Structure of the Cobalt-Based Oxide and Its Catalytic Performance in Oxidation of Toluene*

HU Xiuzhen,JIANG Chongwen,ZHU Jundong,LONG Jie,SONG Quzhi

(Department of Chemical Engineering,College of Chemistry and Chemical Engineering,Central South University,Changsha 410083,China)

1 Introduction

Volatile organic compounds (VOCs),emitted from industrial process and biogenic systems,are a great threat to the human health and the atmosphere[1-4].The strategies for elimination and treatment have attracted increasingly attention of many researchers[5-6].Catalytic oxidation of VOCs into harmless CO2and H2O is considered as one of the most environment-friendly and cost-effective strategies in the past few decades[7-9].Supported noble metals (Pt,Pd,and Au catalysts[10-13]) are well-known as their high activities for deep oxidation of VOCs.Thus,they have been widely used in industrial processes for abatement of exhausts.However,the high cost of noble metals has increased the interest in the development cheaper transition metal oxide catalysts with comparable activity of noble metal catalyst.Recently,Co3O4has received a high degree of interest because of significantly higher catalytic performance in oxidation of VOCs[14-16],[17]11 447.T Garcia et al[18]found Ordered Co3O4oxides performed well in the deep oxidation of a series representative VOCs.They demonstrated that the existence of a high concentration of Co2+ion on the surface explained the higher intrinsic activity.HU F Y et al[19]proposed that the excellent catalytic performances of Co3O4were associated with large surface area and high surface oxygen concentration.Jiang S J et al[20]conducted the oxidation of toluene over a series of Co3O4catalysts and found that Co3O4/CNTs (Carbon nano tubes) exhibited the best catalytic activity compared with cobalt oxides on other supported material (Beta Zeolite,ZSM-5,SBA-15),with a complete toluene conversion temperature at 257 ℃.They confirmed that the morphology or crystal plane of Co3O4could remarkably alter their catalytic performance.However,the combination of cobalt oxide with other different oxides often affects the textural,morphological,redox and catalytic activity.The synergistic effects have been found in different composite oxides such as cobalt,copper,manganese and chromium catalysts on which their catalytic activities are significantly promoted[21],[22]1 655,[23-24].Higher bulk oxygen mobility (through Mars-Van Krevelen mechanism) and formation of highly active oxygen species activated by the oxygen vacancies are widely recognized as the factors influencing the catalytic activity of Co3O4and Co3O4-CeO2binary oxides for VOCs catalytic oxidation[25]3 094.

Although numerous studies point out the successful use of cobalt in the oxidation VOCs,how the structure of the cobalt-based oxide is correlated with its catalytic the oxidation of VOCs is not clear.It is noted that the physical chemical properties and activity of catalysts are directly associated to their preparation method.Sol-gel method was preferred due to the advantages of obtaining mixed oxides with a good control of stoichiometry and nano size.In the present work,mixed oxides of Ce-Co,Mn-Co,and Cu-Co were synthesized through sol-gel method.The catalytic activity of these catalysts was evaluated through the oxidation of toluene,which is a very common VOCs in petrochemical and chemical industries.The attention focused on the maintenance of the catalyst stability was more than 50 hours.The prepared catalysts were characterized by using X-ray diffractometry (XRD),nitrogen adsorption-desorption,scanning electron microscope (SEM),H2temperature programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS).An attempt to elucidate the structural and surface composition with the catalytic activity will be studied.

2 Experimental

2.1 Preparation of Catalysts

Mixed-oxide catalysts were prepared by a citric acid sol-gel method.Briefly,nitrates with different cations were used as precursors (Cu(NO3)2·3H2O,50%(W.%) of Mn(NO3)2·4H2O,Ce(NO3)3·6H2O,Co(NO3)2·6H2O) in stoichiometric ratio of A∶Co=1∶1,which A is Mn,Ce and Cu.In a typical synthesis,6.04 g Cu(NO3)2·3H2O and 7.28 g Co(NO3)2·6H2O solution were added into 50 mL deionized water to obtain a mixed solution (1.0 mol/L metal ion concentration).Then,10.51 g citric acid was added into the nitrate solution.The obtained solution was stirred at 80 ℃ for 4 hours to form a gel.This gel was then dried at 110 ℃ for 10 hours and calcined at 500 ℃ (the heating rate of 5 ℃/min) for 3 hours to ensure the removal of carbonaceous residues and the formation of the mixed oxides labeled as Ce-Co.In addition,the single oxide of Co,the binary mixture of Cu-Co and Mn-Co were prepared by the same procedure.

