Preparation,Characterization and Catalytic Activity for ortho-Dichlorobenzene Ozonation of Fe-,Mn-Doped Xerogels

HUANG Yan ZHANG Xing-Wang LEI Le-Cheng

(Institute of Industrial Ecology and Environment,Department of Chemical and Biological Engineering,Zhejiang University,Hangzhou 310027,P.R.China)

Preparation,Characterization and Catalytic Activity for ortho-Dichlorobenzene Ozonation of Fe-,Mn-Doped Xerogels

HUANG Yan ZHANG Xing-Wang LEI Le-Cheng*

(Institute of Industrial Ecology and Environment,Department of Chemical and Biological Engineering,Zhejiang University,Hangzhou 310027,P.R.China)

Metal-doped xerogels were synthesized by the sol-gel method and their catalytic performance was evaluated for the catalytic ozonation of simulated micro-polluted water containing ortho-dichlorobenzene(o-DCB).The obtained samples were characterized by X-ray photoelectron spectroscopy(XPS),transmission electron microscopy(TEM),X-ray diffraction(XRD),scanning electron microscopy(SEM),mercury porosimetry and nitrogen adsorption.Results show that Fe3+oxides and Mn3+oxides are present in the Fe-doped xerogel(XFe)and the Mn-doped xerogel(XMn),respectively.No obvious diffraction peaks corresponding to a metallic phase were present in the XRD patterns of the xerogels.Also,the metallic particles were found to be small;therefore,the metallic particles were well distributed over the xerogel supports.Catalytic ozonation experiments indicated that the removal rate of o-DCB was 44%with single ozonation,and this increased to 58%and 72%with catalytic ozonation using XFe and XMn,respectively,which suggests that metal-doped xerogels are promising catalysts for the ozonation process.We also investigated metal leaching from the metal-doped xerogel to the water solution to test the stability of the catalysts,and about 2%and 8%metal leaching for XFe and XMn was obtained within 5 h.

Sol-gel method; Xerogel; Characterization; Catalytic ozonation; ortho-Dichlorobenzene

Heterogeneous catalytic ozonation is an effective approach to decompose recalcitrant contaminants,such as chlorinated aromatic compounds,without extra addition of chemicals and energy.The main catalysts used in this process are activated carbon (AC)[1-2],metaloxides(e.g.,TiO2,CeO2-MnOx,CeO2-Co3O4,FeOOH, Co3O4[3-6]),and supported catalysts(e.g.,CoOx/Al2O3,Cu-Al2O3, Cu-TiO2,Cu/AC,Fe/AC,Mn/AC,Pt/AC,Ru/AC,Fe2O3/Al2O3, CeO2/AC[6-11]).Supported catalysts are more attractive to researchers,due to the synergistic effect of their supports and active components in the reaction.Thus,researchers are always interested in the development of efficient and stable supported catalysts.

Xerogels/aerogels(classified by drying methods)[12-15],which were first developed in the late 1980s,have been used as adsorbents and catalysts because of their large surface area and pore texture flexibility[16].Recently,transition-metal doped aerogels with tunable porosity were prepared by addition of the corresponding metal salts to the initial mixture[17].The metal precursor can catalyze the polymerization or gelation process,which can in turn affect the morphology and the pore texture of the xerogel/ aerogel[16].Furthermore,it is possible to combine the pore texture of the xerogel/aerogel with the acid character of the metallic oxides in catalytic reactions[18].Cr-,Mo-,W-,Ni-,and Pd-doped aerogels/xerogels have been prepared and used as catalysts in the isomerization reaction of 1-butene and the hydrogenation reaction of ethylene[17,19].

The application of metal-doped xerogels for ozonation has been seldom reported[20-21].In addition,iron and manganese are usually used as catalysts in ozonation processes,because of not only their original existence in natural waters but also their catalytic abilities.Therefore,in this work,we prepared Fe-and Mn-doped xerogels and tested their catalytic activity for the ozonation of o-DCB,a common micro-pollutant in water.The relationship between the catalytic activity and the properties of xerogel samples was explored.Meanwhile,the amount of metal leaching from the xerogel catalysts was also determined.

