In Situ Study of the Oxygen-Induced Transformation of Pyrochlore

Article Cite This: Chem. Mater. 2017, 29, 9218-9226

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In Situ Study of the Oxygen-Induced Transformation of Pyrochlore Ce2Zr2O7+x to the κ‑Ce2Zr2O8 Phase Sven Urban,† Igor Djerdj,‡ Paolo Dolcet,§ Limei Chen,∥ Maren Möller,† Omeir Khalid,† Hava Camuka,† Rüdiger Ellinghaus,† Chenwei Li,†,⊥ Silvia Gross,§ Peter J. Klar,∥ Bernd Smarsly,*,† and Herbert Over*,† †

Physical Chemistry Institute, Justus Liebig University Giessen, Heinrich-Buff-Ring 17, 35392 Giessen, Germany Department of Chemistry, Josip Juraj Strossmayer University of Osijek, Cara Hadrijana 8/a, HR-31000 Osijek, Croatia § Istituto di Chimica della Materia Condensata e di Tecnologie per l’Energia, ICMATE-CNR, Dipartimento di Scienze Chimiche, Università degli Studi di Padova via Francesco Marzolo, 1, I-35131 Padova, Italy ∥ I. Physikalisches Institut, Justus Liebig University Giessen, Heinrich-Buff-Ring 16, 35392 Giessen, Germany ⊥ Key Laboratory for Advanced Materials, Research Institute of Industrial Catalysis, School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237, P.R. China ‡

S Supporting Information *

ABSTRACT: Temporary storage of oxygen in a solid catalyst is imperative for many important industrial oxidation reactions in the gas phase, for instance the post-treatment of automotive exhaust gas. A peculiar mixed Ce−Zr (1:1) oxide, the ordered κCe2Zr2O8 phase, is a promising catalytic material exhibiting an extraordinarily high oxygen storage capacity (OSC) and high thermal and chemical stability. We elucidate the temperature-dependent transformation between the pyrochlore pyr-Ce2Zr2O7 and κ-Ce2Zr2O8 phase upon oxygen uptake by in situ X-ray diffraction, X-ray absorption, and in situ Raman spectroscopy, providing insights into the electronic and structural changes on the atomic level, which are at the heart of the extraordinarily high OSC. We demonstrate that the Ce3+ concentration can be followed during transformation in situ by Raman spectroscopy of the electronic spin flip in the f-shell of Ce3+. The catalytic activity of the κ-Ce2Zr2O8 phase was investigated without an additional active component such as noble metals (Pt, etc.). While the high OSC of the κ-phase is beneficial for the oxidation of CO, the oxidation of HCl turns out to be unaffected by the high OSC.

1. INTRODUCTION Mixed Ce−Zr oxides in the form of solid solutions, in particular possessing equal amounts of Ce and Zr such as in t-Ce0.5Zr0.5O2 (tetragonal structure), are commonly employed as oxygen buffer in the post-treatment of automotive exhaust gas. Mixed Ce−Zr oxides supply lattice oxygen for the oxidation of residual CO and hydrocarbons under fuel-rich conditions and store oxygen under lean reaction conditions of the engine. Zirconium fulfills two important tasks in the mixed oxide Ce0.5Zr0.5O2. First, Zr thermally stabilizes CeO2 against sintering, and second, the oxygen storage/release capacity (OSC) of CeO2 is significantly increased.1 The underlying mechanism of oxygen uptake and release is closely associated with the superb redox chemistry of Ce that can readily switch its oxidation state between Ce3+ and Ce4+.2 © 2017 American Chemical Society

Previously, it was demonstrated that ordering the cationic sublattice of t-Ce0.5Zr0.5O2 into the so-called κ-Ce2Zr2O8 phase leads to a profound improvement of the OSC.4−11 For the preparation of the κ-Ce2Zr2O8 phase, the solid-solution tCe0.5Zr0.5O2 is reduced by hydrogen at high temperatures (up to 1500 °C), leading to the pyrochlore phase pyr-Ce2Zr2O7+x with a well-ordered cation sublattice (cf. Figure 1a). The successful ordering of the cationic sublattice was visualized in a previous high-resolution transmission electron microscopy (HRTEM) study, employing atomic-resolved chemical mapping.12 Subsequently, pyr-Ce2Zr2O7+x is mildly oxidized (T ≤ Received: July 22, 2017 Revised: October 5, 2017 Published: October 5, 2017 9218

DOI: 10.1021/acs.chemmater.7b03091 Chem. Mater. 2017, 29, 9218−9226

Article

Chemistry of Materials

conditions as well as X-ray absorption (XAS) performed ex situ on samples calcined at different temperatures. The oxygen storage capacity is correlated with the catalytic activity of the bare κ-Ce2Zr2O8 phase (without additional active component) in the oxidation of CO and HCl.

