Investigation of the Oxygen Exchange Property and Oxygen Storage

J. Phys. Chem. C 2009, 113, 6921–6928

6921

Investigation of the Oxygen Exchange Property and Oxygen Storage Capacity of CexZr1-xO2 Nanocrystals Jing Ouyang and Huaming Yang* Department of Inorganic Materials, School of Resources Processing and Bioengineering, Central South UniVersity, Changsha 410083, People’s Republic of China ReceiVed: April 28, 2008; ReVised Manuscript ReceiVed: March 3, 2009

CexZr1-xO2 nanocrystals were prepared via a coprecipitation and subsequent solvothermal treatment. Temperature-programmed hydrogen reduction (H2-TPR) was applied to evaluate the reduction properties of the as-prepared materials. The oxygen storage capacities (OSC) of CexZr1-xO2 nanocrystals with various amounts of ceria were evaluated by measuring mass changes of samples before and after a severe hydrogenreduction process. The oxygen exchange property of the as-prepared samples was studied by differential scanning calorimetry (DSC) and thermogravimetric analysis (DSC/TG) detection in a programmed atmosphere and temperature protocol. Surface status evolution of the samples before and after reduction was traced by in situ Fourier transformation infrared spectroscopy (in situ FTIR). Phase evolution was followed by X-ray diffraction (XRD) in combination with Rietveld refinement. The results indicated that nanoscale phase segregation emerged when zirconia became dominant in chemical content, and the solubility limit was revealed to be 33.1% of ZrO2 in CeO2 lattice. Zirconium ions in CZ can be reduced into trivalence or lower ones, which was rarely reported before. Among the series of samples containing different ceria contents, Ce0.5Zr0.5O2 has the superiority of a lower beginning reduction temperature and has an appreciable OSC of 0.44 mol of O2/mol of Ce. Repeatable oxygen release and adsorption were highlighted based on the DSC/TG measurement. Oxygen was released from the lattice of the samples during the reduction, leading to a crystal phase change of about 54% in volume fraction from simple fluorite into pyrochlore-II type. In addition, adsorption of hydroxyls was found to accompany the reduction process, both Zr and Ce can be coordinated in situ in the process, and fresh surface favored the adsorption. Cerium ions can be reduced at lower temperature than Zr ions, according to TPR and IR results. Introduction Ceria based materials have attracted more and more attention in recent years due to their significant three-way catalytic (TWC) properties in the car exhaust cleaning field.1-3 The so-called three-way catalysis was originated from the nature of the outstanding repetitive redox behavior of Ce3+/Ce4+ pairs in CeO2 based materials, through which the NOx, COx, and CHx complexes can be changed into nontoxic matters when simply supplying the exhausts to the catalysts without excessive air or oxygen consumption.4,5 Countering the utilization condition of CeO2 catalysts (continuous exhaust supply and above 700 °C), their thermal, chemical, and structural stabilities act as the key factors for determining the performance of these catalysts. It has been found in recent decades that introduction of Zr4+ ions into CeO2 can stabilize the host lattice at high temperatures and enhance the catalytic performances of CeO2 samples,6,7 and ceria-zirconia (CexZr1-xO2, CZ) solid solutions have been intensively studied by many groups.8-13 Also, CZ catalysts were found to possess the superior properties of lower starting temperature of reduction reaction as well as larger oxygen storage capacity (OSC) than pure ceria.14 These superior properties were attributed to the higher mobility of lattice oxygen atoms in the partially distorted lattice of CZ solid solutions, and therefore the participation of both surface and bulk oxygen atoms in redox reactions, compared with pure fluorite ones, * To whom correspondence should be addressed. Phone: +86-731-8830 549. Fax: +86-731-8710 804. E-mail: [email protected].

because only surface oxygen atoms were involved for the latter, which was verified by isotope technique.15 To fully optimize catalytic performances of CZ materials, many preparation methods and various kinds of compositions of CZ catalysts have been studied. XRD in combination with Raman spectroscopy were frequently adopted to characterize structures of the samples. Temperature-programmed reduction (TPR), temperature-programmed desorption (TPD), and FTIR spectroscopy were used to evaluate their catalytic activities.16-18 On the basis of the aforementioned techniques and systematic investigations, it has been recognized that the methodology of the synthesis processes and postsynthesis treatments can significantly affect the structures and performances of CZ samples. In addition, many efforts have been devoted to establishing a relationship between the structure and properties of CZ samples, but the situation seems to be sophisticated due to the nature of various kinds of structures in zirconia contained materials. Single and combined m, t, t′, t′′, and c type structures may exist in one sample19-23 and several chemical compounds also may exist in the binary CeO2-ZrO2 equilibrium phase diagram.18,20 Furthermore, a pyrochlore structure could be obtained when CZ samples were reduced in severe conditions, and a k-phase can be obtained when the pyrochlore was reoxidized under mild oxidizing circumstances.9,24,25 An excellent review by Monte and Kaspar on the CZ catalyst gave in detail the structure and property of CZ matters.26 The review included results of reducing intensity for a series of samples with gradually elevated Ce contents, and a possible mechanism of structural changes in the reducing

10.1021/jp808075t CCC: $40.75  2009 American Chemical Society Published on Web 04/06/2009

