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Oxygen storage properties of La SrFeO for chemical-looping reactions – an in-situ neutron and synchrotron X-ray study Daniel D. Taylor, Nathaniel J. Schreiber, Benjamin D. Levitas, Wenqian Xu, Pamela S. Whitfield, and Efrain E. Rodriguez Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b01274 • Publication Date (Web): 16 May 2016 Downloaded from http://pubs.acs.org on May 18, 2016

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Chemistry of Materials

Oxygen storage properties of La1-xSrxFeO3-δ for chemicallooping reactions – an in-situ neutron and synchrotron X-ray study Daniel D. Taylor,1 Nathaniel J. Schreiber,1ǁ Benjamin D. Levitas,2 Wenqian Xu,3 Pamela S. Whitfield,4 and Efrain E. Rodriguez1,2* 1

Department of Materials Science and Engineering, 2 Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742-2115, United States

3

X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, United States

4

Spallation Neutron Source, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830, United States

ABSTRACT: Oxygen storage materials (OSMs) provide lattice oxygen for a number of chemical-looping reactions including natural gas combustion and methane reforming. La1-xSrxFeO3-δ has shown promise for use as an OSM in methane reforming reactions due to its high product selectivity, fast oxide diffusion, and cycle stability. Here, we investigate the structural evolution of the series La1-xSrxFeO3-δ for x = 0, 1/3, 1/2, 2/3, and 1, using in-situ synchrotron X-ray and neutron diffraction, as it is cycled under the conditions of a chemical-looping reactor (methane and oxygen atmospheres). In the compositions x = 1/3, 1/2, 2/3, and 1, we discover an ‘envelope’, or temperature range, of oxygen storage capacity (OSC), where oxygen can easily and reversibly be inserted and removed from the OSM. Our in-situ X-ray and neutron diffraction results reveal that while samples with higher Sr contents had a higher OSC, those same samples suffered from slower reaction kinetics and some, such as the x = 1/2 and x = 2/3 compositions, had local variations in Sr content, which led to inhomogeneous regions with varying reaction rates. Therefore, we highlight the importance of in-situ diffraction studies, and propose that these measurements are required for the thorough evaluation of future candidate OSMs. We recommend La2/3Sr1/3FeO3-δ as the optimal OSM in the series because its structure remains homogenous throughout the reaction and its OSC ‘ envelope ’ is similar to that of the higher doped materials.

1. Introduction In chemical-looping reactions an oxygen storage material (OSM) is used in a closed loop to transport oxygen between the two half reactions of the oxidation of a fuel1,2 – shown in Figure 1. Such a process has the potential to efficiently capture CO2 in the case of chemical-looping combustion (CLC),3–5 or generate syngas (CO + 2H2) in the case of chemical-looping reforming (CLR).6,7 Which of these two processes proceeds is determined largely by the OSM’s product selectivity – whether it favors the complete (CLC) or partial (CLR) oxidation of a fuel. Although recent studies have linked this to oxygen availability on the OSM’s surface,8–10 it is not yet well understood how to control which reaction is favored. Many of the OSMs already studied have been the binary transition metal oxides of Mn, Fe, Co, Ni, and Cu because of their natural abundance and variable oxidation states.11,12 Their binary oxides are able to provide large amounts of oxygen to the reaction, known as oxygen storage capacity (OSC), as they can easily convert between their various structures with different oxygen contents (e.g. Fe2O3, Fe3O4, FeO, and Fe).13 However,

these materials suffer from poor cycling stability due to their tendency to sinter under the conditions of a chemical-looping reactor (700-900 °C)14,15 and poor product selectivity due to their drastically changing oxygen availability throughout the cycle.16 In contrast to the binary transition metal oxides, the perovskite La1-xSrxFeO3-δ - a mixed ionic and electronic conductor (MIEC)17,18 that has already seen use in solid oxide fuel cells (SOFCs)19–23 and heterogeneous catalysis24,25 – processes with similar mechanistic underpinnings to the chemical-looping reactions, has shown promise as a potential OSM because of its high product selectivity for chemical-looping reforming and good cycling stability.26–29 Whether used for chemicallooping or other related applications such as SOFC’s, it is important to understand how the structure and composition of perovskite oxides evolve during oxidation and reduction reactions.

