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Effect of Al substitution on structural stability and topotactic oxygen release rate of LaNi AlO with perovskite structure 1-x

x

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Yoshihiro Goto, Akira Morikawa, T. Tanabe, and Masaoki Iwasaki ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b02235 • Publication Date (Web): 05 Apr 2019 Downloaded from http://pubs.acs.org on April 7, 2019

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ACS Applied Energy Materials

Effect of Al Substitution on Structural Stability and Topotactic Oxygen Release Rate of LaNi1−xAlxO3 with Perovskite Structure Yoshihiro Goto,* Akira Morikawa, Toshitaka Tanabe, and Masaoki Iwasaki Toyota Central R&D Labs., Inc., 41-1 Yokomichi, Nagakute, Aichi 480-1192, Japan KEYWORDS: oxygen storage material; cationic substitution; perovskite; LaNi1−xAlxO3; structural stability; oxygen release rate

ABSTRACT: Oxygen storage materials (OSMs) composed of transition metals are promising in the field of environment protection and energy conversion. In this study, we synthesized LaNi1−xAlxO3 (0 ≤ x ≤ 1.0) solid solutions with a rhombohedral perovskite structure and investigated the effect of Al substitution on their structural stability and oxygen release rate. Xray diffraction measurements and thermogravimetric analysis revealed that Ni-containing LaNi1−xAlxO3 released/stored oxygen atoms topotactically and reversibly via the Ni3+  Ni2+ redox reaction. LaNiO3 decomposed into La2O3 and Ni0 at 470 °C in a reducing atmosphere (H2). However, Al-substituted LaNi1−xAlxO3 maintained their perovskite structure even at high temperatures (>800 °C for 0.6 ≤ x ≤ 1.0). The high structural stability of these compounds can be attributed to the suppression of oxygen vacancy ordering. Furthermore, LaNi0.2Al0.8O3 (x = 0.8)

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exhibited higher oxygen release rate (9.35 × 10−3 mol-O mol-Ni−1 s−1) than LaNiO3 (x = 0; 2.59 × 10−3 mol-O mol-Ni−1 s−1) at 400 °C because of its lower activation energy. This is consistent with the decrease of bond strength of the oxygen atoms as expected based on bond valence sum calculations. Such substitution effects of irreducible cations offer a wide opportunity to develop transition metal OSMs with desired structural stability and oxygen release rate.

1. Introduction Reversible release/storage of atoms from/in the host lattice is a potentially useful phenomenon in solid-state chemistry. Such phenomena in inorganic compounds have not only resulted in new materials,1 but have also demonstrated various beneficial properties such as selective separation of heavy metals (xCd + Ti2PTe2  CdxTi2PTe2),2 energy storage (xLi+ + e− + Li1−xCoO2  LiCoO2),3 and oxygen storage (1/2O2 + Ce2Zr2O7  Ce2Zr2O8).4 Oxides that possess oxygen storage ability are referred to as oxygen storage materials (OSMs). These materials can control the oxygen concentration in the gas phase since they can release/store oxygen atoms in response to changes in temperature and oxygen partial pressure.5,6 OSMs have been extensively studied for energy conversion applications such as thermochemical water splitting,7 solid-state fuel cells,8 and chemical looping combustion.9 In order to realize these applications the development of novel OSMs with high oxygen release/storage rates is imperative. Ce-based oxides with precious metals, which are insensibly investigated for automotive exhaust catalysts because of their high thermal stability, release/store oxygen atoms reversibly via the Ce4+  Ce3+ redox reaction.5,10 Meanwhile, OSMs composed of transition metals (e.g. Cr, Mn, Fe, and Co) have also been investigated since they can release oxygen atoms at low temperatures


