Low-Temperature Oxygen Storage of CrIV–CrV Mixed-Valence YCr1

Article pubs.acs.org/JACS

Low-Temperature Oxygen Storage of CrIV−CrV Mixed-Valence YCr1−xPxO4−δ Driven by Local Condensation around Oxygen-Deficient Orthochromite Yoshitaka Aoki,*,†,¶ Kosuke Kuroda,‡ Satoshi Hinokuma,§,¶ Chiharu Kura,‡ Chunyu Zhu,‡ Etsushi Tsuji,⊥ Aiko Nakao,# Makoto Wakeshima,∥ Yukio Hinatsu,∥ and Hiroki Habazaki† †

Faculty of Engineering, ‡Graduate School of Chemical Sciences and Engineering, and ∥Faculty of Science, Hokkaido University, N13W8 Kita-ku, Sapporo, 060-8628, Japan § Faculty of Advanced Science and Technology, Kumamoto University, 2-39-1 Kurokami, Chuo-ku, Kumamoto, 860-8555, Japan ⊥ Department of Chemistry and Biochemistry, Graduate School of Engineering, Tottori University, 4-101 Koyama-cho Minami, Tottori, 680-8522, Japan # Institute of Physical and Chemical Research (RIKEN), 2-1 Hirosawa, Wako, Saitama, 351-0198, Japan ¶ JST-PRESTO, 4-1-8 Honcho, Kawaguchi, 332-0012, Japan S Supporting Information *

ABSTRACT: The oxygen storage capability and related defect structure of tetrahedral orthochromite(V) compound YCr1−xPxO4 (x = 0, 0.3, 0.5, and 0.7) were investigated by employing thermal gravimetry and in situ X-ray spectroscopy for reversible oxygen store/release driven by heating−cooling cycles in the temperature range from 50 to 600 °C. YCr1−xPxO4 started releasing oxygen as heated from 50 °C under ambient atmosphere, with reduction of CrV to CrIV, while the reduced YCr1−xPxO4−δ phase was significantly reoxidized via absorbing oxygen by cooling to 50 °C under ambient atmosphere, recovering the original stoichiometric phase. Operando X-ray adsorption spectroscopy and firstprinciples calculations demonstrate that nonstoichiometric YCr1−xPxO4−δ phases were stabilized by forming linking polyhedral CrIV2O76− via corner sharing between oxygen-deficient CrIVO32− and adjacent CrIVO44−. YCr1−xPxO4 was found to have an extremely low reduction enthalpy of about 20 kJ mol−1 probably due to the relatively high reduction potential of high-valence-state Cr(V)/Cr(IV) redox pairs, thereby resulting in reversible oxygen storage in such a low-temperature region.

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INTRODUCTION Recent global concerns regarding energy and environmental issues have drawn a lot of academic and industrial interest to the development of functional materials that enable precise control of the redox reactions for energy conservation and environmental protection. Oxygen storage materials (OSMs), which are capable of performing reversible oxygen intake/ release as a response to temperature or atmosphere, are one of the targets. In the case of thermochemical storage reactions, metal oxides are reduced at higher temperature while releasing pure oxygen, whereas the reduced phase is reoxidized by capturing oxygen at lower temperature, thereby recovering the original oxide phase. CeO2−ZrO2 solid solutions are the most well known oxygen storage materials and have been used for NOx and CO removal catalysts in automobiles.1−3 Meanwhile, there still exists a strong demand for more efficient OSMs, because they also play a crucial role in a wide variety of energyrelated applications, such as commercial air separation,4−6 solar water splitting,7−9 solar CO2 splitting,10−12 and chemical looping.13,14 Hence it is strongly motivated to develop new © 2017 American Chemical Society

artificial OSMs with tailored redox properties, i.e., large oxygen storage capacity and preferable reaction temperatures,15,16 by using various Mn+/Mm+ redox couples for related applications. Redox properties of high-valence-state metal oxides comprising CrV, FeVI, CoIV, MnV, etc., have been the focus of many investigations because of their important roles in super ion batteries,17 biological reactions,18 and electrocatalysis.19−21 They would be also attractive as a redox-active oxide for OSMs, since these are readily reduced by heating due primary to their high reduction potentials and finally decomposed to lower valence state oxide phases while releasing a large amount of oxygen. Unfortunately, such oxides are not recovered by a simple thermal treatment under ambient atmosphere once decomposed, which causes difficulty for direct use in OSMs. Zircon-type rare earth orthochromites(V), RECrO4 (RE: rare earth metal(III)), are high-valence-state CrV oxides and have been synthesized by calcination of Cr(III)−RE(III) mixed Received: May 26, 2017 Published: July 23, 2017 11197

DOI: 10.1021/jacs.7b05429 J. Am. Chem. Soc. 2017, 139, 11197−11206

Article

Journal of the American Chemical Society precursors in an oxidative atmosphere.22−27 RECrVO4 have been reported to reductively decompose at temperatures above 600 °C in air, converting to thermally stable perovskite-type RECrIIIO3, and thus the reoxidation to RECrO4 does not occur under ambient atmosphere.23,25,26 Here, we demonstrated that solid solutions of the metastable yttrium orthochromite(V) (YCrO4) and its isomorphous yttrium orthophosphate (YPO4) cause reversible uptake/release of oxygen in the extremely low temperature region due to the thermochemical redox between CrV and CrIV states, aided by defect-induced local condensation of orthochromite(IV) tetrahedral units. The mixed metal oxides possessed a relatively low reduction enthalpy of ∼20 kJ mol−1, thereby enabling reversible oxygen release/intake triggered by heating and cooling in the temperature range from 25 to 600 °C under ambient atmosphere.

