ACr2O4尖晶石氧化物(A=Co，Zn，Mn，Cu)上二氯甲烷的催化燃燒：A位陽離子的影響

陳家喜，張婷婷，魯繼青，賈愛平，羅孟飛

(浙江師范大學(xué)物理化學(xué)研究所，先進(jìn)催化材料教育部重點(diǎn)實(shí)驗(yàn)室，浙江 金華 321004)

ACr2O4尖晶石氧化物(A=Co，Zn，Mn，Cu)上二氯甲烷的催化燃燒：A位陽離子的影響

陳家喜，張婷婷，魯繼青*，賈愛平，羅孟飛

(浙江師范大學(xué)物理化學(xué)研究所，先進(jìn)催化材料教育部重點(diǎn)實(shí)驗(yàn)室，浙江 金華 321004)

采用溶膠-凝膠法制備4種不同ACr2O4尖晶石氧化物(A=Co，Zn，Mn，Cu),考察A位陽離子對(duì)ACr2O4尖晶石氧化物的性質(zhì)以及對(duì)二氯甲烷催化燃燒性能的影響,并對(duì)催化劑進(jìn)行SEM、HRTEM、H2-TPR、NH3-TPD以及XPS等表征。結(jié)果表明，A位離子顯著影響催化劑的可還原性和表面酸性，催化劑催化活性順序?yàn)镃oCr2O4>ZnCr2O4>MnCr2O4>CuCr2O4。結(jié)合表征結(jié)果，認(rèn)為催化劑活性與其可還原性能和表面酸性存在密切關(guān)系。CoCr2O4由于具有最佳的可還原性和較高的表面酸性，具有最高的催化活性；而CuCr2O4由于具有最低的表面酸性導(dǎo)致其催化活性最低。

催化化學(xué)；尖晶石氧化物；催化燃燒；含氯揮發(fā)性有機(jī)化合物；表面酸性；可還原性

由于含氯揮發(fā)性有機(jī)化合物(CVOCs)對(duì)環(huán)境和人體健康危害嚴(yán)重，因此,消除CVOCs是目前環(huán)境領(lǐng)域的重要課題[1]。催化燃燒是一種有效的CVOCs消除手段，與傳統(tǒng)燃燒相比，催化燃燒具有操作溫度低及消除效率高等優(yōu)點(diǎn)[2]。CVOCs催化燃燒常用的催化劑為貴金屬催化劑(如Pt和Pd)[3-6],與其相比，過渡金屬氧化物如MnOx[7]、CeO2[8]、CrOx[9-19]、鈣鈦礦型氧化物[20-22]和分子篩[23-26]等更具經(jīng)濟(jì)性，并有更好的抗Cl中毒能力。其中，CrOx氧化物中因高價(jià)Cr物種(Cr6+)的存在，對(duì)CVOCs催化燃燒具有很高的活性[16]。由于在反應(yīng)中活性Cr物種易流失并容易生成劇毒的鉻酰氯物種[23]，實(shí)際應(yīng)用受到很大限制。

最近，含Cr尖晶石氧化物在CVOCs催化燃燒中因催化活性高且Cr離子被限域在堅(jiān)固的尖晶石晶格中，克服了Cr物種流失的問題[27-29]而備受關(guān)注。Liu J D等[30]研究了CoCr2O4尖晶石氧化物的微結(jié)構(gòu)性質(zhì)對(duì)二氯甲烷催化燃燒性能的影響，結(jié)果發(fā)現(xiàn)，高溫焙燒有利于Cr3+/Cr6+進(jìn)入尖晶石結(jié)構(gòu)中的八面體位，增強(qiáng)了氧化物的可還原性能和表面酸性，催化活性提高。