2.2 Catalytic Performance Tests

Catalytic performance evaluation experiments were performed in a continuous flow quartz fixed-bed reactor (L=52 cm,Di=0.8 cm) at atmospheric pressure.About 500 mg of catalysts (40～60 mesh) was packed at the isothermal zone of the reactor.The reaction temperature was continuously monitored by a thermocouple tied on the reactor and positioned in the middle of the catalyst bed.5.0×10-3toluene with a total flow of 155 mL/min was generated by passing a N2flow through a bottle containing pure toluene chilled in an ethyl alcohol isothermal bath at 5 ℃.Reactive mixture was introduced over the catalysts with heating at a ramp rate of 5 ℃/min from room temperature to 300 ℃.The concentration of toluene was analyzed via a Shimadzu 2010 gas chromatograph equipped with a flame ionization detector (FID).All experimental runs were repeated three times under steady state conditions and the average conversion of toluene was employed.The complete conversion value of toluene (ηtoluene%) was calculated by the following equation:

where [toluene]inand [toluene]outare the inlet and outlet concentrations of toluene,respectively[26].

2.3 Characterization of Catalysts

XRD patterns of the catalysts were recorded on a RigakuMiniFlex 600 X-ray power diffractometer equipped with a Cu Kα radiation (λ=0.154 056 nm,40 kV).The XRD data was then generally collected in the 2θrange of 10°～80° with a step size of 0.01°.The BET surface area and pore size distributions of all samples were obtained with Nitrogen adsorption-desorption method at -196 ℃ through a Micromeritics ASAP2460 instrument.Before the measurement,samples were outgassed at 200 ℃ for 3 hours under primary vacuum to remove water and other atmospheric contaminants.The specific surface area (SSA) of each catalyst was determined according to the Brunaure-Emmett-Teller (BET) methods.The porous volume and the pore size distribution were obtained via the Barrett-Joyner-Halenda (BJH) methods.Temperature-programmed reduction (TPR) measurements were carried out with Auto Chem.II 2920 equipment (Micromeritics,USA),which the samples (200 mg) were packed into a quartz tube and heated in a 5 vol.% H2/Ar reducing gas mixture at 30 mL/min from room temperature to 900 ℃ by a heating rate of 10 ℃/min,and the detector signal was recorded continuously.The SEM images were taken by a Hitachi 1 080 apparatus.The working voltage was 15 kV.The sample powders were mounted on a double-side adhesive tape and observed at different magnification.XPS measurements were carried out on a 300 W Thermo Fisher Scientific apparatus with an Al Kα line radiation source.Each powder sample was pressed into the double-faced adhesive tape and degassed to remove the volatile contaminants.It was then transferred to the analyzing chamber for XPS analysis.Binding energies (BE) values were calibrated relative to the C 1s peak at 284.8 eV.

3 Results and Discussions

3.1 Catalytic Activities

Fig. 1 Toluene Oxidation Through Cu-Co,Mn-Co,Ce-Co Mixed Oxides and Co Oxide

Fig. 1 shows the catalytic performances of the catalysts in combustion of toluene within the temperature range 140 to 300 ℃.We found that the complete conversion temperature of toluene was lower than 300 ℃ for all the Co based catalysts,which showed better performance with respect to previous reports[27-28].It is worth noting that the binary mixing oxides showed higher activities than pure Co3O4catalyst,suggesting that there exists a certain kind of synergistic effect between Co and other metal oxides.This synergistic effect can significantly improve the catalytic activities for the oxidation of toluene.However,Ce-Co oxide are the most active catalyst in combustion of toluene witht10andt90values of 221 and 238 ℃ from Table 1,respectively.Consequently,the efficiency decreases in the order:Ce-Co>Mn-Co>Cu-Co>Co3O4.In addition,the stability of the Ce-Co oxide was evaluated at 99% of toluene conversion at 240 ℃ for 50 hours in Fig. 2.The conversion of toluene oxidation still maintained at about 99% after 50 hours.Therefore,the Ce-Co mixed oxide catalyst exhibits not only excellent catalytic performance but also good stability.

