Low-Temperature Oxygen Storage of CrIVâCrV Mixed-Valence YCr1

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Low temperature oxygen storages of Cr -Cr mixed valence YCr PO 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 J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b05429 • Publication Date (Web): 23 Jul 2017 Downloaded from http://pubs.acs.org on July 24, 2017

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Journal of the American Chemical Society

Low temperature oxygen storages of CrIV-CrV mixed valence YCr1-xPxO4- driven by local condensation around oxygen-deficient orthochromite

Yoshitaka Aoki *,a,g, Kosuke Kuroda b, Satoshi Hinokuma

c,g

, Chiharu Kura b, Chunyu Zhu b,

Etsushi Tsuji e, Aiko Nakao f, Makoto Wakeshima d, Yukio Hinatsu d and Hiroki Habazaki a

a Faculty of Engineering, Hokkaido University, N13W8 Kita-ku, Sapporo, 060-8628 Japan. b Graduate School of Chemical Sciences and Engineering, Hokkaido University, N13W8 Kitaku, Sapporo, 060-8628 Japan. c Faculty of Advanced Science and Technology, Kumamoto University, 2-39-1 Kurokami, Chuo-ku, Kumamoto, 860-8555 Japan. d Faculty of Science, Hokkaido University, N13W8 Kita-ku, Sapporo, 060-8628 Japan. e Department of Chemistry and Biochemistry, Graduate School of Engineering, Tottori University, 4-101 Koyama-cho Minami, Tottori, 680-8522 Japan. f Institute of Physical and Chemical Research (RIKEN), 2-1 Hirosawa, Wako, Saitama, 351-0198 Japan. g JST-PRESTO,4-1-8 Honcho, Kawaguchi 3320012, Japan.

Corresponding: Yoshitaka Aoki Tel : +81-11-706-6752 E-mail : [email protected]

Key word: high valence state metal oxide, oxygen storage materials, defect chemistry

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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 heatingcooling cycles in the temperature range from 50C 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- phase was significantly reoxidized via absorbing oxygen by cooled down to 50C under ambient atmosphere, recovering to the original stoichiometric phase. Operando X-ray adsorption spectroscopy and first-principle calculations demonstrates that nonstoichiometric YCr1-xPxO4- phases were stabilized by forming linking polyhedral CrIV2O76via corner sharing between oxygen-deficient CrIVO32- and adjacent CrIVO44-. YCr1-xPxO4 was found to have the 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 the reversible oxygen storage undergoing in such a low temperature region.


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Introduction Recent global concerns regarding energy and environmental issues has drawn a lot of academic and industrial interests to development of the functional materials which enables a 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 by response to temperature or atmosphere, are one of the targets. In case of thermochemical storage reactions, metal oxides are reduced at higher temperature while releasing a pure oxygen, whereas the reduced phase is re-oxidized by capturing oxygen at lower temperature, thereby recovering to the original oxide phase. CeO2-ZrO2 solid solutions are most famous oxygen storage materials and has been used for the NOx and CO removal catalysts in automobiles.1-3 Meanwhile, there exists still a strong demand for more efficient OSMs, because they also make a crucial role in a wide variety of energy-related 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 artificial OSMs with tailored redox properties, i.e. large oxygen storage capacity and preferable reaction temperatures15,16 by using various Mn+/Mm+ redox couples for the related applications. Redox properties of high valence-state metal oxides comprising CrV, FeVI, CoIV, MnV etc. has been focus of many investigations because of its important roles in super ion battery,17 biological reactions18 and electro-catalysis19-21. 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 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 the Cr(III)-RE(III) mixed 3 ACS Paragon Plus Environment


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precursors in 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 occurs 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 ~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.

Experimental 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 the modify of the method reported by Amezawa et al to prepare YPO4.28 In briefly, 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 0.4 M Y/Cr = 1/1-x mixed solution. In separate, (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 vigorously stirring at room temperature, and subsequently the white precipitates were formed. After filtration and rinsing, the obtained precipitates were heated at 150C in vacuum and finally calcined at 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: Perkin-Elmer ICP-OES222) with 4 ACS Paragon Plus Environment

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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) were conducted with a Thermo Fischer Scientific EscaLab 250 equipped with a specially-designed high temperature probes. 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 were 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 Fe(II) solution with using a 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 Cr K-edge was obtained on the BL01B1 station of SPring-8, Japan Synchrotron Radiation Research Institute (JASRI). The sample placed in the temperaturecontrollable 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- was characterized by the spin-polarized densityfunctional 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 5 ACS Paragon Plus Environment


