Significant Lanthanoid Substitution Effect on the Redox Reactivity of

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Significant lanthanoid substitution effect on the redox reactivity of the oxygen-storage material BaYMnO 2

5+#

Teruki Motohashi, Makoto Kimura, Yuji Masubuchi, Shinichi Kikkawa, Janine George, and Richard Dronskowski Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b01501 • Publication Date (Web): 29 May 2016 Downloaded from http://pubs.acs.org on June 1, 2016

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Chemistry of Materials

Significant lanthanoid substitution effect on the redox reactivity of the oxygen-storage material BaYMn2O5+δδ Teruki Motohashi,a,b* Makoto Kimura,b Yuji Masubuchi,b Shinichi Kikkawa,b Janine George,c Richard Dronskowskic,d a

Department of Materials and Life Chemistry, Kanagawa University, Yokohama, Japan

b

Faculty of Engineering, Hokkaido University, Sapporo Japan Institute of Inorganic Chemistry, RWTH Aachen University, Aachen, Germany d Jülich-Aachen Research Alliance (JARA-HPC), RWTH Aachen University, Aachen, Germany c

ABSTRACT: The redox characteristics of oxygen storage materials BaLnMn2O5+δ with Ln = La, Nd, Gd, and Y were investigated employing the reductive water dissolution by the deoxygenated δ = 0 form. The Ln = La, Nd, and Gd compounds were found to show a capability to produce hydrogen gas through the water dissolution at 500 °C, whereas the Ln = Y compound was unreactive to water. It was also revealed that the reactivity obviously depend upon the Ln species: the larger the ionic size of Ln3+ is, the higher reactivity

the

BaLnMn2O5.0

samples

exhibit.

The

experimentally-derived

thermodynamic parameters of the BaLnMn2O5+δ series were compared with those obtained by first- principles calculations.

*Corresponding author. Department of Materials and Life Chemistry, Kanagawa University, 3-27-1 Rokkakubashi, Kanagawa-ku, Yokohama 221-8686, Japan. E-mail: [email protected]

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1. INTRODUCTION Redox is one of the most fundamental reactions that is involved in a wide variety of chemical processes. Recent global concerns regarding the energy and environmental issues have evoked the importance of functional materials that enable to precisely control redox reactions for energy conservation and environmental protection. “Oxygen storage materials” (OSMs), which show a rapid and reversible oxygen intake/release capability, are within this class.1-14 With OSMs, various redox applications are promising such as redox catalysts/agents for promoting specific chemical reactions. In fact, a CeO2–ZrO2 solid solution, the best known OSM so-called CZ, has already been used in three-way catalysts for the effective removal of NOx, CO, and hydrocarbons from automobile exhausts.1 Meanwhile, studies on OSMs for other research areas are rather new and have received growing interest since they cover various energy-related topics: for example, chemical looping processes3,15,16 including hydrogen production through water dissolution,7,17,18 oxygen reduction/evolution reactions (ORR/OER) in solid-oxide fuel cells (SOFC),19 metal-air batteries,20,21 and electrolysis of water.21

To realize the aforementioned applications, it is important to establish the strategy to tailor “on-demand” OSMs optimized for the target redox reaction. For each OSM, the oxygen intake/release processes are triggered by valence changes of the redox species. This implies that the characteristics of the OSMs are essentially linked with the constituent redox species (Ce3+/Ce4+ redox couple for CZ), and fine tuning may be achieved through “isovalent” substitutions for redox-inactive cations. It should be noted that isovalent substitutions have widely been examined for electronic materials such as superconductive copper oxides22 and magnetoresistive manganese oxides.23 Since the 2


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modified electronic properties of such materials are often accounted for the local atomic environment around transition metals, the influence of isovalent substituents on the redox characteristics is also believed to be noteworthy.

