Enhanced Thermal Stability and Oxygen Storage Capacity for Ce

Dec 14, 2006 - ... Joint Laboratory on Rare Earth Materials and Bioinorganic Chemistry, Peking .... Peking University. ...... ed.; Dunod: Paris, 1964;...
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J. Phys. Chem. C 2007, 111, 787-794

787

Enhanced Thermal Stability and Oxygen Storage Capacity for CexZr1-xO2 (x ) 0.4-0.6) Solid Solutions by Hydrothermally Homogenous Doping of Trivalent Rare Earths Rui Si,† Ya-Wen Zhang,*,† Li-Min Wang,† Shi-Jie Li,† Bing-Xiong Lin,† Wang-Sheng Chu,‡ Zi-Yu Wu,‡ and Chun-Hua Yan*,† Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Rare Earth Materials Chemistry and Applications & PKU-HKU Joint Laboratory on Rare Earth Materials and Bioinorganic Chemistry, Peking UniVersity, Beijing 100871, China, and Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Beijing 100039, China ReceiVed: May 19, 2006; In Final Form: October 26, 2006

Trivalent rare-earth (RE ) La, Pr, Nd, Y)-doped CexZr1-xO2 (x ) 0.4-0.6) solid solutions were synthesized via a urea hydrolysis-based hydrothermal method, followed by calcination at 1000 °C for 100 h. The X-ray absorption near-edge structure (XANES) tests for the Ce LIII edge determine that the chemical valence of the Ce atom in either undoped or RE-doped CeO2-ZrO2 samples was mainly +4. The measurements of X-ray fluorescence (XRF) and energy-filtered transmission electron microscopy (EFTEM) testify to the good compositional homogeneity for Ce, Zr, and RE atoms in both bulk and microdomain of the as-calcined CexZr1-x-yREyO2-z (x ) 0.4-0.6, y ) 0-0.16, and z is the number of oxygen vacancy) samples. The characterizations with X-ray diffraction (XRD) and vis-Raman demonstrate the high structural homogeneity for ternary CeO2-ZrO2-RE2O3 solid solutions after calcination at 1000 °C for 100 h. The aliovalent substitution of the RE3+ ion for the Ce4+ or Zr4+ ion has been concluded to stabilize the pseudo cubic t′′-structure, to improve the surface area and to enhance the oxygen storage capacity (OSC) for the binary CeO2-ZrO2 system. The reducibility of the as-calcined CexZr1-x-yREyO2-z by CO seems dependent upon the nature and concentration of the RE dopant. The highest CO-OSC value obtained in this work appeared for Ce0.5Zr0.42Nd0.08O2-z in 558 µmol CO g-1 at 700 °C, while the undoped samples gave a value of less than 460 µmol CO g-1.

Introduction As one of the most important functional rare-earth oxides, ceria (CeO2) has wide applications in catalysis,1-9 electrochemistry,10 and optics11,12 due to its unique physical and chemical properties. For instance, CeO2 has attracted great interest in many catalytic processes, including H2 production from hydrocarbons,1 water-gas-shift (WGS) reaction,2 sulfur abatement in the FCC process,3 catalytic water de-pollution,4 and so forth. Particularly, the most extensive application of CeO2-based materials may be in automotive pollution control, as promoters of three-way catalysts (TWCs), due to their high oxygen storage capacity (OSC), rich oxygen vacancies, and low redox potential between Ce3+ and Ce4+.5-9 Since 1995, CeO2-ZrO2 mixed oxides have gradually replaced pure CeO2 as the OSC promoters in TWCs to reduce the emission of toxic pollutants (CO, NOx, hydrocarbons, etc.) from automobile exhaust, because of their enhanced reduction behaviorandimprovedthermalstabilityatelevatedtemperatures.5-9,13 For such compounds to achieve desirable and optimal properties, the close-coupled converter is normally indispensable to reduce the emissions during the start-up of the engine, which exposes the CeO2-ZrO2 catalyst to extremely high temperatures (10001100 °C).6,14 Therefore, the loss of the OSC values during the high-temperature long-term sintering for the current catalytic components is one of the major deactivation pathways for the * Correspondingauthors.Fax: +86-10-6275-4179.E-mail: [email protected]. † Peking University. ‡ Institute of High Energy Physics.

TWCs, and thus the high thermal stability in the CeO2-ZrO2 system is a requisite for developing the advanced TWCs.5-9 Nowadays, there is a general agreement that the presence of a single-phase solid solution is preferable for the TWC application, compared to the microdomain or phase-segregated nonhomogeneous CeO2-ZrO2 mixed oxides because the former generally leads to better textural stability and redox properties.7 So, many research groups are devoting themselves to the synthesis and characterization of CeO2-ZrO2 solid solutions with high chemical and structural homogeneity.6,7,13,15-21 Recently, our group prepared homogeneous CexZr1-xO2 (x ) 0.21) solid solutions via a urea hydrolysis-based hydrothermal method and observed that the highest CO-OSC values for the as-calcined samples appeared in x ) 0.4-0.6.21 However, CexZr1-xO2 (x ) 0.4-0.6) solid solutions more easily undergo a phase separation upon long-term (g100 h) sintering at about 1000 °C, as also reported by many other groups.6,7,9 For obtaining the best-performing CexZr1-xO2 (x ) 0.4-0.6) OSC materials, it is thus essential to check their actual crystal structure, to improve their thermal stability at elevated operation temperatures or in reduction atmospheres by doping with foreign cations, to clarify the complex phase-diagram and diverse phasetransitions in both binary and doped CeO2-ZrO2 systems, and to eliminate the chemical-inhomogeneity-derived OSC loss by choosing a proper synthetic route.6,7,9 Primarily, the introduction of trivalent cations (La3+, Y3+, Sc3+, etc.) into the CeO2- and/or ZrO2-based materials can enhance their thermal stability and properties during the hightemperature long-term anneal22-28 and can suppress the phase decomposition of CeO2-ZrO2 into t + c/t′′ phases,26,27 but the

