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Evaluation of Fe substitution in perovskite LaMnO3 for the production of high purity syngas and hydrogen
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Evaluation of Fe substitution in perovskite LaMnO3 for the production of high purity syngas and hydrogen Yajing Wang a, Yane Zheng a, b, *, Yuhao Wang b, c, Hua Wang b, c, Xing Zhu b, c, Yonggang Wei b, c, Yaming Wang a, Lihong Jiang a, Zhiyuan Yang a, Kongzhai Li b, c, ** a b c
Faculty of Chemical Engineering, Kunming University of Science and Technology, Kunming, 650093, China State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization, Kunming University of Science and Technology, Kunming, 650093, China Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming, 650093, China
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� The doping of Fe to LaMnO3 promotes the production of syngas and pure hydrogen. � La–Mn–Fe–O oxygen carriers possess higher reactivity and ideal H2/CO molar ratio. � La0.85MnFe0.15O3 exhibits the optimal performance during 20 redox cycles.
A R T I C L E I N F O
A B S T R A C T
Keywords: Chemical looping reforming of methane Fe ion substitution LaMnO3 Syngas Hydrogen Carbon deposition
Chemical looping reforming of methane (CLRM) technology is a new approach for syngas and pure hydrogen generation. During the whole reaction, methane is first partially oxidized to syngas by the lattice oxygen of the oxygen carrier, and then the lattice oxygen of the reduced oxygen carrier is recovered by thermal water splitting, with pure hydrogen produced. LaMnO3 is used as an oxygen carrier for the CLRM. It obtains a high resistance to carbon deposition, while a lower CH4 conversion due to the unstable structure. Fe ion substitution in LaMnO3 can be used to improve its reactivity and thermal stability. La1-xMnFexO3/LaMn1-xFexO3 (x ¼ 0.05, 0.1, 0.15, 0.2) oxygen carriers own higher reactivity and better resistance to carbon deposition, producing high purity syngas and hydrogen. La0.85MnFe0.15O3 sample exhibits the highest CO selectivity (~99%), syngas productivity (3.78 mmol g 1) and H2 productivity (1.76 mmol g 1) due to the higher oxygen vacancy concentration, and its H2/CO molar ratio is maintained at the ideal ratio of 2.0 (1.93–2.07) during the whole process. Notably, no carbon deposition is generated over LaMnO3, La0.85MnFe0.15O3 and LaMn0.9Fe0.1O3 oxygen carriers even after 20 redox cycles, and La0.85MnFe0.15O3 exhibits superior resistance to carbon deposition due to the better structural and thermal stability.
* Corresponding author. Faculty of Chemical Engineering, Kunming University of Science and Technology, Kunming, 650093, China. ** Corresponding author. Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming, 650093, China. E-mail addresses: [email protected] (Y. Zheng), [email protected] (K. Li). https://doi.org/10.1016/j.jpowsour.2019.227505 Received 23 August 2019; Received in revised form 22 November 2019; Accepted 25 November 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Yajing Wang, Journal of Power Sources, https://doi.org/10.1016/j.jpowsour.2019.227505
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1. Introduction
oxidation [25]. Moreover, LaMnO3 also exhibits high reactivity in chemical looping combustion (CLC). Since partial substitution A or B sites of the perovskite oxides can result in the formation of crystal lattice defects and thus affect the catalytic activity, the reactivity of LaM nO3-based perovskite can be adjusted by doping metal cations like Fe and Cu at A or B site [26,27]. It has been reported that replacing cations in CaMnO3 with iron in chemical looping with oxygen uncoupling (CLOU) and CLC processes can remarkably improve the oxygen absorption-release properties and stability of perovskite type oxygen carriers [28,29]. He et al. [30] found that the Fe doped on MgAl2O4 exhibited a high reactivity and redox ability in chemical-looping pro cesses. Therefore, the combination of perovskite LaMnO3 with Fe may create a promising oxygen carrier for CLRM technology to produce both syngas and hydrogen. In this work, LaMnO3 and a series of La1-xMnFexO3/LaMn1-xFexO3 (x ¼ 0.05, 0.1, 0.15, 0.2) perovskite-type oxygen carriers were prepared by sol-gel method. Moreover, the performances of oxygen carriers in the CLRM process were investigated. The substitution of Fe could improve the activity of pure LaMnO3 and the influences of Fe content on the performances of La1-xMnFexO3/LaMn1-xFexO3 were mainly studied. The La0.85MnFe0.15O3 achieved higher selectivity, productivity, and ideal H2/CO ratio with no carbon deposition. The superior carbon resistance led to the generation of pure syngas and hydrogen. In addition, we also explored the reactivity and stability of oxygen carriers for CLRM in successive redox cycles.
