(College of Chemistry and Materials Engineering,Bohai University,Jinzhou,Liaoning 121013,China)

Abstract:CoxZr1-xO2catalysts were prepared by precipitation,hydrothermal,thermal decomposition and impregnation methods.The physicochemical properties of the catalysts were characterized by thermogravimetric(TG),X-ray diffraction(XRD),scanning electron microscope(SEM),energy dispersive X-ray spectroscopy(EDS),N2adsorption/desorption test.The results showed that the cobalt species and zirconium species on the catalysts prepared by hydrothermal method had good dispersion,uniform spherical particles,and large specific surface area up to 102 m2·g-1,and well-developed pore structure.The activity of oxidized toluene and formaldehyde on the catalyst CoxZr1-xO2-H(1∶1)prepared by hydrothermal method consisting of Co and Zr was better than that of other catalysts.The apparent activation energies were 65.2 and 53.6 kJ·mol-1.The excellent catalytic performance of CoxZr1-xO2-H(1∶1)was associated with the species dispersion,strong synergistic effect,surface area and pore structure.The well-controlled method for creation of Co-Zr catalysts with good physicochemical properties could be adopted for the generation of other catalytic materials with mixed metal oxides.

Keywords:heterometallic compound oxide;heterogenous catalysis;porous materials;volatile organic compounds oxidation;synthesis methods

With the rapid development of urbanization and industrialization,more and more volatile organic compounds(VOCs)are emitted into the atmosphere,whichnot only pollute the environment,but also seriously endanger human health[1-2].At present,strict standards and legislation have been established to control VOCsemission.At the same time,researchers are also developing new VOCs terminal processing technologies,such as activated carbon absorption[3],non thermal plasma removal[4],biological filtration[5]and catalytic combustion[6-7].Catalytic combustion has become a hotspot research field due to its low light-off temperature,low energy consumption,high efficiency and no secondary pollution,etc[7-8].Common catalysts are noble metal[9-10]and transition metal oxide[11-13]catalysts.Although the activity of transition metal oxide catalyst is lower than that of noble metal catalyst,it has gained more attention due to its advantages such as low prices,easy availability and good reduction performance at low temperature[11-15].Different compositions and preparation methods of catalysts affect their physicochemical properties such as bulk phase and surface composition,active site distribution,crystal phase structure,particle size and pore structure,reduction and adsorption performance[16-21],as well as catalytic removal performance of VOCs[22-26].Different preparation methods of the same catalyst components and different impregnation sequence of the active components of multi-component catalysts will also affect the catalytic performance[24-27].Therefore,it is of great significance to select a suitable preparation method for the catalyst.

Toluene and formaldehyde are the main components of the VOCs.A study on the catalytic oxidation of toluene and formaldehyde by metal oxide catalyst has been very active.For example,Li et al.[28]reported the catalyticperformanceoftolueneonCe Mntwo component catalyst,and found a strong synergistic effect between cerium species and manganese species.Yan et al.[29]prepared Co3O4nanoflower clusters by lowtemperature hydrothermal method and evaluated the catalytic removal performance of toluene.They found that the good crystallinity and porous structure of Co3O4 nanoflower clusters were the main factors influencing the oxidation of toluene.Zhao et al.[30]prepared CuCe0.75Zr0.25/TiO2catalyst by impregnation method,which can achieve complete oxidation of toluene at 234℃,which is related to reactive oxygen species,low temperature reducibility and species dispersion of the catalyst.Feng et al.[31]also prepared the Co/Sr CeO2catalyst by dipping method.The doping of Sr increased the specific surface area of the carrier CeO2,promoted the synergistic effect of Co and Ce and the dispersion of Co,and achieved the complete oxidation of toluene at 330℃.Formaldehyde,a typical oxygen-containing volatile organic compound,is widely existed in indoor air due to its extensive application of building materials.Zhang et al.[32]synthesized Al-rich β zeolite supported Pt catalyst for the formaldehyde oxidation,and found that 1%(mass fraction)Pt/H-β-SDS-4 could completely oxidize HCHO at room temperature,which was due to the rich acid level and high dispersion of Pt.

