石 勇,李 橙,黃 磊,熊 巍,肇啟東,孫健恒,丁 越

Ti-Ni1-x-MOFs的制備及其CO選擇性催化還原NO研究

石 勇1*,李 橙1,黃 磊1,熊 巍1,肇啟東2,孫健恒1,丁 越1

(1.大連理工大學環(huán)境學院,工業(yè)生態(tài)與環(huán)境工程教育部重點實驗室和精細化工國家重點實驗室,遼寧 大連 116024；2.大連理工大學盤錦校區(qū),化工學院盤錦分院,遼寧 盤錦 124221)

采用溶劑熱法和微波法合成了不同比例的Ti-Ni1-x-MOFs材料,并用于以CO為還原劑的選擇性催化脫硝反應.結(jié)果表明,雙金屬Ti-Ni1--MOFs的NO還原率顯著高于單金屬Ni-MOF,且反應溫度窗口更寬,其中,Ti0.2-Ni0.8-MOF表現(xiàn)出最佳的脫硝效率,在200～400oC溫度范圍達到100%的轉(zhuǎn)化率.通過XRD,FT-IR,SEM,TGA,XPS,N2吸脫附等表征手段發(fā)現(xiàn),Ti摻雜Ni-MOF后有利于改善原子分散性,Ti、Ni間金屬的相互作用有利于產(chǎn)生豐富的高效Ni-O-Ti位點,加強Ni2++Ti4+? Ni3++Ti3+氧化還原循環(huán),從而明顯提高了NO+CO催化反應性能.與溶劑熱法相比,微波法制備Ti0.2-Ni0.8-MOF具有合成效率高、結(jié)晶度好、晶粒細小均勻的優(yōu)勢,并進一步提高了其低溫脫硝效果.

CO-SCR；金屬有機骨架；Ti-Ni1-x-MOFs；微波法；溶劑熱法

化石燃料的大量燃燒導致氮氧化物(NO)的過度排放,從而引起了如溫室效應、光化學煙霧、酸雨和臭氧損耗等環(huán)境及健康問題.工業(yè)中廣泛采用的選擇性催化還原技術(SCR)是通過使用還原劑與NO發(fā)生化學反應,生成氮氣與水,具有二次污染小、凈化效率高等優(yōu)點.其中CO-SCR是以煙氣中與NO同時存在的CO作為SCR反應的還原劑,不僅可以同時去除這2種有害氣體達到“以廢治污”的目的,還能有效解決還原氣體的成本和運輸儲存等問題,所以被認為是目前最具吸引力和應用前景的脫硝技術.為了避免煙塵中重金屬和SO2引起的磨損或中毒效應,SCR脫硝設備通常置于除塵器和脫硫裝置下游,這使得脫硝反應溫度低于300°C,所以研發(fā)高效穩(wěn)定的低溫CO-SCR催化劑成為當今脫硝領域內(nèi)的重點課題[1-4].

金屬-有機骨架(MOFs)是金屬離子連接有機配體形成的新型多孔功能材料.由于MOFs材料的高表面積、有序的多孔結(jié)構以及可靈活修飾功能化等優(yōu)異性能在低溫催化脫硝領域引起廣泛關注.MOFs材料不僅有利于底物反應分子的吸附和富集,而且雜化結(jié)構可形成多催化中心.由于Ni-MOF成本低、比表面積大、熱穩(wěn)定性強、電子傳輸效率高等特點[5-7],可在寬溫度窗口下NO轉(zhuǎn)化率超過90%,優(yōu)于同構型的Co-MOF,Cu-MOF,Fe-MOF等材料[8-13].但單金屬Ni-MOF也存在Ni分散性差且還原溫度高的問題,可通過引入另一種過渡金屬到骨架中來改善單金屬催化劑缺陷,使兩種金屬原子在同一晶胞中共存產(chǎn)生更規(guī)則的結(jié)構、強的相互作用以及提高原子分散度.Ti金屬能提供豐富的酸性位點,TiNi金屬間的協(xié)同作用不僅可以產(chǎn)生新的活性位點增強活性,還使材料具有良好的活性物質(zhì)均勻分散性[14-17],有利于催化反應的進行.已有研究通過控制Ni2+/Ti4+的物質(zhì)的量之比進而調(diào)節(jié)金屬團簇組分,使得Ni-Ti雙金屬MOFs不僅具有大量反應位點,而且高電子親和度的Ni可加快電荷轉(zhuǎn)移速率進而增強CO2還原催化活性[18].然而,Ni-Ti雙金屬MOFs催化劑在CO-SCR中的應用研究較少,利用Ni-Ti雙金屬摻雜改善Ni-MOF金屬催化劑缺陷,優(yōu)化其性能結(jié)構并提高材料的低溫催化性能仍是很大的挑戰(zhàn).