6922 J. Phys. Chem. C, Vol. 113, No. 17, 2009 processes was proposed. The other review included the in situ structural evolution of CZ, observed by in situ XRD, in a temperature-programmed protocol, and found a lattice extraction during the heating process.3 Identification of the reduced structures often depends on XRD in combination with Rietveld refinement and Raman spectroscopy.25,27 From the previously published articles, one can learn that many experiments emphasized pure CZ solid solutions or pyrochlore materials, which are the fully reduced CZ matters after their preparation and reduction processes, but direct observation of the transient status of cubic (or tetragonal) CZ to the pyrochlore structure is still lacking. Also the repeatability of oxygen exchange is still in vogue, with the exception of repeatable mass change in reductive protocol detected by Reddy et al., but the dynamic parameters were not included.28 Efforts in this work involved the preparation and characterization of CZ and its oxygen exchange behaviors with different chemical compositions. Solvothermal treatment at the selfgenerated pressure in hot ethanol was adopted in the preparation process to avoid the traditionally followed calcinations processes and to avoid the formation of hard aggregation of the products. H2-TPR was used to characterize the reductive properties. A new protocol for detecting the repeatability of oxygen exchange in the as-prepared CZ samples and a severe reduction protocol for measuring OSC of these samples were established to evaluate the possibility of using CZ in such fields as oxygen buffers or containers both in inert and redox circumstances. In situ FTIR was adopted to monitor the surface hydroxyls formed in the reducing process. The results showed that zirconium can be reduced in H2 circumstance, the ceria-rich samples displayed superiority in OSC, and the Ce0.5Zr0.5O2 sample reached the maximum. Oxygen exchange was totally repeatable and the desorption process was revealed to be an endothermal process. A transient status in the structure of CZs in the reducing procedure was observed, which was rarely found in the literature. Experimental Section CexZr1-xO2 (abbreviated as C10x hereafter, such as C7 denote the Ce0.7Zr0.3O2 sample, where 7 ) 10 × 0.7) nanocrystals with gradually elevated cerium content (from 0.1 to 1.0) were prepared via a coprecipitation method followed by a solvothermal treatment process and were collected by freeze-drying. All chemicals were used without further purification. In a typical synthesis process, 5 mmol of raw sources composed of the desired ratio of Ce(NO)3 · 6H2O (99.0%, Sinopharm Chemical) and ZrOCl2 · 8H2O (99.0%, Shanghai Shiyi Chemical) was dissolved into 100 mL of deionized water under strong stirring; 1 g of polyvinyl pyrrolidone (PVP, 98.0%, Xiangzhong Chemical) was also dissolved into the solvent. Then 1 M NaOH was dropped into the salt solvent at a speed of 3 mL/min; the titration was not ended until a pH value of the mixture was around 9.0 and a pink suspension was obtained. The suspension was stirred continuously for another 2 h and its color turned to fresh yellow before it was concentrated by centrifugation and washed by deionized water until no Cl- could be detected by AgNO3 solution. Solvothermal treatment of the washed sediment was performed in a Teflon-lined stainless steel autoclave that was filled to 75% of its total volume by ethanol. The autoclave was kept at 190 °C for 18 h before the product was washed and dispersed into deionized water, then the as-obtained suspension was frozen at -40 °C by a Sanyo MDF-V333 biomedical freezer and lyophilized by a LGJ-10 freeze-drying machine at -53 °C under a pressure of 38 Pa. The samples were structurally characterized by a RIGAKU D/max-2550VB+ 18 kW powder diffractometer with Cu KR