Non-noble metal oxides are commonly used as active components of catalysts for the removal of VOCs.Cobalt oxide and zirconia are also commonly used as support of catalysts[24,33]and as one of the active components of catalysts to remove VOCs[24-26,34].Wyrwalski et al.[23-24]prepared highly dispersed Co/ZrO2catalyst and studied its oxidation performance of toluene.It was found that a new cobalt species was formed on the surface of Co/ZrO2catalysts,which showed good reducibility at low temperature.Dong et al.[27]had studied the preparation of CuO CeO2 ZrO2catalyst by sol gel,co precipitation,one-step impregnation,and two step impregnation methods.They found that different preparation methods affected the dispersion degree of active components and the interaction between the oxides,which further affected the catalytic oxidation of CO.

In this work,CoxZr1-xO2catalysts were prepared by precipitation method,hydrothermal method,thermal decomposition method and impregnation method.The physicochemical properties were investigated by thermogravimetric(TG),X-ray diffraction(XRD),scanning electron microscope(SEM),energy dispersive X ray spectroscopy(EDS),N2adsorption/desorption test and the performance of oxidized toluene or formaldehyde was evaluated.

1 Experimental

1.1 Preparation of catalysts

1.1.1Precipitation method

According to the molar ratio of nCo∶nZr=3∶1,2.91 g Co(NO3)2·6H2O and 1.43 g Zr(NO3)4·5H2O were com-pletely dissolved in 100 mL distilled water,and ammonia was added drop by drop while stirring until solution pH=9.The precipitate was obtained after 12 hours at room temperature and filtered and washed with distilled water and ethanol for three times,respectively.The precursor sample was obtained dried in an oven at 60℃.It was further calcined in air at 500℃for 3 h with a heating rate of 1℃·min-1.The catalyst was obtained,which was designated as CoxZr1-xO2-P.

1.1.2Hydrothermal method

The same amount(nCo∶nZr=3∶1)of Co(NO3)2·6H2O and Zr(NO3)4·5H2O were measured and dissolved in distilled water.Then,3.0 g urea was added to continue stirring.Subsequently,the mixed solution was transferred to a 100 mL Teflon-lined stainless steel autoclave,which was placed in an oven at 60℃for 6 hours.After cooling,the mixture was centrifuged and separated,washed with distilled water and ethanol for three times respectively and dried to obtain precursor sample,CoxZr1-xO2-H was obtained under the same calcination conditions as precipitation method.Catalysts CoxZr1-xO2-H(1∶1)and CoxZr1-xO2-H(1∶2)were prepared by changing the molar ratio of Co and Zr to 1∶1 and 1∶2.

1.1.3Thermal decomposition method

The same amount of Co(NO3)2·6H2O and Zr(NO3)4·5H2O were ground in an agate mortar.3.0 g citric acid(C6H8O7·H2O)was added,and the mixture was further ground to powder to get precursor sample.It was calcinedunderthesameconditionstoprepareCoxZr1-xO2-T.

1.1.4Impregnation method

According to the molar ratio of nCo∶nZr=3∶1,2.91 g Co(NO3)2·6H2O was accurately weighed and dissolved in 100 mL deionized water,then 0.41 g ZrO2(The decomposition product of zirconium nitrate at 500℃)was added.Ammonia was dropped with stirring until the solution pH=9,and allowed to stand for 12 hours.The precipitate was subjected to the same treatment to obtain CoxZr1-xO2-I.

1.2 Catalyst characterization

TG curves were measured by the Pyris Diamond thermal analyzer.Test conditions are as follows:the temperature was raised from room temperature to800℃with a rate of 10℃·min-1in an air atmosphere of 20 mL·min-1.

XRD patterns of the samples were collected on a Rigaku UltimaⅣType instrument operated at 40 kV and 40 mA using Cu Kα radiation source and nickel filter(λ=0.154 06 nm).The scanning range was 10°～80°.The XRD pattern was compared with the PDF standard database.

SEM images of the samples were recorded on the S-4800 apparatus operating at 20 kV.EDS DX-4 instrument was used to obtain the energy dispersion X ray spectrum(EDS)for chemical composition and distribution analysis.