含Cr尖晶石氧化物的化學(xué)通式為ACr2O4，其中，A位離子占據(jù)四面體位，Cr離子占據(jù)八面體位[31]且暴露于表面，并被認(rèn)為是氧化反應(yīng)中的活性位[32]。ACr2O4尖晶石氧化物的物化性質(zhì)受到A位離子的影響，進(jìn)一步影響其催化活性。

本文采用溶膠-凝膠法制備4種含Cr的尖晶石氧化物ACr2O4(A=Co,Zn,Mn，Cu)，并考察對(duì)二氯甲烷催化燃燒的性能。

1 實(shí)驗(yàn)部分

1.1 催化劑制備

化學(xué)試劑均購自國藥化學(xué)試劑有限公司，無需進(jìn)一步純化。

ACr2O4尖晶石氧化物(A=Co,Zn,Mn,Cu)采用溶膠-凝膠法制備，金屬硝酸鹽為前驅(qū)體。一定量Co(NO3)2·6H2O(99.0%)、Cr(NO3)3·9H2O(99.0%)和檸檬酸溶于去離子水，n(Co)∶n(Cr)∶n(檸檬酸)=1∶2∶6。溶液在90 ℃加熱攪拌至形成凝膠，180 ℃ 烘干3 h,獲得蓬松固體，并在空氣氣氛600 ℃焙燒4 h。其他尖晶石氧化物采用類似方法制備，前驅(qū)體分別為Zn(NO3)2·6H2O(99.0%)、Mn(NO3)2(49.0%水溶液)或Cu(NO3)2·3H2O(99.0%)。

1.2 催化劑表征

催化劑比表面積采用Quanta chrome Autosorb-1型N2物理吸附儀在液氮溫度(77 K)測定，測定前催化劑在真空和120 ℃條件下預(yù)處理6 h。

X射線粉末衍射實(shí)驗(yàn)在PANalytical X′pert PRO MPD 粉末X射線衍射儀上進(jìn)行,CuKα，工作電壓40 kV，工作電流40 mA,掃描范圍10°～90°，掃描速率0.15°·s-1，所得譜圖采用JADE 6.5軟件用全譜擬合方法分析催化劑的晶粒尺寸和晶格參數(shù)。

催化劑形貌采用Hitachi S-4800型掃描電子顯微鏡觀測，工作電壓5.0 kV。

高分辨透射電鏡采用JEOL JEM-2100F型透射電子顯微鏡測試，工作電壓200 kV。

催化劑還原性由氫氣程序升溫還原實(shí)驗(yàn)測定，將25 mg催化劑放入石英反應(yīng)管，在氮?dú)饬魉?0 mL·min-1和300 ℃條件下預(yù)處理1 h，除去吸附水和碳酸鹽，將樣品在流速為20 mL·min-1的氮?dú)庵欣鋮s至50 ℃，然后切換至流速為20 mL·min-1的5%H2-95%N2混合氣，以10 ℃·min-1升溫速率從50 ℃升至900 ℃。用熱導(dǎo)檢測器氣相色譜儀(TCD)測定氫氣消耗量，并通過已知量的CuO粉末樣品進(jìn)行標(biāo)定。

催化劑表面酸性由氨氣程序升溫脫附實(shí)驗(yàn)測定。將50 mg催化劑放入石英反應(yīng)管，在流速20 mL·min-1氮?dú)鈿夥蘸?00 ℃條件下預(yù)處理30 min，然后冷卻至50 ℃。在樣品上吸附流速為20 mL·min-1的氨氣15 min，升溫至100 ℃通流速為20 mL·min-1的氮?dú)獯祾?0 min，除去物理吸附的NH3。再以20 ℃·min-1速率由100 ℃升至800 ℃，氨氣的脫附信號(hào)由氣相色譜儀(TECHTEMP GC 7890Ⅱ) TCD檢測器進(jìn)行檢測。

催化劑的X射線光電子能譜在ESCALAB 250Xi型儀器上進(jìn)行，操作條件：真空度約2×10-7Pa，電壓20 eV， AlKα為X射線源(1 486.6 eV)，所得譜圖的結(jié)合能通過Cls=284.8 eV校正。

