摘要]以CeZ (CO )3 } ZrOCh 0 8HZ0和H2C2O4        0 2H2O为原料，第一次成功地采用了机械力活化固相化学法制备纳米Ceo. }s Zro. zs Oz粉体。以XRD, TEM分析及XPS等测试手段对Ceo. }s Zro. zs Oz粉体的结构和形貌进行了表征，结果表明，产物为单一立方相的球形粉体，平均粒径小于20nm，比表面积为85.4m2/g。通过TG-DTA分析，对合成过程中可能发生的化学反应机理进行了分析。对Ceo. }sZro.zs O:氧化物固溶体在三效催化剂中的活性进行了评价。   

【关键词】氧化饰;氧化错;固相化学法;纳米

1  Introduction

N anometer Ce,一xZrxOz materials have drawn extraordinary research interests in recent years due notonly to its good mechanical and electrical proper- tiesl 1}}，but also to the potential applications in dif-ferent areas, such as the catalysts’n，solid oxide e-lectrolyte materialsl 8‘。]，advanced ceramics“13}，etc.There have been many methods to prepare nanometerCel一、ZrxOz  solid  solution,  such  as  coprecipitati-        sol-gels 16-1'i，high  temperature  calcina- tions} 18}，high energy mechanical millings 19}  and hy-drothermal methods Z0}.However,  most of these ap-proaches were complicated and have limitations thatwere not suitable for large scale applications.   

Although substantial efforts have been exertedto the development of new synthetic methodologiesfor  making  more  efficient  nanometer  Cep-x ZrxOzpowders, a grand challenge still exists in the rationaldesign and facile synthesis of nanometer materials inlarge scale at low cost.So far, the mechanically acti-vated solid state chemical reaction method has gradu-ally become a novel synthesis route and a rapidly de-veloping research field}Zli.In this work,  nanometerCeo.}sZro.zsOz powder was firstly prepared by the me-chanically  activated  solid  state  chemical  reactionmethod, which was considered as a more simple andgeneral preparation procedure.The characterizationand properties of nanometer Ceo.}s Zro.zs Oz powderwas investigated and determined. Its catalysis effi-ciency was also checked at the aspect of potential ap-plications·

2  Experimental

2 .1   Preparation

2 .1 .1   Synthesis of nanometer Cep-xZrxOz powders   

In this work,  nanometer Ceo.}s Zro.zs Oz powderwas prepared by mechanically activated solid statechemical reaction method for  the  first  time.Thepreparation procedure was fully illustrated in Fig. 1.At first,  precursors  were prepared by  mixing ofZrOC1Z。8HZ0,  CeZ (C03):and HZCZ04  0 2HZ0 atthe molar ratio of ZrOCIz。8HZ0:Cez(C03  }3CzHzOa 0 2Hz0=0. 25:0.375:0.375.And then,the milling speed of ball mill (model No.XQM-4L)was 150r/min for two hours,  the ratio of grindingmedia to material was 15:1 (i.e.，15g of balls to 1gof powder).The surfactant containing Swt.%wasadded to the ball milling pot after milling an hour.Last, the white precursors after milling were calcined at 600 }C for 3 hours.Thus,  the yellow nanometerCeo.}sZro.2s02 powder was obtained.

2 .1 .2   Preparation of catalysts  

Preparation ofcatalyst A:Catalyst A was composed of the two no-ble metals 0. 40wt.%，promoter 30wt.%and sup-port 69.6wt.%} 22-23}0.Pd and Rh took 0.25%and0.15%respectively.Here the preparation was brief-1y outlined because the details of the catalytic prepa-ration processes could be found elsewhere}20}.First-1y, Ceo.}sZro.2s02 particles were blended with Y-A1203under  mechanically  mixing;  Secondly,  to preparemixture of water solutions of PdC12 and RhC13 at themass ratio of Pd2十and Rh3十一5:3;  Thirdly,  themixture of water solutions of Pd- and Rh-containedsalts was loaded into the mixture of Ceo.}sZro.2s 02 andY人1203  realizing  necessary  reactions  by  incipientwetness method; Finally, the composite was calcinedat SOO}C for 2 hours and subsequently reduced at500 }C under an atmosphere of H2 for 2 hours.    Preparation  of  catalyst  B;   the  preparationprocess was  almost the same as  catalyst A.Thedifference was that nanometer Ceo.}s Zro.2s 02 powderwas prepared by  coprecipitation  method,  and  theprocess was as follows:Firstly,  Ce (N03 )3  0 8H20and ZrOC12  0 8H20 were dissolved in water respec-tively,  to form water solutions both in 0.2mo1/ L,and then they are mixed together; Secondly, aqueousammonia in lmol/L was dropped into the solutionmixtures until the pH = 9.8 under continuously stir-ring; Finally, precipitate was washed and dried thencalcined at 600 }C for 3hr.