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Package (VASP),30 The supercell consists of 2  2  2 YP0.5Cr0.5O4- unit cells (~ 192 atoms) with neutrally charged defects. Cutoff energy for the plane-wave basis was set to 600 eV. A Γcentered 3×3×3 Monkhorst–Pack special k-point grid31 for the first Brillouin zone using Gaussian smearing model of σ=0.05 eV was used. X-ray crystallographic data of YCrO422,32 are used as an initial sets of lattice constants and atomic coordinates. Atomic positions were optimized until the forces on each atom were smaller than 0.02 eV/Å.

Results and Discussion Thermochemical oxygen sorption and desorption We first conducted a broad survey of the optimal conditions for annealing the coprecipitates in order to prepare a single phase of zircon type yttrium orthochromite(V) and its phosphatesubstitutes, i.e. YCr1-xPxO4 (x = 0, 0.3, 0.5 and 0.7) by means of X-ray diffraction (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 3h in air and at 700C for 3h in air, respectively (Supplementary Fig. 1). The chemical composition, that is, Y/Cr/P molar ratios of the samples thus prepared were determined by inductive-coupled-plasma optical emission analysis (ICP), which are in agreement with the mixing ratios of the precursor solutions, confirming that single phase zircon type oxides are obtained in all x. YCr1-xPxO4 powders are made of submicron-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 6 ACS Paragon Plus Environment

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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.

Figure 1 Thermal gravimetric change of YCr0.5P0.5O4 for temperature cycle in dry air. 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.

The thermal gravimetry (TG) clarifies that YCr0.5P0.5O4 is capable of conducting reversible intake/release of oxygen, triggered by heating and cooling in the temperature ranges below 400C. Figure 1 shows a TG curve of YCr0.5P0.5O4 in dry air when specimens are firstly 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 1.1wt% by oxygen sorption. The masses, however, start decreasing as temperature is raised from 50C (step d,) 7 ACS Paragon Plus Environment


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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 as a total mass gain by the sorption in the quenching process. High temperature XRD clarified that YCr0.5P0.5O4 preserved zircon type structure at 700C regardless of its large oxygen deficiency (supplementary Fig. 2).

Table 1 Weight variable (W), oxygen deficiency () at 600C and Cr‐oxidation state of YCr1‐ xPxO4 for the thermochemical oxygen intake/release by temperature cycles between 50 and

600C. Numbers in parenthesis after CrV/CrIV molar ratios show the formal oxidation state of chromium.



X

0

0.3

0.5

0.7

W / %

0.3

0.9

1.1

1.3



0.046

0.11

0.13

0.15

CrV/CrIV at 25C

100/0 (+5.0)

84/16 (+4.8)

91/9 (+4.9)

76/24 (+4.8)

CrV/CrIV at 400C

50/50 (+4.5)

66/34 (+4.7)

57/43 (+4.6)

40/60 (+4.4)

CrV/CrIV at 25C

100/0 (+5.0)

100/0 (+5.0)

97/3 (+5.0)

95/5 (+5.0)

CrV/CrIV at 600C

90/10 (+4.9)

69/31 (+4.7)

44/56 (+4.4)

0/100 (+4.2)

Determined by high temperature XPS. Determined by chemical titration. Calculated from

W of TG.

In 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 quenched at 200, 100 and 50C, respectively (Fig. 2a). Whereas, the amounts desorbed is not largely changed in T  400C, since YCr0.5P0.5O4 lose 1.0 wt% at 400C and 1.1 wt% in T  500C (Fig. 2b). These features clarify that the oxygen deficiency increases with temperature in T ≤


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400C while is remained constant in T > 400C in air.

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 1h 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 dot lines the temperature programs.

Figure 3 shows the mass changes of YCr1-xPxO4 (x = 0, 0.3, 0.5 and 0.7) when the temperature 9 ACS Paragon Plus Environment


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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, with causing large mass gains/losses in T ≤ 400C while small ones in T > 400C. The absorption and desorption cycles are completely repeatable, since the deviations of the masses equilibrated at 50C or 600C for every cycle are within 10%. Weight variable (W) is defined here as the mean value of the gaps between the weights equilibrated at 50C and 600C and the values of x = 0, 0.3, 0.5 and 0.7 equal 0.3, 0.9, 1.1 and 1.3 %, respectively (Table 1), disclosing that YCr1xPxO4

can accept higher oxygen deficiency with increasing x.

