电镜

Journal of Alloys and Compounds 494 (2010) 340–346Contents lists available at ScienceDirectJournal of Alloys andCompoundsj o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /j a l l c omInfluence of thermal treatment on catalytic performance of Pd/(Ce,Zr)O x –Al 2O 3three-way catalystsBo Zhao a ,b ,Chunqing Yang a ,Qiuyan Wang a ,Guangfeng Li a ,Renxian Zhou a ,∗a Institute of Catalysis,Zhejiang University,Hangzhou 310028,PR ChinabSchool of Pharmaceutical and Chemical Engineering,Taizhou University,Taizhou 317000,PR Chinaa r t i c l e i n f o Article history:Received 25August 2009Received in revised form 7January 2010Accepted 9January 2010Available online 18 January 2010Keywords:Sol–gelSupercritical drying Three-way catalysts CZA mixed oxidesa b s t r a c tCeO 2–ZrO 2–Al 2O 3(CZA)mixed oxides were synthesized using sol–gel and supercritical drying methods,and their physicochemical properties were characterized by BET surface area (BET),X-ray diffraction (XRD),H 2-temperature-programmed reduction (H 2-TPR)and O 2-temperature-programmed desorption (O 2-TPD)techniques.The catalytic performances of Pd/CZA catalysts with different thermal treatment were also investigated.The results indicate that catalytic activities are affected by thermal treatment of support or catalyst and Pd/CZA catalysts exhibit good thermal stability.H 2-TPR results show that the effect of thermal treatment on the dispersion state of PdO is different between catalyst and support after calcination.However,O 2-TPD results show that the influence of thermal treatment on the desorption performance of oxygen is similar in both cases.Desorption of surface oxygen is closely related to surface area of the support.During the calcination of the supports or the catalysts at high temperature,the surface oxygen desorption capacity and the mobility of lattice oxygen decrease due to the sintering of supports,which is harmful to the dispersion of active species,corresponding to the decrease of three-way catalytic activities.Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved.1.IntroductionThree-way catalysts (TWCs)are today’s most successful exhaust after-treatment system for gasoline engines.With a warmed up catalyst and at steady-state conditions conversions of more than 95%are reached for the three pollutants CO,NO and HC [1].In recent years,typical TWC formulations have included Pd as the active metal,fluorite-type oxides,such as ceria–zirconia,as pro-moters and alumina as support as well as other minor components mainly present in order to enhance thermal stability [2–4].In mod-ern TWC formulations,the promotion by ceria has been extended to other structurally related oxide systems in order to increase or maintain the durability of the TWC while enhancing the oxygen-storage capacity (OSC)properties of the catalyst [5,6].In this sense,Ce–Zr mixed oxide systems have attracted much attention as they provide greater OSC after thermal sintering,potentially increasing the useful lifetime of the catalytic system [7–9].Concerning the active metal,the use of Pd as the only active metal component in TWC has received considerable attention [5,10].Part of this interest has been purely economic,resulting from the high cost and scarcity of Rh.On the other hand,recently,Pd has been widely used in TWC∗Corresponding author.Tel.:+8657188273290;fax:+8657188273283.E-mail address:zhourenxian@ (R.Zhou).for its good performance of HC oxidation and durability at high temperatures [11].Automotive emission standards are becoming increasingly stringent which requires continuous improvements in the exhaust after-treatment systems [12].Among them,achievement of lower light-off temperature during the cold-start phase is of the highest relevance,considering that more than 50%of the total emission is produced during that period [13].Most studies available in the literature have concentrated on improving the light-off behavior [14]and it is necessary to place the TWC closer to the engine in order to obtain its light-off activity immediately after the start up of the engine.Therefore,the catalyst must be able to have better thermal stability [15].Thus,the development of new-generation TWC requires that the oxygen-storage component exhibits better thermal stability and higher OSC.However,for the CeO 2–ZrO 2(CZ)compounds,the thermal stability of texture is not good enough,for example,after calcinations at 1273K,the specific surface area is lower than 10m 2g −1,which cannot meet the requirement of TWC [16].The “diffusion barrier concept”was presented through intro-ducing Al 2O 3into CeO 2–ZrO 2solid solution on a nanometer scale by Morikawa et al.