Mesoporous Co3O4and Au/Co3O4Catalysts for
Low-Temperature Oxidation of Trace Ethylene Chun Yan Ma,†Zhen Mu,†Jin Jun Li,†Yong Gang Jin,‡Jie Cheng,†Gao Qing Lu,‡
Zheng Ping Hao,*,†and Shi Zhang Qiao*,‡
State Key Laboratory of En V ironmental Chemistry and Ecotoxicology,Rearch Center for
Eco-En V ironmental Sciences,Chine Academy of Sciences,Beijing100085,P.R.China,and
ARC Centre of Excellence for Functional Nanomaterials,Australian Institute for Bioengineering and Nanotechnology,The Uni V ersity of Queensland,QLD4072,Australia
Received August2,2009;E-mail:zpinghao@rcees.ac;s.qiao@uq.edu.au
Abstract:Low-temperature catalysts of mesoporous Co3O4and Au/Co3O4with high catalytic activities for the trace ethylene oxidation at0°C are reported in this paper.The catalysts were prepared by using the nanocasting method,and the mesostructure was replicated from three-dimensional(3D)cubic KIT-6silicas. High resolution transmission electron microscopy(HRTEM)studie
电化教学s revealed that{110}facets were the expod active surfaces in the mesoporous Co3O4,whereas the Co3O4nanosheets prepared by the precipitation method exhibited the most expod{112}facets.We found that the mesoporous Co3O4was significantly more active for ethylene oxidation than the Co3O4nanosheets.The results indicated that the crystal facet{110}of Co3O4played an esntial role in determining its catalytic oxidation performance. The synthesized Au/Co3O4materials,in which the gold nanoparticles were asmbled into the pore walls of the Co3O4mesoporous support,exhibited stable,highly disperd,and expod gold sites.Gold nanoparticles prent on Co3O4readily produced surface-active oxygen species and promoted ethylene oxidation to achieve a76%conversion at0°C,which is the highest conversion reported yet.
1.Introduction
Chemists and material scientists are driven to improve the performance of materials for technological applications.Materi-als’functions depend on their compositions,crystal structures, and morphologies.The properties of materials with the same composition but different structure or morphologies can vary substantially.1-4Scientists directed increasingly more effort on nanostructural organization to design functional materials.5The fundamental understanding has showed that structure and morphology control of ba transition-metal oxides allows preferential exposure of cata
lytically active sites.6
Co3O4has been reported to be an effective catalyst in the oxidation reaction and has also been ud as a support for noble metals.Co3O4is a versatile oxide that is involved in many advanced physical applications(magnetic properties)and in various heterogeneous catalysis process,for example,NO x reduction,7CO oxidation(with8,9or without gold promoter10),and vapor-pha oxidation of organic molecules.11Co3O4with different morphologies or structures such as nanospheres, nanocubes,nanorods,and mesoporous structures has been reported.9,12-16The properties of the as-synthesized Co3O4 materials strongly depend on their morphologies,crystal sizes, and expod crystal facets.6,9,12,17-19However,Co3O4catalysts are usually compod of assorted polycrystals with different expod crystal facets,posssing veral kinds of active sites, which exhibit different reactivities and usually with lower catalytic activities.9,12Recently,Hu et al.12reported that the unusually high index{112}crystal facets of Co3O4nanosheets were more reactive than the{011}facets of Co3O4nanobelts
†Chine Academy of Sciences.
‡The University of Queensland.
(1)Iijima,S.Nature1991,354,56–58.
部中(2)Pai,R.A.;Humayun,R.;Schulberg,M.T.;Sengupta,A.;Sun,J.N.;
Watkins,J.J.Science2004,303,507–510.
(3)Yang,H.G.;Sun,C.H.;Qiao,S.Z.;Zou,J.;Liu,G.;Smith,S.C.;
Cheng,H.M.;Lu,G.Q.Nature2008,453,639–641.
(4)Yang,H.G.;Liu,G.;Qiao,S.Z.;Sun,C.H.;Jin,Y.G.;Smith,S.C.;
Zou,J.;Cheng,H.M.;Lu,G.Q.(Max)J.Am.Chem.Soc.2009, 131,4078–4083.
(5)Co¨lfen,H.;Mann,S.Angew.Chem.,Int.Ed.2003,42,2350–2365.
(6)Cao,A.-M.;Hu,J.-S.;Liang,H.-P.;Song,W.-G.;Wan,L.-J.;He,
X.-L.;Gao,X.-G.;Xia,S.-H.J.Phys.Chem.B2006,110,15858–15863.
(7)Wichterlova´,B.Top.Catal.2004,28,131–140.
(8)Xu,X.Y.;Li,J.J.;Hao,Z.P.;Zhao,W.;Hu,C.Mater.Res.Bull.