Ni-Ti-MOFs的性能通常取決于材料粒徑、結(jié)晶度、分散性和均勻性,這些性質(zhì)受制備工藝的影響,因此通過優(yōu)化合成方法調(diào)控MOFs材料的微觀形貌、晶體尺寸具有一定的可行性.傳統(tǒng)溶劑熱法具有操作簡單步驟少的特點,但反應時間較長[19].通過微波法制備的雙金屬M/Fe-MOFs(M=Ni,Mg,Sn),不僅反應速率快,可控制反應參數(shù),而且有效減少副反應,提高選擇性[20-21].然而,微波合成法和溶劑熱法對Ni-Ti-MOFs的晶粒尺寸、形貌及熱穩(wěn)定性能的影響還未有過系統(tǒng)研究.因此,通過研究對比溶劑熱法和微波合成法Ni-Ti雙金屬MOFs的結(jié)構性能,有利于優(yōu)化合成工藝來進一步提高該材料的熱穩(wěn)定性及低溫催化性能.

本文首先通過溶劑熱法制備出系列Ti改性Ni基MOFs材料Ti-Ni1-x-MOFs(x代表Ti:(Ti+Ni)的物質(zhì)的量之比),對其進行NO+CO催化性能研究.然后根據(jù)活性結(jié)果篩選出最優(yōu)比例的Ti0.2-Ni0.8-MOF進行微波法制備.利用XRD、BET、TGA、SEM、FT-IR、XPS技術針對材料物理化學性質(zhì)進行了表征,比較研究了溶劑熱法和微波法這兩種材料合成方法對雙金屬Ti0.2-Ni0.8-MOF性能的影響規(guī)律和反應機理.

1 材料與方法

1.1 溶劑熱法制備Tix-Ni1-x-MOFs催化劑

按照不同的Ti物質(zhì)的量占比將總物質(zhì)的量為5mmol的Ti(SO4)2·9H2O(AR,國藥集團),NiCl2·6H2O (AR,國藥集團)與1,3,5-苯三甲酸(5mmol,1.051g)溶于50mLN,N-二甲基甲酰胺和10mL無水乙醇的混合溶液中,Ti:(Ti+Ni)的物質(zhì)的量比值為0, 0.14, 0.2, 0.33, 0.5, 1.0,超聲攪拌直至混合均勻.然后將溶液轉(zhuǎn)移到100mL的特氟隆內(nèi)襯的高壓反應釜中,放置在150°C烘箱內(nèi)反應24h.降至室溫后,取出淡綠色粉末狀樣品,采用N,N-二甲基甲酰胺與乙醇重復洗滌3次去除樣品表面的溶劑分子,再將樣品放在乙腈溶液中浸泡2d,最后放置在真空烘箱中100°C干燥24h,得到淡綠色樣品并依次標記為Ti-Ni1-x-MOFs催化劑.

1.2 微波法制備Ti0.2-Ni0.8-MOF-M催化劑

采用微波輻射法制備Ti0.2-Ni0.8-MOF催化劑進行實驗對比.將Ti(SO4)2·9H2O(1mmol,0.240g),NiCl26H2O(4mmol,0.951g)與1,3,5-苯三甲酸(5mmol, 1.051g)依次加入到30mL N,N-二甲基甲酰胺溶劑中,充分混合均勻并超聲分散30min.然后將該混合溶液密封在100mL的反應釜中,放置在100W功率下的微波反應器加熱2h,待乳白色懸浮液自然降至室溫后分別用乙醇和N,N-二甲基甲酰胺溶劑洗滌3次進行分離純化,放入乙腈溶液浸泡48h,最后放置在真空烘箱中100℃干燥24h,得到淡綠色樣品,標記為Ti0.2-Ni0.8-MOF-M催化劑.

