两步沉淀法制备耐高温和优异还原性能CeO2材料

陈山虎 闫朝阳 曹毅 兰丽 赵明 龚茂初 陈耀强＊

(四川大学化学学院，绿色化学与技术教育部重点实验室，成都610064)

两步沉淀法制备耐高温和优异还原性能CeO2材料

陈山虎 闫朝阳 曹毅 兰丽 赵明 龚茂初 陈耀强＊

(四川大学化学学院，绿色化学与技术教育部重点实验室，成都610064)

分别采用两种沉淀方法制备了CeO2：以传统的氨水为沉淀剂，在氨水沉淀法前引入碳铵沉淀步骤(两步沉淀法)。采用热重-差热(TG-DTA)、傅里叶变换红外(FTIR)、X光电子能谱(XPS)等手段对沉淀及其分解过程进行了研究。结果表明，在两步沉淀法中的第一步，碳酸物种为主要沉淀物种，而在第二步中被氢氧根取代。X射线衍射(XRD)和透射电子显微镜(TEM)结果表明，两步沉淀法生成的沉淀颗粒粒径更大。通过两步沉淀法制备的CeO2与氨水沉淀相比具有更好的抗高温老化性能和还原性能。经过900℃焙烧3 h后，仍然具有25 m2·g-1和0.11 cm3·g-1的比表面和孔容。

两步沉淀法；传统沉淀法；耐高温；氧化还原性能

Abstract:Two series of CeO2samples were prepared by two different synthetic routes:one was conventional precipitation route employing ammonia as reactant,and the other was a two-step precipitation procedure,using ammonium carbonate((NH4)2CO3)as precipitant at the first step and ammonia as reagent at the second step.The precipitates and their decomposition processes were characterized by thermogravimetric and differential thermal analysis(TG/DTA),Fourier transform infrared spectroscopy(FTIR),and X-ray photoelectron spectra(XPS).The results show that the precipitate produced at the(NH)4CO3precipitation step mainly consists of carbonate species, however,after the second precipitation step by ammonia,the carbonate species are replaced by hydroxyl species. By introducing the intermediate carbonate precipitation process,the nucleation rate of CeO2can be controlled.X-ray diffraction(XRD)and transmission electron microscopy(TEM)results indicate that the grain size of the precipitate prepared by two-precipitation route is larger than that of the precipitate prepared by conventional route.CeO2prepared by this two-precipitation route exhibits higher thermal stability and better reduction property than that obtained by traditional procedure.After the heat treatment at 900℃for 3 h,the surface area and pore volume are 25 m2·g-1and 0.11 cm·g-1,respectively.

Key words：two-step precipitation;conventional precipitation;high thermal stability;redox properties

CeO2-based materials have important roles in many commercial catalytic processes.For example, CeO2has been used for catalytic wet oxidation[1],for removalofsootfrom dieselengine exhaust[2]and forfuel celltechnology[3].In addition,they have been one ofthe mostimportantcommercialsupports for the purification of exhaust gases[4].Generally,a three-way catalyst (TWC)is required to simultaneously convert the hydrocarbons,CO and NO x present in the automotive exhaust to harmless H2O,CO2and N2[5].However,high conversion ofthe pollutants can be achieved only when the air-to-fuel(A/F)ratio oscillates around the stoichiometric value(14.6)[5].Adding of CeO2-based materials can balance the oxygen concentration due to their abilities to store and release oxygen under lean and rich operation conditions,respectively[6].Under usual TWC working conditions,the above-mentioned performance is essentially related to the surface area of the CeO2support[7-8].As soon as significantsintering of CeO2particles occurs,both redox property and metalsupportinteractions appearinhibited[9-10].So,to improve the thermal stability of CeO2-based materials used in TWC is a challenge for researchers and TWC companies.

Differentstrategieshave been used to prepare CeO2-based materials,including precipitation method[11-12], hydrothermal route[13-14],sol-gel techniques[15-16], surfactant-assisted approach[17-18],and combustion synthesis[19-20].Among these methods,the most convenient one is precipitation.Usually,the precipitation can be carried out via the reaction of cerium(ⅢorⅣ)saltsolution and a base solution such as ammonia[21],ammonium carbonate[22],and alkaline hydroxide[23].It has been established that,surface area, particle morphology and even the lattice structure of CeO2-based materials are strongly affected by the synthesis conditions[24].Woodhead[25]firstly developed an H2O2-assitated method based on precipitation method in order to produce CeO2,and after that,the method was adopted by many researchers[26-29]. Investigations have also been carried out by some groups to study the effectof H2O2on the preparation of CeO2-based materials.Djurii et al[26]found that the presence of H2O2could change the precursor of CeO2, leading to differentdecomposition processes.Scholes et al[23,27]investigated the effect of the amount of H2O2on the physico-chemical properties of precursors and proposed the formation of Ce(O2)(OH)2on the basis of titration results.