[17].Recently,the effects of Al 2O 3addition on the thermal stability and reduction behavior of mixed oxides have also been reported in literatures [13,16].The CeO 2–ZrO 2–Al 2O 3(CZA)materials are hopeful to simultaneously maintain high-surface area of Al 2O 3and good OSC of CZ oxygen-storage materials [18].Fur-thermore,the catalyst containing CZA could significantly reduce0925-8388/$–see front matter.Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved.doi:10.1016/j.jallcom.2010.01.031B.Zhao et al./Journal of Alloys and Compounds494 (2010) 340–346341the NO x emission[19].So investigation of CZA mixed oxide is significant to enhance performance of TWC.Moreover,these mate-rials would eventually help to decrease the cold-start emissions mainly by permitting the catalyst to be located closer to the engine [20–23].The chemical and structural characteristics of CZA mainly depend on the preparation method.The main preparation methods include sol–gel[24],coprecipitation[12]and impregnation[25,26]. However,further research should be done to improve the thermal stability of CZA mixed oxide.Therefore,there is currently a great deal of interest in prepar-ing CZA effectively.In this paper,high-surface area of CZA mixed oxides were synthesized using sol–gel and supercritical drying methods.TWC supported on CZA were prepared and the catalytic performances of0.5wt%Pd/CZA were also investigated.Reactant mixture simulated to the actual automotive exhaust was adopted to evaluate the catalytic performance of the catalysts.The redox properties of the materials were studied by means of temperature-programmed reduction(H2-TPR)and temperature-programmed desorption(O2-TPD)experiments.2.Experimental2.1.Catalyst preparationCZA mixed oxide was prepared by sol–gel methods combined with super-critical drying techniques.The solution of aqueous NH3was slowly dropped into quantitative mixed aqueous solutions of Ce(NO3)3,ZrO(NO3)2and Al(NO3)3under continuous stirring and the pH value was controlled to be9.5.The molar ratio of Ce:Zr was3:1and the content of CeO2–ZrO2in CZA was18wt%.After aging at room temperature for12h,the precipitate wasfiltered and washed with deionized water. Then the precipitate was dried under supercritical condition in alcohol(250◦C, 7.5MPa)for3h.Then the samples were calcined at500,800,900,1000and1100◦C, respectively,for4h in air.These as-prepared mixed oxides were denoted as CZA5, CZA8,CZA9,CZA10and CZA11,respectively.CZ-free alumina was also prepared by a similar procedure and was calcined at500or1100◦C for4h.The Pd-only three-way catalysts with different CZA mixed oxides as supports were prepared by incipient impregnation method with aqueous solutions of H2PdCl4 as metal precursors(Sinopharm Chemical Reagent Co.,Ltd.).Pd nominal loading was0.5wt%.The impregnated samples were reduced with hydrazine hydrate,and washed with a large amount of deionized water until no Cl−ions was detected in thefiltered solution.The catalysts were dried at110◦C for12h and then calcined at 500◦C for2h.These as-prepared fresh catalysts,denoted as Pd/CZA-5,were further calcined at800,900,1000and1100◦C,respectively,for4h in air.The obtained aged catalysts were then denoted as Pd/CZA-8,Pd/CZA-9,Pd/CZA-10and Pd/CZA-11.Pd/Al2O3catalysts were also prepared by a similar procedure and were calcined at500and1100◦C for2h,respectively.The obtained catalysts were denoted as Pd/Al2O3-5and Pd/Al2O3-11.2.2.Characterization techniques2.2.1.BET surface areaSpecific surface area of the samples was measured by N2adsorption using an OMNISORP100CX apparatus.Prior to adsorption measurements,samples were degassed at200◦C for2h under vacuum,and N2adsorption was carried out at the liquid nitrogen temperature.2.2.2.X-ray powder diffraction(XRD)The powder X-ray diffraction experiment was recorded on a Rigaku D/Max-IIIB diffractometer employing monochromatic Cu K␣radiation.The X-ray tube was operated at45kV and40mA.2.2.3.Transmission electron microscopy(TEM)The size of the metallic particles on the supported Pd catalysts was checked with transmission electron microscopy using a JEM-2010apparatus operated at200kV. For the TEM analysis,the