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with neutrally charged defects. The cutoff energy for the plane-wave basis was set to 600 eV. A Γ-centered 3 × 3 × 3 Monkhorst−Pack special k-point grid31 for the first Brillouin zone using a Gaussian smearing model of σ = 0.05 eV was used. X-ray crystallographic data of YCrO422,32 are used as an initial set of lattice constants and atomic coordinates. Atomic positions were optimized until the forces on each atom were smaller than 0.02 eV/Å.

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RESULTS Thermochemical Oxygen Sorption and Desorption. We first conducted a broad survey of the optimal conditions for annealing the co-precipitates in order to prepare a single phase of zircon-type yttrium orthochromite(V) and its phosphate substitutes, i.e., YCr1−xPxO4 (x = 0, 0.3, 0.5, and 0.7), by means of XRD measurements of the samples annealed in air with adjusting the temperatures and annealing periods. A single phase of zircon-type oxides was obtained by annealing for 1 h at 600 °C for x = 0 and at 700 °C for x = 0.3, 0.5, and 0.7, while a secondary phase of perovskite-type YCrIIIO3 was formed due to the partial decomposition of CrV oxide moieties by annealing at T > 620 °C for x = 0 and at T > 750 °C for others. Consequently, the optimal annealing conditions for YCrO4 and YCr1−xPxO4 (x = 0.3, 0.5, 0.7) were determined to be at 600 °C for 3 h in air and at 700 °C for 3 h in air, respectively (Supplementary Figure 1). The chemical composition, that is, Y/Cr/P molar ratios of the samples thus prepared, were determined by ICP analysis, which are in agreement with the mixing ratios of the precursor solutions, confirming that singlephase zircon-type oxides are obtained for all x. YCr1−xPxO4 powders are made of submicrometer-sized particles, as confirmed by scanning electron microscopy (SEM), and so they have relatively large BET surface areas, SBET, equaling 18, 36, 46, and 49 m2 g−1, for x = 0, 0.3, 0.5, and 0.7, respectively. The XRD peaks of the phosphate substitutes, i.e., x = 0.3, 0.5, and 0.7, are clearly broadened compared to the end members (YCrO4 and YPO4), with the broadening being more evident at higher x, indicating that local atomic displacements would be involved in the YCrO4−YPO4 solid solutions probably due to the size mismatch between Cr and P cations. The thermal gravimetry clarifies that YCr0.5P0.5O4 is capable of conducting reversible intake/release of oxygen, triggered by heating and cooling in the temperature range below 400 °C. Figure 1 shows a TG curve of YCr0.5P0.5O4 in dry air when specimens are first kept at 700 °C for 1 h (step a), cooled to 50 °C at 20 °C min−1 (step b), kept at 50 °C for 1 h (step c), and finally heated again to 700 °C at 20 °C min−1 (step d). The masses successively increase with decreasing temperature in the temperature range of T ≤ 400 °C (step b) and are immediately stabilized by keeping at 50 °C (step c), which yields a total mass gain of 1.1 wt % by oxygen sorption. The masses, however, start decreasing as the temperature is raised from 50 °C (step d) and are stabilized in a region of T ≥ 400 °C, leading to a total loss of 1.1 wt % by the oxygen desorption throughout steps c and d, which is equivalent to a total mass gain by the sorption in the quenching process. Hightemperature XRD clarified that YCr0.5P0.5O4 preserved the zircon-type structure at 700 °C regardless of its large oxygen deficiency (Supplementary Figure 2). In the case of YCr0.5P0.5O4, the amounts absorbed clearly increase with decreasing the temperature of quenching so that YCr0.5P0.5O4 gains 0.25, 0.6, and 1.1 wt % oxygen by quenching at 200, 100, and 50 °C, respectively (Figure 2a), whereas the