2006,41,406–413.
(9)Xie,X.;Li,Y.;Liu,Z.-Q.;Haruta,M.;Shen,W.Nature2009,458,
746–749.
(10)Wang,C.B.;Tang,C.W.;Gau,S.J.;Chien,S.H.Catal.Lett.2005,
101,59–63.
(11)Vetrivel,S.;Pandurangan,A.J.Mol.Catal.A:Chem.2005,22,269–
278.
(12)Hu,L.;Peng,Q.;Li,Y.J.Am.Chem.Soc.2008,130,16136–16137.
(13)He,T.;Chen,D.R.;Jiao,X.L.;Xu,Y.Y.;Gu,Y.X.Langmuir
2004,20,8404–8408.
(14)Wang,Y.Q.;Yang,C.M.;Schmidt,W.;Spliethoff,B.;Bill,E.;
学习粤语
Schu¨th,F.Ad V.Mater.2005,17,53–56.
(15)Lakshmi,B.B.;Patrissi,C.J.;Martin,C.R.Chem.Mater.1997,9,
2544–2550.
(16)Feng,J.;Zeng,H.C.Chem.Mater.2003,15,2829–2835.
(17)Tian,Z.R.R.;Voigt,J.A.;Liu,J.;McKenzie,B.;McDermott,M.J.;
Rodriguez,M.A.;Konishi,H.;Xu,H.F.Nat.Mater.2003,2,821–826.
(18)Hu,J.S.;Ren,L.L.;Guo,Y.G.;Liang,H.P.;Cao,A.M.;Wan,
L.J.;Bai,C.L.Angew.Chem.,Int.Ed.2005,44,1269–1273. (19)Li,W.Y.;Xu,L.N.;Chen,J.Ad V.Funct.Mater.2005,15,851–857
.
Published on Web02/08/2010
10.1021/ja906274t 2010American Chemical Society 26089J.AM.CHEM.SOC.2010,132,2608–2613
and{001}facets of Co3O4nanocubes for methane catalytic combustion.While Xie et al.9reported that the Co3O4nanorods, which predominantly expod their{110}facets,favoring the prence of active Co3+species at the surface,exhibited a much higher activity for CO oxidation than that of conventional nanoparticles which mainly expod the{001}and{111}facets, containing only inactive Co2+sites.Therefore,the lective synthesis of nanostructured Co3O4catalysts with highly reactive crystal facets under nanoscale is a key to exploring different catalytic properties and applications.
Many efforts have been made to obtain porous Co3O4particles with controlled dimensions via ,by crystallizing inside the pores of both aperiodic and periodic silicas(com-mercial monoliths,MCM,SBA).10,11,20-25Basic issues on the nanocasting strategy have been recently reviewed.26However, very few published results are available on the reactive crystal planes of Co3O4materials prepared using the nanocasting method and their correlation with catalytic reaction activity. Highly disperd Au nanoparticles supported on metal oxides can show exceptionally high a
ctivities for veral kinds of reactions including CO oxidation,ozone decomposition,oxida-tion of hydrogen,and the CO+NO reaction.27-30Many studies have focud on the unusual low-temperature activity and the activation mechanism of gold catalysts.31-34The efficiency of supported Au nanoparticles for a low-temperature reaction depends on a variety of factors including,for example,the sizes of Au particles,the properties of supports,and their preparation procedures and pretreatment conditions.Although the support has activity in some reactions such as CO oxidation,the highly disperd Au nanoparticles appear to be responsible for the high activity of supported catalysts.35,36
Ethylene,a low molecular weight volatile organic compound (VOC)that posss both C-Cσand C-Cπbonds,is harmful,causing anesthetic illness and enhancing photochemical pollution.It is thus worthwhile to remove trace ethylene from some environments.For example,in fruit stores(such as refrigerated warehous),ethylene relead from fruits can accelerate aging and spoiling of produce.37,38In order to maintain freshness,the elimination of trace ethylene at low temperature(0°C)is required.Although the oxidation of ethylene is a thermodynamically favored process,ethylene is thermally stable and usually oxidized in the prence of catalysts. Until now,only limited solid thermal catalysts have been reported,but their catalytic activities are not sufficient and th
e operating temperatures are higher than100°C.38To the best of our knowledge,there has not been any report on ethylene oxidation over solid thermal catalysts at0°C.It is very difficult to activate and break the C-Cσbond of ethylene(unlike HCHO and CO)at a low reaction temperature of0°C;thus,more powerful catalytic materials are desirable.