The present study focuses on a manganese-based OSM, BaYMn2O5+δ. This oxide is categorized as an A-site ordered double-perovskite which contains smaller yttrium and larger barium ions in alternate layers (see the illustration in Graphical Abstract).24-26 The oxygen sites within the yttrium layer are readily filled/unfilled in response to variations in temperature and/or the surrounding atmosphere, resulting in a large oxygen nonstoichiometry ranging 0 ≤ δ ≤ 1. Our previous work revealed the rapid and perfectly reversible oxygen intake/release of BaYMn2O5+δ at 500 °C under switching oxidative/reductive atmospheres.8 The oxygen intake/release processes are found to always involve phase separation into three distinct forms with δ ≈ 0, 0.5, and 1; details of the structural evolution upon oxygen intake/release are discussed in Ref. 27. Taking into account the structural feature of BaYMn2O5+δ, substitutions of the lanthanoid (Ln) series at the yttrium site could be most remarkable, because yttrium is neighboring the active site for oxygen intake/release. Recent reports on the Ln-substituted derivatives BaLnMn2O5+δ suggested28-31 that their properties are indeed related to the ionic size of the constituted Ln species, but detailed discussion based on thermodynamics considerations has never been made.

Here, we report a systematic study on a BaLnMn2O5+δ series with Ln = La, Nd, Gd, and Y. The redox characteristics of BaLnMn2O5+δ were investigated employing the following reductive water dissolution by the deoxygenated (δ = 0) form: 3



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2BaLnMn2O5.0(s) + H2O(g) → 2BaLnMn2O5.5(s) + H2(g) (“s” and “g” denote solid and gas phases, respectively). This reaction is of particular interest from not only the scientific but also engineering points of view, as it provides a strategy of materials tailoring for realizing thermochemical hydrogen production from water. Our water dissolution experiments indicated that the reductive reactivity obviously depends on the Ln species: the larger the ionic size of Ln3+ is, the higher reactivity the BaLnMn2O5.0 samples exhibit. The experimentally-derived thermodynamic parameters of the BaLnMn2O5.0 series were compared with those obtained by first-principles calculations.

2. EXPERIMENTAL SECTION Synthesis and Basic Characterization. Samples of BaLnMn2O5+δ (Ln = Y, Gd, Nd, and La) were synthesized via a citrate precursor route combined with the oxygen-pressure-controlled encapsulation technique.31 Ln2O3 (Ln = Y, Gd, and Nd, 99.9%, Wako Pure Chemical; Ln = La, 99.99%, Kanto Chemical; fired at 1000 °C overnight prior to use), Ba(NO3)2 (99.9%, Wako Pure Chemical), and Mn(NO3)2⋅6H2O (99.9%, Wako Pure Chemical) were used as starting materials. Appropriate amounts of these reagents were dissolved in diluted HNO3 (for Ln2O3) or Milli-Q water [for Ba(NO3)2 and Mn(NO3)2⋅6H2O] to prepare Ln, Ba, and Mn nitrate solutions. These solutions were mixed in a crucible in which an equal mole of citric acid (98%, Wako Pure Chemical) was subsequently added as a complexing agent. The citrate solution was stirred and heated at ca. 70 °C to promote polymerization. The gelatinous product was prefired at 450 °C in air for 1 h and then at 1000 °C in flowing N2 gas for 24 h. The resultant precursor powder was pressed into pellets and placed in an evacuated silica ampule together with an equal amount of FeO powder, which acts as a getter for excess 4


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oxygen. The silica ampule was heated at 1100 °C for 24 h, followed by quenching into ice water.

The X-ray diffraction (XRD) data indicated that all the as-synthesized samples were essentially phase-pure of the deoxygenated δ ≈ 0 form. Both the tetragonal a- and c-axis lengths of each sample are in agreement with those in the previous literature. The oxygen content (5+δ) values determined by iodometric titration were close to 5.00 for all the Ln-samples. Observations of scanning electron microscopy (SEM) revealed that all the samples consist of coarse particles with a similar size of 1 – 2 µm, suggesting that the influence of grain morphology is negligible. The data of sample characterization are summarized in Ref. 31.

