Perovskite Promoted Mixed Co-Fe Oxides for Enhanced Chemical

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Perovskite Promoted Mixed Co-Fe Oxides for Enhanced Chemical Looping Air Separation Jian Dou, Emily Krzystowczyk, Amit Mishra, Xingbo Liu, and Fanxing Li ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03970 • Publication Date (Web): 01 Oct 2018 Downloaded from http://pubs.acs.org on October 6, 2018

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ACS Sustainable Chemistry & Engineering

Perovskite Promoted Mixed Co-Fe Oxides for Enhanced Chemical Looping Air Separation Jian Dou,† Emily Krzystowczyk,† Amit Mishra,† Xingbo Liu,‡ and Fanxing Li*,† †

Department of Chemical and Biomolecular Engineering, North Carolina State University, 911 Partners Way, Raleigh, North Carolina 27695-7905, United States ‡

Department of Mechanical and Aerospace Engineering, West Virginia University, 1374 Evansdale Drive, Morgantown, WV 26506, USA *Email: [email protected]

Abstract: Chemical looping air separation (CLAS) is a promising approach to produce high purity oxygen from air. Redox kinetics and oxygen carrying capacity of oxide based oxygen carrier materials play a critical role in the overall performance of CLAS. In view of the fast lattice oxygen transport property of mixed-conductive perovskite materials, composites of La0.8Sr0.2CoxFe1-xO3 (LSCF) perovskite and mixed Co-Fe oxides (CF) were investigated for chemical looping air separation. The effects of Fe and perovskite addition were systematically examined by varying Co/Fe and LSCF/CF ratios. Increase of Fe in mixed Co-Fe oxides significantly increases oxidation kinetics of LSCF-CF composites while decreases the rate of oxygen release. An optimized average redox rate was achieved by balancing the oxygen uptake (oxidation) and release (reduction) rates through tuning Co/Fe ratio, with the maximum occurring at a ratio of 9 : 1. Unpromoted Co-Fe mixed oxide exhibited a working oxygen capacity of 1.6 w.t.% at 850 oC. With the addition of 10-30 w.t.% LSCF, the oxygen capacity more than doubled to 4.1-4.2%. The enhanced oxygen storage/release is attributed to well

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dispersed Co-Fe mixed oxide within LSCF, which assists fast lattice oxygen diffusion to and from Co-Fe mixed oxide. The LSCF-CF composite exhibited satisfactory stability and activity over 50 redox cycles at 850 oC.

Keywords Chemical looping, air separation, oxygen carrier, redox reactions, mixed oxide

Introduction Concentrated oxygen from air separation finds a variety of applications ranging from the biomedical industry to metallurgy and to waste water treatment.1 More recently, concentrated oxygen was used for fossil fuel combustion, in the context of the oxy-fuel combustion process, to allow efficient CO2 capture when compared to amine-based CO2 capture from flue gases generated via conventional combustion.2 With an increasing need to reduce greenhouse gas emissions and energy usage also comes a need to develop more efficient methods for air separation.3–5 Current methods for air separation can be categorized into two classes, i.e. cryogenic and non-cryogenic methods. The first, and most widely used approach is cryogenic air separation. Cryogenic air separation units (ASUs) utilize a multi-column distillation process to produce oxygen from compressed air. It also incurs significant energy penalty as it uses electric motordriven equipment to compress the air before it is sent to the cold box and distillation columns.6 Analysis based on second law of thermodynamics indicates that cryogenic air separation utilizes 4 times the required minimum energy for air separation, rendering a second law efficiency of merely 25%.7 Despite of the large energy penalty and the high capital cost, cryogenic ASU