2 .2  Characterization  

 Thermo-gravimetric/differential thermal analysis(TG/DTA)was performed using a SPA409PC ana-lytic apparatus operated at the 30m1/min airflow rateand the 5 }C/min heating rate.X-ray powder diffrac-tion (XRD)measurements were made with a RigakuD/Max-3B    diffractometer  employing  Cu-Ka  radia-tion. The average crystallite size of the washed pow-der was estimated from the peak breadth using theSchemer equation. The tetragonal volume fraction ofthe washed powder was derived from the integralpeak intensities using the equations given by Torayaet al. } 24i.The particle size and morphology were ex-amined using a JEM-100CXII transmission electron microscope(TEM).Particle diameters were meas-ured from enlarged photographs.Particle size distri-bution histograms  were  obtained  on  the basis  ofmeasurements from about 300 particles.X-ray photo-electron spectra (XPS)were recorded on a scanningESCA microprobe (Quantum-2000, PHI)photoelec-tron spectrometer using Al-Ka radiation under a vac-uum of 1 X 10一6 Pa. All binding energy values werereferred to carbon (Cm一284.6eV).The specific sur-face area of the powder was measured by five pointBET gas adsorption using an ASAP2010M full-auto-matic adsorption instrument.

2 .3  Experiment of catalytic evaluation   

As for the activity of catalysts, it was evaluatedby monitoring the contents of five components suchas NO, CH, CO, OZ，and COz in automobile exhaustusing a gas chromatograph (GC 9790)and the auto-mobile exhaust analytic system  (FGA-4100)，withthe reference exhaust composed of }(NO)一0.1%，小(CH)一0 .1%}C3H8:C3H}=1:1)，} ( CO)一1 .5%，and } (OZ >=1 .37%in volume fraction.Theinstruments were run with He as buffer gas at S.V.- 40000h一‘and air/fuel (A/F)ratio of 14. 6.Theheating rate was set at 10 }C per minute.The rate ofconversion could be calculated as follow: X denotes the rate of conversion,氏andvolumestand thestarting volume fraction and finalgases, respectively.fraction of

3  Results and discussion

3 .1  Determination of solid solution manner   

Fig.2 and Table 1 displayed that XIRD patternsof the final products resulted from different ratios ofraw materials, and the corresponding crystal cell vol-umes respectively.As can be seen,  the crystal cellvolume decreases gradually with increasing the con-centration of Zr4十ions.In general,  there are twoways that Zr‘十ions could be doped into the crystallattice of CeOz，one is that Zr4+ is filled into thecrystal lattice in the form of interstitial ions,  theexistence of Zr4十ion thus causes a vacancy of Ce4十，leading to enlargement of the crystal cell volume of

 3 .2  TG-DTA Analysis   

The TG-DTA traces were plotted in Fig. 3.TheTG curve  showed  three main  stages  through  thewhole process.Loss of free water from precursorsurfaces firstly took place at about 116 }C. A furtherweight loss followed,  as indicated by the exothermicpeak on the DTA curve at 170 }C. Continuous waterlosing of oxalate and heat decomposition of excessiveoxalic acid resulted in the weight loss .The rate ofweight loss was up to about 22%after this step.Atlast, the heat decomposition of oxalate began and las-ted from 240 }C to 600 }C, reducing the weight of pre-cursors by 26.5%，which was basically consistentwith the theoretical weight loss of 27.7%.Based onTG-DTA analysis,  the following chemical reactionscould occur during heat decomposition;    Chemical Eq.  (2)and (3)stood for the heat de-composition reactions,  while Eq.(4)showed thestoichiometric reaction without loss of weight.Asshown in Fig. 2,  Cez(COs )s  did not react withHZCZ04 0 2HZ0,  only ZrOC1Z  0 8HZ0 reacted withHzCz04 0 2Hz0 during the process of ball milling.

