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La1-xSrxFeO3 perovskite-type oxides for chemical-looping steam methane reforming: Identification of the surface elements and redox cyclic performance
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 1 0 2 6 5 e1 0 2 7 6
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La1-xSrxFeO3 perovskite-type oxides for chemicallooping steam methane reforming: Identification of the surface elements and redox cyclic performance Fang He a,b,*, Jing Chen b,c, Shuai Liu d, Zhen Huang b, Guoqiang Wei b, Guixia Wang a, Yan Cao d,e, Kun Zhao b,** a
College of Chemistry and Bioengineering, Guilin University of Technology, Guilin 541004, PR China Key Laboratory of Renewable Energy, Chinese Academy of Sciences, Guangdong Key Laboratory of New and Renewable Energy Research and Development, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, China c Institute of Nano Science and Technology, University of Science and Technology of China, Suzhou 215123, China d College of Chemistry & Chemical Engineering, Anhui University, Hefei 230601, China e Department of Chemistry, Western Kentucky University (WKU), Bowling Green, KY 42101, USA b
article info
abstract
Article history:
We report a family of perovskite-type oxides La1-xSrxFeO3 (x ¼ 0.1, 0.3, 0.5, 0.7, 1.0) prepared
Received 1 January 2019
by combustion method as effective redox catalysts for methane partial oxidation and
Received in revised form
thermochemical water splitting in a cyclic redox scheme. The effect of Sr-doping on the
21 February 2019
characterizations and properties of these perovskite-type oxides were studied by means of
Accepted 3 March 2019
X-ray diffraction (XRD), hydrogen temperature-programmed reduction (H2-TPR), X-ray
Available online 26 March 2019
photoelectron spectroscopy (XPS), and scanning electron microscope (SEM). All the asprepared and regenerated samples with various Sr substitutions exhibited pure crystal-
Keywords:
line perovskite structure. The oxygen carrying capacity of the La1-xSrxFeO3 perovskites was
Perovskites
improved by doping Sr into the La-site. Besides, Sr-substitution has obvious effects on the
Oxygen species
valences of the Fe cations in the B-site and the oxygen species distribution of the La1-
XPS
xSrxFeO3
Chemical looping
series because it gives the maximum Ola/Oad (Ola and Oad stand for lattice oxygen and
Methane reforming
adsorbed oxygen species, respectively.) ratio of 3.64:1, which can be regarded as a criterion
Water thermochemical splitting
for the reactivity and selectivity of partial oxidation of methane into syngas of the oxygen
perovskites. We recommend La0.7Sr0.3FeO3 as the optimal oxygen carrier in the
carriers. Up to 80% CH4 conversion in the methane partial oxidation step and 96% of H2 concentration in the water splitting step were achieved in ten successive redox tests conducted in a fixed bed reactor at 850 C with La0.7Sr0.3FeO3 as a redox catalyst. The electronic properties of the original LaFeO3 cell and its lattice substituted by Sr were calculated based on the density functional theory method. Electronic structure analysis demonstrates that doping of Sr makes LaFeO3 more electric conductive and its electron is prone to be excited. This is in agreement with the test results that La0.7Sr0.3FeO3 exhibited better performance in chemical looping reactions. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. College of Chemistry and Bioengineering, Guilin University of Technology, Guilin 541004, PR China. ** Corresponding author. E-mail addresses: [email protected] (F. He), [email protected] (K. Zhao). https://doi.org/10.1016/j.ijhydene.2019.03.002 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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Introduction Chemical-looping steam methane reforming (CL-SMR) [1e3], in which methane is selectively oxidized into syngas by the lattice oxygen of an oxygen carrier in a reforming reactor, and then the reduced oxygen carrier is oxidized by steam to replenish its lattice oxygen thereby generating hydrogen in a steam reactor, is a novel technology co-producing syngas and hydrogen, as shown in Fig. 1. The success of CL-SMR approach is highly dependent on finding suitable oxygen carriers (also called redox catalysts) with high methane conversion, good activity for water splitting to produce hydrogen, and good reactivity in successive redox cycles [4]. Perovskite-type oxides with chemical formula ABO3 have been extensively investigated for many potential energy and environmental related applications due to their uniquely tunable bulk and surface properties for oxygen vacancy formation and elimination, mixed ion and electron conduction, surface/bulk oxygen evolution, band structure, and ability to accommodate large number of dopants in the cationic (A/B) sites [5e7]. Among various perovskites, LaFeO3-based oxides have been attracted much attention since they have potential as candidate materials for versatile applications in advanced technologies, such as catalysts, oxygen carriers, chemical sensors, cathode materials in solid oxide fuel cells etc due to their high oxygen mobility and stable capability of hosting large concentrations of vacancies in the structure [8e10]. Generally, metal cations in B-site are regarded as the key factor affecting the chemical reactivity, magnetic properties, and electrical conductivity of the perovskite-type oxides. Therefore, doping various metal ions into the B-site of LaFeO3 has been extensively in previous literature [11e13]. Zhang et al. [14] studied the catalytic combustion of acrylonitrile over Cu-doped LaBO3 (B¼ Fe, Co, and Mn) perovskites and found that LaFe0.8Cu0.2O3 exhibited the best performance due to an easy transformation from Cu2þ to Cu0 at low temperatures. Zhao et al. [15] prepared LaFe1-xCoxO3 (x ¼ 0.1, 0.3, 0.5, 0.7, 1.0)
Fig. 1 e CL-SMR for syngas and hydrogen production.
perovskites and found that Co-doping in the B-site could obviously improve the reactivity and inhibit carbon formation in the chemical looping steam methane reforming reactions. Taylor et al. [16] examined the viability of a range of divalent B-site dopants for promoting ionic and electronic conductivity of LaFeO3 by using ab initio methods. They concluded that cobalt, nickel and magnesium are the most favorable B-site dopants, due to low solution energies and low binding energies to oxygen vacancies, along with minimal distortion, particularly for nickel, to the LaFeO3 lattice. In Comparison to extensive studies of the effects of different metal cations doping in B-site of LaFeO3 on its properties, investigations on effects of metal cations doping in the A-site on its performance have limitedly been reported. Previous literature reported that the metals in A-site are noncatalytic, however, they have effects on the concentration of oxygen vacancies and the valence states of the B-site metals, thus consequently affecting the reactivity of the perovskitetype oxides [17,18]. The valences of M ion in Lal-xSrxMO3 (where M is a first-row transition metal) might be adjusted by partial substitution of La3þ by Sr2þ, thereby generating a number of interesting changes in their physical and chemical properties. According to our previous studies [19], Sr substitution would inhibit methane decomposition and enhance the reactivity of La1-xSrxFeO3 in reactions of partial oxidation of methane. Effect of Sr2þ and/or Ce2þ substitution in the A-site of LaFeO3 perovskite on its catalytic activity for oxidation of NO and CH4 have been studied, respectively [20,21]. Jones and the co-workers [22] found that Sr2þ substitution on the La3þ site gives a minimum in the binding energy between Sr2þ ions and vacancy clusters, which would be beneficial to oxide ion conductivity and oxygen migration of the perovskites. Taylor et al. [23] reported that La2/3Sr1/3FeO3 exhibits promise for use as an oxygen storage material in methane reforming reactions due to its high product selectivity, fast oxide diffusion, and cycle stability because its structure remains homogenous throughout the reaction. Evdou et al. [24] used Lal-xSrxMnO3 as redox catalyst for two-step steam reforming of methane towards the production of high purity hydrogen, and found that a production of 60 mmol H2 per 500 mmol water passed over 200 mg La0.3Sr0.7MnO3 at 1273 K. However, the mechanism of the enhancement of Sr-doping on the reactivity of La1-xSrxFeO3 perovskites was not yet discussed in detail. The interaction effects of Sr substitution on the element compositions on the near-surface of the perovskites are still unknown. As a powerful technique to study the near-surface properties of materials, X-ray photoelectron spectroscopy (XPS) technology is sensitive to the chemical environment and the XPS spectra can give information about the oxidation state of the surface elements of solid materials. In this work, a detailed investigation of the metal valence variation and oxygen species distribution on the near-surface of the particle is made by mean of XPS measurement to determine the best candidate of La1-xSrxFeO3 (x ¼ 0.1, 0.3, 0.5, 0.7, 1.0) for CL-SMR. Then ten successive redox reactions were performed to investigate the thermal stability of the La0.7Sr0.3FeO3 sample in a fixed bed reactor. We recommend La0.7Sr0.3FeO3 as the optimal oxygen carrier in the series because of its high reactivity, selectivity, as well as structure stability.
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Experimental Synthesis of the La1-xSrxFeO3 The La1-xSrxFeO3 (x ¼ 0.0, 0.1, 0.3, 0.5, 0.7, 0.9, and 1.0) perovskites in this work were synthesized by combustion method. Stoichiometric amounts of La(NO3)3$6H2O, Sr(NO3)2, and Fe(NO3)3$9H2O were weighed and put into a beaker to mix with an amount of deionized water to make a solution. Then glycine (H2NCH2CO2H) was added into the mixed nitrates solution to reach a glycine/nitrates molar ratio of 1.05. The resulting solution was allowed to evaporate by stirring in a 100 mL beaker at 70 C until a viscous gel with brown color was obtained. The gel was then put in a muffle furnace at 300 C to combust for generation of a porous precursor. The powdered precursor was calcined at 500 C for 3 h, and then at 900 C for 6 h to get the fresh perovskite-type oxides of La1-xSrxFeO3d (x ¼ 0.0, 0.1, 0.3, 0.5, 0.7, 0.9, and 1.0).
Material characterization The crystal phases of the perovskites were identified by XRD in a Japan Science D/max-R diffractometer with Cu Ka radiation (l ¼ 0.15406 nm), operating voltage of 40 kV and current of 40 mA, and the diffraction angle (2q) was scanned from 10 to 80 . Hydrogen-temperature programmed reduction (H2-TPR) tests were conducted in 5.0 vol% H2 balanced with helium at a flow rate of 60 ml/min from room temperature to 900 C with a heating rate of 10 C/min. X-ray photoelectron spectroscopy (XPS) was used to probe the near-surface composition of the oxides. The equipment was Thermo Fisher Scientific Inc with an Al Ka X-ray source at an operating voltage of 20 kV and a current of 10 mA, under the conditions of 20eV and 100eV pass energy for the survey spectra and the single element spectra. The XPS analysis software used in this work is Avantage (version 5,932,0,6055) provided by Thermo Fisher Scientific. The micro-morphology of the as-prepared and regenerated perovskite samples were studied by scanning electron microscopy (SEM) on a Hitachi S4800 instruments.
Successive redox reactivity The successive redox cyclic reactivity was tested in a fixed-bed quartz reactor (30 mm i.d.) under atmospheric pressure at 850 C by using 2 g of oxygen carrier in each test. The experiments of partial oxidation of methane were performed in a gas flow of 40.0 vol% methane with nitrogen as a balance gas at flow rate of 50 ml/min. The product gases out of the reactor were collected with gas bags and analyzed by a gas chromatograph (Shimadzu GC-2010plus).When the methane conversion step finished, pure N2 (50 ml/min) was fed into the reactor purging the reactor for 30 min to avoid mixing of gases from the two steps. After that, steam generated by injecting demineralized water in an electric furnace at 400 C using a micro pump was introduced into the reactor for 20 min with N2 as carrier gas (50 ml/min). The flow rate of the water was controlled at 0.2 ml/min. The temperature at the gas inlet of the reactor is around 300 C, therefore, the total flow rate of inlet gases in the water-splitting stage is 628 ml/min including the
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nitrogen and steam. The real flow rate of inlet gases may lower than this value because a fraction of steam will condense into water in some cases. Next, dry air (50 ml/min) was introduced into the reactor for 10 min to guarantee the lattice oxygen consumed for methane oxidation completely replenishment. The above partial oxidation of methane, water-splitting, and air oxidation cycle was repeated for 10 times. To reduce the experimental error, the successive redox cyclic reactivity tests were repeated 3 times and then averaged the results.