1 Environment ACS Paragon Plus


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thoroughly characterize the structure and availability of lattice oxygen throughout the chemical-looping cycle. 2. Experimental 2.1 Material Synthesis. All samples were prepared using a citrate gel method to ensure similar sample morphology and particle size. For each sample, citric acid was first dissolved in stirring DI water, and appropriate amounts of La(NO3)3·6H2O, Sr(NO3)2, and Fe(NO3)3·9H2O were dissolved into the same solution, with a 1:3 molar ratio of metal ion to citric acid. 5 mL of ethylene glycol were added and the solution stirred while gently heating for several hours until a thick gel formed. The gel was heated at 300 °C overnight (~12 hours). The resulting brown powder was ground in an agate mortar and pestle before being heated at 1200 °C for 48 hours, which led to dark grey powders. Figure 1. Schematic of the chemical-looping process. By selecting the oxygen storage material, it is possible to control the reaction products giving either pure CO2 and H2O (combustion), or CO and H2 (reformation).

In-situ diffraction can help us understand how a material’s structure and composition evolve throughout the chemical-looping process.30 Previous in-situ diffraction experiments of OSMs during chemical-looping reactions have already provided some insight into how the extent of reduction in NiO affect reaction rate and product selection,31 and to observe the topochemical insertion and removal of oxygen from perovskites.32,33 However, these experiments were performed on laboratory X-ray diffractometers and were not able to collect complete diffraction patterns while the materials were cycled under the conditions of a chemical-looping reactor. Since these cycling reactions can be on the order of minutes, a faster diffraction technique, such as synchrotron X-ray diffraction, is needed to collect complete diffraction patterns while the samples are reacting.34 To better understand how to design future OSMs, we need to improve our understanding of how factors such as structure and composition evolve during these reactions, and how they affect properties such as product selectivity and OSC. The overall performance of OSMs for chemicallooping reactions seems to depend primarily on two factors: the transport of oxygen to and from the bulk, and the reaction of the fuel or oxygen on the surface of the OSM. With a focus on the bulk properties, we performed a series of in-situ diffraction experiments, both neutron and synchrotron X-ray, to observe how the structure and composition of the materials in La1-xSrxFeO3 evolve under the conditions of chemical-looping reactors. The quickness of the synchrotron X-ray data collection allows for us to observe any non-trivial structural details of the OSMs as they progress through cycling, and to determine the kinetics of the reactions by following the changes in volume. Moreover, neutron diffraction allows us to more

2.2 In-situ Neutron Powder Diffraction. Highresolution time-of-flight (TOF) neutron powder diffraction (NPD) data were collected at the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory using the BL-11A POWGEN beam line covering a dspacing range of approximately 0.45 to 5 Å (Q-range of 1.3 to 14 Å-1). Temperature was controlled between approximately 135 °C and 835 °C using the ILL vacuum furnace with a gas handling insert – temperatures reported were corrected with calibration run. Mass flow controllers were used to control the gas flow and switch between 20% O2/N2 (air) and 15% CH4/N2 (methane). The total flow rate was maintained at 500 mL/min throughout the experiment. Blank diffraction patterns of the quartz basket sample holder were collected at each temperature and subtracted from sample diffraction patterns – an example of this process is shown in ESI Figure S1. La1-xSrxFeO3-δ for x = 0, 1/2, 2/3, and 1 was heated under a methane flow with diffraction patterns collected every 100 °C from 135 °C to 835 °C. The furnace was heated at 3 °C/minute between temperatures and held at constant temperature for ~45 minutes to collect each pattern. The furnace was then cooled (~3 hours) and after reaching a temperature below 200 °C, the gas flow was switched to air and the heating process repeated with patterns collected starting at 235 °C. It is important to note that the patterns under air were collected after the patterns under methane. Therefore, all refined parameters for the samples under air represent the re-oxidized samples. Select parameters for all refined patterns, performed with TOPAS 5,35 are provided in Tables S1-S12 in the ESI. Plots of representative refinements for the x = 1/2, 2/3, and 1 samples are provided in ESI Figures S2-S4. 2.3 In-situ Synchrotron X-ray Powder Diffraction. Insitu synchrotron X-ray powder diffraction (SXRD) experiments were performed in transmission geometry on the 17-BM beamline at the Advanced Photon Source (APS) at Argonne National Laboratory. A 2D Perkin Elmer a-Si Flat Panel detector was used with an average wavelength of 0.72768 Å. Diffraction images were integrated with GSAS-II36 giving patterns with a Q-range of approximately 0.3 to 6.6 Å-1. A flow-cell/furnace sample holder37 was used to control sample temperature and