samples were grounded in a mortar and then deposited on a Cu grid covered with a perforated carbon membrane.2.2.4.Temperature-programmed reduction(H2-TPR)Hydrogen temperature-programmed reduction(H2-TPR)experiments were car-ried out in aflowing system to observe the reducibility of the sample.Typically0.05g of sample was employed.The standard pretreatment consisted of heating the sam-ple in air from room temperature to300◦C,holding at that temperature for0.5h. After cooling to−20◦C in the same atmosphere,the sample was heated underflow-ing H2(5%in Ar,40ml min−1)at a rate of10◦C min−1.The outlet gases were analyzed on-line by a gas chromatograph(GC,KX100)with a thermal conductivity detector (TCD),and the effluent H2O formed during H2-TPR was adsorbed with a5A molec-ular sieve,the latter not influencing the transient response of the TCD.Based on the amount of H2consumption observed over a standard CuO sample in similar TPR procedures,the amount of H2consumption for the samples were obtained.2.2.5.Temperature-programmed desorption(O2-TPD)Temperature-programmed desorption(O2-TPD)experiments were employed to investigate the O2desorption with temperature.The sample(0.10g)wasfirst pretreated inflowing O2(5%in He)at300◦C for1h in a quartzfixed-bed micro-reactor.After cooling to room temperature in the same atmosphere,the sample was purged inflowing He(40ml min−1)for30min.After that,the sample was heated at a rate of20◦C min−1to1000◦C.The outlet gases were analyzed on-line by a gas chromatograph(GC,KX100)with TCD.2.2.6.Catalytic testsThe three-way catalytic activity was evaluated in thefixed-bed reactor with the simulated exhaust gas containing CO(6800ppm),HC(C3H8,C3H6)(3300ppm),NO (1000ppm),NO2(300ppm),O2(1.75%)and Ar(balance)and theirflow rates were controlled by massflow controllers before entering the blender.The concentration of CO,NO,NO2and total HC(C3H6and C3H8)were analyzed by a Bruker EQ55FTIR spectrometer coupled with a multiple reflection transmission cell(Infrared Analysis Inc.).The space velocity was48,000h−1and0.2ml of catalysts(40–60mesh)were loaded in the reactor.The curves about the relationship between conversion and temperature were obtained.3.Results and discussionThe BET surface area of the CZ-free alumina calcined at500◦C is 300m2g−1.In contrast,the CZA support calcined at500◦C has large surface area and the BET value is362m2g−1.On the other hand,the CZA sample prepared by sol–gel and supercritical drying methods presents higher surface area in comparison with material obtained by microemulsion(about180m2g−1)method[27].Surface areas of CZA supports decrease obviously with increasing calcination tem-perature.The value of BET specific surface area is213m2g−1for CZA8,which is higher than that of CZA materials calcined at700◦C and prepared by coimpregnation(about120m2g−1)and sol–gel (about170m2g−1)methods[24].The values of BET specific sur-face area are133,121m2g−1for CZA9and CZA10,respectively. Moreover,the CZA11sample exhibits surface area of97m2g−1after calcination at1100◦C for4h,which is obviously higher than that of CZ-free alumina(35m2g−1)calcined at1100◦C.These results indicate that CZA mixed oxide prepared by sol–gel and supercritical drying methods shows higher heat resistance.Fig.1shows the XRD patterns of CZA mixed oxides calcined at different temperature.From Fig.1,it can be seen that the cubic and tetragonal phases of Ce x Zr1−x O2solid solutions are observed for all the samples and no pure CeO2diffraction peaks aredetected.Fig.1.XRD patterns of CZA pretreated at different temperature:(a)CZA5;(b)CZA8;(c)CZA9;(d)CZA10;(e)CZA11.342 B.Zhao et al./Journal of Alloys and Compounds494 (2010) 340–346Fig.2.TEM photographs of catalysts:(A)Pd/Al 2O 3-5;(B)Pd/Al 2O 3-11;(C)Pd/CZA-5;(D)Pd/CZA-11.The Ce x Zr 1−x O 2solid solutions show mainly the cubic phase due to high Ce content,which also occurs in CeO 2–ZrO 2/Al 2O 3speci-mens prepared by coimpregnation via organic precursors [24].The cell parameter of CZA5,CZA8,CZA9,CZA10and CZA11is 0.53601,0.53251,0.53103,0.53086and 0.53215nm,respectively,which is smaller than pure CZ and CeO 2(0.54nm)[27,28].It may be attribute to the fact that the ion radius of Al 3+(0.057nm)is smaller than that of Ce 4+(0.097nm)/Ce 3+(0.114nm)and Zr 4+(0.084nm).Incorpora-tion Al 3+into the CZ lattice by replacing of Ce 4+/Ce 3+or Zr 4+leads to the shrinkage of the lattice parameter.Moreover,with