DFT calculations DFT calculations on electronic properties of the synthesized perovskite samples were carried out using a module of Materials studio 8.0 (CASTEP, Cambridge Serial Total Energy Package). Perovskite has a face-centered cubic lattice of space group Pm-3m, number 221 in the international tables. To simplify, a primitive cell of LaFeO3 was first built and then the metal La was partially substituted by metal Sr in proportion. Generalized Gradient Approximation-Perdew Burke Ernzerhof (GGA-PBE) was selected as exchange-correlation functional in all calculations, therein using the GGA to represent the atoms and PBE representing solids [25]. An energy cutoff at 340 eV was adopted after a convergence study over the range of 450e750 eV. The k-point separation was specified within 0.015 Å1 in the geometry optimization and the convergence criteria for the calculation were set to the energy tolerance at A, 1.0 105 eV/atom, the maximum force tolerance at 0.04 eV/ and the self-consistent field tolerance at 1.0 106 eV/atom.
Results and discussion XRD The XRD patterns of the as-prepared La1-xSrxFeO3 samples are shown in Fig. 2 (a). It can be seen that all the as-prepared samples with various Sr-doping degrees in the La-site are well crystallized with orthorhombic perovskite structure. No impurity peak appears with Sr substitution, indicating that all Sr-doped samples retain the perovskite crystal structure. It is noted that the diffraction peaks of the samples with Sr-doping in La site are a bit broaden than those of LaFeO3, suggesting that the Sr2þ incorporated into the La3þ site of the LaFeO3 perovskite lattice without causing phase change. In addition, the broadening of the full width at half maximum (FWHM) at approximately 32.5 (2q) means that the Sr-doping generates lattice contraction and particle size decrease of the doped samples. A part of La3þ is substituted by Sr2þ causing an electronic unbalance in the perovskite lattice that can be compensated by the formation of oxygen vacancies and/or oxidation of a fraction of Fe3þ ions to Fe4þ (even Fe5þ). The ionic radius of Fe4þ (0.058 nm) is smaller than that of Fe3þ (0.0645 nm) explaining the lattice contraction and smaller grain size [15]. Moreover, the decreasing of the crystalline size may be due to the slowing the crystal growth process because doping LaFeO3 by Sr would possibility of blockade of crystal growth, giving rise decrease grain size [26]. The regenerated samples after successive redox reactions were also examined by XRD. The XRD patterns of the La1-xSrxFeO3 samples of
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Fig. 3 e H2-TPR profiles of La1-xSrxFeO3-d. Temperature ramping rate is 10 C/min.
Fig. 2 e XRD patterns of La1-xSrxFeO3 perovskites of (a) fresh, (b) regenerated. x ¼ 0.1, 0.3, and 0.5 were selected and shown in Fig. 2 (b). The XRD patterns of the spent samples are very similar to those of the as-prepared samples. This means that the perovskite oxides have good regenerability. Furthermore, we found that the splitting of the strongest characteristic peaks (at 2q ¼ 32.5 ) of the regenerated perovskites become less obvious, indicating that the crystal structure becomes purer with better integration as the samples were subjected to multi-cyclic redox reactions.
H2-TPR Because oxygen carriers utilize lattice oxygen to convert methane into syngas, it is important to understand the reduction behaviors of the oxygen carriers by reducing gases or fuels. To explore the oxygen release properties of the oxygen carriers under a reducing environment, H2-TPR was performed to characterize the reducibility of the La1-xSrxFeO3 perovskites with x ¼ 0.0, 0.1, 0.3, 0.5, and 0.9. As shown in Fig. 3, the LaFeO3 without Sr-doping has two reduction peaks with defined maximums at about 482 C and 680 C, respectively. In a previous study, we analyzed the Fe-based perovskite type oxides at different reduction stages using XRD and XPS. Results showed that the reduction of Fe ions in B-site is a stepwise reduction pathway as Fe4þ/Fe5þ/Fe3þ/Fe2þ [27]. The first peak corresponds to reduction of adsorbed oxygen which can happen at a relative low temperature, combing with the transitions in iron valence states from Fe4þ/Fe3þ to Fe3þ/Fe2þ. The second peak is ascribed to the bulk reduction of lattice oxygen and the deep reduction of Fe3þ/Fe2þ to Fe2þ and even metallic Fe. Zhang and the co-workers [28] reported similar H2-TPR results of Fe-based perovskites. The TPR profiles of Sr-doped La1-xSrxFeO3 were compared those of
undoped LaFeO3. La0.9Sr0.1FeO3 also exhibits two reduction peaks with defined maximums at 388 C and 747 C, respectively. In comparison to the peaks of pure LaFeO3, the first peak of La0.9Sr0.1FeO3 shifts near 100 C to lower temperatures, whereas the second peak shifts slightly to higher temperatures. The La0.7Sr0.3FeO3 appears triplet reduction peaks: one major peak at about 470 C and two minor peaks at 590 and 770 C, respectively. With increasing Sr-doping degrees above 0.3, the samples of La1-xSrxFeO3 (x ¼ 0.5 and 0.9) exhibit a single reduction peak at 445 and 475 C no appearing the second peaks until the temperature reaching at 800 C. It is noted that the first major reduction peak becomes more intense and shifts slightly to higher temperatures with increasing of Sr-substitution increasing compared to that of La0.9Sr0.1FeO3. The emergence of single reduction peak for La1xSrxFeO3-d (x ¼ 0.5 and 0.9) indicates the simultaneous reduction of surface absorbed oxygen and bulk lattice oxygen without clear distinction [29]. It means that the bulk lattice oxygen inside the particle of the oxygen carriers could transfer to the surface as rapidly as to supply the oxygen vacancies as the surface oxygen is consumed, indicating a high lattice oxygen mobility [30]. Besides, the reduction peak areas which represent the hydrogen consumption increase as the Sr substitution degrees increase suggesting that Sr-doping increases the hydrogen consumption capacity of the oxygen carriers.
XPS XPS analysis was performed to determine the nature of the near-surface cation compositions and oxygen species of the La1-xSrxFeO3 samples. As shown in Fig. 4, intensive signals of La3d, Sr3d, Fe2p and O1s are observed within the binding energy (BE) of 0e1400 eV. In addition to these expected elements, three spectrum peaks of C1s for contamination typically CeC, CeOeC and OeC]O components are detected, representing the carbon formation that is aroused by exposing the samples to methane atmosphere during the methane reforming stage. Fig. 5 shows the La3d, Sr3d, and Fe2p spectra for the La1xSrxFeO3 samples by fitting the XPS curves. The XPS spectra of La3d exhibit two doublet peaks namely La3d3/2 and La3d5/2 with two splitting peaks (doublets denoted by aa’ and bb’), which are similar with the form of standard trivalent La3d presenting the typical compounds of La3þ. The first doublet peak aa’, namely La3d94f0, are related to the almost uncharged
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Fig. 4 e XPS survey spectrum of five samples.
state of La3þ ion, corresponding to the LaFeO3 crystal phase. Moreover, the peak bb’, namely La3d94f1, was believed ascribing to the electron transferring from the 2p orbital of oxygen ligand to the empty La 4f shell [29], indicating the existence of LaSrFeO4 phase or/and surface hydroxylation or carbonization of lanthanum sites [31]. The binding energy (EB) and s(LaeO) of the main peak La3d5/2 (Table 1) shows no significant change with the increase of Sr doping, being consistent with that of La2O3. It is suggested that the substitution of Sr in LaFeO3 has little effect on the chemical state of La3d from the spectra and binding energy. The Sr3d spectra exhibit two obvious peaks of Sr3d3/2 and Sr3d5/2 with spin-orbit splitting. Deconvolution of the Sr3d peaks exhibit three chemical states labeled as Sra in the range of BE of Srþ in low oxidation state; Srb being associated with the La1-xSrxFe3þO3-d crystal phase and Src which is due to the La1-xSrxFe4þ/5þO3 phase [32]. There is an obvious increase of peak Src as the Sr-doping degree increases, which means that the Sr doping not only affect the content of Sr on the oxides, but also affect the chemical states of the perovskite structure, consequently giving rise to influences on the valence state of B-site elements. For the Fe2p spectra, an asymmetry feature with doublepeaked spectrum with two small satellite peaks is observed, indicating the existence of the multiple components of Fe cations with different chemical valence states in the La1xSrxFeO3 perovskites. The peak positions of Fe2p3/2 for all the five samples are located at 710.1e710.3 eV, which is typical of Fe ions in high oxidation states being attributed to both of Fe3þ and Fe4þ oxidation states [33,34]. Due to the unstable property of Fe4þ, the unfilled Fe 3 d orbit (t32ge1g) can easily lose the electron on eg band to form Fe5þ (t32g) or to get another electron to be half-filled, meanwhile, to generate abundant oxygen vacancies to maintain the electro-neutrality [35]. The existence of Fe4þ or Fe5þ is also confirmed by researchers who € ssbauer technique to characterize the perovskites used mo particles [32,36]. The Fe3þ/(Fe4þþFe5þ) ratios of the five samples are determined by dividing the integrated areas of peaks, as shown in Table 1. A clear decrease of the Fe3þ/(Fe4þþFe5þ) ratios can be seen as the increase of the Sr substitution. This indicates that doping of Sr has an obvious effect on valence states of Fe ion, whereas the Fe3þ ions are still the primary component of the B-site for all the samples.
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The activity of the perovskite-type oxides is strongly affected by the properties of near-surface oxygen species, which can be revealed by a detailed O1s XPS scan. Fig. 6 shows the O1s scans of the five La1-xSrxFeO3 samples. As can be seen, two major peaks and two minor peaks are identified on all the five La1-xSrxFeO3 samples. The major peak with lower binding energy at around 528.0 eV is assigned to lattice oxygen species (denoted as OI) [37]. A secondary major peak at around 531.5 eV is assigned to physical adsorbed oxygen (denoted as OIII), i.e. hydroxyls species OH or carbonate species CO2 3 [38]; An minor intermediate peak appeared at around 529.0e529.5 eV is associated with surface adsorbed oxygen species, O2 2 /O (denoted as OII) [39]. And another minor peak with higher binding energy at about 533.0 eV is assigned to adsorbed molecular water on the surface of the solid particles [40]. It is noteworthy that the relative intensities of the first major peak corresponding to the lattice oxygen were obviously suppressed for the La1-xSrxFeO3 samples when the Srdoping degree (x) is above 0.5. In general, the lattice oxygen OI is conducive to partial oxidation of methane into syngas. It is observed that the XPS patterns of La1-xSrxFeO3 (x ¼ 1.0) are significantly different from those of the rest four samples. SrFeO3 give the lowest intensity peak at 528.0 eV, indicating that it has the lowest lattice oxygen content among the five oxides. This could explain the poor syngas selectivity observed in SrFeO3. As above mentioned, the bulk lattice oxygen is prone to methane partial oxidation into H2 and CO. Therefore, the Ola/ Oad (O la and Oad stand for lattice oxygen and adsorbed oxygen species, respectively.) ratio from XPS results can be taken as a comparison criterion for the performance of the oxygen carriers in chemical looping reforming process where selective oxidation of CH4 into syngas by O2 in the lattice is desirable. It can be seen from Table 2 that La0.7Sr0.3FeO3-d has the highest Ola/Oad ratio among the five perovskites, whereas the Ola/Oad ratios exhibit decrease as the Sr-doping degrees further increase when x 0.5. Besides, the oxygen vacancies concentrations, namely oxygen nonstoichiometry parameter d, increase as the Sr substitution increases. Meanwhile, d increase is accompanied with an increase of adsorbed oxygen and a decrease of Fe3þ. It means that Sr2þ substitution for La3þ generates an electronic unbalance of the perovskite structure, and as a consequence, oxygen vacancies or an increase in the oxidation state of Fe3þ to Fe4þ/Fe5þ can be produced to preserve charge neutrality [41]. As demonstrated earlier, Sr substitution in the La1-xSrxFeO3 perovskites changes the coordination environments and electron density of oxygen anions generating two effects on the lattice properties of the perovskites: one is an increase of the oxygen vacancies in the bulk of the oxides and the other is formation of hypervalent B-site cation concentrations in the perovskites. The former effect facilitates the total oxidation of hydrocarbons into CO2 and H2O, while the latter one is beneficial to partial oxidation of hydrocarbons into CO and H2. Thus, the performance of the La1-xSrxFeO3 perovskites toward chemical looping reforming reactions can be tailored by adjusting the Sr substitution contents. In one of our previous works, we found that the reactivity of the La1-xSrxFeO3 perovskites decreased as the degree of Sr substitution x > 0.5, whereas Sr doping can inhibit methane decomposition in the
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Fig. 5 e XPS deconvolution spectra of La3d, Sr3d, and Fe2p.