1.3 催化劑活性評(píng)價(jià)

催化劑活性評(píng)價(jià)在微型固定床反應(yīng)器上進(jìn)行，石英反應(yīng)管內(nèi)徑9 mm。將(40～60)目催化劑1 g用石英砂均勻稀釋至2 mL后裝入反應(yīng)管。二氯甲烷由含水汽的空氣通過冰浴保持的0 ℃的液態(tài)二氯甲烷帶入,二氯甲烷濃度為3 000×10-6，水蒸汽濃度為12 000×10-6，總流量為500 mL·min-1，空速為15 000 h-1。二氯甲烷轉(zhuǎn)化率采用日本島津公司GC-14C氣相色譜儀進(jìn)行分析，F(xiàn)ID檢測器檢測。出口尾氣通過0.1 mol·L-1的NaOH溶液進(jìn)行中和吸收。

轉(zhuǎn)化率按下式計(jì)算：

式中，[CH2Cl2]in和[CH2Cl2]out分別為入口和出口二氯甲烷氣體體積分?jǐn)?shù)，%。

反應(yīng)動(dòng)力學(xué)研究在同一反應(yīng)器上進(jìn)行，試驗(yàn)中保證二氯甲烷轉(zhuǎn)化率小于15%時(shí)(微分模式)，反應(yīng)條件與在催化測試中相同。同時(shí)，采用Weisz-Prater和Mears判據(jù)確認(rèn)動(dòng)力學(xué)研究過程中反應(yīng)不受傳質(zhì)和傳熱的影響。

2 結(jié)果與討論

制備的CoCr2O4、ZnCr2O4、MnCr2O4和CuCr2O4催化劑的XRD圖如圖1所示。由圖1可見，各氧化物中均含有各自的尖晶石物相 (JCPDS 80-1668對(duì)應(yīng)于CoCr2O4,JCPDS 87-0028對(duì)應(yīng)于ZnCr2O4,JCPDS 75-1614對(duì)應(yīng)于MnCr2O4，JCPDS 72-1212對(duì)應(yīng)于CuCr2O4)。對(duì)于制備的CoCr2O4、ZnCr2O4和CuCr2O4樣品，XRD圖中僅觀察到純尖晶石相，但MnCr2O4樣品中含有Cr2O3晶相(JCPDS 82-1484)，其特征衍射峰為2θ=24.4°、33.5°、36.3°、41.6°、50.1°和54.8°。各樣品的微結(jié)構(gòu)分析數(shù)據(jù)表明，CoCr2O4、ZnCr2O4和MnCr2O4樣品為立方相結(jié)構(gòu)，CuCr2O4為四方相結(jié)構(gòu)。

圖 1 催化劑的XRD圖Figure 1 XRD patterns of the catalysts

表1為催化劑的比表面積、晶胞參數(shù)、平均晶粒尺寸和表面/體積比。由表1可以看出，各樣品的表面/體積比(假設(shè)樣品為密堆積球形結(jié)構(gòu))隨著晶粒尺寸增大而減小。