Table 1 Reaction Temperature for 10% and

Fig. 2 Stability Test of Toluene Oxidation Test Through Ce-Co Mixed Oxide

3.2 XRD Characterization

●—Co3O4；▲— Cu0.76Co2.24O4；◆— (Co,Mn)(Co,Mn)2O4;□— CuO；○— CeO2.Fig.3 XRD Patterns of Ce-Co,Mn-Co,Cu-Co Mixed Oxides and Co Oxide

The crystal structure of each type of catalyst was obtained by comparison with the standard powder diffraction files (PDF) from the International Centre for Diffraction Data (ICDD).The XRD patterns of all the catalysts are shown in Fig. 3.In the XRD pattern of pure cobalt oxides,a series of intensive and sharp diffraction peaks were observed which may ascribed to the cobalt oxide phase (PDF 42-1467).For Ce-Co oxide,the main crystal phases are visible at 2θ=28.6°,33.1°,47.6°,56.4°,which are ascribed to the (111),(200),(220),(311) crystal planes of CeO2(PDF 43-1002),respectively.Weak diffraction peak of,(311),(440),(511),(440) Co3O4(PDF 43-1003) was observed at 2θ=36.8°,45°,59°,65.3°,respectively.The typical diffraction peaks of spinel structure broaden and the intensity also decrease.In the Mn-Co oxides XRD pattern,a typical diffraction peak around 18.3°,29.4°,33.0°,36.5°,44.8°,59.1°,60.8° can be contributed to the (111),(202),(113),(311),(400),(511),(404) crystal plane of spinel structure (Co,Mn)(Co,Mn)2O4(PDF 18-0408),respectively.Diffraction peak of (111),(311),(440) Co3O4(PDF 43-1003) was observed at 2θ=19.0°,36.8°,65.3°,respectively.In terms of Cu-Co oxides,the appearance of intense peaks at 2θ=35.5°,38.7° revealed the presence of tenorite (111) CuO (PDF 48-1548),the peaks in 2θ=19.0°,31.3°,36.8°,44.7°,55.6° are ascribed to the presence of (111),(220),(311),(400),(422) Co3O4phase (PDF 43-1003),respectively.In addition to CuO and Co3O4phase,a mixed spinel oxide Cu0.76Co2.24O4(PDF 36-1189) in Cu-Co sample was detected.The characteristic peaks of this phase at 2θ=59.3°,65.1° ascribe to (511),(440) crystal plane.The obtained values in detail are listed in Table 2.The crystallite size of Ce-Co oxide is 6.3 nm,which is the smallest among the prepared catalysts,the mean crystallite sizes were further estimated according to the Debye-Scherer equation.More active sites over the particles on the Ce-Co oxide seemed generating from the small size of the crystallite[2]112.L F Liotta et al[27]and M M Schubert et al[29]also proposed that oxygen vacancies on the surface of the Co3O4played an important and favorable role in accelerating the adsorption and dissociation of oxygen molecules resulting in the formation of highly active electrophilic O-species.Although the active phase in the oxidation of toluene may involve Co3O4,the interaction between the CeO2and Co3O4phase facile would enhance the catalytic activity in the oxidation of toluene since both CeO2and Co3O4possess strong redox ability.

Table 2 Physicochemical Properties of Ce-Co,Mn-Co,Cu-Co Mixed Oxides and Co Oxide

3.3 N2 Adsorption Desorption Isotherms

BET surface areas,pore size and pore volume are presented in the Table 2.With respect to Co oxide,Mn-Co and Cu-Co,Ce-Co mixed oxide shows the highest surface area value (31 m2/g).The corresponding average pore size of Ce-Co,Mn-Co,Cu-Co mixed oxide and Co oxide are 6.9,15.9,24.7,18.7 nm,respectively.The Ce-Co oxide with the highest surface area shows the most active catalytic performance in the oxidation of toluene in the Fig. 1.The excellent catalytic performances of catalysts were associated with large surface area,because higher surface area could provide more active sites for catalytic reaction[2]115.

3.4 Scanning Electron Microscope (SEM)

Fig. 4 shows the morphology and particle size distribution of prepared catalysts.All these oxides consist of nano-sized aggregates.The mean particle size of Ce-Co mixed oxide is obviously smaller than that of Mn-Co,Cu-Co and Co3O4oxide.As we expected there is a reverse relation between particle size and specific surface area of sample (Table 2).

