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A mixed spinel oxygen carrier with both high reduction degree and redox stability for chemical looping H2 production
international journal of hydrogen energy xxx (xxxx) xxx
Available online at www.sciencedirect.com
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A mixed spinel oxygen carrier with both high reduction degree and redox stability for chemical looping H2 production Dewang Zeng 1, Dongxu Cui 1, Yulin Lv, Yu Qiu, Min Li, Shuai Zhang, Rui Xiao* Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy and Environment, Southeast University, Nanjing, 210096, PR China
highlights A mixed spinel Co0$5Fe0$5(FeAl)Ox material is prepared as an oxygen carrier. The material performs a stable and high hydrogen yield of 8.53 mmol/g. The reversible exsolution and dissolution of active phase improve redox stability.
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abstract
Article history:
Developing the oxygen carrier of high reduction degree and redox stability has key
Received 17 September 2019
importance to the chemical looping hydrogen production. However, high reduction degree
Received in revised form
of oxygen carrier often leads to severe sintering and poor redox stability. Therefore, the
4 November 2019
trade-off between such two aspects of it must be considered. In this paper, a Co0$5Fe0$5(-
Accepted 6 November 2019
FeAl)Ox material was reported as an oxygen carrier and an investigation on the perfor-
Available online xxx
mance of its chemical looping hydrogen production was conducted. It exhibited a stable and high hydrogen yield of 8.53 mmol/g during 20 cycles even at the reduction degree of
Keywords:
0.75. Upon comparison, it was found that the obvious deactivation occurred in Fe2O3 ma-
Hydrogen production
terial when its reduction degree was higher than 0.25. Upon various characterization
Chemical looping
techniques, we found that the high performance of such material was rightly because of
Oxygen carrier
the reversible exsolution and dissolution of its active compositions from and into the
Spinel material
parent spinel support. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen is an ideal clean energy for carbon free economy because it has a high specific energy density and water is the only production of its combustion [1,2]. The steam methane
reforming technology, being the predominant way for producing hydrogen nowadays, has still certain apparent disadvantages such as need of multiple reactors, high energy penalty for hydrogen purification, and coke deposition resulting in catalysts deactivation [3e5]. Therefore a
* Corresponding author. E-mail addresses: [email protected] (D. Zeng), [email protected] (R. Xiao). 1 These people contributed equally to this work. https://doi.org/10.1016/j.ijhydene.2019.11.062 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Zeng D et al., A mixed spinel oxygen carrier with both high reduction degree and redox stability for chemical looping H2 production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.062
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technology capable of efficiently producing pure hydrogen is highly desirable [6]. Chemical looping hydrogen production (CLH) has attracted wide attention because it produces pure hydrogen without additional energy consumption [7]. Different from traditional steam methane reforming process, the transition metal oxides are employed as oxygen transport carriers for cycling between fuel and steam reactors. The oxygen carrier is firstly reduced in the fuel reactor whilst the fuel is oxidized, then the oxygen-depleted oxygen carrier is re-oxidized in the steam reactor and the hydrogen is released in this process simultaneously [8,9]. After the water condenses, pure hydrogen can be obtained without energy consumption of separation and purification [10,11]. The oxygen carrier plays a very important role in the CLH process [12,13]. In the past several years, many kinds of oxygen carriers, mainly comprising Ni [5,14,15], Mn- [16,17], Co[18e20], and Fe- [9,21e23] oxides, had been used. Due to the high oxygen capacity, compatibility to environment, good mechanical strength, and low cost, iron oxides are widely used as oxygen carriers [9,24,25]. In the CLH of iron oxides, two major stoichiometric redox pathways have been employed and the specific one depends on different reduction degrees (X is defined as the ratio of practical weight loss and the value in terms of the complete reduction). One of which is the partial reduction to wustite (reduction: Fe3O4þCO/3FeO þ CO2, reoxidation: 3FeO þ H2O/Fe3O4þH2), and the other one of which is complete reduction to metallic iron (reduction: Fe3O4þ4CO/3Feþ4CO2, re-oxidation: 3Feþ4H2O/Fe3O4þ4H2) [9,26]. Theoretically, the hydrogen yield of the latter pathway is 4 times higher than that of the former one, signifying that for the practical chemical looping system, the material usage or solid flux would be decreased to a quarter. Unfortunately, it has reported that the high reduction degree of oxygen carrier has negative effect on its redox stability [27,28]. Bohn et al. [29,30] explored the relationship between redox stability and reduction degree of the oxygen carrier; in which the oxygen carrier rapidly sintered at first few redox cycles when the oxygen carrier was reduced from Fe2O3 to Fe (X ¼ 1). As a contrast, when the oxygen carrier is operated from Fe2O3 to FeO (X ¼ 0.33), an obvious stability improvement can be observed. Qiu et al. [31] and Diego et al. [32] found that this phenomenon was resulted from the high lattice stress generated by the phase transition. The density of Fe2O3, Fe3O4, and FeO is respectively 5.26 g/cm3, 5.18 g/cm3, and 5.75 g/cm3. The Fe density is 7.87 g/cm3 which is about 1.5 times than that of Fe2O3. When the phase transition is between Fe2O3 and Fe, the large density change of oxygen carrier would generate the stronger lattice stress to cause structural damage and severe sintering [33]. Some researchers proposed to use promoter materials to solve above issues, such as ZrO2 [3,34], MgO [35,36], SiO2 [19,37,38], CeO2 [39,40] and Al2O3 [41,42]. These promoter materials, as oxygen conduction channels or physical barriers, keep reaction activity and prevent iron oxides from sintering [43]. However they sacrifices parts of the loadings of the active phase and subsequently hydrogen yield on one hand, and they are not always inert during redox cycles on the other hand. Under a high operating temperature 750e950 C, it is inevitable that phase interaction between the active and inert compositions in them leads to inert spinel
formation [7]. This formation further hinders the final hydrogen production process. Here in this paper a Co0$5Fe0$5(FeAl)Ox spinel material was employed as an oxygen carrier for chemical looping hydrogen production and the redox stability was tested in varied reduction degrees (X ¼ 0.25, 0.5, 0.75 and 1), the results showed that the oxygen carrier exhibited good redox stability even at the reduction degree of 0.75.