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remains as the most cost-effective technology for mass production of oxygen, nitrogen, and argon with high purity. Nevertheless, further research needs to be performed in order to develop simpler and more energy efficient approaches for air separation to drive down the energy penalty and capital costs.8–10 Adsorption represents a promising non-cryogenic air separation method. Both natural and synthetic sorbent materials have been explored to preferentially absorb nitrogen, facilitating O2N2 separation via cyclic adsorption-desorption schemes. Zeolites are the most commonly investigated sorbent materials due to their non-uniform electric field in the various spaces inside the material, which causes preferential adsorption of molecules.11,12 When air is blown through a bed of these zeolites, nitrogen is more readily adsorbed due to its higher quadrupole moment comparing to that of O2,13,14 and the resulting exit stream is oxygen rich. Since sorbent performance is the key to adsorption based air separation processes, significant efforts have been made in terms of sorbent development. Bed size factor (BSF), defined as the amount of sorbent required to produce oxygen at a specific rate, is often used to characterize sorbent performance. It was reported that sorbents with BSFs below 37.5 kg adsorbent/ kg of O2 per hour have the potential to be economically feasible.13,15 Further decrease of BSF can be achieved by reducing the total cycling time to seconds (i.e., ~ 10 seconds or less).16,17 Despite of the continued improvements in sorbent performance, adsorption based air separation is not expected to displace cryogenic separation especially for fossil energy and chemical related applications due to the lack of ability to achieve higher than 95% oxygen purity. An alternative non-cryogenic method uses ionic or mixed ionic-electronic conductive (MIEC) membrane for air separation.18,19 MIEC materials are composed of oxide ceramics which create high purity oxygen streams via oxygen ions transport through the crystal structures at elevated temperatures. Fabrication of airtight



MIEC membrane, however, can be costly. Its high operating temperature, complexity, and long term stability are also of concern.20–22 These challenges limit the commercial implementation of the membrane based air separation processes. Although relatively less studied, chemical looping air separation (CLAS) represents a high temperature air separation approach that has shown promise1,15,23. CLAS utilizes an oxygen carrier material, typically a metal oxide, that selectively “sorb” and “desorb” oxygen due to elimination or creation of oxygen vacancies and/or phase transitions induced by temperature or oxygen partial swings.21,23,24 CLAS is typically carried out in two steps under a cyclic redox scheme. During the first step, reduced metal oxide is exposed to a high oxygen partial pressure environment (typically from air) to scavenge oxygen from the air. Once oxidized, the oxygen carrier is subjected to low oxygen partial pressures (e.g. with steam purge or vacuum) so the metal oxide releases its lattice oxygen into the gas phase while being reduced to complete the loop23,25–28. Continuous air separation can be achieved with a multi-packed bed reactor design by switching air and sweeping gas between reactors. Process modeling performed by Moghtaderi et al. indicates that the specific power for the process was 0.08 kWh/m3 with consideration of heat losses to ambient, approximately 26% of the power requirement of an advanced cryogenic systems.21 It was therefore concluded that CLAS has the potential to replace cryogenic system as it can be suitable for mass production of oxygen with a reduced energy penalty. It is noted, however, that the above analysis did not account for practical issues such as oxygen carrier degradation and kinetic limitations and the metal oxides are considered hypothetical solids for the energy calculations21. This highlights the importance of oxygen carrier development and optimizations for CLAS. Among the various options for oxygen carrier selection, metal oxides consisted of Cu, Co, and Mn are the most thermodynamically favorable. Song et al. studied the


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effects of these oxides with additional supports of Al2O3 and SiO215. Their study suggested that CuO/SiO2 had an oxygen capacity of 4.77 w.t.% and a rate of oxygen release of 5.1 w.t.% min-1 at 950 oC. Long term studies indicated a 10.3% decrease in oxygen capacity for the 41 redox cycles tested. Cluster growth or sintering was proposed to be the primary cause of capacity loss. Further improvements in oxygen carrier stability, oxygen capacity, and oxygen release/uptake rates are highly desirable. Chemical looping oxygen uncoupling (CLOU) represents a process that is closely related to CLAS23,26,29–31. In CLOU, a carbonaceous fuel such as coal is introduced into the oxygen release step for fuel combustion and CO2 generation.31 Similar to CLAS, CLOU makes use of Cu, Co and Mn containing oxides due to their ability to change valence states under varying oxygen partial pressure environments.30,32 Among them, copper oxide is more extensively studied since the CuO/Cu2O redox pair exhibits suitable thermodynamic properties and high oxygen capacity. A challenge facing copper is its low sintering resistance especially when reduced to a metallic form. This can lead to agglomeration of the oxides.30 While inert support addition can increase sintering resistance of copper oxide based oxygen carriers, it also lowers the oxygen carrying capacity.33–35 Moreover, high operation temperature (i.e., >950 oC) is generally required to achieve significant amount of oxygen release in CLOU, which limits its potential applications.34,36 Unlike copper oxides, monometallic oxides of Mn and Co (i.e. Mn2O3 and Co3O4) are not thermodynamically suitable for CLOU or CLAS due to their tendency to spontaneously decompose，resulting from their high equilibrium partial pressure within the typical chemical looping temperature range37,38. This can make it challenging to re-oxidize the reduced monometallic oxides (i.e. Mn3O4 and CoO). A frequently adopted approach to address this