Ouyang and Yang radiation (λ1 ) 1.5405983 Å, λ2 ) 1.5444939 Å, intensity ratio of the two is Iλ1:Iλ2)2:1). Data were collected from 10° to 90° of 2θ with a step width of 0.02°. Phases were identified by using the Search/Match capabilities of the JADE 5.0 program along with the ICDD (International Center for Diffraction Data) powder diffraction file (PDF) database. Rietveld refinement was performed with MAUD software. Instrumental calibration was performed by using diffraction data of standard silica (which play the role of outer reference and the data were collected by the same instrument) imported into the program; KR1 and KR2 of XRD sources were all included in the refinement. The Caglioti Pseudo-Voigt function as the default was used to simulate the diffraction peaks. The background of the obtained XRD patterns was extracted by using a 6-point polynomial curve. A step-bystep refinement was performed through switching the “fix” and “free” setting of the MAUD program because of the uncertainty of structure: background and space group were first identified; then lattice parameters, atom positions, and peak shape parameters were freed in the refinement with the other factors fixed; site occupancy and isotropic displacement parameters of all of the atoms were freed while the other factors were fixed in the next step to refine the fine structures; in the last step, all of the aforementioned parameters were freed again to obtain the final structure of the sample. Sizes of the nanocrystals were calculated according to Scherrer’s equation based on the half-width at halfmaximum (FWHM) of the fluorite (111) diffraction peak from Rietveld refinement results. Oxygen storage capacities (OSC) of the CZ samples were detected by reducing the samples in flowing pure hydrogen atmosphere at 800 °C for 4 h and then cooled to room temperature (RT) in H2 circumstance. Batches of CZ samples were stowed in covered Φ 5 mm corundum crucibles with a hole in the cover. The OSC value was detected by weighing the crucibles before and after loading, before and after reduction, respectively, by a Mettler Toledo MX5 electronic balance. Before the samples were subjected to hydrogen reduction, the loaded crucibles were kept at 550 °C for 3 h in a flowing air atmosphere to fully oxidize the materials and to remove the absorbed surface hydroxyls and surfactants. The protocol is schematically illustrated by Figure 1A; OSC was calculated by subtracting the mass of the sample at t1 from that at t2 in the program. Temperature-programmed hydrogen reduction (H2TPR) of the as-prepared CZ samples was measured under a settled protocol on a TPR instrument produced by Xianquan Ltd., China. In a typical measurement procedure, 100 mg of CZ sample was used in a corundum crucible, the temperature of the TPR chamber was elevated gradually to 120 °C in flowing N2 circumstance to eliminate the adsorbed water and residual O2, then the chamber was allowed to cool to room temperature at the same atmosphere and the reductive property of the sample was measured by recording the hydrogen consumption in flowing H2 atmosphere at a heating rate of 10 deg/min. Among the CZ samples, structural characterization of reduced C5 with use of X-ray diffraction was performed immediately after the TPR test was finished. Also the XRD pattern of C5 was surveyed after OSC measurement in H2 atmosphere for 4 h. Oxygen adsorption/desorption of the as-prepared C5 was monitored with a NETZSCH STA 449C thermal analyzer by detecting thermal gravity mass loss and differential scanning calorimetry (TG/DSC) of the sample in the following temperature and atmosphere protocol: The sample was heated in Ar+10%O2 atmosphere to 500 °C and kept for 90 min to minimize the affect of adsorbed surface hydroxyl and capping surfactant. Then the temperature was allowed to cool down to

Investigation of CexZr1-xO2 Nanocrystals

J. Phys. Chem. C, Vol. 113, No. 17, 2009 6923

Figure 1. Temperature and atmosphere protocols for (A) oxygen desorption measurement, (B) full oxidation and sever reduction of the samples, and (C) in situ FTIR measurement.

100 °C and was kept for 30 min in the same atmosphere, followed by elevating to 800 °C and kept for 120 min in pure Ar to maximum desorption of oxygen for the first time, then the chamber was cooled to 100 °C in Ar+10%O2 atmosphere and kept for 30 min to make sure that the released oxygen from the surface and bulk of the sample can be fully regenerated. Thereafter, the desorption and regeneration circle was repeated another three times, with the exception that the holding time at 800 °C was reduced to 30 min, the last cycle was performed only in pure Ar, and the cooling was sustained to room temperature (RT). The full protocol is schematically illustrated by Figure 1B. No reductive agents such as H2 or CO were used in the protocol for the purpose of better understanding the oxygen exchange mechanism. In situ FTIR spectra for hydroxyl adsorption on C5 were recorded by a Nicolet Nexus 670 FTIR spectrometer equipped with a MCT/B detector. A total of 256 scans were obtained for each spectrum with a resolution of 4 cm-1. A high-temperature DRIFTS reactor cell with a ZnSe window (Nexus Smart Collector) connected to a purging/adsorption gas control system was used for in situ adsorption measurements. Powder catalysts were packed into the sample vessel of the reactor cell and preoxidized with 10%O2/Ar at 400 °C, followed by Ar purging at 400 °C for 30 min to eliminate the residual O2, then 5%H2/ Ar flow of 20 mL/min was allowed to pass through the reactor cell to reduce C5. The sample was reoxidized and then rereduced at 400 °C and cooled to RT. IR spectra were collected before each atmosphere switching. The protocol was illustrated in Figure 1c, where a-f were the settled scanning points which were designed to monitor surface as well as bonding status of C5 in the repetitive redox program. Before C5 was pressed into the in situ FTIR cell, it was dried at 110 °C to eliminate the physically adsorbed water. Results and Discussion Evaluating the structure of CexZr1-xO2 mixed oxides is a sophisticated procedure, and XRD in combination with Raman

Figure 2. XRD patterns of a series of CZ samples.

spectroscopy was often introduced to distinguish the t, t′, t′′, and c structures. The difference between solid solution and mixed oxides is clear because the different XRD lines of CeO2 and c-ZrO2 oxides are distinguishable, let alone that between CeO2 and t- or m-structured ZrO2. Structures of a series of CZ samples characterized by XRD (Figure 2) showed that the intensity of diffractions increased with the elevation of ceria content, and the zirconia-rich (C1 and C3) samples even displayed C5 asymmetric diffractions, and some distinctive peaks belonging to the individual ZrO2 and CeO2 components can be found, indicating that segregation between the two phases occurred in these samples. Asymmetric peaks located near 50°, which should originate from the segregated phases, were identical, although the peaks were broadened due to the similar position of diffraction lines for fluorite ceria and cubic zirconia phases and to the nature of broadened XRD lines of nanometerscale particles. However, no unexpected diffractions can be noticed on XRD patterns of ceria-rich samples (C7, C9, and C10) for which symmetric shapes on their diffraction peaks were