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atmosphere, and diffraction patterns were collected every 6.5 seconds. Mass flow controllers were used to switch the gas flow between 20% O2/He (air), 100% He (helium), and 15% CH4/He (methane). The overall flow rate was maintained at 15 mL/min. Samples were diluted in fused quartz powder to minimize beam absorption. La1-xSrxFeO3-δ for x = 0, 1/3, 1/2, 2/3, and 1 was first heated to the selected temperature under air. Then, as the temperature was held constant, the atmosphere was cycled between helium, methane, and air to simulate the conditions within a chemical-looping reactor. Two complete cycles (reduction then re-oxidation) were performed for each material at 500 °C, 600 °C, and 700 °C. Rietveld refinement of all diffraction patterns, performed with TOPAS 5,35 were used to track unit cell symmetry and volume throughout the experiments. Plots of representative refinements for the x = 1/3, 1/2, 2/3, and 1 samples are provided in ESI Figures S5-S8. 3. Results 3.1 Structural investigation with in-situ NPD 3.1.1 Oxygen storage in x = 0 and 1 Example TOF NPD results under chemical-looping conditions for SrFeO3-δ are presented in Figure 2. A series of phase transitions are apparent as the material was heated under flowing methane. First, the material went from pseudocubic (oxygen vacancy ordered Sr8Fe8O23; I4/mmm)38 to cubic perovskite (oxygen vacancy disordered SrFeO3-δ; Pm-3m)39 between 235 °C and 335 °C, and then transitioned to the brownmillerite structure (oxygen vacancy ordered Sr2Fe2O5-δ; Icmm)40 between 535 °C and 635 °C. Finally, the reduced material reverted to the cubic perovskite structure (oxygen vacancy disordered SrFeO3-δ; Pm-3m) above 735 °C. It is important to note that this is the first observation of the reduction of cubic SrFeO3-δ to the brownmillerite Sr2Fe2O5-δ with in-situ NPD.

Figure 3. Refined unit cell volume, normalized by formula unit, from in-situ NPD data for La1-xSrxFeO3-δ for x = 1/2, 2/3, and 1. Each sample was heated first under flowing methane (15% CH4/N2; golden circles). Then, after cooling, the material was heated under flowing air (20% O2/N2; blue triangles). Error bars for the volume were obtained from the standard uncertainty from the Rietveld refinement but are smaller than the markers. Vertical dashed lines designate locations of the phase transitions.

The refined unit cell volume and oxygen content for SrFeO3-δ are given in the bottom panels of Figures 3 and 4, respectively. For the patterns collected at 235 °C and 335 °C, the unit cell volume and oxygen content were similar under both atmospheres. As the temperature increased, the sample under methane began to reduce causing the volume to expand while the oxygen content decreased. For the patterns collected at 635 °C and 735 °C, the unit cell volume continued to increase while oxygen content remained almost constant. When the material reverted back to the cubic perovskite, at 835 °C, both the unit cell volume and oxygen content decreased. Upon reoxidation, small impurity peaks were visible (ESI Figure S9) suggesting that SrFeO3-δ is not stable for long periods of time under the reducing conditions of the fuel reactor. The refined unit cell volume for LaFeO3 is given in Figure S10, provided in the ESI. There was no significant difference between the unit cell volume at each temperature for the sample under methane and air indicating that the sample did not react with methane. Figure 2. In-situ time-of-flight NPD patterns of SrFeO3-δ collected as the material was heated, under flowing methane, from 135 °C to 835 °C.



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Figure 5. In-situ SXRD (λ = 0.72768 Å) collected for SrFeO3-δ at 700 °C as the atmosphere was cycled between helium, methane (15% CH4/He), and air (20% O2/He). Complete diffraction patterns were collected every 6.5 seconds. The crystal structures shown to the right represent the structure of the material at each step. Figure 4. Refined oxygen content from in-situ NPD data for La1-xSrxFeO3-y for x = 1/2, 2/3, and 1. Each material was heated first under flowing methane (15% CH4/N2; golden circles). After cooling, the material was heated under flowing air (20% O2/N2; blue triangles). Vertical error bars represent the standard uncertainty from the Rietveld refinement. Vertical dashed lines designate locations of the phase transitions. The Fe oxidation state, determined by charge balancing with the refined oxygen content, is provided on the right y-axis.