EXPERIMENTAL SECTION

YCrO4 and YCrO4−YPO4 solid solutions (YCr1−xPxO4; x = 0.3, 0.5, and 0.7) were prepared by the co-precipitation technique, which was a modification of the method reported by Amezawa et al. to prepare YPO4.28 In brief, Y(NO3)3·H2O (99.9% purity, Kanto) and Cr(NO3)3· 6H2O (99.9% purity, Kanto) were dissolved into Milli-Q water at a stoichiometric Y/Cr molar ratio in order to prepare a 0.4 M Y/Cr = 1/ 1 − x mixed solution. Separately, (NH3)2HPO4·H2O (99.9% purity, Kanto) was dissolved into a 0.1 M NH3 solution by adjusting the P concentration. Both solutions were mixed with vigorous stirring at room temperature, and subsequently white precipitates were formed. After filtration and rinsing, the obtained precipitates were heated at 150 °C under vacuum and finally calcined under appropriate conditions. Phase purities were determined by X-ray diffraction (XRD; Rigaku RINT2200 diffractometer with Cu Kα radiation), and chemical compositions were examined by inductively coupled-plasma optical emission analysis (ICP: PerkinElmer ICP-OES222) with samples dissolved into 0.1 M H2SO4 solutions. Thermal gravimetry (TG) was performed using a Rigaku Evo-2 under flowing dry gases prepared by mixing pure Ar and O2. X-ray photoelectron spectroscopy (XPS) was conducted with a Thermo Fischer Scientific EscaLab 250 equipped with a specially designed high-temperature probe. High-temperature XRD was measured by using a Rigaku Ultima-II diffractometer equipped with an electrical furnace. The average oxidation state of Cr in pristine YCr1−xPxO4 specimens was determined by redox titration using a standard Fe(II) solution. The pristine powders were dissolved in 0.1 M H2SO4 solution with a disproportionation by 3Cr5+ → 2Cr6+ + Cr3+, and thus Cr6+ in the solution was redox-titrated by an Fe(II) solution using an XO orange indicator (Aldrich).26 BET surface areas were determined by measuring nitrogen adsorption isotherms at 77 K (Microtrackbel BELSORP Mini-II). Operando XAFS for the Cr K-edge was obtained on the BL01B1 station of SPring-8, Japan Synchrotron Radiation Research Institute (JASRI). The sample placed in the temperature-controllable cell was heated and/or cooled between room temperature and 700 °C at a heating rate of 10 °C min−1 in air (flow rate: 50 cm3 min−1). A Si(111) double-crystal monochromator was used. The incident and transmitted X-rays were monitored in ionization chambers filled with N2 and 85% N2 + 15% Ar. Quick EXAFS in the continuous scanning mode was recorded from 5674 to 7506 eV (5 min scan−1). YP0.5Cr0.5O4, YP0.7Cr0.3O4, and reference samples (Cr2O3, CrO2, and YCrO4) were mixed with boron nitride (BN) powder to achieve an appropriate absorbance at the edge energy. The XAFS data were processed using the IFEFFIT software package (Athena and Artemis). The electronic state of YP0.5Cr0.5O4−δ was characterized by the spinpolarized density-functional-theory (DFT) calculations with the generalized gradient approximation (GGA) using Perdew−Burke− Ernzerhof (PBE) exchange−correlation functions,29 which are implemented in the plane-wave and projector-augmented wave method, the Vienna ab Initio Simulation Package (VASP).30 The supercell consists of 2 × 2 × 2 YP0.5Cr0.5O4−δ unit cells (∼192 atoms) 11198

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°C (Figure 2b). These features clarify that the oxygen deficiency increases with temperature at T ≤ 400 °C, while it remained constant at T > 400 °C in air. Figure 3 shows the mass changes of YCr1−xPxO4 (x = 0, 0.3, 0.5, and 0.7) when the temperature is repeatedly swung between 50 and 600 °C in dry air. All YCr1−xPxO4 members provoke reversible release and intake of oxygen, driven by heating and cooling cycles, causing large mass gains/losses at T ≤ 400 °C while small ones at T > 400 °C. The absorption and desorption cycles are completely repeatable, since the deviations of the masses equilibrated at 50 or 600 °C for every cycle are within 10%. Weight variable (ΔW) is defined here as the mean value of the gaps between the weights equilibrated at 50 and 600 °C and the values of x = 0, 0.3, 0.5, and 0.7 equal to 0.3, 0.9, 1.1, and 1.3%, respectively (Table 1), disclosing that YCr1−xPxO4 can accept a higher oxygen deficiency with increasing x. Valence Change of Chromium. High-temperature Cr 2p3/2 XPS spectra were measured in order to validate the valence changes of Cr (Figure 4) by temperature. YCrO4 shows a single 2p3/2 peak at 578.5 eV at 25 °C, which is in agreement with the binding energies of the Cr(V) oxides.25,26 YCr1−xPxO4 shows an intensive CrV peak, together with a small shoulder peak appearing at around 576.3 eV at 25 °C, which remarkably grows up by heating at 400 °C and thus would be assigned to either Cr(III) oxide (576.6 eV)33,34 or Cr(IV) oxide (576.4 eV)35 moieties formed by the reduction of Cr(V) oxides via oxygen release. It has been well known that CrIII cations in oxides exhibit a strong tendency to adopt an octahedral coordination because of their 3d3 4A2 ground state with all t2g spin-up states filled.36,37 As mentioned below, extended X-ray adsorption fine structure (EXAFS) measurements clarified that oxygen-deficient YCr1−xPxO4−δ retained a tetrahedral coordination environment around Cr atoms, indicating that CrIV valence states are favored in oxygen-deficient YCr1−xPxO4−δ, and thus reversible oxygen release/intake can be attributed to the thermochemical CrV/CrIV redox reactions. The CrIV/CrV molar ratios determined by peak fitting to XPS are listed in Table 1. In Kröger−Vink notation, incorporation of an oxygen vacancy into YCr1−xPxO4 is expressed by

Figure 1. Thermal gravimetric change of YCr0.5P0.5O4 versus temperature cycle in dry air. The temperature program is as follows: step a: keeping at 700 °C for 1 h, step b: cooling to 50 °C at 20 °C min−1, step c: keeping at 50 °C for 1 h, and step d: heating to 700 °C at 20 °C min−1.