2. EXPERIMENTAL DETAILS To prepare the Ce0.5Zr0.5O2 solid-solution, 2.5 mmol Ce(NO3)3·6H2O and 2.5 mmol ZrO(NO3)2·xH2O are dissolved in 40 mL of deionized H2O using ultrasonic bath (60 °C, 240 W, 37 Hz) for 120 min (shaking it every 30 min to stir up the sediment). The slightly opaque solution is divided into 2 centrifuge tubes (20 mL each), and 15 mL of concentrated acetic acid is added per tube. Again, an ultrasonic bath (60 °C, 240 W, 37 Hz) is used for 60 min. The solution is heated in an open beaker at 130 °C for 24 h, resulting in a xerogel-like foam. After the sample is ground and heated to 500 °C (heating rate 3 K/min) in a ceramic crucible with a cover (not airtight) for 4 h, a yellow powder is obtained. t-Ce0.5Zr0.5O2 is then heavily reduced by hydrogen at high temperatures (1500 °C). In this way, O-vacancies are generated, which facilitate the mobility of the cations and therefore the ordering of the cationic sublattice. This high temperature (which is at least 200 K higher than that typically reported in the literature16) is required to warrant good ordering of the cation sublattice along the ⟨110⟩ direction and transform all Ce4+ into Ce3+. In general, the synthesis of the κ-Ce2Zr2O8 phase is based on a mild reoxidation of pyr-Ce2Zr2O7.3 at 600 °C under atmospheric conditions. In situ XRD measurements were performed on a PANalytical Empyrean diffractometer with a Cu Kα X-ray source (1.54060 Å, Ni filter for suppressing the Cu Kβ radiation). Heating of the sample was performed in an XRD reactor sample stage (Anton Paar XRK 900) using either ambient or pure nitrogen atmosphere. The heating rate was 3 K/min. Before every measurement, z-axis-correction was performed. The Rietveld refinement of powder XRD patterns was performed using the software FULLPROF.17 The modified Thompson−Cox−Hastings pseudo-Voigt method, which is known to allow for facile size analysis, was chosen as profile function, while the background has been modeled using the polynomial function of sixth degree with refinable coefficients. In this approach, we assumed that line broadening of the deconvoluted profile is a result of the small crystallite size and lattice microstrain. Isotropic temperature parameters and corresponding occupancies were separately refined to avoid a strong correlation thereof. The quality of the refinement was assessed by the values of the discrepancy factor (profile weighted residual error), Rwp, and the goodness-of-fit, GoF. The XAS measurements were carried out at beamline XAFS at Elettra synchrotron facility (Trieste, Italy). The ring energy was 2.4 GeV, while ring current was 160 mA. A Si(111) double crystal monochromator was used for measurements at the Ce L3-edge (5723 eV). The spectra were recorded in transmission mode at room temperature using ionization chamber detectors. Energy calibration was performed with a standard commercial CeO2 sample. In the temperature dependent XAS experiments, the samples were calcined in an oven up to a specific temperature and then cooled to room temperature. The powder samples were pressed into self-supporting pellets using polyvinylpyrrolidone (PVP) as a binder. For the analysis of XAS data, data reduction and analysis were performed using the freeware package Demeter.18 The surface area was determined by Kr physisorption performed at 77 K (Autosorb iQ, Quantachrome Instruments, Boynton Beach, FL), applying the BET method, assuming values of p0 = 2.63 Torr for the saturation pressure and σ(Kr) = 20.5 Å2 for the cross-sectional area. We are aware of the fact that absolute values of the BET approach determined from Kr sorption might suffer from the fact that the area of Kr on CeO2 surfaces is not reported yet; however, in our study, the comparison of these values determined for the same type of Ce−Zr mixed oxides is sufficient for a normalization of the catalytic data with respect to the surface area.

Figure 1. Structural transformation of pyr-Ce2Zr2O7 (Ce16Zr16O56, a) to the κ-Ce2Zr2O8 phase (Ce16Zr16O64, b). Figure is adopted from ref 3. The O-vacancies in pyr-Ce2Zr2O7 are highlighted in yellow.

600 °C) to form the κ-Ce2Zr2O8 phase under conservation of the cation sublattice ordering. At higher temperatures of 800 and 900 °C, a strong disordering in the cationic lattice can be induced, as reported in a previous TEM study.13 Several studies proposed intermediate structures upon oxidation of pyrCe2Zr2O7 into κ-Ce2Zr2O8. For instance, Achary et al.14 showed by neutron diffraction studies that pyr-Ce2Zr2O7+x can transform into the κ-Ce2Zr2O8 structure via an intermediate Ce2Zr2O7.5 lattice. The κ-phase κ-Ce2Zr2O8 is stable at ambient pressure but can be transformed to a rhombohedral phase by high-pressure treatment.15 The unit cell of the ideal pyrochlore phase pyr-Ce2Zr2O7 consists of 96 sites (cf. Figure 1a) that are occupied by 16 Ce, 16 Zr, and 56 O atoms (Ce16Zr16O56 corresponds to pyrCe2Zr2O7). The oxide ions are tetrahedrally coordinated to the metal cations. Forty-eight O atoms out of 56 are coordinated to 2 Zr and 2 Ce atoms (Wyckoff symbol: 48f), while 8 O atoms (Wyckoff symbol: 8a) are attached to four Ce sites. The residual eight sites (Wyckoff symbol: 8b) in the unit cell (cf. Figure 1) are coordinated to four Zr and are either vacant in the pyrochlore phase or occupied by oxygen in the κ-Ce2Zr2O8 phase (Ce16Zr16O64) (cf. Figure 1b). The Ce ions are either completely in the 3+ (pyr-Ce2Zr2O7) or in the 4+ (κCe2Zr2O8) oxidation state. In this study, we elucidate the temperature-dependent transformation between the pyrochlore pyr-Ce2Zr2O7 and κCe2Zr2O8 phase upon oxygen uptake, employing X-ray diffraction (XRD) and Raman spectroscopy under in situ 9219