1.3 CO-SCR反應活性測試

稱量0.2g催化劑樣品,用壓片機將其粒徑壓至20～40目,然后將制備好的催化劑放置在內(nèi)徑為8mm的U型石英管中,在Ar氣氛下預處理2h用以去除表面雜質(zhì),同時降低煙氣分析儀的本底濃度以提高監(jiān)測的靈敏性.通過氣體配比模擬實際情況下的煙氣,總氣體流量為100mL/min,氣體空速約為30,000h-1.反應初始入口氣體濃度為:[NO]=500× 10-6, [CO]=1000×10-6,氬氣作為反應平衡氣.采用溫控加熱儀進行溫度調(diào)節(jié),反應在25～400℃溫度范圍內(nèi)進行.使用煙氣分析儀(Testo 350)記錄進氣口與出口氣體中的NO和CO濃度.NO轉(zhuǎn)化率依據(jù)以下公式計算:

1.4 催化劑性能表征

X射線衍射(XRD)表征使用Rigaku D/MAX 2500v/PC儀器進行,以分析催化劑結(jié)晶性和結(jié)構.場發(fā)射掃描電子顯微鏡(SEM)用于表征材料的微觀形貌和表面結(jié)構,使用日本日立公司生產(chǎn)的S-4200場發(fā)射掃描電子顯微鏡.N2吸附脫附的表征使用NOVA 1200(Quanta Chrome)吸附儀在液氮溫度77K下進行得出催化劑的孔徑分布.熱重分析(TGA)表征使用TGA/SDTA851型熱重分析儀.使用Thermo 139ESCALAB 250XI型電子能譜儀對催化劑進行X-射線光電子能譜(XPS)表征.使用布魯克VERTEX-70型傅立葉變換紅外光譜儀對催化劑進行原位紅外測試(FT-IR),分析樣品的分子結(jié)構和官能團.

2 結(jié)果與討論

2.1 催化劑活性測試

如圖1(a)所示,雙金屬Ti-Ni1-x-MOFs的NO還原率都顯著高于單金屬Ni-MOF,且反應溫度窗口更寬.隨著Ti占比增加,雙金屬Ti-Ni1-x-MOFs的催化活性先升高后下降.其中,Ti0.2-Ni0.8-MOF在200～400℃的寬溫度窗口內(nèi)達到100%的最高NO還原率,200℃時的催化效率較Ni-MOF提高36%,說明適量的Ti在Ni基MOFs材料中的摻雜起到了促進劑的作用.這說明Ni和Ti兩種物質(zhì)之間具有良好的分散性和協(xié)同效應[15],不僅Ni-O-Ti位點高度分散,而且有利于提高晶格氧活性,促進氧化還原循環(huán),從而降低反應能壘,改善NO和CO的吸附和活化.隨著Ti含量進一步增加,Ti-Ni1-x-MOFs的脫硝性能未呈正相關上升趨勢,可能是由于團聚造成的底物與材料接觸面積下降.

活性測試表明,Ti0.2-Ni0.8-MOF催化劑在整個溫度范圍內(nèi)具有最優(yōu)NO還原效率,所以用微波法合成Ti:(Ti+Ni)=0.2的雙金屬材料,記為Ti0.2- Ni0.8-MOF-M.如圖1(b)所示,在100～150℃溫度范圍內(nèi),微波法制備的MOF材料低溫效果更佳,125℃時的NO還原率提高16%,除此之外,微波法還使反應熱時間由24h縮減至2h,加快了Ti0.2-Ni0.8- MOF的結(jié)晶速率,這是由于微波通過分子與電磁場的相互作用將能量直接傳遞給反應溶液,并在開始成核時局部超熱,從而快速加熱結(jié)晶,縮短了合成時間.