3.1.2 Oxygen storage in x = 2/3 The refined unit cell volume and oxygen content for La1/3Sr2/3FeO3-δ are given in the middle panels of Figures 3 and 4, respectively. The structure was refined in the rhombohedral (R-3c)41 space group for the pattern collected under methane at 135 °C. Above this, the sample under both atmospheres took on the cubic perovskite (Pm-3m) structure. As seen with SrFeO3-δ, the oxygen content was similar under both methane and air for the scans from 235 °C through 435 °C. Above 435 °C the sample began to reduce under methane. 3.1.3 Oxygen storage in x = 1/2 The refined unit cell volume and oxygen content for La1/2Sr1/2FeO3-δ are given in the top panels of Figures 3 and 4, respectively. The structures for the sample under both methane and air were refined in the rhombohedral (R-3c) space group at low temperatures. The structure transitioned to the cubic perovskite structure (Pm-3m) at 535 °C and 635 °C for the material under methane and air, respectively. As seen with La1/3Sr2/3FeO3-δ, above 435 °C the sample began to reduce under methane.

3.2 Chemical-looping cycles with in-situ SXRD 3.2.1 Reaction kinetics for x = 0 and 1 Figure 5 shows example in-situ SXRD patterns for an experiment with SrFeO3-δ. Here we observe how the diffraction patterns evolved as the atmosphere was cycled at 700 °C - the corresponding crystal structures and symmetries are shown to the right of the patterns. When the atmosphere switched to methane, the sample gradually transitioned from the cubic perovskite (Pm-3m) into the brownmillerite structure (Icmm). After approximately 13 minutes under methane followed by purging the chamber with helium, the material was reoxidized to the cubic perovskite (Pm-3m), within one scan (6.5 seconds), by switching to air. A contour plot of the SXRD patterns for two complete cycles of SrFeO3-δ at 700°C is provided in Figure 6 along with the refined unit cell volume (normalized per formula unit) for the first cycle. The sample began to reduce when the atmosphere was switched to helium. This is evident from the shift in the main perovskite peak (011) and the increasing unit cell volume. When the atmosphere was switched to methane, the reduction accelerated and SrFeO3-δ transitioned to the brownmillerite phase (Sr2Fe2O5-δ). After this transition, the reaction appeared to be essentially complete as the unit cell volume was nearly constant while the sample was held under methane. When the atmosphere was switched back to air, the sample almost immediately re-oxidized to the cubic perovskite phase. A comparison of the refined unit cell volume for SrFeO3-δ cycling at 500 °C, 600 °C, and 700 °C is provided in Figure S12 in the ESI. The sample’s ability to cycle was strongly


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Figure 6. Contour plot of in-situ SXRD for SrFeO3-δ for two complete cycles at 700 °C (left) along with the refined unit cell volume (right) normalized by formula unit. The sample first began to reduce under 100% He. When the atmosphere was switched to methane (15% CH4/He), the sample abruptly transitioned to the brownmillerite structure (Icmm) and the unit cell expanded discontinuously. The sample quickly re-oxidized when the atmosphere was switched to air (20% O2/He). Error bars

for the volume were obtained from the standard uncertainty from the Rietveld refinement but are smaller than the markers. dependent on temperature with its response becoming much faster and larger at higher temperatures. Clear transitions between the cubic perovskite and brownmillerite structures were only observed at 700 °C. Numerous cycling experiments were attempted for LaFeO3-δ at 800 °C, however, there was no response and it was determined to be essentially inert under these conditions – in agreement with the neutron diffraction results. A contour plot of one such experiment is given in ESI Figure S11. 3.2.2 Reaction kinetics for mixed A-site samples (x = 1/3, 1/2, and 2/3) Contour plots of the SXRD patterns for two complete cycles of La1/3Sr2/3FeO3-δ, La1/2Sr1/2FeO3-δ, and La2/3Sr1/3FeO3-δ at 700 °C are provided in Figures S13-S15. All three samples behaved similarly. Although there did not appear to be any phase transitions, the peaks all shifted to lower angles as the samples were reduced under methane. La1/3Sr2/3FeO3-δ and La1/2Sr1/2FeO3-δ both showed considerable peak broadening during this reduction while La2/3Sr1/3FeO3-δ did not. When the samples were re-oxidized in air, the peaks shifted back to higher angles and, in the cases of La1/3Sr2/3FeO3-δ and La1/2Sr1/2FeO3-δ, sharpened. A comparison of the refined unit cell volume for La2/3Sr1/3FeO3-δ cycling at 500 °C, 600 °C, and 700 °C is provided in Figure 7. Similar plots for La1/3Sr2/3FeO3-δ and La1/2Sr1/2FeO3-δ are provided in Figures S16 and S17, respectively. As seen with SrFeO3-δ, the response to cycling was strongly dependent on temperature. At 500 °C the volume increased slightly during the reduction step for La1/3Sr2/3FeO3-δ and La2/3Sr1/3FeO3-δ whereas there