increasing the incorporation of Al 3+,the lattice parameter becomes smaller.Significant sharpening of the XRD patterns is normally observed with the increase of calcination temperature,which is due to the growth and sintering of the crystallites.Especially for the sample calcined at 1100◦C,the peaks of CZA solid solution become sharper and more intense,corresponding to an increase in the crystalline particle size and a decrease of the BET area of the supports.The major problem with CeO 2is the loss of OSC due to sintering of the particles and forming of CeAlO 3with ␥-Al 2O 3when the exhaust temperature exceeds 850◦C [29].However,in contrast with the results of Kozlov et al.[24]reported,no CeAlO 3diffraction peaks are detected during calcination of the mixed oxides at high tem-perature in our research.Moreover,it is interesting to note that ␥-Al 2O 3phase was transformed into ␦-Al 2O 3after CZA calcined at temperature higher than 800◦C.The results show that the pres-ence of ceria–zirconia mixed oxide inhibits the transformation of ␥-alumina into the low-surface-area ␣-phase,which is typical at this temperature [30].On the basis of these results a conclusion can be drawn that the interaction between CZ and Al 2O 3in the CZA mixed oxides is stronger,which is the main reason for big surface areas and high thermal stability of CZA mixed oxides.Fig.2shows TEM pictures of Pd/Al 2O 3and Pd/CZA catalysts calcined at 500and 1100◦C.For the catalysts calcined at 500◦C,PdO x particles exhibit well-dispersed state and shows small parti-cle size on the surface of supports.The average particle size of Pd on Pd/Al 2O 3-5and Pd/CZA-5is 6.0and 5.0nm,respectively.The micrograph shows that the CZA mixed oxides exhibit small sphere while free Al 2O 3exhibit filiform.The sintering of Pd particles and supports occurs obviously after calcined at 1100◦C for all catalysts.From Fig.2(B),it can be seen that for Pd/Al 2O 3-11catalyst seri-ous sintering and encapsulation phenomena of Pd particles occur due to significant agglomeration of support.On the opposite,for the Pd/CZA-11catalyst,PdO x particles still exhibit good dispersed state and no encapsulation phenomena exist.The average parti-cle size of Pd on Pd/Al 2O 3-11and Pd/CZA-11is 34.0and 19.5nm,respectively.That indicates that the addition of CZ would inhibit the sintering of PdO x particles in Pd/CZA catalyst and thus improve the thermal stability of the supported Pd catalyst because of the increased thermal stability of its support,which is in agreement with the BET and XRD results.Results obtained from the light-off tests under simulated exhaust gas over Pd/CZA catalysts are shown in Fig.3(catalyst was calcined at different temperature)and Fig.4(CZA support was calcined at different temperature).The light-off (T 50)and full-conversion (T 90)temperature of CO,HC,NO and NO 2over different Pd/CZA catalysts are shown in Table 1.T 50(corresponding to the temperature at which 50%conversion of a given compound is com-pleted)and T 90(corresponding to the temperature at which 90%conversion of a given compound is completed)are used to evalu-ate the activities of different catalysts.From Fig.3and Table 1we can see that the catalytic activities for both oxidation and reduction reactions decrease with the increasing of calcination temperature.B.Zhao et al./Journal of Alloys and Compounds494 (2010) 340–346343Fig.3.Three-way conversion as a function of reaction temperature over the Pd/CZA catalysts(catalysts calcined at different temperature).Fig.4.Three-way conversion as a function of reaction temperature over the Pd/CZA catalysts(supports calcined at different temperature).344 B.Zhao et al./Journal of Alloys and Compounds494 (2010) 340–346Table1Light-off(T50)and full-conversion(T90)temperature of CO,HC,NO and NO2over different Pd/CZA catalysts.Catalysts T50(◦C)T90(◦C)CO HC NO NO2CO HC NO NO2Pd/CZA-5171300352230191337358348 Pd/CZA-8174303354235200340363354 Pd/CZA-9182305357264204350364358 Pd/CZA-10191310360281219348368358 Pd/CZA-11205310359287218350368362 Pd/CZA5171300352230191337358348 Pd/CZA8178321363265202373368355 Pd/CZA9184326368274215383374372 Pd/CZA10195340373290222397378390 Pd/CZA11204350374305226411378399For CO conversion,the T50and T90increase more obviously com-pared with those of HC and NO.Moreover,the catalyst calcined at 1100◦C still present acceptable catalytic activity,which indicates that Pd/CZA