2.2 催化劑表征

如圖2所示,觀察到5個主要衍射峰分別為2=7.3°,11.0°,14.8°,18.4°和22.5°,衍射峰位置與文獻所報道的Ni-MOF模擬峰位一致[22],分別對應于Ni-MOF的(001)、(110)、(213)、(150)和(143)晶面,表明兩種制備方法使得Ti均勻摻雜到Ni基MOFs中,晶體結(jié)構仍保持穩(wěn)定.雙金屬MOFs的衍射峰強度比單金屬更高,證明雙金屬材料結(jié)晶度較強.與溶劑熱法相比,微波法制備的Ti0.2-Ni0.8-MOF的峰更窄,更尖銳,在14.8°和22.5°處峰強較弱可能是由于MOFs中TiNi金屬組分及有機配體對微波功率的吸收程度不同產(chǎn)生選擇性生長導致的[23-24].

如圖3所示,在25～275℃低溫階段內(nèi),Ti0.2-Ni0.8- MOF-M的質(zhì)量損失最小,表明微波制備使得低溫下MOFs材料的熱穩(wěn)定性更強.失重曲線可分為P1、P2、P3共3個階段.50～170℃溫度范圍下,催化劑的5%～10%質(zhì)量損失是MOFs材料表面以及孔道內(nèi)部吸附的溶劑分子揮發(fā)所致.在170～320℃的質(zhì)量損失是由于金屬活性位點結(jié)合的水分子脫除,促使金屬活性位點暴露,催化劑的反應活性隨之增強,其中, Ti0.2-Ni0.8-MOF-M的質(zhì)量損失(35%)略大于Ti0.2- Ni0.8-MOF(32%),表明微波法制備的雙金屬材料暴露出更多的Ni-O-Ti位點,形成豐富的催化活性中心,從而明顯促進NO+CO反應的吸附與還原.320～ 400℃的P3階段產(chǎn)生的質(zhì)量損失是由于高溫階段有機配體的受熱分解坍塌[25-27].

圖2 催化劑XRD譜圖

圖3 催化劑TGA譜圖

表1 Tix-Ni1-x-MOFs催化劑的孔隙結(jié)構參數(shù)

由表1可知,Ti摻雜Ti-Ni1-x-MOFs材料的比表面積均高于單金屬材料,當Ti占比x = 0～0.2時,比表面積和孔容與Ti含量成正比,Ti0.2-Ni0.8-MOF的比表面積(167.30m2/g)和孔容(0.36cm3/g)達到最大.催化劑的高比表面積不僅產(chǎn)生對NO與CO的高效吸附,還可有效提供NO+CO與Ni-O-Ti位點相互作用的場所[28-29],從而提高催化效率.如圖4,Ti0.2- Ni0.8-MOF-M催化劑吸脫附等溫線與III型等溫線相似,在高相對壓力段(P/P0 = 0.7～1.0)吸附量迅速上升,達到最高值,反映出多分子層吸附現(xiàn)象.Ti0.2- Ni0.8- MOF-M催化劑孔徑分布主要集中在2～5nm范圍,說明其內(nèi)表面活性位點主要分布在晶粒堆積的空隙及晶內(nèi)孔道,這種空隙結(jié)構更有利于反應過程中的擴散傳質(zhì)[30].

如圖5所示,2種方法制備的雙金屬MOFs材料均呈現(xiàn)類似的層狀長方體結(jié)構,其中,溶劑熱法制備的晶粒長度分布在0.5～3um,而優(yōu)化制備后的晶粒尺寸分布在1～1.5um.這是由于溶劑熱法的反應時間長,所以晶粒生長尺寸較大.微波制備反應時間短,可較好克服高溫條件下團聚引起的粒徑增大卻不阻礙大量成核位點的產(chǎn)生[24,31],故產(chǎn)物平均粒徑減小且更加均勻.

圖5 催化劑SEM圖

(a)Ti0.2-Ni0.8-MOF, (b)Ti0.2-Ni0.8-MOF-M

如圖6所示,3條MOFs材料的紅外譜圖中均觀察到(－C=O)伸縮振動特征峰從1712cm-1藍移至1610cm?1,表明配體與金屬(Ni)絡合,制備所得金屬有機骨架成鍵增強并且結(jié)構趨于穩(wěn)定.在3496cm-1處的伸縮振動表明制備的材料中存在吸附與配位結(jié)合的(H2O)分子,而在1338,1386和1445cm?1處3個較強的特征峰是由金屬離子與水分子中的羥基結(jié)合形成的(M-OH)鍵的彎曲振動.此外,對比雙金屬MOFs和Ni-MOF紅外譜圖,在722cm?1處的特征峰歸因于(Ni-O)鍵的振動[32-33],雙金屬的峰強度增加,研究表明,Ti、Ni相互作用使得Ni原子電子云密度增加導致瞬間偶極矩變化增大,振動加強,活性升高.在雙金屬Ti0.2-Ni0.8-MOF-M和Ti0.2-Ni0.8-MOF紅外譜圖中的500～700cm?1的特征峰歸因于(Ti-O)鍵的伸縮振動帶,Ti摻雜使材料產(chǎn)生了新絡合峰,更有利于NO+CO的吸附.