Our recent work revealed that the Co3O4catalyst prepared by a precipitation method had no catalytic activity of trace ethylene oxidation at20°C,and the Au/Co3O4catalyst,with 2%gold loading prepared by a deposition-precipitation method, converted7.4%of trace ethylene at20°C.30Thus,this catalyst is not suitable in the elimination of trace ethylene becau of its insufficient activity at low temperature,as it cannot efficiently activate ethylene and break its C-Cσbond and produce carbon dioxide.Furthermore,the catalytic activation mechanism of Co3O4and Au/Co3O4catalysts in ethylene oxidation was still not clearly discusd.
Herein,we report the development of low-temperature catalysts of mesoporous Co3O4and Au/Co3O4and their excellent catalytic performance on the oxidation elimination of trace ethylene at low temperatures.We found more enhanced catalytic activity for ethylene oxidation over mesoporous Co3O4prepared by the nanocasting method compared with Co3O4nanosheets prepared by the precipitation method.The roles of the active crystal facets{110}of Co3O4catalysts on ethylene oxidati
on are discusd in this paper.Furthermore,we found that the mesoporous Au/Co3O4catalyst exhibited well-distributed Au nanoparticles on the porous Co3O4matrix,which provided a greater number of active gold sites and led to high catalytic activity for ethylene oxidation(76%conversion)at0°C.This is the highest reported conversion of ethylene oxidation at a low temperature.The study of the catalytic activation mechanism prented here is critically important to enhancing low-temper-ature catalytic activities in various oxidation reactions.
祝福结婚的成语2.Results and Discussion
2.1.Synthesis and Characterization of Co3O4and Au/ Co3O4Materials.The synthesis of mesoporous Co3O4material involves three substeps:(1)the impregnation and thermal decomposition of a cobalt precursor in the pore of three-dimensional(3D)cubic KIT-6silica with an ia3d mesostructure (hard template,e Figure S1-S3in the Supporting Information for relative characterizations);(2)the repeated impregnation and decomposition steps;(3)the removal of the hard template by NaOH solution etching.The obtained Co3O4grows along the direction of the KIT-6channels and replicates the3D meso-porous structure of KIT-6.The preparation of the mesoporous Au/Co3O4sample follows the same steps as in the synthesis of mesoporous Co3O4above but also involves impregnation of cobalt and gold precursors together.It is widely recognized that active pha dispersion is greatly infl
uenced by the affinity of
(20)Martinez,A.;Lopez,C.;Marquez,F.;Dias,J.J.Catal.2003,220,
486–499.
(21)Ohtsuka,Y.;Takahashi,Y.;Noguchi,M.;Arai,T.;Tarasaki,S.;
Tsubouchi,N.;Wang,Y.Catal.Today2004,89,419–429. (22)Khodakov,A.Y.;Griboval-Constant,A.;Bechara,R.;Zholobenko,
V.L.J.Catal.2002,206,230–241.
(23)Tian,B.Z.;Liu,X.;Solovyov,L.;Liu,Z.;Zang,H.;Zhang,Z.;Xie,
S.;Zhang,F.;Tu,B.;Yu,C.;Terasaki,O.;Zhao,D.J.Am.Chem.
Soc.2004,126,865–875.
(24)Crowley,T.A.;Ziegler,K.J.;Lyons,D.M.;Erst,D.;Olin,M.;Morin,
M.A.;Holmes,J.D.Chem.Mater.2003,15,3518–3522.
(25)Smatt,J.H.;Spliethoff,B.;Ronholm,J.B.;Linden,M.Chem.
Commun.2004,2188,2189.
(26)Liu,A.H.;Schu¨th,F.Ad V.Mater.2006,18,1793–1805.
(27)Haruta,M.;Tsbota,S.;Kobayashi,T.;Kageyama,H.;Genet,M.J.;
Delmon,B.J.Catal.1993,144,175–192.
(28)Hao,Z.P.;An,L.D.;Zhou,J.L.;Wang,H.L.React.Kinet.Catal.
Lett.1996,59,295–300.
(29)Hao,Z.P.;Cheng,D.Y.;Guo,Y.;Liang,Y.H.Appl.Catal.,B2001,
33,217–222.
(30)Li,J.J.;Ma,C.Y.;Xu,X.Y.;Yu,J.J.;Hao,Z.P.;Qiao,S.Z.
En V iron.Sci.Technol.2008,42,8947–8951.
(31)Tsai,H.-Y.;Lin,Y.-D.;Fu,W.-T.;Lin,S.D.Gold Bull.2007,43,
184–191.
(32)Date´,M.;Okumura,M.;Tsubota,S.;Haruta,M.Angew.Chem.,Int.
Ed.2004,43,2129–2132.
(33)Chen,Z.;Gao,Q.Appl.Catal.,B2008,84,790–796.