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Figure 2. Thermogravimetric analysis during the temperature sweep (gray) in reducing atmosphere (pure H2: 1 bar) of the κ-Ce2Zr2O8 phase (red) in comparison with the solid-solution t-Ce0.5Ze0.5O2 (black). Subsequently, the previously reduced samples were reoxidized in ambient atmosphere (broken lines) during the temperature sweep. The Raman spectra were recorded in backscattering geometry using a Renishaw inVia Raman microscope system with a HeNe laser (633 nm) for excitation. For the in situ measurements, the powder sample was mounted in a Linkham microscope stage which allows us to vary the sample temperature in a controlled way between room temperature and 600 °C in different atmospheres. We used either air or N2 to establish an oxidative or oxygen-free atmosphere, respectively. The temperature-dependent Raman measurements were performed applying a heating rate of 10 K/min between sampling temperatures. At each sampling temperature, we allowed additional 10 min waiting time for the sample to equilibrate and an acquisition time of 15 min for a spectrum. The ex situ measurements were performed at room temperature in ambient air. The thermogravimetric analysis (TGA) measurements were carried out using a SETARAM SETSYS Evolution TGA. The catalytic tests of the oxidation of CO and HCl were carried out in a fixed bed flow reactor. The homemade design comprises the gas supply, the quartz tube reactor, heated by the furnace, mass flow controller, the UV/Vis spectrometer for analytic chlorine quantification, and an X-STREAM-CO2 analyzer (Rosemont) for the CO2 quantification.19

First-principles studies ascribed the facile formation of O 8b vacancies, i.e. the transformation of κ-Ce2Zr2O8 to pyrCe2Zr2O7, to the local relaxation of six neighboring 48f O atoms displaced toward this vacancy. This localized relaxation pattern reduces the total energy by 1.2 eV if comparing κCe2Zr2O8 with the solid-solution of the same composition (tCe0.5Zr0.5O2). The number of such possible local relaxations is maximized for the pyrochlore phase with an ordered array of cations,23 thus explaining the outstanding oxygen storage capacity of κ-Ce2Zr2O8. Clearly, the reversible transformation of the pyrochlore phase into the κ-Ce2Zr2O8 phase lies at the heart of the outstanding OSC (cf. Figure 2). Yet, the structural adaptation of the lattice during oxygen incorporation is hardly understood, most notably because practically all reported studies prepare the pyr-Ce2Zr2O7 by reduction of the solid-solution t-Ce0.5Zr0.5O2 at 1300 °C or below, a temperature which is not sufficient to warrant a well-defined starting structure.24 This issue becomes quite obvious from X-ray absorption near edge (XANES) spectrum at the Ce L3 edge (cf. Figure 3) that clearly indicates that such a temperature is insufficient to fully transform the starting solid solution to pyr-Ce2Zr2O7. The spectrum for the sample reduced at 1300 °C (black line) presents a split white line (peaks at 5731 and 5739 eV), typical for Ce4+ in CeO2.

3. RESULTS AND DISCUSSION The facile and reversible oxygen release of the κ phase is traced back to the reduction of κ-Ce2Zr2O8 toward the pyrochlore phase pyr-Ce2Zr2O7 paralleled by the reduction of most of Ce4+ to Ce3+. In TGA experiments (Figure 2), we observe that the oxygen release of κ-Ce2Zr2O8 in H2 atmosphere sets in already at 205 °C, while for the solid-solution t-Ce0.5Zr0.5O2, the corresponding on-set temperature is as high as 335 °C. The κCe2Zr2O8 phase loses 2.7% weight during the reduction. This loss is nicely reconciled with a reduction from Ce2Zr2O8 to Ce2Zr2O7 which is in broad accordance with a previous TGA study.20 In the literature, the oxygen storage and release has mainly been studied in the presence of an additional supported active metal component such as Pt. It is reported that the oxygen storage is independent of the Pt particles, but Pt plays a crucial role in the oxygen release.11,21,22 In ref 22, the reduction of κ-Ce2Zr2O8 without Pt (and with 1% Pt) sets in at 350 °C (150 °C), which is a significantly higher temperature than that shown in Figure 2. In a pure Ar-atmosphere, only little oxygen is released in the temperature range up to 600 °C, indicating a high thermal stability of both κ-Ce2Zr2O8 and t-Ce0.5Zr0.5O2 (cf. Figure S1). The reoxidation of previously reduced κ-Ce2Zr2O8 and t-Ce0.5Zr0.5O2 sets in for both materials around 80 and 100 °C, respectively (cf. Figure 2).

Figure 3. XANES spectra at Ce L3 edge. pyr-Ce2Zr2O7 is formed by reducing the solid solution t-Ce0.5Zr0.5O2 at high temperature in 5% H2. Room-temperature spectra of samples after reduction at 1300 °C (black) and 1500 °C (red), respectively. 9220