10.1021/jp0630875 CCC: $37.00 © 2007 American Chemical Society Published on Web 12/14/2006

788 J. Phys. Chem. C, Vol. 111, No. 2, 2007 origin or explanation responsible for such improvement still remains confusing. Therefore, the main targets of the present work are to utilize a urea hydrolysis-based hydrothermal method to synthesize homogeneous CexZr1-xO2 (x ) 0.4-0.6) solid solutions doped with trivalent rare earths (RE), to investigate their chemical and structural homogeneity against the calcination at a high temperature (1000 °C) for a long period of time (100 h), and to determine their crystal structures, microstructures, and textures by multiple modern characterization techniques. In this paper, the doping effects of RE2O3 on the structural stability and oxygen storage ability of the CeO2-ZrO2 solid solutions are systematically studied and discussed. Experimental Section 1. Synthesis. RE(NO3)3 (RE ) La, Pr, Nd, Y) was prepared by dissolving RE2O3 (>99.9%) in dilute nitric acid solution (1:1 by volume). To obtain CexZr1-x-yREyO2-z (x ) 0.4-0.6, y ) 0-0.16, z is the number of oxygen vacancy), an 80-mL quantitative aqueous solution of (NH4)2Ce(NO3)6 (g99.0%, Beijing Chemical Corp., China), ZrO(NO3)2 (A. R. grade, Beijing Liulidian Chemical Plant, China), RE(NO3)3, and urea (>99.0%, Beijing Chemical Corp., China) in the molar ratio of x:(1 - x - y):y:2 was used as the stock solution with the fixed metal concentration at 0.1 mol L-1 in a Teflon bottle (inner volume: 100 mL), which was kept in a stainless-steel autoclave. After the autoclave was sealed tightly, it was placed in a temperature-controlled electric oven. The vessel was heated to 80 °C in 0.5 h and then kept for 6-h duration at this temperature. Then, the temperature of the vessel was raised to 180 °C in 1 h and was kept for another 24 h at that temperature. After the hydrothermal reaction, the as-obtained precipitates were separated by centrifugation, washed with deionized water and absolute ethanol several times, and then dried at 80 °C in air overnight. The as-dried powders were ground and then calcined in still air at 1000 °C (heating speed: 10 °C min-1) for 100 h; the final products with yields higher than 85% were thus obtained. 2. Characterization Methods. X-ray fluorescence (XRF) data were determined from a Bruker S4 Explorer spectrometer under an input power of 1 kW. Vis-Raman spectra were acquired on a Renishaw spectrometer with an Ar ion laser of 514.5-nm excitation wavelength. Back-scattering geometry was adopted for the measurement under a laser power of 20 mW and a resolution of 4 cm-1. The BET specific surface area (S) was measured by nitrogen adsorption at 78.3 K, using an ASAP 2010 analyzer (Micromeritics Co. Ltd.). The measurement was performed after outgassing the sample at 150 °C for more than 4 h in vacuum until the residual pressure was better than 10 µmHg. The powder X-ray diffraction (XRD) patterns were recorded on a Rigaku D/ MAX-2000 diffractometer with a slit of 1/2° at a scanning rate of 1° min-1, using Cu KR radiation (λ ) 1.5406 Å). With the software “LAPOD” of least-squares refinement of cell dimensions from powder data by Cohen’s Method,29,30 the lattice constants were calculated. The grain size (D) was estimated according to the Scherrer equation,31

D ) 0.90λ/(β cos θ) where θ is the diffraction angle of the (111) peak of the cubic phase, and β is the full width at half-maximum (fwhm) of the (111) peak (in radian), which is calibrated from high-purity silicon. The microstrain () in the lattice of the as-calcined crystallites was estimated by the single-line method for analysis of XRD line broadening using a pseudo-Voigt profile function.32