The nitrogen adsorption and desorption isotherms were measured at-196℃on a Micromeritics ASAP 2020 apparatus.The samples were degassed at 110℃for 24 hours before the measurement.BET(Brunauer-Emmett Teller)andBJH(Barrette Joyner Halenda)method was used to calculate the specific surface area and pore size distribution of the catalysts,respectively.

1.3 Evaluation of catalyst activity

The catalytic oxidation performance of the catalyst was evaluated by using the oxidation of toluene or formaldehyde as probe.100 mg of catalyst was loaded in the middle of the quartz tube and placed inside a temperature-controlled thermocouple furnace with a temperature range from room temperature to 400℃.The volume fraction of toluene or formaldehyde concentration was 0.05%.The reaction gas was composed of toluene(or formaldehyde)and oxygen at a molar ratio of 1∶400 with a space velocity of 20 L·g-1·h-1.The outlet gases were analyzed using an on-line gas chromatograph(Agilent 6890N)to determine gas compositions.Theinlettemperature,columntemperatureand detector temperature were 190,180 and 200℃,respectively.

2 Results and discussion

2.1 TG analysis

Fig.1 shows the TG analysis curve of the precursor samples prepared by different methods.Different preparation methods lead to different TG behaviors.As can be seen from the Fig.1a,precursor sample pre-pared by precipitation method had a slow weight loss region when the temperature was lower than 180℃.At the temperature range of 160～280℃the weight loss was 33%.The maximum weight loss reached 46%when the temperature exceeded 400℃.The TG curve of the precursor obtained by hydrothermal method is shown in Fig.1b.A relatively obvious weight loss peak appears at 208,280 and 370℃,respectively.The maximum weight loss reached 28%after 430℃.Fig.1c shows the TG curve of the precursor obtained by thermal decomposition,which has two obvious loss weight regions.The first stage appeared in the range of 30～170℃,with weight loss reaching 52%.The second stage appeared in the range of 170～400℃,with weight loss of 24%.The peak temperature of weight loss was 139 and 335℃,respectively.There is no weight loss after 400℃.The thermal analysis curve of precursor obtained by impregnation method is shown in Fig.1d.There are two small weight loss peaks at 160～190℃and 190～240℃,with weight loss of 8%and 5%,respectively.The peak temperatures appeared at 171 and 208℃.Above 300℃,there is no weight loss.The total weight loss was 13%.In conclusion,the precursor samples of Co-Zr oxide catalysts prepared by four different methods can be completely decomposed at 430℃or above.In the following experiments,the calcining temperature is chosen as 500℃to ensure the complete decomposition of precursor samples.

Fig.1 TG curves of catalyst precursor of(a)CoxZr1-xO2-P,(b)CoxZr1-xO2-H,(c)CoxZr1-xO2-T and(d)CoxZr1-xO2-I

2.2 Crystal structure

The XRD patterns of the 2θ range of 10°～80°are shown in Fig.2 for catalysts prepared by different methods.The right figure in Fig.2 shows a pattern zoomed in on the region of strong diffraction peak.The strongest diffraction peak in Fig.2a appears at 2θ=30.78°,and the diffraction peak in Fig.2d appears at 2θ=30.27°and 31.27°.While the diffraction peak in this region in Fig.2b and c shows a certain deviation.Compared with the standard pattern,the XRD patterns of CoxZr1-xO2by different methods are constituted of peaks attributed to ZrO2phase(PDF No.50-1089)and Co3O4phase(PDF No.43 1003).The characteristic peaks of ZrO2and Co3O4were indexed at 2θ=30.27°and 31.27°respectively,corresponding to the respective crystallographic planes(011)and(220).It is indicated that the sample of CoxZr1-xO2-P prepared by precipitation method is a cobalt zirconium solid solution,since the cobalt ion radius(Co2+:0.079 nm，Co3+:0.068 nm)less than the radius of zirconium ions(Zr4+:0.086 nm),and cobalt ions is incorporated into the tetragonal lattice of ZrO,forming a single strong diffraction peak at 2θ=30.78°.A similar phenomenon also occurs in the vicinity of 2θ=60°[35].However,samples CoxZr1-xO2-H,CoxZr1-xO2-H(1∶1),CoxZr1-xO2-H(1∶2),CoxZr1-xO2-T and CoxZr1-xO2-I are composite oxides of zirconium andcobalt and it can be seen that the characteristic peaks of zirconia and cobalt oxide are obvious in the patterns.The diffraction peaks at 2θ=30.27°,35.26°,50.38°,59.61°and 60.21°correspond to the crystallographic plane of(011),(110),(112),(013)and(121)respectively,which is consistent with the simple tetragonal ZrO2(PDF No.50-1089).And the diffraction peaks at 2θ=19.05°,31.27°,36.85°,59.35°and 65.23°correspond to the crystallographic plane of(111),(220),(311),(511)and(440),respectively,which is accordance with face-centered cubic Co3O4(PDF No.43-1003).