limitation is through the formation of mixed oxides containing Mn or Co. For example, Shulman et al. tested Mn based oxides with Fe, Ni, and Si supports. From their studies, Mn/Fe and Mn/Ni systems were able to be reoxidized in 10% O2 at 900 °C. However, equilibrium partial pressure of Mn/Si was still too high.37 Shafiefarhood et al. noted a marked decrease in equilibrium partial pressures of both Mn and Co oxides when Fe was added. However, the redox kinetics and oxygen carrying capacity for mixed Mn-Fe and Co-Fe oxides are limited. It would be desirable to enhance the oxygen capacity and activity for these oxygen carriers while keeping the equilibrium PO2 within a desirable range.36 Previous research has indicated that perovskites, a family of mixed oxides with a general stoichiometry of ABO3, can operate both individually as a CLOU oxide and as a support owing to their faster redox rates and lower operating temperature.39,40 For instance, CaMnO3 has been extensively studied as a standalone oxygen carrier for CLOU because of both the ease of synthesis and its fast oxygen donation kinetics. However, it suffers from long-term stability issues as it tends to undergo irreversible phase changes to spinel (CaMn2O4) and Ruddlesden– Popper (Ca2MnO4) phases

23,36,41

. The presence of sulfur in coal conversion further destabilizes

CaMnO342. To enhance the stability of CaMnO3, metal ion substituents were added to the A and/or B sites of the parent CaMnO3 perovskite.41 By mixing compatible metal cations on either site, the perovskite structure is distorted, potentially minimizing the formation of unwanted phases25,41–52 Galinsky et al. noted that the addition of Sr to CaMnO3 stabilized the perovskite structure so that irreversible phase transitions were minimized for 100 redox cycles at 850 °C.23 Similar stabilization effects were observed when Mn was partially substituted with Fe or Ti29,53. Besides stability improvements, Sr and Fe substitution also significantly enhanced the redox activities of CaMnO3 based oxygen carriers especially at lower temperatures (c.a. 650 –


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750 °C)23,29. Nevertheless, single-phase perovskites exhibit low oxygen capacity (typically 10%), the oxygen capacity calculated from TPD testing is slightly higher than that calculated from redox cycling. This is because that higher Fe loading further reduces oxygen release rate of LSCF-CF composites at 850 oC, which has less impact during TPD with heating up to 1000 oC. It is noted that a similar volcano shaped correlation between Fe content and oxygen capacity was observed using TPD analysis. The TPD peak temperature of LSCF-CF


Oxidation rate (mg O2/g⋅ min)

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Reduction rate (mg O2/g⋅ min)


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composites in Ar was measured at the maximum weight change during heating LSCF-CF composites to 1000 oC in Ar. Shown in Figure 4b and Table 1, the peak temperature of oxygen release for L0.8S0.2C1.0F0.0-C1.0F0.0 is 878 oC. The peak temperature shifted up by 11 oC to 889 oC with 10 mol% Fe, which further increased to 903-904 oC with above 20 mol% Fe loading. The increase in peak temperature of oxygen release with increasing Fe content further confirms that the addition of Fe inhibits the reduction of Fe-Co mixed oxide. This is consistent with the decrease in equilibrium PO2 as well as the trend of oxygen release rate and Fe content observed from the redox cycling tests.



a)

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Figure 4. (a) Weight loss and (b) derivative weight of LSCF-CF composites (La0.8Sr0.2Co1-xFexCo1-xFex) with Fe/(Co+Fe) ratio = 0, 0.1, 0.2, and 0.3 during temperature programmed reduction in argon.

0.8 Der. weight (mg/min)

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Figure 5. Derivative weight of La0.8Sr0.2Co0.9Fe0.1-Co0.9Fe0.1 composites with LSCF loading of 0%, 10%, 20%, 30%, 40%, 50%, 70%, and 100% during O2-TPD in argon.