2(CrO4 )Cr ′ O4× → (Cr′O3)CrO4 ′ + (Cr′O4 )CrO4 ′ +

1 O2 (g) 2 (1)

Here, the subscript formula indicates an occupation site and superscripts ′ and • denote negative and positive charges, respectively. According to reaction 1, one oxygen vacancy gives rise to two CrIV cations in YCr1−xPxO4, leaving a defect pair of tetrahedral (Cr′O4)CrO4• and O-deficient (Cr′O3)CrO4•, both of which correspond to CrIVO44− and Cr IVO32− oxoanions, respectively. In past decades, several authors reported on the defect structures of the tetrahedral orthometallate anion compounds, such as acceptor-doped orthoniobate,38,39 orthophosphate,40,41 and orthogallate.42,43 In these, incorporation of an oxygen vacancy results in the formation of polyhedral (M2O7)2MO4•• aided by corner oxygen sharing between Odeficient MO3 and adjacent MO4 units. On the basis of this framework, (Cr′O3)CrO4• defects are expected to associate with neighbor (Cr′O4)CrO4• defects via corner sharing as follows:

Figure 2. (a) TG curves for the oxygen sorption of YCr0.5P0.5O4, measured in dry air, by swinging the temperature as (i) 600 → 50 → 600 °C, (ii) 600 → 100 → 600 °C, and (iii) 600 → 200 → 600 °C with holding for 1 h at each temperature step. (b) TG curves for the oxygen desorption of YCr0.5P0.5O4, measured in dry air, by swinging the temperature as (i) 50 → 700 → 50 °C, (ii) 50 → 600 → 50 °C, (iii) 50 → 500 → 50 °C, and (iv) 50 → 400 → 50 °C with holding for a few hours at each temperature step. Both drop and ramp rates are 10 °C min−1. Solid lines show TG, and dotted lines the temperature programs.

amount desorbed is not largely changed at T ≥ 400 °C, since YCr0.5P0.5O4 loses 1.0 wt % at 400 °C and 1.1 wt % at T ≥ 500

2(CrO4 )CrO4× → (Cr′2 O7 )2CrO4× + 11199

1 O2 (g) 2

(2)

DOI: 10.1021/jacs.7b05429 J. Am. Chem. Soc. 2017, 139, 11197−11206

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Figure 3. TG curves of YCr1−xPxO4 during temperature cycles between 50 and 600 °C in dry air. x = 0 (a), 0.3 (b), 0.5 (c), and 0.7 (d). One temperature cycle consists of four steps: temperature rise from 50 to 600 °C at a rate of 20 °C min−1, hold at 600 °C for 1 h, temperature fall from 600 to 50 °C at a rate of 20 °C min−1, and hold at 50 °C for 1 h. Here the cycles are repeated 5 times. Red solid lines show TG curves, and black dotted lines temperature programs. The large weight loss at the step of the first temperature rise is attributed to water desorption.

Table 1. Weight Variable (ΔW), Oxygen Deficiency (δ) at 600 °C, and Cr-Oxidation State of YCr1−xPxO4 for the Thermochemical Oxygen Intake/Release by Temperature Cycles between 50 and 600 °Ca X ΔW/% δ CrV/CrIV CrV/CrIV CrV/CrIV CrV/CrIV

at at at at

0

25 °Cb 400 °Cb 25 °Cc 600 °Cd

0.3 0.046 100/0 50/50 100/0 90/10

0.3 0.9 0.11 84/16 66/34 100/0 69/31

(+5.0) (+4.5) (+5.0) (+4.9)

(+4.8) (+4.7) (+5.0) (+4.7)

0.5 1.1 0.13 91/9 (+4.9) 57/43 (+4.6) 97/3 (+5.0) 44/56 (+4.4)

0.7 1.3 0.15 76/24 (+4.8) 40/60 (+4.4) 95/5 (+5.0) 0/100 (+4.2)

a c

Numbers in parentheses after CrV/CrIV molar ratios show the formal oxidation state of chromium. bDetermined by high-temperature XPS. Determined by chemical titration. dCalculated from ΔW of TG.

The defect reaction 2 is rewritten by the following CrIV/CrV redox reactions.

respectively (Table 1), which reveals that redox efficiencies of Cr cations in YCr1−xPxO4 increase with x and 56% and 100% Cr atoms are redox active in YCr0.5P0.5O4 and YCr0.3P0.7O4, respectively (Table 1). Defect Structures. In order to elucidate the local rearrangements around Cr atoms by thermochemical reactions, operando Cr K-edge XAFS was measured for YCr1−xPxO4 (x = 0.5 and 0.7) in temperature cycles of rt → 700 °C → rt. Figure 5a shows the normalized X-ray adsorption near-edge structure (XANES) spectra of YCr 0.5 P0.5O 4 together with three references (Cr2O3, CrO2, and YCrO4). Pristine YCr0.5P0.5O4 reveals similar features to those of YCrO4 (Cr5+) in the XANES region, giving rise to the large pre-edge (approximately 5981 eV) peak due to the quadrupolar 1s → 3d transitions arising from the hybridization of the 3d levels of Cr in the presence of surrounding oxygen ligands.44,45 This pre-edge feature is characteristic of tetrahedrally coordinated Cr atoms,45,46 and

Y(Cr VO4 )1 − x (PO4 )x T

→ Y(Cr VO4 )1 − x − 2δ (Cr 2IVO7 )δ (PO4 )x +

δ O2 2

(3)