was no response for La1/2Sr1/2FeO3-δ. At 600 °C, all three samples showed a larger volume increase with reduction. At 700 °C, each sample first began to reduce when the atmosphere switched to helium with more rapid reduction beginning when the atmosphere switched to methane. At both 600 °C and 700 °C, the samples began to re-oxidize during the air step, however, only La2/3Sr1/3FeO3-δ fully re-oxidized before the second cycle began. Table 1 provides the volume expansion with reduction for each sample. 4. Discussion 4.1 Bulk oxygen storage properties Through our in-situ NPD experiments, we probed the structural parameters of La1-xSrxFeO3-δ for x = 0, 1/2, 2/3, and 1 under the operating conditions of a chemicallooping reactor. Of these four samples, three were successfully reduced with methane: x = 1/2, 2/3, and 1. Figure 8a.) presents overlays of the refined oxygen content for each sample. As the two lines, methane and air, represent the two stages of the chemical-looping cycle, it is their difference that determines the amount of lattice oxygen available for cycling, i.e. the OSC (Figure 8b.). Table 1. Normalized volume expansion during the first reduction for La1-xSrxFeO3-δ. Temperature

x = 1/3

x = 1/2

x = 2/3

x=1

500 °C

0.04%

0.02%

0.15%

0.20%

600 °C

0.22%

0.15%

0.39%

1.2%

700 °C

0.73%

0.81%

0.98%

3.3%



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structure at 835 °C, it was able to support more oxygen vacancies under methane and the OSC increased to 2.1 wt%. This discontinuous OSC with temperature emphasizes the need to account for phase transitions within the active temperature range of an OSM. At 735 °C and 835 °C, La1/3Sr2/3FeO3-δ had an OSC of 2.3 wt%, the largest at these temperatures for the series. Interestingly, the OSC of both La1/3Sr2/3FeO3-δ and La1/2Sr1/2FeO3-δ decreased from 735 °C to 835 °C indicating that the OSC may continue to decrease for these samples if the temperature were to raise further. Since chemical-looping reactors can operate at temperatures over 900°C,12 decreasing OSC at elevated temperatures is an important consideration. A plot of the difference in oxygen content under methane and air for each of these samples is provided in Figure S18 of the ESI. The oxygen content changes were mirrored in the volume data with the unit cell expanding (Δvol) for each sample as they were reduced – shown in Figure S19 of the ESI. As large volume changes throughout the cycle can lead to particles breaking apart, thus hurting the long term stability of a material, such volume changes should be considered when designing an OSM. 4.2 Comparison to other OSMs Due to their large oxygen content and natural abundance, primary metal oxides have received the most attention for use as OSMs in chemical-looping reactions. For example, Fe2O3 contains approximately 35 wt% oxygen. While in practice this entire amount is not utilized, cycling between Fe2O3 and Fe3O4 still yields about 3.4 wt%.12 However, because of the tendency for iron oxides to sinter and agglomerate, stable supports need to be used to improve their cycling stability further lowering the wt% of available lattice oxygen. For example, an OSM composed of 60 wt% Fe2O3 and 40 wt% Al2O3 would have an OSC of approximately 2 wt%.13 La1-xSrxFeO3-δ does not need a support to function as an OSM and therefore the entire wt% determined here would be available for cycling.

Figure 7. a.) Refined unit cell volume, normalized by formula unit, from in-situ SXRD experiments for La2/3Sr1/3FeO3-δ for two complete cycles at 500 °C, 600 °C, and 700 °C. b.) The normalized unit cell volume expansion. Error bars for the volume were obtained from the standard uncertainty from the Rietveld refinement but are smaller than the markers.

