Stabilization of Brownmillerite type SrCoO2.5 by a Cost-effective

Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Stabilization of Brownmillerite-Type SrCoO2.5 by a Cost-Effective Quenching Method for Oxygen-Scavenging Applications Aswathy M. Narayanan, Rajasekar Parasuraman, and Arun M. Umarji* Materials Research Centre, Indian Institute of Science, Bengaluru 560012, India

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ABSTRACT: Brownmillerite (BM)-type oxide sorbents have gained attention recently for producing oxygen-enriched streams. Herein, a cost-effective method of quenching with the use of an Al foil pad is adapted for the synthesis of brownmillerite SrCoO2.5. The oxygen storage capacity of this oxide has been investigated using a simple home-built volumetric setup. The oxygen-rich phase was formed by a pressurized heat-treatment of a BM sample. The oxygen storage capacity of the sample has been calculated from the pressure change during desorption. The effect of oxygen pressure on the amount of oxygen stored inside the sample has also been evaluated. Furthermore, selective absorption of oxygen is confirmed by performing the absorption in compressed air. The results indicate that 15.28 cm3 O2 g−1 can be stored in the sample at STP. The change in oxygen content in SrCoO2.5+δ varied reversibly up to a δ value of 0.26, which is confirmed by iodometric titration. It is shown that the new method of quenching proposed does not deteriorate the oxygen storage property of the material.

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INTRODUCTION Oxygen is one of the most widely used gases in industry, with applications in medical fields, chemical processing, combustion enhancement, wastewater treatment, and fuel cells. Conventionally, separation of oxygen from the air is carried out by cryogenic distillation of liquefied air.1 Even though it provides high-purity oxygen, this method requires a large amount of energy and thus is capital-intensive.1,2 Hence, novel costeffective methods to replace cryogenic oxygen separation are sought-after. Physisorption of gases using zeolites or carbon molecular sieves has been studied for air separations through pressure swing adsorption (PSA) or temperature swing adsorption (TSA). 1,3 However, lack of selectivity in physisorption limits the purity of the separated gas. Transition-metal oxides which respond to the change in oxygen partial pressure of the environment have an absolute selectivity toward oxygen.4−7 This is mainly attributed to oxide ion vacancies in the lattice due to the presence of mixed valence cations.8 Rather than physisorption, the mechanism involves absorption of oxygen to the oxide ion sublattice to fill the vacancies. Recently, brownmillerite-type oxides have gained attention as oxygen storage materials (OSMs) because of their fast response to a change in surrounding temperature or partial pressure of oxygen.9−13 Strontium cobaltite (SrCoO3−δ) is one such material that has potential applications in oxygen storage and separation.10 Because of the highest nonstoichiometry (δ = 0.5) and lowest molecular mass among other reported perovskites for the application of oxygen storage, SrCoO3−δ possess maximum theoretical oxygen storage capacity per gram of the material. © XXXX American Chemical Society

This material has three major crystallographic forms, such as hexagonal perovskite, brownmillerite, and cubic perovskite.14 Brownmillerite (SrCoO2.5, BM, Ima2) is a metastable phase which is obtained by quenching cubic perovskite (SrCoO3, C, Pm3̅m) from temperatures above 1173 K.14−17 This metastable phase is the oxygen-deficient vacancy ordered form of cubic SrCoO3. This order−disorder transition can be exploited for oxygen separation applications if the brownmillerite phase is stabilized at room temperature. Stabilization of this phase requires quenching of the sample to liquid nitrogen (LN2) temperature from high temperatures.14−22 Generally, liquid nitrogen is used for quenching hightemperature phases to stabilize them in the metastable form. This liquid quenching method has advantages as it provides an inert atmosphere and can be used for materials with any shape. However, at the time of contact between the liquid and the surface of the ceramic, the temperature gradient between the surface and bulk of the sample is very high. This creates thermal stress and leads to cracks in the sample. Even though the liquid nitrogen temperature is very low, the gas blanket formed around the sample at the time of contact acts as a thermal barrier and reduces the heat transfer. Hence, an attempt was made to use a milder quenchant like Al foil pad, which could sufficiently extract the heat conductively and help stabilize the metastable phase. This technique was adopted by Received: Revised: Accepted: Published: A