Experiments Synthesis of the Co0·5Fe0·5(FeAl)Ox materials Co0$5Fe0$5(FeAl)Ox spinel was synthesized by sol-gel method. Typically, the metal nitrates were dissolved in 150 ml deionized water with molar ratio of Co(NO3)2$6H2O, Fe(NO3)3$9H2O, Al(NO3)3$9H2O of 0.5:1:1. Then the FeCl2 was mixed to solution with molar ratio of Fe3þ:Fe2þ of 2:1. The citric acid was added to solution with stoichiometry of it and metal cation of 1.2:1. Subsequently the polyethylene glycol (Mw ¼ 400) was added to solution with stirring in 95 C oil bath and kept up 8 h. The molar ratio of citric acid: polyethylene glycol was 4:1. Finally, the gel precursor was calcined at air atmosphere with 900 C (10 C/min) for 4 h. As a contrast, Fe2O3 oxygen carriers were synthesized by the same sol-gel method.
Material characterization The crystalline phases of the material were characterized by X-ray diffraction (XRD) on a Rigaku SmartLab 9 diffractometer with scanning scope (2q) of 5 to 80 , where Cu Ka radiation was generated at 40 kV and 150 mA. The Raman spectrum was tested in a HORIBA Evolution spectrometer with a 532 nm laser source. For observing morphology of the materials, scanning electron microscopy (SEM) was employed. The materials with a thin layer of gold were observed by FEI 400FEG electron microscope. The CO2eTPO was used to test activity of the depleted oxygen material in water splitting reaction. Before test, the 20 mg material was treated in an argon atmosphere for 30 min at 100 C. Subsequently, the sample was heated to 900 C in a 15 vol% CO2 of Ar atmosphere with a ramp rate of 10 C/min. The nanoscale structure of material was observed in a Tecnai G2 F30 S-Twin TEM with an Energy Dispersive Spectrometer. The phase transition of the Co0$5Fe0$5(FeAl)Ox material during redox cycle was studied in a thermogravimetric analyzer (TGA) with a constant temperature of 900 C. The reducing agent was CO (5 vol% in N2), and the oxidation agents were CO2 (10 vol% in N2) and air. The gas flow rate was 40 ml/min.
Chemical looping tests The Co0$5Fe0$5(FeAl)Ox (0.5 g) materials were reduced by 5% CO and re-oxidized by 30% steam in a laboratory scale fixed bed reactor under 900 C. The total gas flow rate was 1000 ml/min. The stability and reactivity of the material were tested by measuring the molar amount of CO2 and H2 production. The detailed experimental procedure and apparatus could gain from our previous study [44].