The average oxidation states of Cr in pristine YCr1−xPxO4 are determined to be +4.7 to +5.1 by redox titration using a standard Fe(II) solution, indicating that the CrV cationic state is predominant in all pristine samples at room temperature (Table 1). On this basis, the relatively large intensity of the CrIV XPS peak in phosphate substituents indicates that the surfaces of YCr1−xPxO4 are easily reduced under vacuum even at 25 °C. Assuming that all Cr cations of YCr1−xPxO4 are solely 5+ at 50 °C in air, the oxygen deficiency (δ) at 600 °C is calculated from ΔW to be 0.051, 0.11, 0.13, and 0.15 for x = 0, 0.3, 0.5, and 0.7, 11200

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Figure 4. Cr 2P3/2 XPS spectra of YCr1−xPxO4−δ, measured at 25 and 400 °C. x = 0 (a), 0.3 (b), 0.5 (c), and 0.7 (d). Black dots denote the observed, and red solid lines the curve fitting.

so the octahedrally coordinated Cr2O3 (corundum type) and CrO2 (rutile type) do not show such a strong pre-edge peak. Heating at 700 °C lowers the intensity of the pre-edge peak slightly, disclosing that the local coordination environments around Cr atoms are modified by oxygen deficiency at elevated temperatures. Moreover, the oxidation state of Cr is found to be in a range between +4 and +5 at 700 °C because the adsorption edge shifts by −1 eV as it approaches that of CrIVO2, which is in agreement with the average oxidation state (+4.4) determined by TG measurements (Table 1). By cooling to 25 °C, pre-edge intensity and absorption edge position are recovered to those before heating, indicating that the tetrahedral coordination environment around the Cr atoms is restored via oxygen absorption. The combined results of TG, XPS, and XAFS unequivocally demonstrate that YCr1−xPxO4 releases oxygen due to the thermal reduction of CrV to CrIV, while nonstoichiometric YCr1−xPxO4−δ is readily reoxidized to the original YCr1−xPxO4 with recovering CrV states by cooling to room temperature in air. Fourier transform EXAFS and the corresponding curvefitting analysis determine more precisely the change of the local structure environment around Cr by reduction at elevated temperatures (Table 2 and Figure 5b and c). Since these data

Figure 5. (a) Operando Cr K-edge XANES spectra of YCr1−xPxO4 (x = 0.5 and 0.3) measured in a temperature cycle between 25 and 700 °C in dry air. In the temperature program, the pristine samples are heated to 700 °C at 5 °C min−1 with holding at this temperature for 3 h and then cooling to 25 °C at 5 °C min−1. The measurements at each temperature were conducted 1 h after temperature equilibration. The spectra measured at 25 °C before and after heating are denoted as “25 °C” and “700 °C → 25 °C”, respectively. The spectra of YCrVO4, CrIII2O3, and CrIVO2 references are also shown. Fourier transform EXAFS of (b) YCr0.5P0.5O4 and (c) YCr0.3P0.7O4. The first and second coordination shells are attributed to the first neighbor oxygen and second neighbor yttrium. In (b) and (c), the arrows indicate new coordinates formed by the corner oxygen sharing between oxygendeficient CrO3 and the CrO4 neighbor. The details are described in the text. 11201

DOI: 10.1021/jacs.7b05429 J. Am. Chem. Soc. 2017, 139, 11197−11206

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Journal of the American Chemical Society Table 2. Fitting Parameters Obtained from Cr K-Edge EXAFS Analysis for YCr1−xPxO4 (x = 0.5 and 0.7)a sample YCr0.5P0.5O4 YCr0.5P0.5O4 YCr0.5P0.5O4 YCr0.3P0.7O4 YCr0.3P0.7O4 YCr0.3P0.7O4 Cr2O3 CrO2 YCrO4 a

(RT) (700 °C) (700 °C → RT) (RT) (700 °C) (700 °C → RT)

shell Cr−O Cr−O Cr−O Cr−O Cr−O Cr−O Cr−O Cr−O Cr−O

r/Åb 1.69 1.69 1.69 1.69 1.68 1.69 1.96 1.89 1.69

± ± ± ± ± ± ± ± ±

0.02 0.02 0.02 0.02 0.03 0.02 0.02 0.06 0.02

CNc 3.9 3.1 3.8 4.0 2.8 3.8 6.0 6.0 4.0

± ± ± ± ± ±

0.2 0.2 0.2 0.4 0.4 0.4

σ2/10−2 Å2d

R/%

± ± ± ± ± ± ± ± ±

1.0 0.74 1.5 0.7 1.8 1.1 2.5 4.5 4.7

0.20 0.29 0.10 0.28 0.25 0.23 0.75 1.08 0.28

0.20 0.27 0.25 0.23 0.37 0.27 0.15 0.40 0.10

Interval of k-space to r-space of FT is 2.0−15.0 Å−1. bAtomic distance. cCoordination number. dDebye−Waller factor.

Figure 6. Schematic representation of local geometry around (a) a CrO4 unit in the stoichiometric phase, (b) (Cr′O3)CrO4• defects created by an oxygen vacancy, and (c) a (Cr′2O7)2CrO4× defect complex formed via association of (Cr′O3)CrO4• and (Cr′O4)CrO4′. The dashed lines in (a) show the closest O−O contacts between neighboring CrO4 tetrahedra. The dotted gray rectangle shows a unit cell in (a)−(c).