August 2, 2018 October 10, 2018 October 10, 2018 October 10, 2018 DOI: 10.1021/acs.iecr.8b03652 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research us earlier23 to control oxygen nonstoichiometry in the oxide. In this work, a similar approach is extended to freeze the structural transformation and stabilize the metastable brownmillerite phase of SrCoO2.5. Oxygen storage capacity measurement of a material is reported in the literature with temperature-programmed desorption (TPD)4,24,25 or thermogravimetric analysis (TGA).4,12,24,26 The intake/release is measured by heating the sample under high/low oxygen partial pressure and monitoring the respective change. Employment of high pressure increases surface chemisorption, and this may lead to an increase in oxygen absorption. Only a few authors report a pressurized condition for sorption.27 This is because it needs a specially designed system, since a commercial TGA instrument, or the quartz tube reactor cannot be used for high-pressure sorption. The rate of desorption with the help of a sweep gas is reported to be low as compared to the one in a chamber with reduced pressure.27 Keeping the abovementioned details in mind, a simple volumetric apparatus has been designed and fabricated that can be used for highpressure absorption and low-pressure desorption. It can be used to study the oxygen storage property of different materials in a wide operational pressure range. The fabrication and calibration of the setup and the oxygen desorption characteristics of the BM SrCoO2.5 samples are discussed. Selective oxygen absorption capability of SrCoO2.5 has been studied by performing oxygen storage studies in both air and oxygen.

binder and sintered at 1223 K for 4 h. The quenching of the sample was carried out using a multilayer Al foil pad preheated at about 373−473 K to stabilize the intact pellet of hightemperature brownmillerite phase. For comparison, pellets with the same heat treatment as mentioned above were quenched in air and in liquid nitrogen. Phase evolution of the sample in each synthesis step was analyzed using powder X-ray diffraction (XRD, PANalytical X’Pert-PRO, Cu Kα radiation) in the angular range 10−90° with a step size of 0.02°. Rietveld refinement of the patterns was performed using GSAS II software30 to calculate the lattice parameters. XRD data collected at a scan rate of 1.3°/min with the same step size and the angular range mentioned above was used for the refinement. Prior to the refinements, XRD data collected for a standard polycrystalline silicon sample with minimum crystallite size broadening and strain broadening was refined to obtain the instrumental parameters. This was used for the refinement of the samples’ XRD pattern to get reliable results. A pseudo-Voigt function was used to generate profile shape, and the irregular background was fitted by selecting points manually and with the Chebyshev function. The microstructure of the sample was analyzed using field emission scanning electron microscopy (FESEM, FEI Inspect F50). Chemical analysis was done with an energy dispersive analysis of X-rays (EDAX) apparatus equipped with a FESEM instrument (Oxford Instruments). The density of the sample was measured using Archimedes’s principle.31 Oxygen content in the sample was determined using iodometric titration.32 The oxygen absorption−desorption characteristics of the sample were investigated using a home-built volumetric apparatus. A detailed description of the apparatus is given below. Description of the Setup. Figure 1 shows the schematic diagram of the apparatus used for analyzing oxygen intake/

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EXPERIMENTAL SECTION Synthesis and Characterizations. The brownmillerite phase of strontium cobaltite (SrCoO2.5) was synthesized by a three-step synthesis process including solution combustion synthesis (SCS), heat treatment, and quenching. Scheme 1 Scheme 1. Scheme of Synthesis of BM SrCoO2.5

Figure 1. Schematic Diagram of the setup for oxygen gas absorption− desorption study.

shows a schematic representation of the steps involved in the synthesis. SrCO3 (SD Fine Chemicals, 99%) and CoC2O4· 2H2O28 were used as metal sources, and Oxalyl Di Hydrazide (ODH)29 was used as the fuel. Stoichiometric amounts of the precursors were dissolved in hot dilute nitric acid. The solution was introduced into a preheated furnace at 773 K. The black powder obtained from a self-propagating combustion reaction was mixed well and calcined at 1173 K for 8 h. The resulting residue was ground using a mortar and pestle and compacted to form a pellet 10 mm in diameter. Poly(vinyl alcohol) (2 wt %) in distilled water was used as binder, and compaction was carried out using cold uniaxial press by applying a pressure of 477 MPa. The pellet was soaked at 523 K for 3 h to remove the

release characteristics. The setup consists of a stainless-steel (SS) tube (outer diameter, 10 mm; length, 225 mm) closed at one end as the sample tube, four valves, and a semiconductor pressure sensor (Free scale semiconductors, MPX2100AP). The necessary connections between them were accomplished using SS (316) tubes, Swagelok joints (straight, T, and cross), and nuts. If the volume of the system is small, the increase in oxygen partial pressure hinders desorption. Hence, a dead volume was included to increase the total volume of the system. This avoids a considerable increase in oxygen partial B

DOI: 10.1021/acs.iecr.8b03652 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Figure 2. XRD Pattern of SrCoOx samples after (a) SCS, (b) calcination (Sr6Co5O15, H), (c) quenching in Al foil pad (SrCoO2.5, BM), (d) air quenching (BM and H), and (e) liquid nitrogen quenching (BM). The starred (∗) peaks correspond to Co3O4, and hash marks (#) correspond to Sr(NO3)2.

temperature in the same atmosphere. To know the effect of partial pressure of oxygen on absorption, loading was carried out with varying oxygen pressure but with the same heat treatment. Desorption. The sample chamber was evacuated, and a known amount of the absorbed sample was heated to 693 K. It was soaked at 693 K for 1 h and cooled to room temperature. During the desorption step, the pressure change in the chamber as a function of temperature and time was recorded. A heating rate of 6 K/min was used in both steps. XRD and iodometric titration were used to characterize the samples after absorption and desorption.