Fig.2 XRD patterns of(a)CoxZr1-xO2-P,(b)CoxZr1-xO2-H,(b-1)CoxZr1-xO2-H(1∶1),(b-2)CoxZr1-xO2-H(1∶2),(c)CoxZr1-xO2-T,and(d)CoxZr1-xO2-I catalysts

2.3 Catalyst surface morphology,elemental analysis and surface area

Fig.3showsSEMimagesofCoxZr1-xO2 P,CoxZr1-xO2-H,CoxZr1-xO2-T and CoxZr1-xO2-I catalysts.It can be seen from Fig.3a that the catalyst is composed of a number of like-spherical microparticles with accumulation holes between them.The particles of hydrothermal catalysts can be form regular spheres as depicted in the Fig.3b and the inset.On the surface of spherical particles,the regular worm-like pore structure was observed.SEM photograph of the catalyst obtained bythermal decomposition(Fig.3c)shows that the sample has an irregular structure and the particle surface is loose.Fig.3d is the SEM photograph of the catalyst prepared by the impregnation method,which shows that the sample is composed of a homogeneous accumulation of small particles and the surface of the sample is unevenly distributed.

Fig.4 shows EDS analysis results,which can confirm that the prepared catalysts contain Zr,Co and O elements but no miscellaneous elements.In Fig.4a,there are obvious detection peaks of Zr and Co elements,as shown in the XRD results in Fig.2a,the diffraction peak of crystalline cobalt is very weak,and the characteristic diffraction peak of zirconium is shifted to high angle.It can be inferred that in the catalyst prepared by precipitation method,cobalt ions with small atomic radius enter the crystal lattice of zirconia to form a solid solution.It can be seen from the curve in Fig.4b,b-1 and b-2 that there is more cobalt content on the surface.With the increase of zirconium content,the strength of zirconium detection peak also increased.The mass fraction of zirconium content was 35.4%,57.5%and 63.6%,while the cobalt content was 47.8%,28.3%and 19.6%,respectively.The detection peaks of Zr and Co in Fig.4c and d are evenly distributed,indicating that the dispersion of zirconia and cobalt oxide species in b is better than that in c and d.

Fig.3 SEM images of(a)CoxZr1-xO2-P,(b)CoxZr1-xO2-H,(c)CoxZr1-xO2-T and(d)CoxZr1-xO2-I catalysts

Fig.4 EDS spectra of(a)CoxZr1-xO2-P,(b)CoxZr1-xO2-H,(b-1)CoxZr1-xO2-H(1∶1),(b-2)CoxZr1-xO2-H(1∶2),(c)CoxZr1-xO2-T,and(d)CoxZr1-xO2-I catalysts

Fig.5 shows the nitrogen adsorption/desorption isotherms of the six catalysts.The isotherm has a process of increasing adsorption capacity in the area with relatively low pressure.The isotherms of CoxZr1-xO2-H,CoxZr1-xO2-H(1∶1)and CoxZr1-xO2-H(1∶2)catalyst are the most obvious in the process of adsorption rise with I type isotherm in the lower relative pressure,and it can be known that there are certain micropores in the sample[36].In the region with higher relative pressure,theisothermsofCoxZr1-xO2 T,CoxZr1-xO2 Iand CoxZr1-xO2 P have no obvious saturation adsorption,while the isotherm of CoxZr1-xO2-H,CoxZr1-xO2-H(1∶1)and CoxZr1-xO2-H(1∶2)catalyst appears saturation adsorption,indicating that the pore structure of catalysts with different preparation methods was quite different.The microporous structure is beneficial to the adsorption of reactants and can affect the physical and chemical properties of materials[37].In general,micropores are the main route for the adsorption of VOCs,while mesoporous pores promote the diffusion of these sub-stances[38].The specific surface area of each catalyst is determined by BET method in order of CoxZr1-xO2-H(1∶1)＞CoxZr1-xO2-H＞CoxZr1-xO2-H(1∶2)＞CoxZr1-xO2-T＞CoxZr1-xO2-I＞CoxZr1-xO2-P.The surface areas are 102,82,58,45,30 and 19 m2·g-1,respectively.