are shown without phase-shift corrections, the observed peaks could give shorter values with respect to the true atomic distances. Nevertheless, for three references and YCr0.5P0.5O4, the first coordination shells corresponding to an intense peak at around 1.6−2.0 Å can be attributable to the shortest Cr−O bonds. The curve-fitting analysis of the first coordination shell of Cr 2 O 3 and CrO 2 yields a Cr−O distance (r) of approximately 1.96 and 1.89 Å, respectively, together with both coordination number (CN) around Cr of 6. These are in agreement with the crystallographic data of corundum Cr2O347 and rutile CrO2.48 The profiles of YCrO4 are fitted very well with the CN of 4 and r of 1.69 Å, which are identical to the Xray crystallographic data for zircon-type YCrVO4 (CN 4, Cr−O 1.74 Å).22,32 On the basis of the crystallographic data of YCrO4,22,24,32 the second neighbor shell at a radial distribution around 2.7 Å can be assigned to the coherence between Cr and the second neighbor yttrium (3.1 Å). The curve-fitting analysis for YCr0.5P0.5O4 at room temperature yields the approximate CN of 4 and r of about 1.69 Å, indicating that the local coordination environment of Cr atoms is similar as that of YCrO4 (Table 1 and Supplementary Figure 3), whereas the best fitting for the first Cr−O shell at 700 °C is achieved by a smaller CN (3.2) with the same r (1.69 Å) in comparison with both of the room-temperature phases (Table 1 and Supplementary Figure 3). Moreover, a new coordination peak is evident at a distance around 2.1 to 2.5 Å at 700 °C but disappears on return to 25 °C (Figure 5b), revealing that the incorporation of oxygen vacancies induces the displacement of atoms surrounding the Cr cation, creating additional

coordination toward Cr atoms. Similar local structure changes are also observed in YCr0.3P0.7O4 during the temperature cycle (Figure 5c). In the zircon-type structure, every CrO4 tetrahedron has four CrO4 neighbors, and they are adjacent to each other on approaching the corner oxygens toward the neighbors, which gives rise to the closest O−O contact distance of 2.6 Å among neighboring CrO4 tetrahedra (Figure 6a). This structural feature implies that the O-deficient CrO3 units associate easily with the neighbor CrO4 by corner-sharing along the diagonal Cr(vac)−Cr(a) line, with attracting a corner oxygen of the CrO4 neighbor (Figures 6b,c). This structural deformation is in agreement with the local structure changes observed by EXAFS, because the association between CrO3 and CrO4 creates a new oxygen coordinate with a relatively long Cr−O bond distance. To validate the aforementioned defect models, the structure relaxation and defect formation energy (ΔEdef) were calculated for the oxygen-deficient YCr0.5P0.5O4‑δ by DFT with 2 × 2 × 2 unit cells. Here, the [Y32Cr16P16O128] supercell is used for a stoichiometric YCr0.5P0.5O4 phase, and the [Y32Cr16P16O120] supercell is considered for the oxygen-deficient YCr0.5P0.5O3.9375 phase, which is prepared by removing one corner oxygen of a CrO4 tetrahedron for every YCr0.5P0.5O4 unit cell. Figure 7 shows relaxed atomic configurations for YCr0.5P0.5O4 and YCr0.5P0.5O3.9375 unit cells, indicating that reconstruction of associated polyhedra around the vacancy occurs. Cr that loses an oxygen coordinate is labeled by Cr(vac), Cr(a) is the neighbor of Cr(vac), and O(s) is an oxygen that is shared in the (Cr′2O7)2CrO4× defect complex. 11202

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DISCUSSION The theoretical and experimental results mentioned above unambiguously demonstrate that the reversible oxygen release/ intake of YCr1−xPxO4 is due to the thermal reduction and reoxidation of chromium between CrV and CrIV states aided by association between oxygen-deficient (Cr′O3)CrO4• and the adjacent (Cr′O4)CrO4′ defects into (Cr′2O7)2CrO4× defect complexes and vice versa. These reactions proceed in the relatively low temperature region, indicating that reduction enthalpy of YCr1−xPxO4 is significantly small, which is supported by the DFT defect formation energy of an oxygen vacancy (∼14 kJ mol−1). When reduction 3 spontaneously occurs at a temperature, the reaction free energy, ΔG0(T), must be negative as follows. ΔG 0(T ) = ΔH 0 − T ΔS 0 ≤ 0

(5)

Here, ΔH and ΔS are the reduction enthalpy and entropy at a standard pressure p0 = 101 kPa, respectively. Since the equilibrium of reaction 3 is dependent on oxygen partial pressure (pO2), the ΔG(T, pO2) at pO2 can be represented with ΔG0(T) as follows: 0

0

ΔG(T , pO2 ) = ΔG 0(T ) + 1/2 × RT ln(pO2 /p0 )

Figure 7. Relaxed geometry and isosurfaces of the partial charge derived from a normalized wave function near Fermi energy levels for (a) YCr0.5P0.5O4 and (b) YCr0.5P0.5O3.9375, calculated by DFT. (c) and (d) highlight the configuration near (200) planes in (a) and (b), respectively, which clarifies the rearrangement of O(s) atoms around the oxygen vacancy. Gray planes in (c) and (d) depict the (200) plane.