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Effects of LSCF on the Redox Properties of Co-Fe oxides The contribution of perovskite and mixed Co-Fe oxides for the oxygen capacity and redox rate was further examined by varying the weight percentage of LSCF from 0 to 100%, while keeping the optimized Fe/(Co+Fe) ratio at 0.1. For the pure metal oxide sample, the oxygen release peak appeared at 920 oC (Figure 5). After incorporating 10 w.t.% of LSCF, the oxygen release peak became significantly sharper and downshifted by 18 oC to 902 oC. Further increasing the loading of LSCF, the oxygen release peak continued shifting to lower temperature. For instance, with LSCF loading in the range of 20-30%, the peak temperature of oxygen release for LSCF-CF composites occurred at around 899-902 oC. At 40-50% LSCF, the oxygen release peak further shifted to 888-889 oC. While with a pure LSCF sample, the oxygen release peak was observed at 837 oC. The peak area for the pure LSCF sample was significantly smaller due to its limited oxygen capacity. The trend of oxygen release temperature with LSCF loading clearly shows that LSCF enhances oxygen release of LSCF-CF composites. The oxygen capacity of LSCF-CF composites was examined by redox cycling under Ar and 20%O2/Ar at 850°C. For the mixed oxide at 9:1 Co:Fe ratio, the oxygen capacity was 1.6% (Figure 6a and Table 1). With the addition of 10 w.t.% LSCF in the LSCF-CF composites, the oxygen capacity more than doubled to 4.1 w.t.%. The high oxygen capacity of LSCF-CF composites remained almost unchanged when increasing the LSCF content from 10 to 40%.. Particularly, at 30 w.t.% of LSCF loading, the oxygen capacity maximized at 4.2 w.t.%. At above 30 w.t.% of LSCF loading, the oxygen capacity gradually decreased. For 50 and 70 w.t.% LSCF loading, the oxygen capacity was 3.4 and 2.2 w.t.%, respectively. For the pure LSCF sample, the oxygen capacity was merely 0.5 w.t.%. These results indicated that: i. the oxygen capacity of pure LSCF (0.5 w.t.%) is significantly lower than that for Co-Fe mixed oxide due



mainly to thermodynamic limitations; ii. the oxygen capacity of pure Co-Fe mixed oxide are significantly lower than the capacities of the composite materials, which is likely to be due to kinetic limitations as will be further elaborated. These results indicate a synergistic effect between the perovskite and mixed Co-Fe oxide phases, which corresponds well with the volcano shaped trends for the redox properties as illustrated in Figure 6. As an additional evidence, the oxygen capacity of LSCF-CF composites is significantly higher than the corresponding physical

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mixture of LSCF and CF (Figure 6a, red bars vs green dots).

Oxygen capacity (%)

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0.0 0.0

0.2 0.4 0.6 0.8 LSCF/(LSCF+CF) ratio

1.0

Figure 6. Effect of LSCF/(LSCF+CF) ratio on (a) oxygen capacity (the green dots with dashed connected line represents the calculated oxygen capacity based on physical mixture of LSCF and CF without synergistic effect), and (b) oxygen release rate, oxidation rate, and average redox rate of La0.8Sr0.2Co0.9Fe0.1-Co0.9Fe0.1 composites during chemical looping air separation at 850 oC. The synergistic effect between LSCF and CF was also evident when examining the oxygen release rate, oxidation rate, and average redox rate within each cycle. As shown in Figure 6b, the oxygen release rates for pure LSCF and CF were 0.17 and 0.62 mg O2/g·min, respectively. However, with 10 w.t.% of LSCF loading, the oxygen release rate increased to 1.74 mg O2/g·min, nearly 3 times as high as that for CF, and 10 times higher than the LSCF sample. The oxygen release rate further increased with increasing LSCF loading, with a maximum rate reached at 50