表 1 催化劑的比表面積、晶胞參數(shù)、平均晶粒尺寸和表面與體積比

①假定為緊密堆積球形模式，則比率等于6/D(D為平均晶粒尺寸);②單位質(zhì)量催化劑的表面酸量;③比表面積歸一化后的單位面積催化劑的表面酸量

催化劑的SEM照片如圖2所示。由圖2可以看出，樣品為堆積的納米結(jié)構(gòu)，且無孔道結(jié)構(gòu)存在。

圖 2 催化劑的SEM照片F(xiàn)igure 2 SEM images of the catalysts

圖3為催化劑的HRTEM照片。

圖 3 催化劑的HRTEM照片F(xiàn)igure 3 HRTEM images of the catalysts

由圖3可見，催化劑均為結(jié)晶度良好的晶體。通過測量晶面間距進(jìn)一步驗(yàn)證了尖晶石氧化物的存在。

樣品中各元素的化學(xué)狀態(tài)通過XPS測量，圖4為催化劑的Cr2p和O1s的XPS譜圖。

圖 4 催化劑的Cr2p和O1s的XPS譜圖Figure 4 XPS spectra of Cr2p and O1s of the catalysts

由圖4(a)可以看出，樣品中的譜峰可擬合為3個(gè)峰，結(jié)合能為575.6 eV、577.0 eV和579.1 eV，分別歸屬為Cr(OH)3或Cr2O3[33],Cr3+(占據(jù)八面體位)[32]和Cr6+[34]。ACr2O4尖晶石氧化物中Cr6+的存在文獻(xiàn)已有報(bào)道，并且該物種一般富集于表面[34]。由圖4(b)可見，譜峰可擬合為兩個(gè)位于530.2 eV和532.2 eV處的峰，分別歸屬于晶格氧(Olatt)和吸附氧(Oads)[35]。Oads的譜峰是由于表面吸附氧物種或者表面羥基的存在而引起[36]。

各樣品的A位離子XPS譜圖如圖5所示。

圖 5 Co2p、Zn2p、Mn2p和Cu2p的XPS譜圖Figure 5 Co2p,Zn2p,Mn2p and Cu2p XPS spectra

由圖5可以看出，Co2p3/2譜圖可擬合為位于781.5 eV、785.4 eV和788.6 eV的3個(gè)譜峰，分別歸屬于Co2+、Co3+和Co2+的衛(wèi)星峰[37-38]。Zn2p3/2譜圖可擬合為位于1 021.4 eV和1 023.2 eV的兩個(gè)譜峰，分別歸屬于四面體和八面體位的Zn2+[39]。Mn2p3/2譜圖可擬合為位于640.8 eV、642.7 eV和646.2 eV的3個(gè)譜峰，分別歸屬于Mn2+、Mn3+和Mn4+[40]。對(duì)于Cu2p3/2譜圖，譜峰可擬合為位于930.2 eV、934.6 eV和937.0 eV的3個(gè)譜峰，分別歸屬于Cu+、四面體位Cu2+和八面體位Cu2+[41-42]。

催化劑表面元素組成見表2。由表2可見，催化劑中Oads/Otot為0.33～0.44，但CuCr2O4含有較高的吸附氧物種。Crtot/Atot(A=Co,Zn,Mn,Cu)代表了催化劑中金屬離子的分布，約為1.64～2.17。該數(shù)值與體相Crtot/Atot相當(dāng)， 表明在尖晶石結(jié)構(gòu)中各金屬元素分布較為均勻，無明顯的表面富集情況。對(duì)于表面Cr6+物種，各樣品中的比例較為接近，約為0.25。基于XPS數(shù)據(jù)和樣品的表面與體積比(見表1)，計(jì)算出金屬元素的表面密度(見表2)。金屬離子的表面密度隨著比表面積減小和晶粒尺寸增大而增加。CuCr2O4含有最高的表面Cu離子密度(200.4 μmol·m-2) 和表面Cr離子密度(329.7 μmol·m-2)，可歸因于該樣品最小的比表面積(3.7 m2·g-1) 和最大的晶粒尺寸(42.5 nm)。