(34)Dobrosz-Go´mez,I.;Kocemba,I.;Rynkowski,J.M.Appl.Catal.,B
2008,83,240–255.
(35)Dobrosz-Go´mez,I.;Kocemba,I.;Rynkowski,J.M.Appl.Catal.,B
2008,83,240–255.
(36)Chen,Z.;Gao,Q.Appl.Catal.,B2008,84,790–796.(37)Park,D.R.;Ahn,B.-J.;Park,H.-S.;Yamashita,H.;Anpo,M.Korean
J.Chem.Eng.2001,18,930–934.
(38)Ahn,H.G.;Choi,B.M.;Lee,D.J.J.Nanosci.Nanotechnol.2006,
6,3599–3603.
J.AM.CHEM.SOC.9VOL.132,NO.8,20102609
Mesoporous Co3O4and Au/Co3O4Catalysts A R T I C L E S
the precursor to the support,and that this strong affinity facilitates good dispersion.39When mesoporous Au/Co 3O 4is synthesized,hydrolysis and aging of the gold chloride (HAuCl 4)solution at different pH conditions may vary the concentration of various gold species [Au(OH)n Cl 4-n ]-(n )1-3).The strongly adsorbing species Au (OH)3Cl -can be obtained at pH 7.Therefore,in this work the well-disperd gold nanoparticles attached to the Co 3O 4support were synthesized at pH 7.In the impregnation process,the solution mixture containing Au(OH)3Cl -and Co 2+precursor was introduced into the pores of KIT-6.After the samples were calcined and the hard template KIT-6was removed,ordered mesoporous Co 3O 4was obtained as a faithful replica of the template KIT-6and the gold nanoparticles were strongly adsorbed/embedded on the meso-porous Co 3O 4ba.
The low-angle X-ray diffraction (XRD)patterns of Co 3O 4and Au/Co 3O 4samples,with different Au c
ontents,synthesized using KIT-6as templates are shown in Figure 1A.Figure 1A(a)shows well-resolved diffraction peaks (211)and (332),and the shoulder peak (220),which are characteristic of a cubic ia 3d mesostructure,indicating a high degree of ordering of the mesoporous Co 3O 4material.Mesoporous Co 3O 4material is well-resolved in comparison to the characteristic diffraction peaks from its template KIT-6(e Figure S1in Supporting Informa-tion),indicating that the mesostructure of the Co 3O 4samples is a negative replica of KIT-6.A decrea of the structural ordering with an increa of the gold content can also be found for various Au/Co 3O 4catalysts.It shows that the adjustment of the pH value and the production of [Au(OH)n Cl 4-n ]-may interrupt the impregnation and decomposition of a cobalt precursor in the silica pore.Large angle XRD patterns of mesoporous Co 3O 4and Au/Co 3O 4materials exhibit peaks at 31.3°,36.9°,38.2°,44.5°,55.6°,59.4°,and 65.3°(2θ),40indicating that the cobalt precursor is turned completely into crystalline cobalt oxide (Figure 2).The diffraction peak at 38.2°corresponds to the Co 3O 4(222)plane.Becau the Au 0(111)diffraction peak overlaps with the Co 3O 4(222)diffraction peaks,it is difficult and not meaningful to calculate the gold particle size using the Scherrer equation.
The N 2sorption results (Figure 1B)show that all of the mesoporous Co 3O 4and Au/Co 3O 4materials exhibit type IV isotherms (IUPAC classification),indicating the prence of mesopores.41,
42However,the capillary condensation step of mesoporous Au/Co 3O 4is not very pronounced,indicating the relatively small sizes of ordered domains.43Nitrogen physisorp-tion data are in good agreement with the low-angle XRD results.The Barrett -Joyner -Halenda (BJH)pore size distributions of the mesoporous Co 3O 4and Au/Co 3O 4materials are shown in the int of Figure 1B.The Co 3O 4and Au/Co 3O 4materials have two primary mesopore diameters of 3.4and 11.5nm,respectively.A broad peak of pore size distribution at 11.5nm is probably attributed to interspace of the samples.Table 1shows structure parameters of the Co 3O 4and Au/Co 3O 4catalysts.The mesopore diameters of Au/Co 3O 4samples do not decrea remarkably with the increa of gold loading,indicating that most Au particles are studded on the surface or embedded in the pore wall.The Brunauer -Emmett -Teller (BET)surface areas (Table 1)of the mesoporous Co 3O 4and Au/Co 3O 4materials synthesized by the nanocasting method are much larger than tho of the Co 3O 4samples prepared by the precipitation method (about 15m 2·g -1).30
High angular annular dark-field scanning transmission elec-tron microscopy (HAADF-STEM)images of the mesoporous 2.5%Au/Co 3O 4materials are shown in Figure 3.The support Co 3O 4has Ia 3d symmetry and ordered mesostructures.The gold nanoparticles (smaller than 5nm),imaged as white luminous dots,are prented as pudospherical particles and incorporated in the Co 3O 4matrix.Such a mesoporous structure can thus
(39)Lee,S.-J.;Gavriilidis,A.J.Catal.2002,206,305–313.