Water Dissolution Experiments. The water dissolution experiments were carried out with a home-made reactor system schematically illustrated in Fig. S1 of the Supporting Information. A 0.1 g portion of the BaLnMn2O5.0 products was placed in a quartz tube (4mmφ inner diameter) which was built into a closed circuit of the reactor system. The sample was first heated at 500 °C in flowing 5% H2 / 95% N2 gas mixture to remove residual excess oxygen of the sample. The p(H2) / p(H2O) pressure ratio in this procedure is estimated at > 103 such that the recovery to the fully deoxygenated δ = 0 form is ensured. Then, the closed circuit was evacuated by a rotary pump, and subsequently filled with a humidified N2 gas. The initial inner pressure was set at 5.5 kPa with respect to the atmospheric pressure. The water vapor content of the injected N2 gas was controlled by bubbling into an isothermal water bath operated at a constant temperature. Gas tubes and cylinder of the closed circuit were warmed with a rubber 5



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heater to minimize the adsorption of water at the inner surface. The injected gas was circulated using an electromagnetic diaphragm pump at a flowing rate of 50 mL min-1, while the sample temperature was kept at 500 °C.

The hydrogen concentration in the closed circuit was analyzed with gas chromatography (Inficon 3000 Micro GC). In each run, approximately 2 mL of the inner gas was automatically pumped out, so that the inner pressure gradually decreased and finally reached a value equal to the atmospheric pressure. The water vapor content was measured at this point utilizing a digital humidity sensor (Sensirion SHT75): the accuracy of the humidity sensor is guaranteed only under the atmospheric pressure.

3. RESULTS AND DISCUSSION Figure 1 shows time-dependent variations of the hydrogen concentration in the closed circuit with BaLnMn2O5+δ (Ln = La, Nd, Gd, and Y) as the reductants. In these experiments, the initial water vapor content in the injected N2 gas was set at 1.2 vol %, which was achieved by bubbling into an isothermal water bath operated at 10 °C.32 It can be seen that hydrogen gas was indeed produced when the Ln = La, Nd, and Gd samples were exposed to the humidified N2 gas, while hydrogen was never detected for Ln = Y. As evidenced by these plots, the hydrogen concentration is immediately saturated within 30 min, and importantly, the saturated value depends significantly upon the Ln species. Obviously, the larger the ionic size of Ln3+ is, the higher reactivity the BaLnMn2O5.0 samples exhibit.

Since the saturated hydrogen to water vapor pressure ratio [p(H2)/p(H2O)] was found 6


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to be independent of the initial water vapor content (see the result for the Nd sample in the Supporting Information), we assume that the water dissolution reaction has reached its equilibrium in the present experimental condition. As demonstrated later, the water dissolution reaction proceeds only when the oxygen content 5+δ is lower than 5.5, implying that divalent manganese is exclusively active (and Mn3+/Mn4+ are inactive). The reaction formula is thus written in the following reversible form: 2BaLnMn2O5.0(s) + H2O(g) ↔ 2BaLnMn2O5.5(s) + H2(g). From the fact that the oxygen intake/release processes of BaLnMn2O5+δ always involve phase separation into three distinct forms with δ ≈ 0, 0.5, and 1,27 the equilibrium constant Kp may be simply represented as:

Kp ≡

2 a Ba LnMn 2O 5.5 (s) × a H 2 (g)

a

2 BaLnMn 2O5.0 (s)

× a H2O(g)

=

p(H 2 ) , p(H 2 O)

where aX denotes activity of the X phase.

Table 1 summarizes the resultant Kp values for the Ln samples, and also the standard Gibbs energy (∆G°) readily calculated as:  p (H 2 )   , ∆G o = − RT ln K p = − RT ln   p (H 2 O) 

where R is the gas constant. For the Y sample, the hydrogen amount was below the detection limit (~ 2 Pa), so that we only present the highest and lowest limits for Kp and ∆G°, respectively. The ∆G° value is significantly different among the four samples, ranging approximately 50 kJ mol-1 (~ 0.5 eV mol-1) despite their similar chemical composition and crystal structure. The plot of ∆G° vs Ln ionic radius in Fig. 2 indicates that the ∆G° value systematically decreases with the increasing ionic size. Remarkably, the sign of ∆G° changes to negative for the largest Ln = La, implying that the reaction 7



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will proceed spontaneously even in the standard condition.

