氧化铈纳米粒子的制备ppt课件.ppt

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1、Members: Xianhong Rui Yu Chen Litao Yan Huamin Yao Liangjun YiDepartment of Materials Science and EngineeringUniversity of Science and Technology of ChinaJan 3, 200813. 2CeO2属于萤石型氧化物。属于萤石型氧化物。 CeO2晶胞中的晶胞中的Ce4+按面心按面心立方点阵排列，立方点阵排列，O2-占据所占据所有的四面体位置，每个有的四面体位置，每个Ce4+被被8个个O2-包围，而每个包围，而每个O2-则与则与4个个Ce4+配位。配

2、位。1. Structure of CeO22.功能特性功能特性 CeO2的结构中有的结构中有1/2立方体空隙，可称之为敞立方体空隙，可称之为敞型结构。敞型结构允许离子快速扩散。经高温型结构。敞型结构允许离子快速扩散。经高温(T950)还原后，还原后，CeO2转化为具有氧空位、非化转化为具有氧空位、非化学计量比的学计量比的CeO2-X氧化物氧化物(0 x0.5)，而在低温下，而在低温下(T450) CeO2可形成一系列组成各异的化合物。可形成一系列组成各异的化合物。 值得注意的是，即使从晶格上失去相当数量值得注意的是，即使从晶格上失去相当数量的氧，形成大量氧空位之后，的氧，形成大量氧空位之后，

3、CeO2仍然能保持萤仍然能保持萤石型晶体结构，这种亚稳氧化物暴露于氧化环境石型晶体结构，这种亚稳氧化物暴露于氧化环境时又易被氧化为时又易被氧化为CeO2，因而，因而CeO2具有优越的储存具有优越的储存和释放氧功能及氧化还原反应能力，同时和释放氧功能及氧化还原反应能力，同时CeO2也也有着良好的化学稳定性和高温快速氧空位扩散能有着良好的化学稳定性和高温快速氧空位扩散能力。力。 3Applications of CeO2 玻璃脱色剂氧化铈大颗粒氧化铈磨料氧化铈抛光粉/液晶显示屏氧化铈抛光粉氧化铈抛光轮CeO2 Slurry 此外，此外， CeO2还用作催化材料、高温氧敏材料还用作催化材料、高温氧敏

4、材料、 pH传感材料、电化学池中膜反传感材料、电化学池中膜反应器材料应器材料、燃料电池的中间材料、中温固体氧化物燃料电池燃料电池的中间材料、中温固体氧化物燃料电池(SOFC)用电极材料用电极材料4Synthesis of CeO21. Direct precipitationprecipitationStir and ageing stageScouring and dryingto calcine precursorThe power of CeO2Ce3+ or Ce4+technology of direct precipitationprecipitantNitrate: Ce(NO3

5、)3 or (NH4)2Ce(NO3)6Precipitant: ammonia or NH4HCO3 Surface active agent: PEG-4000Process: nitrate and PEG-4000 were dissolved in distilled wate.Then ammonia or NH4HCO3 solution was added dropwise under vigorous stirring till the pH reached 9. The precipitate was filtered, washed thrice with distill

6、ed water and alcohol and dried at 80 over night.5(a)(b)(c)(d)Results and discussion10203040506070 2(a)(b)(c)(d)SEM photoes of precursorXRD of precursor(a): Ce(NO3)3 + NH3H2O(b): (NH4)2Ce(NO3)6 + NH3H2O(c): Ce(NO3)3 + NH4HCO3 (d): (NH4)2Ce(NO3)6 + NH4HCO3610203040506070050010001500200025003000 2(a)(c

7、)XRD of CeO2 synthesized at 70010203040506070 (a)(b)(c)(d)XRD of CeO2 synthesized at 50010203040506070 2(a)(c)XRD of CeO2 synthesized at 6007(a)(c)SEM photoes of CeO2 calcined at 6008Microwave homogeneous precipitationMicrowave reaction equipmentNitrate: Ce(NO3)3 or (NH4)2Ce(NO3)6Precipitant: urea S