Below 735 °C the OSC scaled with Sr content; SrFeO3-δ had an OSC of 2.3 wt% at 635 °C. However, its OSC decreased sharply to 1.6 wt% at 735 °C as the formation of the brownmillerite (Sr2Fe2O5-δ) under methane, at 635 °C, limited the amount of oxygen vacancies stable in the reduced sample over this temperature range. When the sample transitioned back to the cubic perovskite Table 2. Oxygen storage properties of select complex transition metal oxides and their reaction conditions Conditions

Oxygen storage capacity

Compound

Temperatures

Cycling atmospheres

wt%

mmol O2/gram

Reference

BaYMn2O5+δ

300 °C to 600 °C

5% H2 and 100% O2

3.7

1.2

38

Dy0.7Y0.3Mn O3+δ

200 °C to 400 °C

air and 100% O2

2.o

0.62

39

HoMnO3+δ

300 °C

air and 100% O2

1.7

0.54

40

Sr3Fe2O7-δ

950 °C

5% H2 and air

2.0

0.62

41

La0.5Sr0.5Co0.5Fe0.5O3-δ

400 °C to 600 °C

5% H2 and air

3.6

1.1

42

BaYCo4O7+δ

350 °C

N2 and 100% O2

3.5

1.1

43,44

2.2

0.69

45

-4

LuFe2O4+δ

200 °C to 400 °C

5% H2 and 2×10 atm pO2

Ca2(AlxMn1-x)2O5+δ

300 °C to 700 °C

100% N2 and 100% O2

3.0

0.94

46

Ce0.7Cu0.3O2+δ

700 °C

5% H2 and air

3.2

1.0

47

La1-xSrxFeO3+δ

600 °C to 835 °C

15% methane and air

2.3

0.7

This work


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Figure 8. a.) Oxygen content and b.) oxygen storage capacity from in-situ NPD experiments under methane (15% CH4/N2; circles) and air (20% O2/N2; triangles). The dashed horizontal line in the oxygen content plot represents an oxygen content of three - ideal for the perovskite structure. In a.) vertical error bars represent the standard uncertainty from the Rietveld refinement. In b.) vertical error bars represent the quadratic sum of the standard uncertainty from the Rietveld refinement for each sample under methane and air.

Furthermore, La1-xSrxFeO3-δ can itself act as a support for other OSMs.42–44 These systems form a composite material combining the high product selectivity of La1xSrxFeO3-δ with the high OSC of primary metal oxides. A number of other complex metal oxides have been explored for use as oxygen carriers. Some of these materials, along with their cycling conditions, are outlined in Table 2. Because of the need to maintain the overall structure throughout the cycle, each of these materials is only viable over a set temperature and atmospheric range. Outside of these ranges, the materials are either not reactive enough or the reactions proceed beyond the topotactic range and lead to overall cation rearrangement or sample decomposition – thus limiting their cycling stability. Furthermore, a large OSC does not necessarily coincide with favorable reaction kinetics. For example, BaYCo4O7 required 5 hours to achieve its reported OSC of 3.5 wt% at 350 °C.45,46 This material also decomposes under air above 600 °C. Such slow kinetics and poor sample stability prevent this material from being used in chemical-looping reactors.

Figure 9. Normalized unit cell volume for two complete cycles of La1-xSrxFeO3-δ for x = 1/3, 1/2, 2/3, and 1 at 700 °C – data from in-situ SXRD. SrFeO3-δ had the largest unit cell expansion when reduced (>3%) and the shortest reaction time. Furthermore, SrFeO3-δ transitioned between the perovskite and brownmillerite structures with cycling, while the others remained in the perovskite structure throughout the cycle. Only La2/3Sr1/3FeO3-δ and SrFeO3-δ completely re-oxidized before the beginning of the second reduction step. Error bars for the volume were obtained

from the standard uncertainty from the Rietveld refinement but are smaller than the markers. It is also worth noting that many of these materials have only been tested with hydrogen, nitrogen, or even air as the reducing agent. While this is a reasonable way to approximate OSC, their ability to oxidize methane is unknown and their product selectivity still needs to be assessed. 4.3 Non-reactivity of LaFeO3 LaFeO3 did not show a response in any of our experiments suggesting it will not function as an OSM under these conditions. This is surprising as the oxidation state of Fe is nominally +3 in LaFeO3 and would reduce towards +2 in the reduction step of the chemical-looping cycle. Both of these oxidation states should be readily obtainable under the conditions of a chemical-looping reactor. Furthermore, LaFeO3 has been investigated for use in chemical-looping reforming reactions and found to have good cycling stability and product selectivity.47–49 However, these studies did not determine the OSC. The lack of reactivity in LaFeO3 can be understood by considering the formation energy for an oxygen vacancy. Computational investigations suggest that the energy of



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formation for an oxygen vacancy is large in LaFeO3 approximately 4-5 eV.50 This energy penalty limits both the reactivity of the lattice oxygen and the transport of oxygen from the bulk of the material.