Please cite this article as: Zeng D et al., A mixed spinel oxygen carrier with both high reduction degree and redox stability for chemical looping H2 production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.062
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Results and discussion Material characterization Fig. 1 shows the XRD patterns of Co0$5Fe0$5(FeAl)Ox material in which main diffraction peaks situated at 30.6 , 36.3 and 44.0 can be observed. The lattice parameter of Co0$5Fe0$5(FeAl)Ox was 8.22 A. To study insight structural of this material, CoAl2O4, CoFe2O4, and FeAl2O4 (AB2O4) spinel materials were referred with the lattice parameter 8.10 A (CoAl2O4, PDF: A (FeAl2O4, 44e0160), 8.38 A (CoFe2O4, PDF: 03e0864), and 8.15 PDF: 34e0192), respectively. Since the radius of B site Al3þ cation (0.068 nm) and A site Co2þ cation (0.072 nm) are less than that of Fe3þ (0.069 nm) and Fe2þ (0.077 nm), the decrease of Co0$5Fe0$5(FeAl)Ox lattice parameter could be ascribed to the replacement of Fe3þ and Fe2þ with Al3þ and Co2þ in that spinel structure. Therefore, it was reasonable to deduce that the material was produced from the partial replacement of A and B sites elements in CoAl2O4 spinel. The Raman spectrum of the Co0$5Fe0$5(FeAl)Ox material is shown in Fig. 2, in which five peaks can be found. The Td-site mode located at ~700 and ~608 cm1 reflecting the local lattice effect in the tetrahedral sublattice, Oh-site mode situated at ~500, ~300 and ~220 cm1 reflecting the local lattice effect in the octahedral sublattice. As a conclusion of this Raman spectrum, this material was a cubic inverse spinel structure based on the group theory [45]. The surface morphologies of the fresh Co0$5Fe0$5(FeAl) Ox material are shown in Fig. 3. The material particle was in the size of 50e600 nm.
Fig. 1 e XRD patterns of the prepared Co0·5Fe0·5(FeAl)Ox with spinel structural. The crystalline patterns correspond to CoAl2O4 (PDF: 44e0160, vertical blue lines), CoFe2O4 (PDF: 03e0864 vertical black lines) and FeAl2O4 (PDF: 34e0192 vertical red lines), respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 2 e The Raman spectra of the as-prepared Co0·5Fe0·5(FeAl)Ox material.
Chemical looping performance A typical CLH cycle profile is shown in Fig. 4. At the beginning of the reduction process, large amount CO2 was produced. Then, CO began to breakthrough and CO2 rapidly deceased at ~1 min. At about 34 min, CO2 was almost undetected by the gas analyzer, at which the end of reduction was determined. Subsequently, the reactor was purged by N2 and steam was fed to produce hydrogen. When the reduction was completely performed, the hydrogen yield was 12.1 mmol/g. To explore the effect of reduction degree on materials stability, we operated the samples in a fixed bed with varied X of 1, 0.75, 0.5 and 0.25, corresponding the CO reduction time of 2.2, 6.1, 16.1 and 34.1 min (Fig. 5). The pure Fe2O3 reference sample was also prepared and subjected to the same reduction condition. Typically, under low reduction degree condition, two materials both exhibited good redox stability with the hydrogen yield less than 4 mmol/g. When the reduction degree increased, the corresponding hydrogen yield was enhanced but redox stability obviously decreased. In terms of the high reduction degree, it should be noted that the redox stability of Co0$5Fe0$5(FeAl)Ox material was much better than that of Fe2O3 sample. For instance，at X ¼ 0.75，the hydrogen yield of Co0$5Fe0$5(FeAl)Ox material was 9.01 mmol/g, in contrast to 11.1 mmol/g for Fe2O3. But after 20 cycles, the hydrogen yield of Co0$5Fe0$5(FeAl)Ox material remained as high as 8.53 mmol/g but it decreased to 5.2 mmol/g for Fe2O3. The above results indicated Co0$5Fe0$5(FeAl)Ox material can be stably operated at high reduction degree, implying the materials usage and solid flux could be decreased for a practical chemical looping system.
Mechanism insights To get mechanism insights of the observed performance, the phase transition was firstly investigated. As is shown in Fig. 6, only the 2q value of spinel peaks can be
Please cite this article as: Zeng D et al., A mixed spinel oxygen carrier with both high reduction degree and redox stability for chemical looping H2 production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.062
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Fig. 3 e The SEM images of the fresh Co0·5Fe0·5(FeAl)Ox material.
Fig. 4 e A typical CLH cycle profile reduction in 5% CO, subsequent oxidation in 30% steam at 900 C with a 0.5 g charge of Co0·5Fe0·5(FeAl)Ox. (X1 ¼ 0.25, X2 ¼ 0.5, X3 ¼ 0.75 and X4 ¼ 1).
