Article pubs.acs.org/cm
Role of Multivalent Pr in the Formation and Migration of Oxygen Vacancy in Pr-Doped Ceria: Experimental and First-Principles Investigations Kiyong Ahn,†,‡ Dong Su Yoo,‡ D. Hari Prasad,† Hae-Weon Lee,† Yong-Chae Chung,*,‡ and Jong-Ho Lee*,† †
High-Temperature Energy Materials Research Center, Korea Institute of Science and Technology, 136-791 Seoul, Republic of Korea Department of Materials Science and Engineering, Hanyang University, 133-791 Seoul, Republic of Korea
‡
S Supporting Information *
ABSTRACT: We combined first-principles calculations with several experimental studies to investigate the complex role for high oxygen storage capacity (OSC) in multivalent Pr-doped ceria. TPR and Raman spectra were measured for confirming oxygen vacancy concentration and oxygen mobility. The coordination number was fitted via EXAFS spectra, and it was the correlated DFT calculation that has been corrected as effective U (5.3 eV) to well express the reducing state (4+ → 3+) for both Ce and Pr elements. In our study, when Pr is incorporated into pure ceria, Pr3+ and Pr4+ ions are incorporated as majority and minority ions, respectively. Pr3+ ions play a key role to create oxygen vacancies and induce a local distortion, which improves oxygen mobility, and Pr4+ can contribute to diminishing reduction energy and a respectable OSC via the formation of an additional redox couple. KEYWORDS: ceria, density functional calculations, oxygen storage capacity, solid-state ionics, lattice distortion
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INTRODUCTION Enhancing the oxygen storage capacity (OSC) of ceria-based materials is one of the key means of expanding their successful use as a metal-oxide material in catalysis beyond the scope of three-way catalysts (TWCs) and solid oxide fuel cell (SOFC) applications.1−5 The capability of ceria-based materials to store and release oxygen is known to be intricately linked to the facile formation and diffusion of oxygen vacancies in ceria,6−8 which can be further enhanced by the addition of dopants (isovalent or alio/multivalent) into the ceria lattice.3−9 Among the known ceria-based materials, ceria-zirconia (CZO) solid solution is specifically considered for various applications due to its high OSC and relatively rapid oxygen-exchange kinetics.10−13 Despite the fact that substitution of Ce4+ by the isovalent Zr4+ is limited in terms of self-generation of oxygen vacancies via the charge neutrality criterion, the oxygen vacancies can be generated by large local relaxation in the crystalline lattice.11 Recently, certain multivalent cation-doped CeO2 species have been considered as outstanding catalysts for TWC applications or as electrode materials for SOFCs.14−19 Among the various multivalent elements, praseodymium is expected to be the most appropriate for dissolution into the ceria matrix to form a solid solution, due to its analogous fluorite structural nature and ionic radius to the Ce4+ ions.8,17−20 The presence of multivalent praseodymium (Pr3+/4+) in Pr-doped ceria (CPO) materials is known to enhance the formation and migration of oxygen vacancies, and thus, the OSC of these materials approaches magnitudes comparable to the level of CZO materials.14−16 However, many ambiguities surround these materials. For example, what is changed in the physicochemical nature of the © 2012 American Chemical Society
CPO material in the coexistence of the multivalent cations Pr and Ce? What are the energetic differences in the formation of oxygen vacancies during the replacement of Ce4+ with Pr3+ or reduction of the tetravalent cation (Pr4+ or Ce4+) to the trivalent state (Pr3+ or Ce3+)? More interestingly, what is the origin of the excellent OSC of these CPO materials? To clarify all of these issues, herein, we undertake a first-principles investigation into the structural and redox properties of CPO materials in conjunction with certain supporting experiments to determine the origin of the superior OSC of CPO materials. Details of the synthesis procedure and the computational methods utilized are given in the experimental section.