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Chemistry of Materials The Ce3+ content gives rise to the shoulder centered at 5727 eV. The sample reduced at 1500 °C (red line) is indeed mainly composed of trivalent cerium ions. A well-defined starting structure is, however, essential in the investigation of this transformation. In our XRD experiments, we did not observe an ordered intermediate state of Ce2Zr2O7.520 during the oxidation of pyr-Ce2Zr2O7. In the present paper, the pyr-Ce2Zr2O7 phase was prepared by high-temperature reduction of the solid-solutions tCe0.5Zr0.5O2 at 1500 °C. Actually, when keeping the highly reduced κ-Ce2Zr2O8 sample, i.e. pyr-Ce2Zr2O7, under ambient atmosphere at room temperature, pyr-Ce2Zr2O7 takes up 30% oxygen after 1−2 days. Therefore, in the following, we refer to our pyrochlore material as pyr-Ce2Zr2O7.3. The structural transformation of pyr-Ce2Zr2O7.3 to the κ-Ce2Zr2O8 phase was followed in situ as the temperature was increased from room temperature to 600 °C under ambient atmosphere (for comparison also under nitrogen atmosphere: cf. Figure S7), applying in situ techniques of XRD, Raman spectroscopy, and online TGA. These in situ measurements were complemented by ex situ XAS and ex situ Raman spectroscopy at room temperature. In Figure 4a we present a temperature series of in situ XRD data in the enlarged 2θ range from 78 to 82°, starting from the pyr-Ce2Zr2O7.3 phase (the series of full 2θ scans are provided in the Supporting Information, Figure S2); recall that the higher the diffraction angle, the higher also is the sensitivity to changes in the unit cell dimensions, i.e. the lattice parameters. It is obvious that up to 120 °C, no significant changes are discernible in the 2θ scans: both the reflection positions and the full-width at half-maximum (fwhm) values remain unaltered. Above 120 °C, the reflections start to shift to higher diffraction angles, a behavior that is qualitatively reconciled with a shrinking size of the unit cell. Furthermore, the shape of the reflections changes in that they become asymmetric. A significant broadening of the signals starts at 210 °C, while a dramatic shift of the broadened signals toward higher 2θ values sets in at 270 °C and terminates already at 300 °C. Around 360 °C, the XRD features of κ-Ce2Zr2O8 phase are fully developed, and from this temperature up to 600 °C, the reflections shift continuously to lower diffraction angles without altering their shapes. This continuous shift in position is indicative of thermal expansion of the fully developed κ-Ce2Zr2O8 phase. When the same heating experiment was conducted in a pure nitrogen atmosphere (cf. Figure S7), no structural changes were discernible, clearly indicating that the reduction in lattice parameter is unambiguously ascribed to oxygen incorporation. As seen by in situ XRD (cf. Figure 4a), the heat-treatment of pyr-Ce2Zr2O7.3 in ambient atmosphere is associated with significant structural alterations which were studied by Rietveld analyses. The XRD patterns for low temperatures were indexed in the cubic space group Fd 3̅ m (diamond space group) being appropriate for the pyrochlore structure (cf. Figure 1a), while κCe2Zr2O8 crystallizes in P213 space group with a cubic unit cell. High-resolution XRD data of the pyr-Ce2Zr2O7.3 phase and the κ-Ce2Zr2O8 phase are shown in Figure S3 together with the corresponding Rietveld refinement plot. Figures S3a and b display Rietveld refinement plots of pyr-Ce2Zr2O7.3 and κCe2Zr2O8, respectively, where, based on the shape of the difference curves, one can notice the validity of the chosen structural models. Moreover, merged Rietveld plots given in Figure S3c accompanied by the enlarged angular range reveal a clear difference between pyr-Ce2Zr2O7.3 and κ-Ce2Zr2O8

Figure 4. Transformation of the pyrochlore Ce2Zr2O7.3 phase to the κCe2Zr2O8 phase when the sample was stepwise heated under ambient atmosphere (210 mbar of O2): (a) in situ temperature series of XRD scans at high diffraction angles from 30 to 600 °C in temperature steps of 30 °C. (b) Ex situ Ce-L3 XANES spectra at room temperature of samples treated at various calcination temperatures. (c) In situ Raman spectra in the phonon region and the vicinity of 2100 cm−1 for various oxidation temperatures. The Raman signal at 2100 cm−1 corresponds to an electronic spin-flip in the f-shell of Ce3+ between 2F7/2 and the 2 F5/2 state. It does not occur for Ce4+ and is indicative of the presence of Ce3+.

phases; the latter is corroborated by the appearance of additional reflections. Structural parameters from detailed Rietveld analyses of pyr-Ce2Zr2O7.3 phase and κ-Ce2Zr2O8 phase are compiled in Tables S1 and S2, respectively, while selected interatomic distances are given in Table S3. These structural parameters are in reasonable agreement with recent studies.14,15 One particularly important type of information which can be extracted from such an analysis is the ordering of the cationic sublattice. The analysis performed on XRD data of the 9221


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Chemistry of Materials pyrochlore phase (T = 30 °C) revealed that 95% of the Ce and Zr sites are indeed occupied by Ce and Zr, respectively, and only 5% antisite disorder CeZr or ZrCe is present. The ordering of the cationic sublattice does not vary with calcination temperature, being consistent with the expected low mobility of cations below 600 °C. These results are coherent with earlier Rietveld analyses3,10,25 and ab initio calculations.23 As a main finding of the in situ XRD study, the variation of the lattice parameter as a function of the oxidation temperature (cf. Figure 5a) reveals four temperature regions (ranges I−IV).

the pyrochlore lattice. However, with the uptake of oxygen Ce3+ transforms to Ce4+, thereby shrinking in ion size from 1.14 to 0.87 Å.26 Within the Kröger−Vink notation, the oxygen incorporation is given by 2Ce3 + + VO° ° +

