預處理氣氛對CuO/CeO2/γ-Al2O3催化劑表面性質及“NO+CO”反應性能的影響

朱 捷 葛奉娟

(徐州工程學院化學化工學院，徐州221111)

預處理氣氛對CuO/CeO2/γ-Al2O3催化劑表面性質及“NO+CO”反應性能的影響

朱 捷＊葛奉娟

(徐州工程學院化學化工學院，徐州221111)

用不同的預處理氣氛制備了CeO2/γ-Al2O3載體以調節表面Ce的價態，并以Cu(CH3COO)2為前驅體制備了CuCeAl催化劑。XRD和H2-TPR的結果表明在還原氣氛下處理的CeO2/γ-Al2O3載體具有更多的活性氧原子，因此相應的CuCeAl催化劑表面有更多分散態的Cu2+/Cu+物種。NO+CO反應的結果表明分散態的Cu2+/Cu+是NO轉化的活性物質，而Cu0在低溫下具有較好的N2選擇性。因此，同時含有分散態Cu2+/Cu+和少量晶相Cu0的催化劑具有最好的催化性能。

多相催化；氮氧化物；污染防治；預處理氣氛；CeO2/γ-Al2O3；“NO+CO”反應

0 Introduction

NO is the major source of air pollution,for it has the ability to generate secondary contaminants through its interaction with other primary pollutants[1].Catalytic reduction of NO in the presence of CO is one of the mostimportantreactionsoccurringinautomotive catalyticconverters,wherebothreactantsare undesirablepollutants[2].Supportednoblemetal catalysts have been considered as the most efficient catalystsfortheirresponsibleactivity,good hydrothermal and resistance to impurities such as SO2[3]. However,the scarcity and the high cost have limited their application.Copper-containing catalysts show apotential activity for the treatment of exhaust gas from automobiles and have been extensively investigated during the past decades[2,4-5].

CeO2is an important support and has attracted more attention recently due to its outstanding oxygen storage capacity and unique redox properties[6-8].A number of reports have shown that the conversion between Ceand Ceoxidation states is likely the key factor leading to the high mobility of lattice oxygen and providing activation centers in some redox reactions[9-11].In our work,the CeO2/γ-Al2O3supports werepreparedandthenpretreatedindifferent atmospheres to adjust the valence state of surface Ce element,the surface states of CuCeAl samples were characterized by XRD and H2-TPR and the catalytic activity for NO+CO reactions was evaluated.The effect of pretreating atmosphere was discussed on the surface property and the NO+CO activity of the cat alysts.

1 Experimental

1.1 Sample Preparation

γ-Al2O3waspurchasedfromFushun Petrochemical Institute(China)with BET surface area of 140 m2·g-1and was calcined at 700℃for 5 h before use.

CeO2/γ-Al2O3samples were prepared by impregnating the γ-Al2O3with an aqueous solution containing required amount of Ce(NO3)3.The impregnated sample was dried at 100℃for 12 h and then divided into three parts and calcined at 500℃for 2 h in various atmospheres,respectively:H2-Ar mixture(7.3%H2by volume),N2(≥99.5%by volume)and O2(≥99.9%by volume),the sample was denoted correspondingly as CeAl-H2,CeAl-N2,CeAl-O2.The Ce loading was 0.07 mmol/100m2γ-Al2O3.

The CuO/CeO2/γ-Al2O3samples were prepared by impregnating the CeO2/γ-Al2O3support above with an aqueoussolutioncontainingrequiredamountof copper acetate,then dried at 100℃for 12 h.The samples were all calcined in flowing N2(≥99.5%by volume)at 450℃for 5 h.For simplicity,the resultant samples were denoted as xCuCeAl-H2,xCuCeAl-N2and xCuCeAl-O2,respectively,where x stands for the loading amount of copper acetate(mmol/100m2γ-Al2O3).For comparison,the xCuO/γ-Al2O3sample was also prepared by the same method.

1.2 Characterization

X-ray diffraction(XRD)patterns were obtained with a Philips Xpert Pro diffractometer using Ni filtered Cu Kα radiation(λ=0.154 18 nm).The X-ray tube was operated at 40 kV and 40 mA.

BET surface area was measured by nitrogen adsorption at 77 K on a Micromeritics ASAP-2020 adsorption apparatus.

Temperature-programmed reduction(TPR)was carried out in a quartz U-tube reactor,and 100 mg samplewasusedforeachmeasurement.Before reduction,the sample was pretreated in N2stream at 100℃for 1 h and then cooled to room temperature. After that,a H2-Ar mixture(7%H2by volume)was switchedonandthetemperaturewasincreased linearly at a rate of 10℃·min-1.Athermal conductivity cell was used to detect the consumption of H2on stream.

