梁　輝　 歸柯庭

(東南大學(xué)能源熱轉(zhuǎn)換及其過程測控教育部重點(diǎn)實(shí)驗(yàn)室, 南京 210096)

納米γ Fe2O3催化劑SCR脫除NOx化學(xué)反應(yīng)動力學(xué)

梁輝 歸柯庭

(東南大學(xué)能源熱轉(zhuǎn)換及其過程測控教育部重點(diǎn)實(shí)驗(yàn)室, 南京 210096)

摘要:為了研究納米γ Fe2O3催化劑選擇性催化還原法(SCR)脫硝反應(yīng)機(jī)理,采用微分反應(yīng)器測量了納米γ Fe2O3催化劑上SCR反應(yīng)的動力學(xué)參數(shù),并構(gòu)建了SCR反應(yīng)動力學(xué)模型.實(shí)驗(yàn)數(shù)據(jù)分析結(jié)果表明,NH3,NO和O2的反應(yīng)級數(shù)分別為0,0.78～0.93和0.09～0.11,反應(yīng)活化能為57.3 kJ/mol.原位紅外漫反射光譜(DRIFTS)實(shí)驗(yàn)結(jié)果表明:NH3能夠強(qiáng)吸附到催化劑表面并達(dá)到飽和,進(jìn)一步增加NH3的濃度并不能增加NO的轉(zhuǎn)化速率;NO在有氧條件下能吸附到催化劑表面生成吸附態(tài)NO2和亞硝酸鹽;在低于270 ℃的情況下SCR反應(yīng)遵循Langmuir-Hinshelwood(L-H)反應(yīng)機(jī)理,在高于270 ℃的情況下則主要遵循Eley-Rideal (E-R)反應(yīng)機(jī)理.

關(guān)鍵詞:納米γ Fe2O3;化學(xué)反應(yīng)動力學(xué);活化能;原位紅外漫反射光譜;選擇性催化還原;脫硝

doi:10.3969/j.issn.1001-0505.2015.05.002

中圖分類號:TK121

文獻(xiàn)標(biāo)志碼:??碼: A

文章編號:??號: 1001-0505(2015)05-0918-05

Abstract:In order to study the selective catalytic reduction (SCR) reaction mechanism over γ Fe2O3nano-catalyst, the kinetic parameters of the SCR reaction on γ Fe2O3nano-catalyst were investigated by the differential reactor, and a reaction kinetic model was established. The experimental data analysis results show that the reaction orders are 0, 0.78 to 0.93 and 0.09 to 0.11 for NH3, NO and O2, respectively and the activation energy is 57.3 kJ/mol. The experimental results of diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) show that NH3can be strong adsorbed to the catalyst surface and the adsorption sites can be saturated by NH3. The conversion rate of NO does not increase with the further increase of the NH3concentration. At the presence of O2, NO can be adsorbed to catalyst surface and converted to adsorbed NO2and nitrite species. The SCR reaction mainly follows the Langmuir-Hinshelwood (L-H) mechanism at the temperature below 270 ℃, and follows Eley-Rideal (E-R) mechanism at the temperature above 270 ℃.

收稿日期:2014-12-30.

作者簡介:??: 梁輝(1988—),男,博士生;歸柯庭(聯(lián)系人),男,博士,教授,博士生導(dǎo)師,ktgui@seu.edu.cn.

基金項(xiàng)目:國家自然科學(xué)基金資助項(xiàng)目(51276039).

Chemical reaction kinetics of SCR of NOxover γ Fe2O3nano-catalyst

Liang Hui Gui Keting

(Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, Southeast University, Nanjing 210096, China)

Key words: γ Fe2O3nano-catalyst;chemical reaction kinetics;activation energy;diffuse reflectance infrared fourier transform spectroscopy (DRIFTS);selective catalytic reduction;denitration

引用本文: 梁輝,歸柯庭.納米γ Fe2O3催化劑SCR脫除NOx化學(xué)反應(yīng)動力學(xué)[J].東南大學(xué)學(xué)報(bào):自然科學(xué)版,2015,45(5):918-922. [doi:10.3969/j.issn.1001-0505.2015.05.017]

氨法選擇性催化還原(selective catalytic reduction,SCR)脫硝技術(shù)具有高效性和經(jīng)濟(jì)性,廣泛應(yīng)用于燃煤電廠、工業(yè)爐窯等固定源煙氣的NOx脫除中.V2O5/TiO2系催化劑為當(dāng)今商業(yè)應(yīng)用最廣泛的一種SCR脫硝催化劑,但價(jià)格昂貴、回收處理困難.Fe基催化劑雖然具有原料來源廣泛、無污染、高選擇性等優(yōu)點(diǎn),但大多工作于中高溫度段[1].本課題組研究發(fā)現(xiàn)γ Fe2O3在中低溫度段具有良好的SCR脫硝效果[2].中低溫Fe基催化劑的開發(fā)仍需研究其SCR脫硝反應(yīng)機(jī)理.