We then studied the influence of temperature on the reactivity. For the La sample, the amount of hydrogen through water dissolution systematically increased as the temperature was lowered. This behavior is in contrast to the cases of Ln = Nd and Gd, where the reactivity gets reduced at lowering temperatures. Such distinct features are demonstrated by positive and negative slopes of the ln Kp vs 1/T plots as shown in Fig. 3. The ln Kp vs 1/T relation is known as the van’t Hoff equation represented as: ln K p = −

∆H o ∆S o , + RT R

where ∆H° and ∆S° are the standard enthalpy and entropy changes, respectively, both of which are assumed to be independent of temperature. The positive (negative) slope of the van’t Hoff plot gives negative (positive) sign for ∆H°. Obviously, the water dissolution reaction is exothermic only for the La sample and endothermic for the others. Taking into account the fact that this reaction does not involve overall molar changes in the gas phase, the entropy term may be small and thereby the enthalpy term is the likely source of the significantly different reactivity. Unfortunately, we were unable to accurately determine the ∆H° and ∆S° values because of the narrow temperature range to examine the van’t Hoff equation.

Our water dissolution experiments evidenced a significant role of Ln species in the redox characteristics of BaLnMn2O5+δ. Next, we theoretically deal with the thermodynamic energetics on the basis of quantum-chemical calculations.34,35 The Gibbs energy for the process concerned, defined as ∆G°(BLMO-H2O), can be related to those for the BaLnMn2O5.0-to-BaLnMn2O5.5 oxygenation (∆G°(O5.0/O5.5)) and simple 8


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thermal dissolution of water (∆G°(H2O/H2)): BaLnMn2O5.0(s) + 1/4O2(g) → BaLnMn2O5.5(s); H2O(g) → H2(g) + 1/2O2(g);

∆G°(O5.0/O5.5)

∆G°(H2O/H2)

∆G°(BLMO-H2O) = 2∆G°(O5.0/O5.5) + ∆G°(H2O/H2) The theoretically estimated ∆G°(O5.0/O5.5) values are −85.4 kJ and −123.9 kJ mol-1 at 800 K (= 527 °C) for Ln = Y and La, respectively. It should be emphasized that the value for the Ln = La compound is somewhat larger in negative sign, meaning that the process is more energetically favorable than that for the Y compound. Using a literature value of ∆G°(H2O/H2) = +203.6 kJ mol-1 at 800 K,36 the ∆G°(O5.0/O5.5) values readily lead to Gibbs energies of the overall reaction, ∆G°(BLMO-H2O) = +32.8 and −44.2 kJ mol-1 for Ln = Y and La, respectively. Thus, our theoretical work has reproduced the significantly different water dissolution reactivity of BaLnMn2O5.0, and remarkably, the sign change of ∆G°(BLMO-H2O) between the Ln = Y and La compounds (Fig. 2).

The Ln-dependent redox characteristics of BaLnMn2O5+δ may be qualitatively explained from the structural chemistry point of view. It is generally known that cations with larger ionic sizes preferably form larger coordination numbers (CN). This trend is indeed applicable to Ln3+ species in lanthanoid sesquioxides (Ln2O3), which crystallize into three distinct types; the so-called A-type for larger Ln = La-Nd, B-type for moderate Ln = Sm-Dy, and C-type for smaller Ln = Y, containing 7-, 7-/6-, and 6-coordinated Ln sites, respectively.37 Taking into account the fact that the O5.0-to-O5.5 oxygenation process is directly linked to the increased CN at the Ln site, the oxygenated O5.5 form (CN = 10) with larger Ln members is possibly more stabilized with respect to the deoxygenated O5.0 form (CN = 8), giving rise to more exothermic oxygenation 9



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enthalpy and thereby stronger reductive reactivity.