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EXPERIMENTAL SECTION
Powder Synthesis. PrxCe1−xO2 (x = 0, 0.1, 0.2, 0.3, 0.4, and 0.5) powders were prepared via the glycine nitrate process (GNP). This process is a self-combustion synthesis technique that can produce fine and homogeneous metal oxide powders.21 Cerium(III) nitrate hexahydrate (Ce(NO 3 ) 3 ·6H 2 O, 99.9%, Alfa Aesar) and praseodymium(III) nitrate hexahydrate (Pr(NO3)3·6H2O, 99.9%, Alfa Aesar) were used as the Ce and Pr precursors, respectively. These chemical compounds were dissolved in distilled water. The solutions were mixed in appropriate proportions to provide the nominal compositions of the CPO, and glycine (Junsei Chemicals) was used as a combustion fuel for GNP with a G/N ratio of 0.55. The obtained powder was calcined at 600 °C for 1 h to remove all traces of Received: July 16, 2012 Revised: October 6, 2012 Published: October 18, 2012 4261
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carbon. Furthermore, to reduce the particle size and improve uniformity, the CPO powders were ball-milled for 12 h. Computational Method. For structural and electronic optimization, first-principles calculations were performed using the density functional theory (DFT) with the exchange-correlation energy functional treated by Perdew−Burke−Ernzerhof (PBE) analysis of the generalized gradient approximation (GGA).22 Using the Vienna Ab initio Simulation Package (VASP) employing projected augmented wave (PAW), the self-consistent electronic density functional and the total energy were calculated. The cutoff energy of the plane-wave basis was set to 450.0 eV (33.07 Ry).23−25 The self-consistent loop was iterated until the total energy difference of the systems between the adjacent iterating steps became less than 10−5 eV. All constituent atoms were fully relaxed for structural optimization until the maximum Hellmann−Feynman forces became within the range of ±5.0 meV/Å. The ionic relaxation scheme is based on the conjugate gradient (CG) method.26 The nonshifted and Γ-point centered Monkhorst−Pack scheme of 6 × 6 × 6 k-point grids was used for Brillouin zone sampling.27 A linear tetrahedron method with Blöchl correction for the Brillouin zone integration was also applied.28 GGA+U calculation by employing Dudarev’s approach was used to describe the valence change (3+/4+) of the Ce element.24 In this method, two parameters, U and J, which, respectively, reflect the strength of the on-site Coulomb interaction and the strength of the exchange interaction, are combined into a single parameter (Ueff = U − J). Ueff was optimized as 5.3 eV when the lattice constant ratio of the hexagonal unit cell (c0/a0) of Ce2O3 was assumed to be 1.55, which was in good agreement with the experimental value.29 Since it has been suggested that a DFT hybrid approach with the Heyd−Scuseria−Ernzerhof functional (HSE) can describe the exact electronic structure,30 the convergence test for Ueff was performed with the HSE method and the electronic structure obtained from the HSE method was compared with that from the GGA+U method (in Figure S4, Supporting Information). After confirming the effectiveness of the GGA+U method, it was applied to the Pr element using the same Ueff value. Instrumentation. X-ray diffraction (XRD) patterns were obtained with a D/MAX-2500 X-ray diffractometer (Cu Kα radiation; Rigaku, Japan) with high power (18 kW). The intensity data were collected over the 2θ range of 20−80° with a 0.02° step size and a counting time of 1 s per point. Raman spectra were acquired by two different means. The Raman spectra in Figure 1 were measured using a conventional back-scattering geometry with a Raman spectrometer (T 64000, Jobin-Yvon, France), which consists of a triple polychromator and a charged-coupled device (CCD) detector. The excitation source was an Ar+-ion laser (λ = 514.23 nm), and the laser power was 10 mW at the sample point. The Raman spectra in Figure S8 (Supporting Information) were acquired with a Raman spectrometer (RFS 100/S FT-Raman Spectrometer,