8、urface active agent: PEG-4000CO(NH2)2 + H2O CO2 + 2NH3NH3 + H2O NH4+ + OH-CO2 + H2O CO32- + 2H+ 水解生成的构晶水解生成的构晶离子离子OH-、CO32-, ,在微波辐照作用在微波辐照作用下下, ,与与Ce3+、Ce4+等结合生成不溶等结合生成不溶前驱物前驱物 9Results and discussion10203040506070 2XRD of precursor calcined at 500(a)(b)(c)10203040506070050100150200250 2XRD of precu

9、rsor (a)Mean:(a)0.093um(b)0.171um(c)0.210umLS of CeO2 calcined at 600 (a) Ce(NO3)3 + urea, without PEG-4000 (b) Ce(NO3)3 + urea + PEG-4000(c) (NH4)2Ce(NO3)6 + urea + PEG-400010102030405060700500100015002000 2600700XRD of CeO2 synthesized at 600、700 SEM photo of CeO2 calcined at 600SEM photo of precu

10、rsor(a)11Hydrothermal synthesis of CeO2 nano-particles1. Cerium(IV) hydroxide precursorA.I.Y. Tok ,et al (Nanyang Technological University), Journal of Materials Processing Technology 190 (2007) 217222H2O2 + cerium(III) nitrate , stirred for 5 min under heat to convert Ce3+ to Ce4+ammonia (pH =8.8),

11、 stir continuously at 80 for 1 hthe pale yellow precipitates (Ce(OH)4) were washed ,the conductivity of the supernatant =2ms30 ml of the washed precipitates (pH=10) were placed into the Teflon vessel of the hydrothermal bomb, then placed in the oven and heated at the respective durations (024 h)The

12、final products were re-washed, conductivity=2ms, dried at 75 122. Ceria acetate precursorhydrous cerium oxide stabilized by acetate ions (cerium acetate gel) was dissolved in deionized water to yield acetate stabilized colloidal ceria and will be identified as ceria acetateceria acetate was diluted,

13、 placing 30 ml of the solution into the Teflon vesselthe bomb was then placed in the oven and heated to 250 at different treatment timesthe products were later centrifuged and dried at 75 13Fig. 1. DTA/TG of Ce(OH)4 precursorResults and discussionThe total measured weight loss from 25 to 900 was 11.

14、64%, while the theoreticalweight loss for the decomposition of cerium hydrate oxide is 17.3%, i.e. Ce(OH)4/CeO22H2O to CeO2The decomposition of the precursor is a form of dehydration process of the hydrated CeO2the difference in weight loss observed could be due to the following reasons: (a) precipi

15、tate consisting of a partially hydrated form of ceria, (i.e. CeO2xH2O), for which a 11.64% weight loss on decomposition corresponds tox = 1.35 or (b) the precipitate consisted of a mixture of phasesl i k e C e O2 2 H2O + C e O214Fig. 2 DTA/TG of ceria acetate precursorThe precursor measured a total

16、weight loss of 12.55% with four distinct temperature peaksThe first endothermic peak was detected at around 100. This is attributed to the release of the water molecules present in the precursorFrom 100 to 200, the weight loss was attribute to the removal of the surface acetate groups and later the

17、formation of the acetic acid when surface acetate hydrolysis occurs. This also explains the very weak endothermic peak detected at 200There was a sharp weight loss from 200 to 400 and a corresponding exothermic peak. This exothermic peak suggests the formation of oxyacetate and dioxocarbonate comple

18、xes with cerium, Ce(OH)(CH3COO) andCe2O2CO3As temperature increased to 700, the Ce2O2CO3decomposed endothermally to produce the final product CeO215Fig. 3 DTA/TG for CeO2 synthesized from ceria acetate: (a) after 6 h treatment;(b) after 24 h treatmentafter 6 and 24 h of hydrothermal treatment, weigh

