯、硫酸鑭、硫酸鈦[52-54].總之,SO2中毒處理后,金屬硫酸鹽的形成會導致催化劑氧化還原性的降低,而低的氧化還原性有利于提高催化劑的N2選擇性[54].

中毒后催化劑的NH3-TPD譜圖如圖5(b)所示.相較于新鮮的催化劑,在SO2中毒處理后,四個催化劑的NH3的脫附曲線明顯發(fā)生改變,主要表現為脫附溫度窗口變寬并伴隨著新的脫附峰的出現.SO2中毒后,會在催化劑上沉積硫酸氫銨/硫酸銨,部分活性位點也會被硫酸化形成金屬硫酸鹽.一般情況下,大部分硫酸氫銨/硫酸銨會在400℃前分解,硫酸銅會在637℃開始分解,而硫酸鋯?硫酸鑭和硫酸鎢在800℃前不會分解[43,55-56].所以根據SO2處理及再生后催化劑的NH3吸附量,一定程度上能夠反映催化劑形成的硫酸氫銨/硫酸銨的量的多少.結合H2-TPR的結果,進一步證明SO2中毒后催化劑上金屬硫酸鹽和硫酸氫銨/硫酸銨的形成.之前的研究指出形成的金屬硫酸鹽和硫酸氫銨/硫酸銨可以為催化劑提供新的酸性位點進而提高催化劑NH3吸附性能0.根據i-SCR機理,NH3轉化的NO可以與NH3通過SCR的方式轉化為N2和H2O,因此SO2中毒為催化劑提供了新的NH3吸附位點,進而提高了N2選擇性,降低NO的生成.此外,表1中SO2處理后催化劑的NH3吸附量與N2選擇性并不相符,因為中毒后催化劑表面沉積的硫酸氫銨/硫酸銨分解會產生NH3,所以中毒后催化劑的NH3-TPD結果并不能準確地反應催化劑的NH3吸附能力.

2.4 熱再生后

2.4.1 熱再生后催化劑的NH3-SCO性能 進一步考察了熱再生對SO2中毒催化劑的影響,經過400℃熱再生后的CuM/Ti催化劑的NH3-SCO活性如圖6所示.結果顯示,再生后催化劑的活性得到部分恢復,在300℃下,催化劑的NH3氧化活性與新鮮樣品相當(除CuZr/Ti-r),表明熱再生處理能有效去除催化劑表面部分金屬硫酸鹽和硫酸氫銨/硫酸銨等,實現催化劑活性的恢復.

圖6 再生CuM/Ti催化劑的NH3-SCO活性

反應條件:5×10-4NH3, 5×10-2O2, N2平衡氣, GHSV=65000h-1

2.4.2 H2-TPR和NH3-TPD 再生后CuM/Ti催化劑的H2-TPR和NH3-TPD譜圖如圖7(a)和圖7(b)所示.再生后Cu/Ti, CuW/Ti, CuZr/Ti, CuLa/Ti催化劑部分金屬硫酸鹽的特征峰消失,且歸屬于Cu物種還原的特征峰出現,表明熱再生處理導致金屬硫酸鹽的分解,伴隨著活性位點的釋放.在NH3-TPD中,經過再生處理后催化劑的酸性出現了降低,這是由于硫酸氫銨/硫酸銨和小部分金屬硫酸鹽的分解導致的.但是相較于新鮮樣品,再生后的催化劑依然表現出高的NH3吸附性能.

3 結論

3.1 WO3, ZrO2和La2O3的添加能夠顯著提高Cu/Ti催化劑NH3-SCO反應的N2選擇性.其中WO3使N2選擇性提高了20%～40%,效果最好.這可能是因為WO3的添加降低了Cu/Ti催化劑的氧化性能,同時提高了其NH3吸附能力.

3.2 SO2中毒能夠顯著提高CuM/Ti催化劑對NH3的吸附能力.硫酸根的存在抑制了催化劑在低溫下(250℃及以下)的NH3-SCO性能,卻提高了其在高溫下的N2選擇性.

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Effect of additives and SO2on selective catalytic oxidation of NH3over CuO/TiO2catalysts.

CHEN Hao-hui1, WANG Ya-zhou2, YU Jie1, QIU Lei1, YIN Yi-meng1, WANG Chi-zhong1, CHANG Hua-zhen1 *

(1.School of Environment and Natural Resources, Renmin University of China, Beijing 100872, China；2.Beijing Municipal Research institute of Eco-Environmental Protection, Beijing 100037, China)., 2023,43(10)：5123～5130

A series of CuO-MO/TiO2(M=W, Zr, La) catalysts were prepared by impregnation method for the selective catalytic oxidation of NH3, and the effects of SO2poisoning on the oxidation of NH3over Cu/Ti-based catalysts were investigated. The results show that adding of transition metal oxides decreased the NH3conversion efficiency of Cu/Ti catalyst slightly, but significantly improved the N2selectivity. WO3was the best promoter among those additives, with an increase of N2selectivity by 36% at 300℃ in comparison to Cu/Ti catalyst. The H2-TPR and NH3-TPD results indicate that adding of WO3significantly increased the number of acid sites on the surface of Cu/Ti catalyst, and promoted the adsorption of NH3. But it affected the redox performance of the catalyst and inhibited the oxidation of NH3to NO. After SO2poisoning, the N2selectivity of CuO-MO/TiO2catalyst was further improved. The characterization results shows that the increase of acid sites and the reduction of redox performance are the key factors to improve the N2selectivity.

Cu/Ti-based catalysts；NH3selective catalytic oxidation；metal oxide adding；SO2poisoning

X511

A

1000-6923(2023)10-5123-08

2023-03-17

國家自然科學基金資助項目(22176217)

* 責任作者, 教授, chz@ruc.edu.cn

陳浩輝(1999-),男,河南駐馬店人,中國人民大學碩士研究生,研究方向為大氣污染控制.1020878834@qq.com.

陳浩輝,王亞洲,余 杰,等.助劑添加對CuO/TiO2催化劑NH3-SCO性能的影響研究 [J]. 中國環(huán)境科學, 2023,43(10):5123-5130.

Chen H H, Wang Y Z, Yu J, et al. Effect of additives and SO2on selective catalytic oxidation of NH3over CuO/TiO2catalysts [J]. China Environmental Science, 2023,43(10):5123-5130.