Fig. 4 SEM Images of Ce-Co (a),Mn-Co (b),Cu-Co (c) Mixed Oxides and Co3O4 (d)

3.5 H2 Temperature Programmed Reduction (H2 -TPR)

Fig. 5 H2-TPR Profiles of Ce-Co,Mn-Co,Cu-Co Mixed Oxides and Co Oxide

H2-TPR was carried out to investigate the reducibility of the series of catalysts.In Fig. 5,a broad overlapping peak appears around 441 ℃ in the H2-TPR curve of pure cobalt oxide,indicating to the reduction of cobalt oxide:Co3O4→CoO→Co[17]11 453.The H2-TPR of Ce-Co oxide yields two reduction peaks at temperatures of 280 and 512 ℃,respectively,the lower temperature is associated with the reduction reactions of Co3O4,the peak at 512 ℃ is ascribed to the reduction of CeO2[30-33].In the case of H2-TPR curve of Mn-Co oxide,a first peak at around 456 ℃ indicates to the reduction of Mn3O4in spinel environment to MnO.The weak reduction peak around 645 ℃ ascribes to the reduction of Co2+metallic cobalt[22]1 658.In terms of Cu-Co oxides,the broad overlapping peak at 300° can be assigned to the reduction of Cu2+→Cu0and Co3+→Co2+[25]3 093,[34].The total H2consumption in the temperature range of 100～800 ℃ of cobalt oxide-base catalysts was given in Table 2.However,the catalytic activity of toluene oxidation corresponds to the less total H2consumption of TPR.We conclude that total H2consumption of TPR involves surface oxygen and bulk oxygen in the catalyst,and the toluene oxidation is mainly governed by the amount of available surface oxygen species.The most activity of Ce-Co catalyst may relate to the highest surface oxygen species in spite of the least total H2consumption.

3.6 X-Ray Photoelectron Spectroscopy (XPS)

Fig. 6 O 1s XPS Spectra of Ce-Co,Mn-Co,Cu-Co Mixed Oxides

Fig. 6 is the O 1s XPS spectra of the Ce-Co,Mn-Co and Cu-Co mixed oxides,which show O 1s level peaks in which two oxygen species can be distinguished by deconvolution.The dominant peak at lower binding energy of about 529.3,529.8 and 529.9 eV represents the lattice oxygen (Olatt).The higher binding energy at 531.3 and 531.8 eV is attributed to the adsorbed oxygen (oxygen vacancy,Oads).It has been elucidated that oxygen vacancy is very important for catalyst's physicochemical property because of its more surface active oxygen species for oxidation reaction[35]4.The ratio of Oads/Olattof Ce-Co,Mn-Co and Cu-Co mixed oxide is 0.5,0.45,0.33,respectively.High oxygen vacancy might be caused from the strong interaction between CeO2and Co3O4.The presence of CeO2in the mixtures enhanced the mobility of the lattice oxygen species offers more surface-active oxygen species for toluene oxidation reaction,which agreed with the result of H2-TPR.These results could explain that the Ce-Co exhibited the highest catalytic activity for toluene oxidation among all the prepared catalysts.

Mars-Van Krevelen mechanism is generally accepted to take account of the oxidation mechanism of toluene,which involves adsorption of toluene,and its subsequent oxidation by lattice oxygen and adsorbed oxygen atoms.This mechanism can be depicted in Fig. 7.In terms of oxidation of toluene,Ce4+/Ce3+and Co3+/Co2+redox couples may facilitate the surface oxygen species and high mobility of the oxygen.The generation of active oxygen species activated by oxygen vacancies and oxygen mobility and are the main factors that affect the catalytic activity of Co3O4-CeO2binary oxides for toluene catalytic oxidation[25]3 094,[35]4,[36].

Fig. 7 Illustration of Synergistic Effect of CeO2 and Co3O4 in Oxidation of Toluene

4 Conclusion

In summary,different composites of cobalt oxide-based catalysts were successfully synthesized via sol-gel method.The binary mixing oxides show higher activities than pure Co3O4,involving in the synergistic effect between Co3O4and other metal oxides.The Ce-Co oxide catalyst shows the best catalytic activity with thet90of 238 ℃ and good stability in the oxidation of toluene.The characteristics obtained suggest that the toluene oxidation over the cobalt oxide-based catalysts is governed by the amount of surface oxygen species in the mixed oxides.Ce4+/Ce3+and Co3+/Co2+redox couples in the Ce-Co catalyst may facilitate the mobility of oxygen and improve the reducibility of surface oxygen accounting for the synergetic effect in the oxidation of toluene.

:

[1] HUANG H B,XU Y,FENG Q Y,et al.Low Temperature Catalytic Oxidation of Volatile Organic Compounds:A Review[J].Catalysis Science & Technology,2015,5(5):2 649-2 669.