catalysts exhibit better thermal stability.From Fig.4and Table1,it is worth pointing out that light-off activities of the catalysts are significantly affected by the calcina-tion temperature of the support.The catalytic activities decrease obviously with increasing calcination temperature of the supports. Compared with the catalyst supported on CZA calcined at500◦C, the T90for different reactants increases clearly in the range of 20–80◦C over the catalyst supported on CZA calcined at1100◦C. The data indicate that high temperature calcination of the support results in the decrease of catalytic activity due to sintering.Fur-thermore,the dispersion of active species is deteriorated with the decreasing of BET surface area of the support[31].Fig.5shows the H2-TPR profiles of the Pd/CZA catalysts calcined at different temperature.The H2consumption and peak tempera-ture maxima are listed in Table2.From Fig.5it can be seen that three positive peaks of hydrogen consumption at about40,70and 105◦C are found in all catalysts.These peaks are designated by␣,␤and␥.According to the previous work in our research group,we suggest that the peaks␣is ascribed to the reduction of the PdO dispersed on alumina-rich grains and the peak␤is ascribed to the reduction of the PdO species dispersed on Ce–Zr-rich grains[32]. For the peak␥,it is suggested that the stable PdO species are present on the Pd/CZA catalysts due to the strong interaction between PdO and ZrO2[33].The negative peak results from the decomposition of palladium hydride which is also seen in the reduction of bulk PdO [34].Palladium oxide is reduced to palladium metal on exposure to H2andthis metal absorbs hydrogen,forming the hydride phase, which decomposes near80◦C[35,36].In addition,from Table2 it can be seen that the peak␣shifts to higher temperature with increasing calcination temperature of the catalysts.On the other hand,H2consumption decreases.It is interesting to note that the trend of the peak␣with temperature increasing is similar to that of decreasing of their catalytic activities.This suggests that the cat-alytic activity may be related to the reducibility of␣-PdO species, and the higher the reducibility of the PdO species,the higher the catalytic activities.From Fig.5we can also see that for the catalyst Fig.5.TPR profiles of the catalysts calcined at different temperature:(A)Pd/CZA-5;(B)Pd/CZA-8;(C)Pd/CZA-9;(D)Pd/CZA-10;(E)Pd/CZA-11.calcined at1100◦C,the H2consumption of␤-PdO species increases obviously.But for the catalyst calcined below1100◦C,the changes for the H2consumption and reduction temperature of␤-PdO and ␥-PdO species are not obvious,indicating that strong interaction exists between PdO species and support.These results indicate that the essential change of dispersion state of surface PdO species can not be seen during calcination of the catalysts.Moreover,the Pd/CZA catalysts exhibit high heat resistance.H2-TPR profiles of the Pd/CZA catalysts with supports calcined at different temperature are shown in Fig.6.The influence of sup-ports calcination temperature on the reduction performance of the Pd/CZA catalysts is significant.Although all catalysts exhibit three peaks of hydrogen consumption and a negative peak of pal-ladium hydride decomposition,great changes have happened for the dispersion state of PdO species in Fig.6compared with those in Fig.5.In addition,it can be seen that the trend of the peak␣-PdO with temperature increasing and the decreasing amount of H2con-sumption is similar to that of their catalytic activities decreasing. From Fig.6,wefind that the peak temperature for␤-PdO slightly changes but the amount of H2consumption increases obviously.In particular,for the supports calcined at1000or1100◦C,their TPR profiles exhibit two peaks of hydrogen consumption at60–100◦C, which indicates that a new dispersion state of PdO species appears. From Fig.6we can also see that the decomposition peak of pal-ladium hydride obviously shifts to higher temperature and shows less intense negative peak with the increasing of calcination tem-perature of the supports.Decreased intensity of the hydride peak suggests that less Pd was reduced on initial exposure to H2for the catalyst with support calcined at higher temperature,and conse-quently less hydride was formed.This,along with the increasedTable2H2consumption and temperature of TPR peaks for the catalysts calcined at