In the present work,we developed a two-step precipitation route based on the H2O2-asistated method to prepare CeO2material,aiming atpreparing CeO2with improved thermal stability and reduction property.In this two-step route,ammonium carbonate and ammonia were used as reagents at the first and second steps, respectively.The formation process of the sample was also investigated.By introducing an carbonate intermediate precipitation procedure,the crystallite size ofthe precipitate,as wellas the pore size oftargetCeO2, were enlarged,which are crucial for preparing CeO2-based materials with improved thermalstability and red uction property[28-29].

1 Experimental

1.1 Synthesis

1.1.1 Preparation ofprecipitate by conventionalroute

Ce(NO3)3·6H2O was dissolved into distilled water, and H2O2(30wt%)was added.The molar ratio of Ce (NO3)3∶H2O2was 1∶1.A ammonia solution(25wt%)was added into the saltsolution to adjustthe pH value to 12 under stirring.The obtained precipitate was filtered out and washed with distilled water untilno changes in pH value,and dried at 80℃for 5 h to obtain CeC precipitate.

1.1.2 Preparation of precipitate by ammonium carbonate

The preparation procedure was according to experiment(1.1.1),except the base solution was ammonium carbonate(25wt%)and the pH value was adjusted to 8.2.The precipitate was denoted as CeM.

1.1.3 Preparation of precipitate by two-step precipitation route

First,a solution ofammonium carbonate(25wt%) was added into the salt solution prepared according to experiment(1.1.1),tillthe pH value to 8 under stirring, and keptatroom temperature for 2 h.Then an ammoniasolution(25wt%)was added to adjusted the pH value to 12,and then stirred for 12 h．The precipitate was filtered outand washed with distilled water;then itwas dried at80℃for5 h to produce CeT precipitate．

1．1．4 Calcination ofprecipitates

The CeT and CeC precipitates were calcined at different temperatures in the range of 500～900℃for 3 h．The target samples were labeled as CeT-t and CeC-t where t stands for the calcination temperature．1.2 Characterizations

TG/DTA was carried out by a HCT-2 analyzer (Beijing Science Apparatus Factory,Beijing,China) under a flowing N2atmosphere(30 mL·min-1)．The sample was heated to 600℃with a heating rate of10℃·min-1．The alumina was taken as the reference material．

FTIR spetra were recorded atroom temperature in the range of 400～4 000 cm-1using KBr pellet with a Nicolet6700 FT spectrometer．

X-ray photoelectron spectra data were collected on a XSAM 800 spectrometer(KRATOS Corp．)with a Mg Kαsource working at 13 kV and 20 mA,and the C1s peak was used as an internal standard for calculating binding energy values．

The nitrogen adsorption-desorption isotherms were measured on Quantachrome SIinstrument．The specific surface areas and pore size distribution were calculated according to Brunauer-Emmett-Teller(BET)method and Barret-Joyner-Halenda(BJH)method,respectively．The measurement was carried outat-196℃,after the sample was degassed at300℃for3 h undervacuum．

The X-ray diffraction patterns were determined on a D/max-rA diffractometer(RIGAKU Corporation) equipped with Cu Kα(λ=0．154 18 nm)radiation and Graphite monochromator．The anode X-ray target was operated at40 kV and 25 mA．The X-ray was detected by scintillation counter and recorded for 2θvalues between 10°and 80°with a step of0．03°．

The size of the precipitates was observed with transmission electron microscopy(TEM)using a Tecnai G2F20 S-TWIN apparatus operated at200 kV．

TPR profiles were determined in a conventional reactor equipped with a thermal conductivity detector． Allsamples(100 mg)were pretreated in a flow ofN2at 450℃for 45 min,and then cooled down to room temperature．The reduction was carried outin a flow of 20 mL·min-1of5%H2/N2from room temperature to 900℃with a heating rate of10℃·min-1．

2 Results and discussion

2.1 Formation processes ofprecipitates

Composition of the precipitates depends on the kinds of cations and anions present in the solution．In this work,Ce3+is employed as metal salt precursor, while OH-or CO32-is used as precipitants,in the presence of H2O2．When ammonia is utilized as precipitant,the following equilibria are expected in the precipitation process[27]:

Whereas,in the case of two-step precipitation process,the reaction is very complex since different anions might enter into the precipitate．In the first precipitation step,although the carbonate species is used as reagent,considering the following hydration process[30]:

We can reasonably assume that the component of the precipitate is Ce(O2)(OH)x(CO3)1-x/2．In the second precipitation step,the OH-is added,thus the CO32-species is entirely replaced by OH-:

The dehydration process might occur in the operation procedure due to the O22-and OH groups are notstable enough[26,29]．

Fig．1 presents the thermalbehavior ofprecipitates prepared by differentmethods．From Fig．1a,we can see that the precipitate prepared by ammonia only shows one continuous decomposition process．The totalweight loss is 13．13%,which does not correspond to the decomposition of Ce(OH)4or Ce(O2)(OH)2,indicating that the dehydration event has occurred under our operation condition．The DTA curve shows one endothermic peak atabout80℃due to the elimination of physical-adsorbed water[26]．It seems that the crystallization of the hydroxide is a slow process,thus the exothermal phenomenon,as reported in previous study[26],can notbe observed．Fig．1b shows the thermal decomposition of the precipitate prepared by ammonium carbonate．The evolution behavior proceeds through three stages at the temperature ranges of 50～200℃,250～350℃and 350～450℃,respectively．By comparing with literatures[23,26],we attribute the firstone to the desorption ofwater and the decomposition of O22-containing species,and the second one to the crystallization of hydroxide particles,while the last one to the decomposition of carbonate species．Fig．1c displays the thermalevents ofsample CeT．Itis clearly observed that its decomposition process is very similar to that of CeC,indicating they possess the same or similar chemical components．Interestingly,the total weightloss of CeT is 12．7%,which is very close to that of CeC(13%)．In addition,the DTA behavior of CeT is also in accordance with CeC,presenting an endothermic peak at about 80℃．From the above results,we can conclude that in the second precipitation step of two-step precipitation route,the carbonate species are replaced by OH-groups,indicating the occurrence of following reactions:Ce(O2)(OH)x(CO3)1-x/2+OH-→Ce(O2)(OH)2/Ce(OH)4→CeO2·n H2O．

The FTIR spectra for CeO2precipitates prepared by differentmethods are shown in Fig．2．Generally,the intense bands around 3 000～3 650 cm-1region are attributed to the O-H stretching of H-bound hydroxyl groups or molecularly chemisorbed water[31-32]．The bands at this region of CeC and CeT show three different adsorption signals centered at about 3 540, 3 480 and 3 420 cm-1,respectively,indicating the presence of mono-coordinated,bi-coordinated and tricoordinated hydroxylgroups[31-32]．However,in the case of CeM,only a characteristic broad adsorption can be observed．The distinction ofthis region suggests thatthe content of surface OH in CeC and CeT is higher than thatin CeM．The band at1 620 cm-1is assigned to the adsorption of H2O[33]．On the basis of literatures[31,34-35], the bands around 1 555,1 347,1 142,1 056,913,851 and 722 cm-1are representative ofcarbonate species．It should be noted thatthe bands around 1 555 and 1 347 cm-1are likely attributed to the carbonate species generated from the precipitant,since they are not observed in CeC and CeT,while the others are related to the carbonate species caused by the interaction of precipitates with atmospheric carbon dioxide[36]．It is obvious that the IR spectra of CeC and CeT are almost the same,indicating that they possess similar chemical structure,which is consistentwith the TG-DTA results．The results further prove that,in the secondprecipitation step of two-step precipitation method,theis superseded by OH-groups.

The XPS spectra for precipitates are shown in Fig.3.In the spectrum of Ce3d,six peaks are all present for the samples from different synthesis routes. According to the literature[37],all the peaks may be attributed to the diversified states of Ce4+,indicating that Ce3+ions are all oxidized by H2O2′in agreement with the previous report[23].In the O1s of precipitates (Fig.4),the peak around 529.7 eV is assigned to the oxygen in the lattice[37],while the binding energy around 531.5 and 532.7 eV are related to the hydroxyl and C=O oxygen,respectively[38-39].It is noted that,the 532.7 eV signal of CeM is much stronger than that of CeC and CeT,suggesting the existence of larger amount of carbonate species,which provides another evidence for the occurrence of the substitution of CO32-by OH-,in accordance with DTA-TG and IR results.The relative amounts of Ce,O and C are listed in Table 1.The amount of CeC and CeT is almost the same,implying that they possess the same chemical compositions,whereas the atom ratio is much different for CeM.The content of carbon in CeM is significantly higher than the other two samples due to the participation of carbonate during precipitation process. The amount of carbon cannot be evaluated accurately because the adventitious organic compounds generated from vacuum system are not easily excluded[23].