It should be noted that the above argument is practically true for the ideal A-site ordered double-perovskite structure. Meanwhile, it is widely known that Ba/Ln ions are subject to statistical distribution at the A-site in BaLnMn2O5+δ with larger Ln3+ ions, especially the largest Ln = La.38 Our BaLaMn2O5.0 sample seems to be of nearly perfect Ba/La order showing a large tetragonal anisotropy,31 being comparable to that for the fully-ordered sample reported in Ref. 38. Nevertheless, the exact value for the degree of Ba/La order is unclear for our sample, because of the difficulty in differentiating between Ba and La by XRD analysis. The influence of Ba/Ln order on the redox characteristics is open to dispute and merits further studies.

To gain deeper insight into the Ln-dependent redox characteristics of BaLnMn2O5+δ, we closely look at its electronic structure constructed by our quantum-chemical calculations. A comparison of the chemical bonding by means of Crystal Orbital Hamilton Population (COHP) analysis obtained with the LOBSTER package39-42 reveals that the manganese–oxygen interactions are very similar between the Ln = Y and La compounds (Fig. 4), even for the limiting cases BaLnMn2O5.0 and BaLnMn2O6.0 which can hardly be distinguished with the naked eye since only the Fermi level is affected by the O concentration; also, BaLnMn2O5.5 is an atomic mixture of the latter cases and lies in-between. Previous electronic-structure studies on the BaLnMn2O5+δ system evidenced the appearance of various magnetic/electronic phases depending on the oxygen content as well as the Ln species. The fully-deoxygenated O5.0 form was found to show a charge-ordered Mn2+/Mn3+ state at lower temperatures,25,26,38 10


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suggesting a decreased density of states near the Fermi level. Another detailed theoretical study on the electronic structure of BaLnMn2O5+δ was carried out by Vidya et al.43 who reported that the energy level diagram is indeed sensitive to the magnetic structure assumed in the calculation. Such low-temperature magnetic states are irrelevant, however, for the calculation of free energies at high temperatures. Hence, we described all materials with diamagnetic ground states and, as expected, the experimental thermochemistry is well described by this approach.

The significantly different thermodynamic energetics cannot be explained on the basis of their covalent bonding effects. Another view from electrostatic approaches should be meaningful. Again, employing LOBSTER and integrating local density of states for each atom, effective charges were calculated to compute Madelung energies within the VESTA software.44 It appears that the resultant Madelung energy grows by 10% more strongly from BaLaMn2O5.0 to BaLaMn2O6.0 than the Ln = Y case, indicating a primary role of electrostatics on the energetic difference. Again, we focus on the limiting cases (O5.0 vs O6.0) for which the energetic trend is more pronounced.

This finding is somewhat surprising, as the simplest ionic model assuming a formal valence for each element gives an opposite conclusion, that is, a smaller negative value of the Madelung energy for the La compound than the Y compound, because of larger cation-anion distances in the former. Roughly speaking, lanthanum is less electronegative than yttrium such that oxygenating BaLaMn2O5.0 to BaLaMn2O6.0 translates more strongly into ionic bonding. In fact, Pauling’s electronegativity (χP) is similar but surely increases with the decreasing ionic size in the order of La, Nd, Gd, 11



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and Y (χP = 1.10, 1.14, 1.20, and 1.22, respectively).32,45 It is worthwhile to point out that the basic property of the Ln2O3 series, revealed by temperature-programmed CO2 desorption,46 may also be explained by the electronegativity trend. While both the enhanced reductive reactivity of BaLnMn2O5.0 and the stronger basicity of Ln2O3 with larger Ln members are in good agreement with the smaller electronegativity of Ln, the effect is much more pronounced for the former ranging approximately 50 kJ mol-1, which is an order of magnitude larger than the CO2 desorption energy for the latter ranging ~200 K ≈ 2 kJ mol-1.46 The much stronger Ln substitution effect for BaLnMn2O5+δ is probably attributed to direct chemical interactions between Ln3+ and active oxide ions locating at the nearest site.