methane partial oxidation reactions. From the point of view of the reactivity, resistance to carbon formation, as well as oxygen donation ability, an optimal range of the Sr substitution degree is x ¼ 0.3e0.5 [19]. In one previous work [19], we studied the effects of Srdoping degree on the reactivity of La1-xSrxFeO3 perovskites on the basis of experiments conducted with a thermalgravimetrical analysis reactor and a fixed bed reactor. The sample x ¼ 0.3 exhibited a better performance for partial oxidation of methane into H2 and CO than others. The main purpose of the present work is exploring insight into the effect of Sr-
substitution on the oxygen species and cation ions distributions on the near surface of the redox catalysts. Thus, we focused on identification of elements on the surface of the oxygen carriers in this work. Taking account of the chemical state of La3d, Sr3d, Fe2p, and Ola/Oad ratio of the XPS analysis, combining with the experimental results in our previous study through TG analysis and fixed-bed tests, it can be observed that La0.7Sr0.3FeO3-d exhibits the best performance for chemical looping partial oxidation of CH4 among the five perovskite samples. Therefore, we recommend La0.7Sr0.3FeO3 as the optimal oxygen carrier in the series because it gives the
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Table 1 e Near-surface atomic ratio from XPS. Sample
x x x x x
¼ ¼ ¼ ¼ ¼
0.1 0.3 0.5 0.7 1.0
Sr3d5/2
La3d5/2 EB
s(LaeO)
Peak area/%
834.00 833.05 833.55 833.81 e
304.90 303.98 303.98 304.03 e
12.23 9.71 5.40 2.68 e
Fe2p3/2
EB
s(OeSr)
Peak area/%
EB
s(FeeO)
Fe3þ:(Fe4þþFe5þ)
Peak area/%
132.82 132.84 132.80 132.70 133.03
396.28 396.23 396.77 397.08 397.61
5.06 6.19 13.13 17.71 20.53
710.11 710.14 710.14 710.15 710.32
181.01 181.07 180.57 180.37 179.68
63.9:36.1 61.9:38.1 61.4:38.6 61.3:38.7 60.2:39.8
15.60 17.88 11.31 11.00 9.57
s(M-O) ¼ EB(M 3d5/2 or M 2p3/2)-EB(O1s), where M ¼ La, Sr, Fe. Fe3þ/(Fe4þþFe5þ) ratios were calculated based on the assumption that Fe3þ, Fe4þ and Fe5þ ions were presented in the samples. The spectra at lower binding energy at ~710 eV are attributed to Fe3þ, whereas the spectra at higher BE value are assigned to Fe4þ or/and Fe5þ.
Fig. 6 e XPS deconvolution spectra of O1s.
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Table 2 e O1s XPS peak deconvolution results and the oxygen vacancies at the surface. Sample La0.9Sr0.1FeO3-d La0.7Sr0.3FeO3-d La0.5Sr0.5FeO3-d La0.3Sr0.7FeO3-d SrFeO3-d
O2(lattice)
O, O2 2 (adsorbed)
OH, CO23
H2O
Ola/Oad
d
44.36 47.21 37.01 26.41 9.70
16.17 12.97 23.46 14.56 24.81
31.17 26.71 33.09 54.11 60.95
8.30 13.11 6.44 4.92 4.54
2.74 3.64 1.58 1.81 0.40
0.05 0.15 0.25 0.35 0.50
d is calculated by assuming all the Fe ion as trivalent. 2Oad refer to adsorbed oxygen (O, O2 2 ) and Ola to lattice oxygen O
maximum Ola/Oad ratio of 3.64:1, which can be regarded as a criterion for the reactivity and selectivity of the oxygen carriers for chemical-looping steam methane reforming reactions.
Reactivity in successive redox cycles To further observe the reactivity of the La0.7Sr0.3FeO3 in chemical looping reforming of methane and in water splitting reactions, we studied its performance of co-production of syngas and hydrogen in multiple successive redox cycles by exposing it in an alternating methane reducing and steam oxidizing in a fixed bed reactor. Fig. 7 shows the methane conversion, H2/CO molar ratio in methane chemical looping partial oxidation stage and the H2 yields in steam splitting stage of the tests. As shown in Fig. 7 (a), CH4 conversions reached above 80% in most of the cycles in methane stage, and the average H2/CO molar ratio was close to 2.0. This indicates that methane was mainly converted into syngas via partial oxidation on the peroviskite particles, and methane decomposition was not significantly observed because methane cracking results in a high H2/CO molar ratio above 2.0. When La0.7Sr0.3FeO3 perovskite was reduced by methane, the oxygen species of the perovskite were consumed generating oxygen vacancies, metallic phase and/or metal oxides with lower oxygen content in the bulk of the perovskite. Fig. 8 shows the XRD patterns of the reduced La0.7Sr0.3FeO3. It can be observed that the reduced sample is primarily composed of a composite of La2SrOx, Fe3O4 and FeO. In order to prevent occurrence of agglomeration of the oxygen carriers, the sample was not deeply reduced into metallic phase in the methane stage in this work.
In the water splitting stage, the Fe3O4, FeO and/or Fe (if formed) in the reduced perovskite sample is re-oxidized by steam to replenish its lattice oxygen and to re-fill the oxygen vacancies thereby generating pure hydrogen. Fig. 7 (b) shows the amount of hydrogen generated in the steam oxidation stage as a function of the number of cycles. The hydrogen productivity appears obvious variation from the 1st to the 4th redox cycles giving the highest hydrogen yield of 100.8ml/goc at the 3rd cycle (OC denotes oxygen carrier). Afterward, the hydrogen productivity remained stable at around 90 ml/goc in the resting cycles. The average concentration of the hydrogen in the ten cycles is more than 96% with a minor fraction of CO and CO2 which were produced from gasification of carbon deposition formed during methane reforming. As above mentioned, the inlet gases in the water-splitting stage are steam and N2, therefore, the generation of CO and CO2 in this stage is attributed to the carbon formation in the methane reforming stage. Carbonation of the oxygen carriers was not detected in this work. To avoid the formation of carbon in methane reforming stage, the reduction of the redox catalyst should be stopped before 500 s [42]. Fig. 9 shows a H2 production profile in a typical redox cycle in water splitting stage. The peak H2 production rate was 2.12 mmol/(goc$min) at 3 min of the reaction time. After 6 min, the H2 production rate decreased significantly to 0.56 mmol/(goc$min) and to 0.04 mmol/(goc$min) till at 20 min. It is noted that minor fractions of CO and CO2 were detected in the gas products at the initial 6 min due to the carbon formation in the methane reforming stage. In summary, the reduced La0.7Sr0.3FeO3 perovskite exhibited faster oxidation kinetics in water splitting stage. Near 80% of the hydrogen was produced in the initial 6 min period during the whole 20 min water splitting stage. In other
Fig. 7 e Catalytic performance of sample La0.7Sr0.3FeO3 in ten cycles. (a) methane reduction stage, (b) steam oxidation stage.
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DFT calculations
Fig. 8 e XRD patterns of La0.7Sr0.3FeO3 reduced by methane. :-La2SrOx; *-Fe3O4; △-FeO.
Fig. 9 e H2 production rate in water splitting stage in a typical redox cycle.
words, 80% of the active lattice oxygen of the reduced La0.7Sr0.3FeO3 sample could be replenished in such a short regeneration period of 6 min. To determine the micro morphology of the perovskite sample during the redox cyclic reactions, SEM images have been obtained as shown in Fig. 10. The as-prepared sample exhibits micro morphology with uniform aggregates of small granules with interconnected pores existing on the particle surface. Different sizes of holes distribute in the grain boundaries with various particle sizes. After successive redox cycles, the used sample shows a morphology slightly rough due to the accumulative impacts during the altering methane reduction and steam oxidation leading to crack and interstice of the particles. Aggregation between small particles can be found on the surface, suggesting that slight sintering occurred in the course of the successive cycles. However, the slight sintering of the particles did not significantly affect the reactivity of the perovskite. To prevent agglomeration and sintering of the perovskite redox catalysts, adding an inert oxide such as Al2O3 as a supporter or additive may be a good approach to improve the agglomeration resistance of the redox catalysts [43].
In general, the electron behavior near the Fermi level at the top of valance band can reflect the most significant characterization of the studied materials. The electronic band structures of the six perovskites La1-xSrxFeO3 (x ¼ 0, 0.1, 0.3, 0.5, 0.7, 1.0) were calculated in this work. The band structures of LaFeO3, La0.7Sr0.3FeO3, and SrFeO3 were typically shown in Fig. 11. It is observed that the band gap near Fermi level of La0.7Sr0.3FeO3 is smaller than them of the LaFeO3 and SrFeO3 samples, which means that partial doping of Sr for La making the energy distribution of electrons. It indicates that more electrons occupy orbitals with higher energy in the cell of Srdoped La1-xSrxFeO3 perovskites; thus, these electrons are more prone to be excited and the cell as an electron donor to oxidize the fuels easier [44]. Recalling their reducibility performed in the experiments above, the electronic continuity enforces the reactivity of the Sr-doped samples, especially the La0.7Sr0.3FeO3, in chemical looping reactions. To further investigate the properties of the LaFeO3 with and without Sr-doping, the partial density of states (PDOS) were calculated. The PDOS profiles of the LaFeO3, La0.7Sr0.3FeO3, and SrFeO3 were selected and shown in Fig. 12. It is observed that the dominant electronic character near the Fermi level is due to Fe 3 d states, La 3 d states, and a noticeable mixing with O 2p states. In the pure unit cell of LaFeO3, divalent La cations constitute an octahedral coordination with six oxygen ions and the trivalent iron cations coordinated with twelve oxygen atoms located in the octahedron cavity. Partial substitution of Sr in La sites increased the transition intensity of Fe 3d-O2p orbit and more electrons were excited near Fermi level. It suggests that the electron density of Fe atom increase for the doped samples around Fermi level both in conduction band and valence band. Compared to LaFeO3, the Fe-d orbitals and O-p orbitals are located in higher energies in the Sr-doped materials and thus the electrons occupied those orbitals became more likely to be activated. This is again consistent with the experimental results.
Reaction pathway of water splitting on the surface of oxygen carrier particles Reaction mechanism of chemical looping reforming of methane with oxygen carriers were studied in a number of previous literature [27,45,46]. Li et al. [46] believed that oxygen vacancies and the resulting Fe2þ/Ce3þ ions acts as the active sites for methane activation in chemical looping reforming reactions when CeO2/LaFeO3 is used as oxygen carriers. CeH bond is activated and dissociated on the surface of the oxygen carrier particles generating CH3 groups with significant radical character and weak interactions with the surface at the transition state. CO2 and steam were first formed in the initial stage, whereas CO and H2 increasingly became the main products in the main stage when the most reactive oxygen species were consumed. However, the reaction mechanism of the water thermochemical splitting reaction on the reduced La1-xSrxFeO3 perovskites is not well understood. On the basis of the XPS analysis, and the resulting gas products of the steam oxidation stage, the reaction mechanism model of the water splitting reaction is proposed, as shown in Fig. 13. As
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Fig. 10 e SEM images of (a) fresh and (b) spent sample after 10 cycles.
Fig. 11 e Band structure of (a) LaFeO3 (b) La0.7Sr0.3FeO3 (c) SrFeO3.
Fig. 12 e The partial density of states (PDOS) for (a) LaFeO3 (b) La0.7Sr0.3FeO3 (c) SrFeO3. The dashed vertical lines represent the position of the Fermi level.
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Fig. 13 e Reaction mechanism model of water splitting over reduced La0.7Sr0.3FeO3 perovskite.
water vapor is introduced on the surface of the reduced perovskite, the H2O molecule is adsorbed onto a metal surface, which acts as an electron donor and an active site of catalyst for HeO bond breaking, forming an OH and a Hþ. An OH will react with adjacent OH to generate H2 meanwhile forming an O2. Then the O2 ions would be taken by the oxygen vacancies in the oxygen carrier, along with the oxidation of metals and/or metal cations with lower valence in the B-site of perovkite. As the B-site metals are oxidized, electrons will be released and transferred to two Hþ to generate H2 as well. A multi-step mechanism that reflects the oxidation of reduced perovskite by water vapor is expressed below: H2O(g) / OH þ Hþ
(1)
2OH / O2 þ H2(g)
(2)
Fe(bulk in the B-site of La0.7Sr0.3FeO3-d 2Fe2þ(bulk in
perovskite)
- 2e / Fe2þ
the B-site of La0.7Sr0.3FeO3-d perovskite)
- 2e / 2Fe3þ
(3) (4)
2Hþ þ 2e / H2(g)
(5)
La0.7Sr0.3FeO3-d þdH2O(g) / La0.7Sr0.3FeO3 þdH2(g)
(6)
Here, the reaction (6) represents the global reaction of water splitting on the surface of the reduced La0.7Sr0.3FeO3 perovskite.