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w.t.% LSCF loading correlating to 2.67 mg of oxygen released per gram of LSCF-CF composite per minute. Similar synergic effects were observed for oxidation rates for LSCF-CF composites. The oxidation rates for LSCF and CF were 5.60 and 0.22 mg O2/g·min, respectively. With 10 w.t.% LSCF loading, the oxidation rate reached a maximum at 21.31 mg O2/g·min. Further increasing LSCF content led to a decrease in oxidation rate. At 70 w.t.%, the oxidation rate decreased to 6.64 mg O2/g·min, but is still higher than that of pure LSCF or CF samples. a) 5

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CF LSCF-CF LSCF

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Figure 7. Rates of (a) oxygen release in Ar and (b) oxidation in 20%O2/Ar at 850 oC for CF (Co0.9Fe0.1), LSCF-CF (La0.8Sr0.2Co0.9Fe0.1-Co0.9Fe0.1, LSCF/CF=3/7 weight ratio), and LSCF (La0.8Sr0.2Co0.9Fe0.1) samples. It is known that perovskites such as LSCF are mixed ionic-electronic conducting (MIEC) materials,66 which assist in fast O2- diffusion. Incorporation of LSCF within LSCF-CF composite is believed to enhance dispersion of Co-Fe mixed oxide and facilitates oxygen transport to and from CF surface, which is further supported by the time on course study of pure CF, LSCF, and LSCF-CF composite shown in Figure 7. For the pure LSCF sample, the initial oxygen release within the first minute was much faster than CF and LSCF-CF due to its mixed conductivity, which facilitated initial oxygen removal. As the activation energy of oxygen removal increased with oxygen vacancy concentration in the perovskite, the oxygen release became slower for



LSCF. At 3 min, around 0.3 w.t.% oxygen was released from LSCF. Over the next 27 min, only 0.2 w.t.% additional oxygen was removed from LSCF due to its low oxygen capacity. For Co-Fe mixed oxide, ~ 0.2 w.t.% oxygen desorbed within the first 3 min, which was 30% less than the LSCF sample. However, the oxygen release rate remained nearly unchanged for 20 min, and around 1.7 w.t.% oxygen was removed overall. By incorporating Co-Fe mixed oxide with LSCF, the oxygen release was significantly enhanced. At 3 min, ~0.6 w.t.% oxygen was released which is roughly two times higher comparing to LSCF or CF. The fast release of oxygen continued for about 10 min with 2.5 w.t.% oxygen desorbed. The total amount of oxygen released approached 4.2 w.t.% at end of 30 min oxygen release. Enhancement of oxygen uptake was similarly observed for LSCF-CF sample during the oxidation stage. a) 5

b)

53-75 um 150-250 um LSCF-CF CF(70%)

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Weight gain (%)

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3 2

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Figure 8. Rates of (a) oxygen release in Ar and (b) oxidation in 20% O2/Ar at 850 oC for LSCFCF (La0.8Sr0.2Co0.9Fe0.1-Co0.9Fe0.1, LSCF/CF = 3/7 weight ratio), LSCF-CF (La0.8Sr0.2Co0.9Fe0.1Co0.9Fe0.1, LSCF/CF = 3/7 weight ratio, CF size: 53-75 µm), LSCF-CF (La0.8Sr0.2Co0.9Fe0.1Co0.9Fe0.1, LSCF/CF = 3/7 weight ratio, CF size: 150-250 µm), and CF (Co0.9Fe0.1,) samples. The weight loss/gain for CF was based on 70% of CF in order to compare with LSCF-CF samples containg 70% CF.


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Figure 9. (a-c) SEM images and (d-h) EDX mapping of LSCF-CF composite (La0.8Sr0.2Co0.9Fe0.1-Co0.9Fe0.1, LSCF/CF = 3:7 weight ratio). Previous work on TiO2 supported iron oxide shows that TiO2 enhances oxygen anion transport instead of lowering activation energy for surface reactions with iron oxide based oxygen carriers.57,58 We propose that the synergistic effect between LSCF and Co-Fe mixed oxide was similarly resulted from facilitated O2- conduction to and from the Co-Fe oxide, which is responsible for the oxygen storage capacity, via the MIEC LSCF, which provides the O2- and electronic conduction pathways. As such, well mixing between LSCF and Co-Fe oxide at a



submicrometer level is necessary to achieve such a synergistic effect. To validate the hypothesis, two LSCF-CF composite samples were prepared using CF with different particle sizes (i.e., 5375 µm vs 150-250 µm), while keeping the LSCF loading the same at 30 w.t.%. For the LSCF-CF composite with Co-Fe mixed oxide of 150-250 µm sample, the average oxygen release rate was 0.60 mg O2/g·min for the first 15 min, and 1.2% oxygen was released at 30 min (Figure 8). By reducing the size of Co-Fe mixed oxide to 53-75 µm, the oxygen release rate was increased by 16% to 0.71 mg O2/g·min, with up to 1.4% oxygen was released at 30 min. The oxygen release was even lower for pure CF assuming 70% CF loading. This demonstrates the critical role of LSCF for promoting oxygen release from the composite oxygen carriers.