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(< 500 °C) without precious metals.6,11−16 In particular, transition metal OSMs with the perovskite structure (e.g. BaYMn2O6 and SrFeO2.81)12,16 show high oxygen release rates. These OSMs maintain their original cationic frameworks even after the release of oxygen atoms from their lattice. Such a transformation is often referred to as “topotactic transformation”, which is crucial for achieving high oxygen release rates. Since ABO3-type perovskites exhibit wide compositional flexibility, cationic substitution is an effective strategy to tune their chemical and physical properties. For example, Ti-substituted SrFe1−xTixO3−d show both a high resistance to coexisting CO2 and a high oxygen storage rate in comparison to SrFeO3.16 Hence, the substitution of irreducible cations can tune the chemical resistance and oxygen release/storage rate of OSMs with perovskite structure, but the detailed mechanism has not been fully elucidated. In this study, we focused on perovskite LaNiO3, which exhibits the highest volume of released oxygen among the ABO3-type perovskites (A = La, Ba, Sr, Ca, and B = Cr, Mn, Fe, Co, Ni, Cu).17 The primary objective was Al substitution of a Ni site in LaNiO3 because of the equivalent valence states of Al3+ and Ni3+ cations in addition to the identical crystal structures of LaAlO3 and LaNiO3 (rhombohedral perovskite structure; space group of R-3c).18,19 Since Al3+ is an irreducible cation, Al substitution is expected to enhance the chemical resistance and oxygen release/storage rate of LaNiO3. Herein, we synthesized LaNi1−xAlxO3 (0 ≤ x ≤ 1.0) solid solutions using the polymerized complex method. The solid solutions released/stored oxygen atoms reversibly and topotactically via the Ni3+  Ni2+ redox reaction. Moreover, Al substitution enhanced both the structural stability in a reducing atmosphere (H2) and the oxygen release rate of LaNi1−xAlxO3 by suppressing the oxygen vacancy ordering and weakening the bond strength


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of oxygen atoms, respectively. These identified factors that determine the structural stability and oxygen release rate may lead to a more comprehensive understanding of transition metal OSMs.

2. Experimental Section 2.1. Sample Preparation. Polycrystalline samples of LaNi1−xAlxO3 (x = 0, 0.2, 0.4, 0.6, 0.8, and 1.0) solid solutions were synthesized using the polymerized complex method. La(NO3)36H2O (99.9%, FUJIFILM Wako Chemicals), Ni(NO3)26H2O (99.9%, FUJIFILM Wako Chemicals), and Al(NO3)39H2O (99.997%, Sigma-Aldrich) were dissolved in a minimum amount of deionized water in a molar ratio of 1:(1−x):x and the resulting mixture was stirred at room temperature for 10 min. Citric acid (6 equimolar, 99%, Alfa Aesar) and ethylene glycol (6 equimolar, 99.0%, FUJIFILM Wako Chemicals) were then added to the solution and the mixture was stirred at 90 °C for 30 min to promote polymerization. The gelatinous product obtained was preheated at 400 °C for 2 h and calcined twice at 800 °C for 5 h in air. 2.2 Characterization. High-resolution synchrotron X-ray diffraction (SXRD) measurements were performed using a Debye-Scherrer camera with a two-dimensional detector (PILATUS 100K) installed at the Aichi Synchrotron Radiation Center. The incident beams from the bending magnet were monochromatized to λ = 0.690473 Å. The powder samples were placed in Pyrex capillaries (0.2 mm i.d.), which were rotated during the measurement process to reduce any preferential orientation. The collected SXRD patterns were analyzed using the Le Bail fitting program Jana200620 to calculate the lattice constants of the samples. Laboratory X-ray diffraction (XRD) measurements were performed using an Ultima IV X-ray diffractometer (Rigaku) with Cu-Kα radiation (λ = 1.54056 Å).


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2.3. Evaluation of Oxygen Release Properties. The oxygen release properties of the samples were examined using a thermogravimetric analyzer (TGA-50, Shimadzu) connected to a dual gas supplying system. A platinum pan was used as the sample holder. The temperature dependence of the thermogravimetric analysis (TGA) curves of the samples were recorded up to 800 °C at a heating rate of 10 °C/min under 5% H2/N2 (100 mL/min) flow. The decomposition temperatures of the samples were determined from the first derivative of the TGA curves. The isothermal TGA curves of the samples were recorded under a flow of 5% O2/N2 and 5% H2/N2 at 300, 350, and 400 °C. The oxygen release amount for each sample was estimated from the weight loss in 5% H2/N2 after 5 min. The oxygen release rates of the samples were determined from their initial weight loss assuming a zeroth-order reaction for the reduction up to 20% of Ni3+ to Ni2+, or 0 ≤ α ≤ 0.2, where α the is fraction of reducible Ni3+ (α = [Ni2+]/([Ni2+]+[Ni3+])).