1 O2 → OO x + 2Ce 4 + 2

(1) x

where VO are oxygen vacancies on 8b sites and Oo denotes O2− on regular anionic positions. The uniform contraction of the unit cell leads to the formation of cracks in the κ-Ce2Zr2O8 phase observed in scanning electron microscopy (SEM).24 From 350 to 600 °C (range IV), the lattice parameter increases steadily with temperature with a constant slope of 1.1 × 10−4 Å/K that corresponds to the thermal expansion coefficient of 10.3 × 10−6 K−1. TGA data (cf. Figure 5b) suggest that above 350 °C, oxygen uptake from the oxide lattice starts dying out. Therefore, the observed increase in the lattice parameter is ascribed mostly to a thermally induced lattice expansion, whose expansion coefficient is in good agreement with that found for pure CeO2 (12 × 10−6 K−1).27 The linear variation of the lattice parameter with temperature is also recognized in the cooling curve (cf. Figure 5a, diffraction data are shown in Figure S4). Once the κ-Ce2Zr2O8 phase is formed in an air environment at high temperature, this phase persists upon cooling to room temperature, i.e., the transformation of the pyr-Ce2Zr2O7.3 to κ-Ce2Zr2O8 phase is irreversible. By a microstrain analysis (cf. Figure S5), the phase transformation from the pyr-Ce2Zr2O7 phase to the κCe2Zr2O8 phase can be clearly identified in the temperature range above 200 °C up to 350 °C. Rietveld refinement of powder XRD patterns is, in general, not very sensitive to the occupancies of oxygen positions due to the low values of atomic scattering factors for light atoms such as O. However, we conclude from our detailed Rietveld analysis that below 300 °C the occupation of 8a (O is coordinated by 4 Ce) is 100%, while the 8b sites (O is coordinated by 4 Zr) are occupied by ca. 30%. After the phase transformation at 320 °C, the equivalent oxygens coordinated by four Ce cations in the related 4a sites are occupied by 100%, while oxygens coordinated by four Zr cations (4a sites) are occupied by 90%. (cf. Tables S1 and S2) Using an identical temperature protocol as the one used in the in situ XRD experiments, the mass uptake during oxidation was monitored by in operando TGA measurements (cf. Figure 5b). Mass increase due to oxygen incorporation is observed only for temperatures higher than 120 °C. Around 350 °C, the slope of the TGA curve changes and begins to approach zero, indicating that the oxygen uptake of the pyrochlore phase becomes saturated. A similar behavior is evident from the XRD data (cf. Figure 4a), which do not change above 350 °C except for a continuous lattice expansion. The stoichiometry change in oxygen composition (oxygen uptake: 1.65 wt %) can be estimated to be about 0.6, fitting remarkably well to the proposed O occupancies derived by the Rietveld refinement. Both the development of the lattice parameter derived from XRD and the mass increase are in qualitative agreement with recent studies, e.g. Achary et al.14 and Sasaki et al.3,20 In these studies, two separate mass uptake steps were observed upon oxidation of the pyr-Ce2Zr2O7.0 phase, which are related to the oxidation of the pyrochlore structure pyr-Ce2Zr2O7.0 to Ce2Zr2O7.5 and the oxidation of Ce2Zr2O7.5 to the κ-phase κCe2Zr2O8. Interestingly, the corresponding temperature intervals described by Achary et al. agree with the temperature ranges II and III found in the present study.

Figure 5. (a) Lattice parameter as a function of the reaction temperature during mild oxidation of the pyrochlore phase. (b) Thermogravimetric analysis during the temperature sweep in oxidizing atmosphere. Four temperature ranges (I, II, III, and IV) are clearly discernible with distinct variations in the structural parameters and the O uptake. (c) Ce3+ fraction of XAS (left y-axis) and relative intensity of the Ce3+ Raman signal (right y-axis) at 2100 cm−1 obtained from ex situ measurements at room temperature on the same set of samples as a function of the maximum calcination temperature of the corresponding samples.

Up to 120 °C (range I), the lattice parameter is practically constant with a value of 10.65 Å, consistent with TGA data (cf. Figure 5b) that do not indicate any oxygen uptake in this temperature range. In the temperature interval of 120−200 °C (range II), the lattice parameter decreases with a slope of −2.7 × 10−4 Å/K, while in the temperature interval from 200 to 300 °C (range III), the lattice parameter decreases with an even larger slope of −7.8 × 10−4 Å/K. At first glance, the contraction of the lattice parameter is somehow counterintuitive because oxygen is incorporated into 9222


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spectroscopy suggests that this transformation into the κCe2Zr2O8 phase already takes place around 100 °C, a transformation that XRD cannot detect due to its low sensitivity for light elements such as oxygen (in comparison with Ce and Zr), very small sizes of coherently scattering domains of the second phase, or even due to the absence of long-range ordering of regions with the second phase. Furthermore, intermediate structures such as the cubic Ce2Zr2O7.5 describe by Achary et al.14 might form temporarily which, however, is hardly discernible by X-ray diffraction. With increasing temperature and increasing oxygen uptake, the spectrum varies mainly in intensity until the κ-Ce2Zr2O8 phase is fully developed. The observed (small) energy shifts of the Raman features above 150 °C have two contributions, mostly temperature-induced and partially due to the structural changes. This is manifested in the temperature-dependence of the mode positions on heating and cooling, which differ in the temperature ranges I, II, and III, where the structural transformation into κ-Ce2Zr2O8 is still in progress during heating in air, but is completed during cooling (cf. Figure S6), in agreement with the XRD results in Figure 5a. Additional evidence for reordering of the oxygen sublattice was reported by Thomson et al., who studied the intercalation of oxygen into the pyrochlore phase by neutron diffraction when using a 0.5 M solution of sodium hypobromite NaOBr in distilled water as the oxygen source.25 In this study, only the initial oxidation was considered. Because not a gas-phase oxidation with molecular O2, but rather oxidation in solution is considered, a direct comparison may be hampered. Oxide compounds with Ce3+ exhibit a characteristic Raman signal at 2100 cm−1 originating from electronic Raman scattering which arises from a spin-flip of the electron in the inner f-shell between the levels 2F7/2 and 2F5/229,30 and is not observed for Ce4+. In the in situ Raman spectra shown on the left of Figure 4c, the corresponding signal starts to decrease above 120 °C and disappears at about 360 °C. Similar to the signals in the phonon region, it is better to compare room temperature spectra taken on samples after calcination to different maximum temperatures. Figure S8 shows room temperature spectra of the Ce3+ signal of four such samples indicative for the temperature regions I−IV. The Ce3+ signal is detectable in regions I to III, but not anymore in region IV, in agreement with the XAS results and confirming the full conversion of Ce3+ into Ce4+. We have also measured the room-temperature Raman spectra of the same samples as used for the ex situ XAS experiments. We added a corresponding plot of the Raman intensity of the Ce3+ signal normalized to the intensity of the phonon region vs calcination temperature to Figure 5c. Both curves in Figure 5c show the same temperature behavior within the experimental uncertainties, strongly suggesting that in situ Raman spectroscopy may be profitably employed for monitoring the Ce3+ concentration of the catalyst under reaction conditions when properly calibrated (here against the XAS signal). More recently, the dynamics of the oxygen storage and release of ordered CeO2−ZrO2 supported Pt catalysts was studied by time-resolved X-ray absorption fine structure (XAFS) spectroscopy,11 indicating a rapid oxygen exchange already at 300 °C. However, the pyrochlore phase was prepared by reduction of t-Ce0.5Zr0.5O2 at 1200 °C, which we consider as a too low temperature to allow for complete ordering of the cationic sublattice. The oxygen diffusion process involved in this transformation has recently been imaged by nano-XAFS