Figure 3 TG curves of YCr1‐xPxO4 during temperature cycles between 50C and 600C in dry air.


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x = 0 (a), 0.3 (b), 0.5 (c) and 0.7 (d). One temperature cycle is consisting of the 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, and here the cycles are repeated 5 times. Red solid lines show TG curves and black dot lines temperature programs. Large weight loss at the step of 1st temperature‐rise is attributed to the water desorption.

Thermochemical reduction of chromium High temperature Cr 2p3/2 XPS spectra were measured in order to validate the valence changes of Cr (Fig. 4) by temperature. YCrO4 shows a single 2p3/2 peak in 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 whether 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 strong tendency to adopt octahedral-coordination because of its 3d3 4A2 ground state with all t2g spinup states filled.36,37 As mentioned below, extended X-ray adsorption fine structure (EXAFS) measurements clarified that oxygen-deficient YCr1-xPxO4- retained tetrahedral coordination environment around Cr atoms, indicating that CrIV valence states are favored in oxygendeficient YCr1-xPxO4-, 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.



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Figure 4 Cr 2P3/2 XPS spectra of YCr1‐xPxO4‐, 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.

In Kröger-Vink notation, incorporation of oxygen vacancy into YCr1-xPxO4 is expressed by 2(CrO4)CrO4  (CrO3)CrO4 + (CrO4)CrO4 + 1/2 O2 (g)

(1)

Here, subscript formula indicates an occupation site and superscripts  and  denote a negative and positive charges, respectively. According to reaction (1), one oxygen vacancy gives rise to two CrIV cations in YCr1-xPxO4 with leaving a defect pair of tetrahedral (CrO4)CrO4 and Odeficient (CrO3)CrO4, both of which are corresponding to CrIVO44- and Cr IVO32- oxoanions, 12 ACS Paragon Plus Environment

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respectively. In past decades, several authors reported on the defect structures of the tetrahedral orthometallate anion compounds, such as acceptor-doped orthoniobate,38,39 orthophosphate40,41 and orthogallate.42,43 In these, incorporation of oxygen vacancy results in the formation of liking polyhedral (M2O7)2MO4 aided by corner oxygen sharing between O-deficient MO3 and adjacent MO4 units. Based on this framework, (CrO3)CrO4 defects are expected to associate with neighbor (CrO4)CrO4 defects via corner sharing as follows: 2(CrO4)CrO4  (Cr2O7)2CrO4 + 1/2 O2 (g)

(2)

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

P

→Y

Cr

P

(3)

The average oxidation states of Cr in pristine YCr1-xPxO4 are determined to be +4.7~+5.1 by redox titration using a standard Fe(II) solution, indicating that CrV cationic state is predominant in all pristine samples at room temperature (Table 1). Based on this, the relatively large intensity of CrIV XPS peak in phosphate substitutes 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, oxygen deficiency () at 600C is calculated from W to be 0.051, 0.11, 0.13 and 0.15 for x = 0, 0.3, 0.5 and 0.7, respectively (Table 1), which reveals that redox efficiencies of Cr cations in YCr1-xPxO4 increases 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 as rt  700C  rt. Figure 5a shows the normalized X-ray adsorption near edge structure (XANES) spectra of YCr0.5P0.5O4 together with three references (Cr2O3, CrO2 and YCrO4).



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Pristine YCr0.5P0.5O4 reveal the similar features as 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 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 pre-edge peak slightly, disclosing that the local coordination environments around Cr atoms are modified by oxygen deficiency at elevated temperature. 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 with approaching to 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 both before heating, indicating that tetrahedral coordination environment around Cr atoms are 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- is readily reoxidized to original YCr1-xPxO4 with recovering CrV states by cooled to room temperature in air.


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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C and 700C in dry air. In temperature program, the pristine 15 ACS Paragon Plus Environment


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samples are heated to 700C at 5C min‐1 with kept at the temperature for 3 h and cooled down to 25C at 5C min‐1. The measurements at each temperature were conducted after 1 h from temperature equilibrated. 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. 1st and 2nd 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 oxygen‐deficient CrO3 and the CrO4 neighbor. The details are described in a main text.