[2] TANG W X,WU X F,LI S D,et al.Co-Nanocasting Synthesis of Mesoporous Cu-Mn Composite Oxides and Their Promoted Catalytic Activities for Gaseous Benzene Removal[J].Applied Catalysis B:Environmental,2015,162:110-121.

[3] JI Y M,ZHAO J,TERAZONO H,et al.Reassessing the Atmospheric Oxidation Mechanism of Toluene[J].Proceedings of the National Academy of Sciences of the United States of America,2017,114(31):8 169-8 174.

[4] LEE-TAYLOR J,MADRONICH S,AUMONT B,et al.Explicit Modeling of Organic Chemistry and Secondary Organic Aerosol Partitioning for Mexico City and Its Outflow Plume[J].Atmospheric Chemistry and Physics,2011,11(24):13 219-13 241.

[5] ROKICINSKA A,DROZDEK M,DUDEK B,et al.Cobalt-Containing BEA Zeolite for Catalytic Combustion of Toluene[J].Applied Catalysis B:Environmental,2017,212:59-67.

[6] YI H H,YANG Z Y,TANG X L,et al.Promotion of Low Temperature Oxidation of Toluene Vapor Derived from the Combination of Microwave Radiation and Nano-Size Co3O4[J].Chemical Engineering Journal,2018,333:554-563.

[8] SIHAIB Z,PULEO F,GARCIA-VARGAS J M,et al.Manganese Oxide-Based Catalysts for Toluene Oxidation[J].Applied Catalysis B:Environmental,2017,209:689-700.

[9] TANG W X,WU X F,LIU G,et al.Preparation of Hierarchical Layer-Stacking Mn-Ce Composite Oxide for Catalytic Total Oxidation of VOCs[J].Journal of Rare Earths,2015,33(1):62-69.

[10] ABOUKAIS A,SKAF MS,HANY S,et al.A Comparative Study of Cu,Ag and Au Doped CeO2in the Total Oxidation of Volatile Organic Compounds (VOCs)[J].Materials Chemistry and Physics,2016,177:570-576.

[11] HE C,LI J R,ZHANG X Y,et al.Highly Active Pd-Based Catalysts with Hierarchical Pore Structure for Toluene Oxidation:Catalyst Property and Reaction Determining Factor[J].Chemical Engineering Journal,2012,180(1):46-56.

[12] SANZ O,DELGADO J J,NAVARRO P,et al.VOCs Combustion Catalyzed by Platinum Supported on Manganese Octahedral Molecular Sieves[J].Applied Catalysis B:Environmental,2011,110(45):231-237.

[13] SOLSONA B,AYLO'N E,MURILLO R,et al.Deep Oxidation of Pollutants Using Gold Deposited on a High Surface Area Cobalt Oxide Prepared by a Nanocasting Route[J].Journal of Hazardous Materials,2011,187(1/2/3):544-552.

[14] CAI T,HUANG H,DENG W,et al.Catalytic Combustion of 1,2-Dichlorobenzene at Low Temperature over Mn-Modified Co3O4Catalysts[J].Applied Catalysis B:Environmental,2015,166/197:393-405.

[15] REN Q,MO S,PENG R,et al.Controllable Synthesis of 3D Hierarchical Co3O4Nanocatalysts with Various Morphologies for the Catalytic Oxidation of Toluene[J].Journal of Materials Chemistry A,2018,6(2):498-509.

[16] CARABINEIRO S A C,CHEN X,KONSOLAKIS M,et al.Catalytic Oxidation of Toluene on Ce-Co and La-Co Mixed Oxides Synthesized by Exotemplating and Evaporation Methods[J].Catalysis Today,2015,244:161-171.

[17] GOMEZ D M,GALVITA V V,GATICA J M,et al.TAP Study of Toluene Total Oxidation over a Co3O4/La-CeO2Catalyst with an Application as a Washcoat of Cordierite Honeycomb Monoliths[J].Physical Chemistry Chemical Physics,2014,16(23):11 447-11 455.

[18] GARCIA T,AGOURAM S,SANCHEZ-ROYO J F,et al.Deep Oxidation of Volatile Organic Compounds Using Ordered Cobalt Oxides Prepared by a Nanocasting Route[J].Applied Catalysis A:General,2010,386(1/2):16-27.

[19] HU F Y,CHEN J J,PENG Y,et al.Novel Nanowire Self-Assembled Hierarchical CeO2Microspheres for Low Temperature Toluene Catalytic Combustion[J].Chemical Engineering Journal,2018,331:425-434.