characterized during the initial reduction stage in spite of the slight peak shift, indicating that the compositions were mainly changed between different spinels. The overlapping of metallic Co with spinel led to absence of the reduced metal, but the reduced Co metallic could be confirmed by the increased intensity of peaks at ~44 . With further reduction, the presence of CoFe alloy can be characterized, indicating the exsolution of CoFe alloy from the parent spinel. At full reduction stage, only characteristic peaks of CoFe can be found. The peak of Al2O3 was not observed perhaps in its amorphous state. In the subsequent oxidation process, the peak of CoFe alloy gradually declined and the peak of spinel increased indicating the CoFe dissolved into the supports. The formation of Co0$5Fe0$5(FeAl)Ox spinel was characterized after fully oxidized. Following the results of phase transition, the phase in each reduction and oxidation half cycles with different reduction degree was studied. As shown in Fig. 7, two different redox regimes can be observed. For the sample with complete reduction, the materials were cycled between spinel and alloy/Al2O3. When it comes to the lower reduction degree it changed to the cycles between spinel and alloy/spinel. In spite of the different reduction degree, the phase change during 20 cycles was reversible in all
Fig. 5 e The hydrogen yield of the (a) Co0·5Fe0·5(FeAl)Ox and (b) Fe2O3 materials operated with varied X at 900 C. Please cite this article as: Zeng D et al., A mixed spinel oxygen carrier with both high reduction degree and redox stability for chemical looping H2 production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.062
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Fig. 6 e The phase transition of the Co0·5Fe0·5(FeAl)Ox materials with a redox cycle at 900 C. (a) TGA, (b) XRD.
Fig. 7 e XRD patterns of the Co0·5Fe0·5(FeAl)Ox with varied X at 900 C. (a) X ¼ 1, (b) X ¼ 0.75, (c) X ¼ 0.5, (d) X ¼ 0.25.
cases. But under complete reduction, the alloy peak became weaker along with the cycling, indicating the part of oxygen carrier was no longer reducible. This result may be ascribed to the growth of CoFe alloy particles, which resulted in the diffusion resistance increase of lattice oxygen. With a fixed reduction time, the oxygen carrier would
not be reduced completely so that the hydrogen yield decreased. The TPO results shown in Fig. 8 confirmed the growth of active CoFe alloy particles. As the cycle number increased, the re-oxidation temperature gradually increased and the intensity of oxidation peak decreased when the X was 1.
Please cite this article as: Zeng D et al., A mixed spinel oxygen carrier with both high reduction degree and redox stability for chemical looping H2 production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.062
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Fig. 8 e TPO curves of the Co0·5Fe0·5(FeAl)Ox with varied X. (a) X ¼ 1, (b) X ¼ 0.75, (c) X ¼ 0.5, (d) X ¼ 0.25.
Fig. 9 e The crystallite size evaluation of the Co0·5Fe0·5(FeAl) Ox with varied X.
Although the slight increase of re-oxidation temperature was observed when the X was less than 0.75, the obvious change of peak intensity was not tested. The crystallite size was also calculated by Scherrer equation (Fig. 9). It can be found that as cycle number increase, the size of CoFe alloy was virtually not changed in terms of lower reduction degree, but it rapidly increased to ～85 nm for the sample with full reduction. For further explored this phenomenon, the samples with varied X were characterized by TEM. The exsolved CoFe alloy particles were embedded in spinel supports when the X was less than 0.75, which hindered the immigration of the active alloy phase and material sintering. The embedded interface structure could be observed even though after 20 cycles (Fig. 10eeh and i-l). Therefore, the Co0$5Fe0$5(FeAl)Ox performed a good redox stability in prolonged tests. In contrast, when the oxygen carrier was fully reduced with X of 1, the exsolved CoFe alloy particles were deposited on amorphous state of Al2O3 (Fig. 10aec). In the 20th cycle, the agglomeration of CoFe alloy particle could be obviously observed so that the sizes of CoFe alloy increased and material sintering.
Please cite this article as: Zeng D et al., A mixed spinel oxygen carrier with both high reduction degree and redox stability for chemical looping H2 production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.062
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Fig. 10 e TEM images of the Co0·5Fe0·5(FeAl)Ox oxygen carriers. (aec) fresh exsolution with X ¼ 1, (d) 20th exsolution with X ¼ 1, (eeg) fresh exsolution with X ¼ 0.75, (h) 20th exsolution with X ¼ 0.75, (iek) fresh exsolution with X ¼ 0.5, (l) 20th exsolution with X ¼ 0.5.
Conclusions In this study, we prepared a Co0$5Fe0$5(FeAl)Ox material as an oxygen carrier and studied its redox stability with varied X in chemical looping hydrogen production process. The XRD along with Raman spectra confirmed the material of a pure Co0$5Fe0$5(FeAl)Ox and a cubic inverse spinel structure. This oxygen carrier has exhibited good redox stability in fixed bed tests even at X ¼ 0.75. The mechanism study showed the CoFe active phase reversibly exsolved and dissolved from and into the spinel support during the reduction and oxidation process. Upon the TEM images, the embedded structure was observed to hinder from sintering and aggregation of active compositions. The ability of the material to stably operate under high X conditions can effectively improve hydrogen yield and decrease the usage and solid flux of oxygen carriers, and enhance the economy of the chemical looping hydrogen technology.
Acknowledgements The authors gratefully acknowledge the National Natural Science Foundation of China (Grant No. 51906041), the Natural Science Foundation of Jiangsu Province (Grant
No. BK20190360) and National Science Foundation for Distinguished Young Scholars of China (Grant No. 51525601).