Further information about the oxidation state and the coordination geometry around the metallic centers can be gained from ex situ X-ray absorption spectroscopy (XAS) measurements, recorded at room temperature, of samples calcined at different temperatures starting from room temperature (RT: pyr-Ce2Zr2O7.3) up to 600 °C (κ-Ce2Zr2O8 phase). In Figure 5c, we show the evolution of the Ce L3-edge XANES with the calcination temperature. Up to a calcination temperature of 150 °C, the Ce absorption spectrum changes very little, consistent with negligible oxygen uptake. However, for samples calcined between 270 and 420 °C, the Ce-L3 absorption spectra change profoundly, indicating a substantial oxygen-induced change in the oxidation number of Ce. For samples treated at temperatures higher than 420 °C, the corresponding Ce-spectra almost show no further changes. Altogether, this behavior is fully consistent with the observed in situ XRD and TGA data, demonstrating that the 270 to 350 °C range is characterized by intense structural rearrangements due to oxygen incorporation into the lattice. In agreement with literature reports,9 the room temperature Ce-spectrum of pyr-Ce2Zr2O7.3 is dominated by Ce3+, while that of the κ-Ce2Zr2O8 phase (after calcination at 600 °C) is dominated by Ce4+. Therefore, it is possible to estimate the Ce3+ concentrations of all samples in between, i.e. calcined up to different intermediate temperatures between room temperature and 600 °C, by fitting the corresponding absorption spectra with a linear superposition of the room-temperature spectra of pyr-Ce2Zr2O7 and κ-Ce2Zr2O8, thus providing the atomic ratio of Ce3+ with respect to all Ce species (Ce3+/Ce3+ + Ce4+) (cf. Figure 4b). We need to recall that the concentration of Ce3+ is directly related to the concentration of incorporated oxygen via reaction 1. In agreement with TGA, the oxygen stoichiometry increased by 0.65 (cf. Figure 5c) which, in turn, is consistent with the Rietveld refinements of the T-dependent XRD data, thus providing a coherent picture. Raman spectroscopy in the phonon region yields additional information about the symmetry of the crystal lattice and structural phase transitions. For this class of oxides, Raman spectroscopy is a particularly sensitive probe of the oxygen sublattice.28 In addition, electronic Raman scattering may yield useful information about the electronic band structure or, as for these materials, about the oxidation state of the cerium ions when considering the specific Raman signal at 2100 cm−1. Figure 4c depicts a series of in situ Raman spectra describing the transformation of pyr-Ce2Zr2O7.3 starting material into κCe2Zr2O8 during heating from room temperature to 570 °C in ambient atmosphere. The left graph of Figure 4c shows the evolution of the phonons. Quite surprisingly, in the low temperature range around 100 to 150 °C, dramatic changes in the Raman phonon spectra are evident. Because we know that significant oxygen uptake occurs only above 150 °C (cf. Figure 5b) and that the cationic sublattice is not altered at such low temperatures, the Raman spectra may indicate a reordering in the oxygen sublattice. This structural change in the temperature range of 100 to 150 °C is irreversible in air (cf. Figure S6) and does not occur when heating in N2 atmosphere (cf. Figure S7). However, we have to bear in mind that the cross section for Raman scattering is higher by a factor of 100 for the κCe2Zr2O8 than for pyr-Ce2Zr2O7 (cf. Figure 4c, left). Therefore, even small regions of transformed κ-Ce2Zr2O8 in the sample will dominate the resulting Raman spectrum. From the Rietveld analysis, we know that about 30% of oxygen sits in 8b position and may be able to form locally κ-Ce2Zr2O8. Raman 9223


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Chemistry of Materials (X-ray absorption fine structure) for a single particle of Pt/ CeZrO7+x.22 In our study, we demonstrate that bare pyrochlore is able to take up oxygen even at 100 °C without the action of additional active metal component such as Pt. This finding may be of utmost interest for the catalyzed HCl oxidation reaction (Deacon process) where bare mixed Ce−Zr oxides have already been demonstrated to be active and stable catalysts,31−33 and for pure CeO2, the oxygen storage capacity is considered important for this kind of reaction.34 Because the κ-Ce2Zr2O8 phase has demonstrated to be very active in the combustion of chlorinated compounds,35,36 we presume that the κ-Ce2Zr2O8 phase is a promising candidate as catalyst for the Deacon process with high activity and stability. In Figure 6, we summarize activity experiments in the HCl oxidation reaction over κ-Ce2Zr2O8 in comparison with that

Figure 7. Inherent activity in the CO oxidation reaction of the κCe2Zr2O8 phase in comparison with the corresponding solid solution t-Ce0.5Zr0.5O2: the space time yield is normalized to the BET surface area (STYa) that was determined by krypton physisorption experiments.

temperature range 300−450 °C is beneficial for the CO oxidation reaction.