Ramis等[3]認(rèn)為NO以氣態(tài)或弱吸附態(tài)直接參與SCR反應(yīng),反應(yīng)遵循E-R反應(yīng)機(jī)理;Marbán等[4]則認(rèn)為NO首先吸附到催化劑表面,然后再參與SCR反應(yīng);Liu等[1,5]和Yang等[6]發(fā)現(xiàn)在Fe基催化劑表面同時(shí)存在2種反應(yīng)機(jī)理.研究Fe基催化劑的SCR反應(yīng)機(jī)理,需要通過反應(yīng)動力學(xué)來分析SCR脫硝反應(yīng)速率與各過程變量之間的關(guān)系.Huang等[7]研究得到在Fe-ZSM-5催化劑上NO,NH3和O2的反應(yīng)級數(shù)分別為1,0和0.5,SCR反應(yīng)在此催化劑的表觀活化能為54 kJ/mol.Li等[8]發(fā)現(xiàn)在Fe-Mo/ZSM-5催化劑上NO,NH3和O2的反應(yīng)級數(shù)分別為0.74～0.99,0.01～0.13和0,反應(yīng)活化能為65 kJ/mol;熊志波等[9]得到Fe0.95Ce0.05Oz催化劑上NO,NH3和O2的反應(yīng)級數(shù)分別為1,0和0.5,表現(xiàn)活化能為42.6 kJ/mol.由此可見,不同的Fe基催化劑表現(xiàn)出不同的催化反應(yīng)動力學(xué)規(guī)律和表觀活化能.目前未見關(guān)于納米γ Fe2O3的SCR脫硝化學(xué)反應(yīng)動力學(xué)的研究報(bào)道.

本文利用微分反應(yīng)器考察了不同濃度下NO,NH3和O2對NOx轉(zhuǎn)化速率的影響,并對實(shí)驗(yàn)數(shù)據(jù)進(jìn)行處理得到相應(yīng)的反應(yīng)級數(shù),根據(jù)Arrehenius方程計(jì)算氨法SCR脫硝反應(yīng)在此催化劑上的表觀活化能.原位紅外漫反射光譜結(jié)果表明,不同溫度下納米γ Fe2O3的催化反應(yīng)機(jī)理是不同的.

1實(shí)驗(yàn)

1.1　催化劑制備

實(shí)驗(yàn)采用的商用納米γ Fe2O3粉末由北京嘉安恒科技有限公司提供.首先,將其于400 ℃下煅燒3 h,然后利用粉末壓片機(jī)于4 MPa壓力下壓制成塊,最后采用破碎篩選出粒徑為230～425 μm的γ Fe2O3顆粒作為催化劑.

1.2　化學(xué)反應(yīng)動力學(xué)實(shí)驗(yàn)

催化劑的化學(xué)反應(yīng)動力學(xué)研究在微分反應(yīng)器中進(jìn)行,反應(yīng)器為內(nèi)徑8 mm的石英管,內(nèi)置納米γ Fe2O3催化劑顆粒質(zhì)量W=0.06～0.20 g.實(shí)驗(yàn)所采用的模擬煙氣中φ(NO)=φ(NH3)=0～0.1%,φ(O2)=0～5%,且N2為平衡氣,NO,NH3和N2的氣體純度均大于99.99%,O2純度大于98.5%;氣體各組分的流量均由質(zhì)量流量計(jì)(七星華創(chuàng)公司,精度為1%)控制,模擬煙氣總流速qv=1.55 L/min.將反應(yīng)后的氣體通入德國產(chǎn)ECOM便攜式煙氣分析儀中,在線監(jiān)測NOx(NO和NO2)的濃度,精度為±0.5%.氮氧化物轉(zhuǎn)化率的計(jì)算公式為

(1)

式中,Cin,Cout分別為進(jìn)、出口處煙氣中NOx的體積分?jǐn)?shù).