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Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
A mixed spinel oxygen carrier with both high reduction degree and redox stability for chemical looping H2 production Dewang Zeng 1, Dongxu Cui 1, Yulin Lv, Yu Qiu, Min Li, Shuai Zhang, Rui Xiao* Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy and Environment, Southeast University, Nanjing, 210096, PR China
highlights A mixed spinel Co0$5Fe0$5(FeAl)Ox material is prepared as an oxygen carrier. The material performs a stable and high hydrogen yield of 8.53 mmol/g. The reversible exsolution and dissolution of active phase improve redox stability.
article info
abstract
Article history:
Developing the oxygen carrier of high reduction degree and redox stability has key
Received 17 September 2019
importance to the chemical looping hydrogen production. However, high reduction degree
Received in revised form
of oxygen carrier often leads to severe sintering and poor redox stability. Therefore, the
4 November 2019
trade-off between such two aspects of it must be considered. In this paper, a Co0$5Fe0$5(-
Accepted 6 November 2019
FeAl)Ox material was reported as an oxygen carrier and an investigation on the perfor-
Available online xxx
mance of its chemical looping hydrogen production was conducted. It exhibited a stable and high hydrogen yield of 8.53 mmol/g during 20 cycles even at the reduction degree of
Keywords:
0.75. Upon comparison, it was found that the obvious deactivation occurred in Fe2O3 ma-
Hydrogen production
terial when its reduction degree was higher than 0.25. Upon various characterization
Chemical looping
techniques, we found that the high performance of such material was rightly because of
Oxygen carrier
the reversible exsolution and dissolution of its active compositions from and into the
Spinel material
parent spinel support. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen is an ideal clean energy for carbon free economy because it has a high specific energy density and water is the only production of its combustion [1,2]. The steam methane
reforming technology, being the predominant way for producing hydrogen nowadays, has still certain apparent disadvantages such as need of multiple reactors, high energy penalty for hydrogen purification, and coke deposition resulting in catalysts deactivation [3e5]. Therefore a
* Corresponding author. E-mail addresses: [email protected] (D. Zeng), [email protected] (R. Xiao). 1 These people contributed equally to this work. https://doi.org/10.1016/j.ijhydene.2019.11.062 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Zeng D et al., A mixed spinel oxygen carrier with both high reduction degree and redox stability for chemical looping H2 production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.062
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technology capable of efficiently producing pure hydrogen is highly desirable [6]. Chemical looping hydrogen production (CLH) has attracted wide attention because it produces pure hydrogen without additional energy consumption [7]. Different from traditional steam methane reforming process, the transition metal oxides are employed as oxygen transport carriers for cycling between fuel and steam reactors. The oxygen carrier is firstly reduced in the fuel reactor whilst the fuel is oxidized, then the oxygen-depleted oxygen carrier is re-oxidized in the steam reactor and the hydrogen is released in this process simultaneously [8,9]. After the water condenses, pure hydrogen can be obtained without energy consumption of separation and purification [10,11]. The oxygen carrier plays a very important role in the CLH process [12,13]. In the past several years, many kinds of oxygen carriers, mainly comprising Ni [5,14,15], Mn- [16,17], Co[18e20], and Fe- [9,21e23] oxides, had been used. Due to the high oxygen capacity, compatibility to environment, good mechanical strength, and low cost, iron oxides are widely used as oxygen carriers [9,24,25]. In the CLH of iron oxides, two major stoichiometric redox pathways have been employed and the specific one depends on different reduction degrees (X is defined as the ratio of practical weight loss and the value in terms of the complete reduction). One of which is the partial reduction to wustite (reduction: Fe3O4þCO/3FeO þ CO2, reoxidation: 3FeO þ H2O/Fe3O4þH2), and the other one of which is complete reduction to metallic iron (reduction: Fe3O4þ4CO/3Feþ4CO2, re-oxidation: 3Feþ4H2O/Fe3O4þ4H2) [9,26]. Theoretically, the hydrogen yield of the latter pathway is 4 times higher than that of the former one, signifying that for the practical chemical looping system, the material usage or solid flux would be decreased to a quarter. Unfortunately, it has reported that the high reduction degree of oxygen carrier has negative effect on its redox stability [27,28]. Bohn et al. [29,30] explored the relationship between redox stability and reduction degree of the oxygen carrier; in which the oxygen carrier rapidly sintered at first few redox cycles when the oxygen carrier was reduced from Fe2O3 to Fe (X ¼ 1). As a contrast, when the oxygen carrier is operated from Fe2O3 to FeO (X ¼ 0.33), an obvious stability improvement can be observed. Qiu et al. [31] and Diego et al. [32] found that this phenomenon was resulted from the high lattice stress generated by the phase transition. The density of Fe2O3, Fe3O4, and FeO is respectively 5.26 g/cm3, 5.18 g/cm3, and 5.75 g/cm3. The Fe density is 7.87 g/cm3 which is about 1.5 times than that of Fe2O3. When the phase transition is between Fe2O3 and Fe, the large density change of oxygen carrier would generate the stronger lattice stress to cause structural damage and severe sintering [33]. Some researchers proposed to use promoter materials to solve above issues, such as ZrO2 [3,34], MgO [35,36], SiO2 [19,37,38], CeO2 [39,40] and Al2O3 [41,42]. These promoter materials, as oxygen conduction channels or physical barriers, keep reaction activity and prevent iron oxides from sintering [43]. However they sacrifices parts of the loadings of the active phase and subsequently hydrogen yield on one hand, and they are not always inert during redox cycles on the other hand. Under a high operating temperature 750e950 C, it is inevitable that phase interaction between the active and inert compositions in them leads to inert spinel
formation [7]. This formation further hinders the final hydrogen production process. Here in this paper a Co0$5Fe0$5(FeAl)Ox spinel material was employed as an oxygen carrier for chemical looping hydrogen production and the redox stability was tested in varied reduction degrees (X ¼ 0.25, 0.5, 0.75 and 1), the results showed that the oxygen carrier exhibited good redox stability even at the reduction degree of 0.75.