The activities of the catalysts for NO+CO reaction were evaluated under steady state,involving a feed stream with a fixed composition,NO 3.33%,CO 6.67% and He 90%by volume as diluter.A quartz tube was employed as the reactor and the required quantity of catalysts(50 mg for each test)was used.The catalysts were pretreated in N2stream at 100℃for 1 h and then heated to reaction temperature,after that,the mixed gases were switched on.The reactions were carried out at 300℃with the same space velocity of 30 000 mL·g-1·h-1.Two columns and thermal conductivity detector (TCD)were used for the purpose of analyzing the products.Column A was a stainless steel(9.525 mm (Φ3)×1.75 m)Porapak Q for separating N2O and CO2, and Column B was a stainless steel(9.525mm(Φ3)×1.75 m)13X molecular sieve(30～60 Mesh(250～595 μm))for separating N2,NO and CO.

2 Results and discussion

2.1 XRD

Fig.1 XRD patterns of 0.3CuAl and 0.3CuCeAl samples

Fig.1 showstheXRDpatternsof0.3CuAl, 0.3CuCeAl-O2,0.3CuCeAl-N2and0.3CuCeAl-H2samples.No crystalline peaks of ceria are observed for allCe-modifiedsamples,implyingthepossibility (albeit not complete proofing)of well dispersed CeO2on the surface of γ-Al2O3[12-14].For 0.3CuAl sample, characteristic peaks of Cu0metal are observed at 43° and 50°,indicating that there are many Cu0arising from the decomposition of Cu(CH3COO)2.According to our previous results,the dispersion capacity of CuO on the surface of γ-Al2O3is about 0.75 mmol/100m2[15].The result indicates that the Cu0species are difficult to disperse on the surface of γ-Al2O3in our experimental condition of Cu(CH3COO)2precursor and N2atmosphere.

Thepossiblereasonsshouldbeasfollows. According to the literatures[16-18],for γ-Al2O3,the(110) plane is the preferentially exposed plane with the octahedral and tetrahedral vacancies,and this plane consistsofC-andD-layers,whichhaveequal exposure probabilities.When CuO are dispersed on the surface of γ-Al2O3,the Cu2+cations incorporate intotheoctahedralvacantsites.Meanwhile,the capping oxygen atoms cover the Cu2+to balance the charge,forming steady octahedral structures,as shown by Figures 2a and 2b.The average density of the octahedral vacant sites is 2 sites/unit mesh(0.443 5 nm2,while 0.14 nm is taken as the radius of O2-anion)of the C-and D-layers,corresponding to 0.75 mmol/100 m2γ-Al2O3.Thus,the theoretical dispersion capacity of CuO is 0.75 mmol/100 m2γ-Al2O3.When γ-Al2O3is impregnated by Cu(CH3COO)2aqueous solution,Cu2+cations will occupy the octahedral vacant sites on the surface of γ-Al2O3and the two accompanying CH3COO-anions would stay at the topof the occupied site as capping anions,compensating theextrapositivecharges.Thus,therepulsion between acetate anions may hinder the dispersion of Cu2+into the adjacent vacant sites of the γ-Al2O3, leadingtothedecreaseofdispersioncapacity. Another possible reason is that the decomposition gas of Cu(CH3COO)2is reductive and the calcination atmosphere is N2,thus the Cu0metal is the main decomposition product.The radius of Cu0atom is about 117 pm,while the radius of octahedral vacant sites is only 58 pm.Thus,Cu0is too large to incorporate steadily into the surface vacant sites,as shown in Fig.2c.Furthermore,as Cu0metal has no positive charges,the absence of electrostatic attraction makes it difficult to disperse.

Fig.2 Scheme for Cu2+/Cu0species dispersion on the surface of γ-Al2O3.O(1):surface oxygen,O(2):capping oxygen.

However,it is noteworthy that,for 0.3CuCeAl-N2and 0.3CuCeAl-H2,no any crystalline peaks of Cu species could be observed.As analyzed before,Cu0can hardly incorporate into the surface vacant sites of γ-Al2O3.Thus,CeO2should promote the production of Cu species with higher valence states,i.e.,Cu+or Cu2+,in the 0.3CuCeAl-N2and 0.3CuCeAl-H2samples. While for 0.3CuCeAl-O2,the crystalline peaks of Cu0metal could be observed obviously and the peak intensity is comparative with that in 0.3CuAl sample, implying that the CeO2in 0.3CuCeAl-O2sample has little effect on the valence states of Cu species and most of Cu species are still crystalline Cu0metal.The result indicates that the calcination atmosphere of CeAl support is very important for its surface structure and properties.