BRUKER, Germany), which consists of a high-sensitivity InGaAs detector and an optical ultra-high-sensitivity Ge detector (liquid N2 cooled). A proprietary Rayleigh line rejection filter was used for simultaneously recording the Stokes and anti-Stokes shift. The excitation source was a Nd:YAG laser (λ = 1064 nm), and the laser power was 20 mW at the sample point. For hydrogen temperature programmed reduction (H2-TPR), approximately 0.03 g of the catalyst sample was placed in a quartztube reactor. The sample was pretreated at 300 °C under a pure argon atmosphere at a flow rate of 30 mL min−1 for 30 min. After cooling to room temperature, the gas atmosphere was switched to 5 vol % H2/Ar, and the reactor was programmatically heated to 800 °C at the rate of 5 °C min−1. The consumption of hydrogen was monitored by an in situ thermal conductivity detector (TCD) in a Micromeritics AutoChem II 2920 apparatus. Extended X-ray absorption fine structure (EXAFS) determinations of the Ce LIII- and Pr LIII-edges were performed at 10B XRS KISTPAL beamline of the Pohang Accelerator Laboratory (PAL). During the measurement, the beamline was operated at 2.5 GeV with a maximum storage current of 200 mA. This beamline is monochromatized by a Si(111) double-crystal monochromator detuned from 30% to 40% in order to reduce the higher-order harmonic content from the beam. For data analysis, the EXAFS spectra were subjected to subtraction of the atomic absorption using the AUTOBK program.31 The model for fitting was interpreted via Athena 0.8.056.32 Fourier transform (FT) formation and background removal procedures for isolating the oscillatory portion of the absorption coefficient were arranged using the Athena program. To obtain the coordination number of the cations, the EXAFS data were simulated via the Artemis 0.8.012 program with the IFEFFIT package, version 1.2.11c.32,33 The oxygen release characteristics of the powders were evaluated in the temperature range of 300−800 °C. The weight change of the sample was monitored by thermogravimetry (TG) under cycling heat treatments in flowing nitrogen or under dry air conditions. A commercial Q-600 TG-DTA analyzer was used for this purpose. The heat-treatment cycling schedule involved heating the sample to 800 °C, cooling to 150 °C, and heating again to 800 °C. A heating and cooling rate of 5 °C min−1 was used throughout. The weight loss of the sample during the second heating cycle was used to measure the oxygen storage capacity of the sample. This evaluation technique for the oxygen release characteristics is essentially similar to that described previously.34
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RESULTS AND DISCUSSION Raman spectra of the CPO samples, presented in Figure 1, show the presence of two characteristic peaks. The peak at lower wavenumber (∼465 cm−1) can be attributed to the F2g phonon mode of the fluorite structure, and the peak at higher wavenumber (∼570 cm−1) can be ascribed to a localized vibration mode of the intrinsic oxygen vacancies conjugated with Ce3+. The latter can also be assigned to the oxygen vacancies extrinsically introduced into CPO for maintaining the charge neutrality.20 Interestingly, with the increase in Pr content, there is a slight, but systematic, shift of the F2g band to lower wavenumber accompanied by a decrease in the peak intensity. This is due to the larger ionic radius of Pr3+ (1.266 Å) relative to Ce4+ (0.97 Å), which, as seen from XRD analysis (Supporting Information, Figures S1 and S2), results in a slight shift of 2θ to lower values and an increase in the lattice parameter with increasing Pr content. In contrast, the second peak at around 570 cm−1 in the Raman spectra increases drastically as the Pr content in the CPO samples increases. Generally, the most common oxide form of cerium (Ce) is CeO2, which is preferentially formed in air atmosphere. However, the most common form of praseodymium oxide is Pr6O11, in which the preferential oxidation state of Pr is less
Figure 1. Raman spectra of Ce1−xPrxO2−δ oxides calcined at 600 °C. 4262
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than +4. Thus, it can be expected that the CPO samples would a have relatively higher oxygen vacancy concentration due to the effect of doping with lower-valence Pr3+ on CeO2.35 Hence, the monotonic increase of the 570 cm−1 peak intensity with respect to Pr content in Figure 1 is indicative of the increased oxygen vacancy concentration in the Pr-doped cerium oxide. However, even though it is conclusive that the oxygen vacancies in CPO increase with the Pr content, the exact nature of the alteration of the oxygen vacancy concentration in CPO and its effect on the redox or OSC properties are still not elucidated. The redox properties of the CPO material were probed using H2-TPR experiments, and the results are shown in Figure 2. All
Table 1. Weight Loss (%) and Total Oxygen Storage Capacity (OSC) Obtained from Thermogravimetric Analysis (TGA) samples
weight loss [%]
μmol of O2/g
CeO2−δ Ce0.9Pr0.1O2−δ Ce0.8Pr0.2O2−δ Ce0.7Pr0.3O2−δ Ce0.6Pr0.4O2−δ Ce0.5Pr0.5O2−δ
0.16 0.23 0.42 0.98 1.82 2.02
50 72 131 306 569 631