(40)Li,J.J.;Xu,X.Y.;Hao,Z.P.;Zhao,W.J.Porous Mater.2008,15,
163–169.
(41)Sing,K.S.W.;Evrett,D.H.;Haul,R.A.W.;Moscou,L.;Pierotti,
R.A.;Rouqe ´rol,J.;Siemieniewska,T.Pure Appl.Chem.1985,57,603–619.
(42)Kruk,M.;Jaroniec,M.Chem.Mater.2001,13,3169–3183.(43)Rumplecker,A.;Kleitz,F.;Salabas,E.-L.;Schu ¨th,F.Chem.Mater.
2007,19,485–496
.
Figure 1.(A)Low-angle XRD patterns and (B)N 2adsorption/desorption
isotherms (int:pore size distribution calculated from desorption branch)of the mesoporous Co 3O 4and Au/Co 3O 4materials with different gold loadings:(a)Co 3O 4,(b)1.0%Au/Co 3O 4,(c)2.5%Au/Co 3O 4.The isotherm curves b and c in panel B are shifted by 15and 30cm 3·g -1,STP,respectively,for
clarity.
Figure 2.Large-angle XRD patterns of the mesoporous Co 3O 4and Au/Co 3O 4materials.(a)Co 3O 4,(b)1.0%Au/Co 3O 4,(c)2.5%Au/Co 3O 4.Table 1.Ethylene Oxidation Activities and Physical Properties of
Co 3O 4and Au/Co 3O 4Catalysts a
catalysts
C 2H 4conversion (%)S BET b (m 2/g)V p c (cm 3/g)淡淡幽香
D p d (nm)
Co 3O 4(3D)
30(0°C)840.14 3.6/11.71.0%Au/Co 3O 4(3D)50(0°C)940.17 3.4/11.42.5%Au/Co 3O 4(3D)76(0°C)1000.18 3.4/11.5Co 3O 4e (P)
0(20°C)---2%Au/Co 3O 4e (DP)
7.4(20°C)---
a
Initial concentration of ethylene is 50ppm.b BET specific surface areas determined from the linear part of the BET equation (P /P 0)0.05-0.25).c Total pore volumes obtained at P /P 0)0.99.d Pore size determined from the desorption branch using the BJH method.e
Reproduced with permission from ref 7.P:precipitation method,DP:deposition -precipitation method.
2610
J.AM.CHEM.SOC.
9
VOL.132,NO.8,2010
A R T I C L E S Ma et al.
effectively prevent gold nanoparticles from both aggregation and leaching becau of the confineme
nt of the channels.We can also note that the mesoporous channels are not blocked by the gold nanoparticles,thus greatly facilitating the transport of both reactant and product.For mesoporous Au/Co 3O 4materials,gold particles enter pores,embed into pore walls,and drill through the Co 3O 4walls,which may offer more active sites for the catalytic reaction.
Figure 4shows the O 2-temperature-programmed desorption (O 2-TPD)profiles of the fresh catalysts.The peak below 300°C is ascribed to the desorption of surface-active oxygen species such as O 2-and O -,and the desorption peak above 350°C is attributable to the desorption of lattice oxygen in Co 3O 4.44Generally speaking,the larger the corresponding desorption peak area of surface-active oxygen species at the low-temperature range,the higher the catalytic ability for oxidation reaction.29A broad and strong desorption peak from 50to 300°C for the 2.5%Au/Co 3O 4sample is evident.In contrast,a small desorption peak can be detected in Co 3O 4in a similar temperature range.Both surface-active oxygen species and lattice oxygen desorption peaks can also be obrved in the O 2-TPD profiles of the 1.0%Au/Co 3O 4samples.It is clear that the surface-active oxygen species of the Au/Co 3O 4catalysts increa with increasing gold content.