由圖7、表2可知,其中雙金屬MOFs中均存在Ti、Ni、O等元素且Ti:Ni原子比接近1:4.

圖6 代表性的FT-IR譜圖

圖7(b)中結(jié)合能在458.0～458.6eV和463.7～ 464.4eV分別對應于Ti2p3/2和Ti2p1/2峰.其中,將Ti 2p3/2分為458.97和459.33eV兩處特征峰,分別屬于Ti3+和Ti4+物種.與Ti0.2-Ni0.8-MOF相比, Ti0.2-Ni0.8- MOF-M的Ti3+峰的結(jié)合能增加0.2eV,這是由于微波法制備的雙金屬材料具有電子親和性更強的Ti-O-Ti鍵,有利于產(chǎn)生良好的Ti、Ni間誘導和協(xié)同效應[34-35].微波法通過快速均勻地加熱材料促進晶粒生長,優(yōu)化Ni-Ti-MOFs的粒徑、結(jié)晶度、分散性和均勻性.

圖7(c)中結(jié)合能在856.4和873.8eV的特征峰分別歸屬于Ni的2p3/2和2p1/2軌道.通過高斯方法擬合分峰,得到Ni2p3/2光譜在856.0和856.8eV處的兩個特征峰,分別為Ni2+和Ni3+物種.此外,在861.2～ 861.8eV處還出現(xiàn)了震激峰,這是由于鎳內(nèi)層電子光電離發(fā)射后的弛豫效應.研究表明Ni3+是促進氧化還原性質(zhì)增強的有利物種[36-37],Ni電子云密度的增加可以促進催化劑表面CO的吸附和氧化,從而獲得高催化性能.通過表2可知,Ni3+/(Ni3++Ni2+)的比例按以下順序遞減:Ti0.2-Ni0.8-MOF-M(42.5%)>Ti0.2- Ni0.8-MOF(41.7%)>Ni-MOF(29.1%),與CO-SCR活性測試結(jié)果相吻合.此外,相較于Ni-MOF,Ti0.2- Ni0.8-MOF和Ti0.2-Ni0.8-MOF-M的Ni3+特征峰向著更低的結(jié)合能分別偏移了0.5和0.2eV,研究表明, Ti4+/Ti3+氧化還原電子轉(zhuǎn)移使Ni原子電子云密度增加[38],推斷在催化反應中發(fā)生了Ni2++Ti4+?Ni3++ Ti3+氧化還原循環(huán),引發(fā)并促進了Ti、Ni之間的協(xié)同效應,增強MOFs材料的低溫脫硝活性.

圖7 催化劑XPS譜圖

表2 基于XPS表征的Tix-Ni1-x-MOF的化學成分(%)

O1s的XPS能譜圖被擬合成如圖7(d)所示的兩個峰.高結(jié)合能(532.4～532.8eV)處的峰屬于表面化學吸附氧(Oα,如OH-、CO32-),低結(jié)合能(531.3～ 531.7eV)處的峰屬于表面晶格氧(Oβ).如表2所示,雙金屬Ti0.2-Ni0.8-MOF中的Oβ/(Oα+Oβ)的比值(微波法63.8%、溶劑熱法62.3%)大于單金屬Ni-MOF(51.7%).晶格氧Oβ比Oα具有更好的反應活性[39-41],較高的Oβ/(Oα+Oβ)比值促進Ni-Ov-Ti中氧空位(Ov)的產(chǎn)生[42-43],有效提高NO+CO催化循環(huán)效率.