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still increasing at the end of the cycle indicating that these reactions were still ongoing.

Electron doping (e.g. substituting Sr for La) would increase the reactivity of LaFeO3 by lowering the formation energy for oxygen vacancies.51 This explains how substituting 33% Sr for La, as in La2/3Sr1/3FeO3-δ, led to such a large increase in reactivity as seen in our experiments. 4.4 Kinetic considerations When studying the availability of lattice oxygen, we must also consider kinetic factors. This is highlighted by comparing the NPD (Figure 8b) and SXRD (Figure S16) data for La1/3Sr2/3FeO3-δ. While the NPD results suggested it had a similar OSC at 635 °C and 735 °C, the SXRD data showed the reduction reaction rates were very different at 600 °C and 700 °C. Therefore, even though approximately the same amount of lattice oxygen is available over this temperature range, the reaction rates are likely to be too slow around 600 °C to be practical in actual chemical-looping reactions. The key difference here is in the nature of the experiments. While SXRD was able to collect diffraction patterns every 6.5 seconds during isothermal cycling, NPD experiments required the material to be slowly heated (~3 °C/min) while under a fixed atmosphere then held for ~45 minutes while the patterns were collected. Therefore, in the NPD experiments the sample was given much more time to reduce and even equilibrate, while in the SXRD experiments the material was observed in a kinetically limited state. From the in-situ SXRD data of the samples (Figures 5-7 and S12-S17) it is immediately apparent that the reoxidation reaction was much faster than the reduction step for each oxide in our studies. Furthermore, in our in-situ NPD experiments, all materials were fully reoxidized prior to the patterns collected at 235 °C. Therefore, we can conclude that reduction is much slower than oxidation in these chemical-looping processes and requires more attention when optimizing the OSM. Figure 9 shows the normalized unit cell volume for La2/3Sr1/3FeO3-δ, La1/2Sr1/2FeO3-δ, La1/3Sr2/3FeO3-δ, and SrFeO3-δ for two complete cycles at 700 °C allowing us to compare the reaction kinetics as a function of Sr content. There were two important differences between the cycling reactions of SrFeO3-δ and the others. First, SrFeO3-δ passed through a clear 1st order transition during reduction and subsequent re-oxidation while the other samples retained the perovskite structure throughout cycling. Second, both the reduction and reoxidation reactions of SrFeO3-δ appeared to be much faster than those of La2/3Sr1/3FeO3-δ, La1/2Sr1/2FeO3-δ and La1/3Sr2/3FeO3-δ. With SrFeO3-δ, the volume was nearly constant after the first few minutes of the reduction step indicating that the reduction was essentially complete, while for the La containing oxides, the volumes were

Figure 10. Overlay of SXRD data for La1-xSrxFeO3-δ for x = 1/3, 1/2, and 2/3. One pattern is plotted approximately every 2.5 minutes to cover an entire cycle. There was more pronounced peak splitting with reduction for increasing Sr content.

Furthermore, the first and second cycles of SrFeO3-δ at 700 °C exhibited some subtle yet important differences. While in the first cycle the material did not transition to the brownmillerite phase until after the atmosphere was switched to methane. In the second cycle, the material began transitioning to the brownmillerite phase during the purge with helium causing an extended period of phase coexistence. Both the cubic perovskite and brownmillerite phases were present for about 1 minute whereas in the first cycle both phases were only present for about 15 seconds. These two changes could be explained by a phase separation reaction occurring during the reduction stage of the cycle. Although no secondary phases can be seen in these patterns, the results from our in-situ NPD experiments revealed the presence of small impurity peaks in the patterns of the re-oxidized SrFeO3-δ (ESI figure S9). If SrFeO3-δ is unstable during the reduction, then any phase separation must be reversed during the re-oxidation step of the cycle to maintain its long-term performance – such a process increases the amount of time required to fully re-oxidize the material during the cycle. For La2/3Sr1/3FeO3-δ, La1/2Sr1/2FeO3-δ, and La1/3Sr2/3FeO3-δ, both the reduction and re-oxidation reactions were dependent on Sr content. While none of the three samples completely reduced by the end of reduction step, only La2/3Sr1/3FeO3-δ was able to completely reoxidize by the end of the first cycle. This can be seen by considering the slope of the volume vs. time curve for each sample, which can be treated as a reaction coordinate for a 2nd order transition. The reactions slow as they move towards completion and the slope becomes increasingly flat.