3. Results and Discussion 3.1. Structural stability in reducing atmosphere. The SXRD results revealed that all the synthesized samples had a rhombohedral perovskite structure with the R-3c space group (Fig. 1a). The x = 0, 0.2, and 0.4 samples exhibited a small amount of NiO (the peak intensities were less than 2% of that of the highest peak at around 2θ = 14.5° in their rhombohedral phases), while no impurity was observed in the x = 0.6, 0.8, and 1.0 samples. The estimated lattice constants of the end members were a = 5.45501(8) and c = 13.1631(4) Å for x = 0 and a = 5.36676(7) and c = 13.1182(2) Å for x = 1.0 (Table 1). These values are in agreement with those reported for LaNiO3 (a = 5.4573(1) and c = 13.1462(3) Å)18 and LaAlO3 (a = 5.3647(1) and c = 13.1114(3) Å),19 respectively. As shown in Figure 1b and Table 1, the lattice constants for the a


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and c axes of the main rhombohedral phases varied linearly with the Al content (Vegard’s law). This is indicative of the formation of LaNi1−xAlxO3 solid solutions. The decrease in the cell volume with an increase in the Al content (x) (Fig. 1c) supports the assertion that the Ni site of LaNiO3 was successfully substituted by Al because the ionic radius of trivalent Al (0.54 Å) is smaller than that of Ni (0.61 Å).21

Figure 1. (a) SXRD patterns of LaNi1−xAlxO3 (0 ≤ x ≤ 1.0). Asterisks (*) indicate the peaks corresponding to NiO. (b) Lattice constants and (c) cell volumes of LaNi1−xAlxO3 as a function of Al content (x).


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Table 1. Physical characteristics of LaNi1−xAlxO3 (0 ≤ x ≤ 1.0)

Lattice constantsa (Å)

Compound

Decomposition temperature (°C)

Oxygen release amountc

Oxygen release rate per Ni

(wt%)

(μmol-O μmol-Ni−1 s−110−3)

b

a

c

LaNiO3

5.45501(8)

13.1631(4)

LaNi0.8Al0.2O3

5.4405(3)

LaNi0.6Al0.4O3

Eae (kJ/mol)

300 °C

350 °C

400 °C

300 °C

350 °C

400 °C

470

0.37

1.57

2.89

0.20d

0.86

2.59

82  2

13.1532(6)

610

0.70

2.16

2.48

0.48

1.87

3.83

67  9

5.4235(2)

13.1437(4)

700

0.72

1.75

1.81

0.65

2.49

4.85

65  9

LaNi0.4Al0.6O3

5.39997(13)

13.1355(3)

> 800

0.87

1.13

1.08

1.39

3.44

6.02

47  4

LaNi0.2Al0.8O3

5.38121(7)

13.1300(3)

> 800

0.52

0.63

0.67

2.18

5.24

9.35

47  3

LaAlO3

5.36676(6)

13.1182(2)

> 800

0

0

0

-

-

-

aCalculated bDetermined

-

on the basis of the rhombohedral perovskite structure (Space group: R-3c; Z = 6).

from the first derivative of the TGA curves obtained under the flow of 5% H2/N2

(Fig. S1). cEstimated from the weight loss under the flow of 5% H2/N2 for 5 min. dEstimated from the weight loss in the fraction range of 0 ≤ α ≤ 0.1. eActivation energy for the oxygen release reaction. Figure 2 shows the TGA curves of LaNi1−xAlxO3 that were recorded upon heating to 800 °C under the flow of 5% H2/N2. LaAlO3 (x = 1.0) showed no weight loss up to 800 °C, whereas all the Ni-containing samples (0 ≤ x ≤ 0.8) exhibited weight loss at 270–400 °C because of the oxygen release. These weight losses correspond to the reduction of Ni3+ to Ni2.10–2.23+. All the XRD patterns of Ni-containing samples quenched at 400 °C in the presence of 5% H2/N2 could be indexed to single perovskite phases (Fig. 3a), indicating that LaNi1−xAlxO3 (Ni3+) stoichiometrically transform into LaNi1−xAlxO3−δ (Ni2+; δ = (1 – x)/2) along with the release of oxygen. The structural changes along with the oxygen release are estimated from the XRD patterns (Fig. 3a). LaNiO3 transformed into a monoclinic perovskite structure (space group: C2/c) which has the same crystal structure as that of La2Ni2O5 (Fig. 4).22,23 However, Al-substituted LaNi1−xAlxO3 (x ≥ 0.2) maintained its original rhombohedral perovskite structure (space group: R-3c) even