由Ni2++Ti4+? Ni3++Ti3+氧化還原循環(huán)引發(fā)的雙金屬協(xié)同效應,使Ni-O-Ti活性位點提供晶格氧來吸附CO和NO物種.在此過程中產(chǎn)生的晶格氧空位Ov可以與N-O鍵斷裂產(chǎn)生游離的O結(jié)合,并再次生成,從而重新啟動催化循環(huán).

2.3 催化劑循環(huán)性測試

對Ti0.2-Ni0.8-MOF-M,Ti0.2-Ni0.8-MOF和Ni- MOF重復進行5次CO-SCR測試,如圖8所示,經(jīng)過循環(huán)測試后,Ni-MOF的NO還原率保持在80%以上,說明催化劑結(jié)構發(fā)生了局部的破壞和失穩(wěn).而2種方法制備的Ti0.2-Ni0.8-MOF均保持在初次測量的90%以上,這可能是由于Ti的適量摻雜加強了原子分散度導致金屬骨架結(jié)構更加穩(wěn)定.

圖8 催化劑循環(huán)測試

3 結(jié)論

3.1 Ti在Ni基MOF材料中的適量摻雜不僅增大雙金屬Ti0.2-Ni0.8-MOF的比表面積和孔容,有利于Ni-O-Ti活性位點高度分散,并且Ti4+/Ti3+間的氧化還原電子轉(zhuǎn)移促進了Ni3++Ti3+?Ni2++Ti4+循環(huán)反應,降低了反應能壘.其中Ti0.2-Ni0.8-MOF在200～400oC溫度范圍可達到100%的轉(zhuǎn)化率.

3.2 采用微波法替代溶劑熱法,不僅提高了合成效率,而且晶粒更加小且均勻,使Ti0.2-Ni0.8-MOF-M材料的比表面積增大,暴露出更加豐富高效的Ni-O- Ti活性位點,催化效率、熱穩(wěn)定性及循環(huán)性能均明顯提高.

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Preparation of Ti-Ni1-x-MOFs and their selective catalytic reduction of NOby CO.

SHI Yong1*, LI Cheng1, HUANG Lei1, XIONG Wei1, ZHAO Qi-dong2, SUN Jiang-heng1, DING Yue1

(1.Key Laboratory of Industrial Ecology and Environmental Engineering and State Key Laboratory of Fine Chemicals, School of Environmental Science and Technology, Dalian University of Technology, Dalian 116024, China；2.Panjin Branch of School of Chemical Engineering, Dalian University of Technology Panjin Campus, Panjin 124221, China)., 2022,42(11)：5080～5087

Ti-Ni1-x-MOFs catalysts with different proportions were successfully synthesized by solvothermal method and microwave method, applied for selective catalytic reduction reaction of NOwith CO. The doping of Ti significantly improved NOreduction performance of Ni-MOF catalysts and widened reaction temperature window. Ti0.2-Ni0.8-MOF showed the best denitration efficiency and reached 100% conversion in the temperature range of 200～400℃. Multiple characterizations were conducted to ascertain the properties of bimetallic Ti-Ni1-x-MOFs materials (e.g., TGA, XRD, SEM, FT-IR, XPS and BET). Ti-doping in Ni-MOF can improve the atomic dispersion, and indicate a strong metal-metal interaction between Ti and Ni which was conducive to produce more efficient Ni-O-Ti sites and oxygen vacancies, strengthen the Ni2++Ti4+? Ni3++Ti3+redox cycle, and thus improve the catalytic performance of NO reduction reaction by CO. Compared with solvothermal method, the preparation of Ti0.2-Ni0.8-MOF by microwave method exhibits the advantages of high synthesis efficiency, good crystallinity, fine and uniform grains, which further enhancing its low-temperature denitration effect.

CO-SCR；metal-organic frameworks；Ti-Ni1-x-MOFs；microwave method；solvothermal method

X511

A

1000-6923(2022)11-5080-08

石 勇(1975-),男,河南南陽人,副教授,博士,在大連理工大學從事環(huán)境催化材料合成、污染物監(jiān)測及大氣污染控制等方面的研究.發(fā)表論文70余篇.

2022-04-25

國家自然科學基金資助項目(21677022,22006007);工業(yè)生態(tài)與環(huán)境工程教育部重點實驗室開放基金資助項目(KLIEEE-21-03)

* 責任作者, 副教授, yongshi@dlut.edu.cn