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witnessed, compared with their zirconia-rich counterparts, suggesting that the homogeneous solid solutions between CeO2 and ZrO2 were obtained. As it has been deduced that the homogeneous solid solution of CZ mixed oxides could not always be reached, a solubility limit of zirconia in ceria lattice was often detected in the CZ systems. Kim has proposed an avenue to deduce the solubility limit based on the square of Vegard’s slope of their provided empirical formula,29 which was initiated to acquire the lattice constant (a) of CZ solid solutions with various ceria contents. The empirical formula was also adopted by Leitenburg et al.30 to calculate the lattice constants of CZ solid solutions. According to the literature, the solubility limit should be deduced by adapting the dCe value, which is actually the observed lattice constant (in nanometer) of the asprepared sample, to the following formula to solve the variable mZrO2, which denotes the actual dosage (in percentage) of ZrO2 solute in CeO2 lattice.

mZrO2 )

dCe - 0.5414 0.0220(rZr - rCe)

Figure 3. H2-TPR curves of a series of CZ samples. Dotted ellipse highlight the reduction at around 800 °C for C1. The arrow points to the distinguishable reducing in C5.

(1)

where rZr and rCe (in nanometer) denote the effective ionic radii of Zr and Ce cations, respectively. The formula was deduced from the Kim provided empirical formula counting on the fact that ZrO2 was the only dopant in our cases. The lattice constant of the as-prepared C5 was revealed to be 0.5324 nm, which was a little larger than that calculated from the empirical formula (0.5271 nm), suggesting that zirconia in C5 was not fully incorporated into the CeO2 lattice. Therefore, the solubility limit (mZrO2) should be 33.12% when adopting dCe as 0.5324 into eq 1. The limit was exactly in accordance with what was indicated in Figure 2, from which the solubility limit located in the range of 30-50% zircinia in ceria lattice can be deduced based on the transformation of diffraction peaks from asymmetric (C5) to symmetric ones (C7), as the former indicates the anharmonicity of the sample and the latter is representative of homogeneous compounds. Crystal sizes of the ceria-rich samples were revealed to be an analogue of each other and the average value was ca. 8 nm according to the Rietveld refinement results. Lattice parameters of C7 through C10 samples increased from 5.4182 Å, 5.4219 Å to 5.4261 Å, respectively, which were found to be slightly larger than those calculated from the empirical formula. The enlargement of lattice constants can be attributed to the residual hydroxyls in the lattice (which would be verified by DSC/TG measurements), but dosage of hydroxyls in the as-prepared samples was revealed to be much lower than those prepared via the hydrothermal route, because the a parameters of our products were much smaller than those of the corresponding hydrothermally prepared samples.20 The phenomenon should to a certain degree verify the superiority of the solvothermal route compared with those using aqueous solution as the reaction media, which frequently introduced a large number of hydroxyls into the products. Utilization of organic solvents (such as ethanol) may have partly overcome this shortcoming. Furthermore, lattice contraction is a common issue in zirconia doped ceria materials due to the smaller ionic radii of Zr4+ (rZr4+ ) 84 pm) than Ce4+ (rCe4+ ) 97 pm), and also the thermodynamic and crystallographic instability of the pure CeO2 lattice. An ideal fluorite lattice has a cubic close packing structure by anions and cations stand in the body center of the cubic and have an 8-fold coordination, therefore, the ideal radii ratio of r+/r- should equal 0.732. But the ratio for pure CeO2 is 0.693

(rather than 0.782 provided by Sergent et al.31), a value smaller than the ideal one, therefore, the lattice of ceria tends to contract to a new more compacting one. The introduction of even smaller Zr4+ ions will accelerate the contracting process, and this was verified by Raman and EXAFS results, which suggested that there exist distorted tetragonal lattices in CZ samples;28,32-34 the lattice distortion can also be regarded as the seeking of a more stable form through contraction or deformation. In addition, Lemaux et al. introduced a lamellar structure to interpret the high oxygen storage capacity of Ce0.5Zr0.5O2 materials;35 the layer stacked Ce4+ and Zr4+ should also be regarded as lattice contraction of a ceria lattice for the purpose of pursuing higher thermodynamic stability. Hydrogen reduction properties of the as-prepared CZ nanocrystals were evaluated by TPR test, which yielded some interesting results (Figure 3). Pure zirconia as the reference prepared by the same method (with the exception of no surfactant being used) was also included. The major downward peaks at around 460 °C were found on all of the tested samples with the exception of pure ZrO2. The peak should belong to the carbonization of residual surfactant on the surface. All of the samples displayed a major H2 consumption peak in the temperature range of 450-620 °C; usually the consumption was attributed to the conversion of surface and near-surface Ce4+ (sometimes the bulk one) to Ce3+ in the reductive circumstance. An intriguing phenomenon was found in the case of pure ZrO2 sample that a reducing signal was found in the region of 500-600 °C on the TPR curve, indicating that the tetravalence zirconium ions in our sample can be reduced into trivalence or even lower ones, too. This phenomenon was often neglected in the previous literature because pure ZrO2 was often considered to be quite stable and could not be reduced; the temperature region for Zr4+ reduction being exactly located in the Ce4+ region also will lead to the neglecting of this phenomenon. In addition, the Zr4+ reduction in our sample could be an exhibition of high activity of nanocrystals. The other reductive behavior of different CZ samples was the high temperature reduction above 780 °C. CZ nanocrystals all exhibited deeper reductions when temperature in the TPR chamber reached above 780 °C. The deep reduction of CZ was related to the Ce3+ being reduced to Ce2+ and matters alike;26,36 the end point of this reductive reaction cannot be monitored due to the limitation of the instrument. But some different phenomena were discovered for the C1 and C5 samples. C1