Finally, we show that Ln substitutions for BaLnMn2O5+δ have a great influence on the reaction kinetics of water dissolution. A 0.2 g portion of fully-deoxygenated BaLnMn2O5.0 was placed in a quartz tube and reacted with a humidified N2 gas (H2O/N2 = 2.3/97.7; flowing rate = 11.6 mL min-1) at 500 °C. The composition of the outlet gas was analyzed with the micro GC in every two minutes. The total hydrogen amount was estimated from integration of the hydrogen concentration at each measurement (Fig. 5). The hydrogen formation is again evident for the La, Nd and Gd samples, whereas hydrogen is scarcely detected for the Y sample, in good agreement with the result for the closed reaction system. The XRD patterns for the La and Nd samples completely reacted with water vapor are essentially identical to the O5.5 products obtained through partial oxygenation of the O5.0 phases in a flowing O2/N2 gas mixture. Thus, the water dissolution reaction mainly leads to a pure oxide form, and the incorporation of OH group may be ruled out. For these samples, the hydrogen amount seems to be saturated 12


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at approximately 5 mL, which corresponds to an oxygen content variation ∆δ ≈ 0.5. In fact, the δ values determined by iodometric titration are ≈ 0.5 for the samples after the water dissolution reaction, indicating that the reaction terminates at δ = 0.5 where the average manganese valence is equal to +3.

Noticeably, the hydrogen formation rate obviously depends upon the Ln species with a systematic trend with respect to the Ln3+ ionic size. The reaction rate of the La sample is respectively an order and two orders of magnitude higher than those of the Nd and Gd samples. We believe that the influence of grain morphology of the samples is negligible, because all the samples were found to consist of coarse particles with a similar size of 1−2 µm.31 Thus, the much higher reaction rate for the La sample is likely correlated with larger equilibrium constant Kp. The correlation between the reaction kinetics and thermodynamic equilibrium is widely known as “the linear free energy relation”, signifying that as the reaction becomes thermodynamically more favorable, its rate constant increases.47

4. CONCLUSIONS In summary, the present work demonstrated that BaLnMn2O5+δ with a fully-deoxygenated (δ ≈ 0) form has a capability to produce hydrogen gas through the reductive water dissolution at 500 °C. The reactivity obviously depends on the Ln species: the larger the ionic size of Ln3+, the higher reactivity the BaLnMn2O5.0 samples exhibit. The theoretical study on the electronic structure of the Ln = La and Y compounds suggested that their different reactivity may be reasonably explained on the basis of electrostatics in the crystal lattice, which emphasizes the important role of 13



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effective charges at the Ln site in the redox reactivity of this oxide.

The strongly Ln-dependent redox characteristics of BaLnMn2O5+δ seems to be attributed to the structural feature of this oxide, in which the Ln ions are neighboring the active sites for oxygen intake/release such that chemical characters of the Ln species are maximally highlighted by the direct chemical interactions between Ln3+ and active oxide ions. This finding thus provide a prospective strategy for the design of redox catalysts, which implies that redox reactivity can be controlled through isovalent substitutions neighboring the active sites, even though the redox species (transition metals) has remained untouched.

ASSOCIATED CONTENT Supporting Information (1) Details of the experimental setup, (2) water dissolution experiments with varying water vapor contents, and (3) methodology and results of quantum-chemical calculations. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENTS The present work was supported by Grant-in-Aid for Science Research (Contract No. 26288104) from Japan Society for the Promotion of Science. J.G. thanks the Fonds der Chemischen Industrie for a PhD scholarship.