Fossil fuels, as the major source of energy today, present serious economic and environmental problems, such as limited resources, high prices and the generation of greenhouse gas (CO2) after combustion [1]. Therefore, the exploration of clean energy has become a key issue. Hydrogen energy plays a vital role due to its low cost and cleanliness. It can also provide energy for hydrogen fuel cell vehicles and portable electronic devices [2–4]. At present, most of the H2 industrially gener ated comes from fossil fuel gasification/reforming, but it refers to a lot of gas separation and purification steps and high energy consumption [5, 6]. Therefore, a natural, sustainable and clean technology of water decomposition is developed to produce hydrogen [7]. In recent years, there are several techniques to produce hydrogen from water decom position such as electrochemical, photocatalytic and thermal decom position of water [8–10]. The hydrogen production through the electrochemical process is simple and pollution-free, however, its low conversion efficiency and high power consumption result in high cost and limited application [11]. During the photocatalytic process, the search and optimization of high-efficiency photo-catalysts has become a major issue [12]. The traditional thermal decomposition of water con tains only one-step, while this process requires quite high temperatures (above 2500 K) and is also subject to the effective separation of hydrogen and oxygen [13]. Therefore, we urgently need to find a new type of technology to achieve more economical, efficient and clean hydrogen. Chemical looping reforming of methane (CLRM) as a novel two-step process, couples the partial oxidation of CH4 to syngas and hydrogen production by water splitting. In the first step of the CLRM process, syngas was generated, which can be used for the production of industrial chemicals. Meanwhile, the pure hydrogen produced in the second step can be applied to fuel cells or synthetic ammonia, etc. [14]. The CLRM process can be expressed as follows: � (r.1) sCH4 þ Mex Oy →Mex Oy s þ s CO þ 2H2 Mex Oy s þ sH2 O→Mex Oy þ sH2
2. Experimental 2.1. Oxygen carriers preparation In this paper, LaMnO3 and La1-xMnFexO3/LaMn1-xFexO3 series oxy gen carriers were prepared by sol-gel method. The chemicals La (NO3)3⋅6H2O, Mn(NO3)2 (50 wt % in H2O), Fe(NO3)3⋅9H2O and C6H8O7⋅H2O of analytical grade were purchased from Aladdin. At room temperature, the appropriate amount of La(NO3)3⋅6H2O, Mn(NO3)2 (50 wt % in H2O), Fe(NO3)3⋅9H2O and C6H8O7⋅H2O (CA) dissolved in deionized water to form a solution, in which the molar ratio of total metal ions to citric acid was 2:1. In other words, the molar ratios of components in LaMnO3 and La1-xMnFexO3/LaMn1-xFexO3 series oxygen carriers were La/Mn/CA ¼ 1:1:1 and (La þ Fe)/Mn/CA ¼ 1:1:1 or La/ (Mn þ Fe)/CA ¼ 1:1:1, respectively. The solution then reacted in water bath at 70 � C for heating and stirring for 4 h until the gel was formed. After that, the gel was dried in an oven at 120 � C for 24 h. Eventually, the obtained precursor was calcined at 300 � C for 2h and then at 800 � C for 2 h. The particle sizes of the oxygen carriers were in the range of 20–40 mesh.