Investigation of CexZr1-xO2 Nanocrystals

Figure 4. OSC values and degree of reduction as a function of ceria content in flowing H2 atmosphere. The horizontal dotted line indicates the theoretical OSC value calculated according to the formula Zr2Ce2O6.2.

displayed a much lower beginning temperature (750 °C) followed by lower ending point (about 850 °C) than all of the other CZ samples. Pure ZrO2 cannot be further reduced above 620 °C, which confirmed the reliability of TPR results. C5 in the test exhibited a shoulder at about 820 °C (see the arrow in Figure 3), indicating the C5 possessed a different reducing behavior compared with the others. The easier transformation of Ce3+ into divalence or even lower ones should have happened in the C5 sample according to the TPR measurement. In other words, the CZ solid solutions containing one-half of ceria may have a better reductive ability than the other CZ powders at normal high temperatures (about 800 °C in our discussion). For the purpose of making analogues with practical circumstances and making the samples reliable for industrial applications, a severe hydrogen reduction of the samples was undertaken to evaluate their actual OSC values. Mass losses for a series of the as-prepared CZ nanocrystals were measured following protocol A in Figure 1 and the results were presented in Figure 4 in the form of the OSC value as a function of cerium content. The OSC data were calculated from the mass loss of the samples, which was regarded as the oxygen releasing from the lattice of CZ solutions under hydrogen reduction conditions. The relative mass loss percentages according to the initial weight at the t1 point in Figure 1A were also included in Figure 4 for better understanding of our results. The OSC value of C5 in the severe reduction process was revealed to be 0.44 mol of O2/mol of Ce, which was the maximum except for C1 among the tested samples, although the as-prepared C5 was found to be a mixture of CZ solid solution and segregated zirconia according to the XRD results. In addition, the OSC value was much closer to the theoretical one (0.45) if pyrochlore-II was regarded as the destination of the reduction process, which will be discussed in the following part. Relative mass changing for the C5 sample also demonstrated the biggest one and was revealed to be 4.77% of weight loss from its fully oxidized and surfactant-decomposited parent sample. Ceria-rich samples in our experiments possessed smaller OSC values than those of zirconia-rich ones, but their relative mass loss percentages were more appreciable. It should be notified that the OSC value (1.23) of C1 was tremendous (because a OSC value larger than 1.0 is not acceptable if the unit mol of O2/mol of Ce was used for ceria-based TWCs), but should be reasonable, which will be discussed in the following part. An intriguing result lies in the OSC values of C7, C5, and C3 samples. In other words, if reducing Ce2Zr2O8 into Ce2Zr2O7

J. Phys. Chem. C, Vol. 113, No. 17, 2009 6925

Figure 5. XRD pattern of C5 after the TPR test (d) and Rietveld refinement curve of H2 reduced C5 (c), the broad shoulder in the 2θ range of 20° to 30° on pattern c should belong to the amorphous glass that was used as the sample supporter for XRD measurement due to the rather smaller amount of the sample. The solid red line is the calculated curve and the blank dotted line is the observed data. Red sticks are the permitted diffraction positions of the two phases and the bottom is the deviation between the calculated and observed data. The inset shows the shift of the main XRD diffraction of (a) C7, (b) C5, (c) reduced C5, and (d) C5 after TPR test, the solid lines under (b) and (c) in the inset are Gaussian-Lorentz deconvolved lines of the peaks. For pattern b, the low angle line should belong to the CZ solid solutions and the other one to the cubic zirconia phase; for pattern c, the low angle line should belong to the CZ solid solutions and the other one to the pyrochlore-II phase.

(pyrochlore-I, in which Ce4+ was totally converted into Ce3+) was concerned, the maximum OSC value should be 0.25, but those values of the three samples were much larger than 0.25. The much larger OSC value suggested that more Ce4+ and Zr4+ were reduced into Ce3+ and Zr3+ or lower valence, and more lattice oxygen atoms escaped in our severe reducing process, although a certain degree of sintering might have happened for samples after thermal treatment in the full oxidizing process. For C7, C5, and C3 samples, their reduced forms may be Ce2Zr2O7-y (y > 0). The results can be interpreted by the more recently published work of Baidya and co-workers, who found a pyrochlore-II (abbreviated as P-II hereafter) type compound (Fd3jm, a ) 10.6463 Å) with a chemical formula of Ce2Zr2O6.236 when reducing CZ samples at 800 °C for 120 h. Theoretical mass loss of P-II is 4.876% and that of OSC is 0.45 mol of O2/mol of Ce, which is in a good agreement with that observed in our experiments, indicating that the formula of the reduced C5 should be Ce2Zr2O6.2, in which the neutralization of charge balance through releasing oxygen originated not only from Ce3+ or lower but also from Zr3+ or lower ones. The assignment can be strongly supported by the XRD result of the reduced C5 shown in Figure 5, in which the XRD pattern of C5 after the TPR test was also included. Because C5 was proved to have all the superior properties in the tests, the following experiments were all designed to apply to C5 only. The crystal structure of C5 after the TPR test (Figure 5d) revealed a pure fluorite with no phase segregation; even a symmetric shape of the main diffraction peak can be found, indicating that the thermal treatment at 900 °C has induced a better uniformity of the CZ solid solution. The reduction of cerium ions from tetravalence to lower ones may also contribute to the better uniformity, because reducing Ce4+ to Ce3+ will induce the enlargement of the effective ion radii from 97 pm to 114 pm, which makes the r+/r- value much closer to 0.732.