Figure 5shows the diffu reflectance infrared Fourier transform (DRIFT)spectra of surface species on mesoporous Co 3O 4and Au/Co 3O 4samples at 25°C in a flow of 50ppm C 2H 4/22%O 2/He.Se
veral IR peaks are obrved in the range of 1500to 4000cm -1.The bands appearing at 2340and 2360
cm -1are ascribed to the asymmetric stretch,νasym (OCO),of CO 2molecules adsorbed on the catalysts.45The bands obrved around 1641and 3246cm -1are attributed to the stretching vibration of carbon -carbon double bonds,ν(C d C),and the vibration of carbon -hydrogen bonds,ν(C -H),of C 2H 4mol-ecules,respectively.45,46Compared with Co 3O 4,the intensity of peak at 1641cm -1on Au/Co 3O 4increas,which suggests a relatively strong adsorption of reactant C 2H 4.Apart from C 2H 4and CO 2,no other ethylene oxides are detected in DRIFT measurements on mesoporous Co 3O 4and Au/Co 3O 4catalysts,thus indicating that only the complete oxidation of ethylene to CO 2occurs.
2.2.Low-Temperature Activity of Ethylene Oxidation.An ethylene concentration of 50ppm in the initial gas was ud to investigate catalyst activity in this study.Table 1also exhibits an ethylene conversion over Co 3O 4and Au/Co 3O 4catalysts.It is evident that the preparation method has a significant influence on catalytic performance.The 2.0%Au/Co 3O 4catalyst prepared by the deposition -precipitation method was less active at 0°C and can oxidize only 7.4%ethylene at 20°C.The Co 3O 4catalyst (without nanogold loading)prepared by the precipitation method does not have any ethylen
e oxidation activity even at 20°C.In contrast,the mesoporous Co 3O 4catalysts prepared using the nanocasting method drastically enhance the activity of ethylene oxidation,with an ethylene conversion of 30%at 0°C.In addition,the activity of mesoporous Au/Co 3O 4catalysts prepared using the nanocasting method is further enhanced when nan-ogold is introduced.The percentage of ethylene conversion is 50%over mesoporous 1.0%Au/Co 3O 4catalyst at 0°C.The mesoporous 2.5%Au/Co 3O 4catalyst shows much higher activity at 0°C (76%conversion of ethylene),which is around ten times the activity of the 2.0%Au/Co 3O 4catalyst at 20°C prepared by the deposition -precipitation method.
2.3.Correlation between Catalyst Structure and Activity.In this work,the mesoporous Co 3O 4samples replicate the 3D mesostructure of the KIT-6.Mesoporous Co 3O 4shows around 30%ethylene conversion to CO 2at 0°C.However,the Co 3O 4nanosheets prepared by the precipitation method have no activity of ethylene oxidation at 20°C.This indicates that structural control of Co 3O 4allows the preferential exposure of catalytically active sites.Figures 6and 7show HRTEM images of the Co 3O 4nanosheets prepared by the precipitation method and the mesoporous Co 3O 4and Au/Co 3O 4synthesized via nanocasting,respectively.The Co 3O 4nanosheets are hexagonal in shape with size ca.20-50nm.Both a t of {220}planes with a lattice
(44)Xue,L.;Zhang,C.;He,H.;Teraoka,Y.Appl.Catal.,B 2007,75,
167–174.
(45)Berna ´,A.;Kuzume,A.;Herrero,E.;Feliu,J.M.Surf.Sci.2008,602,
84–94.
(46)Chen,X.-M.;Yang,X.-F.;Zhu,A.-M.;Fan,H.-Y.;Wang,X.-K.;Xin,
Q.;Zhou,X.-R.;Shi,C.Catal.Commun.2009,10,428–432
.
Figure 3.HAADF-STEM images of the mesoporous 2.5%Au/Co 3O 4
materials obrved along different
directions.
Figure 4.O 2-TPD profiles of mesoporous Co 3O 4and Au/Co 3O 4with
different gold
化妆品保质期一般是多久
loadings.
Figure 5.DRIFT spectra of ethylene oxidation on mesoporous Co 3O 4and 2.5%Au/Co 3O 4catalysts.
J.AM.CHEM.SOC.