of higher mobile oxygen vacancies and the corresponding RE4+/RE3+ (RE = Ce and Pr) redox couple. Since the local structure around a probe atom can be evaluated by studying its fine structure above the absorption edge using EXAFS analysis,36 this technique was used to elucidate the role of multivalent Pr on the crystal structure and the defective nature of CPO. LIII-edge EXAFS analyses of Ce and Pr in Ce1−xPrxO2−δ samples were performed to determine the cation primarily involved in oxygen vacancy formation. Figure 3a,b shows normalized EXAFS spectra of Ce1−xPrxO2−δ samples. The EXAFS fluctuation is clearly distinguished in each spectrum. From the radial distribution functions (Figure 3c,d) corresponding to the Fourier transform amplitude with the krange from 3 to 9 Å−1, the valence state (4+ or 3+) of Ce and Pr can be deduced based on the number of O2− defects (oxygen vacancies in this case) around the corresponding element. The EXAFS study clearly demonstrates that the valence states of either or both Ce and Pr ions are changed from 4+ to 3+ with increasing Pr content. However, it was not possible to differentiate the extent of the valence state change of each cation and its associated oxygen vacancy formation. Evaluation of the EXAFS results using the open source analysis package, IFEFFIT, version 1.2.11c, was performed to estimate the extent of oxygen vacancy formation from each cation. Figure 4 shows the variation in the oxygen coordination number (CN) of each cation (NCe−O and NPr−O) based on the Pr content; the oxygen coordination number for a perfect crystal structure having no oxygen vacancy is 8. The inserted figure (Figure 4) shows the octahedral coordination scheme around the cation in the fluorite crystal structure. It is evident from the figure that, with the increase in Pr content, there is a distinctive decrease in NPr−O compared with NCe−O. Thus, it can be concluded that the oxygen vacancies are preferentially generated around the Pr ion than the Ce ion. To apply the correct DFT method for calculating the electronic structure of CPO containing oxygen vacancies, Ce2O3 was selected as the extreme case of a defective CeO2 structure and the density of states (DOS) was calculated using the PBE method under various effective U conditions.37 From the iterative calculation of DOS with varying U − J values, 5.3 eV was selected as the best effective U value for calculating the DOS of Ce2O3 (Supporting Information, Figure S4). The specific area charge density distributions of both stoichiometric CPO (Ce0.96875Pr0.03125O2) and defective CPO (Ce0.96875Pr0.03125O1.96875) (Supporting Information, Figure S5) were calculated using this corrected DFT method. The difference between the calculated oxygen deficit (δ) of the two compounds was used to determine whether Ce or Pr played the dominant role in generating the oxygen vacancies. As shown in Figure 5, the plot of the differential charge density calculated in this way shows a contour of oxygen vacancy
Figure 2. H2-TPR profiles of Ce1−xPrxO2−δ oxides calcined at 600 °C.
the samples show two typical characteristic reduction peaks; the low-temperature peak is assigned to the easily reducible surfacecapping oxygen, whereas the high-temperature peak is due to the removal of bulk oxygen. With the increase in the Pr content, both the surface and the bulk reduction peaks shift to lower temperatures. Notably, the lowering of the bulk reduction temperature with increasing Pr content is particularly pronounced, possibly due to the increased oxygen vacancy concentration (as evidenced from Raman spectra results), which enhances oxygen migration in the lattice. Moreover, Pr addition also induces an ordered arrangement of vacancies, thereby creating an easy pathway for oxygen diffusion.14 All of these factors would promote oxygen ion diffusion from the bulk to the surface, facilitating bulk reduction at lower temperatures. OSC mainly relies on transport properties, such as oxygen diffusion at the surface and/or in the bulk of oxides, which is mainly determined by the presence, concentration, and mobility of lattice defects, such as oxygen vacancies. Table 1 shows the OSC of the CPO samples obtained by the thermogravimetric method. The data in this table corroborate the observation that the incorporation of Pr into the ceria lattice clearly promotes bulk reduction (as evidenced from Figure 2) and thus enhances the OSC. Because of the conceivable valence change of both Ce and Pr from 3+ to 4+ or from 4+ to 3+, the up−down swings of the oxygen defects accompanied with the valence charge can be achieved. Thus, it will have a high diffusivity of oxygen and the ability to store more oxygen as well. Hence, with the increase of Pr content, an increase of OSC is observed due to the simultaneous presence 4263
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Figure 3. Normalized EXAFS spectra of (a) Ce LIII-edge and (b) Pr LIII-edge and radial distribution functions corresponding to Fourier transformed amplitude of (c) Ce LIII-edge and (d) Pr LIII-edge of Ce1−xPrxO2−δ oxides calcined at 600 °C.
