Activation of Oxygen in Ce2Zr2O7+x across Pyrochlore to Fluorite

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Activation of Oxygen in CeZrO Across Pyrochlore to Fluorite Structural Transformation: First-Principles Analysis Asha Gupta, Anil Kumar, Umesh V. Waghmare, and Manjanath Subraya Hegde J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b12264 • Publication Date (Web): 09 Dec 2016 Downloaded from http://pubs.acs.org on December 13, 2016

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The Journal of Physical Chemistry

Activation of Oxygen in Ce2Zr2O7+x across Pyrochlore to Fluorite Structural Transformation: First-principles Analysis Asha Gupta1,2,*, Anil Kumar3,4, U. V. Waghmare3, M. S. Hegde5,* 1

Materials Research Centre, Indian Institute of Science, Bangalore 560012, India

2

Texas Materials Institute, The University of Texas at Austin, Austin, Texas 78712, USA.

3

Theoretical Sciences Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur

Campus, Bangalore 560064, India 4

Theoretical Division, Los Alamos National Laboratory, Los Alamos, NM 87544, USA

5

Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India

*

To whom the correspondence may be addressed. Email: [email protected],

[email protected]

Keywords: Ceria-zirconia, DFT, oxygen storage capacity, fluorite, pyrochlore, bond valence sum

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Abstract Cationic substitution of zirconium in ceria (CeO2) greatly improves its oxygen storage capacity (OSC) and thermal stability. While the fluorite structure of Ce0.5Zr0.5O2 (Ce2Zr2O8) exhibits good oxygen storage and release properties, its reduced counterpart Ce2Zr2O7 in the pyrochlore structure derived from the fluorite structure does not. Here, we present analysis of the structural evolution of Ce2Zr2O7+z from pyrochlore-Ce2Zr2O7 (z=0) to fluorite-Ce2Zr2O8 (z=1) using first-principles density functional theoretical calculations and bond-valence theory and correlate the consequent activation of oxygen to the observed oxygen storage capacity. By gradual addition of oxygen atoms to the otherwise ordered vacant tetrahedral sites for anions in pyrochlore-Ce2Zr2O7 structure will lead to a transition to fluorite-Ce2Zr2O8 structure and we demonstrate that this transition involves an increase in the number of weakly bonded, activated oxygen sites that are pertinent to higher OSC observed for fluorite-Ce0.5Zr0.5O2 structure. The structural descriptors of OSC demonstrated here will facilitate understanding and rational design of oxide materials with improved OSC, which is key to catalyzing a number of reactions for various applications.


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Introduction Solid solutions of CeO2–ZrO2 have attracted considerable interest in the last few decades due to their excellent oxygen storage/release property that is useful in the three–way catalysis widely used to control or minimize exhaust emission of automobiles.1–8 The oxygen storage capacity (OSC) of ceria is key to its use as a catalyst in various other industrially important reactions such as water-gas shift reaction, preferential oxidation of CO (PROX), thermo–chemical splitting of water, steam reformation of natural gas, which are crucial to establishment of future hydrogen fuel economy.9–11 Improved OSC is observed in a number of compounds having composition Ce1–xMxO2 (where M = Zr, Ti, Sn, transitions metal ions, noble metal ions, rare-earth ions, etc.).12-27 Early reports on the enhancement of OSC on adding ZrO2 to CeO2 date back to early 1990s,28 followed by a number of studies devoted to understanding the origin of high OSC in Ce1–xZrxO2.12-14,

29-32

General structural feature correlating with enhanced OSC of modified-CeO2 was identified using first-principles density functional theoretical (DFT) calculations with focus on transition metal ions, noble metal ions and rare-earth ions doped ceria.22 Dopant cation, with ionic radii comparable to or larger (in case of rare-earth elements) than Ce4+ ion induce weaker local structural distortions than the ones arising from substitution of transition-metal ions such as Zr4+, Ti4+, Sn4+, Cu2+,22,33 which have ionic radius smaller than that of Ce4+ ion. According to Vlaic et al. Zr ion is too small to accommodate eight oxygen atoms in its first coordination sphere, favoring displacement of one of the coordinating oxygen atoms to a non-bonding distance from Zr ion;29 this structural order or disorder in the oxygen sublattice generated by Zr ion in CeO2 enhances the reducibility of mixed oxides. Liu et al. reported that O K–edge XANES spectra for Ce1–xZrxO2 compounds show a distinctive line shape that cannot be attributed to a sum of CeO2 and ZrO2 features, supporting the