different temperature.Catalysts␣Peak␤Peak␥PeakH2consumption (␮mol/g cat)Peak temperature(◦C)H2consumption(␮mol/g cat)Peak temperature(◦C)H2consumption(␮mol/g cat)Peak temperature(◦C)Pd/CZA-559.1±0.582710.8±0.0945 5.6±0.3698 Pd/CZA-858.6±0.6146 1.1±0.676212.1±0.21103 Pd/CZA-956.7±0.57480.89±0.5862.514.8±0.37102 Pd/CZA-1053.2±0.2550 1.0±0.5562.514.9±0.35103 Pd/CZA-1151.9±0.4954 5.4±0.117015.4±0.35106B.Zhao et al./Journal of Alloys and Compounds494 (2010) 340–346345Fig.6.TPR profiles of the catalysts(supports calcined at different temperature):(A) CZA5;(B)CZA8;(C)CZA9;(D)CZA10;(E)CZA11.temperature of PdO reduction,is consistent with a stronger inter-action between the metal and the support for the catalyst calcined at higher temperature of its support[37,38].At the same time, the results further show that the dispersion state of active species changes during calcination of the support due to the sintering and the decreasing of the BET surface area of CZA.Fig.7shows the O2-TPD profiles of the Pd/CZA catalysts cal-cined at different temperature.The TPD profiles of all catalysts display four peaks of oxygen desorption at about150,415,675 and975◦C,respectively.The peaks at about150and415◦C are ascribed to desorption of surface oxygen species.The peak at about 675◦C is due to decomposition of PdO species and the peak at about 975◦C is attributed to desorption of support bulk lattice oxygen [39].It is well known that the introduction of ZrO2into CeO2cre-ates a certain amount of oxygen vacancies(V o)via the formation of[Ce3+–V o–Ce3+]clusters[40].Fig.7indicates that the support of Pd/CZA-5can release a lot of oxygen.This is possibly due to the oxygen mobility increases with an increasing of V o concentration in support[41].From Fig.7,we can also see that oxygen desorp-tion area decreases obviously with the increasing of calcination temperature of the catalyst,especially for the desorption peaks at150and415◦C.This suggests that desorption of surface oxygenisFig.7.O2-TPD profiles of the catalysts calcined at different temperature:(A) Pd/CZA-5;(B)Pd/CZA-8;(C)Pd/CZA-9;(D)Pd/CZA-10;(E)Pd/CZA-11.Fig.8.O2-TPD profiles of the catalysts(supports calcined at different temperature): (A)CZA5;(B)CZA8;(C)CZA9;(D)CZA10;(E)CZA11.closely related to the surface area of support.With the decreasing of support surface area,desorption of surface oxygen is deteriorated. Furthermore,with the particle size increasing during high temper-ature calcination of the catalysts,the interaction between PdO and supports is enhanced,which results in decreased decomposition of PdO species.O2-TPD profiles of the Pd/CZA catalysts with supports calcined at different temperature are shown in Fig.8.From Fig.8,it can bee seen that the peak areas of desorption at different temperature decrease obviously with the increasing of calcination temperature of CZA, which is similar to that of Fig.7.These results indicate that physic-ochemical properties of the supports,such as surface area,particle size,structure and stability play a key role in catalytic performance of catalysts.During calcination of the supports or the catalysts at high temperature,the surface oxygen desorption capacity and the mobility of lattice oxygen decrease due to supports sintering,which is harmful to the dispersion of active species,as a result,three-way catalytic activities decrease correspondingly.4.ConclusionsThe catalytic activities are affected by thermal treatment of sup-port or catalyst.Moreover,Pd/CZA catalysts in our research exhibit good thermal stability.H2-TPR results show that strong interaction exists between PdO species and the support.Moreover,different effect on the dispersion state of PdO can be seen between calci-nation of catalyst and support.The essential change of dispersion state of surface PdO species cannot be seen during calcination of the catalyst but significant influence can be seen on the samples with supports calcined at different temperature.The amount of H2consumption of the peak␣-PdO decreases obviously with the increasing of calcination temperature of the supports.But for the peak␤-PdO the trend is opposite.O2-TPD results indicate that the treatment temperature of catalysts or supports has almost the same effect on the desorption behavior of surface and bulk oxygen, depending mainly on the anti-sintering performance of supports. 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