Conclusions The substitution of Sr2þ for La3þ in perovskites La1-xSrxFeO3d generates an electronic unbalance of the perovskite structure, and as a consequence, oxygen vacancies and/or an increase in the oxidation state of Fe3þ to Fe4þ/Fe5þ can be produced to preserve charge neutrality. The Ola/Oad ratio, which can be taken as a criterion for comparison of the reactivity of partial oxidation of methane, obtains the maximum of 3.645:1 for La0.7Sr0.3FeO3-d and decreases as x 0.5. La0.7Sr0.3FeO3-d exhibits high catalytic stability during the successive redox process for the continuous production of syngas and hydrogen achieving up to 80% CH4 conversion in the methane partial oxidation step and 96% of H2 concentration in the water splitting step in ten successive redox tests at 850 C. It is a desirable oxygen carrier candidate for CL-SMR.
Acknowledgements The financial support of National Key Research and Development Program of China (2017YFE0105500) is gratefully acknowledged. This work was also supported by the National Natural Science Foundation of China (51876205, 51776210) and the Guangxi Natural Science Foundation (2018JJD120017), R&D Project of Guilin University of Technology (GLUTQD2018027).
references
[1] Richter HJ, Knoche KF. Reversibility of combustion processes. ACS Symp Ser 1983;235:71e85. [2] Zhu X, Wei Y, Wang H, Li K. CeeFe oxygen carriers for chemical-looping steam methane reforming. Int J Hydrogen Energy 2013;38:4492e501. [3] Lee DH, Cha KS, Kim HS, Park CS, Kim YH. Syngas and hydrogen production via stepwise methane reforming over Cu-ferrite/YSZ. Int J Energy Res 2014;38:1522e30. [4] Zhu X, Zhang M, Li K, Wei Y, Zheng Y, Hu J, Wang H. Chemical looping water splitting over ceria-modified iron oxide: performance evolution and element migration during redox cycling. Chemcial Engineering Science 2018;179:92e103. [5] Huang H, Liu Y, Tang W, Chen Y. Catalytic activity of nanometer La1xSrxCoO3 (x ¼ 0, 0.2) perovskites towards VOCs combustion. Catal Commun 2008;9:55e9. [6] Parvaneh EA, Khodadadi A, Hessam ZA, Mortazavi Y. Effects of excess manganese in lanthanum manganite perovskite on lowering oxidation light-off temperature for automotive exhaust gas pollutants. Chem Eng J 2011;169:282e9. n M, Leion H, Mattisson T, Lyngfelt A. Combined oxides [7] Ryde as oxygen-carrier material for chemical-looping with oxygen uncoupling. Appl Energy 2014;113:1924e32. [8] Tang M, Xu L, Fan M. Progress in oxygen carrier development of methane-based chemical-looping reforming: a review. Appl Energy 2015;151:143e56. [9] Mihai O, Chen D, Holmen A. Catalytic consequence of oxygen of lanthanum ferrite perovskite in chemical looping reforming of methane. Ind Eng Chem Res 2011;50:2613e21. [10] Haron W, Thaweechai T, Wattanathana W, Laobuthee A, Manaspiya H, Veranitisagul C, Koonsaeng N. Structural characteristic and dielectric properties of La1-xCoxFeO3 and LaFe1-xCoxO3 synthesized via metal organic complexes. Energy Procedia 2013;34:791e800. [11] Varandili SB, Babaei A, Ataie A, Khodadadi AA, Kazerooni H. Nano-structured Pd doped LaFe(Co)O3 perovskite; synthesis,
10276
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
[29]
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 1 0 2 6 5 e1 0 2 7 6
characterization and catalytic behavior, Materials chemistry and Physics. Mater Chem Phys 2018;205:228e39. Kuhn JN, Matter PH, Millet J-MM, Watson RB, Ozkan US. Oxygen exchange kinetics over Sr- and Co-doped LaFeO3. J Phys Chem 2008;112:12468e76. Jeong JH, Song CG, Kim KH, Sigmund W, Yoon JW. Effect of Mn doping on particulate size and magnetic properties of LaFeO3 nanofiber synthesized by electrospinning. J of Alloy Compd 2018;749:599e604. Zhang R, Li P, Xiao R, Liu N, Chen B. Insight into the mechanism of catalytic combustion of acrylonitrile over Cudoped perovskites by an experimental and theoretical study. Appl Catal BEnviron 2016;196:142e54. Zhao K, Fang H, Huang Z, Wei GQ, Zheng AQ, Li HB, Zhao ZL. Perovskite-type oxides LaFe1-xCoxO3 for chemical looping steam methane reforming to syngas and hydrogen coproduction. Appl Energy 2016;168:193e203. Taylor FH, Buckeridge J, Catlow RAC. Screening divalent metals for A- and B-site dopants in LaFeO3, Chemistry of Materials. Chem Mater 2017;29:8147e57. Shafiefarhood A, Galinsky N, Huang Y, Chen YG, Li FX. Fe2O3@LaxSr1-xFeO3 core-shell redox catalyst for methane prtial oxidation. Chem Catal Chem 2014;6:790e9. Neal LM, Shafiefarhood A, Li FX. Dynamic methane partial oxidation using a Fe2O3@LaxSr1-xFeO3 core-shell redox catalyst in the absence of gaseous oxygen. ACS Catal 2014;4:3560e9. Fang H, Li XA, Zhao K, Huang Z, Wei G, Li H. The use of La1xSrxFeO3 perovskite-type oxides as oxygen carriers in chemical-looping reforming of methane. Fuel 2013;108:465e73. Qin Y, Sun L, Zhang D, Huang L. Role of ceria in the improvement of SO2 resistance of LaxCe1xFeO3 catalysts for catalytic reduction of NO with CO. Catal Commun 2016;79:53e7. Evdou A, Zaspalis V, Nalbandian L. La1xSrxFeO3d perovskites as redox materials for application in a membrane reactor for simultaneous production of pure hydrogen and synthesis gas. Fuel 2010;89:1265e73. Jones A, Islam MS. Atomic-scale insight into LaFeO3 perovskite: defect nanoclusters and ion migration. J Phys Chem C 2008;112:4455e62. Taylor DD, Schreiber NJ, Levitas BD, Xu W. Oxygen storage Properties of La1-xSrxFeO3 for chemical-looping reactionsean in situ neutron and synchrotron X-ray study. Chem Mater 2016;28:3951e60. Evdou A, Zaspalis V, Nalbandian L. Lal-xSrxMnO3 perovskite as redox materials for the production of high purity hydrogen. Int J Hydrogen Energy 2008;33:5554e62. Iwanowski RJ, Heinonen MH, Pracka I, Kachniarz J. XPS characterization of single crystalline SrLaGa3O7:Nd. Appl Sur Sci 2013;283:168e74. Cyza A, Kopia A, Cieniek L, Kusinski. Structural characterization of Sr doped LaFeO3 thin films prepared by pulsed electron deposition method. J. Mate Today Proc 2016;3:2707e12. Zhao K, Zheng A, Li H, He F, Huang Z, Wei G, Shen Y, Zhao Z. Exploration of the mechanism of chemical looping steam methane reforming using double perovskite-type oxides La1.6Sr0.4FeCoO6. Appl Catal B Environ 2017;219:672e82. Zhang R, Villanueva A, Alamdari H, Kaliaguine S. Cu- and Pdsubstituted nanoscale Fe-based perovskites for selective catalytic reduction of NO by propene. J Catal 2006;237(2):368e80. Atribak I, Bueno LA, Garcia GA. Combined removal of diesel soot particulates and NOx over CeO2eZrO2 mixed oxides. J Catal 2008;259:123e32.
[30] Zheng Y, Li K, Wang H, Wang Y, Tian D, Wei Y, Zhu X, Zeng C, Luo Y. Structure dependence and reaction mechanism of CO oxidation: a model study on macroporous CeO2and CeO2-ZeO2 catalysts. J Catal 2016;344:365e77. [31] Mickevicious S, Grebinskij S, Bondarenka V, Vengalis B, Sliuziene K, Orlowski BA, Osinniy V, Drube W. Investigation of epitaxial LaNiO3x thin films by high-energy XPS. J Alloy Compd 2006;423:107e11. [32] Stathopoulos VN, Belessi VC, Bakas TV, Neophytides SG, Costa CN, Pomonis PJ, Efstathiou AM. Comparative study of LaeSreFeeO perovskite-type oxides prepared by ceramic and surfactant methods over the CH4 and H2 lean-deNOx. Appl Catal B Environ 2009;93:1e11. [33] Widatallah HM, Al-Rawas AD, Johnson C, Al-Harthi SH, Gismelseed AM, Moore EA, Stewart SJ. The formation of nanocrystalline SrFeO3-d using mechano-synthesis and € ssbauer studies. J subsequent sintering: structural and mo Nanosci Nanotechnol 2009;9:2510e7. [34] Al-Rawas AD, Widatallah HM, Al-Harthi SH, Johnson C, Gismelseed AM, Elzain ME, Yousif AA. The formation and structure of mechano-synthesized nanocrystalline € ssabuer and XPS analyses. Mater Sr3Fe2O6.4: XRD Rietveld, Mo Res Bull 2015;65:142e8. [35] Zhao W, Yang Q, Cui J. XPS study of surface absorbed oxygen of ABO3 mixed oxides. J Rare Earths 2008;26:511e4. [36] Russo U, Nodari L, Faticanti M, Kuncser V, Filoti G. Local interactions and electronic phenomena in substituted LaFeO3 perovskites. Solid State Ionics 2005;176:97e102. [37] Dudric R, Vladescu A, Rednic V, Neumann M, Deac IG, Tetean R. XPS study on La0.67Ca0.33Mn1xCoxO3 compounds. J Mol Struct 2014;1073:66e70. [38] Fierro JLG. Structure and composition of perovskite surface in relation to adsorption and catalytic properties. Catal Today Off 1990;8:153e74. [39] Yamazoe N, Teraoka Y, Seiyama T. TPD and XPS study on thermal behavior of absorbed oxygen in La1-xSrxCoO3. Chem Lett 1981;10:1767e70. [40] Chen L, Wang W, Zhao N, Liu Y, He B, Hu F, Chen C. Structure properties and catalytic performance in methane combustion of double perovskites Sr2Mg1xMnxMoO6d. Appl Catal B Environ 2011;102:78e84. [41] Merino NA, Barbero BP, Grange P, Cadu´s LE. La1xCaxCoO3 perovskite-type oxides: preparation, characterisation, stability, and catalytic potentiality for the total oxidation of propane. J Catal 2005;231:232e44. [42] Ebrahimi Hadi, Rahmani Mohammad. Modeling chemical looping syngas production in a microreactor using perovskite oxygen carriers. Int J Hydrogen Energy 2018;43(10):5231e48. [43] Brackmann R, Perez CA, Schmal M. LaCoO3 perovskite on ceramic mololiths-pre and post reaction analyzes of the partial oxidation of methane. Int J Hydrogen Energy 2014;39:13991e4007. [44] Liu S, Xiang D, Xu Y, Sun Z, Cao Y. Relationship between electronic properties of Fe3O4 substituted by Ca and Ba and their reactivity in chemical looping process: a first-principles study. Appl Energy 2017;202:550e7. [45] Neal LM, Shafiefarhood A, Li F. Dynamic methane partial oxidation using a [email protected] core-shell redox catalyst in the absence of gaseous oxygen. ACS Catal 2014;4:3560e9. [46] Zheng Y, Li K, Wang H, Tian D, Wang Y, Zhu X, Wei Y, Zheng M, Luo Y. Designed oxygen carriers from macroporous LaFeO3 supported CeO2 for chemical-looping reforming of methane. Appl Catal B Environ 2017;202:51e63.