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idea that O atoms in the substituted ceria are in a chemical environment different from that of the host oxide.30 To understand how oxygen storage capacity develops in Zr substituted CeO2, the study of Ce0.5Zr0.5O2 composition is useful although Ce0.75Zr0.25O2 is the preferred composition for its catalytic applications. A compound with pyrochlore structure is represented by a general formula A2B2O7. Pyrochlore structure can be described as an ordered cubic close-packed (fcc) array of cations with the oxygen anions occupying seven of the eight tetrahedral sites between the cations. In the pyrochlore structure, Ce ion is in +3 oxidation state and 8–fold coordinated, while Zr ion is in +4 oxidation state and 6–fold coordinated. The pyrochlore structure Ce2Zr2O7 is related to the fluorite–CeO2 structure: ordered cations along and ordered vacancies in one-eighth of the tetrahedral anion sites (which is otherwise occupied by an oxygen ion in fluorite structure). Thus, a change from Ce2Zr2O8 to Ce2Zr2O7 involves a transition from fluorite to pyrochlore, that is accompanied by introduction of ordered oxide ion vacancies and reduction of Ce4+ to Ce3+. Equimolar CeO2–ZrO2 solid solution is an interesting system that provides a platform to determine structural origin of OSC in fluorite structure because (a) Ce2Zr2O7 in the pyrochlore structure does not exhibit any oxygen buffering property, while (b) its oxidized counterpart, Ce2Zr2O8 in fluorite structure has a very high OSC. This is represented by the following equation: Ce2Zr2O8 + H2 → Ce2Zr2O7 + H2O; and Ce2Zr2O7 + ½ O2 → Ce2Zr2O8. Bruce and co-workers were the first to report the formation of oxygen excess pyrochlore, Ce2Zr2O7.97 with a composition close to fluorite, by controlled oxidation of Ce2Zr2O7. In a recent report, Wang et al. showed that maximizing both the local relaxation and the number of such relaxations are the key factors to achieve high OSC for Ce2Zr2O8 solid solutions.12 Similarly, large positional disorder and greater spatial distribution of the oxides ions in Ce0.5Zr0.5O2 was considered as a factor relevant to its higher catalytic activity.32 In 4 ACS Paragon Plus Environment

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another report authors claimed that the formation energy for O vacancy for a series of Ce1–xZrxO2 is lowest for 50% Zr-doped composition and, therefore, ease of formation of O vacancy is the key to high OSC observed for Ce2Zr2O8 (or Ce0.5Zr0.5O2).13 To gain an insight into the atomistic origin of OSC in Ce0.5Zr0.5O2, we undertook to examine the changes in the local geometry of Ce2Zr2O7+x that arise during a transition from pyrochlore-Ce2Zr2O7 (z=0) to fluorite-Ce2Zr2O8 (z=1). We use a combination of first-principles DFT calculations and bond valence analysis to demonstrate that weaker oxygen bonds are generated upon transiting from the pyrochlore-Ce2Zr2O7 to fluoriteCe2Zr2O8. Our analysis shows the absence of activated oxide ions (with bond valence sum < 2) in pyrochlore-Ce2Zr2O7 structure, and an increase in their concentration on transiting through intermediate structures (from Ce2Zr2O7  Ce2Zr2O7.25  Ce2Zr2O7.75  Ce2Zr2O8); the fluoriteCe2Zr2O8 structure shows the highest local structural distortion, which is necessary for its higher OSC observed.