Si et al. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) characterizations were carried out on a Philips Tecnai F30 FEG-TEM operated at 300 kV. The samples were supported on carbon-coated copper grids by dropping the ethanol suspension containing uniformly dispersed powders on the grid. An energy-filtered TEM (EFTEM) equipped with a Gatan image filter (GIF) was used for the elemental mapping. The microscope has a point-to-point resolution of 0.2 nm, and the space resolution of EFTEM is better than 1 nm. X-ray absorption fine structure (XAFS) spectra for Ce LIII and Zr K edges were measured in transmission mode by using synchrotron radiation with a Si(111) double-crystal monochromator at the XAFS station (Beam line 4W1B) of the Beijing Synchrotron Radiation Facility. The storage ring was run at a typical energy of 2.2 GeV with an electron current of higher than 60 mA. To suppress the unwanted harmonics, the angle between the monochromator crystal faces was adjusted to mistune the incident beam by about 50% or 30% for the Ce LIII or Zr K edge, respectively. The incident beam intensities were monitored and recorded using a pure nitrogen gas flowing ionization chamber. The output beam intensities were monitored and recorded using a 50% argon-doped nitrogen or pure argon flowing ionization chamber for the Ce LIII or Zr K edge, respectively. The spectrum was scanned in the range of 56255900 or 17850-18900 eV for the Ce LIII or Zr K edge, respectively. Energy resolution was 1.5 eV for X-ray absorption near-edge structure (XANES) and 3.0 eV for extended X-ray absorption structure (EXAFS) measurements. The EXAFS spectra were analyzed by the UWXAFS package.33 After background subtraction, k3-weighted EXAFS functions were Fourier transformed into R-space using the Bessel window with the same parameter dk ) 1 eV for all the spectra. Then the isolated Zr-O EXAFS oscillations were extracted by inverse Fourier transformed with a rectangular window, also weighted by k3. The curve fittings were carried out in the k-space. The spectral range values in k-space for Fourier transformation and in R-space for inverse Fourier transformation were 2.1-13 Å-1 and 1.1-2.2 Å, respectively. The number of parameters which may be determined by EAXFS was limited by the number of independent data points:34 Nind ∼ (2 ∆k ∆R)/π, where ∆k and ∆R are, respectively, the ranges in k- and R-space over which data are analyzed. In our case, it gave Nind ∼ 7.6 for the EXAFS due to the Zr-O shell. The back-scattering amplitude and phase shift were calculated by the FEFF8 code.35 A crystallographic structure with a space group of P42/nmc was used as a model system for EXAFS theoretical calculation. In fact, there are three bond distances lying in 2.09 Å (P42/nmc), 2.13 Å (Fm3m), and 2.36 Å (P42/nmc) for the undoped samples, because they show a mixed phase of cubic (Fm3m) and tetragonal (P42/nmc). However, the nearly same phase shift for both the bonds of 2.09 and 2.13 Å has been found by the calculation using FEFF8 code. Therefore, for undoped samples, the same phase shift of Zr-O at 2.09 Å in tetragonal structure has been used for both the bonds, and our EXAFS fitted for the isolated Zr-O EXAFS includes the two distances. In order to get the effective amplitude reduction factor S02 and the photoelectron energy origin E0, we fitted the EXAFS spectrum of the standard 8 mol % yttriastabilized zirconia (YSZ) sample in pure fluorite cubic structure (space group: Fm3m), which was prepared via a two-step urea hydrolysis-based hydrothermal method from Y(NO3)3 (0.015 mol L-1) and ZrO(NO3)2 (0.085 mol L-1) aqueous solutions (i.e., treated at 80 °C for 12 h and then at 180 °C for 48 h in turn, followed by calcinations at 800 °C for 2 h and then at 1400 °C for 24 h).24 We found the nearest coordination number

CexZr1-xO2 (x ) 0.4-0.6) Solid Solutions

J. Phys. Chem. C, Vol. 111, No. 2, 2007 789 TABLE 1: Experimental Compositions of the As-Calcined CexZr1-x-yREyO2-z (RE ) La, Pr, Nd, Y; x ) 0.4-0.6; y ) 0-0.16) Samples Determined by XRF

Figure 1. Ce LIII-edge XANES spectra of Ce0.5Zr0.5O2 and Ce0.5Zr0.42La0.08O2-z.

is indeed eight and the distance of the Zr-O bond was little changed. The obtained S02 and E0 were 0.72 and -2.0 eV, respectively. Therefore, there were six alterable parameters in our EXAFS fit for all the undoped and doped samples, that is, coordination numbers, distances, and Debye-Waller factors for the two Zr-O subshells. The uncertainties in the parameters were estimated from the standard EXAFS method.36,37 Error bars for coordination numbers, distances, and Debye-Waller factors were limited in (0.5, (0.01, and (0.001, respectively. By pulse technique with some modifications,38 the OSC experiments were performed with a GC-7890T gas chromatograph (Shanghai Tianmei Instrument Co. Ltd., China) as the analytical device. The samples were oxidized at 700 °C in flowing O2 (5.29%)/He injected (2 mL) periodically at intervals of 2 min and repeated five times; they were then reduced at 700 °C with pulses of 5 mL of CO (5.0%)/He. The dynamic CO-OSC values were determined by the amount of CO consumed during the CO pulse. Each measurement was repeated five times and the results were averaged. Results and Discussion 1. Chemical Valence and Compositional Homogeneity. The employed two-step urea hydrolysis-based hydrothermal synthesis via a heterogeneous nucleation growth mechanism can readily guarantee the homogeneous precipitation of Ce4+, ZrO2+, and RE3+ ions in spite of the different solubility products of their hydroxides.24 Under appropriate experimental parameters, such as the molar ratio of metal to urea, the total concentration of metalcations,andtherampprogram,homogeneousCexZr1-x-yREyO2-z (RE ) La, Pr, Nd, Y; x ) 0.4-0.6; y ) 0-0.16) solid solutions could be obtained after the calcination at a high temperature (1000 °C) for a long time (100 h). The as-calcined samples appeared with characteristic colors of both Ce4+ and RE3+ ions, i.e., light yellow for the Ce-La or Ce-Y system, gray-yellow for the Ce-Pr system, and purple-yellow for the Ce-Nd system. XANES spectroscopy was carried out to detect the chemical valence of the Ce atom in the CexZr1-x-yREyO2-z samples. Figure 1shows the Ce LIII edge XANES spectra of Ce0.5Zr0.5O2 and Ce0.5Zr0.42La0.08O2-z. For comparison, pure (NH4)2Ce(NO3)6 and Ce(NO3)3·6H2O were selected as the Ce4+ and Ce3+ references, respectively. The standard Ce4+ spectrum contains two peaks of A at about 5737 eV with a final state of Ce [2p54f05d1] O 2p6 and B at about 5730 eV with a final state of Ce [2p54f15d1] O 2p5, while the standard Ce3+ spectrum contains only one peak of C at about 5726 eV with a final state of Ce [2p54f15d1] O 2p6.37 For both undoped (Ce0.5Zr0.5O2) and REdoped (Ce0.5Zr0.42La0.08O2-z) samples, the observation of the much more intense peaks of A and B indicates that the chemical