Figure 10. (a-c) SEM images and (d-f) EDX mapping of CF oxide (Co0.9Fe0.1).


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Figure 11. (a-c) SEM images and (d-h) EDX mapping of LSCF-CF (53-75 µm) composite (La0.8Sr0.2Co0.9Fe0.1-Co0.9Fe0.1, LSCF/CF = 3:7 weight ratio, CF size = 53-75 µm used for preparing LSCF-CF composite).

The morphology of LSCF-CF composite and CF metal oxide was examined by scanning electron microscopy (SEM). Shown in Figure 9a and 9b, the size of LSCF-CF particles was roughly 150250 µm, as the sample was sieved into this size range. Each particle was an assembly of small grains with a size range of 2-3 µm. Mapping of the composite particles showed that all the elements (i.e., Co, Fe, La, and Sr) were well dispersed, and no observable phase segregation



based on the current resolution (Figure 9d-9h). This clearly demonstrates that LSCF is well mixed with CF, at a sub-micro level, through the solid-state reaction synthesis route. For Co-Fe oxide, the grain size was ~ 10 µm, which was significantly larger than CF dispersed in the LSCFCF composite (Figure 10). It is likely that the presence of LSCF physically inhibits the growth of Co-Fe oxide during solid-state reaction synthesis. For LSCF-CF composites with CF size of 5375 and 150-250 µm, two types of grains were observed in a single particle cluster: small grains with size of 2-3 µm and larger grains with size of 10+ µm (Figure 11 and S4). From EDX mapping, the bigger grains were rich of Co, while the small grains were mainly La. It is consistent with the preparation procedures, as 53-75 or 150-250 µm Co-Fe oxide particles were mixed with LSCF powders (i.e., < 53 µm), followed by pelletizing and sintering. The trend of decreasing size for CF > LSCF-CF (53-75 ), LSCF-CF (150-250 ) > LSCF-CF (as-synthesized through ball milling and sintering) correlates well with the observed increase in oxygen storage and release rates for these samples. It shows that oxygen transport is the rate-limiting step, while the addition of LSCF reduces the size of CF crystals and enhances oxygen diffusion to and from CF particles. Effect of oxygen concentration and stability of LSCF-CF Effects of oxygen partial pressure on the oxygen content in the LSCF-CF composite and pure CF samples were investigated by heating the reduced sample at 850 oC in the presence of O2/Ar flow with various O2 concentration (i.e., 0-50%). Shown in Figure 12, negligible weight change was observed at below 7.5% O2 for the LSCF-CF sample. At 7.5% O2, the weight increased sharply from 47.75 to 49.92 mg, corresponding to a 4.3 w.t.% oxygen capacity. This is consistent with the capacity (i.e., 4.2 w.t.%) measured by redox cycling experiment. Additional 0.3 w.t.% oxygen was stored by increasing oxygen concentration up to 50%. For Co-Fe oxide, significant


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oxygen storage was observed at 5% O2. However, the oxidation kinetics is very slow with ~1.9 w.t.% oxygen storage within 1,300 min. To compare, 4.2 w.t.% oxygen uptake was observed within ~300 min for the LSCF-CF sample, further confirming the effectiveness of LSCF for promoting oxygen storage and redox kinetics.

Figure 12. Oxygen content for (a) LSCF-CF (La0.8Sr0.2Co0.9Fe0.1-Co0.9Fe0.1, LSCF/CF = 3:7 weight ratio) and (b) CF (Co0.9Fe0.1) samples with varying oxygen concentration from 0 to 50% at 850 oC.