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TABLE 1: Structure Parameters of Pyrochlore-II (Ce2Zr2O6.2) Lattice According to Rietveld Refinementa atom

site

x

y

z

occupancy

Biso (Å2)

Ce1 Zr1 Zr2 Ce2 O1 O2 O3

16c 16c 16d 16d 48f 8a 8b

0 0 0.5 0.5 0.3816 0.1250 0.3752

0 0 0.5 0.5 0.1250 0.1250 0.3746

0 0 0.5 0.5 0.1249 0.1249 0.3752

0.9001 0.0899 0.9001 0.0899 0.9000 0.5501 0.3001

0.021 0.022 0.020 0.018 0.039 0.028 0.042

a

Space group ) Fd3jm, a ) 10.4264 Å.

Therefore, lattice contraction induced by smaller zirconium would be compensated, which resulted in the uniform crystal structures. The structure of C5 after severe reduction was revealed to be quite different from the original one. The Rietveld refinement of the reduced C5 in Figure 5c displayed the phase segregation of CeO2-ZrO2 mixed oxides from pseudo-fluorite structure to the mixture of CZ and pyrochlore-II. According to the results of Baidya et al., some main diffraction peaks located at 29°, 34°, 48°, 57.5°, and 60° in 2θ should belong to the P-II type lattice. Rietveld refinement of the XRD plot based on a cubic Fd3m model for P-II and Fm3m for Ce0.5Zr0.5O2 solid solution was performed and the “quantitative analysis” function of MAUD software was adopted here to evaluate the phase contents in the mixture. The analysis results demonstrated a near 1:1 ratio of the two phases, a 46% in volume fraction for Ce0.5Zr0.5O2 phase and 54% for P-II was obtained. The lattice parameters of cubic P-II were listed in Table 1. Actually, the P-II structure is the contracted cubic CZ solid solution with better homogeneity and with oxygen vacancies left in the 8b sites of the lattice after the oxygen release,36 as the occupancy of 8b sites was 0.3001 according to the refinement results. However, the phase contents of the sample suggested an incomplete crystal structure conversion from CZ solid solution to P-II (Figure 5). This may be a hint that a large dosage of oxygen vacancies may exist in the CZ lattice after the reduction, the vacancies occupied the original oxygen sites, and led to the cubic lattice observed by XRD, but actually it should be the pseudocubic structure because a large number of oxygen vacancies existed in it. The results also indicated that CZ solid solutions in our reduced samples could not completely contract into P-II when reduced at 800 °C for only 4 h compared with pure P-II obtained at 800 °C by Baidya et al.; the reason lies in the fact that their temperature holding was prolonged too long to 120 h, and their beginning materials prepared through a solution combustion route may also lead to the deviation. Obvious maxima shifts for diffractions near 29° in 2θ were observed from 28.2° for the as-prepared C7 to 28.7° for C5, to 29.1° for C5 after TPR and to 29.7° for C5 after OSC measurement, which were shown in the inset of Figure 5, and two Gaussian-Lorentz lines could be obtained to fully deconvolve the asymmetric diffraction peaks of the original and severely reduced samples. The shift for C7 and C5 should originate from the lattice parameter changing due to the different composition, while that for the reduced sample was regarded as the evidence of two phases coexisting in the sample. The TG/DSC results were shown in Figure 6A in the plot of real time temperature, DSC, and gravimetric signal vs. time, respectively. DSC signal and the relative mass of the sample in the initial stage of protocol A in Figure 1 were shown in Figure 6B, which in fact was the exaggerated full-oxidizing part of Figure 6A. A 2.38% mass loss on the TG curve before 220 °C

Figure 6. (A) Temperature-programmed oxygen adsorption/desorption curves of C5 sample: (a) the real-time temperature, which is in good agreement with our settled program in Figure 1B, (b) the TG curve, and (c) the DSC signal. (B) DSC/TG curves in the corresponding full oxidation period in part A.