sample

Ce (atom %)

Zr (atom %)

RE (atom %)

Ce0.4Zr0.52La0.08O2-z Ce0.5Zr0.42La0.08O2-z Ce0.6Zr0.32La 0.08O2-z Ce0.5Zr0.44La 0.06O2-z Ce0.5Zr0.34La 0.16O2-z Ce0.5Zr0.42Pr0.08O2-z Ce0.5Zr0.42Nd0.08O2-z Ce0.5Zr0.42Y0.08O2-z

38 48 58 49 48 47 48 48

55 44 35 46 37 45 44 43

7.3 7.3 7.3 5.4 15 7.6 7.8 8.8

valence of the Ce atom in both binary CeO2-ZrO2 and ternary CeO2-ZrO2-RE2O3 systems was mainly +4.39 The XRF instrument was utilized to check the metal compositions of the as-calcined CexZr1-x-yREyO2-z powders. Table 1shows that the experimental contents of Ce, Zr, and RE elements were in good agreement with the target ones for each CeO2-ZrO2-RE2O3 sample. The EFTEM characterization was used to investigate the elemental compositions of the RE-doped CeO2-ZrO2 crystallites in microdomain. Figure 2 is the zeroloss image with the corresponding elemental maps for Ce, O, and La of Ce0.5Zr0.42La0.08O2-z. The atomic distributions of Ce, La, and O were highly uniform over the shown crystallites, indicating the high chemical homogeneity of the as-calcined Ce0.5Zr0.42La0.08O2-z sample. 2. Crystal Structure and Structural Homogeneity. Generally speaking, CeO2-ZrO2 mixed oxides have a quite complex phase diagram, containing three stable phases (monoclinic (m), tetragonal (t), and cubic(c)) and two metastable tetragonal phases (t′ and t′′).40-43 Among them, the t-phase with a space group of P42/nmc is formed through a diffused phase decomposition, and the t′-phase with the same space group of P42/nmc is obtained through a diffusionless transition; the pseudo-cubic t′′phase with a space group of P42/nmc is an intermediate one between t′ and c, and its structure is very close to that of c, except for the displacement of the involved oxygen atoms.40-43 To accurately describe and determine these similar crystal structures, comprehensive investigations with multiple characterization methods are necessary. Herein, for binary CeO2-ZrO2 and ternary CeO2-ZrO2-RE2O3 systems, we employ XRD as a conventional characterization to assign the phases, check the crystallinity, and to calculate the grain sizes; we use vis-Raman as a powerful technique to define the lattice structure more exactly in combination with the XRD data and probe the positions of oxygen anions, and we apply EXAFS as an effective mean to detect the local atomic structures of certain atoms. The as-calcined CexZr1-xO2 and CexZr0.92-xLa0.08O2-z (x ) 0.40.6) powders are selected as typical examples for discussion. Figure 3a shows the XRD patterns of CexZr1-xO2 (x ) 0.40.6). The main diffractive peaks are split and can be separated into the t (JCPDS card no. 38-1439)- and c (JCPDS card no. 38-1436)-phases. In our previous report, the homogeneous CexZr1-xO2 (x ) 0.4-0.6) solid solutions with the pseudo cubic t′′-structure were stabilized after calcination at 1000 °C for 4 h.21 So the present structural transition from t′′- to (t + c)-phases for the undoped CeO2-ZrO2 during the high-temperature (1000 °C) long-term (100 h) sintering seems to be controlled by a kinetic process.7 Figure 3b shows the XRD patterns of CexZr0.92-xLa0.08O2-z (x ) 0.4-0.6). All the diffractive peaks are highly symmetric and can be indexed to the cubic c- or t′′-phase with lattice constants a of 5.3000, 5.3237, and 5.3481 ÅforCe0.4Zr0.52La0.08O2-z,Ce0.5Zr0.42La0.08O2-z,andCe0.6Zr0.32La0.08O2-z, respectively (see Table 2).

790 J. Phys. Chem. C, Vol. 111, No. 2, 2007

Si et al.

Figure 2. Zero-loss image (a) and elemental maps (b-d) of Ce0.5Zr0.42La0.08O2-z. (b) Ce; (c) O; (d) La.

Figure 3. XRD patterns of (a) CexZr1-xO2 and (b) CexZr0.92-xLa0.08O2-z (x ) 0.4-0.6).

Figure 4. Raman spectra of (a) CexZr1-xO2 and (b) CexZr0.92-xLa0.08O2-z (x ) 0.4-0.6).