4. CONCLUSIONS The pyr-Ce2Zr2O7.3 phase was prepared by high temperature reduction of the solid-solutions t-Ce0.5Zr0.5O2 at 1500 °C. Only this high reduction temperature ensures a good ordering of cationic sublattice as revealed by XAS (cf. Figure 3), thereby establishing a well-defined starting point for the present transformation study. The temperature-induced transformation between the pyrochlore pyr-Ce2Zr2O7+x and κ-Ce2Zr2O8 phase upon oxygen uptake in ambient atmosphere was studied in situ by X-ray diffraction, X-ray absorption, and Raman spectroscopy (cf. Figure 4). From TGA, the stoichiometry value amounts to x = 0.3, i.e. the starting material has already taken up 30% of the maximum additional oxygen at room temperature. The further transformation in ambient atmosphere starts at 100−120 °C and is accompanied by a characteristic shrinkage of the lattice parameters upon oxygen uptake due to the size-reducing oxidation of Ce3+ to Ce4+. The actual transformation is accompanied by a maximum in the microstrain. Upon oxygen uptake, the Ce3+ concentration diminishes as quantified by Ce L3-edge XANES, but more easily, this very same trend can be followed by in situ Raman spectroscopy of the electronic spin flip in the f-shell of Ce3+. By TGA, it was shown that the pyrochlore pyr-Ce2Zr2O7 is able to take up oxygen in the temperature range 80−330 °C, and the resulting κ-Ce2Zr2O8 phase releases oxygen in H2 atmosphere between 200 and 400 °C without the action of an additional active metal component such as platinum. This facile oxygen exchange is considered beneficial for promoting catalytic oxidation reactions. For the catalyzed HCl oxidation reaction (Deacon process), we found that the high OSC of κ-Ce2Zr2O8 does not improve the activity. Instead, our catalytic tests show that the inherent activity of the κ-Ce2Zr2O8 phase is virtually identical to that of corresponding solid solution t-Ce0.5Zr0.5O2. Quite in contrast, the catalyzed CO oxidation indicates inherent activity of the κ-Ce2Zr2O8 phase substantially higher compared to that of Ce0.5Zr0.5O2.

Figure 6. Inherent activity in the HCl oxidation reaction of the κCe2Zr2O8 phase in comparison with the corresponding solid solution t-Ce0.5Zr0.5O2: the STY is normalized to the BET surface area that was determined by krypton physisorption experiments.

over the solid solution t-Ce0.5Zr0.5O2. We normalized the space time yield (STY) to the BET specific surface area (inherent activity), which was determined by krypton physisorption experiments (which is more sensitive to low surface area samples than nitrogen physisorption; cf. Figure S9). From these experiments, it is evident that the inherent activity of κCe2Zr2O8 in the Deacon reaction is not higher than that of tCe0.5Zr0.5O2, i.e., the higher oxygen storage capacity of κCe2Zr2O8 has no beneficial effect on the activity. In a second reaction, we focused on the CO oxidation reaction over κ-Ce2Zr2O8 in comparison with that of the solid solution t-Ce0.5Zr0.5O2 (cf. Figure 7) for various temperatures in the range from 300 to 450 °C. The CO oxidation is considered a prototypical reaction for probing the oxidation activity of ceria-based materials.37−39 The CO oxidation over ceria-based catalysts takes place via a Mars-van Krevelen type of reaction mechanism, during which the solid surface is reduced and Ovacancies are formed, which subsequently are replenished by gas-phase oxygen. Clearly, from inspection of Figure 7, the BET normalized STY of the κ-Ce2Zr2O8 is about 10 times higher than that of t-Ce0.5Zr0.5O2. From the slope of the corresponding Arrhenius plots, the apparent activation energies are 75 ± 4 and 72 ± 3 kJ/mol for the κ-Ce2Zr2O8 and t-Ce0.5Zr0.5O2 phase, respectively. Obviously, the higher OSC of κ-Ce2Zr2O8 in the 9224