9李小龙是怎么死的
VOL.132,NO.8,20102611
Mesoporous Co 3O 4and Au/Co 3O 4Catalysts A R T I C L E S
space of 0.286nm and a t of {111}planes with a square crossing lattice space of 0.467nm were obrved for Co 3O 4nanosheets (Figure 6b).The dominant expod planes of Co 3O 4nanosheets are {112},which are perpendicular to both the first t of (220)planes and the cond t of (222)planes.12Figure 7shows HRTEM images of typical mesoporous Co 3O 4and Au/Co 3O 4.The upper left int in Figure 7a shows the fast Fourier transform (FFT)diffractogram from the mesoporous Co 3O 4with [110]direction.Figure 7a shows that the lattice planes of mesoporous Co 3O 4are both {111}with a lattice space of 0.467nm and {220}with a lattice space of 0.286nm.The {111
}planes are believed to be inactive crystal planes.9Furthermore,{220}planes with a lattice space of 0.286nm were also obrved for mesoporous Au/Co 3O 4(Figure 7b).For the mesoporous Co 3O 4and Au/Co 3O 4,the expod active planes are {110}planes parallel to {220}.The {110}planes are compod mainly of Co 3+cations,and the abundant Co 3+cations on the {110}planes provide sufficient sites for ethylene and oxygen adsorption and are regarded as the active sites for ethylene oxidation.When the reactants pasd through and adsorbed into the pores of mesoporous Co 3O 4,they were activated by the expod facets.Thus,we can conclude that the {110}planes are the mainly active planes for ethylene oxidation.This is in accord with recently published significant rearch,in which low-temperature oxidation of CO on Co 3O 4nanorods was systematically studied.9
In the preparation of mesoporous Au/Co 3O 4,the precursor AuCl(OH)3-is adsorbed into the pores of silica template,and the pore channels limit the growth of Au particles during the calcination process.Following removal of the hard template,the Au particles are embedded or partly enter the Co 3O 4pore walls.Au particles offer more active sites by entering pores,embedding into pore walls,and drilling through the walls of Co 3O 4(Figure 3).Seemingly,the well-disperd gold nanopar-ticles may be responsible for the enhancement of catalytic activity.It is well-known that active surface oxygen sp
ecies always exert a great influence on the catalytic activity in low-temperature oxidation.47The activity of the Au/Co 3O 4catalyst correlates with the surface-active ,the more a surface-active oxygen species occurs on the catalyst,the higher the catalyst activity.Au nanoparticles disperd on Co 3O 4play an important role in promoting the production of active oxygen species on the catalyst surface,which apparently leads to the enhanced oxidation activity of ethylene.It is obvious that the complete oxidation of ethylene to CO 2occurs on both Co 3O 4and Au/Co 3O 4catalysts,which is evidenced by the DRIFT measurements.Conquently,the gold nanoparticles improve the production of surface-active oxygen to increa catalytic activity but do not alter the reaction path of ethylene oxidation.
3.Conclusions
In summary,mesoporous Co 3O 4and Au/Co 3O 4catalysts have been successfully synthesized using the nanocasting approach and found to be highly active toward eliminating trace ethylene at 0°C.Our results show that 30%ethylene conversion on mesoporous Co 3O 4occurs at 0°C,but the Co 3O 4nanosheets prepared by the precipitation method do not have any catalytic activity at 20°C.Further investigation reveals that the structure control of Co 3O 4allows improvement of the catalytic ,mesoporous Co 3O 4,with reactive planes {110},are more active than Co 3O 4na
nosheets with {112}planes.Furthermore,the highest activity is found over mesoporous 2.5%Au/Co 3O 4catalyst (76%)becau of the surface-active oxygen species produced easily by the active sites of nanogold on porous Co 3O 4.The obtained mesoporous Co 3O 4and Au/Co 3O 4materials are effective low-temperature catalysts,which can break the C -C σand C -C πbonds at a low reaction temperature,even at 0°C.The catalytic materials have great potential applications for the efficient elimination of trace ethylene in,for example,warehou storage.The activation mechanism study in this work helps to understand the improvement of oxide catalyst activity.
4.Experimental Section
Synthesis of KIT-6Silicas.KIT-6mesoporous silicas were synthesized using tetraethoxysilane (TEOS)as a silica source and Pluronic P123(EO 20PO 70EO 20)as a structure-directing agent.48In a typical synthesis,P123(0.17mmol,1.0g),n -butanol (13.5mmol,1.0g),and HCl (35mL,0.6M)were stirred at 35°C until a homogeneous mixture formed.TEOS (2.08g)was then added and stirred for another 24h at the same temperature,followed by a hydrothermal treatment in an autoclave at 100°C for 24h.Following synthesis,the mixture was washed with distilled water until there was no foam in the water ud.The sample was then dried and calcined at 550°C for 3h to absolutely remove the P123template.The resulting white powder is 3D cubic KIT-6mesopo-rous silica.
Nanocasting Preparation of Mesoporous Au/Co 3O 4and Co 3O 4.Porous Au/Co 3O 4materials were prepared using 3D cubic KIT-6as a hard template.Typically,an aqueous solution of HAuCl 4·3H 2O (16.09mg Au/mL)was adjusted by 1M NaOH solution to pH 7and was then disperd in 21.5g of 7wt %solution of Co(NO 3)2·6H 2O in ethanol and stirred at room temperature for 2h.Subquently,the KIT-6was added into the solution containing gold and cobalt precursors.Ethanol was removed by the evaporation at 70°C for 12h,and the resulting powder was calcined at 300°C for 3h to completely decompo the nitrate species.The impregna-tion and decomposition steps were repeated once in order to achieve
(47)Chavadej,S.;Saktrakool,K.;Rangsunvigit,P.;Lobban,L.L.;
Sreethawong,T.Chem.Eng.J.2007,132,345–353.