pressure inside the chamber. The pressure sensor has its operating range from 1 to 1000 mbar, and this limited the pressure range of operation. Thus, the valve V1 (needle valve, Parker Hannifin, 6000 psig max pressure range) was introduced, which allows higher pressures of absorption without disturbing the sensor. The valve V2 (needle valve, Hoke, 6000 psig max pressure range) was used to keep the system gastight throughout the experiment. Valves V3 and V4 (ball valves, Whitey Research Tool Co., 3000 psig max pressure range) allow the switching between gas inlet and pump. A home-built tubular resistive furnace with a temperature controller (Selec PR502-0-0-1) was used to heat the sample holder with controlled heating rates. Alumel−Chromel (Ktype) thermocouple was used to measure the temperature of the furnace. The furnace had a constant temperature region of ∼3 cm length. The sample was ensured to be in this region, and thus the maximum sample volume used was 1.5 cm3. The pressure sensor was connected to a pressure readout, and pressure change in the chamber and temperature of the chamber were monitored through a Keithley 2010 multimeter. This setup was interfaced with a computer for data logging using LabVIEW 7 (National Instruments). The pressure change as a function of temperature and time was recorded. A standard volume was used to calibrate the volume of the chamber. The pressure fluctuations within the chamber were monitored with an accuracy of ±0.1 mbar. The whole chamber was verified to be gastight throughout the experiment. The temperature profile of the furnace and the gas leakage rate is given in Figure S1. Oxygen Storage Studies. The oxygen storage measurements consisted of mainly two steps: Absorption (Loading). The BM sample was pressurized with pure oxygen/air (80% N2 and 20% O2) and heated to 673 K. It was soaked at 673 K for 1 h and then cooled to room

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RESULTS AND DISCUSSION

Synthesis and Structural Characterizations. The aqueous combustion mixture introduced into the preheated furnace has undergone auto combustion to form a black powder which contains a mixture of strontium nitrate (JCPDSICDD No. 00-025-0746) and cobalt(II, III) oxide (JCPDSICDD No. 00-042-1467) (Figure 2a). This indicates that the required phase is not achievable by a one-step solution combustion synthesis and requires further heat treatment. The powder was then calcined, and the XRD pattern of the calcined powder (Figure 2b) shows the presence of two phases: the cobalt-deficient form of strontium cobaltite (Sr6Co5O15 (H), space group R3̅33) and cobalt oxide. The peaks of cobalt oxide in the as-synthesized powder are very broad, indicating its nanocrystalline nature. The high surface area of this nano crystalline cobalt oxide helped in reducing the heat-treatment time and temperature than reported.14,15,21 The powder sample was then pressed into disc form and quenched to room temperature. Single-phase brownmillerite (BM) SrCoO2.5 which crystallizes in Ima2 space group16 was obtained with both liquid N2 and Al quenching (Figure 2c,e). However, the sample quenched in air was a mixture of C


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Industrial & Engineering Chemistry Research two phases, namely BM and H, as given in Figure 2d. Rietveld refinement of the XRD data was carried out using the space group, atomic positions, and occupancies taken from ref 16. The refined XRD pattern of BM SrCoO2.5 shows a good fit (Figure S2) between observed and calculated profiles. The obtained lattice parameter values (Table S1) are in agreement with the literature.17 Mechanism of Heat Extraction on Quenching. The choice of using an Al foil pad as a quenchant and its advantages are discussed in this section. Heat removal on quenching using liquid quenchant (e.g., LN2) involves mainly three steps.34 Once the hot ceramic comes in contact with the liquid quenchant, the surrounding liquid gets evaporated and forms a stable vapor blanket. This formed vapor blanket acts as a thermal insulator and imparts a slow cooling by conduction and radiation. This stage persists until the surface temperature reaches the boiling point of the quenchant. In the second stage, the vapor blanket is removed and the wetting of the surface takes place. The third stage is cooling of the bulk sample to liquid temperature, and this stage is mainly responsible for the deformation caused in the sample. In this stage, heat is removed by conduction and convection. As the thermal conductivity of the ceramic is very low, faster removal of heat from the surface creates thermal stress in the sample and leads to cracks. In these aforementioned steps, the first and second steps are responsible for the stabilization of the metastable phase. Hence, the ideal quenchant should have faster first and second steps and a slow third step. As mentioned, in the first step of the liquid nitrogen quenching, heat transfer is through the vapor blanket formed, and it is expected to be low. The third step in liquid nitrogen quenching is relatively faster, and poor thermal conduction in oxide ceramics creates the cracks (Figure 3a,b and inset of panel a). Quenching in air removes heat by convection, which is not sufficient to stabilize the metastable brownmillerite phase (Figure 2d). In this regard, employing the metal foil with high thermal conductivity could help with sufficient heat removal and stabilization of the phase.