Experiments Synthesis of the Co0·5Fe0·5(FeAl)Ox materials Co0$5Fe0$5(FeAl)Ox spinel was synthesized by sol-gel method. Typically, the metal nitrates were dissolved in 150 ml deionized water with molar ratio of Co(NO3)2$6H2O, Fe(NO3)3$9H2O, Al(NO3)3$9H2O of 0.5:1:1. Then the FeCl2 was mixed to solution with molar ratio of Fe3þ:Fe2þ of 2:1. The citric acid was added to solution with stoichiometry of it and metal cation of 1.2:1. Subsequently the polyethylene glycol (Mw ¼ 400) was added to solution with stirring in 95 C oil bath and kept up 8 h. The molar ratio of citric acid: polyethylene glycol was 4:1. Finally, the gel precursor was calcined at air atmosphere with 900 C (10 C/min) for 4 h. As a contrast, Fe2O3 oxygen carriers were synthesized by the same sol-gel method.
Material characterization The crystalline phases of the material were characterized by X-ray diffraction (XRD) on a Rigaku SmartLab 9 diffractometer with scanning scope (2q) of 5 to 80 , where Cu Ka radiation was generated at 40 kV and 150 mA. The Raman spectrum was tested in a HORIBA Evolution spectrometer with a 532 nm laser source. For observing morphology of the materials, scanning electron microscopy (SEM) was employed. The materials with a thin layer of gold were observed by FEI 400FEG electron microscope. The CO2eTPO was used to test activity of the depleted oxygen material in water splitting reaction. Before test, the 20 mg material was treated in an argon atmosphere for 30 min at 100 C. Subsequently, the sample was heated to 900 C in a 15 vol% CO2 of Ar atmosphere with a ramp rate of 10 C/min. The nanoscale structure of material was observed in a Tecnai G2 F30 S-Twin TEM with an Energy Dispersive Spectrometer. The phase transition of the Co0$5Fe0$5(FeAl)Ox material during redox cycle was studied in a thermogravimetric analyzer (TGA) with a constant temperature of 900 C. The reducing agent was CO (5 vol% in N2), and the oxidation agents were CO2 (10 vol% in N2) and air. The gas flow rate was 40 ml/min.
Chemical looping tests The Co0$5Fe0$5(FeAl)Ox (0.5 g) materials were reduced by 5% CO and re-oxidized by 30% steam in a laboratory scale fixed bed reactor under 900 C. The total gas flow rate was 1000 ml/min. The stability and reactivity of the material were tested by measuring the molar amount of CO2 and H2 production. The detailed experimental procedure and apparatus could gain from our previous study [44].