Figure 13. (a) Stability of LSCF-CF composite (La0.8Sr0.2Co0.9Fe0.1-Co0.9Fe0.1, LSCF/CF = 3:7 weight ratio) during 50 redox cycles between 0.5%O2/Ar and 10%O2/Ar at 850oC. (b) XRD patterns of fresh LSCF-CF, spent LSCF-CF after 50 cycles redox test in reduced form, and spent LSCF-CF after 50 cycles redox test in oxidized form. Considering practical CLAS applications which require reasonable air conversion and purge gas consumption, the long-term stability of LSCF-CF composite for air separation was investigated



by redox cycling between 0.5%O2/Ar and 10%O2/Ar at 850 oC for 50 cycles (Figure 13). This PO2 range is significantly narrower compared to that between Ar and air. Shown in Figure 13, for the first cycle, the oxygen capacity was 3.6 w.t.%. The oxygen capacity gradually increased to 3.8 w.t.% after 10 cycles. The oxygen capacity remained almost unchanged at 3.7 w.t.% for up to 30 cycles. At the end of 50 cycles, the oxygen capacity slightly dropped to 3.5 w.t.%. These results indicate that the LSCF-CF composite is quite robust for air separation under redox cycling condition at 850 oC. XRD indicates that the structure remained a mixture of LSCF and CF, no other phase impurities were observed. Furthermore, the structure of the spent LSCF-CF after 50 cycles shows spinel oxide after oxidation in 20%O2 and cobalt/iron monoxide phase after reduction in Ar at 850 oC (Figure 13b), as anticipated. The sizes of LSCF and CF particles of spent LSCF-CF after 50 cycles calculated by Scherrer equation are 16 nm (LSCF) and 28 nm (CF) accordingly, which are similar as those of as-prepared LSCF-CF sample with 16 nm LSCF and 26 nm CF. In addition, the average redox rate corresponds to bed size factor of ~ 10.4 kg adsorbent per kg of O2 per hour, which is well below 37.5 kg adsorbent/ kg of O2 per hour to be economically feasible.13 It is, however, noted that such a bed size factor criteria is typically used for low temperature adsorption processes and may not be directly applicable to CLAS applications.

Conclusions In summary, LSCF-CF composite materials with varying Fe and LSCF contents were prepared via a solid state reaction method and investigated for chemical looping air separation. Increase in Fe content in the LSCF-CF composite significantly increases their oxygen uptake rates while decreases the oxygen release rates. The oxygen release and uptake rates were balanced through optimization of Fe content to achieve maximized redox rate and oxygen capacity at significantly


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lower temperatures (~850 °C) when compared to copper based oxygen carriers (~950 °C). The addition of LSCF into Co-Fe mixed oxides through mixing the respective precursors in the synthesis is critical to form well-dispersed LSCF with CF and reduce the particle size of CF. The well mixing between LSCF and CF inhibits the growth of CF particles and enhances lattice oxygen transport from and to CF particles for air separation application. As a result, the oxygen capacity and redox rate more than doubled when compared to either pure LSCF or CF. Furthermore, the LSCF-CF composite has an equilibrium oxygen partial pressure of around 75 mbar (~7.5%) at 850 oC, which renders this material potentially suitable for practical pressure swing air separation under a CLAS scheme. The LSCF-CF composite was stable and active under redox cycling between 0.5% and 10% O2 for 50 cycles, with oxygen carrying capacity of ~ 3.5 w.t.%. The rates for oxygen release and uptake are also potentially feasible when compared to the typical criteria used for adsorption based air separation processes.

ASSOCIATED CONTENT Supporting Information. TGA data; Equilibrium oxygen pressure; SEM images and EDX mapping.

Author Information Corresponding Author *E-mail: [email protected]

Acknowledgements This work was supported by the U.S. Department of Energy (Award No. FE0031521), National Science Foundation (CBET-1510900), and the North Carolina State University Kenan Institute



for Engineering, Technology, and Science. The authors acknowledge the use of the Analytical Instrumentation Facility (AIF) at North Carolina State University, which is supported State of North Carolina and the National Science Foundation.

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Synopsis: Significantly enhanced oxygen storage/release is achieved using perovskite promoted Co-Fe oxide for chemical-looping air separation.


Perovskite Promoted Mixed Co-Fe Oxides for Enhanced Chemical

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