表 2 催化劑表面元素組成

①Crtot=Cr3++Cr6+

催化劑的可還原性能通過H2-TPR測定，結(jié)果如圖6所示。

圖 6 催化劑的H2-TPR譜圖Figure 6 H2-TPR profiles of the catalysts

由圖6可以看出，CoCr2O4在(190～300) ℃出現(xiàn)的重疊峰歸屬于Cr6+到Cr3+的還原或Co3+到Co2+的還原[43-44]。ZnCr2O4在284 ℃出現(xiàn)的還原峰也歸屬于Cr6+到Cr3+的還原[45]。MnCr2O4在(200～500) ℃出現(xiàn)一個(gè)寬的還原峰，其中，295 ℃的還原峰歸屬于Cr6+到Cr3+的還原[46],而356 ℃和435 ℃出現(xiàn)的肩峰分別歸屬于MnO2和Mn2O3的還原[47]。CuCr2O4的還原較為復(fù)雜，大致分為4個(gè)還原峰，242 ℃的還原峰歸屬于Cr6+的還原[43]或CuO和Cu2O還原為Cu0[48];320 ℃和412 ℃的還原峰歸屬于CuCr2O4的體相還原；(560～595) ℃出現(xiàn)的弱峰歸屬于CuCrO2(銅鐵礦型氧化物)的還原[48]。對(duì)比表1可以看出，CoCr2O4、ZnCr2O4、MnCr2O4以及CuCr2O4的耗氫量分別為0.55 mmol·g-1、0.33 mmol·g-1、0.90 mmol·g-1和3.46 mmol·g-1。從圖6還可以看出，各催化劑的起始還原溫度順序?yàn)镃oCr2O4≈CuCr2O4

圖7為催化劑的NH3-TPD譜圖。由圖7可以看出，CoCr2O4、ZnCr2O4和MnCr2O4在(150～500) ℃出現(xiàn)NH3脫附峰，表明其具有不同性質(zhì)的表面酸性位，而CuCr2O4僅在(200～400) ℃出現(xiàn)微弱的脫附峰，表明表面酸性很弱。CoCr2O4、ZnCr2O4、MnCr2O4和CuCr2O4催化劑的表面酸量分別為0.11 mmol·g-1、0.14 mmol·g-1、0.06 mmol·g-1和0.005 mmol·g-1。計(jì)算得ZnCr2O4表面酸量最高，為3.32 μmol·m-2，而CuCr2O4的表面酸量僅為1.35 μmol·m-2。

圖 7 催化劑的NH3-TPD譜圖Figure 7 NH3-TPD profiles of the catalysts

圖8為催化劑對(duì)二氯甲烷催化燃燒催化活性曲線(a)和面積比速率(b)。由圖8(a)可以看出，催化劑活性順序?yàn)椋篊oCr2O4>ZnCr2O4>MnCr2O4>CuCr2O4,T50分別為251 ℃、279 ℃、311 ℃和475 ℃。選擇性較高，最終反應(yīng)產(chǎn)物為CO2、H2O和HCl/Cl2(已被NaOH中和吸收)，未檢測到其他有機(jī)副產(chǎn)物如CH3Cl。CoCr2O4活性在相同反應(yīng)條件下，與文獻(xiàn)[29]采用溶膠-凝膠法制備的CoCr2O4尖晶石氧化物活性接近(T50=259 ℃)，但略低于共沉淀法制備的CoCr2O4尖晶石氧化物活性(T50=210 ℃)[30]。由圖8(b)可以看出，CoCr2O4催化劑面積比速率最高，為2.07×10-8mol·s-1·m-2。

圖 8 催化劑的催化活性曲線(a)和面積比速率(b)Figure 8 Light-off curves (a) and area specific reaction rates (b) of the catalysts

催化劑氧化二氯甲烷的Arrhenius圖如圖9所示。

圖 9 催化劑氧化二氯甲烷的Arrhenius圖Figure 9 Arrhenius plots of CH2Cl2 oxidation over the catalysts

反應(yīng)活化能順序： CoCr2O4(121.0 kJ·mol-1)

考察CoCr2O4和ZnCr2O4催化劑的穩(wěn)定性，結(jié)果見圖10。由圖10可以看出，反應(yīng)10 h，CoCr2O4和ZnCr2O4催化劑性能均保持穩(wěn)定。