1 Experimental

1.1 Materials

All chemicals used were of analytical grade unless stated otherwise.Acetone,formaldehyde(mass fraction of 37%)and orthodichlorobenzene(o-DCB,CP)were obtained from Sinopharm Chemical Reagent Co.,Ltd.(China).Resorcinol was purchased from Tianjin Damao Chemical Reagent Factory(China).Manganese acetate tetrahydrate was purchased from Fisher Scientific Worldwide Co.,Ltd.(China).Ferrous acetate(mass fraction of 95%)was purchased from Acros Organics(Geel,Belgium).Activated carbon(AC)(Yixin Industrial Co.,Ltd.,China)utilized in the experiments was prepared from olive stones by direct steam activation.All the catalysts in the context were crushed and sieved to particles with a size of 37-74 μm prior to reaction.

1.2 Xerogel preparation

Metal-doped xerogels were prepared by the method reported by Moreno-Castilla and his coworkers[22].In brief,resorcinol(R) and formaldehyde(F)were dissolved in water(W)containing manganese acetate or iron acetate.The stoichiometric R/F and R/W molar ratios were 0.5 and 0.13,respectively.And the amount of acetates added was 1%(w)of the metal in the initial solution.The samples above were denoted as X followed by the metal existing in the samples:XMn and XFe.Additionally,another xerogel was prepared in the same way but without any metal precursor,denoted as X.

The mixtures were stirred to obtain homogeneous solutions that were cast into special glass molds and cured for a certain period of time.The cure cycle included 1 d at 298 K,1 d at 323 K,and 5 d at 353 K.The obtained gel rods were cut into uniform small pellets and exchanged the water in the particle with acetone for 2 d.Then the pellets were dried within a given temperature range in nitrogen atmosphere.The obtained samples were denoted as X followed by the metal existing in the samples.

Mn-doped activated carbon was prepared by the classical impregnation method.Activated carbon(size,37-74 μm)and manganese acetate tetrahydrate were used as carriers and precursors,respectively.The amount of manganese acetate needed was 1%(w)of manganese in the final catalyst.Thus,the corresponding acetate was dissolved in the minimum amount of water and then the solution was added drop by drop on the activated carbon support.After impregnation,the sample was dried in the same way as xerogel samples.The obtained sample was denoted as Mn-AC.

1.3 Characterization

The morphology analysis of samples was carried out by SEM with a SIRION-100 system(FEI,Holand).The internal morphology and the distribution of metallic phases were detected with a JEM-200 CX transmission electron microscope(JEOL, Japan).XRD patterns were obtained with a X′Pert Pro XRD system(PANalytical,Holland)using Cu Kαradiation.XPS experiments were carried out with a RBD upgraded PHI-5000C ESCA system(Perkin Elmer,USA)using Mg Kαradiation(hν=1253.6 eV),and the XPS data of metal 2p3/2spectra were fitted using the AugerScan software.The C 1s signal,the peak at a binding energy(EB)of 284.6 eV,was chosen as reference to the deconvolution of the spectra.

Mercury porosimetry was obtained under the pressure up to 307 MPa using AutoPore IV 9510 equipment(Micromeritics, USA).With this technique,the following parameters were obtained:the pore size distribution(PSD)of pores with a diameter larger than 3.7 nm;pore volume of pores with a diameter between 3.7 and 50 nm,or mesopore volume(note that mesopore volume range is classically defined as 2-50 nm),V2;pore volume of pores with a diameter larger than 50 nm,or macropore volume,V3.

N2equilibrium adsorption at 77 K was carried out on ASIC-2 measuring instrument(Quantachrome,USA)and the nitrogen surface area(SBET)was calculated by using the Brunauer-Emmett-Teller(BET)equation.Prior to adsorption measurements,all the samples were outgassed at 383 K overnight under high vacuum.