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after the oxygen release. These observations indicate that LaNiO3 and Al-substituted LaNi1−xAlxO3 both released oxygen topotactically from their perovskite lattices; however, the locations of oxygen vacancy generation were different. The oxygen vacancies present in LaNiO3 were one-dimensionally ordered (Fig. 4a), whereas that in Al-substituted LaNi1−xAlxO3 were disordered (Fig. 4b). The random Al substitution for Ni in LaNi1−xAlxO3 suppressed the oxygen vacancy ordering. The TGA curve of x = 0 sample showed a further weight loss at 470–550 °C. The weight loss for temperatures in excess of 400 °C (5.76 wt%) was 2.13 times larger than that observed up to 400 °C (2.70 wt%). This value is consistent with Ni2+ → 0/Ni3+ → 2+ = 2 via sequential reduction of LaNiO3 (Ni3+) to La2Ni2O5 (Ni2+) followed by decomposition into La2O3 and Ni (Ni0). This behavior is indicated by the in-situ SXRD measurements (Fig. S2). The slight deviation of 2.13 from 2 can be attributed to the presence of NiO impurity in the initial state, as observed from the SXRD pattern (Fig. 1a). Note that the decomposition temperature of LaNi1−xAlxO3 increased with an increase of the Al content (black arrows in Fig. 2). The temperatures for the x = 0, 0.2, and 0.4 samples were 470, 610, and 700 °C, respectively (Table 1). No weight loss was observed in the case of the x = 0.6, 0.8, and 1.0 samples. The XRD patterns of the samples quenched at 800 °C in the presence of 5% H2/N2 (Fig. 3b) revealed that the perovskite phase of the x = 0 sample decomposed completely into La2O3 and Ni0. However, the x = 0.2 and 0.4 samples exhibited only partial decomposition of the perovskite phase. Phase separation was not observed in the x = 0.6, 0.8, and 1.0 samples. These results clearly indicate that Al substitution enhanced the structural stability of LaNi1−xAlxO3 in a reducing atmosphere.


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Figure 2. TGA curves of LaNi1−xAlxO3 (0 ≤ x ≤ 1.0) during heating in 5% H2/N2 flow. Black arrows indicate the onset decomposition temperature.

Figure 3. XRD patterns of LaNi1−xAlxO3 (0 ≤ x ≤ 1.0) quenched at (a) 400 °C and (b) 800 °C in 5% H2/N2.


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Figure 4. Schematic of the structural changes following the oxygen release/storage reactions in (a) LaNiO3 and (b) LaNi1−xAlxO3 (x ≥ 0.2). The white, green, blue, and red spheres represent La, Ni, Al, and O, respectively. The enhancement of the structural stability was likely related to the arrangement of oxygen vacancies. As shown in Figures 4a and 4b, LaNiO3 formed NiO4 square planes (coordination number, CN = 4) after the release of oxygen because of the oxygen vacancy ordering, while LaNi1−xAlxO3 (x ≥ 0.2) mainly formed Ni(Al)O5 pentahedra (CN = 5) because of the disordered arrangement of their oxygen vacancies. This suggests that Ni2+ cations with larger CN are more resistant to further reduction (to Ni0) which leads to the phase decomposition. The correlation of CN with the chemical reactivity of OSMs has been reported previously. In perovskite SrFe1−xTixO3−d, an increase in the Ti content suppresses the formation of SrCO3 on the surface in a reducing atmosphere (containing CO2), because Ti substitution suppresses the formation of less coordinated Sr (CN = 8) and reduces the CO2 reactivity.16 In both LaNi1−xAlxO3


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and SrFe1−xTixO3−d, the oxygen vacancy ordering that results in the formation of less coordinated cations is suppressed by the random substitution of irreducible cations (Al and Ti) at the redox site (Ni and Fe) in the ABO3-type perovskites. This strategy can also be applied to other transition metal OSMs that require chemical resistance against specific molecules (e.g. H2, CO2, and H2O).

3.2. Oxygen release rate. The isothermal TGA curves of LaNi1−xAlxO3 under the flow of 5% O2/N2 and 5% H2/N2 are shown in Figure 5a. For all the Ni-containing samples (0 ≤ x ≤ 0.8), the decrease in weight in 5% H2/N2 was immediately restored to the original values when the atmosphere was changed to 5% O2/N2. This suggests that LaNi1−xAlxO3 could reversibly release/store oxygen atoms from/in their perovskite lattices (Fig. 4) as the following reactions: LaNi1−xAlxO3 + δH2 → LaNi1−xAlxO3−δ + δH2O LaNi1−xAlxO3−δ + δ/2O2 → LaNi1−xAlxO3

(1) (2)

The oxygen release amount for LaNi1−xAlxO3 at 400 °C decreased with an increase in Al content (Table 1 and Fig. S3). This is consistent with their decreased Ni content. At low temperatures, however, some Al-substituted samples demonstrated a larger amount of released oxygen compared to LaNiO3 (300 °C: 0.2 ≤ x ≤ 0.8, 350 °C: 0.2 ≤ x ≤ 0.4) in spite of the loss of Ni content. For example, the oxygen release amount of the x = 0.6 sample (0.87 wt%) at 300 °C was approximately 2.4 times larger than that of the x = 0 sample (0.37 wt%). This indicates that the oxygen release amount in 5 min depends on the oxygen release rates, which in turn depends on the Al content.