Previously, our group attempted to control oxygen nonstoichiometry in oxides through Al quenching. This similar approach is extended, anticipating the arrest of structural transformation and stabilization of the metastable brownmillerite SrCoO2.5. Al has a very high thermal conductivity (κ = 237 W m−1 K−1), which could help in fast removal of heat by conduction. Hot ceramic is pressed with Al foil pad for the fast removal of heat from the sample and to suppress the transformation. As this Al foil pad removes heat by conduction, it is observed to be sufficient to stabilize the metastable phase at room temperature. To avoid fast cooling of the sample in the third step, the Al foil pad is heated to 373−473 K and was used as a quenchant. Heat dissipation is very fast, which kinetically affects the melting of the Al foil pad, leading to a contamination- and crack-free sample (Figure 3c,d and inset of panel c). Furthermore, the morphology and oxygen storage characteristics of the sample have been studied and compared with the liquid nitrogen-quenched samples. Morphology and Elemental Composition. Scanning electron micrographs of the quenched pellets are given in Figure 4a−c. Both show globular interconnected particles with

Figure 4. Scanning electron micrographs of BM SrCoO2.5 samples: (a and b) liquid N2-quenched and (c) Al-quenched. (d) EDAX spectra of Al-quenched sample showing the presence of Sr, Co, and O. The elemental composition as obtained from EDAX is also shown.

porous microstructure. The particle size of the Al-quenched sample ranges from 1 to 2 μm. Density of the samples from different batches vary between 60 and 65% of theoretical density, which supports the porous microstructure observed in SEM. But in case of liquid nitrogen-quenched sample, the particle size observed is less than 1 μm (Figure 4 (a-b)). This can be attributed to rapid cooling of the sample along with the expansion of nitrogen (liquid to gas expansion ratio of 1:694 at 293 K and atmospheric pressure35), which results the difficulty in crystallization.36,37 This is confirmed by calculating the crystallite size of both the samples using Williamson-Hall plot (Figure S3). The crystallite size obtained for liquid nitrogenand Al-quenched samples were 80 nm and 0.2 μm, respectively. Figure 4d presents the elemental composition of the Al-quenched sample obtained from EDAX analysis. It confirms the presence of strontium, cobalt and oxygen and as

Figure 3. Optical micrographs of BM SrCoO2.5 samples: (a and b) liquid N2-quenched and (c and d) Al-quenched. Micrographs in panels a and b clearly show the presence of cracks, while panels c and d are crack-free. Inset images in panels a and c show the photographs of liquid N2-quenched pellet and Al-quenched pellet, respectively. D


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phase has taken up oxygen to fill the vacancies and has changed its structure to perovskite. Iodometric titration also indicates an increase in the oxygen content of the sample. Figure 6a shows the desorption curve for the sample absorbed at 16 bar oxygen pressure. The pressure change observed was converted to O2 evolved per gram of SrCoO3 using the ideal gas equation. The temperature versus pressure change (as recorded) is given in Figure S5. From the curve, it is seen that the pressure of the chamber remains constant up to 573 K; beyond that, the pressure slowly increases. When it is held isothermal at 673 K, pressure increases with time, reaches the maximum, and remains unchanged, which denotes the saturation in desorption. This total increase in pressure can be recognized as the release of oxygen from the lattice of SrCoOx. Figure 6b shows the reversible structural changes of SrCoOx during oxygen storage. To validate the usage of the sample as a sponge to absorb oxygen selectively from the air, absorption was carried out by pressurizing the sample with air. Absorption was carried out at 673 K after pressurizing with 16 bar air, and desorption was followed as explained in the Experimental Section. Figure 7

expected for the BM phase, Sr: Co ratio of 1:0.93 was observed. Iodometric titration of the Al and liquid nitrogenquenched samples gave oxygen stoichiometry as 2.43 ± 0.01 and 2.42 ± 0.01 respectively. This is in agreement with an earlier report where BM phase is observed even with an oxygen stoichiometry less than 2.5.15 The correlation proposed by Takeda et al.15 is observed here when oxygen deficiency is compared with the lattice parameter values specified in Table S1. The average oxidation state of cobalt was calculated to be 2.86 (14% Co2+ and 86% Co3+) and 2.84 (16% Co2+ and 84% Co3+ ) for Al- and liquid nitrogen-quenched samples, respectively. Further, the oxygen storage characteristics of the samples were studied using the home-built setup. Oxygen Storage Studies. Oxygen absorption was carried out by pressurizing Al-quenched BM samples with 16 bar oxygen as described in Experimental Section. Figure 5 shows

Figure 5. Indexed XRD pattern of the absorbed BM sample. The inset shows the magnified (200) reflection.