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Such a dependence of reaction rate on Sr content can also explain the appearance of peak splitting during reduction. Figure 10 shows a select region of the diffraction patterns for La2/3Sr1/3FeO3-δ, La1/2Sr1/2FeO3-δ, and La1/3Sr2/3FeO3-δ. While La2/3Sr1/3FeO3-δ showed no peak splitting, La1/2Sr1/2FeO3-δ showed peak broadening, and La1/3Sr2/3FeO3-δ showed clear shoulders forming both to higher and lower angles as the reduction proceeded. Although all three of these samples appeared to be phase pure prior to the reduction reaction, these peak effects suggest there were local variations in Sr content in La1/2Sr1/2FeO3-δ and La1/3Sr2/3FeO3-δ that created regions which reduced at slightly different rates. These regions were only visible while the reduction reaction proceeded and the absence of any peak splitting in the NPD data suggests that given enough time all regions would have reduced to the same final state, that which was observed in the NPD experiments. As all peaks in these patterns showed splitting, not just those along certain crystallographic directions, this must not have been a lowering of symmetry. Instead, these patterns could be understood as having additional cubic perovskite phases with slightly larger or smaller lattice parameters than the main sample phase. Such sample inhomogeneity has been reported in La1xSrxFeO3-δ where Sr was observed to migrate to the surface of the material.22,23 In cases of SOFC cathode materials, this reduced the material’s reactivity with air.52–54 We propose that a similar process affected the cycling reactions in the La1-xSrxFeO3-δ series and that its magnitude was dependent on the Sr content.

reaction kinetics with stability under the reducing conditions of the fuel reactor. We conclude that each material in this series has an ‘envelope’ of OSC over a certain temperature range. Within this region oxygen can easily and reversibly be inserted and removed from the material. Below this region, kinetic limitations keep the lattice oxygen inaccessible to cycling and above this region the difference in oxygen content for the material under oxidizing and reducing conditions is negligible.

ASSOCIATED CONTENT Additional diffraction data and refinement results are provided in the electronic supplementary information (ESI). This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *[email protected]

Present Addresses

ǁ N.J.S.: Materials Science Division, Argonne National Laboratory, Argonne, Illinois, 60439, USA Author Contributions All authors have given approval to the final version of the manuscript.

Funding Sources We acknowledge the Department of Commerce/NIST award 70NANB12H238 for support.

5. Conclusions In summary, La1-xSrxFeO3-δ is a promising candidate for use as an OSM for chemical-looping reforming due to its high oxygen mobility, product selectivity, and cycling stability. Therefore, it is important to characterize its oxygen storage properties under actual operating conditions to better understand the parameters which control the performance of OSMs. Here, we investigated the oxygen storage properties of La1-xSrxFeO3-δ for x = 0, 1/3, 1/2, 2/3, and 1. Designed to mimic the conditions of actual chemical-looping reactors, we performed a series of NPD and SXRD experiments to study the structure and composition of these materials in-situ. While LaFeO3 appeared to be essentially inert under the conditions of these experiments, the other samples each had a maximum OSC of approximately 2.3 wt% between 600 °C and 800 °C. Despite their similar OSCs, the reaction kinetics were strongly dependent on Sr content, and will prove important when implementing these OSMs for chemical-looping applications. Furthermore, our in-situ SXRD experiments revealed the presence of local inhomogeneities in samples with a high Sr content creating regions that reacted at different rates, and further emphasize the significance of Sr content in determining the cycling reaction energetics. We recommend La2/3Sr1/3FeO3-δ as the optimal OSM of this series as it was the only sample to combine fast

Notes Authors declare no competing financial interest.

ACKNOWLEDGMENT Use of the Advanced Photon Source at Argonne National Laboratory was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No.DE-AC02-06CH11357. The research conducted at the Spallation Neutron Source was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, US Department of Energy. The authors also thank C. K. H. Borg and A. Yakovenko for their help with SXRD data collection.

ABBREVIATIONS CLC, chemical-looping combustion; CLR, chemical-looping reforming; OSC, oxygen storage capacity; OSM, oxygen storage material; TOF, time-of-flight; NPD, neutron powder diffraction; SNS, spallation neutron source; SXRD, synchrotron X-ray diffraction; APS, advanced photon source; MIEC, mixed ionic electronic conductor; SOFC, solid oxide fuel cell.

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