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La1-xSrxFeO3 perovskite-type oxides for chemicallooping steam methane reforming: Identification of the surface elements and redox cyclic performance Fang He a,b,*, Jing Chen b,c, Shuai Liu d, Zhen Huang b, Guoqiang Wei b, Guixia Wang a, Yan Cao d,e, Kun Zhao b,** a
College of Chemistry and Bioengineering, Guilin University of Technology, Guilin 541004, PR China Key Laboratory of Renewable Energy, Chinese Academy of Sciences, Guangdong Key Laboratory of New and Renewable Energy Research and Development, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, China c Institute of Nano Science and Technology, University of Science and Technology of China, Suzhou 215123, China d College of Chemistry & Chemical Engineering, Anhui University, Hefei 230601, China e Department of Chemistry, Western Kentucky University (WKU), Bowling Green, KY 42101, USA b
article info
abstract
Article history:
We report a family of perovskite-type oxides La1-xSrxFeO3 (x ¼ 0.1, 0.3, 0.5, 0.7, 1.0) prepared
Received 1 January 2019
by combustion method as effective redox catalysts for methane partial oxidation and
Received in revised form
thermochemical water splitting in a cyclic redox scheme. The effect of Sr-doping on the
21 February 2019
characterizations and properties of these perovskite-type oxides were studied by means of
Accepted 3 March 2019
X-ray diffraction (XRD), hydrogen temperature-programmed reduction (H2-TPR), X-ray
Available online 26 March 2019
photoelectron spectroscopy (XPS), and scanning electron microscope (SEM). All the asprepared and regenerated samples with various Sr substitutions exhibited pure crystal-
Keywords:
line perovskite structure. The oxygen carrying capacity of the La1-xSrxFeO3 perovskites was
Perovskites
improved by doping Sr into the La-site. Besides, Sr-substitution has obvious effects on the
Oxygen species
valences of the Fe cations in the B-site and the oxygen species distribution of the La1-
XPS
xSrxFeO3
Chemical looping
series because it gives the maximum Ola/Oad (Ola and Oad stand for lattice oxygen and
Methane reforming
adsorbed oxygen species, respectively.) ratio of 3.64:1, which can be regarded as a criterion
Water thermochemical splitting
for the reactivity and selectivity of partial oxidation of methane into syngas of the oxygen
perovskites. We recommend La0.7Sr0.3FeO3 as the optimal oxygen carrier in the
carriers. Up to 80% CH4 conversion in the methane partial oxidation step and 96% of H2 concentration in the water splitting step were achieved in ten successive redox tests conducted in a fixed bed reactor at 850 C with La0.7Sr0.3FeO3 as a redox catalyst. The electronic properties of the original LaFeO3 cell and its lattice substituted by Sr were calculated based on the density functional theory method. Electronic structure analysis demonstrates that doping of Sr makes LaFeO3 more electric conductive and its electron is prone to be excited. This is in agreement with the test results that La0.7Sr0.3FeO3 exhibited better performance in chemical looping reactions. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. College of Chemistry and Bioengineering, Guilin University of Technology, Guilin 541004, PR China. ** Corresponding author. E-mail addresses: [email protected] (F. He), [email protected] (K. Zhao). https://doi.org/10.1016/j.ijhydene.2019.03.002 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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Introduction Chemical-looping steam methane reforming (CL-SMR) [1e3], in which methane is selectively oxidized into syngas by the lattice oxygen of an oxygen carrier in a reforming reactor, and then the reduced oxygen carrier is oxidized by steam to replenish its lattice oxygen thereby generating hydrogen in a steam reactor, is a novel technology co-producing syngas and hydrogen, as shown in Fig. 1. The success of CL-SMR approach is highly dependent on finding suitable oxygen carriers (also called redox catalysts) with high methane conversion, good activity for water splitting to produce hydrogen, and good reactivity in successive redox cycles [4]. Perovskite-type oxides with chemical formula ABO3 have been extensively investigated for many potential energy and environmental related applications due to their uniquely tunable bulk and surface properties for oxygen vacancy formation and elimination, mixed ion and electron conduction, surface/bulk oxygen evolution, band structure, and ability to accommodate large number of dopants in the cationic (A/B) sites [5e7]. Among various perovskites, LaFeO3-based oxides have been attracted much attention since they have potential as candidate materials for versatile applications in advanced technologies, such as catalysts, oxygen carriers, chemical sensors, cathode materials in solid oxide fuel cells etc due to their high oxygen mobility and stable capability of hosting large concentrations of vacancies in the structure [8e10]. Generally, metal cations in B-site are regarded as the key factor affecting the chemical reactivity, magnetic properties, and electrical conductivity of the perovskite-type oxides. Therefore, doping various metal ions into the B-site of LaFeO3 has been extensively in previous literature [11e13]. Zhang et al. [14] studied the catalytic combustion of acrylonitrile over Cu-doped LaBO3 (B¼ Fe, Co, and Mn) perovskites and found that LaFe0.8Cu0.2O3 exhibited the best performance due to an easy transformation from Cu2þ to Cu0 at low temperatures. Zhao et al. [15] prepared LaFe1-xCoxO3 (x ¼ 0.1, 0.3, 0.5, 0.7, 1.0)
Fig. 1 e CL-SMR for syngas and hydrogen production.
perovskites and found that Co-doping in the B-site could obviously improve the reactivity and inhibit carbon formation in the chemical looping steam methane reforming reactions. Taylor et al. [16] examined the viability of a range of divalent B-site dopants for promoting ionic and electronic conductivity of LaFeO3 by using ab initio methods. They concluded that cobalt, nickel and magnesium are the most favorable B-site dopants, due to low solution energies and low binding energies to oxygen vacancies, along with minimal distortion, particularly for nickel, to the LaFeO3 lattice. In Comparison to extensive studies of the effects of different metal cations doping in B-site of LaFeO3 on its properties, investigations on effects of metal cations doping in the A-site on its performance have limitedly been reported. Previous literature reported that the metals in A-site are noncatalytic, however, they have effects on the concentration of oxygen vacancies and the valence states of the B-site metals, thus consequently affecting the reactivity of the perovskitetype oxides [17,18]. The valences of M ion in Lal-xSrxMO3 (where M is a first-row transition metal) might be adjusted by partial substitution of La3þ by Sr2þ, thereby generating a number of interesting changes in their physical and chemical properties. According to our previous studies [19], Sr substitution would inhibit methane decomposition and enhance the reactivity of La1-xSrxFeO3 in reactions of partial oxidation of methane. Effect of Sr2þ and/or Ce2þ substitution in the A-site of LaFeO3 perovskite on its catalytic activity for oxidation of NO and CH4 have been studied, respectively [20,21]. Jones and the co-workers [22] found that Sr2þ substitution on the La3þ site gives a minimum in the binding energy between Sr2þ ions and vacancy clusters, which would be beneficial to oxide ion conductivity and oxygen migration of the perovskites. Taylor et al. [23] reported that La2/3Sr1/3FeO3 exhibits promise for use as an oxygen storage material in methane reforming reactions due to its high product selectivity, fast oxide diffusion, and cycle stability because its structure remains homogenous throughout the reaction. Evdou et al. [24] used Lal-xSrxMnO3 as redox catalyst for two-step steam reforming of methane towards the production of high purity hydrogen, and found that a production of 60 mmol H2 per 500 mmol water passed over 200 mg La0.3Sr0.7MnO3 at 1273 K. However, the mechanism of the enhancement of Sr-doping on the reactivity of La1-xSrxFeO3 perovskites was not yet discussed in detail. The interaction effects of Sr substitution on the element compositions on the near-surface of the perovskites are still unknown. As a powerful technique to study the near-surface properties of materials, X-ray photoelectron spectroscopy (XPS) technology is sensitive to the chemical environment and the XPS spectra can give information about the oxidation state of the surface elements of solid materials. In this work, a detailed investigation of the metal valence variation and oxygen species distribution on the near-surface of the particle is made by mean of XPS measurement to determine the best candidate of La1-xSrxFeO3 (x ¼ 0.1, 0.3, 0.5, 0.7, 1.0) for CL-SMR. Then ten successive redox reactions were performed to investigate the thermal stability of the La0.7Sr0.3FeO3 sample in a fixed bed reactor. We recommend La0.7Sr0.3FeO3 as the optimal oxygen carrier in the series because of its high reactivity, selectivity, as well as structure stability.
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Experimental Synthesis of the La1-xSrxFeO3 The La1-xSrxFeO3 (x ¼ 0.0, 0.1, 0.3, 0.5, 0.7, 0.9, and 1.0) perovskites in this work were synthesized by combustion method. Stoichiometric amounts of La(NO3)3$6H2O, Sr(NO3)2, and Fe(NO3)3$9H2O were weighed and put into a beaker to mix with an amount of deionized water to make a solution. Then glycine (H2NCH2CO2H) was added into the mixed nitrates solution to reach a glycine/nitrates molar ratio of 1.05. The resulting solution was allowed to evaporate by stirring in a 100 mL beaker at 70 C until a viscous gel with brown color was obtained. The gel was then put in a muffle furnace at 300 C to combust for generation of a porous precursor. The powdered precursor was calcined at 500 C for 3 h, and then at 900 C for 6 h to get the fresh perovskite-type oxides of La1-xSrxFeO3d (x ¼ 0.0, 0.1, 0.3, 0.5, 0.7, 0.9, and 1.0).
Material characterization The crystal phases of the perovskites were identified by XRD in a Japan Science D/max-R diffractometer with Cu Ka radiation (l ¼ 0.15406 nm), operating voltage of 40 kV and current of 40 mA, and the diffraction angle (2q) was scanned from 10 to 80 . Hydrogen-temperature programmed reduction (H2-TPR) tests were conducted in 5.0 vol% H2 balanced with helium at a flow rate of 60 ml/min from room temperature to 900 C with a heating rate of 10 C/min. X-ray photoelectron spectroscopy (XPS) was used to probe the near-surface composition of the oxides. The equipment was Thermo Fisher Scientific Inc with an Al Ka X-ray source at an operating voltage of 20 kV and a current of 10 mA, under the conditions of 20eV and 100eV pass energy for the survey spectra and the single element spectra. The XPS analysis software used in this work is Avantage (version 5,932,0,6055) provided by Thermo Fisher Scientific. The micro-morphology of the as-prepared and regenerated perovskite samples were studied by scanning electron microscopy (SEM) on a Hitachi S4800 instruments.
Successive redox reactivity The successive redox cyclic reactivity was tested in a fixed-bed quartz reactor (30 mm i.d.) under atmospheric pressure at 850 C by using 2 g of oxygen carrier in each test. The experiments of partial oxidation of methane were performed in a gas flow of 40.0 vol% methane with nitrogen as a balance gas at flow rate of 50 ml/min. The product gases out of the reactor were collected with gas bags and analyzed by a gas chromatograph (Shimadzu GC-2010plus).When the methane conversion step finished, pure N2 (50 ml/min) was fed into the reactor purging the reactor for 30 min to avoid mixing of gases from the two steps. After that, steam generated by injecting demineralized water in an electric furnace at 400 C using a micro pump was introduced into the reactor for 20 min with N2 as carrier gas (50 ml/min). The flow rate of the water was controlled at 0.2 ml/min. The temperature at the gas inlet of the reactor is around 300 C, therefore, the total flow rate of inlet gases in the water-splitting stage is 628 ml/min including the
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nitrogen and steam. The real flow rate of inlet gases may lower than this value because a fraction of steam will condense into water in some cases. Next, dry air (50 ml/min) was introduced into the reactor for 10 min to guarantee the lattice oxygen consumed for methane oxidation completely replenishment. The above partial oxidation of methane, water-splitting, and air oxidation cycle was repeated for 10 times. To reduce the experimental error, the successive redox cyclic reactivity tests were repeated 3 times and then averaged the results.