band in defective CPO compared with that of stoichiometric CPO, which provides further evidence that the oxygen vacancy formation is closely associated with the localization/delocalization of the 4f electron of praseodymium. The required dopant incorporation energy for the various forms of dopant cations (A = Ce3+, Pr3+, and Pr4+, assuming Ce4+ as a host cation) in Ce0.96875A0.03125O2−δ (2 × 2 × 2 supercell of 96 atoms) was determined from ab initio calculations. The incorporation energy was calculated by using eq 1. Ef(A to B) = E(A to B) − E(pure) + μB − μA
(1)
In the above equation, E(A to B) is the energy of the doped system, E(pure) is the energy of the undoped system, and μB and μA are the chemical potentials per atom of A and B bulk crystals. To consider the ionic radius effect in this calculation, the trivalent elements (Ce3+ and Pr3+) were used in the PBE potential for trivalent cations without Ueff. Figure 6 shows the statistical graph of the presence probability of each cationic species based on the Boltzmann distribution, which was estimated from the formation or incorporation energies derived from partition functions, as a function of temperature. On the basis of the results given in Figure 6, Pr3+ is the most energetically favorable cationic species (compared to Ce3+ and Pr4+) to be incorporated into ceria to generate oxygen vacancies up to 1200 °C. This result is consistent with the previously presented EXAFS results (Figure 3c,d) from which it was anticipated that Pr3+ is the most favorable state in CPO.
Figure 4. Coordination number of oxygen ions connected with constituent cations as a function of x = Pr/(Pr + Ce).
concentration around each cation, Ce and Pr. The differential charge density shown in Figure 5 clearly indicates that Pr has the dominant role in creating an oxygen vacancy, evidenced by the deep contour. This further corroborates the CN data presented in Figure 4, calculated from EXAFS analysis. More in-depth insight into the contribution of Pr to oxygen vacancy formation in the CPO samples was sought from the analysis of the density of states of both stoichiometric CPO (Ce0.96875Pr0.03125O2) and defective CPO (Ce0.96875Pr0.03125O1.96875). Figure S6 (Supporting Information) shows that the 4f states of the Pr ion are localized in the valence 4264
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Pr3+. However, after introducing an oxygen vacancy into the crystal lattice, the distorted lattice is relaxed by arbitrarily changing the bond length between the cation/anion and the oxygen vacancy. For example, as seen in Figure S7 (Supporting Information), both Pr3+ and Ce4+ move apart slightly toward the most proximal oxygen in a direction diagonally opposite to the vacancy, whereas the nearest oxygen moves toward the oxygen vacancy. Nevertheless, the lattice distortion induced by Pr3+ does not hinder the incorporation of Pr3+ by local relaxation accompanying the increase in oxygen vacancies. To further elucidate the effect of lattice distortion and relaxation on the oxygen vacancy migration,15 we investigated the lattice instability of CPO by introducing t-Ce2Pr2O8 as a reference structure for illustrating the structural change of CPO after the formation of oxygen vacancies.9,10 The t-Ce2Pr2O8 structure has an ordered cubic fluorite structure (Figure 7a) with a unit cell containing 16 Ce4+, 16 Pr3+, and 64 O2− ions.
Figure 5. 3D and top view of the DFT-calculated differential charge density of Ce1−xPrxO2−δ (x = 0.03125).
Figure 6. Probability of existence of the first-nearest-neighbor cation (Pr3+, Ce3+, and Pr4+) from an oxygen atom at the center of a tetrahedron in Ce0.96875Pr0.03125O2−δ.
Figure 7. (a) Unit cell of the t-Ce2Pr2O8 structure observed along [001], indicating 16 Pr3+ (green), 16 Ce4+ (yellow), and 64 O2− (red). (b) Optimized t-Ce2Pr2O7 structure after the introduction of an oxygen vacancy to a randomly well-distributed position.