1.4 Ozonation of o-DCB

Ozonation experiments were conducted in a semi-batch mode by continuously feeding an ozone-oxygen mixture to the reactor containing the target pollutant and the xerogel catalyst.The reactor basically consists of a sampling port,a thermometer,a gas outlet,a water agitation system,and a cylindrical glass vessel (length,28 cm;diameter,6 cm)equipped with a porous pipe at the bottom to feed the gas mixture.Temperature was kept controlled at(298.0±0.2)K by immersion of the reactor into a thermostatic bath.The ozone gas was generated from pure oxygen by CHYF-6A ozone generator(Hangzhou Rongxin Electronic Equipment,China).The gas flow rate was controlled using a rotameter,and the actual ozone amount was adjusted by the inlet oxygen flow.The ozone dosage was 5.28 mg·h-1when the pure oxygen flow rate was 0.6 L·h-1.

The simulated pollutant o-DCB was dissolved in deionized water to get a solution of 7.5 mg·L-1.The pH value of the solution was 5.6.Once the ozone generator was stabilized,the gas was fed into the reactor containing the reactants.At fixed intervals,samples were withdrawn and a small amount of sodium sulfite solution was added directly into them thereby quenching the residual ozone.All processes,including adsorption,ozonation alone and catalytic ozonation were carried out under the same condition.

Metal leaching experiments were carried out by dipping a certain amount of metal-doped samples in deionized water.The pH value of deionized water was 5.6.At specific intervals,the manganese or iron concentration in the solution was detected using atomicabsorptionspectrophotometerSolaarM6(Thermal,USA). The value of“leaching amount”was defined as the amount of metal leached per 1 g of samples.The actual content of metal that was incorporated in the samples was determined by combustion of the samples(mass,1 g)at 1073 K in air atmosphere. The value of“metal leaching”was defined as[(leaching amount)/(meal content in samples)]×100%,denoted as P.

1.5 Analytical methods

The concentration of o-DCB was determined by a gas chromatograph FULI9790(FuliAnalytical Instrument,China)equipped with a SE-30 capillary column(30 m×0.32 mm,0.33 μm thickness)and an ECD detector.A constant gas flow(2 mL·min-1)of nitrogen was used as a carrier gas.The temperatures of the injector,detector,and column were 523,523,and 423 K,respectively.The aqueous samples were injected into the GC/ECD analyzer by using a headspace sampler DK3001A(Zhongxinghuili, China).The temperatures of the sample room,the sample loopsvalve,and the transport tube were 353,373,and 373 K,respectively.The concentration of gaseous ozone from the ozone generator was detected by iodometry method[23].

2 Results and discussion

2.1 Characterization

XPS patterns of the metal 2p3/2regions are shown in Fig.1. Spin-orbit splitting,multiple oxidation states,satellite structure, and adsorbent surface heterogeneity are known to complicate the analysis of the Fe 2p spectra[24].The Fe 2p3/2spectrum in Fig. 1a comprised the peaks at 710.9 eV(29%),712.7 eV(46%),and 715.2 eV(25%),which was in accordance with the literature [22,25].Both components at 710.9 and 712.7 eV could be ascribed as Fe3+oxides,which have binding energy(EB)values between 710.8 and 711.8 eV[26].The component at 712.7 eV was considered to be FeO(OH)(711.3-711.8 eV)[26],which was also observed in the XRD pattern(Fig.2).Meanwhile,a higher EBvalue of FeO(OH)(0.9 eV)was found in our work,which indicated that the metal particle size in the xerogel was smaller than in the bulk.Because there are fewer neighbor atoms in a dispersed system than in the bulk,there are also fewer electrons.The consequence is a less effective core-hole screening and the EBof the orbit increases[22,27].The third component at 715.2 eV was considered to be iron ions,which chelated electronegative surface ligands[22,25].