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To determine the oxygen release rate per Ni atom, the isothermal TGA curves of LaNi1−xAlxO3 (0  x  0.8) were normalized to the fraction of reducible Ni3+ (α = [Ni2+]/([Ni2+]+[Ni3+])) (Figs. 5b–5d). The initial oxygen release rates that were estimated from the weight loss at 0 ≤ α ≤ 0.2 increased with an increase of the Al content (Fig. 6a and Table 1). The oxygen release rate of the x = 0.8 sample (9.35 × 10−3 mol-O mol-Ni−1 s−1) was approximately 3.6 times higher than that of the x = 0 sample (2.59 × 10−3 mol-O mol-Ni−1 s−1) at 400 °C. This trend was also observed at 300 and 350 °C. The activation energy for the oxygen release reactions as determined from their Arrhenius plots (Fig. S4) based on the oxygen release rates linearly decreased with an increase of Al content (Fig. 6b and Table 1). This suggests that a decrease in the activation energy due to Al substitution increases the oxygen release rates of LaNi1−xAlxO3.


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Figure 5. (a) Isothermal TGA curves of LaNi1−xAlxO3 (0  x  1.0) under the flow of 5% O2/N2 and 5% H2/N2. Fraction of reducible Ni3+ at (b) 300 °C, (c) 350 °C, and (d) 400 °C during the third reduction cycle in 5% H2/N2.

Figure 6. (a) Oxygen release rate per Ni atom and (b) activation energy of LaNi1−xAlxO3 (0 ≤ x ≤ 0.8) estimated from the fraction range of 0 ≤ α ≤ 0.2. The decrease in the activation energy for the oxygen release reactions can be explained by the bond strengths of the oxygen atoms in the crystal lattice of LaNi1−xAlxO3. The bond valence sums (BVSs)24,25 which is often used as a measure of the bond strengths for specific atomic sites were calculated for LaNiO3 and LaAlO3 using reported crystallographic data.18,19 The BVS value for oxygen site (Wyckoff position of 18e) in LaAlO3 (−1.96) was larger than that the value for LaNiO3 (−2.15). This indicates that oxygen atoms in LaAlO3 were weakly bounded to the surrounding cations in comparison to that in LaNiO3. Thus, Al substitution in LaNi1−xAlxO3 weakens the bond strength of oxygen atoms, leading to a decrease in the activation energy for the oxygen release reactions. The increase of the oxygen release rate was also effective even if the unit was converted from per Ni atom (mol-O mol-Ni−1 s−1) into per catalyst mass (mol-O g−1 s−1). The oxygen release rates (per mass) of the samples of 0.2 ≤ x ≤ 0.8 were larger than that of the x = 0 sample at 300


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°C (Fig. S3). Such advantages of Al-substituted LaNi1−xAlxO3 to LaNiO3 at low temperature are beneficial to its practical applications.

4. Conclusions LaNi1−xAlxO3 (0 ≤ x ≤ 1.0) solid solutions with a rhombohedral perovskite structure were successfully synthesized via the polymerized complex method and the effect of Al substitution on their structural stability and oxygen release rate was investigated. Al substitution suppressed the oxygen vacancy ordering in a reducing atmosphere (H2) and increased the decomposition temperature from 470 °C (x = 0) to >800 °C (0.6 ≤ x ≤ 1.0). Moreover, the oxygen release rate of LaNi1−xAlxO3 increased with an increase in their Al content because of the weakening of the bond strengths between oxygen atoms and the surrounding cations. The oxygen release rate of LaNi0.2Al0.8O3 (9.35 × 10−3 mol-O mol-Ni−1 s−1) was found to be approximately 3.6 times higher than that of LaNiO3 (2.59 × 10−3 mol-O mol-Ni−1 s−1) at 400 °C. This study demonstrated that the substitution of irreducible cations at the redox site is an effective approach to enhance structural stability and oxygen release rate of OSMs with perovskite structure. We believe that further investigations of various catalytic activities using LaNi1−xAlxO3 would contribute to the understanding of the correlation between the oxygen release/storage properties and catalytic activity of transition metal OSMs. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI:


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In-situ synchrotron XRD patterns, differentials of weight change in TGA curves, and Arrhenius plots for the oxygen release rate (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors acknowledge Dr. Towata (Aichi Synchrotron Radiation Center, Aichi, Japan) for his contribution in collecting the SXRD data.


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