Table 1 Relative amount of Ce,O,and C,obtained from XPS spectra

2.2 Texturalproperties of CeO2samples

The surface areas and pore volumes of CeO2samples calcined at various temperatures are shown in Fig.5.Itcan be seen thatthe CeO2samples prepared by the two-step precipitation method presentlarger surface area and pore volume than that produced by conventional precipitation method.A severe drop in surface area can be observed for conventionally prepared CeO2at different calcination temperatures, whereas the use of two-precipitation route effectively improves the thermal stability to 900℃,where a surface area of 25 m2·g-1and a pore volume of 0.11 cm3·g-1can still be obtained.To the best of our knowledge,the surface area and pore volume obtained in this work are the highest values reported for CeO2prepared by precipitation method and calcined at such a high temperature[11,17,31].The CeO2samples calcined at 500℃feature isothermals of typeⅣaccording to IUPAC definition,and the curves show a characteristic of cylindrical-ink-bottle-type pores(Fig.6a),through which the gas and thermal diffusion can proceed more easily than any other pore structure[40].BJH pore sizerange of2～8 nm,while for sample CeC,the value is 2～6 nm(Fig．6b)．Note also that,the peak position ofCeC centers at around 2～3 nm,while for CeT,the value is larger(4．0 nm),from which we can conclude that, through the novel two-step precipitation method,larger pores can be created．CeO2-based material with larger pore size exhibits better thermal stability due to its long-range migration while sintering[29]．

X-ray diffraction patterns indicate the presence of a single phase with the cubic fluorite structure typical of CeO2,regardless of preparation routes(Fig．7)．Crystallite sizes of the precipitates were calculated using(111)plane．The crystallite size of CeC is about 1．9 nm,which is smaller than thatof CeT(3．8 nm)．In other words,although the chemical components of CeC and CeT are almost the same the crystal dimensions present a glaring discrepancy．With progressively increasing concentration of the reaction solutions,the mean magnitude for the individual crystals will decrease[41]．In the process of(1．1．1)precipitation,the OH-with high concentration can attack Ce■directly, resulting in crystals with smaller size．However,in the case of two-step precipitation method(1．1．3), crystallization of CeO2proceeds through gradual replacementofCO32-by OH-,thus makes the crystallite size larger,which may facilitates the formation of CeO2with improved thermal resistance because coarser powders require a higher temperature to sinter[28]．

TEM images ofthe precipitates are shown in Fig．8．It is apparent that the particles are approximately sphericaland the grain size ofCeT is larger than thatof CeC．The results confirm the formation of CeO2, indicating thatthe replacementof CO32-by OH-and the crystallization of CeO2(OH)2have occurred．The results are in agreement with XRD outcomes,indicating thatthe growth ofCeO2is affected by the preparation route.

2.3 Reduction behavior of CeO2samples

A crucial requirement of CeO2-based materials, especially when used in three-way catalyst for auto exhaust treatment,is their reduction behavior.Fig.9 displays the TPR profiles of the CeO2samples.The reduction of CeO2samples shows two peaks,the lower one is ascribed to the reduction of surface oxygen species,while the higher one corresponds to the contribution of bulk oxygen species[42].The onset and the reduction peaks ofCeT-t are lowerthan thatofCeC-t,indicating thatthe former is more reducible,which is likely to be related to the disparity of surface areas[22]. Moreover,by comparing the integrated peak area ofthe TPR profiles,we can estimate that the oxygen storage capacity ofCeT-t is larger than thatofCeC-t[43].

3 Conclusions

A two-step precipitation route was developed using ammonium carbonate and ammonia as precipitants successively.The formation mechanism of the CeO2precipitates was also investigated.The chemical compositions were the same for the precipitates prepared by the two-step method and conventional precipitation route employing ammonia as reactant.However,the crystallite size and the pore size of the as-prepared CeO2from two-step procedure are larger,which facilitate producing CeO2with improved thermal stability,textural properties and reduction behavior.After calcination at 900℃for 3 h,the surface area and pore volume are 25 m2·g-1and 0.11 cm3·g-1,respectively,which is the highestreported for CeO2atthe temperature investigated.

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A Two-Step Precipitation Route to CeO2Material with Improved Thermal Stability and Reduction Property

CHEN Shan-Hu YAN Chao-Yang CAO Yi LAN Li ZHAO Ming GONG Mao-Chu CHEN Yao-Qiang＊
(Key Laboratory of Green Chemistry and Technology of the Ministry of Education,College of Chemistry, Sichuan University,Chengdu 610064,China)

O614.33+2

A

1001-4861(2012)05-1001-08

2011-11-11。收修改稿日期：2011-12-23。

国家自然科学基金(No.20803049)和教育部博士点新教师基金(No.20070610026)资助项目。

＊通讯联系人。E-mail：nic7501@scu.edu.cn；Tel/Fax：+86-28-85418451；会员登记号：S06N4556M1006。