TABLE 2: Phases, Lattice Constants (a), Lattice Strains (E), Grain Sizes (D), BET Specific Surface Areas (S), and CO-OSC Values of the As-Calcined CexZr1-x-yREyO2-z (RE ) La, Pr, Nd, Y; x ) 0.4-0.6; y ) 0-0.16) Samples

a

sample

phase

Ce0.4Zr0.6O2 Ce0.5Zr0.5O2 Ce0.6Zr0.4O2 Ce0.4Zr0.52La0.08O2-z Ce0.5Zr0.42La0.08O2-z Ce0.6Zr0.32La 0.08O2-z Ce0.5Zr0.44La 0.06O2-z Ce0.5Zr0.34La 0.16O2-z Ce0.5Zr0.42Pr0.08O2-z Ce0.5Zr0.42Nd0.08O2-z Ce0.5Zr0.42Y0.08O2-z

t+c t+c t+c t t t t t t t t

a (Å)

 (%)

D (nm)

S (m2 g-1)

OSCa (µmol CO g-1)

5.3000(4) 5.3237(2) 5.3481(1) 5.3206(2) 5.3741(4) 5.3110(1) 5.3112(1) 5.2763(1)

0.54b 0.63b 0.62b 0.84 0.71 0.64 0.79 0.71 0.48 0.50 0.77

17b 20b 19b 13 16 17 14 16 22 21 14

5 6 4 18 16 17 15 19 12 11 8

348 456 452 460 527 467 491 513 525 558 486

Measured at 700 °C. b For cubic phase (space group: Fm3m) only.

Figure 4a shows the Raman spectra of CexZr1-xO2 (x ) 0.40.6). Five peaks at 138, 256, 312, 474, and 630 m-1, which are originated from the six Raman active modes (A1g + 2B1g + 3Eg) of the t-phase,44 can be distinctly determined. Meanwhile, the strongest peak at 474 cm-1 can be also attributed to the Raman active mode (F2g) of the c-phase.42 Considering the results of XRD, we determined that the undoped CeO2-ZrO2 mixed oxides were composed of a mixed phase of t- and c-phases.Figure4bshowstheRamanspectraofCexZr0.92-xLa0.08O2-z

(x ) 0.4-0.6). The relatively broadened peaks suggest more defective structures in the ternary CeO2-ZrO2-RE2O3 system than those in the binary CeO2-ZrO2 one.26 Four peaks at 138, 312, 479, and 620 cm-1 can be assigned to the metastable t′′phase.21,26 No observation of peaks at about 256 cm-1, except a very weak band for sample x ) 0.4, indicates the absence of the stable t-phase for the RE-doped CeO2-ZrO2 solid solutions.45 On the basis of the XRD and Raman results, the formation

CexZr1-xO2 (x ) 0.4-0.6) Solid Solutions

J. Phys. Chem. C, Vol. 111, No. 2, 2007 791

Figure 5. (a) EXAFS oscillations multiplied by k3 (left) and Fourier transforms of the corresponding EXAFS spectra (right) extracted from the Zr K-edge absorption of the YSZ reference, CexZr1-xO2 and CexZr0.92-xLa0.08O2-z (x ) 0.4-0.6) samples; (b) The isolated Zr-O EXAFS (solid lines) and the corresponding fitted curves (symbol lines) using two distances.

TABLE 3: Fitting Results of Zr K Edge EXAFS for Zr-O Pair sample Ce0.4Zr0.6O2 Ce0.5Zr0.5O2 Ce0.6Zr0.4O2 Ce0.4Zr0.52La0.08O2-z Ce0.5Zr0.42La0.08O2-z Ce0.6Zr0.32La 0.08O2-z

N

R (Å)

σ2 (Å2)

R (%)

3.6 2.7 4.5 1.8 4.6 1.7 4.0 2.7 4.0 3.1 4.0 2.9

2.09 2.28 2.10 2.31 2.11 2.30 2.10 2.27 2.11 2.24 2.12 2.25

0.003 0.009 0.004 0.005 0.005 0.005 0.001 0.001 0.003 0.004 0.004 0.005

1.9 2.5 1.2 4.7 2.5 2.5

of ternary CeO2-ZrO2-RE2O3 solid solutions can effectively stabilize the pseudo cubic t′′-structure and greatly enhance the structural homogeneity of the CeO2-ZrO2 mixed oxides,26,27 even after being calcined at 1000 °C for 100 h. Figure 5a (right) is the Fourier transformed k3χ data for Zr K edge EXAFS spectra of CexZr1-xO2 and CexZr0.92-xLa0.08O2-z (x ) 0.4-0.6). Two peaks are recognized corresponding to the interatomic distances of Zr-O (first shell) at about 1.6 Å and Zr-(Zr, Ce, La) (second shell) at about 3.2 Å, respectively. Figure 5b shows representative EXAFS fits of the above samples for the Zr-O pair. Clearly, the fitting spectra via the twodistance method were in good agreement with the experimental ones. The calculated structural parameters for each sample have been listed in Table 3. The coordination numbers of the first Zr-O subshells for the undoped and La-doped CeO2-ZrO2 were all around 4, while those of the second subshells decreased greatly. It reveals the presence of oxygen vacancies for both binary CeO2-ZrO2 and ternary CeO2-ZrO2-La2O3 systems. Also from Table 3, it is obvious that the introduction of La3+ (8 atom %) had nearly no changes for the Zr-O distances (2.10-2.12 Å for the first subshell and 2.24-2.27 Å for the second subshell), compared with the undoped samples (2.092.11 Å for the first subshell and 2.28-2.31 Å for the second subshell). Meanwhile, the amplitudes of the Zr-(Zr, Ce, La) shells in the ternary CeO2-ZrO2-La2O3 system are much more reduced, compared with those in the binary CeO2-ZrO2 one. In stabilized ZrO2 with a fluorite cubic structure, the reduced amplitude of the second shell has been attributed to the increase of the Debye-Waller factors, due to the disorder of the cation sublattice caused by the metal substitution.46 Here, it may be due to the presence of anion vacancies and/or differences in ionic radii between Zr4+/Ce4+ and RE3+.47,48