DFT calculations DFT calculations on electronic properties of the synthesized perovskite samples were carried out using a module of Materials studio 8.0 (CASTEP, Cambridge Serial Total Energy Package). Perovskite has a face-centered cubic lattice of space group Pm-3m, number 221 in the international tables. To simplify, a primitive cell of LaFeO3 was first built and then the metal La was partially substituted by metal Sr in proportion. Generalized Gradient Approximation-Perdew Burke Ernzerhof (GGA-PBE) was selected as exchange-correlation functional in all calculations, therein using the GGA to represent the atoms and PBE representing solids [25]. An energy cutoff at 340 eV was adopted after a convergence study over the range of 450e750 eV. The k-point separation was specified within 0.015 Å1 in the geometry optimization and the convergence criteria for the calculation were set to the energy tolerance at A, 1.0 105 eV/atom, the maximum force tolerance at 0.04 eV/ and the self-consistent field tolerance at 1.0 106 eV/atom.
Results and discussion XRD The XRD patterns of the as-prepared La1-xSrxFeO3 samples are shown in Fig. 2 (a). It can be seen that all the as-prepared samples with various Sr-doping degrees in the La-site are well crystallized with orthorhombic perovskite structure. No impurity peak appears with Sr substitution, indicating that all Sr-doped samples retain the perovskite crystal structure. It is noted that the diffraction peaks of the samples with Sr-doping in La site are a bit broaden than those of LaFeO3, suggesting that the Sr2þ incorporated into the La3þ site of the LaFeO3 perovskite lattice without causing phase change. In addition, the broadening of the full width at half maximum (FWHM) at approximately 32.5 (2q) means that the Sr-doping generates lattice contraction and particle size decrease of the doped samples. A part of La3þ is substituted by Sr2þ causing an electronic unbalance in the perovskite lattice that can be compensated by the formation of oxygen vacancies and/or oxidation of a fraction of Fe3þ ions to Fe4þ (even Fe5þ). The ionic radius of Fe4þ (0.058 nm) is smaller than that of Fe3þ (0.0645 nm) explaining the lattice contraction and smaller grain size [15]. Moreover, the decreasing of the crystalline size may be due to the slowing the crystal growth process because doping LaFeO3 by Sr would possibility of blockade of crystal growth, giving rise decrease grain size [26]. The regenerated samples after successive redox reactions were also examined by XRD. The XRD patterns of the La1-xSrxFeO3 samples of
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Fig. 3 e H2-TPR profiles of La1-xSrxFeO3-d. Temperature ramping rate is 10 C/min.
Fig. 2 e XRD patterns of La1-xSrxFeO3 perovskites of (a) fresh, (b) regenerated. x ¼ 0.1, 0.3, and 0.5 were selected and shown in Fig. 2 (b). The XRD patterns of the spent samples are very similar to those of the as-prepared samples. This means that the perovskite oxides have good regenerability. Furthermore, we found that the splitting of the strongest characteristic peaks (at 2q ¼ 32.5 ) of the regenerated perovskites become less obvious, indicating that the crystal structure becomes purer with better integration as the samples were subjected to multi-cyclic redox reactions.
H2-TPR Because oxygen carriers utilize lattice oxygen to convert methane into syngas, it is important to understand the reduction behaviors of the oxygen carriers by reducing gases or fuels. To explore the oxygen release properties of the oxygen carriers under a reducing environment, H2-TPR was performed to characterize the reducibility of the La1-xSrxFeO3 perovskites with x ¼ 0.0, 0.1, 0.3, 0.5, and 0.9. As shown in Fig. 3, the LaFeO3 without Sr-doping has two reduction peaks with defined maximums at about 482 C and 680 C, respectively. In a previous study, we analyzed the Fe-based perovskite type oxides at different reduction stages using XRD and XPS. Results showed that the reduction of Fe ions in B-site is a stepwise reduction pathway as Fe4þ/Fe5þ/Fe3þ/Fe2þ [27]. The first peak corresponds to reduction of adsorbed oxygen which can happen at a relative low temperature, combing with the transitions in iron valence states from Fe4þ/Fe3þ to Fe3þ/Fe2þ. The second peak is ascribed to the bulk reduction of lattice oxygen and the deep reduction of Fe3þ/Fe2þ to Fe2þ and even metallic Fe. Zhang and the co-workers [28] reported similar H2-TPR results of Fe-based perovskites. The TPR profiles of Sr-doped La1-xSrxFeO3 were compared those of
undoped LaFeO3. La0.9Sr0.1FeO3 also exhibits two reduction peaks with defined maximums at 388 C and 747 C, respectively. In comparison to the peaks of pure LaFeO3, the first peak of La0.9Sr0.1FeO3 shifts near 100 C to lower temperatures, whereas the second peak shifts slightly to higher temperatures. The La0.7Sr0.3FeO3 appears triplet reduction peaks: one major peak at about 470 C and two minor peaks at 590 and 770 C, respectively. With increasing Sr-doping degrees above 0.3, the samples of La1-xSrxFeO3 (x ¼ 0.5 and 0.9) exhibit a single reduction peak at 445 and 475 C no appearing the second peaks until the temperature reaching at 800 C. It is noted that the first major reduction peak becomes more intense and shifts slightly to higher temperatures with increasing of Sr-substitution increasing compared to that of La0.9Sr0.1FeO3. The emergence of single reduction peak for La1xSrxFeO3-d (x ¼ 0.5 and 0.9) indicates the simultaneous reduction of surface absorbed oxygen and bulk lattice oxygen without clear distinction [29]. It means that the bulk lattice oxygen inside the particle of the oxygen carriers could transfer to the surface as rapidly as to supply the oxygen vacancies as the surface oxygen is consumed, indicating a high lattice oxygen mobility [30]. Besides, the reduction peak areas which represent the hydrogen consumption increase as the Sr substitution degrees increase suggesting that Sr-doping increases the hydrogen consumption capacity of the oxygen carriers.
XPS XPS analysis was performed to determine the nature of the near-surface cation compositions and oxygen species of the La1-xSrxFeO3 samples. As shown in Fig. 4, intensive signals of La3d, Sr3d, Fe2p and O1s are observed within the binding energy (BE) of 0e1400 eV. In addition to these expected elements, three spectrum peaks of C1s for contamination typically CeC, CeOeC and OeC]O components are detected, representing the carbon formation that is aroused by exposing the samples to methane atmosphere during the methane reforming stage. Fig. 5 shows the La3d, Sr3d, and Fe2p spectra for the La1xSrxFeO3 samples by fitting the XPS curves. The XPS spectra of La3d exhibit two doublet peaks namely La3d3/2 and La3d5/2 with two splitting peaks (doublets denoted by aa’ and bb’), which are similar with the form of standard trivalent La3d presenting the typical compounds of La3þ. The first doublet peak aa’, namely La3d94f0, are related to the almost uncharged
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Fig. 4 e XPS survey spectrum of five samples.
state of La3þ ion, corresponding to the LaFeO3 crystal phase. Moreover, the peak bb’, namely La3d94f1, was believed ascribing to the electron transferring from the 2p orbital of oxygen ligand to the empty La 4f shell [29], indicating the existence of LaSrFeO4 phase or/and surface hydroxylation or carbonization of lanthanum sites [31]. The binding energy (EB) and s(LaeO) of the main peak La3d5/2 (Table 1) shows no significant change with the increase of Sr doping, being consistent with that of La2O3. It is suggested that the substitution of Sr in LaFeO3 has little effect on the chemical state of La3d from the spectra and binding energy. The Sr3d spectra exhibit two obvious peaks of Sr3d3/2 and Sr3d5/2 with spin-orbit splitting. Deconvolution of the Sr3d peaks exhibit three chemical states labeled as Sra in the range of BE of Srþ in low oxidation state; Srb being associated with the La1-xSrxFe3þO3-d crystal phase and Src which is due to the La1-xSrxFe4þ/5þO3 phase [32]. There is an obvious increase of peak Src as the Sr-doping degree increases, which means that the Sr doping not only affect the content of Sr on the oxides, but also affect the chemical states of the perovskite structure, consequently giving rise to influences on the valence state of B-site elements. For the Fe2p spectra, an asymmetry feature with doublepeaked spectrum with two small satellite peaks is observed, indicating the existence of the multiple components of Fe cations with different chemical valence states in the La1xSrxFeO3 perovskites. The peak positions of Fe2p3/2 for all the five samples are located at 710.1e710.3 eV, which is typical of Fe ions in high oxidation states being attributed to both of Fe3þ and Fe4þ oxidation states [33,34]. Due to the unstable property of Fe4þ, the unfilled Fe 3 d orbit (t32ge1g) can easily lose the electron on eg band to form Fe5þ (t32g) or to get another electron to be half-filled, meanwhile, to generate abundant oxygen vacancies to maintain the electro-neutrality [35]. The existence of Fe4þ or Fe5þ is also confirmed by researchers who € ssbauer technique to characterize the perovskites used mo particles [32,36]. The Fe3þ/(Fe4þþFe5þ) ratios of the five samples are determined by dividing the integrated areas of peaks, as shown in Table 1. A clear decrease of the Fe3þ/(Fe4þþFe5þ) ratios can be seen as the increase of the Sr substitution. This indicates that doping of Sr has an obvious effect on valence states of Fe ion, whereas the Fe3þ ions are still the primary component of the B-site for all the samples.
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The activity of the perovskite-type oxides is strongly affected by the properties of near-surface oxygen species, which can be revealed by a detailed O1s XPS scan. Fig. 6 shows the O1s scans of the five La1-xSrxFeO3 samples. As can be seen, two major peaks and two minor peaks are identified on all the five La1-xSrxFeO3 samples. The major peak with lower binding energy at around 528.0 eV is assigned to lattice oxygen species (denoted as OI) [37]. A secondary major peak at around 531.5 eV is assigned to physical adsorbed oxygen (denoted as OIII), i.e. hydroxyls species OH or carbonate species CO2 3 [38]; An minor intermediate peak appeared at around 529.0e529.5 eV is associated with surface adsorbed oxygen species, O2 2 /O (denoted as OII) [39]. And another minor peak with higher binding energy at about 533.0 eV is assigned to adsorbed molecular water on the surface of the solid particles [40]. It is noteworthy that the relative intensities of the first major peak corresponding to the lattice oxygen were obviously suppressed for the La1-xSrxFeO3 samples when the Srdoping degree (x) is above 0.5. In general, the lattice oxygen OI is conducive to partial oxidation of methane into syngas. It is observed that the XPS patterns of La1-xSrxFeO3 (x ¼ 1.0) are significantly different from those of the rest four samples. SrFeO3 give the lowest intensity peak at 528.0 eV, indicating that it has the lowest lattice oxygen content among the five oxides. This could explain the poor syngas selectivity observed in SrFeO3. As above mentioned, the bulk lattice oxygen is prone to methane partial oxidation into H2 and CO. Therefore, the Ola/ Oad (O la and Oad stand for lattice oxygen and adsorbed oxygen species, respectively.) ratio from XPS results can be taken as a comparison criterion for the performance of the oxygen carriers in chemical looping reforming process where selective oxidation of CH4 into syngas by O2 in the lattice is desirable. It can be seen from Table 2 that La0.7Sr0.3FeO3-d has the highest Ola/Oad ratio among the five perovskites, whereas the Ola/Oad ratios exhibit decrease as the Sr-doping degrees further increase when x 0.5. Besides, the oxygen vacancies concentrations, namely oxygen nonstoichiometry parameter d, increase as the Sr substitution increases. Meanwhile, d increase is accompanied with an increase of adsorbed oxygen and a decrease of Fe3þ. It means that Sr2þ substitution for La3þ generates an electronic unbalance of the perovskite structure, and as a consequence, oxygen vacancies or an increase in the oxidation state of Fe3þ to Fe4þ/Fe5þ can be produced to preserve charge neutrality [41]. As demonstrated earlier, Sr substitution in the La1-xSrxFeO3 perovskites changes the coordination environments and electron density of oxygen anions generating two effects on the lattice properties of the perovskites: one is an increase of the oxygen vacancies in the bulk of the oxides and the other is formation of hypervalent B-site cation concentrations in the perovskites. The former effect facilitates the total oxidation of hydrocarbons into CO2 and H2O, while the latter one is beneficial to partial oxidation of hydrocarbons into CO and H2. Thus, the performance of the La1-xSrxFeO3 perovskites toward chemical looping reforming reactions can be tailored by adjusting the Sr substitution contents. In one of our previous works, we found that the reactivity of the La1-xSrxFeO3 perovskites decreased as the degree of Sr substitution x > 0.5, whereas Sr doping can inhibit methane decomposition in the
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Fig. 5 e XPS deconvolution spectra of La3d, Sr3d, and Fe2p.