Huang等[7]指出當(dāng)空速大于2.3×105h-1且轉(zhuǎn)化率控制在20%以內(nèi)時(shí),煙氣量相對催化劑絕對過量,可以忽略擴(kuò)散的影響,可將反應(yīng)器視作微分反應(yīng)器.所以,在本文實(shí)驗(yàn)中,為消除外擴(kuò)散的影響,所采用的空速均大于4.5×105h-1;為消除內(nèi)擴(kuò)散的影響,催化劑顆粒粒徑需足夠小,本文所采用催化劑粒徑為230～425 μm;為保證煙氣相對催化劑絕對過量,Y基本控制在20%以內(nèi).

1.3　原位紅外漫反射光譜

于Nicolet 5700 FT-IR光譜儀外加原位漫反射附件上采集得到原位紅外漫反射光譜(DRIFTS)數(shù)據(jù).FT-IR光譜儀采用液氮冷卻的高靈敏度的MCT檢測器,DRIFTS附件采用ZnSe窗片并裝有筒形加熱器,光譜采集范圍為650～4 000 cm-1,光譜分辨率為4 cm-1,掃描次數(shù)為32.實(shí)驗(yàn)中,φ(NH3)=φ(NO)=1%,φ(O2)=5%,且將N2作為平衡氣.將樣品放入漫反射池中,在400 ℃的條件下采用N2吹掃0.5 h,然后降至室溫,通入模擬煙氣,待氣體吸附飽和后以10 ℃/min的速度升溫,在不同的目標(biāo)溫度下采集扣除相應(yīng)溫度樣品背景的圖譜.

2結(jié)果與討論

在NH3-SCR脫除NO的反應(yīng)體系中,NO轉(zhuǎn)化速率可以寫作各反應(yīng)物濃度的冪方程形式,即[9]

(2)

式中,ka為反應(yīng)速率常數(shù);CNH3,CNO和CO2分別為NH3,NO和O2的摩爾濃度;x,y和z則分別為NH3,NO和O2的反應(yīng)級數(shù).

2.1　NH3反應(yīng)級數(shù)及NH3的吸附

為了測量NH3在γ Fe2O3催化劑上的反應(yīng)級數(shù),于φ(NO)=0.1%,φ(O2)=3%,qv=1.55 L/min,W=0.15 g,空速為6.2×105h-1的條件下,分別在240,270,300 ℃時(shí)考察NH3初始體積分?jǐn)?shù)對NO轉(zhuǎn)化速率的影響,結(jié)果見圖1.由圖可知,在這3種溫度條件下,NO轉(zhuǎn)化速率幾乎不受NH3的濃度影響.由式(2)計(jì)算可得,NH3的反應(yīng)級數(shù)分別為-0.01,-0.02,-0.02.

圖1　不同溫度下NO轉(zhuǎn)化速率隨NH3初始濃度變化

圖2　NH3吸附飽和后程序升溫的原位DRIFTS圖

2.2　NO反應(yīng)級數(shù)及NO的吸附

為了研究NO濃度對SCR反應(yīng)速率的影響,本文在φ(NH3)=0.1%,φ(O2)=3%,qv=1.55 L/min,W=0.15 g,空速為6.2×105h-1的條件下,考察了不同溫度下NO轉(zhuǎn)化速率與NO初始體積分?jǐn)?shù)的變化關(guān)系，結(jié)果見圖3.從圖中可以看出,NO轉(zhuǎn)化速率隨著NO濃度的上升幾乎呈直線上升的趨勢,用式(2)對圖3中的數(shù)據(jù)進(jìn)行擬合,在γ Fe2O3催化劑的SCR反應(yīng)中,NO的反應(yīng)級數(shù)在210,240,270,300 ℃條件下分別為0.79,0.78,0.86,0.93.

圖3　不同溫度下NO轉(zhuǎn)化速率隨NO初始濃度變化

常溫下催化劑吸附NO和O2達(dá)到飽和后,利用N2吹掃,于φ(O2)=5%的氣氛下程序升溫,得到的DRIFTS圖譜如圖4所示.結(jié)果表明,當(dāng)程序升溫高于270 ℃時(shí),NO在催化劑表面吸附生成較為穩(wěn)定的硝酸鹽物種(1 609,1 583,1 547,1 235,1 203 cm-1),NO只能以氣態(tài)形式直接參與SCR反應(yīng),反應(yīng)遵循E-R反應(yīng)機(jī)理,反應(yīng)級數(shù)接近于1;而在溫度低于270 ℃時(shí),NO和O2共吸附在催化劑表面生成了吸附態(tài)的NO2(1 619 cm-1)和亞硝酸鹽(1 434 cm-1),這兩物種是參與L-H機(jī)理反應(yīng)的重要中間產(chǎn)物[5,14-15],所以在溫度低于270 ℃具有L-H機(jī)理反應(yīng),使得反應(yīng)級數(shù)小于1.