the indexed XRD pattern of the sample after absorption. The absorbed sample can be indexed based on the Pm3̅m space group,21 but an unusual broadening was observed for the (200) peak (inset, Figure 5). Detailed analysis of the XRD data was carried out by Rietveld refinement. The observed and calculated patterns were not matching when refined in the Pm3̅m space group. Especially, the (200) reflection is split into two peaks. This may be due to a tetragonal distortion of the unit cell, and thus refinements were carried out with a unit cell based on the P4/mmm space group.38 Details of the refinement are given in Figure S4. Table S1 clearly shows a reduction in cell volume when the oxygen content is high. This change in XRD pattern confirms that the oxygen vacancy ordered BM

Figure 7. Comparison of desorption curves of the sample absorbed in oxygen and air.

shows the comparison of the desorption curves of the sample loaded with oxygen and air. As expected, the desorption curve of the sample absorbed in air also has shown the pressure change as a function of temperature. However, the pressure change observed was less than that of the sample absorbed in oxygen. This may be due to the lower partial pressure of oxygen in air than in pure oxygen. Figure 8a−e shows a comparison of XRD patterns of the as-synthesized sample (a), after loading in O2 and air (b and c), and after desorption (d

Figure 6. (a) Desorption curve of the sample absorbed in oxygen. The right side axis shows the temperature profile. (b) Reversible structural changes in SrCoOx during oxygen storage. The crystal structures were drawn using VESTA software.39 E


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stoichiometry in the sample. Many reports suggest that there is difficulty in preparing a fully stoichiometric SrCoO3 and that the perovskite phase could be stabilized with an oxygen content as low as 2.68, which supports this result.15,40−42 The average oxidation state of cobalt in the samples absorbed in oxygen and air were calculated to be 3.34 and 3.26, respectively. This corresponds to 34% Co4+ and 66% of Co3+ for the sample absorbed in oxygen. For the sample absorbed in the air, the equivalent percentages are 26% (Co4+) and 74% (Co3+). To understand the effect of quenching method on oxygen storage properties, oxygen absorption studies were conducted with the sample quenched in liquid nitrogen. The sample was pressurized with 16 bar air, and absorption was carried out as for the Al-quenched sample. A comparison of the XRD pattern of the absorbed samples are given in Figure 9a,b. After absorption, this sample was also a mixture of perovskite (major) and brownmillerite (minor) phases. A comparison of the desorption curves is given in Figure 9c. There is a slight difference in the oxygen storage capacity of the BM sample obtained by quenching in liquid N2 and Al. Oxygen storage capacity obtained for the liquid nitrogen-quenched sample was 13.20 cm3 of O2 g−1 of the sample and the corresponding δ = 0.22. This is slightly less than that observed for Al-quenched sample and can be attributed to the smaller particle size of the sample. The oxygen intake/release depends on both the surface area and oxide ion diffusivity of the sample. The smaller size may be affecting the ionic diffusivity because of a higher grain boundary scattering of oxide ions. The desorption curves indicate that O2 desorbs out easily from the liquid nitrogenquenched sample also because of its smaller particle size. The effect of partial pressure of oxygen on oxygen storage properties of BM SrCoO2.5 was evaluated by performing loading with varying oxygen pressures. The desorption curves of the BM sample absorbed at different partial pressures of oxygen are depicted in Figure 10a. As shown in Figure 10b, the amount of oxygen stored in the sample increases as the partial pressure of oxygen increases. Most interestingly, apart from partial pressure dependence, a total pressure dependence of oxygen storage was also found. Although the partial pressure of oxygen is almost the same in both 2 bar O2 and 10 bar air (approximately 2 bar), there is a higher absorption in the case of the sample treated in 10 bar air than in 2 bar O2. As already mentioned in the Introduction, higher pressure helps in a higher surface chemisorption, and this leads to a better oxygen absorption. Other gases do not affect the oxygen absorption as they may not be chemisorbed on the surface. In summary, usage of SrCoO2.5 as an oxygen storage material was validated by the desorption characteristics of the sample absorbed in oxygen and air atmospheres. Furthermore, a simple experimental setup has been fabricated to study the oxygen storage capacity of different materials in a wide operating pressure range. This setup could be used to study the oxygen storage capacity of any sample, and for the SrCoO2.5 (BM) phase, it was found to be 15.28 cm3 of O2 g−1 of the

Figure 8. XRD patterns of (a) BM as-synthesized sample and sample absorbed in (b) 16 bar O2 and (c) 16 bar air. The asterisk (∗) shows BM SrCoO2.5 impurity. XRD patterns of desorbed sample (d) absorbed in O2 and (e) absorbed in the air are also shown.