Please cite this article as: Zeng D et al., A mixed spinel oxygen carrier with both high reduction degree and redox stability for chemical looping H2 production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.062
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Results and discussion Material characterization Fig. 1 shows the XRD patterns of Co0$5Fe0$5(FeAl)Ox material in which main diffraction peaks situated at 30.6 , 36.3 and 44.0 can be observed. The lattice parameter of Co0$5Fe0$5(FeAl)Ox was 8.22 A. To study insight structural of this material, CoAl2O4, CoFe2O4, and FeAl2O4 (AB2O4) spinel materials were referred with the lattice parameter 8.10 A (CoAl2O4, PDF: A (FeAl2O4, 44e0160), 8.38 A (CoFe2O4, PDF: 03e0864), and 8.15 PDF: 34e0192), respectively. Since the radius of B site Al3þ cation (0.068 nm) and A site Co2þ cation (0.072 nm) are less than that of Fe3þ (0.069 nm) and Fe2þ (0.077 nm), the decrease of Co0$5Fe0$5(FeAl)Ox lattice parameter could be ascribed to the replacement of Fe3þ and Fe2þ with Al3þ and Co2þ in that spinel structure. Therefore, it was reasonable to deduce that the material was produced from the partial replacement of A and B sites elements in CoAl2O4 spinel. The Raman spectrum of the Co0$5Fe0$5(FeAl)Ox material is shown in Fig. 2, in which five peaks can be found. The Td-site mode located at ~700 and ~608 cm1 reflecting the local lattice effect in the tetrahedral sublattice, Oh-site mode situated at ~500, ~300 and ~220 cm1 reflecting the local lattice effect in the octahedral sublattice. As a conclusion of this Raman spectrum, this material was a cubic inverse spinel structure based on the group theory [45]. The surface morphologies of the fresh Co0$5Fe0$5(FeAl) Ox material are shown in Fig. 3. The material particle was in the size of 50e600 nm.
Fig. 1 e XRD patterns of the prepared Co0·5Fe0·5(FeAl)Ox with spinel structural. The crystalline patterns correspond to CoAl2O4 (PDF: 44e0160, vertical blue lines), CoFe2O4 (PDF: 03e0864 vertical black lines) and FeAl2O4 (PDF: 34e0192 vertical red lines), respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 2 e The Raman spectra of the as-prepared Co0·5Fe0·5(FeAl)Ox material.
Chemical looping performance A typical CLH cycle profile is shown in Fig. 4. At the beginning of the reduction process, large amount CO2 was produced. Then, CO began to breakthrough and CO2 rapidly deceased at ~1 min. At about 34 min, CO2 was almost undetected by the gas analyzer, at which the end of reduction was determined. Subsequently, the reactor was purged by N2 and steam was fed to produce hydrogen. When the reduction was completely performed, the hydrogen yield was 12.1 mmol/g. To explore the effect of reduction degree on materials stability, we operated the samples in a fixed bed with varied X of 1, 0.75, 0.5 and 0.25, corresponding the CO reduction time of 2.2, 6.1, 16.1 and 34.1 min (Fig. 5). The pure Fe2O3 reference sample was also prepared and subjected to the same reduction condition. Typically, under low reduction degree condition, two materials both exhibited good redox stability with the hydrogen yield less than 4 mmol/g. When the reduction degree increased, the corresponding hydrogen yield was enhanced but redox stability obviously decreased. In terms of the high reduction degree, it should be noted that the redox stability of Co0$5Fe0$5(FeAl)Ox material was much better than that of Fe2O3 sample. For instance，at X ¼ 0.75，the hydrogen yield of Co0$5Fe0$5(FeAl)Ox material was 9.01 mmol/g, in contrast to 11.1 mmol/g for Fe2O3. But after 20 cycles, the hydrogen yield of Co0$5Fe0$5(FeAl)Ox material remained as high as 8.53 mmol/g but it decreased to 5.2 mmol/g for Fe2O3. The above results indicated Co0$5Fe0$5(FeAl)Ox material can be stably operated at high reduction degree, implying the materials usage and solid flux could be decreased for a practical chemical looping system.
Mechanism insights To get mechanism insights of the observed performance, the phase transition was firstly investigated. As is shown in Fig. 6, only the 2q value of spinel peaks can be
Please cite this article as: Zeng D et al., A mixed spinel oxygen carrier with both high reduction degree and redox stability for chemical looping H2 production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.062
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Fig. 3 e The SEM images of the fresh Co0·5Fe0·5(FeAl)Ox material.
Fig. 4 e A typical CLH cycle profile reduction in 5% CO, subsequent oxidation in 30% steam at 900 C with a 0.5 g charge of Co0·5Fe0·5(FeAl)Ox. (X1 ¼ 0.25, X2 ¼ 0.5, X3 ¼ 0.75 and X4 ¼ 1).
characterized during the initial reduction stage in spite of the slight peak shift, indicating that the compositions were mainly changed between different spinels. The overlapping of metallic Co with spinel led to absence of the reduced metal, but the reduced Co metallic could be confirmed by the increased intensity of peaks at ~44 . With further reduction, the presence of CoFe alloy can be characterized, indicating the exsolution of CoFe alloy from the parent spinel. At full reduction stage, only characteristic peaks of CoFe can be found. The peak of Al2O3 was not observed perhaps in its amorphous state. In the subsequent oxidation process, the peak of CoFe alloy gradually declined and the peak of spinel increased indicating the CoFe dissolved into the supports. The formation of Co0$5Fe0$5(FeAl)Ox spinel was characterized after fully oxidized. Following the results of phase transition, the phase in each reduction and oxidation half cycles with different reduction degree was studied. As shown in Fig. 7, two different redox regimes can be observed. For the sample with complete reduction, the materials were cycled between spinel and alloy/Al2O3. When it comes to the lower reduction degree it changed to the cycles between spinel and alloy/spinel. In spite of the different reduction degree, the phase change during 20 cycles was reversible in all
Fig. 5 e The hydrogen yield of the (a) Co0·5Fe0·5(FeAl)Ox and (b) Fe2O3 materials operated with varied X at 900 C. Please cite this article as: Zeng D et al., A mixed spinel oxygen carrier with both high reduction degree and redox stability for chemical looping H2 production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.062
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Fig. 6 e The phase transition of the Co0·5Fe0·5(FeAl)Ox materials with a redox cycle at 900 C. (a) TGA, (b) XRD.