圖4　NO和O2共吸附飽和后程序升溫的原位DRIFTS圖

2.3　O2反應(yīng)級數(shù)及氧氣的影響

在φ(NH3)=φ(NO)=0.1%,qv=1.55 L/min,W=0.15 g,空速為6.2×105h-1的條件下,考察不同O2濃度對NO轉(zhuǎn)化速率的影響，結(jié)果見圖5.從圖中看出在無氧條件下,NO轉(zhuǎn)化速率并不為0,這可能是因?yàn)榇呋瘎┲芯Ц裱鯀⑴c了SCR反應(yīng).在反應(yīng)系統(tǒng)中通入微量的O2,NO轉(zhuǎn)化速率即有明顯的提高.通過式(2)對圖5中的數(shù)據(jù)進(jìn)行擬合,在γ Fe2O3催化劑的SCR反應(yīng)中,O2的反應(yīng)級數(shù)在240,270,300 ℃條件下分別為0.09,0.10,0.11.

圖5　不同溫度下NO轉(zhuǎn)化速率隨O2濃度變化

從圖2可以看出,在30～360 ℃溫度范圍內(nèi),隨著溫度的升高,吸附在催化劑表面的NH2物種濃度增大,這是因?yàn)槲皆贚酸位的NH3將Fe3+還原生成Fe2+和NH2,而O2又不斷將Fe2+氧化成Fe3+,從而促使NH2在催化劑表面的生成,進(jìn)而增加E-R機(jī)理反應(yīng)速度.由圖4看出,在30 ℃的低溫條件下,O2能促進(jìn)NO在催化劑表面的吸附,并生成參與L-H機(jī)理反應(yīng)的吸附態(tài)的NO2和亞硝酸鹽.因此，低于270 ℃時(shí),O2能夠促進(jìn)NO吸附生成吸附態(tài)的NO2和亞硝酸鹽,從而增加遵循L-H機(jī)理的SCR反應(yīng)速率;在高于270 ℃的條件下,O2則通過氧化被NH3還原的活性中心來增加遵循E-R機(jī)理的SCR反應(yīng)速率.

2.4　SCR反應(yīng)活化能

綜上所述,NH3,NO和O2的反應(yīng)級數(shù)分別取平均值0,0.84和0.10,式(2)則可寫為

(3)

(4)

對式(4)進(jìn)行積分可得

(5)

令

(6)

式中,CNO,in,CNO,out分別為入口、出口NO摩爾濃度,mol/L.

為了測量納米γ Fe2O3催化劑上SCR反應(yīng)活化能,當(dāng)φ(NH3)=φ(NO)=0.1%,煙氣總流量為1.55 L/min時(shí),測得在不同催化劑用量條件下的NO進(jìn)出口濃度.以W/qv為橫坐標(biāo)、X為縱坐標(biāo)作圖,所得直線(見圖6)的斜率即為表觀反應(yīng)速率常數(shù)k*(見表1).

圖6　不同溫度下X隨W/qv線性變化圖

溫度/℃200220240260280300k*12622383450978581151715021

表觀反應(yīng)速率常數(shù)k*可表示為

(7)

對式(7)兩邊求對數(shù)

(8)

將lnk*對1/T作直線(見圖7),結(jié)合式(8)計(jì)算可得,在納米γ Fe2O3催化劑上NH3-SCR反應(yīng)的活化能為57.3 kJ/mol,是一種較為理想的脫硝催化劑.

圖7　lnk*隨1/T線性變化

3結(jié)語

在微分反應(yīng)器中測得納米γ Fe2O3催化劑上SCR反應(yīng)的NH3,NO和O2的反應(yīng)級數(shù)分別為0,0.78～0.93和0.09～0.11.實(shí)驗(yàn)測得納米γ Fe2O3催化劑上SCR反應(yīng)活化能為57.3 kJ/mol.SCR反應(yīng)在溫度高于270 ℃時(shí)主要遵循E-R反應(yīng)機(jī)理,在溫度等于或低于270 ℃時(shí)遵循L-H機(jī)理反應(yīng).溫度低于270 ℃時(shí)O2通過增加NO吸附生成吸附態(tài)的NO2和亞硝酸鹽,從而增加遵循L-H機(jī)理的SCR反應(yīng)速率;溫度高于270 ℃時(shí)O2則通過氧化被NH3還原的活性中心而增加遵循E-R機(jī)理SCR反應(yīng)速率.

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