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Prepared by Reduction and Successive Oxidation of t’-(Ce0.5Zr0.5O2) Phase. J. Solid State Chem. 1999, 147, 573−583. (8) Kishimoto, H.; Omata, T.; Otsuka-Yao-Matsuo, S.; Ueda, K.; Hosono, H.; Kawazoe, H. Crystal Structure of metastable κ-CeZrO4 phase possessing an ordered arrangement on Ce and Zr ions. J. Alloys Compd. 2000, 312, 94−103. (9) Nagai, Y.; Yamamoto, T.; Tanaka, T.; Yoshida, S.; Nonaka, T.; Okamoto, T.; Suda, A.; Sugiura, M. X-Ray Absorption Fine Structure Analysis of Local Structure of CeO2-ZrO2 mixed oxides with the same composition ratio (Ce/Zr = 1). Catal. Today 2002, 74, 225−234. (10) Suda, A.; Ukyo, Y.; Sobukawa, H.; Sugiura, M. Improvement of Oxygen Storage Capacity of CeO2-ZrO2 Solid Solution by Heat Treatment in Reducing Atmosphere. Nippon Seramikkusu Kyokai Gakujutsu Ronbunshi 2002, 110, 126−130. (11) Yamamoto, T.; Suzuki, A.; Nagai, Y.; Tanabe, T.; Dong, F.; Inada, Y.; Nomura, M.; Tada, M.; Iwasawa, Y. Origin and Dynamics of Oxygen Storage/Release in a Pt/ordered CeO2-ZrO2 Catalysts Studied by Time-Resolved XAFS Analysis. Angew. Chem., Int. Ed. 2007, 46, 9253−9256. (12) Trasobares, S.; Lopez-Haro, M.; Kociak, M.; March, K.; de La Pena, F.; Perez-Omil, J. A.; Calvino, J. J.; Lugg, N. R.; D’Alfonso, A. J.; Allen, L. J.; Colliex, C. Chemical Imaging at Atomic Resolution as a Technique to Refine the Local Structure of Nanocrystals. Angew. Chem., Int. Ed. 2011, 50, 868−872. (13) Lopez-Haro, M.; Perez-Omil, J. A.; Hernandez-Garrido, J. C.; Trasobares, S.; Hungria, A. B.; Cies, J. M.; Midgley, P. A.; BayleGuillemaud, P.; Martinez-Arias, A.; Bernal, S.; Delgado, J. J.; Calvino, J. J. Advanced Electron Microscopy Investigation of Ceria-ZirconiaBased Catalysts. ChemCatChem 2011, 3, 1015−1027. (14) Achary, S. N.; Sali, S. K.; Kulkarni, N. K.; Krishna, P. S. R.; Shinde, A. B.; Tyagi, A. K. Intercalation/Deintercalation of Oxygen: A Sequential Evolution of Phases in Ce2O3/CeO2-ZrO2,. Chem. Mater. 2009, 21, 5848. (15) Errandonea, D.; Kumar, R. S.; Achary, S. N.; Gomis, O.; Manjon, F. J.; Shukla, R.; Tyagi, A. K. New High-Pressure Phase and Equation of State of Ce2Zr2O8. J. Appl. Phys. 2012, 111, 053519. (16) Montini, T.; Hickey, N.; Fornasiero, P.; Graziani, M.; Banares, M. A.; Martinez-Huerta, M. V.; Alessandri, T.; Depero, L. E. Varinations in the Extent of Pyrochlore-Type Cation Odering in Ce2Zr2O8: A t’-κ Pathway to Low-Temperature Reduction,. Chem. Mater. 2005, 17, 1157−1166. (17) Rodriguez-Carvajal, J. FULLPROF-A program for Rietveld Refinement; Laboratoire Leon Brillouin: CEA-Saclay: France, 2000. (18) Ravel, B.; Newville, M. ATHENA, ARTEMIS, HEPHAUESTUS: data analysis for X-ray absorption spectroscopy using IFEFIT. J. Synchrotron Radiat. 2005, 12, 537−541. (19) Kanzler, C.; Urban, S.; Zalewska-Wierzbicka, K.; Hess, F.; Rohrlack, S. F.; Wessel, C.; Ostermann, R.; Hofmann, J. P.; Smarsly, B. M.; Over, H. Electrospun Metal Oxide Nanofibers for the Assessment of Catalyst Morphological Stability under Harsh Reaction Conditions. ChemCatChem 2013, 5, 2621−2626. (20) Sasaki, T.; Ukyo, Y.; Suda, Y.; Sugiura, A.; Kuroda, K.; Arai, S.; Saka, H. Oxygen Absorption Behavior of Ce2Zr2O7+x and Formation of Ce2Zr2O7.5. J. Ceram. Soc. Jpn. 2003, 111, 382−385. (21) Ishiguro, N.; Uruga, T.; Sekizawa, O.; Tsuji, T.; Suzuki, M.; Kawamura, N.; Mizumaki, M.; Nitta, K.; Yokoyama, T.; Tada, M. Visualization of the Heterogeneity of Cerium Oxidation States in Single t/Ce2Zr2Ox Catalyst Particles by Nano-XAFS. ChemPhysChem 2014, 15, 1563−1568. (22) Matsui, H.; Ishiguro, N.; Enomoto, K.; Sekizawa, O.; Uruga, T.; Tada, M. Imaging of Oxygen Diffusion in Individual Platinum/ Ce2Zr2Ox Catalyst Particles during Oxygen Storage and Release. Angew. Chem., Int. Ed. 2016, 55, 12022−12025. (23) Wang, H.-F.; Guo, Y.-L.; Lu, G.-Z.; Hu, P. Maximizing the Localized Relaxation: The Origin of the Outstanding Oxygen Storage Capacity of κ-Ce2Zr2O8. Angew. Chem., Int. Ed. 2009, 48, 8289−8292. (24) Urban, S.; Dolcet, P.; Möller, M.; Chen, L.; Klar, P. J.; Djerdj, I.; Gross, S.; Smarsly, B. M.; Over, H. Synthesis and full Characterization

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b03091. TGA measurements under reducing, oxidizing, and Ar atmosphere; full XRD scans of the T-induced phase transformation; Rietveld refinement of the XRD data of pyr-Ce2Zr2O7.3 phase; temperature-dependent 2θ scans at high diffraction angles; microstrain analysis and temperature factors; in situ Raman spectra in the phonon region taken at various temperatures; series of XRD scans and Raman spectra in an N2 atmosphere; Raman spectra at room temperature in the range of 2100 cm−1 for various annealing temperatures; and Krypton physisorption data of the κ-Ce2Zr2O8 phase (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Igor Djerdj: 0000-0002-0646-4928 Silvia Gross: 0000-0003-1860-8711 Bernd Smarsly: 0000-0001-8452-2663 Herbert Over: 0000-0001-7689-7385 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge financial support by the LOEWE program STORE-E and acknowledge the Center for Materials Research at the JLU (LaMa) for providing experimental platforms; we also express special thanks to BMBF for funding the related research project entitled: “Neuartige Materialien mit Funktion: Eigenschaften, Struktur und Anwendungen”. We acknowledge XAFS beamline at Elettra Sincrotrone Trieste (Italy) for granting access to the experimental facility and Dr. Luca Olivi and Dr. Clara Guglieri Rodriguez for technical support.

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REFERENCES

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In Situ Study of the Oxygen-Induced Transformation of Pyrochlore

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