and e). The sample absorbed with 16 bar O2 was single-phase, while the one absorbed at 16 bar air contains the mixture of BM and perovskite phases. Rietveld refinement was performed including both phases, and the refinement results are given in Figure S4. There is a change in the desorption curves in the range of 525−693 K, showing that O2 desorbs out easily from the sample absorbed with oxygen. This may be because the driving force for desorption, i.e., the oxygen chemical potential gradient, is higher in the case of the sample absorbed in oxygen. Desorbed samples’ XRD pattern shows both the samples have converted completely back to BM (Figure 8d,e). Furthermore, the reusability test of the sample was assessed by performing six absorption−desorption cycles. Figure S6a shows the desorption curve for the sample during the first and sixth cycles, where only a slight decrease in the desorption is observed. The XRD patterns of the desorbed samples (Figure S6b) do not show any detectable impurities. Table 1 presents the pressure change, oxygen stored (cm3 of O2) per gram of the sample with the delta values calculated, and that obtained from iodometric titration. The change in pressure for the same amount of sample (∼803 mg) absorbed in oxygen and air was observed to be 47.2 mbar and 40.9 mbar, respectively. The oxygen stored in the sample after absorption in the oxygen atmosphere was 15.28 cm3 of O2 g−1 of the sample. This is much lower compared to the theoretical value of 28.78 cm3 of O2 g−1. This may be because the sample has not been converted to SrCoO3 and there is still non-

Table 1. Pressure Change, Oxygen Stored, and δ Value from Desorption Experiments and Iodometric Titration from desorption experiment

from iodometric titration −1

sample name

pressure change (m bar)

O2 evolved (cm g )

δ

composition after absorption

composition after desorption

δ

loaded in O2 loaded in air

47.2 40.9

15.28 13.20

0.26 0.22

SrCoO2.67±0.02 SrCoO2.63±0.01

SrCoO2.40±0.01 SrCoO2.41±0.01

0.27 0.22

3

F


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Figure 9. XRD patterns of samples absorbed in 16 bar air: (a) Al-quenched and (b) liquid N2-quenched. The asterisk (∗) shows BM SrCoO2.5 impurity. (c) Desorption curves of Al- and liquid N2-quenched samples absorbed in 16 bar air.

Figure 10. Desorption behavior of the Al-quenched BM sample absorbed at different oxygen pressures: (a) desorption curve and (b) oxygen storage capacity as a function of absorption pressure.

sample. The effect of quenching method and oxygen pressure on oxygen storage capacity of BM SrCoO2.5 were also revealed.

Temperature profile and leak rate of the setup, XRD refined patterns and results of BM SrCoO2.5 and absorbed samples, Williamson−Hall plot for Al and liquid nitrogen-quenched samples, as-recorded temperature versus pressure change desorption curve for Alquenched sample absorbed in air and oxygen, and the results of reusability tests (PDF)

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CONCLUSIONS An economically viable method of obtaining the industrially important brownmillerite SrCoO2.5 phase is reported. A preheated aluminum foil pad was used rather than conventional liquid nitrogen quenching. This Al foil pad quenching could be easily used to stabilize any high-temperature metastable phases. Furthermore, an experimental setup has been fabricated to study oxygen storage capacity of the stabilized BM strontium cobaltite. The amount of oxygen that can be stored in this material was found to be 15.28 cm3 of O2 g−1 of the sample at STP. The change in oxygen content in SrCoO2.5+δ varied reversibly up to a δ value of 0.26, which is confirmed by iodometric titration. A slight difference in oxygen uptake value is seen when compressed air is used. It is also shown that Al quenching does not deteriorate the oxygen storage property of BM SrCoO2.5. Although the setup is demonstrated by studying oxygen absorption characteristics of a specific material, this setup can be extended to study gas intake/release capacities of other materials as well.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Arun M. Umarji: 0000-0002-3167-7060 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Authors acknowledge Supercomputer Education and Research Centre (SERC), IISc for access to LabVIEW 7 program. A.M.N. acknowledges Council of Scientific and Industrial Research (CSIR), Government of India for the financial support in the form of Junior and Senior Research Fellowship. R.P. acknowledges University Grant Commission (UGC), Government of India for the financial support in the form of Junior and Senior Research Fellowship.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.8b03652. G