accompanied by an endothermal peak centered at 190 °C on the DSC signal were observed in Figure 6B, which should be attributed to desorption of chemically adsorbed hydroxyls on the surface of the sample and to the decomposition of lattice hydroxyls. The major endothermal peak centered at 264 °C accompanied by a mass loss of 3.1% was detected before 300 °C, and the mass loss continued until around 500 °C, which was attributed to the chemical decomposition of adsorbed PVP surfactant on the surface of the sample. DSC/TG signals in the repeated heating/cooling as well as oxygen adsorption/desorption circles were revealed to be fairly well repeatable, with the exception of the signal vibration in atmospheric changes, indicative of the reliability of the results. It can be seen that mass of the sample repeatedly rises and falls with the periodical change of atmospheres in the chamber: the mass rose gradually when O2 coexisted with Ar in the chamber and lost when oxygen was totally dispelled by Ar in the cooling processes. This suggested repeatable oxygen release from the sample in inert circumstances and oxygen absorption from the atmosphere in oxidative circumstances by the sample. The DSC signal displayed exothermal character in the oxygen adsorption process and endothermal character in the oxygen desorption in the CZ sample. Similar mass upward and downward changing has been reported recently by Reddy et al.,28 who measured the mass changes of CZ in CO-TPR processes, and recorded the repeatable mass changes of about 1% in TG curves. Although the mass changes were revealed to be only ca. 0.1% in our oxygen adsorption/desorption circles, the fact that the measurement was conducted under an Ar circumstance with no reductive

Investigation of CexZr1-xO2 Nanocrystals reagent should be considered. The protocol in Figure 1A should be regarded as a preferable candidate for studying the oxygen exchange mechanism of CZ samples, because the mechanism has not been fully studied in the literature. Compared with the mass changing of C5 in Figure 6A, the relative mass changing in reductive atmosphere was much bigger than that in oxygen exchange protocol, the deviation clearly originating from the different oxygen release mechanisms in the two testing systems. The energy barrier for H2 reacting with oxygen in ceria lattice in reductive atmosphere should be much smaller than that for chemical desorption of oxygen from oxide lattice in inert circumstance, which means oxygen escaping in H2 conditions and at high temperature is much easier than in inert conditions. Some discussions can be done based on the combination of TPR, OSC, and DSC/TG results. Organic surfactant (PVP in this case) has been adsorbed on the surface of the CZ nanocrystals because all of the above-mentioned results have repeatedly confirmed this evidence. Oxygen was released from the CZ lattice to form -OH or H2O in severe reducing circumstance for the sake of neutralizing the charge produced not only from Ce4+ to Ce3+ (and lower) but also from Zr4+ to Zr3+. This also can be introduced to interpret the ultrahigh OSC value of C1 in Figure 4. Because large amounts of Zr4+ and Ce4+ were reduced, only 10 mol % of cerium was involved in the OSC calculation. In addition, the obviously lowered reduction (at 800 °C) was attributed to the much higher OSC value, because the other samples have not totally finished the reaction. Although the reduction was revealed to be located at 750-850 °C and the OSC was performed at a maximum of 800 °C, it is noted that the TPR test was performed at a heating rate of 10 deg/min; such quick a rate should have delayed the thermal peak on the TPR curve, that is to say, the reduction would be completed when keeping C1 in the 800 °C chamber for 4 h. The same is true for the TPR shoulder of the C5 sample that began at 820 °C, but the reaction also should have been completed at 800 °C after 4 h. Therefore, the relatively higher OSC of C5 should originate from the lower beginning temperature and deeper reducing reactions. Hydroxyls as the tracer in IR spectra to follow the surface changing of CZ matters have been reported by Daturi et al. Hydroxyl is sensitive to the circumstance, especially when H2 as the reducing agent is concerned.36 Stretching of the hydroxyls on the C5 surface was revealed to be sensitive to temperature (Figure 7). Spectra a and f were measured at RT and showed only one broad and asymmetric band in the region of 2550-3750 cm-1, respectively. Changes of -OH stretching on IR spectra are mainly located in the region of 3500-3800 cm-1, in which spectra b through e displayed at least 4 different vibration bands: (1) at ca. 3625 cm-1, (2) ca. 3660 cm-1, (3) ca. 3735 cm-1, and (4) ca. 3764 cm-1. The bands in the wavenumber region smaller than 3700 cm-1 can be ascribed to the asymmetric stretching of hydroxyls coordinated to cerium atoms based on the results of Daturi’s work, because -OH on all of their CeO2-ZrO2 mixed oxide samples was revealed to be coordinated only to Ce atomssno vibration was found in the region above 3700 cm-1 in their work. But the bands at above 3700 cm-1 in our experiments were clearly observed and should be attributed to the stretching of hydroxyls coordinated to zirconium atoms, because -OH on pure zirconia have two bands: one is at ∼3670 cm-1 assigned to -OH monocoordinated to zirconium cations and the other at ∼3770 cm-1 due to the stretching of -OH polycoordinated to Zr cations.37 The assignment can also be deduced from the charge density (dc, calculated by total charger number of the ion and its effective volume) of zirconium species

J. Phys. Chem. C, Vol. 113, No. 17, 2009 6927

Figure 7. In situ FTIR spectra of hydroxyls stretching on the C5 surface in reduction/oxidation protocol. Spectra a through f correspond to the recording points in protocol C in Figure 1; dotted lines 1 through 4 stand for the different variation bands of hydroxyls.