3. Doping Effect on Thermal Stability. In experiments, we found that the nature and concentration of the RE dopant were the key factors for obtaining homogeneous CeO2-ZrO2-RE2O3 solid solutions with high thermal stability upon the hightemperature (1000 °C) long-term (100 h) anneal. Figure 6a shows that various Ce0.5Zr0.5-yREyO2-z samples maintained the singlephase nature under the conditions of RE ) La and y g 0.04 (Figure 6a), RE ) Pr and y g 0.06 (Figure 6b), RE ) Nd and y g 0.06 (Figure 6c), and RE ) Y and y g 0.08 (Figure 6d). Figure 7 displays the linear relationship between the lattice constant (a) and the radius of the doped RE3+ ion (Ri) for Ce0.5Zr0.42RE0.08O2-z (RE ) La, Pr, Nd, Y). According to the Vegard rule, the as-calcined 8 mol % RE-doped CeO2-ZrO2 samples can be attributed to solid solutions, not mixed oxides. Notably, the lowest doping concentration (y) to completely stabilize the t′′-structure for the Ce0.5Zr0.5-yREyO2-z samples increased with the size reduction of the RE3+ ion from La3+ (Ri ) 1.16 Å) to Pr3+ (Ri ) 1.126 Å), to Nd3+ (Ri ) 1.109 Å), and to Y3+ (Ri ) 1.019 Å).49 It can be explained that the stabilization of the pseudo cubic t′′-structure in the present research requires a large unit cell to relax the lattice compression, so that a better doping effect by the substitution of a Zr4+ (Ri ) 0.84 Å)49 or Ce4+ ion (Ri ) 0.97 Å)49 with a larger RE3+ (RE ) La, Pr, or Nd) ion than with a Y3+ ion was shown. The doping effect of RE on the stabilization of the pseudo cubic t′′-structure and on the enhancement of the thermal stability for the as-calcined CexZr1-x-yREyO2-z (RE ) La, Pr, Nd, Y; x ) 0.4-0.6; y ) 0-0.16) solid solutions can be possibly explained by the following mechanism. According to the previous research,19 the t′′-phase for the CeO2-ZrO2 solid solution is of a defective structure containing rich oxygen vacancies and interstitials. In our experiments, the as-obtained CeO2-ZrO2 solid solutions underwent the t′′ f (t + c) phase transition after being calcined at 1000 °C for 100 h (see Table 2 and Figures 3a and 4a). Table 2 also shows that the microstrains () in the lattice of the CexZr1-x-yREyO2-z solid solutions after a high-temperature (1000 °C) long-term (100 h) sintering process were 0.48-0.84%, most of which are higher than those of the undoped CeO2-ZrO2 ones (0.54-0.63%). This result indicates that the RE-doped samples might have more defective structures.21 Moreover, the microstrains in the RE-doped CexZr1-xO2 lattices were greatly increased, compared with those of the undoped ones calcined at 900 °C (0.180.58%).21 In the present work, the aliovalent substitution of the RE3+ ions in the binary CeO2-ZrO2 system can spontaneously generate both a great amount of delocalized oxygen vacancies

792 J. Phys. Chem. C, Vol. 111, No. 2, 2007

Si et al.

Figure 6. XRD patterns of Ce0.5Zr0.5-yREyO2-z (y ) 0.02-0.16) under different RE of (a) La, (b) Pr, (c) Nd, and (d) Y. The arrows point out the split or asymmetric peaks.

Figure 7. Lattice constant a of Ce0.5Zr0.42RE0.08O2-z (RE ) La, Pr, Nd, Y) as a function of ionic radius Ri for the 8-fold coordinated RE3+ ion.

and some highly localized oxygen vacancies, that is, (RE′MV‚‚O) pairs:50,51 ‚‚ RE2O3 T 2RE′M + 3O× O + VO

(1)

where RE′M represents a RE3+ ion occupying the site of a M4+ ‚‚ (M ) Ce or Zr) ion, O× o is a lattice oxygen atom, and VO represents an oxygen vacancy. The EFTEM results show that the doped RE3+ ions were uniformly distributed in the cation sublattice, hinting the homogeneous distribution of the oxygen vacancies over the crystallites of the CexZr1-x-yREyO2-z samples. Therefore, the pseudo cubic t′′-structure can be fully maintained for ternary CeO2-ZrO2-RE2O3 solid solutions, possibly due to the stabilization of the lattice defects (predominately oxygen vacancies caused by the doping of rare earths) and their homogeneous distributions. 4. Textural Properties. Table 2 displays the grain sizes (D) calculated by the XRD data for CexZr1-x-yREyO2-z (RE ) La, Pr, Nd, Y; x ) 0.4-0.6; y ) 0-0.16). The D values of the RE-doped CeO2-ZrO2 samples (13-22 nm) were very close