Fig. 7 e XRD patterns of the Co0·5Fe0·5(FeAl)Ox with varied X at 900 C. (a) X ¼ 1, (b) X ¼ 0.75, (c) X ¼ 0.5, (d) X ¼ 0.25.
cases. But under complete reduction, the alloy peak became weaker along with the cycling, indicating the part of oxygen carrier was no longer reducible. This result may be ascribed to the growth of CoFe alloy particles, which resulted in the diffusion resistance increase of lattice oxygen. With a fixed reduction time, the oxygen carrier would
not be reduced completely so that the hydrogen yield decreased. The TPO results shown in Fig. 8 confirmed the growth of active CoFe alloy particles. As the cycle number increased, the re-oxidation temperature gradually increased and the intensity of oxidation peak decreased when the X was 1.
Please cite this article as: Zeng D et al., A mixed spinel oxygen carrier with both high reduction degree and redox stability for chemical looping H2 production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.062
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Fig. 8 e TPO curves of the Co0·5Fe0·5(FeAl)Ox with varied X. (a) X ¼ 1, (b) X ¼ 0.75, (c) X ¼ 0.5, (d) X ¼ 0.25.
Fig. 9 e The crystallite size evaluation of the Co0·5Fe0·5(FeAl) Ox with varied X.
Although the slight increase of re-oxidation temperature was observed when the X was less than 0.75, the obvious change of peak intensity was not tested. The crystallite size was also calculated by Scherrer equation (Fig. 9). It can be found that as cycle number increase, the size of CoFe alloy was virtually not changed in terms of lower reduction degree, but it rapidly increased to ～85 nm for the sample with full reduction. For further explored this phenomenon, the samples with varied X were characterized by TEM. The exsolved CoFe alloy particles were embedded in spinel supports when the X was less than 0.75, which hindered the immigration of the active alloy phase and material sintering. The embedded interface structure could be observed even though after 20 cycles (Fig. 10eeh and i-l). Therefore, the Co0$5Fe0$5(FeAl)Ox performed a good redox stability in prolonged tests. In contrast, when the oxygen carrier was fully reduced with X of 1, the exsolved CoFe alloy particles were deposited on amorphous state of Al2O3 (Fig. 10aec). In the 20th cycle, the agglomeration of CoFe alloy particle could be obviously observed so that the sizes of CoFe alloy increased and material sintering.
Please cite this article as: Zeng D et al., A mixed spinel oxygen carrier with both high reduction degree and redox stability for chemical looping H2 production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.062
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Fig. 10 e TEM images of the Co0·5Fe0·5(FeAl)Ox oxygen carriers. (aec) fresh exsolution with X ¼ 1, (d) 20th exsolution with X ¼ 1, (eeg) fresh exsolution with X ¼ 0.75, (h) 20th exsolution with X ¼ 0.75, (iek) fresh exsolution with X ¼ 0.5, (l) 20th exsolution with X ¼ 0.5.
Conclusions In this study, we prepared a Co0$5Fe0$5(FeAl)Ox material as an oxygen carrier and studied its redox stability with varied X in chemical looping hydrogen production process. The XRD along with Raman spectra confirmed the material of a pure Co0$5Fe0$5(FeAl)Ox and a cubic inverse spinel structure. This oxygen carrier has exhibited good redox stability in fixed bed tests even at X ¼ 0.75. The mechanism study showed the CoFe active phase reversibly exsolved and dissolved from and into the spinel support during the reduction and oxidation process. Upon the TEM images, the embedded structure was observed to hinder from sintering and aggregation of active compositions. The ability of the material to stably operate under high X conditions can effectively improve hydrogen yield and decrease the usage and solid flux of oxygen carriers, and enhance the economy of the chemical looping hydrogen technology.
Acknowledgements The authors gratefully acknowledge the National Natural Science Foundation of China (Grant No. 51906041), the Natural Science Foundation of Jiangsu Province (Grant
No. BK20190360) and National Science Foundation for Distinguished Young Scholars of China (Grant No. 51525601).
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Please cite this article as: Zeng D et al., A mixed spinel oxygen carrier with both high reduction degree and redox stability for chemical looping H2 production, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.062