3 .3  Influence of surfactant    In order to avoid the aggregation, the influenceof surface active agents on the product was evalua-ted .TEM images of the nano-sized samples producedwith different surface active agents were shown inFig.4 .As can be seen from these figures, the surfac-tams demonstrated different effects on the crystallin-ity and size of the particles.The cationic surfactanthexadecyl trimethyl  ammonium bromide(CTAB)showed no  distinct effect.The  anionic  surfactantneopelex made the particle size rise up to 37.4nm.On the contrary,  the neutral surfactants PEG-400and Tween-60 enabled the particles to maintain rela-tively small sizes of smaller than 25nm and morehomogeneous distributions.The particles prepared with Tween-60 as surfactant displayed properties su-perior to those with other surface active agents .Theydemonstrated to be in more uniformly spherical shapewith homogeneous dispersion.The average size ofparticles was obtained to be smaller than 20nm,  andthe SBET was 85 .4m2/g.   

The experimental results also indicated that theneutral surfactant could effectively restrain the ag-gregation of particles such that the particle size couldbe well  controlled.The  nonionic  surfactant  washardly affected by the inorganic ions during the grindand could facilitate the formation of hydrogen bondwith water,  thus a protectively hydrophilic film wasformed onto the powder surfaces .The hydrophilicfilm enabled the powder to have spatially steric hin-drance  and  electrostatic  effects,  which  preventedproduct particles from aggregation.H owever,  thecationic surfactant could easily react with HzCz04。2Hz0 due to the alkalinity, so that no obvious effectwas brought onto the particles.The anionic surfac-tant could cause the particle size to increase.

3 .4  XPS Analysis  

 The XPS spectra of Zr 3d、Ce 3d and O 1 s for Ceo.}s Zro.zs Oz oxide were displayed in Fig.  5.Thebinding energies of Zr 3d,  Ce 3d and O 1 s in oxideswere listed in Table 2 for reference and compari-song Zsi.Based on the fitting of XPS spectra with aGaussian shape,  we obtained the binding energies(Eb)of Ce3ds}z and Ce3d3}z to be 882.42eV  and900 .85eV  respectively,  comparatively  close to  thereference Eb (Ce3ds}z)=884. 98eV and Eb (Ce3ds}z903 .68eV measured in solid solution of oxides CeOz/(CeOx+YZ 03+ZrOz)and Cez 03/(CeOx+YZ 03+ZrOz)，respectively.So it was reasonable to believethat both Ce3十and Ce‘十exist in the products.Eb ofZr 3d value was measured to be 182.6eV,  which equal to the referenced binding energy of Zr 3d inZrOz/(CeOx+Yz03 }ZrOz).As for the Eh of O islevel,  two binding energies,   529.98eVeV，532.were  obtained,  correspondingtoand 532 .03530. 0  andOeV in solid solution of CeOz/ (CeOx+Yz 03+ZrOz)and Cez 03/(CeOx+ YZ 03+ZrOZ)，respective 1y. These facts proved that the products prepared bymechanically activated solid state chemical reaction tobe in the state of oxide solid solution, which was inagreement with XRD results.3 .5  Evaluation of catalysts    In catalysis experiments, catalyst A and B were used to decontaminate the automotive exhaust.Thecatalysis efficiency was determined by monitoring thecontents of greenhouse gases such as NO,  CH,  and 4  Conclusion    In summary,  Nano-Ceo.}s Zro.zs Oz powder wasprepared by using the solid-solid chemical reactionmethod under the action of mechanical power .Theproducts were determined to be in the single-cubic-phase,  spherical shape,  and the average size smallerthan 20nm,  the specific surface area of 85 .4m2 / g,and more uniform dispersion.   

The effects of surfactants employed in the syn-thesis process of the products have also been investi-gated .The facts prove that Tween-60 and PEG-400have better effects on the control of particle sizes. CO,  through the conversion rate.The conversionrates in percentage of NO, CH and CO have been cal-culated according to Eq.(1)and shown in Fig.6.The light-off temperatures (Tso)of the above gaseswere listed in Table 3.As seen from Fig. 6 and Ta-ble 3,  no obvious difference could be found betweenthe two catalysts,  except for the fluctuation of 1一2}C at the light-off temperature.Although the twocatalysts, prepared by different methods, almost hadthe same catalysis functions, the mechanically activa-ted solid state chemical reaction method still pos-sessed obvious advantages of manufacturing, energy-saving,  and no solvent,  indicating its promisinglycommercial and industrial potentials.    The mechanically activated solid state chemicalreaction  method  possessed  obvious  advantages  inmanufacturing, energy-saving,  and no solvent, indi-eating its promising commercial and industrial poten-tials.

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