Table 2 The fitting parameters obtained from Cr K‐edge EXAFS analysis for YCr1‐xPxO4 (x = 0.5 and 0.7). Samples

Shell

r/Å a

CN b

σ2/10–2 Å2 c

R/%

YCr0.5P0.5O4 (RT)

Cr−O

1.69 ± 0.02

3.9 ± 0.2

0.20 ± 0.20

1.0

YCr0.5P0.5O4 (700 °C)

Cr−O

1.69 ± 0.02

3.1 ± 0.2

0.29 ± 0.27

0.74

YCr0.5P0.5O4 (700 °C→RT)

Cr−O

1.69 ± 0.02

3.8 ± 0.2

0.10 ± 0.25

1.5

YCr0.3P0.7O4 (RT)

Cr−O

1.69 ± 0.02

4.0 ± 0.4

0.28 ± 0.23

0.7

YCr0.3P0.7O4 (700 °C)

Cr−O

1.68 ± 0.03

2.8 ± 0.4

0.25 ± 0.37

1.8

YCr0.3P0.7O4 (700 °C→RT)

Cr−O

1.69 ± 0.02

3.8 ± 0.4

0.23 ± 0.27

1.1

Cr2O3

Cr−O

1.96 ± 0.02

6.0

0.75 ± 0.15

2.5

CrO2

Cr−O

1.89 ± 0.06

6.0

1.08 ± 0.40

4.5

YCrO4

Cr−O

1.69 ± 0.02

4.0

0.28 ± 0.10

4.7

Interval of k-space to r-space of FT is 2.0-15.0 Å–1. a

Atomic distance. b Coordination number. c Debye−Waller factor.

Fourier transform EXAFS and corresponding curve-fitting analysis figure out more precisely the change of the local structure environment around Cr by reduction at elevated temperature 16 ACS Paragon Plus Environment

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(Table 2 and Figs. 5b and c). Since these data 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 Cr2O3 and CrO2 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 X-ray crystallographic data for zircon-type YCrVO4 (CN 4, Cr-O 1.74 Å).22,32 Based on the crystallographic data of YCrO4,22,24,32 the 2nd neighbor shell at a radial distribution around 2.7 Å can be assigned to the coherence between Cr and 2nd neighbor yttrium (3.1 Å). The curve-fitting analysis for YCr0.5P0.5O4 at room temperature yields the approximately 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 Fig. 3). Whereas the best fitting for the first Cr−O shell at 700 °C is achieved by smaller CN (3.2) with the same r (1.69 Å) in comparison with both of the room temperature phase (Table 1 and supplementary Fig. 3). Moreover, a new coordination peak is evident at a distance around 2.1 to 2.5 Å at 700C but disappears by return to 25°C (Fig. 5b), revealing that the incorporation of oxygen vacancies induces the displacement of atoms surrounding Cr cation with creating additional coordination toward Cr atoms. The similar local structure changes are also observed in YCr0.3P0.7O4 during the temperature cycle (Fig. 5c). In zircon type structure, every CrO4 tetrahedron has four CrO4 neighbor and they are adjacent each other with approaching the corner oxygens toward the neighbors, which gives rise to the closest O-O contact of 2.6 Å distance among neighboring CrO4 tetrahedra (Fig. 6a). This 17 ACS Paragon Plus Environment


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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 (Figs. 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.

Figure 6 Schematic representation of local geometry around (a) CrO4 unit in stoichiometric phase, (b) (CrO3)CrO4 defects created by oxygen vacancy, and (c) (Cr2O7)2CrO4 defect complex formed via association of (CrO3)CrO4 and (CrO4)CrO4. The dashed red lines in (a) shows the closest O‐O contact between neighboring CrO4 tetrahedra. Grey rectangular solid shows a unit cell in (a)‐(c).

To validate the aforementioned defect models, the structure relaxation and defect formation energy (Edef) were calculated for the oxygen deficient YCr0.5P0.5O4- by density functional theory (DFT) with 2  2  2 unit cells. Here, [Y32Cr16P16O128] supercell is used for a stoichiometric YCr0.5P0.5O4 phase, and [Y32Cr16P16O120] supercell is considered for the oxygendeficient YCr0.5P0.5O3.9375 phase, which is prepared by removing one corner oxygen of a CrO4