(6)

Under equilibrium, i.e., ΔG(T, pO2) = 0, eq 6 is rewritten by combining with eq 5: (pO2 /p0 )0.5 = exp(ΔS 0/R )exp( −ΔH 0/RT )

(7)

which provides the equilibrium oxygen partial pressure at a given temperature. Both ΔH0 and ΔS0 are a function of oxygen stoichiometry δ, so that eq 7 is rewritten as

O(s) is drawn toward Cr(vac) from the initial position and falls into the location near the center of the diagonal Cr(vac)−Cr(a) line, becoming closer to Cr(vac) (Figure 7c,d). The Cr(a)− O(s) and Cr(vac)−O(s) distances are 1.74 and 4.21 Å, respectively, in the initial position; however these change to 2.32 and 2.81 Å, respectively, after relaxation, which are in close agreement with those observed by EXAFS. Figure 7 also shows isosurfaces of the partial charge densities obtained from a normalized wave function attributed to the Cr 3d states near Fermi energy levels of stoichiometric and oxygen-deficient supercells. The wave functions are clearly localized at the Cr sites, and, moreover, the electron charges are equally distributed in Cr(vac) and Cr(a) atoms (Figure 7b,d), confirming that two CrV cations are equally reduced to CrIV with forming a (Cr′2O7)2CrO4× defect complex by an oxygen vacancy. The spectroscopic measurements and theoretical calculations clearly demonstrate that thermal reduction of YCr1−xPxO4 is progressive via corner-linking between oxygendeficient CrO3 and the adjacent CrO4, and thus oxygen uptake by YCr1−xPxO4 involves the dissociation of the linking polyhedra into two CrO4 tetrahedra. ΔEdef can be defined as a change in total energy according to the incorporation of a defect into a supercell.49 ΔEdef for an O vacancy in the YCr0.5P0.5O4‑δ phase can be expressed by

1 ⎛ pO2 ⎞ ΔH 0 ΔS 0 ln⎜ 0 ⎟ = − + |δ= const 2 ⎝p ⎠ RT R

(8)

Under a fixed pO2, eq 8 suggests that the reduction of metal oxides does not progress more by heating in the region above the temperature at which −ΔH0/RT = 1/2 ln(pO2/p0) − (ΔS0/ R), because lower pO2 is needed to establish equilibrium 8 at higher temperatures, and so the ΔS0 value would set an upper bound on the increment of δ at a fixed pO2. If defect reaction 2 is equilibrated at a temperature above 400 °C, ΔS0 is given by the mixing entropy related to the defect equilibrium between oxygen vacancies and oxygen gases50 and thus can be approximated by the entropy change in release of oxygen gas, i.e., the partial mole entropy of oxygen (0SO2) under constant δ.51 On the basis of this assumption, ΔH0 is roughly estimated to be about 21 k J mol−1, as calculated by using the 0 SO2 value (∼121 J K−1 mol−1) at 673 K.52 This ΔH0 value is consistent with the ΔEdef calculated by DFT (14 kJ mol−1) and is 1 order of magnitude smaller than the corresponding energies of well-known OSMs, such as CeO2 (500 kJ mol−1)53 or CaMnO3 (300 kJ mol−1).54 The aforementioned experimental and theoretical results clearly demonstrate that YCr1−xPxO4 possess a relatively small reduction enthalpy; thereby the reversible oxygen storage reactions occur in the extremely low temperature region. Surprisingly, all of the Cr cations in the YCr0.3P0.7O4 bulk are fully reoxidized by maintaining at 50 °C, disclosing that enhanced diffusion of oxide ions occurs in the bulk even at such low temperatures. On the other hand, isomorphous crystal structure compound ZrSiO4 has been reported to have a very small oxygen diffusion coefficient of about 10−20 m2 s−1 at 1200

ΔEdef = Etot[Y32Cr16P16O120 ] + 4μO2 − Etot[Y32Cr16P16O128] (4)

where Etot[supercell] represents the total energy of each supercell. The chemical potential of O2 denoted by μO2 is derived from Etot[O2] calculated for the isolated oxygen molecules in a vacuum. ΔEdef becomes a small positive value of 14 kJ mol−1, which can be attributed to the reduction enthalpy of YCr0.5P0.5O4, indicating that YCr1−xPxO4 has a relatively small reduction enthalpy. 11203

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Journal of the American Chemical Society °C.55 The diffusivity in oxide lattices must be strongly correlated with the concentration of the oxygen vacancy because the ambipolar diffusion normally proceeds via a vacancy-mediated hopping process of oxide ions. The oxygen vacancy concentration of ZrSiO4 is on the order of 1017 atoms cm−3,56 which is 4 orders of magnitude smaller than that of YCr1−xPxO4‑δ with δ ≈ 0.1 (calculated to be 1021 atoms cm−3). The high concentrations of oxygen vacancies may facilitate the oxide ion diffusion via a site-to-site hopping process in zircontype YCr1−xPxO4‑δ. Table 3 summarizes the recent publications for the oxygen storage materials. The oxygen storage capacity, i.e., δ, of

Several authors reported that Ce redox efficiencies of Ce1−xZrxO2 OSMs become maximum at x = 0.5, because the highly nonstoichiometric phase is more stable with increasing fractions of redox-inert ZrO2.57,58 Similarly, the improvement of Cr redox efficiencies of YCr1−xPxO4 with phosphate substitution would be correlated with the stabilization of the reduced phase because inert phosphate matrices could moderate the structure deformation induced by local rearrangement around oxygen-deficient CrO4 oxoanions. The relatively small reduction enthalpy of YCr0.3P0.7O4−δ must be concerned with the large redox potential of the CrV/CrIV system (2.2 V vs NHE),68 so that various high-valence-state orthometallates15,22−24,69−71 are also attractive for OSMs. For instance, orthoferate(VI) Na 2 Fe VI O 4 70 and orthocobaltate(IV) Ba2CoIVO471 have been reported to decompose to more stable Fe(III) or Co(II) oxides, respectively, at temperatures above 400 °C with releasing amounts of oxygen, and the redox potentials of the related redox pairs are very high for FeVI/FeIII (2.7 V vs NHE) and CoIV/CoIII (2.3 V vs NHE).68 Hence their solid solutions with isomorphous inert oxides are potential candidates for OSMs with low activation energy and large storage capacity. The current results reveal a new direction for the development of tailor-made oxygen storage materials that can facilitate oxygen-related thermochemical conversion processes in relatively low temperature regions.