Methods Our total energy calculations are based on the DFT within a local density approximation (LDA) for the exchange-correlation energy of electrons as implemented in plane-wave selfconsistent field (PWSCF) code.34 The interaction between valence electrons and ionic cores is treated using ultra–soft pseudopotentials,35 which are known for their accuracy efficiency in capturing the structural and electronic properties of materials. We included the semi core states of Ce and Zr in the valence: 5s, 5p, 5d, 6s states of Ce; 4s, 4p, 4d, 5s states of Zr; and 2s, 2p valence states of O. To optimize structure, we minimized the total energy until the Hellman-Feynman forces on each atom are smaller than 0.03 eV/A. Ideally, the inclusion of onsite correlation term with Hubbard U parameter facilitates access to physical ground states, especially the electronic structure 5 ACS Paragon Plus Environment


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when Ce3+ is involved.36

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However, earlier work has shown that the structural optimizations are

generally insensitive to the value of U parameter.37,38 As our focus is on the structural properties here, we did not use U parameter. We use a 2 × 2 × 2 periodic supercell (built from the conventional cubic cell) comprising 96 atoms (i.e., 32 formula units of CeO2) for simulation of CeO2 and Ce2Zr2O7+z. The Brillouin zone integrations were sampled on uniform 4 × 4 × 4 k-point mesh.39 50 % Zr substitution was introduced by keeping the cation ordering the same as that in the pyrochlore structure, with the 1/8 of the tetrahedral sites occupied by the oxygen atom. The resultant supercell, Ce16Zr16O64, is, therefore, a fluorite lattice with an ordered arrangement of Ce and Zr cations along with all tetrahedral sites occupied by oxide ion (see Figure 1a). We then introduce oxygen vacancies (by maximizing the initial vacancy-vacancy distances) in this Ce16Zr16O64 structure to generate Ce16Zr16O56 –pyrochlore structure (shown in Figure 1b) via two intermediate states of Ce16Zr16O62 and Ce16Zr16O58. The M—O bond lengths (M = Ce, Sn Pd) determined from the optimized structures are used to estimate bond valence of oxygen following the bond-valence method described by Brown and O’Keeffe.40 Bond-valence is defined as (a) si = EXP[− (Ri−R0)/B] for Ce—O bond and (b) si = (Ri/R0)−N for Zr—O, where Ri is the ith bond length, R0 is the length of a bond of unit valence, and B and N are the fitted constant parameters. The atomic valence V for the oxide ions and cations are obtained by summing up the valencies of all bonds associated with a particular ion given by V = Σi si.


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A

B

Figure 1. Extended unit-cell of (A) Ce2Zr2O8 (or Ce0.5Zr0.5O2), and (B) Ce2Zr2O7 before structural relaxation. Ce2Zr2O8 consist of 64 O2– (green), 16 Ce4+(red) and 16 Zr4+(blue) ions and Ce2Zr2O7 contains 56 O2– (green), 16 Ce4+(red) and 16 Zr4+(blue). Note that in Ce16Zr16O56 (or Ce2Zr2O7) supercell, oxygen vacancies ordered in a fashion similar to A2B2O7 structure.

Results and Discussion CeO2 is a wide band gap insulator that adopts the cubic fluorite structure with an observed lattice parameter of 5.411 Å.22 Lattice parameter of our theoretical, optimized structure of CeO2 is 5.42 Å, which is close to the experimental value, with all Ce—O bond lengths equal to 2.34 Å. Ce16Zr16O64 model was constructed using fluorite structure by substituting 50% Ce with Zr and was allowed to relax completely. Due to a high concentration of substituent Zr4+ ion, the optimized structure of Ce16Zr16O64 displays a distorted geometry (see Figure 2), with reduced local symmetry 7 ACS Paragon Plus Environment