Fig.3 TEM images of XMn(a),XFe(b),and Mn-AC(c)

The Mn 2p3/2spectrum in Fig.1b comprised the peaks at 641.6 eV(51%),643.6 eV(23%),and 646.2 eV(26%).The component at 641.6 eV was considered as Mn3+oxides,which have EBvalues between 641.2 and 641.7 eV[26].The other two components at 643.6 and 646.2 eV might be considered to be manganese ions,which chelated electronegative surface ligands,as in the case of iron ions.

The dispersion of the metallic species in xerogel samples was analyzed with XRD and TEM technologies.In the XRD pattern of XFe(Fig.2),we could observe a few subtle and wide diffraction peaks of the metallic phases,at 2θ of 21.0°,36.55°,33.12°, 53.06°,and 58.89°,which corresponded to goethite,or FeO (OH)(PDF#29-0713).However,in the case of XMn,no diffraction peaks corresponding to the metallic phase were observed (Fig.2),indicating the incorporation of metal to the polymeric matrix or the occurrence of amorphous manganese oxides or high dispersion of the metallic phases[4,9,28].In addition,no metallic particles were observed in the TEM image of XMn within the available microscope resolution,while in the TEM image of XFe metallic particles with the size of 20-40 nm could be seen (Fig.3(a,b)).A TEM image of Mn-AC was also used for comparison,and the size of the metallic particles in Mn-AC was about 10-20 nm(Fig.3c).Accordingly,it was deduced that high dispersion of metallic phases was achieved in metal-doped xerogels,especially for XMn.

The SEM images of samples are depicted in Fig.4.It can be seen from Fig.4 that the surface morphology of samples X,XFe, and XMn was different.In the SEM image of X,a continuous and highly cross-linked network with open and irregular morphology was observed(Fig.4a),as previously reported[22,29].However,as shown in Fig.4(b,c),the metal-doped xerogel samples (XFe and XMn)were composed of microbead particles,and the particle size was strongly influenced by the character of the metal[22,29].The particle size for XFe was about 1 μm,while the particle size for XMn was difficult to determine within the available microscope resolution since the microbead particles were too tiny.

The textural characteristics of the xerogel samples are listed in Table 1 and the pore size distributions(PSDs)are depicted in Fig.5.Samples X and XFe were macroporous materials,while sample XMn was meso-macroporous one.The PSDs of samples X and XFe presented their maxima at 1500 nm and 850 nm,respectively.However,in the case of XMn,the PSD presented its maximum in the smaller macropore region,with a radius of about 38 nm,and the increase in V2and SBET(Table 1)was due to the development of mesopores.The different characters of the metal salts,which affected the rate of the polymerization of the xerogel,may explain the distinct morphologies and textural characteristics of XMn and XFe[22,30].In addition,the distinct tex-tural characteristics of XMn and XFe may be related to the difference in the dispersion of active components(Fig.3).

Table 1 Pore characteristics of the metal-doped xerogels

Fig.4 SEM images of X(a),XFe(b),and XMn(c)

Fig.5 Pore size distribution of X,XFe,and XMn

2.2 Evaluation of catalysts

2.2.1 Comparison of catalytic activity

Manganese and iron doped xerogels were applied in the ozonation of o-DCB in aqueous solution,and the performance of catalysts,the adsorption experiments,and the control experiments were depicted in Fig.6.Degradation experiments with activated carbon and Mn-doped activated carbon were also carried out for comparison(Fig.6).

According to Fig.6,only 44%removal of o-DCB was reached in ozonation alone after 35 min.The xerogel samples had no effect on the reduction of o-DCB in the absence of ozone.The removal rate of the catalytic ozonation processes using XMn and XFe was about 72%and 58%,respectively,which showed that the metal-doped xerogels had catalytic activity and XMn was better than XFe.The reasons for the different catalytic performances are complicated.Dispersion of metallic particles over the carrier is significant in the catalytic reaction,because high dispersion can enhance active surface area of catalysts[9,31].In the present work,the different catalytic performance between XMn and XFe was ascribed to the difference in the metal dispersion and the metal composition of the catalysts.In addition,better catalytic activity of XMn than Mn-AC was also found in the experiments.These results reveal that metal-doped xerogels are efficient catalysts in ozonation process.