methane partial oxidation reactions. From the point of view of the reactivity, resistance to carbon formation, as well as oxygen donation ability, an optimal range of the Sr substitution degree is x ¼ 0.3e0.5 [19]. In one previous work [19], we studied the effects of Srdoping degree on the reactivity of La1-xSrxFeO3 perovskites on the basis of experiments conducted with a thermalgravimetrical analysis reactor and a fixed bed reactor. The sample x ¼ 0.3 exhibited a better performance for partial oxidation of methane into H2 and CO than others. The main purpose of the present work is exploring insight into the effect of Sr-
substitution on the oxygen species and cation ions distributions on the near surface of the redox catalysts. Thus, we focused on identification of elements on the surface of the oxygen carriers in this work. Taking account of the chemical state of La3d, Sr3d, Fe2p, and Ola/Oad ratio of the XPS analysis, combining with the experimental results in our previous study through TG analysis and fixed-bed tests, it can be observed that La0.7Sr0.3FeO3-d exhibits the best performance for chemical looping partial oxidation of CH4 among the five perovskite samples. Therefore, we recommend La0.7Sr0.3FeO3 as the optimal oxygen carrier in the series because it gives the
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Table 1 e Near-surface atomic ratio from XPS. Sample
x x x x x
¼ ¼ ¼ ¼ ¼
0.1 0.3 0.5 0.7 1.0
Sr3d5/2
La3d5/2 EB
s(LaeO)
Peak area/%
834.00 833.05 833.55 833.81 e
304.90 303.98 303.98 304.03 e
12.23 9.71 5.40 2.68 e
Fe2p3/2
EB
s(OeSr)
Peak area/%
EB
s(FeeO)
Fe3þ:(Fe4þþFe5þ)
Peak area/%
132.82 132.84 132.80 132.70 133.03
396.28 396.23 396.77 397.08 397.61
5.06 6.19 13.13 17.71 20.53
710.11 710.14 710.14 710.15 710.32
181.01 181.07 180.57 180.37 179.68
63.9:36.1 61.9:38.1 61.4:38.6 61.3:38.7 60.2:39.8
15.60 17.88 11.31 11.00 9.57
s(M-O) ¼ EB(M 3d5/2 or M 2p3/2)-EB(O1s), where M ¼ La, Sr, Fe. Fe3þ/(Fe4þþFe5þ) ratios were calculated based on the assumption that Fe3þ, Fe4þ and Fe5þ ions were presented in the samples. The spectra at lower binding energy at ~710 eV are attributed to Fe3þ, whereas the spectra at higher BE value are assigned to Fe4þ or/and Fe5þ.
Fig. 6 e XPS deconvolution spectra of O1s.
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Table 2 e O1s XPS peak deconvolution results and the oxygen vacancies at the surface. Sample La0.9Sr0.1FeO3-d La0.7Sr0.3FeO3-d La0.5Sr0.5FeO3-d La0.3Sr0.7FeO3-d SrFeO3-d
O2(lattice)
O, O2 2 (adsorbed)
OH, CO23
H2O
Ola/Oad
d
44.36 47.21 37.01 26.41 9.70
16.17 12.97 23.46 14.56 24.81
31.17 26.71 33.09 54.11 60.95
8.30 13.11 6.44 4.92 4.54
2.74 3.64 1.58 1.81 0.40
0.05 0.15 0.25 0.35 0.50
d is calculated by assuming all the Fe ion as trivalent. 2Oad refer to adsorbed oxygen (O, O2 2 ) and Ola to lattice oxygen O
maximum Ola/Oad ratio of 3.64:1, which can be regarded as a criterion for the reactivity and selectivity of the oxygen carriers for chemical-looping steam methane reforming reactions.
Reactivity in successive redox cycles To further observe the reactivity of the La0.7Sr0.3FeO3 in chemical looping reforming of methane and in water splitting reactions, we studied its performance of co-production of syngas and hydrogen in multiple successive redox cycles by exposing it in an alternating methane reducing and steam oxidizing in a fixed bed reactor. Fig. 7 shows the methane conversion, H2/CO molar ratio in methane chemical looping partial oxidation stage and the H2 yields in steam splitting stage of the tests. As shown in Fig. 7 (a), CH4 conversions reached above 80% in most of the cycles in methane stage, and the average H2/CO molar ratio was close to 2.0. This indicates that methane was mainly converted into syngas via partial oxidation on the peroviskite particles, and methane decomposition was not significantly observed because methane cracking results in a high H2/CO molar ratio above 2.0. When La0.7Sr0.3FeO3 perovskite was reduced by methane, the oxygen species of the perovskite were consumed generating oxygen vacancies, metallic phase and/or metal oxides with lower oxygen content in the bulk of the perovskite. Fig. 8 shows the XRD patterns of the reduced La0.7Sr0.3FeO3. It can be observed that the reduced sample is primarily composed of a composite of La2SrOx, Fe3O4 and FeO. In order to prevent occurrence of agglomeration of the oxygen carriers, the sample was not deeply reduced into metallic phase in the methane stage in this work.
In the water splitting stage, the Fe3O4, FeO and/or Fe (if formed) in the reduced perovskite sample is re-oxidized by steam to replenish its lattice oxygen and to re-fill the oxygen vacancies thereby generating pure hydrogen. Fig. 7 (b) shows the amount of hydrogen generated in the steam oxidation stage as a function of the number of cycles. The hydrogen productivity appears obvious variation from the 1st to the 4th redox cycles giving the highest hydrogen yield of 100.8ml/goc at the 3rd cycle (OC denotes oxygen carrier). Afterward, the hydrogen productivity remained stable at around 90 ml/goc in the resting cycles. The average concentration of the hydrogen in the ten cycles is more than 96% with a minor fraction of CO and CO2 which were produced from gasification of carbon deposition formed during methane reforming. As above mentioned, the inlet gases in the water-splitting stage are steam and N2, therefore, the generation of CO and CO2 in this stage is attributed to the carbon formation in the methane reforming stage. Carbonation of the oxygen carriers was not detected in this work. To avoid the formation of carbon in methane reforming stage, the reduction of the redox catalyst should be stopped before 500 s [42]. Fig. 9 shows a H2 production profile in a typical redox cycle in water splitting stage. The peak H2 production rate was 2.12 mmol/(goc$min) at 3 min of the reaction time. After 6 min, the H2 production rate decreased significantly to 0.56 mmol/(goc$min) and to 0.04 mmol/(goc$min) till at 20 min. It is noted that minor fractions of CO and CO2 were detected in the gas products at the initial 6 min due to the carbon formation in the methane reforming stage. In summary, the reduced La0.7Sr0.3FeO3 perovskite exhibited faster oxidation kinetics in water splitting stage. Near 80% of the hydrogen was produced in the initial 6 min period during the whole 20 min water splitting stage. In other
Fig. 7 e Catalytic performance of sample La0.7Sr0.3FeO3 in ten cycles. (a) methane reduction stage, (b) steam oxidation stage.
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DFT calculations
Fig. 8 e XRD patterns of La0.7Sr0.3FeO3 reduced by methane. :-La2SrOx; *-Fe3O4; △-FeO.
Fig. 9 e H2 production rate in water splitting stage in a typical redox cycle.
words, 80% of the active lattice oxygen of the reduced La0.7Sr0.3FeO3 sample could be replenished in such a short regeneration period of 6 min. To determine the micro morphology of the perovskite sample during the redox cyclic reactions, SEM images have been obtained as shown in Fig. 10. The as-prepared sample exhibits micro morphology with uniform aggregates of small granules with interconnected pores existing on the particle surface. Different sizes of holes distribute in the grain boundaries with various particle sizes. After successive redox cycles, the used sample shows a morphology slightly rough due to the accumulative impacts during the altering methane reduction and steam oxidation leading to crack and interstice of the particles. Aggregation between small particles can be found on the surface, suggesting that slight sintering occurred in the course of the successive cycles. However, the slight sintering of the particles did not significantly affect the reactivity of the perovskite. To prevent agglomeration and sintering of the perovskite redox catalysts, adding an inert oxide such as Al2O3 as a supporter or additive may be a good approach to improve the agglomeration resistance of the redox catalysts [43].
In general, the electron behavior near the Fermi level at the top of valance band can reflect the most significant characterization of the studied materials. The electronic band structures of the six perovskites La1-xSrxFeO3 (x ¼ 0, 0.1, 0.3, 0.5, 0.7, 1.0) were calculated in this work. The band structures of LaFeO3, La0.7Sr0.3FeO3, and SrFeO3 were typically shown in Fig. 11. It is observed that the band gap near Fermi level of La0.7Sr0.3FeO3 is smaller than them of the LaFeO3 and SrFeO3 samples, which means that partial doping of Sr for La making the energy distribution of electrons. It indicates that more electrons occupy orbitals with higher energy in the cell of Srdoped La1-xSrxFeO3 perovskites; thus, these electrons are more prone to be excited and the cell as an electron donor to oxidize the fuels easier [44]. Recalling their reducibility performed in the experiments above, the electronic continuity enforces the reactivity of the Sr-doped samples, especially the La0.7Sr0.3FeO3, in chemical looping reactions. To further investigate the properties of the LaFeO3 with and without Sr-doping, the partial density of states (PDOS) were calculated. The PDOS profiles of the LaFeO3, La0.7Sr0.3FeO3, and SrFeO3 were selected and shown in Fig. 12. It is observed that the dominant electronic character near the Fermi level is due to Fe 3 d states, La 3 d states, and a noticeable mixing with O 2p states. In the pure unit cell of LaFeO3, divalent La cations constitute an octahedral coordination with six oxygen ions and the trivalent iron cations coordinated with twelve oxygen atoms located in the octahedron cavity. Partial substitution of Sr in La sites increased the transition intensity of Fe 3d-O2p orbit and more electrons were excited near Fermi level. It suggests that the electron density of Fe atom increase for the doped samples around Fermi level both in conduction band and valence band. Compared to LaFeO3, the Fe-d orbitals and O-p orbitals are located in higher energies in the Sr-doped materials and thus the electrons occupied those orbitals became more likely to be activated. This is again consistent with the experimental results.
Reaction pathway of water splitting on the surface of oxygen carrier particles Reaction mechanism of chemical looping reforming of methane with oxygen carriers were studied in a number of previous literature [27,45,46]. Li et al. [46] believed that oxygen vacancies and the resulting Fe2þ/Ce3þ ions acts as the active sites for methane activation in chemical looping reforming reactions when CeO2/LaFeO3 is used as oxygen carriers. CeH bond is activated and dissociated on the surface of the oxygen carrier particles generating CH3 groups with significant radical character and weak interactions with the surface at the transition state. CO2 and steam were first formed in the initial stage, whereas CO and H2 increasingly became the main products in the main stage when the most reactive oxygen species were consumed. However, the reaction mechanism of the water thermochemical splitting reaction on the reduced La1-xSrxFeO3 perovskites is not well understood. On the basis of the XPS analysis, and the resulting gas products of the steam oxidation stage, the reaction mechanism model of the water splitting reaction is proposed, as shown in Fig. 13. As
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 1 0 2 6 5 e1 0 2 7 6
Fig. 10 e SEM images of (a) fresh and (b) spent sample after 10 cycles.
Fig. 11 e Band structure of (a) LaFeO3 (b) La0.7Sr0.3FeO3 (c) SrFeO3.
Fig. 12 e The partial density of states (PDOS) for (a) LaFeO3 (b) La0.7Sr0.3FeO3 (c) SrFeO3. The dashed vertical lines represent the position of the Fermi level.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 1 0 2 6 5 e1 0 2 7 6
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Fig. 13 e Reaction mechanism model of water splitting over reduced La0.7Sr0.3FeO3 perovskite.
water vapor is introduced on the surface of the reduced perovskite, the H2O molecule is adsorbed onto a metal surface, which acts as an electron donor and an active site of catalyst for HeO bond breaking, forming an OH and a Hþ. An OH will react with adjacent OH to generate H2 meanwhile forming an O2. Then the O2 ions would be taken by the oxygen vacancies in the oxygen carrier, along with the oxidation of metals and/or metal cations with lower valence in the B-site of perovkite. As the B-site metals are oxidized, electrons will be released and transferred to two Hþ to generate H2 as well. A multi-step mechanism that reflects the oxidation of reduced perovskite by water vapor is expressed below: H2O(g) / OH þ Hþ
(1)
2OH / O2 þ H2(g)
(2)
Fe(bulk in the B-site of La0.7Sr0.3FeO3-d 2Fe2þ(bulk in
perovskite)
- 2e / Fe2þ
the B-site of La0.7Sr0.3FeO3-d perovskite)
- 2e / 2Fe3þ
(3) (4)
2Hþ þ 2e / H2(g)
(5)
La0.7Sr0.3FeO3-d þdH2O(g) / La0.7Sr0.3FeO3 þdH2(g)
(6)
Here, the reaction (6) represents the global reaction of water splitting on the surface of the reduced La0.7Sr0.3FeO3 perovskite.