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than that of the initial ideal fluorite structure (i.e, Figure 1A). The extent of structural distortions reference to the parent-CeO2 was assessed by extracting the individual Ce—O and Zr—O bonds from the relaxed structure as shown in Figure 3. To monitor the evolution of structural changes we relaxed and analyzed the two intermediate structures, namely, Ce16Zr16O62 and Ce16Zr16O58. Thus, Ce16Zr16O64 and Ce16Zr16O56 serve as two end members of the fluorite to pyrochlore transition with Ce16Zr16O62 and Ce16Zr16O58 as the intermediate states. Distributions of cation–oxygen bond distances extracted from the intermediate structures, Ce16Zr16O62 and Ce16Zr16O58, and the terminal pyrochlore structure, Ce16Zr16O56 and their comparison with that of parent-CeO2 (cubic; space group Fm–3m; a = 5.135 Å) (see Figure 3) show that removal of 1/8 oxide ions generates Ce16Zr16O56 in pyrochlore structure displaying only three types of bond lengths: two Ce—O and one Zr—O bond length. The two types of Ce—O bonds in the optimized structure have bondlengths of 2.32 and 2.46 Å with Ce in 2+6 coordination, and Zr is in 6 coordination with Zr—O bond length of 2.18 Å (see Figure 3B). These are the typical coordination number and bonds lengths expected of the Ce2Zr2O7 pyrochlore structure.14 By adding oxygen to the vacant O2--sites in the pyrochlore-Ce16Zr16O56, Ce16Zr16O58 composition was generated. Ce16Zr16O58 exhibits a distribution of Ce—O and Zr—O bonds in two distinct groups of which Zr—O bonds are more confined and Ce—O bonds are relatively more widely distributed (see Figure 3C). This is because the presence of oxygen vacancies results in cerium assuming two oxidation states, +3 and +4, while Zr remains only in +4 oxidation state. The demand for two different coordination environments and ionic radii of Ce3+ and Ce4+ ions in Ce16Zr16O58 leads to a wider distribution of Ce—O bond compared to Zr—O bonds. Further addition of oxygen at the vacant tetrahedral sites generates Ce16Zr16O62. Bond distances show even wider distribution than that of the compositions with lower oxygen content (see Figure 3D). Ce—O bonds shifts towards the longer bond type and Zr—O


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bonds are more narrowly distributed around the shorter bond type. Finally for the Ce16Zr16O64 structure, distribution of Ce—O and Zr—O bonds span an even wider range of bond lengths (see Figure 3E).

A

B

Figure 2. Optimized structures of (A) Ce16Zr16O64 (96-atoms; Ce2Zr2O8) and (B) Ce16Zr16O56 (88-atoms; Ce2Zr2O7). Note that after complete structural relaxation the periodic lattice symmetry is broken; compare with Figure 1.

These results show that introduction of oxygen atoms in the tetrahedral voids of pyrochlore leads to significant local structural distortions in the lattice. When the stoichiometric composition is achieved with all tetrahedral voids occupied by oxygen, greatest degree of local distortion is observed with Ce—O and Zr—O bonds spanning a wide range of bond lengths. It is mostly due to such structural distortions, the fluorite-Ce1–xZrxO2 solid solution exhibits high oxygen capacitance



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at elevated temperatures. To find rational basis to this correlation, we now use bond valence analysis and identify the concentration of the activated oxygen. Bond valence sum (BVS) of anion is a measure of its oxidation state in these optimized structures, which is also indicative of its chemical activity. BVS for the cations and oxide ions were estimated from the bond lengths in optimized structure. The oxide ions with lower bond valences are relatively loosely bound to surrounding cations in the lattice, and are therefore responsible for higher OSC. For pure CeO2, BVS for all the cerium and oxygen atoms was computed and found to be ~4.06 and ~2.03 respectively, in agreement with their formal oxidation state of +4 and –2. Figure 4 shows that the BVS does not exhibit any distribution due to the highly ordered structure of Ce16Zr16O56 supercell (or Ce2Zr2O7) structure and is close to ~2, while some oxide ions also exhibit a value higher than 2 (BVS of ~2.45). This indicates that the oxide ions are tightly held in their local geometry in the Ce2Zr2O7 lattice. The transformation from Ce16Zr16O56 to Ce16Zr16O58 with introduction of two oxide ion vacancies results in dispersed distributions of BVS of oxygens. While some oxide ions have high BVS of ~2.6–2.7, the presence of many more oxide ions with BVS

Activation of Oxygen in Ce2Zr2O7+x across Pyrochlore to Fluorite

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