However, the larger ionic radius difference between Pr3+ (1.266 Å) and Ce4+ (0.97 Å) may induce structural instability in CPO due to a significant local distortion of the fluorite structure during the replacement of Ce4+ with larger Pr3+. The degree of local distortion induced by Pr3+ was estimated from a comparison of the bond length between each cation and anion for the cases with and without the oxygen vacancy (Supporting Information, Figure S7). It is seen from Figure S7 that the lattice is distorted around Pr3+ when Ce4+ is replaced by larger
As mentioned before, since the large ionic radius difference between Pr3+ (1.266 Å) and Ce4+ (0.97 Å) may induce significant local distortion of the fluorite structure, there would be some counteraction of lattice relaxation by the oxygen vacancy introduced into the CPO lattice for charge compensation. The local lattice structural change induced by removing the eight oxygen ions from the t-Ce2Pr2O8 structure is displayed in Figure 7b. As shown in Figure 7a,b, a remarkable 4265
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CONCLUSIONS The complex role of multivalent Pr in the OSC of ceria has been investigated via empirical evaluation and first-principles calculations. Oxygen vacancies can be generated in ceria by the addition of multivalent Pr ions up to 50 mol %. Bulk oxygen reduction and surface oxygen reduction are facilitated by increasing the Pr content. In this manner, the energy required for dopant incorporation and oxygen vacancy formation in the bulk ceria is much lower during the replacement of Ce4+ with Pr3+ ions compared with the reduction of Pr4+ to Pr3+. Among the incorporated multivalent Pr ions, Pr3+ and Pr4+ are incorporated as majority and minority ions, respectively. It is confirmed that the Pr3+ ions play a key role in the generation of oxygen vacancies during the replacement of Ce4+ with Pr3+ ions. Furthermore, with the addition of Pr3+, a local distortion can be induced near the oxygen vacancy due to a mismatch between the sizes of Pr3+ and Ce4+ and the bond length between each species with oxygen, thereby improving the oxygen mobility. In addition, the remaining Pr 4+ can further contribute a respectable OSC via the formation of an additional redox couple. The individual contributions of Pr3+ and Pr4+ play an essential role in enhancing the OSC property of CPO. Finally, the results of this investigation indicate that the atomic level role of the electronic states of incorporated elements must be considered as an important guideline for the material design of multivalent or transition-metal-doped ceria-based systems in order to develop high-performance catalysts.
displacement occurs in the oxygen sites near the oxygen vacancy, which demonstrates that the Ce4+ and Pr3+ species have a prominent dissimilarity in the binding energy with linked oxygen ions, when an oxygen vacancy is generated. Because of this dissimilarity in oxygen binding, a structural imbalance can be induced in the CPO materials, resulting in the generation of free space for enhanced vacancy migration in the CPO lattice. A further indirect indication of the improved oxygen mobility due to the imbalanced CPO structures can be found from the decreased intensity and shift of the Raman signal with increasing Pr content (Supporting Information, Figure S8). Another notable point from the data in Figure 6 is that, even though the most favorable form of praseodymium is Pr3+, the Pr4+ state is also available in CPO. Therefore, it is expected that not only the host Ce4+ but also these Pr4+ species can contribute to the reducibility of CPO, whereas only Ce4+ can contribute in pure or other lanthanide-doped ceria. The vacancy formation energy was calculated by using eq 2 in order to determine whether Ce4+ or Pr4+ exhibits a higher probability to be reduced for generating additional oxygen vacancies Ev = E(Ce1 − xPrxO2 − δ ) + 1/2E(O2 ) − E(Ce1 − xPrxO2 ) (2)
where E(Ce1−xPrxO2−δ) and E(Ce1−xPrxO2) are the total energies of undoped or Pr-doped CeO2 with and without an oxygen vacancy, respectively, and 1/2E(O2) is the total energy of the oxygen atom in its ground state (O2 molecule). On the basis of the results of the calculation shown in Figure 8, only 1.83 eV is necessitated to create an oxygen vacancy via
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ASSOCIATED CONTENT
S Supporting Information *
Density of states of CeO2−δ and CPO, XRD and Raman spectroscopy spectra, and atomic positions of CPO (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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Figure 8. Calculated oxygen reduction energy of the first-nearestneighbor cations (Pr4+ or Ce4+) from an oxygen atom in the center of a tetrahedron. (a) The oxygen reduction energy in perfect CeO2. (b) The oxygen reduction energy with a single Pr4+ ion embedded into the CeO2 matrix. (c) The oxygen reduction energy with a pair of Pr4+ ions embedded into the CeO2 matrix.
ACKNOWLEDGMENTS This work was supported by the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, Republic of Korea, and the Institutional Research Program of Korea Institute of Science and Technology (KIST).
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the reduction of two tetravalent Pr ions while 3.61 and 4.38 eV are, respectively, needed for oxygen vacancy formation with the reduction of one or two tetravalent Ce ions, which indicates that the Pr4+ species can be preferentially reduced to generate an oxygen vacancy than Ce4+. These results suggest that the reducibility of CPO is enhanced by the presence of Pr4+, which generates a more desirable redox couple with the trivalent species than Ce4+. Overall, it can be concluded that the OSC is improved with increasing Pr content due to the availability of higher-mobility oxygen vacancies and corresponding RE4+/ RE3+ (RE = Ce and Pr) redox couples.
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dx.doi.org/10.1021/cm3022424 | Chem. Mater. 2012, 24, 4261−4267