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(21) Bezdicka, P.; Wattiaux, A.; Grenier, J.; Pouchard, M.; Hagenmuller, P. Preparation and characterization of fully stoichiometric SrCoO3 by electrochemical oxidation. Z. Anorg. Allg. Chem. 1993, 619, 7−12. (22) Le Toquin, R.; Paulus, V.; Cousson, A.; Prestipino, C.; Lamberti, C. Time-resolved in situ studies of oxygen intercalation into SrCoO2.5, performed by neutron diffraction and X-ray absorption spectroscopy. J. Am. Chem. Soc. 2006, 128, 13161−13174. (23) Dasgupta, T.; Sumithra, S.; Umarji, A. A novel method to control oxygen stoichiometry and thermoelectric properties in (RE)BaCo2O5+δ. Bull. Mater. Sci. 2008, 31, 859−862. (24) Kusaba, H.; Sakai, G.; Shimanoe, K.; Miura, N.; Yamazoe, N. Oxygen-sorptive and desorptive properties of perovskite-related oxides under temperature-swing conditions for oxygen enrichment. Solid State Ionics 2002, 152-153, 689−694. (25) Guntuka, S.; Farooq, S.; Rajendran, A. A-and B-Site substituted lanthanum cobaltite perovskite as high temperature oxygen sorbent. 2. Column dynamics study. Ind. Eng. Chem. Res. 2008, 47, 163−170. (26) Räsänen, S.; Parkkima, O.; Rautama, E.-L.; Yamauchi, H.; Karppinen, M. Ga-for-Co substitution in YBaCo4O7+δ:Effect on hightemperature stability and oxygen-storage capacity. Solid State Ionics 2012, 208, 31−l5. (27) Hao, P.; Shi, Y.; Li, S.; Liang, S. Oxygen sorption/desorption kinetics of SrCo0.8Fe0.2O3−δ perovskite adsorbent for high temperature air separation. Adsorption 2018, 24, 65−71. (28) Wisgerhof, E.; Geus, J. W. Morphology and X-ray diffraction pattern of dihydrates of cobalt (II) oxalate. Mater. Res. Bull. 1984, 19, 1591−1598. (29) Patil, K. C.; Hegde, M. S.; Rattan, T.; Aruna, S. T. Chemistry of Nanocrystalline Oxide Materials: Combustion Synthesis, Properties and Applications; World Scientific, 2008. (30) Toby, B. H.; Von Dreele, R. B. GSAS-II: the genesis of a modern open-source all purpose crystallography software package. J. Appl. Crystallogr. 2013, 46, 544−549. (31) Pennings, E.; Grellner, W. Precise nondestructive determination of the density of porous ceramics. J. Am. Ceram. Soc. 1989, 72, 1268−1270. (32) Karppinen, M.; Matvejeff, M.; Salomäki, K.; Yamauchi, H. Oxygen content analysis of functional perovskite-derived cobalt oxides. J. Mater. Chem. 2002, 12, 1761−1764. (33) Sun, J.; Li, G.; Li, Z.; You, L.; Lin, J. Crystal growth and structure determination of oxygen-deficient Sr6Co5O15. Inorg. Chem. 2006, 45, 8394−8402. (34) Avner, S. H. Introduction to Physical Metallurgy; McGraw-Hill, 1964. (35) Cha, M.; Yin, X.; Kneafsey, T.; Johanson, B.; Alqahtani, N.; Miskimins, J.; Patter-son, T.; Wu, Y.-S. Cryogenic fracturing for reservoir stimulation-Laboratory studies. J. Pet. Sci. Eng. 2014, 124, 436−450. (36) Zhang, B.; Zheng, J.-c. Synthesis of Li3V2(PO4)3/C with high tap-density and high- rate performance by spray drying and liquid nitrogen quenching method. Electrochim. Acta 2012, 67, 55−61. (37) Wu, L.; Zhong, S.-K.; Liu, J.-Q.; Lv, F.; Wan, K. High tapdensity and high performance LiFePO 4/C cathode material synthesized by the combined sol spray-drying and liquid nitrogen quenching method. Mater. Lett. 2012, 89, 32−35. (38) James, M.; Wallwork, K.; Withers, R.; Goossens, D.; Wilson, K.; Horvat, J.; Wang, X.; Colella, M. Structure and magnetism in the oxygen-deficient perovskites Ce1−xSrxCoO3−δ (x ≥ 0.90). Mater. Res. Bull. 2005, 40, 1415−1431. (39) Momma, K.; Izumi, F. VESTA 3 for three-dimensional visualization of crystal, volumetric and morphology data. J. Appl. Crystallogr. 2011, 44, 1272−1276. (40) Taguchi, H.; Shimada, M.; Koizumi, M. The effect of oxygen vacancy on the magnetic properties in the system SrCoO3−δ (0< δ < 0.5). J. Solid State Chem. 1979, 29, 221−225. (41) Watanabe, H.; Yamaguchi, Y.; Oda, H.; Takei, H. Magnetic and neutron diffraction study of SrCoO3−x. J. Magn. Magn. Mater. 1980, 15-18, 521−522.