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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 undergoes. Cr which loses an oxygen coordinate is labeled by Cr(vac) and Cr(a) is the neighbor of Cr(vac) and O(s) is an oxygen which is shared in (Cr2O7)2CrO4 defect complex. O(s) is drawn toward Cr(vac) from the initial position and fall into the location near the center of diagonal Cr(vac)-Cr(a) line, becoming closer to Cr(vac) (Figs. 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 changes to 2.32 and 2.81 Å, respectively, after relaxation, which are in close agreement with the observed by EXAFS. Figure 7 also shows isosurfaces of the partial charge densities obtained from a normalized wave functions attributed to the Cr 3d states near Fermi energy levels of stoichiometric and oxygen-deficient supercells. The wave functions are clearly localized at Cr sites and, moreover, the electron charges are equally distributed in Cr(vac) and Cr(a) atoms (Figs. 7b&d), confirming that two CrV cations are equally reduced to CrIV with forming a (Cr2O7)2CrO4 defect complex by oxygen vacancy. The spectroscopic measurements and theoretical calculations clearly demonstrate that thermal reduction of YCr1-xPxO4 are progressive via corner-share linking between oxygen-deficient CrO3 and the adjacent CrO4, and thus oxygen uptake by YCr1-xPxO4 involve the dissociation of the linking polyhedra into two CrO4 tetrahedra.

Edef can be defined as a change in total energy according to the incorporation of a defect into a supercell.49 Edef for O vacancy in YCr0.5P0.5O4- phase can be expressed by Y Cr P O

4

Y Cr P O

(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 vacuum.



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Edef 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 relatively small reduction enthalpy.

Figure 7 Relaxed geometry and isosurfaces of the partial charge derived from a normalized wave functions 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 oxygen vacancy. Grey planes in (c) and (d) depict the (200) plane.

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 oxygen20 ACS Paragon Plus Environment

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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 oxygen vacancy (~14 kJ mol-1). When reduction (3) spontaneously occurs at a temperature, the reaction free energy, G0(T) must be negative as follows.

G0(T) = H0- T S0 ≤ 0

(5)

Here, H0 and S0 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 G(T, pO2) at pO2 can be represented with G0(T) as follows

G(T, pO2) = G0(T) + 1/2  RT ln(pO2 / p0)

(6)

Under equilibrium, i.e. G(T, pO2) = 0, Eq(6) is rewritten by combined with Eq (5) (pO2 / p0)0.5 = exp(S0/R) exp(-H0/RT)

(7)

which provides the equilibrium oxygen partial pressure at a given temperature. Both H0 and

S0 are a function of oxygen stoichiometry , so that eq(7) is rewritten by ∆

∆

=const

(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 (-H0/RT) = 1/2 ln (pO2 / p0) - (S0/R), because lower pO2 is needed to establish equilibrium (8) at higher temperatures, and so S0 value would set an upper bound on the increment of  at a fixed pO2. If defect reaction (2) is equilibrated at a temperature above 400C, S0 is given by the mixing entropy related to the defect equilibrium between oxygen vacancies and oxygen gases,50 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 .51 Based on the assumption, H0 is roughly estimated to be about 21 k J mol-1 as calculated by using the 0SO2 value (~ 121 J K-1 mol-1) at 673 K.52



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This H0 value is consistent with the Edef calculated by DFT (14 kJ mol-1), and is one 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 relatively small reduction enthalpy, thereby the reversible oxygen storage reactions undergoing in the extremely-low temperature region. Surprisingly, all of Cr cations in a YCr0.3P0.7O4 bulk are fully reoxidized by maintained at 50C, disclosing that enhanced diffusion of oxide ion occurs in the bulk even at such low temperatures. On the other hand, isomorphous crystal-structure compound ZrSiO4 has been reported to have very small oxygen diffusion coefficient of about 10-20 m2 s-1 at 1200C.55 The diffusivity in oxide lattices must be strongly correlated with concentration of oxygen vacancy because the ambipolar diffusion normally proceeds via vacancy-mediated hopping process of oxide ions. The oxygen vacancy concentration of ZrSiO4 is in the order of 1017 atoms cm-3,56 which is four orders of magnitude smaller than that of YCr1-xPxO4- with  ~0.1 (calculated to be 1021 atoms cm-3). The high concentrations of oxygen vacancies may facilitate the oxide ion diffusion via site-to-site hopping process in zircon type YCr1-xPxO4-. Table 3 summarizes the recent publications for the oxygen storage materials. The oxygen storage capacity, i.e.  of 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 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C and 50C, respectively, without adjusting pO2 or pH2 in surrounding atmosphere (Fig. 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 perovskite-type 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 22 ACS Paragon Plus Environment