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Oxygen and Cation Ordered Perovskite, Ba2Y2Mn4O11. J. Solid State Chem. 2004, 177, 2122-2128. (27) Motohashi, T.; Ueda, T.; Masubuchi, Y.; Kikkawa, S. Oxygen Intake/Release Mechanism of Double-Perovskite Type BaYMn2O5+δ (0 ≤ δ ≤ 1). J. Phys. Chem. C 2013, 117, 12560-12566. (28) Swierczek, K.; Klimkowicz, A.; Zheng, K.; Dabrowski, B. Synthesis, Crystal Structure and Electrical Properties of A-Site Cation Ordered BaErMn2O5 and BaErMn2O6. J. Solid State Chem. 2013, 203, 68-73. (29) Klimkowicz, A.; Świerczek, K.; Zheng, K.; Baranowska, M.; Takasaki, A.; Dabrowski, B. Evaluation of BaY1-xPrxMn2O5+δ Oxides for Oxygen Storage Technology. Solid State Ionics 2014, 262, 659-663. (30) Klimkowicz, A.; Świerczek, K.; Takasaki, A.; Molenda, J.; Dabrowski, B. Crystal Structure and Oxygen Storage Properties of BaLnMn2O5+δ (Ln: Pr, Nd, Sm, Gd, Dy, Er and Y) Oxides. Mater. Res. Bull. 2015, 65, 116-122. (31) Motohashi, T.; Kimura, M.; Inayoshi, T.; Ueda, T.; Masubuchi, Y.; Kikkawa, S. Redox Characteristics Variations in the Cation-Ordered Perovskite Oxides BaLnMn2O5+δ (Ln = Y, Gd, Nd, and La) and Ca2Al1-xGaxMnO5+δ (0 ≤ x ≤ 1). Dalton Trans. 2015, 44, 10746-10752. (32) CRC Handbook of Chemistry and Physics, ed. W. M. Haynes, CRC Press/Taylor and Francis, Boca Raton, Florida, 95th edn, 2014. (33) Shannon, R. D.; Prewitt, C. T. Effective Ionic Radii in Oxides and Fluorides. Acta Cryst. 1969, B25, 925-946. (34) Gilleßen, M.; Lumeij, M.; George, J.; Stoffel, R.; Motohashi, T.; Kikkawa, S.; Dronskowski, R. Oxygen-Storage Materials BaYMn2O5+δ from the Quantum-Chemical Point of View. Chem. Mater. 2012, 24, 1910-1916. (35) Gilleßen, M.; Lumeij, M.; George, J.; Stoffel, R.; Motohashi, T.; Kikkawa, S.; Dronskowski, R. Oxygen-Storage Materials BaYMn2O5+δ from the Quantum-Chemical Point of View (Addition/Correction). Chem. Mater. 2013, 25, 4460-4460. (36) Barin, I.; Knacke, O. Thermochemical Properties of Inorganic Substances, Springer-Verlag, Berlin, New York, 1973. (37) A. F. Wells, Structural Inorganic Chemistry, Oxford University Press Inc., New York, 5th edn, 1984. (38) Millange, F.; Caignaert, V.; Domengès, Raveau, B.; Suard, E. Order-Disorder Phenomena in New LaBaMn2O6-x CMR Perovskites. Crystal and Magnetic Structure. Chem. Mater. 1998, 10, 1974-1983. (39) Dronskowski, R.; Blöchl, P. E. Crystal Orbital Hamilton Populations (COHP). 17



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Energy-Resolved

Visualization

of

Chemical

Bonding

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Solids

Based

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Density-Functional Calculations. J. Phys. Chem. 1993, 97, 8617-8624. (40) Deringer, V. L.; Tchougréeff, A. L.; Dronskowski, R. Crystal Orbital Hamilton Population (COHP) Analysis As Projected from Plane-Wave Basis Sets. J. Phys. Chem. A 2011, 115, 5461-5466. (41) Maintz, S.; Deringer, V. L.; Tchougréeff, A. L.; Dronskowski, R. Analytic Projection from Plane-Wave and PAW Wavefunctions and Application to Chemical-Bonding Analysis in Solids. J. Comput. Chem. 2013, 34, 2557-2567. (42) Maintz, S.; Deringer, V. L.; Tchougréeff, A. L.; Dronskowski. R. LOBSTER: A Tool to Extract Chemical Bonding from Plane-Wave Based DFT. J. Comput. Chem. 2016, 37, 1030-1035. (43) Vidya, R.; Ravindran, P.; Kjekshus, A.; Fjellvåg, H. Spin, Charge, and Orbital Ordering in RBaMn2O5+δ (R = Y, La; 0 ≤ δ ≤ 1) and Their Dependence on Oxygen Content and Size of the R Constituent. Phys. Rev. B 2007, 76, 195114/1-195114/14. (44) Momma, K.; Izumi, F. VESTA 3 for Three-Dimensional Visualization of Crystal, Volumetric and Morphology Data. J. Appl. Crystallogr. 2011, 44, 1272-1276. (45) Pauling, L. General Chemistry, Dover Publication Inc., New York, 3rd edn, 1988. (46) Sato, S.; Takahashi, R.; Kobune, M.; Gotoh, H. Basic Properties of Rare Earth Oxides. Appl. Catal. A 2009, 356, 57-63. (47) Atkins, P.; de Paula, J. Atkins’s Physical Chemistry, Oxford University Press Inc., United Kingdom, 10th edn, 2014.