19、t loss is dramatically re d u c e d t o 2 . 6 4 a n d 1 . 3 7 %The distinct temperature peaks are similar to that of the precursor. However,the distinct exothermic peak for the hydrothermal treatedsamples is no longer as pronounced a s t h a t o f t h e p r e c u r s o rThis could be due to the amou

20、nt of acetate complexes formation being re d u c e d c o n s i d e r a b l y a f t e r h y d r o t h e r m a l t r e a t m e n t .Traces of cerium acetate complexes were still present in the samples after hydrothermal treatment. The amount is however,significantly lower than that found in the precur

21、sor16Fig. 4 CeO2 using Ce(OH)4 precursor (250 ) as a function of timeFig. 5 CeO2 using ceria acetate precursor (250) as a function of timeFig. 4, the nano-particles exhibited some degree of crystallinity and displayed all of the major peaks of CeO2 with a cubic structure after 6 h treatmentNo signif

22、icant improvement in crystallinity was observed between 6 and 24 h, and the peaks were broad with weak intensities. This trend is similar with the ceria acetate systemFig. 5, the peaks are significantly narrower with higher intensities suggesting larger crystallite sizes at an average of 15.5 nm as

23、calculated and larger degree of crystallinity as compared to the cerium(IV) hydroxide system. The peaks at higher 2 angles can also be clearly observed for all samples17Fig. 6. Lattice constant of CeO2 after hydrothermal treatment at 250 using Ce(OH)4 precursorFig. 7 Lattice constant of CeO2 after h

24、ydrothermal treatment at 250 using ceria acetate precursorthe lattice parameter decreased by about 0.2% after hydrothermal treatment at 250 for 6 h. From 6 to 12 h at the same temperature, the lattice expanded. The lattice constant only varied within a narrow range (|a|/a0.03%) after 12 h, indicatin

25、g that the structure became stable.The lattice constant decreased by about 0.5% after hydrothermal treatment at 250 C for 6 h. Further changes of lattice constant were very small when treatment duration was increased. The variation of lattice constant was less than 0.03%18Fig. 8 CeO2 from Ce(OH)4 (2

26、4 h) heat treated at (a) 500 , (b) 1000Fig. 9 CeO2 from ceria acetate (24 h) heat treated at (a) 500 , (b) 1000 In both figures, it can be seen that the characteristic peaks are sharper and narrowerThe higher 2 peaks for the hydroxide system can also be observed after heat treatment. This crystallit

27、e size after heat treatment at 500 and 1000 grew to 8.8 and 47.4 nm, respectivelyThe samples from the ceria acetate system exhibited a larger degree of crystallinity than cerium hydroxide system. The crystallite size for the ceria acetate system after heat treatment was 17.7 and 53.6 nm at 500 and 1

28、000 , respectively19Fig. 10 TEM and electron diffraction pattern of CeO2 from cerium(IV) hydroxide (a) and ceria acetate (b) after 24 h hydrothermal treatment.Fig. 10 (a) exhibited very fine particles, which were agglomerated. Crystallinity could be observed based on the particles and its correspond

29、ing electron diffraction pattern. Its crystallite size is about 56 nm as estimated from the TEM micrographs. The particles generally shown rounded edges but they are not well-defined due to its small sizeFig. 10 (b), particles are very well-defined and relatively dispersed.Good crystalline faces and

30、 crystallinity state could be observedThe particle sizes, at about 1015 nm, are slightly bigger compared to the cerium(IV) hydroxide system.ceria acetate system appears to be less agglomerated than the cerium(IV) hydroxide system. However, agglomeration of the particles still a p p e a r s t o b e a

31、 p r o b l e m .20Salt-assisted ultrasonic aerosol decompositionSalt-assisted aerosol decomposition (SAD)Conventional aerosol decomposition (CAD)the same operating conditions , the same experimental apparatus, without the saltsprecursor solution: cerium nitratewas dissolved in distilled watera mixtu