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300C,60,61 however, the oxidized phase is not spontaneously reduced in air and thus the oxygen desertion needs to be conducted in inert atmosphere since. The perovskite-related BaYMn2O5+ 62

and Dy0.7Y0.3MnO3+

63

and layered CdI2 type LuFe2O4+

64

also need oxidative/reductive

atmospheric switching for the reversible oxygen storage reactions although their storage capacities are enough high. Recently, several authors reported on low temperature oxygen storage ability of the nanostructured CeO2, which facilitate reversible oxygen store/release at around 150C.65-67 This value is still higher than the oxidation temperature of YCr1-xPxO4-. Accordingly, it can be concluded that YCr1-xPxO4 possess outstanding oxygen storage capability driven by the thermochemical CrV/CrIV redox reactions in extremely low temperature region without controlling the reaction atmosphere.

Table 3 List of recent publications for thermochemical oxygen storage materials. Materials

Oxidation/reduction temperatures (C)

a

ref

CeO2-

400/900

0.1

59

Ce0.5Zr0.5O2-

400/850

0.4

59

SrFe0.95Cu0.05O3-

400/1000

0.3

51

SrCoO3-

600/900

0.1

9

YBaCo4O7+

300(O2)/300(N2)b

1.2

60

Ca0.8Sr0.2MnO3-

400/1200

0.28

54

BaYMn2O5+

500(O2)/500(5%-H2/Ar)b

0.45

61

Ca2AlMnO5+

500/700

0.29

62

0.25

63

b

Dy0.7Y0.3MnO3+

300(O2)/300(/Ar)

LuFe2O4+

500(air)/500(5%-H2/Ar)b

0.5

64

YCr0.3P0.7O4-

50/600

0.15



a

Maximum oxygen deficiency or excess in the reaction temperature ranges. bGases in

parenthesis indicates the oxidation/reduction atmosphere.



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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- must be concerned with the large redox potential of CrV/CrIV system (2.2 V vs NHE),

68

so that various high valence state orthometallates

attractive for OSMs. For instances, oththoferate(VI) Na2FeVIO4 Ba2CoIVO4

71

70

15,22-24,69-71

are also

and orthocobaltate(IV)

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 as 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 of OSMs with low activation energy and large storage capacity. The current results pronounce a new direction for the development of tailor-made oxygen storage materials which can facilitate the oxygen-related thermochemical conversion processes in relatively low temperature regions.

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 the 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 a 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 24 ACS Paragon Plus Environment

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at lower temperatures, a (Cr2O7)2CrO4 defect complexe dissociates into two (CrO4)CrO4 via coupled 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 0.2 p0 in 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 CrV/CrIV system and the stabilization of the oxygen deficient 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.

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

Supporting information Supplemental figures for XRD, high temperature XRD and EXAFS fitting analysis.



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Figure captions Figure 1 Thermal gravimetric change of YCr0.5P0.5O4 for temperature cycle in dry air. 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.

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 1h 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 dot lines the temperature programs.

Figure 3 TG curves of YCr1‐xPxO4 during temperature cycles between 50C and 600C. x = 0 (a), 0.3 (b), 0.5 (c) and 0.7 (d). One temperature cycle is consisting of the 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, and here the cycles are repeated 5 times. Red solid lines show TG curves and black dot lines temperature programs. Large weight loss at the step of 1st temperature‐rise is attributed to the water desorption.

Figure 4 Cr 2P3/2 XPS spectra of YCr1‐xPxO4‐, 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.


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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C and 700C in dry air. In temperature program, the pristine samples are heated to 700C at 5C min‐1 with kept at the temperature for 3 h and cooled down to 25C at 5C min‐1. The measurements at each temperature were conducted after 1 h from temperature equilibrated. 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. 1st and 2nd 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 oxygen‐deficient CrO3 and the CrO4 neighbor. The details are described in a main text.

Figure 6 Schematic representation of local geometry around (a) CrO4 unit in stoichiometric phase, (b) (CrO3)CrO4 defects created by oxygen vacancy, and (c) (Cr2O7)2CrO4 defect complex formed via association of (CrO3)CrO4 and (CrO4)CrO4. The dashed red lines in (a) shows the closest O‐O contact between neighboring CrO4 tetrahedra. Grey rectangular solid shows a unit cell in (a)‐(c).

Figure 7 Relaxed geometry and isosurfaces of the partial charge derived from a normalized wave functions 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 oxygen vacancy. Grey planes in (c) and (d) depict the (200) plane.



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Low-Temperature Oxygen Storage of CrIVâCrV Mixed-Valence YCr1

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