Conclusions The substitution of Sr2þ for La3þ in perovskites La1-xSrxFeO3d generates an electronic unbalance of the perovskite structure, and as a consequence, oxygen vacancies and/or an increase in the oxidation state of Fe3þ to Fe4þ/Fe5þ can be produced to preserve charge neutrality. The Ola/Oad ratio, which can be taken as a criterion for comparison of the reactivity of partial oxidation of methane, obtains the maximum of 3.645:1 for La0.7Sr0.3FeO3-d and decreases as x 0.5. La0.7Sr0.3FeO3-d exhibits high catalytic stability during the successive redox process for the continuous production of syngas and hydrogen achieving up to 80% CH4 conversion in the methane partial oxidation step and 96% of H2 concentration in the water splitting step in ten successive redox tests at 850 C. It is a desirable oxygen carrier candidate for CL-SMR.
Acknowledgements The financial support of National Key Research and Development Program of China (2017YFE0105500) is gratefully acknowledged. This work was also supported by the National Natural Science Foundation of China (51876205, 51776210) and the Guangxi Natural Science Foundation (2018JJD120017), R&D Project of Guilin University of Technology (GLUTQD2018027).
references
[1] Richter HJ, Knoche KF. Reversibility of combustion processes. ACS Symp Ser 1983;235:71e85. [2] Zhu X, Wei Y, Wang H, Li K. CeeFe oxygen carriers for chemical-looping steam methane reforming. Int J Hydrogen Energy 2013;38:4492e501. [3] Lee DH, Cha KS, Kim HS, Park CS, Kim YH. Syngas and hydrogen production via stepwise methane reforming over Cu-ferrite/YSZ. Int J Energy Res 2014;38:1522e30. [4] Zhu X, Zhang M, Li K, Wei Y, Zheng Y, Hu J, Wang H. Chemical looping water splitting over ceria-modified iron oxide: performance evolution and element migration during redox cycling. Chemcial Engineering Science 2018;179:92e103. [5] Huang H, Liu Y, Tang W, Chen Y. Catalytic activity of nanometer La1xSrxCoO3 (x ¼ 0, 0.2) perovskites towards VOCs combustion. Catal Commun 2008;9:55e9. [6] Parvaneh EA, Khodadadi A, Hessam ZA, Mortazavi Y. Effects of excess manganese in lanthanum manganite perovskite on lowering oxidation light-off temperature for automotive exhaust gas pollutants. Chem Eng J 2011;169:282e9. n M, Leion H, Mattisson T, Lyngfelt A. Combined oxides [7] Ryde as oxygen-carrier material for chemical-looping with oxygen uncoupling. Appl Energy 2014;113:1924e32. [8] Tang M, Xu L, Fan M. Progress in oxygen carrier development of methane-based chemical-looping reforming: a review. Appl Energy 2015;151:143e56. [9] Mihai O, Chen D, Holmen A. Catalytic consequence of oxygen of lanthanum ferrite perovskite in chemical looping reforming of methane. Ind Eng Chem Res 2011;50:2613e21. [10] Haron W, Thaweechai T, Wattanathana W, Laobuthee A, Manaspiya H, Veranitisagul C, Koonsaeng N. Structural characteristic and dielectric properties of La1-xCoxFeO3 and LaFe1-xCoxO3 synthesized via metal organic complexes. Energy Procedia 2013;34:791e800. [11] Varandili SB, Babaei A, Ataie A, Khodadadi AA, Kazerooni H. Nano-structured Pd doped LaFe(Co)O3 perovskite; synthesis,
10276
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
[29]
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 1 0 2 6 5 e1 0 2 7 6
characterization and catalytic behavior, Materials chemistry and Physics. Mater Chem Phys 2018;205:228e39. Kuhn JN, Matter PH, Millet J-MM, Watson RB, Ozkan US. Oxygen exchange kinetics over Sr- and Co-doped LaFeO3. J Phys Chem 2008;112:12468e76. Jeong JH, Song CG, Kim KH, Sigmund W, Yoon JW. Effect of Mn doping on particulate size and magnetic properties of LaFeO3 nanofiber synthesized by electrospinning. J of Alloy Compd 2018;749:599e604. Zhang R, Li P, Xiao R, Liu N, Chen B. Insight into the mechanism of catalytic combustion of acrylonitrile over Cudoped perovskites by an experimental and theoretical study. Appl Catal BEnviron 2016;196:142e54. Zhao K, Fang H, Huang Z, Wei GQ, Zheng AQ, Li HB, Zhao ZL. Perovskite-type oxides LaFe1-xCoxO3 for chemical looping steam methane reforming to syngas and hydrogen coproduction. Appl Energy 2016;168:193e203. Taylor FH, Buckeridge J, Catlow RAC. Screening divalent metals for A- and B-site dopants in LaFeO3, Chemistry of Materials. Chem Mater 2017;29:8147e57. Shafiefarhood A, Galinsky N, Huang Y, Chen YG, Li FX. Fe2O3@LaxSr1-xFeO3 core-shell redox catalyst for methane prtial oxidation. Chem Catal Chem 2014;6:790e9. Neal LM, Shafiefarhood A, Li FX. Dynamic methane partial oxidation using a Fe2O3@LaxSr1-xFeO3 core-shell redox catalyst in the absence of gaseous oxygen. ACS Catal 2014;4:3560e9. Fang H, Li XA, Zhao K, Huang Z, Wei G, Li H. The use of La1xSrxFeO3 perovskite-type oxides as oxygen carriers in chemical-looping reforming of methane. Fuel 2013;108:465e73. Qin Y, Sun L, Zhang D, Huang L. Role of ceria in the improvement of SO2 resistance of LaxCe1xFeO3 catalysts for catalytic reduction of NO with CO. Catal Commun 2016;79:53e7. Evdou A, Zaspalis V, Nalbandian L. La1xSrxFeO3d perovskites as redox materials for application in a membrane reactor for simultaneous production of pure hydrogen and synthesis gas. Fuel 2010;89:1265e73. Jones A, Islam MS. Atomic-scale insight into LaFeO3 perovskite: defect nanoclusters and ion migration. J Phys Chem C 2008;112:4455e62. Taylor DD, Schreiber NJ, Levitas BD, Xu W. Oxygen storage Properties of La1-xSrxFeO3 for chemical-looping reactionsean in situ neutron and synchrotron X-ray study. Chem Mater 2016;28:3951e60. Evdou A, Zaspalis V, Nalbandian L. Lal-xSrxMnO3 perovskite as redox materials for the production of high purity hydrogen. Int J Hydrogen Energy 2008;33:5554e62. Iwanowski RJ, Heinonen MH, Pracka I, Kachniarz J. XPS characterization of single crystalline SrLaGa3O7:Nd. Appl Sur Sci 2013;283:168e74. Cyza A, Kopia A, Cieniek L, Kusinski. Structural characterization of Sr doped LaFeO3 thin films prepared by pulsed electron deposition method. J. Mate Today Proc 2016;3:2707e12. Zhao K, Zheng A, Li H, He F, Huang Z, Wei G, Shen Y, Zhao Z. Exploration of the mechanism of chemical looping steam methane reforming using double perovskite-type oxides La1.6Sr0.4FeCoO6. Appl Catal B Environ 2017;219:672e82. Zhang R, Villanueva A, Alamdari H, Kaliaguine S. Cu- and Pdsubstituted nanoscale Fe-based perovskites for selective catalytic reduction of NO by propene. J Catal 2006;237(2):368e80. Atribak I, Bueno LA, Garcia GA. Combined removal of diesel soot particulates and NOx over CeO2eZrO2 mixed oxides. J Catal 2008;259:123e32.
[30] Zheng Y, Li K, Wang H, Wang Y, Tian D, Wei Y, Zhu X, Zeng C, Luo Y. Structure dependence and reaction mechanism of CO oxidation: a model study on macroporous CeO2and CeO2-ZeO2 catalysts. J Catal 2016;344:365e77. [31] Mickevicious S, Grebinskij S, Bondarenka V, Vengalis B, Sliuziene K, Orlowski BA, Osinniy V, Drube W. Investigation of epitaxial LaNiO3x thin films by high-energy XPS. J Alloy Compd 2006;423:107e11. [32] Stathopoulos VN, Belessi VC, Bakas TV, Neophytides SG, Costa CN, Pomonis PJ, Efstathiou AM. Comparative study of LaeSreFeeO perovskite-type oxides prepared by ceramic and surfactant methods over the CH4 and H2 lean-deNOx. Appl Catal B Environ 2009;93:1e11. [33] Widatallah HM, Al-Rawas AD, Johnson C, Al-Harthi SH, Gismelseed AM, Moore EA, Stewart SJ. The formation of nanocrystalline SrFeO3-d using mechano-synthesis and € ssbauer studies. J subsequent sintering: structural and mo Nanosci Nanotechnol 2009;9:2510e7. [34] Al-Rawas AD, Widatallah HM, Al-Harthi SH, Johnson C, Gismelseed AM, Elzain ME, Yousif AA. The formation and structure of mechano-synthesized nanocrystalline € ssabuer and XPS analyses. Mater Sr3Fe2O6.4: XRD Rietveld, Mo Res Bull 2015;65:142e8. [35] Zhao W, Yang Q, Cui J. XPS study of surface absorbed oxygen of ABO3 mixed oxides. J Rare Earths 2008;26:511e4. [36] Russo U, Nodari L, Faticanti M, Kuncser V, Filoti G. Local interactions and electronic phenomena in substituted LaFeO3 perovskites. Solid State Ionics 2005;176:97e102. [37] Dudric R, Vladescu A, Rednic V, Neumann M, Deac IG, Tetean R. XPS study on La0.67Ca0.33Mn1xCoxO3 compounds. J Mol Struct 2014;1073:66e70. [38] Fierro JLG. Structure and composition of perovskite surface in relation to adsorption and catalytic properties. Catal Today Off 1990;8:153e74. [39] Yamazoe N, Teraoka Y, Seiyama T. TPD and XPS study on thermal behavior of absorbed oxygen in La1-xSrxCoO3. Chem Lett 1981;10:1767e70. [40] Chen L, Wang W, Zhao N, Liu Y, He B, Hu F, Chen C. Structure properties and catalytic performance in methane combustion of double perovskites Sr2Mg1xMnxMoO6d. Appl Catal B Environ 2011;102:78e84. [41] Merino NA, Barbero BP, Grange P, Cadu´s LE. La1xCaxCoO3 perovskite-type oxides: preparation, characterisation, stability, and catalytic potentiality for the total oxidation of propane. J Catal 2005;231:232e44. [42] Ebrahimi Hadi, Rahmani Mohammad. Modeling chemical looping syngas production in a microreactor using perovskite oxygen carriers. Int J Hydrogen Energy 2018;43(10):5231e48. [43] Brackmann R, Perez CA, Schmal M. LaCoO3 perovskite on ceramic mololiths-pre and post reaction analyzes of the partial oxidation of methane. Int J Hydrogen Energy 2014;39:13991e4007. [44] Liu S, Xiang D, Xu Y, Sun Z, Cao Y. Relationship between electronic properties of Fe3O4 substituted by Ca and Ba and their reactivity in chemical looping process: a first-principles study. Appl Energy 2017;202:550e7. [45] Neal LM, Shafiefarhood A, Li F. Dynamic methane partial oxidation using a [email protected] core-shell redox catalyst in the absence of gaseous oxygen. ACS Catal 2014;4:3560e9. [46] Zheng Y, Li K, Wang H, Tian D, Wang Y, Zhu X, Wei Y, Zheng M, Luo Y. Designed oxygen carriers from macroporous LaFeO3 supported CeO2 for chemical-looping reforming of methane. Appl Catal B Environ 2017;202:51e63.