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Table 1. Saturated p(H2) and p(H2O) values for the water dissolution experiments (500 °C) with BaLnMn2O5.0 (Ln = La, Nd, Gd, and Y) as the reductants. The resultant equilibrium constant Kp and standard Gibbs energy ∆G° are also given. Ln La Nd Gd p(H2) / Pa p(H2O) / Pa Kp -1

∆G° / kJ mol

Y

863 197

72 861

22 983

+41.0

−6.7(4)

19



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Figure captions Figure 1. Time dependent variations of the concentration of hydrogen gas produced through water dissolution reaction (500 °C) by BaLnMn2O5.0 (Ln = La, Nd, Gd, and Y) as the reductants. Figure 2. Standard Gibbs energy ∆G° for the water dissolution reaction (500 °C) by BaLnMn2O5.0 (Ln = La, Nd, Gd, and Y) against the ionic radius of the Ln3+ ions. The values of the Ln3+ ionic radius (CN = 8) are referred to Ref. 33. Figure 3. Equilibrium constant Kp of the water dissolution reaction by BaLnMn2O5.0 (Ln = La, Nd, and Gd) as a function of inverse temperature (T-1). Figure 4. Crystal orbital Hamilton population (COHP) diagrams of one averaged Mn−O bond calculated for the deoxygenated BaLnMn2O5.0 and oxygenated BaLnMn2O6.0 (Ln = La and Y). Figure 5. Time variations of the total amount of hydrogen gas produced through water dissolution reaction (500 °C) by BaLnMn2O5.0 (Ln = La, Nd, Gd, and Y) as the reductants.

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1.0

0.8 H2 concentration / %

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0.6

Ln = La Ln = Nd Ln = Gd Ln = Y

0.4

0.2

0 0

100

200

300

400

500

600

Time / min

Figure 1. Motohashi et al.

21



50

-1

Y 40

∆G°(H2O/H2) / kJ mol

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

30 Gd 20 Nd 10 0

La

-10 0.100 0.105 0.110 0.115 0.120 Ionic radius (8-fold coordination) / nm


22


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10

550

T / °C 450

500

400

Ln = La Ln = Nd Ln = Gd

5

ln Kp

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

-5

-10 1.2

1.3 T

-1

1.4 -3 -1 / 10 K

1.5

1.6


23



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Figure 4. Motohashi et al. 24


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5

BaLnMn2O5+δ Sample: 0.2 g

4

H2O(g) / N 2 = 2.3 / 97.7 Flowing rate: 11.6 mL / min

H2 amount / mL

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3

Ln = La Nd

2

Gd 1 Y 0 0 10

10

1

102 103 Time / min

104

105

Figure 5. Motohashi et al. 25



Table of Contents image

5

BaLnMn 2O5+δ Sample: 0.2 g

Oxygen intake Ba Ln Mn O

4

H2O(g) / N 2 = 2.3 / 97.7 Flowing rate: 11.6 mL / min

H2 amount / mL

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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3

Ln = La Nd

2

Gd

H 2O BaLnMn2O5.0

1

H2

Y 0 0 10

BaLnMn2O5.5

26


101

102 103 Time / min

104

10 5

Significant Lanthanoid Substitution Effect on the Redox Reactivity of

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