rather than that of cerium ones (dcCe3+/dcZr4+ ) 0.298), as the higher dc involve a higher tendency to share positive charge with the hydroxyls through static force. Bands of hydroxyls coordinated to one or three cerium cations cannot be recognized instead of the I-A and III-A bands found in ref 36. Band 1 at ∼3625 cm-1 was terminated as II-B-Ce and was assigned to the stretching of hydroxyls coordinated to two Ce cations in the proximity of one oxygen vacancy (VO2•); band 2 at ∼3660 cm-1 was terminated as II-A-Ce and was assigned to the -OH coordinated to two normal cerium cations. Likewise, band 3 at ∼3735 cm-1 was terminated as II-B-Zr and was assigned to -OH connected to two zirconium cations incorporated with a VO2•; and band 4 at ∼3764 cm-1 as II-A-Zr stands for -OH connected to two normal surface Zr cations. Formation of II-B series bands should witness the formation of reduction induced oxygen vacancies. Intensities of the four bands changed with the variation of atmospheres and duration time. The signal of the IR bands of freshly formed surface hydroxyls coordinated both to Ce and to Zr cations (see spectra c in Figure 7) were revealed to be quite stronger than the other ones, indicating that the freshly exposed surface favored adsorption of newly formed hydroxyls in hydrogen reducing circles. IR absorption in the region became weakened after sweeping by oxidizing gas, because the surface hydroxyls were partially eliminated. II-B bands could be noticed on all of the spectra, even on the after-oxidized surface, but the II-B bands were enhanced on the after-reduced sample because hydrogen reduction will cause the oxygen to escape from the lattice, which is in agreement with previous discussion. All of the bands were partly strengthened (especially the II-A-Ce bands) but were not fully resumed after the sample was reduced again (Figure 7e), which suggested that there existed a limitation for hydroxyls coordinated onto the surface of the sample under the in situ testing conditions. The in situ IR spectra test was only performed at 400 °C because of the limitation of the instrument; this temperature was not high enough to fully reduce the samples according to H2-TPR results (Figure 3). The asprepared CZ nanocrystals can be partially reduced by hydrogen at 400 °C in the first run of the in situ FTIR test, but the temperature was high enough to sinter the as-prepared nanocrystals in the testing cell, which would handicap reoxidizing and rereducing of the samples in the following redox circles. Therefore, the newly formed hydroxyls coordinated to the

6928 J. Phys. Chem. C, Vol. 113, No. 17, 2009 cations (Ce and Zr) and then were swept away by flowing atmosphere, but the -OH adsorption cannot be fully resumed when the sample was reduced at 400 °C again because the kinetic factor of hydrogen reacting with lattice oxygen was limited. The II-A-Ce bands were strengthened, however, in the rereduced sample (Figure 7e), indicating that the oxygen anions connected to cerium cations have the relatively lower activation energy necessary to react with hydrogen, in comparison with the oxygen connected to zirconium cations, confirming the conclusion that cerium-based materials are a suitable candidate as a kind of three-way catalyst. In summary, a series of characterization techniques were applied to evaluate the redox properties of CZ nanocrystals prepared via a solvothermal route. C5 out of the series of samples was selected according to the TPR and OSC measurements; the following characterizations (DSC/TG, XRD, and in situ FTIR) were all applied to C5 in order to interpret the findings. Repeatable oxygen adsorption and desorption for the as-prepared sample was reported by DSC/TG measurement in programmed changeable atmosphere and temperature protocol; the results indicated that CZ may be suitable for certain kinds of oxygen buffers or containers both in inert and in reductive atmospheres. Severe reduction of a series of CZ samples indicated a maximum oxygen storage capacity of 0.44 mol of O2/mol of Ce for Ce0.5Zr0.5O2 sample in method of partially reducing the sample into P-II type compounds. Cerium in C5 played an important role in redox properties of ceria based TWCs according to the experimental results. Acknowledgment. This work was financially supported by National Natural Science Foundation of China (50774095), Program for New Century Excellent Talents in University (NCET-05-0695), Excellent Youth Foundation of Central South University, and Postgraduate Innovation Foundation of Hunan Province. We thank Prof. Hongbo Liu (School of Materials Science and Engineering, Hunan University, China) for his kind help in H2 reduction experiments. References and Notes (1) Martin, P.; Parker, S. C.; Sayle, D. C.; Watson, G. W. Nano Lett. 2007, 7, 543. (2) Kaspar, J.; Fornasiero, P.; Graziani, M. Catal. Today 1999, 50, 285. (3) Fernan´dez-Garcıa´, M; Martın´ez-Arias, A; Hanson, J. C.; Rodriguez, J. A. Chem. ReV. 2004, 104, 4063. (4) DiMonte, R.; Fornasiero, P.; Desinan, S.; Kaspar, J.; Gatica, J. M.; Calvino, J. J.; Fonda, E. Chem. Mater. 2004, 16, 4273. (5) Fernandez-Garcia, M.; Martinez-Arias, A.; Iglesias-Juez, A.; Belver, C.; Hungria, A. B.; Conesa, J. C.; Soria, J. J. Catal. 2000, 194, 385. (6) Di Monte, R.; Fornasiero, P.; Graziani, M.; Kaspar, J. J. Alloys Compd. 1998, 275-277, 877.

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