to those of the undoped ones (17-20 nm) after being calcined under the same conditions, indicating that the crystal growth of the CexZr1-x-yREyO2-z powders was dependent upon the sintering temperature (1000 °C) and time (100 h), regardless of the doped rare earths. Figure 8a shows that the as-calcined Ce0.5Zr0.42La0.08O2-z powder was composed of somewhat aggregated nanocrystals with an average size of 31 nm, obviously larger than the exhibited XRD grain size (D ) 16 nm). It is consistent with the high level of microstrain (0.71%) in the CeO2-ZrO2-La2O3 lattice, making the Scherrer equation partly invalidated.21 As seen from the HRTEM image in Figure 8b, the as-calcined Ce0.5Zr0.42La0.08O2-z nanocrystals were highly crystallized and exposed by slightly distorted lattice fringes of the (111) and (200) planes of the pseudo cubic t′′-structure. The BET specific surface areas (S) of the as-calcined CexZr1-x-yREyO2-z (RE ) La, Pr, Nd, Y; x ) 0.4-0.6; y ) 0-0.16) powders were 8-19 m2 g-1, higher than those of CexZr1-xO2 in 4-6 m2 g-1 (see Table 2). It can be concluded that the doping of RE into the binary CeO2-ZrO2 system can effectively enhance the surface area of the solid-solution powders during the high-temperature long-term sintering. 5. Oxygen Storage Capacity. To investigate the oxygen storage ability of the as-calcined CexZr1-x-yREyO2-z (RE ) La, Pr, Nd, Y; x ) 0.4-0.6; y ) 0-0.16) solid solutions, the oxidation of CO at 700 °C was selected as a model reaction to test the dynamic OSC value:

CO + 1/2O2 f CO2

(2)

For CexZr1-x-yREyO2-z, we have observed that there is a weak relationship between the grain sizes, BET specific surface areas, or microstrains in lattice and the OSC values (see Table 2, Figure 9a-c). It strongly suggests that the external texture would not be the key factor governing the OSC performance in both undoped and RE-doped CeO2-ZrO2 catalysts. Meanwhile, the strong correlation between the lattice strain and OSC value in

CexZr1-xO2 (x ) 0.4-0.6) Solid Solutions

J. Phys. Chem. C, Vol. 111, No. 2, 2007 793

Figure 8. (a) TEM and (b) HRTEM images of Ce0.5Zr0.42La0.08O2-z.

Figure 9. CO-OSC value of CexZr1-x-yREyO2-z (RE ) La, Pr, Nd, Y; x ) 0.4-0.6; y ) 0-0.16) as a function of (a) grain size D, (b) BET specific surface area S, and (c) microstrain  in the lattice.

the binary CeO2-ZrO2 system was invalidated in the present work for the ternary CeO2-ZrO2-RE2O3 system,21 probably because some of the oxygen vacancies were highly localized around the doped RE3+ ion so that they were inert during the catalytic tests.50,51 Table 2 shows that the CO-OSC values for the RE-doped CexZr1-x-yREyO2-z solid solutions were in the range 460-558 µmol CO g-1, higher than those for the undoped CexZr1-xO2 mixed oxides in 348-456 µmol CO g-1. This result may be due to the higher structural homogeneity and thermal stability, together with better textural properties (larger surface areas, etc.) for the present RE-doped CeO2-ZrO2 samples, which might contain more-active sites (e.g., delocalized oxygen vacancies, weakly bound oxygens, and even oxygen interstitials) for oxygen storage and release in their defective crystal structures.7,19,47 For the ceria-based materials in phase t′′ without the introduction of RE, the above oxygen vacancies were accompanied by interstitial oxygen ions, and it is very likely that these species contribute very efficiently to OSC.19 For our RE-doped CeO2ZrO2 solid solutions, such interstitials might be still present in their more defective structures and somewhat contribute to the OSC.19 We also found that the OSC values of the as-calcined CexZr1-x-yREyO2-z solid solutions relied on the nature and concentration of the RE dopant. The La-, Pr-, and Nd-doped Ce0.5Zr0.42RE0.08O2-z samples displayed higher OSC values in 527, 525, and 558 µmol CO g-1, respectively, higher than the Y-doped one (486 µmol CO g-1). It is likely that the weaker the doping effect of the RE3+ ion on stabilizing the pure t′′phase and on improving the structural homogeneity upon the high-temperature long-term anneal, the lower the OSC values intheternaryCeO2-ZrO2-RE2O3 system.ForCe0.5Zr0.5-yLayO2-z, the samples with y ) 0.08 and 0.16 had CO-OSC values of 527 and 513 µmol CO g-1, respectively, higher than the one with y ) 0.06 (491 µmol CO g-1). It reveals that the lower the doping concentration (y e 0.08), the less effective the doping of RE, and thus the lower CO-OSC values in ternary CeO2-