(48)Shi,Y.;Meng,Y.;Chen,D.;Cheng,S.;Chen,P.;Yang,H.;Wan,
Y.;Zhao,D.Y.Ad V .Funct.Mater.2006,16,561–567
.
Figure 6.(a)TEM image and (b)HRTEM image of Co 3O 4nanosheets
prepared by the precipitation
method.
Figure 7.HRTEM images of mesoporous Co 3O 4(a)and Au/Co 3O 4(b)prepared by the nanocasting method.The inrt in panel a is the FFT diffractogram of the corresponding HRTEM image.
2612
J.AM.CHEM.SOC.
9
VOL.132,NO.8,2010
A R T I C L E S Ma et al.
和邻居
perfect nanocasting,but the amount of HAuCl4·3H2O and Co(NO3)2·6H2O ud in the repeated step was two-thirds of that ud in thefirst step.The silica template was then removed by etching in65mL of2M NaOH aqueous solution at80°C.The black Au/Co3O4material was recovered by centrifugation and dried at room temperature.The prepared Au/Co3O4material was named as mesoporous x%Au/Co3O4(x)1.0,2.5),where x is the calculated loading amount of gold,assuming that all the gold and Co3O4were incorporated in thefinal product.The mesoporous Co3O4samples were prepared by impregnating only Co(NO3)2·6H2O in the nanocasting process.
Material Characterization.Wide-angle XRD patterns were measured on X’pert PRO equipment using Cu K R radiation(λ) 0.15418nm)in the2θrange of10-70°with a scanning step size of0.006°.Small-angle XRD was recorded on a X’pert PRO powder diffraction system using Cu K R radiation in the2θ
range of 0.7-6.0°with a scanning step size of0.0016°.The textural properties of the samples were measured by N2sorption at liquid nitrogen temperature,using a gas adsorption analyzer NOVA1200. HAADF-STEM and HRTEM micrographs were obtained with a Tecnai G2F20u-TWIN instrument at an accelerating voltage of 200kV.The specimens were prepared by ultrasonic dispersion in ethanol,evaporating a drop of the resultant suspension onto a carbon support grid.O2-TPD tests were carried out in Micromeritics Chemisorb2720apparatus.Prior to each TPD run,the catalyst was pretreated in the Heflow at300°C in a quartz reactor.After the reactor temperature was lowered to room temperature,the catalyst adsorbed O2for30min under O2flow of50mL·min-1.Then He gas was fed into the reactor at50mL·min-1for30min to purge any residual oxygen.The catalyst was then heated to750°C at a constant heating rate of10°C·min-1under Heflow of50 mL·min-1.The desorbed oxygen was monitored by the TCD detector.Infrared spectra of the samples were recorded on a Bruker Tensor27using the DRIFT technique and scanned from4000to 600cm-1with256scans at a resolution of4cm-1.Before the DRIFT spectra were recorded,the sample was swept with He gas at150°C for30min,and then the catalyst bed temperature was lowered to25°C.The mixed gas(50ppm C2H4+22%O2in He), prepared using massflow controllers with a total gasflow of60 mL·min-1,was then pasd through the sample cell at25°C for 20min.
Activity Measurement of Catalysts for Ethylene Oxidation. Catalytic tests were performed using afixed bed reactor loading with0.25g catalyst(20-40mesh).The reaction feed consisted of 50ppm ethylene in synthetic air(O2:22%,N2:balance).The combined gasflow rate was maintained at60mL·min-1.The catalyst bed temperature was maintained at0°C,and the reactants and products were analyzed by using a gas chromatograph equipped with an FID detector(Porapak-R column).The gas chromatograph was directly connected to a Ni catalyst converter.Conversion was calculated on the basis of ethylene concentration in the effluent. Acknowledgment.This work wasfinancially supported by Nation-al Natural Science Funds for Distinguished Young Scholar(20725723), National Basic Rearch Program of China(2010CB732300),and the National High Technology Rearch and Development Program of China(2006AA06A310).S.Z.Q.acknowledges UQ Middle Career Rearch Fellowship andfinancial support by the Australian Rearch Council(ARC)through Discovery(DP1095861,DP0987969)and Linkage(LP0882681)programs.
Supporting Information Available:Low-angle XRD patterns, nitrogen physisorption isotherms,and TEM images of the mesoporous silica KIT-6.This material is available free of charge via the Internet at pubs.acs.
JA906274T
J.AM.CHEM.SOC.9VOL.132,NO.8,20102613
Mesoporous Co3O4and Au/Co3O4Catalysts A R T I C L E S