[20] JIANG S J,SONG S Q.Enhancing the Performance of Co3O4/CNTs for the Catalytic Combustion of Toluene by Tuning the Surface Structures of CNTs[J].Applied Catalysis B:Environmental,2013,140:1-8.

[21] DU Y C,MENG Q,WANG J S,et al.Three-Dimensional Mesoporous Manganese Oxides and Cobalt Oxides:High-Efficiency Catalysts for the Removal of Toluene and Carbon Monoxide[J].Microporous and Mesoporous Materials,2012,162:199-206.

[22] HOSSEINI S A,NIAEI A,SALARI D,et al.Nanocrystalline AMn2O4(A=Co,Ni,Cu) Spinels for Remediation of Volatile Organic Compounds—Synthesis,Characterization and Catalytic Performance[J].Ceramics International,2012,38(2):1 655-1 661.

[23] HOSSEINI S A,NIAEI A,SALARI D,et al.Study of Correlation Between Activity and Structural Properties of Cu-(Cr,Mn and Co)2Nano Mixed Oxides in VOC Combustion[J].Ceramics International,2014,40(4):6 157-6 163.

[24] HOSSEINI S A,ALVAREZ-GALVAN M C,FIERRO J L G,et al.MCr2O4(M=Co,Cu,and Zn) Nanospinels for 2-Propanol Combustion:Correlation of Structural Properties with Catalytic Performance and Stability[J].Ceramics International,2013,39(8):9 253-9 261.

[25] LIOTTA L F,WU F,PANTALEO G,et al.Co3O4Nanocrystals and Co3O4-MOxBinary Oxides for CO,CH4and VOC Oxidation at Low Temperatures:A Review[J].Catalysis Science & Technology,2013,3(12):3 085-3 102.

[26] LIAO Y N,ZHANG X,PENG R S,et al.Catalytic Properties of Manganese Oxide Polyhedra with Hollow and Solid Morphologies in Toluene Removal[J].Applied Surface Science,2017,405:20-28.

[27] LIOTTA L F,OUSMANE M,DI CARLO G,et al.Catalytic Removal of Toluene over Co3O4-CeO2Mixed Oxide Catalysts:Comparison with Pt/Al2O3[J].Catalysis Letters,2008,127(3/4):270-276.

[28] AGUILERA D A,PEREZ A,MOLINA R,et al.Cu-Mn and Co-Mn Catalysts Synthesized from Hydrotalcites and Their Use in the Oxidation of VOCs[J].Applied Catalysis B:Environmental,2011,104(1/2):144-150.

[29] SCHUBERT M M,HACKENBERG S,VAN VEEN A C,et al.CO Oxidation over Supported Gold Catalysts-"Inert" and "Active" Support Materials and Their Role for the Oxygen Supply During Reaction[J].Journal of Catalysis,2001,197(1):113-122.

[30] LIU Y Xi,DAI H X,DENG J G,et al.Mesoporous Co3O4-Supported Gold Nanocatalysts:Highly Active for the Oxidation of Carbon Monoxide,Benzene,Toluene,ando-Xylene[J].Journal of Catalysis,2014,309:408-418.

[31] WANG C,ZHANG C H,HUA W C,et al.Catalytic Oxidation of Vinyl Chloride Emissions over Co-Ce Composite Oxide Catalysts[J].Chemical Engineering Journal,2017,315:392-402.

[33] HOSSEINI S A,SALARI D,NIAEI A,et al.Chemical-Physical Properties of Spinel CoMn2O4Nano-Powders and Catalytic Activity in the 2-Propanol and Toluene Combustion:Effect of the Preparation Method[J].Journal of Environmental Science and Health.Part A,Toxic/Hazardous Substance & Environmental Engineering,2011,46(3):291-297.

[34] CARRILLO A M,CARRIAZO J G.Cu and Co Oxides Supported on Halloysite for the Total Oxidation of Toluene[J].Applied Catalysis B:Environmental,2015,164(4):443-452.

[35] MENON U,GALVITA V V,MARIN G B.Reaction Network for the Total Oxidation of Toluene over CuO-CeO2/Al2O3[J].Journal of Catalysis,2011,283(1):1-9.

[36] TODOROVA S,NAYDENOV A,KOLEV H,et al.Mechanism of Completen-Hexane Oxidation on Silica Supported Cobalt and Manganese Catalysts[J].Applied Catalysis A:General,2012,413(4):43-51.