Thedifferencebetween0.3CuCeAl-N2, 0.3CuCeAl-H2and 0.3CuCeAl-O2can be explained by the oxygen deficiency property of CeO2.As Ce content isveryfew(0.07mmol/100m2γ-Al2O3),itis reasonabletoassumethattheCuspeciesare dispersed on the surface of γ-Al2O3.CeO2is actually a nonstoichiometric compound,i.e.,CeO2-x,where oxygen vacant sites are considered to be present in the lattice in a randomized fashion[19].As oxygen deficiency can improve the mobility of oxygen atoms in the lattice, the modified ceria could provide active oxidation sites on the surface of CeAl support.In the calcination process of CeAl supported Cu(CH3COO)2,the active oxygen atoms of ceria react with adjacent Cu atoms and produce Cu+or Cu2+species,which can incorporate into the octahedral vacant sites of γ-Al2O3.While for 0.3CuCeAl-O2,as CeAl-O2support is calcined in O2atmosphere,the CeO2is oxidized greatly and the oxygen deficiency decreases sharply,which lowers the oxygen mobility significantly.Thus,the Cu species mainly exist as Cu0metal,which could not disperse effectively on the surface of γ-Al2O3.

2.2 H2-TPR

For comparison of the amount of dispersed Cu species further,H2-TPR was performed for CuAl and CuCeAl samples and the results are shown in Fig.3.A reduction peak at about 210℃appears in all samples,corresponding to the reduction of CuOxdispersed on γ-Al2O3.As the reduction temperature of Cu species for CeO2modified samples are very close to CuAl sample and the reduction temperature of dispersed CuO on CeO2support is 150～180℃[20],it can be concluded that the Cu species disperse on the surface of γ-Al2O3and CeO2has little promotion effect on the reduction of Cu species in our samples.For CuCeAl-O2,another peak at about 232℃can be observed.According to the literatures[21-22],the reduction of absorbed oxygen on the surface of ceria are about 226 and 268℃,so we attribute it here to the reduction of absorbed oxygen molecules on the surface of CeO2,which are absorbed during the pretreatment in oxygen atmosphere.

Fig.3 TPR profiles of supported Cu catalysts. (a)0.3CuAl(b)0.3CuCeAl-O2(c) 0.3CuCeAl-N2and(d)0.3CuCeAl-H2

The peak areas of the CuCeAl samples are different with the calcination atmosphere as shown in Fig.3.The peak areas for CuOxreduction of 0.3CuAl, 0.3CuCeAl-O2,0.3CuCeAl-N2and 0.3CuCeAl-H2are 14.5,12.1,35.1 and 42.2,respectively,which are orderedas:0.3CuCeAl-H2＞0.3CuCeAl-N2＞＞0.3CuCeAl-O2≈0.3CuAl.It should be noted that the H2consumption of 0.3CuCeAl-H2is about 20%larger than 0.3CuCeAl-N2,indicating that 0.3CuCeAl-H2has more Cux+than 0.3CuCeAl-N2.The reason should be thatCeAl-H2supportpretreatedinreductive atmosphere has more oxygen vacancies at the surface and therefore has more active oxygen atoms.For CuCeAl-O2,the area of CuO reduction peak is very weak,as well as 0.3CuAl,indicating that the CeAl-O2sample has very little oxygen deficiencies and poor oxygen mobility,thus has no promotion effect on the formation of Cu2+/Cu+.

2.3 Catalytic activity and selectivity of NO reduction by CO

Fig.4 is the activity and selectivity results of NO+CO reaction for 0.3CuAl and 0.3CuCeAl samples. The NO conversion displays the following order: 0.3CuCeAl-H2＞0.3CuCeAl-N2＞0.3CuCeAl-O2＞0.3CuAl,whichispositivelycorrelatedwiththe content of Cu2+/Cu+,implying that the dispersed Cu2+facilitates the conversion of NO.The N2selectivity at lower temperature(≤250℃)for 0.3CuCeAl-H2and 0.3CuCeAl-N2is very low,while that of 0.3CuAl is as high as about 80%as seen from the N2selectivity results(Fig.4b),and interestingly,the N2selectivity of 0.3CuCeAl-O2sample is just between the above two. Theresultsindicatethat0.3CuCeAl-H2and 0.3CuCeAl-N2convert NO mostly into N2O.It should be noted that the N2selectivity is in accordance with the content of crystalline Cu0metal.Therefore,it can be concluded that the crystalline Cu0metal is the active species for N2selectivity,i.e.,the Cu0promotes the conversion of N2O to N2[23].