REFERENCES

(1) Smith, A.; Klosek, J. A review of air separation technologies and their integration with energy conversion processes. Fuel Process. Technol. 2001, 70, 115−134. (2) Hashim, S.; Mohamed, A.; Bhatia, S. Oxygen separation from air using ceramic-based membrane technology for sustainable fuel production and power generation. Renewable Sustainable Energy Rev. 2011, 15, 1284−1293. (3) Santos, J.; Cruz, P.; Regala, T.; Magalhaes, F.; Mendes, A. Highpurity oxygen production by pressure swing adsorption. Ind. Eng. Chem. Res. 2007, 46, 591−599. (4) Demizu, A.; Beppu, K.; Hosokawa, S.; Kato, K.; Asakura, H.; Teramura, K.; Tanaka, T. Oxygen Storage Property and Chemical Stability of SrFe1−xTixO3−δ with Robust Perovskite Structure. J. Phys. Chem. C 2017, 121, 19358−19364. (5) Ikeda, H.; Tsuchida, A.; Morita, J.; Miura, N. SrCoxFe1−xO3−δ Oxygen Sorbent Usable for High-Temperature Pressure-Swing Adsorption Process Operating at Approximately 300deg C. Ind. Eng. Chem. Res. 2016, 55, 6501−6505. (6) Sugiura, M. Oxygen storage materials for automotive catalysts: ceria-zirconia solid solutions. Catal. Surv. Asia 2003, 7, 77−87. (7) Lai, K.-Y.; Manthiram, A. Phase Stability, Oxygen-Storage Capability, and Electrocatalytic Activity in Solid Oxide Fuel Cells of (Y, In, Ca)BaCo4−y GayO7+δ. Chem. Mater. 2016, 28, 9077−9087. (8) Pena, M.; Fierro, J. Chemical structures and performance of perovskite oxides. Chem. Rev. 2001, 101, 1981−2018. (9) Yin, Q.; Lin, Y. Effect of dopant addition on oxygen sorption properties of La-Sr-Co-Fe-O perovskite type oxide. Adsorption 2006, 12, 329−338. (10) Jeen, H.; Choi, W. S.; Biegalski, M. D.; Folkman, C. M.; Tung, I.-C.; Fong, D. D.; Freeland, J. W.; Shin, D.; Ohta, H.; Chisholm, M. F.; Lee, H. N. Reversible redox reactions in an epitaxially stabilized SrCoOx oxygen sponge. Nat. Mater. 2013, 12, 1057. (11) Nomura, T.; Zhu, C.; Sheng, N.; Murai, R.; Akiyama, T. Solution combustion synthesis of Brownmillerite-type Ca2AlMnO5 as an oxygen storage material. J. Alloys Compd. 2015, 646, 900−905. (12) Motohashi, T.; Hirano, Y.; Masubuchi, Y.; Oshima, K.; Setoyama, T.; Kikkawa, S. Oxygen storage capability of brownmillerite-type Ca2AlMnO5+δ and its application to oxygen enrichment. Chem. Mater. 2013, 25, 372−377. (13) Wu, H.-C.; Lin, Y. Effects of Oxygen Vacancy Order-Disorder Phase Transition on Air Separation by Perovskite Sorbents. Ind. Eng. Chem. Res. 2017, 56, 6057−6064. (14) Jankovskỳ, O.; Sedmidubskỳ, D.; Vítek, J.; Š imek, P.; Sofer, Z. Phase diagram of the Sr-Co-O system. J. Eur. Ceram. Soc. 2015, 35, 935−940. (15) Takeda, Y.; Kanno, R.; Takada, T.; Yamamoto, O.; Takano, M.; Bando, Y. Phase relation and oxygen-non-stoichiometry of Perovskitelike Compound SrCoOx (2.29 < x > 2.80). Z. Anorg. Allg. Chem. 1986, 540, 259−270. (16) Muñoz, A.; de La Calle, C.; Alonso, J.; Botta, P.; Pardo, V.; Baldomir, D.; Rivas, J. Crystallographic and magnetic structure of SrCoO2.5 brownmillerite: Neutron study coupled with band-structure calculations. Phys. Rev. B: Condens. Matter Mater. Phys. 2008, 78, 054404. (17) De la Calle, C.; Aguadero, A.; Alonso, J.; Fernández-Díaz, M. Correlation between reconstructive phase transitions and transport properties from SrCoO2.5 brownmillerite: A neutron diffraction study. Solid State Sci. 2008, 10, 1924−1935. (18) Nakamura, S.; Tadokoro, Y.; Iida, S. Several New Data on the Electrical Conduction and Magnetism of Highly 0xidized SrCoOx (2.46 ≤ x ≤ 2.80). J. Phys. IV 1997, 07, C1-367. (19) Long, Y.; Kaneko, Y.; Ishiwata, S.; Taguchi, Y.; Tokura, Y. Synthesis of cubic SrCoO3 single crystal and its anisotropic magnetic and transport properties. J. Phys.: Condens. Matter 2011, 23, 245601. (20) Nemudry, A.; Rudolf, P.; Schöllhorn, R. Topotactic electrochemical redox reactions of the defect perovskite SrCoO2.5+x. Chem. Mater. 1996, 8, 2232−2238. H


Article

Industrial & Engineering Chemistry Research (42) Nakatsuka, A.; Yoshiasa, A.; Nakayama, N.; Mizota, T.; Takei, H. Oxygen-deficient strontium cobaltate, SrCo02.64. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 2004, 60, i59−i60.

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