Fig.5 Nitrogen adsorption/desorption isotherms of(a)CoxZr1-xO2-P,(b)CoxZr1-xO2-H,(b-1)CoxZr1-xO2-H(1∶1),(b-2)CoxZr1-xO2-H(1∶2),(c)CoxZr1-xO2-T,and(d)CoxZr1-xO2-I catalysts

Fig.6 is the catalyst pore size distribution curve.It can be seen that the pore size of CoxZr1-xO2-H and CoxZr1-xO2-T catalyst is in bimodal distribution rules.Maximum pore diameter appears at 1.6～1.9 nm and 3.8～4.1 nm.The curve of CoxZr1-xO2-I appears four dis-tributions of 1.5,2.7,3.8 and 6.5 nm,respectively.Pore size distribution curve of the CoxZr1-xO2-P catalyst is irregular shape,and the smallest and maximum aperture appeared in 1.5 and 12.3 nm.

Fig.6 Pore size distributions of(a)CoxZr1-xO2-P,(b)CoxZr1-xO2-H,(c)CoxZr1-xO2-T and(d)CoxZr1-xO2-I catalysts

2.4 Catalyticoxidationoftolueneorformaldehyde

Fig.7 shows the catalytic oxidation performance of toluene or formaldehyde on cobalt-zirconium bicomponent catalysts obtained by different preparation methods.As shown in the Fig.7,with the increase of reaction temperature,the conversion of toluene and formaldehyde oxidation increases.The conversion rate changes slowly at low temperature and rapidly at high temperature.The calculation of the yield of“carbon”before and after the reaction proves that the products of toluene oxidation reaction are CO2and H2O.The blank test showed that there is no oxidation of toluene under this condition.Table 1 lists the reaction temperatures when the conversion rate of toluene or formaldehyde reaches 10%,50%and 90%,and visually compares the activity of catalysts prepared by different methods.For the most active catalyst CoxZr1-xO2 H(1∶1),the reaction temperature is 155 and 100℃for 90%conversion of oxidized toluene and formaldehyde.The reaction temperature of complete oxidation of toluene and formaldehyde is 225 and 115℃,respectively.As can be seen from Fig.7 and Table 1,the reaction performance of oxidized toluene or formaldehyde decreased intheorderofCoxZr1-xO2 H(1∶1),CoxZr1-xO2 H,CoxZr1-xO2 H(1∶2),CoxZr1-xO2 T,CoxZr1-xO2 Iand CoxZr1-xO2-P,which should be affected by the good dispersion of cobalt oxide and zirconia,high specific surface area and strong synergistic effect of zirconium and cobalt composite oxide prepared by hydrothermal method[28,30-31,39].The kinetic analysis of the complete oxidation of VOCs has been reported.Garetto et al.[40]believed that in the case of excess oxygen,the complete oxidation of propane on the supported Pt catalyst followed the first-order reaction process,while Wong et al.[41]also believed that the complete oxidation of ethyl butyrate on AgY and AgZSM-5 catalysts also followed the first-order reaction process.We have reason to believe that the complete oxidation of toluene and formaldehyde also follows the first-order reaction process under the condition of absolute excess oxygen.The inset of Fig.7 illustrates the Arrhenius plots of catalyticoxidationoftolueneandformaldehyde over CoxZr1-xO2-H(1∶1),CoxZr1-xO2-H and CoxZr1-xO2-H(1∶2).The apparent activation energy for the combustion of toluene(inset of Fig.7A)over them was calculated to be 65.2,69.1 and 77.2 kJ·mol-1,respectively.While,for the combustion of formaldehyde(inset of Fig.7B),the apparent activation energy over CoxZr1-xO2-H(1∶1),CoxZr1-xO2-H and CoxZr1-xO2-H(1∶2)catalysts was computed to be 53.6,59.9 and 63.7 kJ·mol-1,respectively.When the molar ratio of Co and Zr is different,the activation energy of oxidized toluene and form-aldehyde is also different.Obviously,when the molar ratio of Co and Zr is equal,the activation energy of oxidized toluene and formaldehyde has the lowest value of 65.2 and 53.6 kJ·mol-1,respectively.It is known that the apparent activation energy for toluene and formaldehyde is mainly on the properties of the catalyst.For example,apparent activation energy is 78.6 kJ·mol-1 overPt@M Cr2O3[42]and105.4kJ·mol-1over Cu0.3Ce0.7Ox[43]for toluene oxidation.It is 69.7 kJ·mol-1 over Co3O4-350[44]for 2-propanol oxidation and 57.1 kJ·mol-1over 1%(mass fraction)Pt/TiO2[45]for formaldehyde oxidation.One can see that the apparent activation energy for the addressed reactions of the Co-Zr catalysts is close to those reported in their literature.