To further investigate the effects of the state of copper species on the activity and selectivity of“NO+ CO”reaction,0.6CuCeAl-H2and 0.6CuAl samples were prepared and their XRD and H2-TPR results are shown in Fig.5.From XRD result(Fig.5a),the diffractionpeaksofcrystallineCu0metalfor 0.6CuCeAl-H2sample appear when the Cu loading increases to 0.6 mmol/100m2,indicating that the modified CeO2with loading of 0.07 mmol/100 m2is not enough to oxidize all Cu0species.Thus,some crystalline Cu0remains in the sample.Furthermore, the H2-TPR result(Fig.5b)shows that the peak area of 0.6CuCeAl-H2is much larger than that of 0.6CuAl, also indicating that 0.6CuCeAl-H2contains much more Cu2+/Cu+than 0.6CuAl.

Figures 5c and 5d are the activity results of NO+ CO reaction for CuCeAl-H2and CuAl samples.The NO conversion of CuCeAl-H2is obviously higher than corresponding CuAl samples as shown in Fig.5c, which further proves that the Cu2+/Cu+species are the active species for NO reduction by CO.In addition, 0.6CuCeAl-H2hasmuchhigherNOconversion activity than that of 0.3CuCeAl-H2.The NO turn-over frequency(TOF)of each copper ion(i.e.the NO turnover number of each copper ion at 250℃per hour)is presentedinTable1.0.6CuCeAl-H2stillkeeps relatively higher TOF.Meanwhile,as Fig.5d shows,both CuAl samples have rather high N2selectivity in low temperatures.The N2selectivity of 0.6CuCeAl-H2also increases obviously compared with 0.3CuCeAl-H2, indicating that the crystalline Cu0metal is the key species for low-temperature N2selectivity.Comparing the N2TOF at 250℃(N2TOF=(NO TOF)·(N2selectivity) (Table 1),it can be concluded that 0.6CuCeAl-H2has higher catalytic efficiency.

Fig.4 NO conversion(a)and N2selectivity(b)of 0.3CuAl and 0.3CuCeAl samples for“NO+CO”reaction

Fig.5 (a)XRD patterns of 0.6CuAl and 0.6CuCeAl-H2;(b)H2-TPR of 0.6CuAl and 0.6CuCeAl-H2; NO conversion(c)and N2selectivity(d)of some CuAl and CuCeAl samples for“NO+CO”reaction

Table1 NO TOF and N2TOF for NO+CO reaction of some CuAl and CuCeAl sample

3 Conclusions

The CeO2/γ-Al2O3supports have been pretreated in different atmospheres.The surface states suggest that the CeO2/γ-Al2O3support pretreated by reducing atmospherehasmoresurfaceoxygenvacancies, consequently has more active oxygen atoms.Therefore, the CuCeAl-H2has more dispersed Cu2+on the surface. NO+CO activity results reveal that dispersed Cu2+/Cu+istheactivespeciesforNOconversion,while crystalline Cu0metal promotes the N2selectivity at low temperatures,i.e.,facilitates the conversion of N2O to N2.Hence,the catalysts containing both dispersed CuO and a few crystalline Cu0species have the best catalytic efficiency.

Acknowledgements:The financial support of General Project of University Science Research of Jiangsu Province (12KJD530002)is gratefully acknowledged.Li Lulu in School of Chemistry,Nanjing University is also gratefully acknowledged for generous technical assistance.

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CuO/CeO2/γ-Al2O3Catalysts:Effect of Pretreating Atmosphere on Surface Properties and Catalytic Performance in Selective Catalytic Reduction of NO with CO

ZHU Jie＊GE Feng-Juan
(School of Chemistry and Chemical Engineering,Xuzhou Institute of Technology,Xuzhou,Jiangsu 221111,China)

The CeO2/γ-Al2O3supports were prepared and pretreated in different atmospheres to adjust the valence state of surface Ce element,then Cu(CH3COO)2was impregnated as the precursor to prepare CuCeAl samples. XRD and H2-TPR results indicate that the CeO2/γ-Al2O3support pretreated by reducing atmosphere has more active oxygen atoms,consequently has more dispersed Cu2+/Cu+species on the surface.The“NO+CO”reaction tests reveal that the dispersed Cu2+/Cu+are the active species for NO conversion,while crystalline Cu0promotes the N2selectivity at low temperatures.Hence,the catalysts containing both dispersed Cu2+/Cu+and a few crystalline Cu0have the optimal catalytic efficiency.

heterogeneous catalysis;nitrogen oxides;waste prevention;pretreating atmosphere;CeO2/γ-Al2O3;“NO+CO”reaction

O611.62

A

1001-4861(2015)01-0191-07

10.11862/CJIC.2015.021

2014-07-03。收修改稿日期：2014-10-15。

江蘇省高校自然科學基金(No.12KJD530002)資助項目。＊

。E-mail：zhujie19@xzit.edu.cn