圖 10 CoCr2O4和ZnCr2O4催化劑的穩(wěn)定性Figure 10 Stability of CoCr2O4 and ZnCr2O4 catalysts

由以上結(jié)果可以看出，盡管ACr2O4尖晶石結(jié)構(gòu)氧化物中，B位離子皆為Cr離子，但A位離子對(duì)催化劑性質(zhì)和催化性能產(chǎn)生很大影響。由于B位Cr離子為催化活性中心[32]，根據(jù)各催化劑中的Cr表面密度(表2)和面積比速率(圖8b)，計(jì)算Cr離子轉(zhuǎn)化頻率，結(jié)果如圖11所示。

圖 11 反應(yīng)溫度290 ℃時(shí)催化劑表面Cr離子轉(zhuǎn)化頻率Figure 11 Turnover frequency based on Cr ions of catalyst surface at reaction temperature 290 ℃

比較轉(zhuǎn)化頻率，可更明顯區(qū)分不同氧化物中Cr離子性能的差異。轉(zhuǎn)化頻率值最高的為CoCr2O4(3.30×10-4s-1)，是CuCr2O4的10倍以上(0.28×10-4s-1)。CVOCs氧化催化體系中，影響反應(yīng)性能的兩個(gè)關(guān)鍵因素為可還原性和表面酸性[49]，結(jié)合兩方面因素，具有最佳還原性能和較高表面酸性的CoCr2O4具有最高的催化活性。

3 結(jié) 論

探討了A位陽離子對(duì)ACr2O4尖晶石氧化物的性質(zhì)以及對(duì)二氯甲烷氧化性能的影響。結(jié)果表明，A位陽離子顯著影響催化劑的可還原性和表面酸性，從而進(jìn)一步影響催化活性。在制備的4種催化劑中，CoCr2O4由于具有最佳的可還原性和較高的表面酸性，因此，催化活性最高。

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Catalytic combustion of dichloromethane over ACr2O4spinel oxides (A=Co,Zn,Mn,Cu):the effects of A site cations

ChenJiaxi,ZhangTingting,LuJiqing*,JiaAiping,LuoMengfei

(Key Laboratory of the Ministry of Education for Advanced Catalysis Materials, Institute of Physical Chemistry,Zhejiang Normal University,Jinhua 321004,Zhejiang,China)

Four Cr-containing spinel oxides ACr2O4(A=Co,Zn,Mn,Cu) were prepared by sol-gel method.The influence of A site cations on the properties of spinel oxides ACr2O4and their performance for dichloromethane catalytic combustion was investigated.The catalysts were characterized by SEM,HRTEM,H2-TPR,NH3-TPD and XPS techniques.The results showed that A site cations had obviously effects on catalyst reducibility and surface acidity.The catalitic activity of the catalysts followed the order of CoCr2O4>ZnCr2O4>MnCr2O4>CuCr2O4.Based on the characterization results,it was believed that the activity of the catalyst was closely related to its reducibility and surface acidity.Among the catalysts,CoCr2O4had the best reducibility and high surface acidity,therefore the best catalytic activity,while CuCr2O4possessed the lowest catalytic activity due to its lowest surface acidity.

catalytic chemictry;spinel oxide;catalytic combustion;chlorinated volatile organic compound;surface acidity;reducibility

O643.36;X701 Document code: A Article ID: 1008-1143(2016)12-0014-09

2016-08-10；

2016-11-05 基金項(xiàng)目：國家自然科學(xué)基金(21173195)資助項(xiàng)目

陳家喜，1991年生，女，廣東省清遠(yuǎn)市人，在讀碩士研究生。

魯繼青，1974年生，男，博士，教授，主要從事催化研究。

10.3969/j.issn.1008-1143.2016.12.003

O643.36;X701

A

1008-1143(2016)12-0014-09

催化劑制備與研究

doi:10.3969/j.issn.1008-1143.2016.12.003