ZrO2-RE2O3 system.Ofallthetestedsamples,Ce0.5Zr0.42Nd0.08O2-z showed the highest CO-OSC value in 558 µmol CO g-1 at 700 °C. Among the trivalent rare-earth elements we doped, praseodymium (Pr) is special because of its mixed valence state of Pr3+ and Pr4+ under ambient conditions (Pr6O11). In previous work, it has been confirmed that the presence of oxygen vacancies associated with Pr3+ ions should contribute to the OSC value of ceria-zirconia efficiently.52 In our case, the OSCs of the Pr-doped materials were similar to those of La- or Nd-doped ones, indicating that the OSC contribution from Pr4+ T Pr3+ is small for them possibly due to the low doping concentration we adopted. Conclusions CexZr1-x-yREyO2-z (RE ) La, Pr, Nd, Y; x ) 0.4-0.6; y ) 0-0.16) solid solutions with good chemical homogeneity have been obtained via a urea hydrolysis-based hydrothermal method, followed by calcination at 1000 °C for 100 h. The chemical valence of the Ce element in each sample was mainly +4. The RE-doped CeO2-ZrO2 solid solutions showed the pseudo cubic t′′-phase with good structural homogeneity and high thermal stability upon the high-temperature long-term anneal. On the basis of multiple analytic techniques, the aliovalent substitution of the RE3+ ion in the binary CeO2-ZrO2 system was investigated and demonstrated to be the following effects: (a) effectively stabilizing the defective t′′-structure; (b) markedly improving the surface areas; (c) considerably enhancing the OSC values. The stabilization of homogeneous CexZr1-x-yREyO2-z solid solutions was dependent upon the nature and concentration of the RE dopant, that is, the RE3+ ion with a larger size and higher y value was preferable in the low-doping concentration (y e 0.08); the introduction of RE3+ ions resulted in the increase of oxygen vacancies that were strongly bound to them, and thus the decrease of mobility for the oxygen vacancies in the high doping concentration (y > 0.16). In the present research, the sample with the highest CO-OSC value appeared for

794 J. Phys. Chem. C, Vol. 111, No. 2, 2007 Ce0.5Zr0.42Nd0.08O2-z in 558 µmol CO g-1 at 700 °C, while the undoped samples gave a value of less than 460 µmol CO g-1. We expect that this work will help the development of a robust synthetic strategy toward homogeneous CexZr1-x-yREyO2-z solid solutions; deepen the understanding of their crystal structure, thermal stability, textural, and redox properties; and further contribute to the design and fabrication of exceptional CeO2ZrO2-based TWCs with higher OSC values and higher thermal stability. Acknowledgment. We greatly appreciate the constructive suggestions by the anonymous reviewers of this work and gratefully acknowledge the financial aid from the MOST of China (Grant No. 2006CB601104) and the NSFC (Grant Nos. 20571003, 20221101, and 20423005). References and Notes (1) Roh, H. S.; Jun, K. W.; Park, S. E. Appl. Catal., A: Gen. 2003, 251, 275. (2) Fu, Q.; Saltsburg, H.; Flytzani-Stephanopoulos, M. Science 2003, 301, 935. (3) Flytzani-Stephanopoulos, M.; Zhu, T. L.; Li, Y. Catal. Today 2002, 62, 145. (4) Imamura, S. Ind. Eng. Chem. Res. 1999, 38, 1743. (5) Kasˇpar, J.; Fornasiero, P.; Graziani, M. Catal. Today 1999, 50, 285. (6) Kasˇpar, J.; Fornasiero, P.; Hickey, N. Catal. Today 2003, 77, 419. (7) Kasˇpar, J.; Fornasiero, P. J. Solid State Chem. 2003, 171, 19. (8) Di Monte, R.; Kasˇpar, J. Catal. Today 2005, 100, 27. (9) Di Monte, R.; Kasˇpar, J. J. Mater. Chem. 2005, 15, 633. (10) Steele, B. C. H. Nature 1999, 400, 619. (11) Li, R. X.; Yabe, S.; Yamashita, M.; Momose, S.; Yoshida, S.; Yin, S.; Sato, T. Solid State Ionics 2002, 151, 235. (12) Patsalas, P.; Logothetidis, S.; Metaxa, C. Appl. Phys. Lett. 2002, 81, 466. (13) Fornasiero, P.; Balducci, G.; Di Monte, R.; Kasˇpar, J.; Sergo, V.; Gubitosa, G.; Ferrero, A.; Graziani, M. J. Catal. 1996, 164, 173. (14) Heck, R. M.; Farrauto, R. J. Appl. Catal., A: Gen. 2001, 221, 443. (15) De Leitenburg, C.; Trovarelli, A.; Zamar, F.; Maschio, S.; Dolcetti, G.; Llorca, J. J. Chem. Soc., Chem. Commun. 1995, 2181. (16) Rossignol, S.; Ge´rard, F.; Duprez, D. J. Mater. Chem. 1999, 9, 1615. (17) Hirano, M.; Miwa, T.; Inagaki, M. J. Solid State Chem. 2001, 158, 112. (18) Madier, Y.; Descorme, C.; Le Govic, A. M.; Duprez, D. J. Phys. Chem. B 1999, 103, 10999. (19) Mamontov, E.; Egami, T.; Brezny, R.; Koranne, M.; Tyagi, S. J. Phys. Chem. B 2000, 104, 11110. (20) Balducci, G.; Islam, M. S.; Kasˇpar, J.; Fornasiero, P.; Graziani, M. Chem. Mater. 2000, 12, 677. (21) Si, R.; Zhang, Y.-W.; Li, S.-J.; Lin, B.-X.; Yan, C.-H. J. Phys. Chem. B 2004, 108, 12481. (22) Vidmar, P.; Fornasiero, P.; Kasˇpar, J.; Gubitosa, G.; Graziani, M. J. Catal. 1997, 171, 160.

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