32、re of potassium and sodium nitrates was added to the precursor solutionthe solution was misted by an ultrasonic transducer (1.7 MHz) into dropletscarried by air into a hot tubular reactor where they were rapidly heated and decomposed to form particles, heating time was less than five secondsCeO2 wer

33、e obtained by washing the product in water to remove the salts or their derivativesB. Xia, I. W. Lenggoro and K. Okuyama, Hiroshima University, Japan, J. Mater. Chem., 2001, 11, 2925292721Results and discussionFig. 1 Submicron to micron CeO2 particles synthesized by the CAD method at 800 : (a) lower

34、 magnification image; (b) higher magnification image of the particle marked A, comprising sintered nano-crystallites.The particles (Fig. 1a) are solid and nearly spherical with a mean particle size of 0.74 umFig. 1 shows the TEM images of the CeO2 particles, which were synthesized by theCAD method a

35、t 800 Consist of nanosized crystallites (Fig. 1b) with mean size of 13.8 nm determined by the X-ray diffraction (XRD) technique.These nanosized crystallites are virtually inseparable due tosintering22Fig. 2 Nanometer nanosized CeO2 particles synthesized by the SAD method at (a) 800 , and (b) a typic

36、al high resolution TEM image of sample (a), showing the crystal lattice of a particleImportant differences between the CAD and the SAD products are indicated below:First, the SAD product (Fig. 2a) is composed of isolated nanoparticles (mean size 51 nm),while the CAD(mean size 0.74 mm) containing sin

37、tered nano-crystallitesSecond, the SAD CeO2 particles are single crystals while the CAD CeO2 particles are polycrystalline (as shown in Fig. 1b) The single crystals are evidenced by the agreement between the particle sizes and the crystallite ones at all synthesis temperatures, as shown in Table 1.

38、The typical crystal latticeimage shown in Fig. 2b confirms the presence of singlecrystalline particlesClearly, the particle sizedistribution of the SAD product has been remarkably narrowed in comparison to the CAD product23Table 1 Comparison of particle and crystallite diameters (in anometers) of Ce

39、O2 synthesized by the CAD and the SAD processesFig. 3 Powder XRD patterns of roducts synthesized at (a) CAD, 800 (CeO2); (b) SAD, 800 (CeO2)Third, the SAD product has a much higher crystallinity than the CAD product, as shown from the sharp peaks in Fig. 3b. The crystallite size of the SAD 800 sampl

40、e is 54.4 nm, as shown in Table 1. This is much larger than the corresponding CAD sampleDetails of the SAD process:CeO2 can participate in dissolution and precipitation in the liquid-state salt media, which can greatly facilitate mass transfer and thus m a t e r i a l f o r m a t i o n a n d crystal

41、lization processes A crystallite grows by depleting its adjacent crystallites and is then isolated from others due to theenergy-favorable interaction of the oxide surface with the salts24Fig. 2 CeO2 particles synthesized by the SAD method at (c) 900 , and (d) 800 with addition of acetic acid to the

42、solution prior to aerosol decompositionAs is shown in Table 1, the CAD CeO2 particle sizes change slightly with synthesis temperature, because it is well understood that the CAD particle size is primarily determined by the droplet size and concentration of the precursor solution.In the SAD process:

43、many factors such as precursor(s), inert salts, additives and process parameters can be used to control particle size and morphologyFor example, Fig. 2c and Table 1: the mean particle size of the SAD product increases from 51 nm (Fig. 2a) to 119 nm when the synthesis temperature is increased f r o m 8 0 0 t o 9 0 0 Fig. 2d shows that the addition of acetic acid as an additive to the precursor solution resulted in a reduction in the final particle size from 51 nm to 21 nm. The broadened XRDpeak is clearly seen in the inset of Fig. 325 2627