Table 3. List of Recent Publications for Thermochemical Oxygen Storage Materials material

oxidation/reduction temperatures (°C)

δa

ref

CeO2−δ Ce0.5Zr0.5O2−δ SrFe0.95Cu0.05O3−δ SrCoO3−δ YBaCo4O7+δ Ca0.8Sr0.2MnO3−δ BaYMn2O5+δ Ca2AlMnO5+δ Dy0.7Y0.3MnO3+δ LuFe2O4+δ YCr0.3P0.7O4−δ

400/900 400/850 400/1000 600/900 300(O2)/300(N2)b 400/1200 500(O2)/500(5%-H2/Ar)b 500/700 300(O2)/300(/Ar)b 500(air)/500(5%-H2/Ar)b 50/600

0.1 0.4 0.3 0.1 1.2 0.28 0.45 0.29 0.25 0.5 0.15

59 59 51 9 60 54 61 62 63 64

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CONCLUSION In summary, tetrahedral orthochromite(V) compound YCr1−xPxO4 induces reversible oxygen release/intake due to the chromium redox between CrV and CrIV states, triggered by heating and cooling at temperatures below 400 °C under ambient atmosphere. In the reduction at elevated temperature, a pair of (Cr′O3)CrO4• and (Cr′O4)CrO4′ defects is inevitably formed by introduction of an oxygen vacancy into YCr1−xPxO4 for charge neutrality, which results in linking polyhedral (Cr′2O7)2CrO4× defect complexes via corner sharing between both. In the reoxidation at lower temperatures, a (Cr′2O7)2CrO4× defect complex dissociates into two (CrO4)CrO4× via coupling to oxide ions, and thus stoichiometric YCr1−xPxO4 is completely recovered. These oxygen storage reactions progress only in the temperature region of T ≤ 400 °C under ambient atmosphere, because the equilibrium oxygen partial pressure of the thermochemical reduction is smaller than 0.2p0 at T > 400 °C due to the relatively small enthalpy (∼20 kJ mol−1), and thus the reduction cannot proceed more in air by heating at T > 400 °C. The outstanding oxygen storage capability of YCr1−xPxO4 is realized by combining the high redox potential of the CrV/CrIV system and the stabilization of the oxygendeficient orthochromites embedded in inert phosphate matrices. The current results propose a new strategy to design low-temperature OSMs based on high-valence-state oxometallates for various energy-related applications.

a

Maximum oxygen deficiency or excess in the reaction temperature ranges. bGases in parentheses indicate the oxidation/reduction atmosphere.

YCr0.3P0.7O4 is smaller than most of the perovskite-related OSMs but is similar to that of the classical oxygen storage material CeO2.2,57 The outstanding feature of the YCr1−xPxO4 system is the extremely lowered reaction temperatures for reversible, thermochemical oxygen storage, in which the release and intake of oxygen readily progress at 600 and 50 °C, respectively, without adjusting pO2 or pH2 in the surrounding atmosphere (Figure 2). Oxygen sorption and desorption of Ce1−xZrxO2 OSMs are progressive at around 400 and 900 °C, respectively,57−59 and the reaction temperatures of perovskitetype SrFe0.95Cu0.05O351 and Ca0.8Sr0.2MnO354 are also much higher than those of YCr1−xPxO4. YBa2Co4O7 has been known to show very high storage capability in the relatively low temperature regions at around 300 °C;60,61 however, the oxidized phase is not spontaneously reduced in air, and thus the oxygen desertion needs to be conducted in an inert atmosphere. The perovskite-related BaYMn2 O 5+δ 62 and Dy0.7Y0.3MnO3+δ63 and layered CdI2-type LuFe2O4+δ64 also need oxidative/reductive atmospheric switching for the reversible oxygen storage reactions, although their storage capacities are high enough. Recently, several authors reported on the low-temperature oxygen storage ability of the nanostructured CeO2, which facilitates reversible oxygen storage/release at around 150 °C.65−67 This value is still higher than the oxidation temperature of YCr1−xPxO4−δ. Accordingly, it can be concluded that YCr1−xPxO4 possess outstanding oxygen storage capability driven by the thermochemical CrV/CrIV redox reactions in the extremely low temperature region without controlling the reaction atmosphere.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b05429. Supplemental figures for XRD, high-temperature XRD, and EXAFS fitting analysis (PDF) 11204

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Journal of the American Chemical Society

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AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Yoshitaka Aoki: 0000-0001-5614-1636 Satoshi Hinokuma: 0000-0002-1764-5089 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by PRESTO-JST “Creation of Innovative Core Technology for Manufacture and Use of Energy Carriers from Renewable Energy” (#JPMJPR1341). XAFS measurements were conducted on the BL01B1 station of SPring-8, Japan Synchrotron Radiation Research Institute (JASRI).

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