Fig.7 Toluene(A)and formaldehyde(B)conversion and Arrhenius plots(inset)as a function of reaction temperature of the(a)CoxZr1-xO2-P,(b)CoxZr1-xO2-H,(b-1)CoxZr1-xO2-H(1∶1),(b-2)CoxZr1-xO2-H(1∶2),(c)CoxZr1-xO2-T and(d)CoxZr1-xO2-I catalysts

Table 1 Performance comparison of toluene or formaldehyde oxidation over various catalysts

Fig.8 Catalytic activity versus on-stream time over CoxZr1-xO2-H(1∶1)catalyst for toluene and formaldehyde oxidation

Compared with the catalysts prepared by the four methods,the morphology of the catalysts has no significant influence on the catalytic performance.However,the morphology of the catalysts prepared by different methods is different,which will affect the specific surface area and surface composition and distribution ofthe catalysts,affect the adsorption-desorption properties of the catalysts and reactants,and further affect the performance of toluene and formaldehyde oxidation.The life test of the catalyst was carried out on CoxZr1-xO2-H(1∶1)catalyst under the condition of space velocity of 20 000 mL·g-1·h-1.Under the condition of 90%conversion of toluene and formaldehyde,that is,at 155 and 100℃respectively,the reaction lasted for 30 hours.The results showed that the catalyst could maintain its catalytic performance for a long time.As shown in Fig.8,the catalytic conversion of toluene and formaldehyde has no significant change.

3 Conclusions

In this paper,CoxZr1-xO2catalyst was prepared by precipitation method,hydrothermal method,thermal decomposition method and impregnation method.The physicochemical properties were characterized by TG,XRD,SEM,EDS and N2adsorption/desorption test,and the catalytic oxidation of toluene or formaldehyde was evaluated.The results showed that the particles of the four catalysts were different in shape and relatively uniform in morphology.The Co-Zr catalyst CoxZr1-xO2-P prepared by precipitation method was a solid solution,while catalysts prepared by hydrothermal,thermal decomposition and impregnation method were composite metal oxides.On the catalyst prepared by hydrothermal method,the cobalt oxide species and the zirconium oxide species had large dispersibility and a strong synergistic effect between Co species and Zr species,the particles were uniformly spherical.The CoxZr1-xO2-H(1∶1)catalyst is composed of equal moles of Co and Zr,and the specific surface area is as large as 102 m2·g-1,and the pore structure is developed,thereby having bet-ter catalytic performance for the oxidation of toluene and formaldehyde.The conversion temperature of fully oxidized toluene and formaldehyde is as low as 225 and 115℃at the 0.05%concentration and the space velocity of 20 000 mL·g-1·h-1.The corresponding apparent activation energy is 65.2 and 53.6 kJ·mol-1 over the CoxZr1-xO2-H(1∶1)catalyst for toluene and formaldehyde,respectively.