Fig.6 Evolution of the dimensionless concentration of o-DCB during adsorption,catalytic and non-catalytic ozonationreaction condition:pH 5.6;o-DCB concentration:7.5 mg·L-1; catalyst dose:50 mg·L-1;ozone dose:5.28 mg·h-1

Fig.7 Percentage of metal leaching(P)from various metaldoped samplesreaction condition:pH 5.6;sample dose:100 mg·L-1

2.2.2 Stability of different catalysts

Catalyst stability is a crucial factor in heterogeneous catalytic reaction.Metal leaching to the water solution is an important deactivation aspect,and it also brings a second pollution source.In our study,we determined the metal leaching from samples XMn, XFe,and Mn-AC.

As depicted in Fig.7,metal leaching from metal-doped xerogels was less than that from metal-doped activated carbon.For sample XMn,metal leaching in 1 h was about 8%and it changed little in the following several hours.However,for sample Mn-AC,metal leaching increased over time and it reached about 12%in 5 h.The difference in metal leaching from the above two samples might be a result of the different preparation methods.In addition,2%metal leaching was found in the solution containing sample XFe.These results suggest that the stability of metal-doped xerogel samples is good.

3 Conclusions

Fe-and Mn-doped xerogel samples(XFe and XMn)were synthesized using the sol-gel method.Fe3+oxides and Mn3+oxides were found to exist in XFe and XMn,respectively,and the metallic particles were well distributed over the xerogel supports.Besides,the catalytic activity of xerogel samples for the ozonation of micro-polluted water containing ortho-dichlorobenzene(o-DCB)was high and the stability of xerogel catalysts was good.The results obtained indicate that metal-doped xerogels are promising catalysts for catalytic ozonation process.

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Fe、Mn摻雜干凝膠的制備、表征及催化臭氧氧化鄰二氯苯性能

黃 燕 張興旺 雷樂成*

(浙江大學化學工程與生物工程學系,工業生態與環境研究所,杭州 310027)

采用溶膠-凝膠法制備Fe、Mn摻雜的干凝膠材料(XFe和XMn),并對其催化臭氧氧化去除鄰二氯苯(o-DCB)的性能進行了研究.通過X射線光電子能譜(XPS)、X射線衍射(XRD)、透射電鏡(TEM)、掃描電鏡(SEM)、壓汞儀和氮氣吸附等技術對Fe、Mn摻雜的干凝膠材料進行表征.結果表明:XFe和XMn中存在三價金屬氧化物等活性物種,金屬相無明顯的衍射峰且金屬顆粒的尺寸較小,表明金屬顆粒在干凝膠載體上獲得較好的分散性;相比于單獨臭氧氧化時44%的o-DCB去除率,XFe和XMn的添加使得o-DCB去除率分別達到58%和72%,XFe和XMn顯示了較好的催化活性;XFe和XMn的金屬溶出狀況不同,在水溶液中浸泡5 h后它們的金屬溶出率分別為2%和8%左右.

溶膠-凝膠法; 干凝膠; 表征; 催化臭氧化; 鄰二氯苯

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Received:February 25,2010;Revised:May 5,2010;Published on Web:July 2,2010.

*Corresponding author.Email:lclei@zju.edu.cn,lablei@zju.edu.cn;Tel/Fax:+86-571-88273090.

The project was supported by the National Natural Science Foundation of China(20836008,U0633003),Science and Technology Department of

Zhejiang Province,China(2007C13061),and National High Technology Research and Development Program of China(2007AA06Z339,2008BAC32B06,2007AA06A409).

國家自然科學基金(20836008,U0633003),浙江省科技計劃項目(2007C13061)和國家高技術研究發展計劃(2007AA06Z339,2008BAC32B06,2007AA06A409)資助

?Editorial office of Acta Physico-Chimica Sinica