(r.2)
In the methane partial oxidation step (see formula. 1), methane re acts with metal oxygen carriers (MexOy) to generate syngas (CO þ H2). During the oxygen carrier regeneration step, H2O as the reactants not only supply oxygen species for the reduced metal oxygen carrier (MexOys) but also provide pure H2 [12,13]. Moreover, the acquisition of pure hydrogen from water splitting eliminates the separation and purification process, reducing the cost of hydrogen production [15]. At present, the suitable oxygen carriers is a key challenge for improving CLRM technology. A suitable oxygen carrier should own high activity, rich oxygen storage capacity, good thermal stability and negligible carbon deposition. Over the optimal oxygen carrier, methane can be converted to H2 and CO with the appropriate molar ratio of H2/ CO (2.0), which can be directly applied to the Fischer-Tropsch synthesis reaction. What’s more, during the second step, the reduced oxygen carriers could still exhibit high reactivity in the presence of H2O [16–18]. Additionally, the required oxygen carriers should also possess low cost and environmental friendliness [19]. The perovskite type oxides own rich oxygen vacancy, good oxygen mobility, high oxygen storage capacity and thermal stability, which are of great significance for the oxygen carriers in the CLRM process [20]. In general, the perovskite type oxides are present in the form of ABO3, where A can be a lanthanide or alkaline earth element, which is located at the center of the dodecahedral structure. B is a transition metal element, which is coordinated with oxygen in the octahedral structure [21,22]. Nitadori et al. [23] found that the activity of ABO3 principally depended on the B site, such as Mn and Co as B sites exhibiting high activity for the oxidation of hydrocarbons and CO [24]. LaMnO3-based perovskite oxides have received extensive attention on CH4 catalytic
2.2. Physical and chemical characterization of oxygen carriers The phase compositions of oxygen carriers were determined by X-ray diffraction (XRD, Rigaku Miniflex 600) using Cu-Kα radiation (λ ¼ 1.54056 Å). XRD patterns were acquired at a scan speed of 1 deg min 1 and a scan range of 20-80� , respectively. The X-ray diffractometer operated at a voltage of 40 kV and a tube current of 15 mA. Raman spectra were acquired on a Thermo Fisher DXRxi Raman spectrometer using an Arþ laser with an incident light wavelength of 532 nm. The laser output power for oxygen carrier testing was 7.0 mw, and the exposure time and scanning range in this process were 80 s and 100-1800 cm 1, respectively. X-ray photoelectron spectroscopy (XPS) was to analyze the surface composition and chemical state of the oxygen carriers. XPS character ization was operated on the PHI5500 X-ray photoelectron spectrometer, which used Al Kα as the radiation source. The internal standard cor rected binding energy of C1s peak on oxygen carrier was 284.8 eV. H2 temperature programmed reduction (H2-TPR) experiment was conducted on the Chembet Pulsar TPR/TPD instrument produced by 2
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Fig. 1. The X-ray diffraction patterns (A, B) and Raman spectra (C, D) of LaMnO3 (a), La0.95MnFe0.05O3 (b), La0.9MnFe0.1O3 (c), La0.85MnFe0.15O3 (d), La0.8MnFe0.2O3 (e), LaMn0.95Fe0.05O3 (f), LaMn0.9Fe0.1O3 (g), LaMn0.85Fe0.15O3 (h) and LaMn0.8Fe0.2O3 (i). Mn 2p (E) and O 1s (F) XPS spectra of LaMnO3 (a), La0.85MnFe0.15O3 (b) and LaMn0.9Fe0.1O3 (c) samples.
Quantanchrome Company. The concentration of exhaust gases was detected via a TCD detector. 50 mg oxygen carriers were purged for 30 min with a helium flow rate of 20 mL min 1 at 120 � C. Volumetric flow is cited under normal conditions for temperature and pressure (NTP), viz. 25 � C, 1 atm. Then it was raised from room temperature to 900 � C at a heating rate of 10 � C min 1 in flowing 10% H2/Ar mixture (20 mL min 1) (NTP). CH4 temperature programmed reduction (CH4-TPR) microactivity tests were carried out in a reaction tube and the exhaust gases were analysed using an online mass spectrometer (QGA, manufactured by Hiden Analytical Co., England). 50 mg oxygen carriers were weighed and put into a fixed bed reactor. After purged with Ar, 5% CH4/Ar mixed gas with a flow rate of 30 mL min 1 (NTP) was introduced, and then the reaction temperature was increased from room temperature to 900 � C at a heating rate of 10 � C min 1.
from Messer Company. The compositions of the gases were analysed using an on-line gas analyzer (GASBOARD-3100, Wuhan cubic opto electronic Co., China) and a gas chromatograph (Agilent 7890A GC System, manufactured by Agilent Co.). The CO selectivity, syngas productivity, H2/CO mole ratio and H2 productivity were calculated throughout the redox process as follows. In the methane reduction step:
2.3. Reactivity tests
Coke productivity ¼
CO selectivityð%Þ ¼
nCH4;i
Syngas productivity ¼
H2
nCO � 100 nCH4;o
(eq.1)
nCO þ nH2 mOC
(eq.2)
� nH CO mole ratio ¼ 2 nCO nCH4;i
(eq.3) nCO
nCO2 mOC
nCH4;o
(eq.4)
In the water splitting step:
Both CH4 reduction and water splitting reaction were carried out in a tubular fixed bed reactor. 1.8 g oxygen carriers with a particle size be tween 20 and 40 mesh were loaded in the middle of the reaction furnace. Firstly, the furnace temperature was increased from room temperature to 850 � C at a heating rate of 10 � C min 1 with introduction of high purity N2 (99.99%). CH4 reduction reaction began when the gas was switched to a 5% CH4/N2 mixture. After 30 min, N2 was introduced to purge the gas. And then steam was brought into the reaction tube to start the water splitting reaction. In the CH4 reduction reaction, the gas flow rate was 150 mL min 1 (NTP). The gases used above were purchased
H2 productivity ¼
nH2 mOC
(eq.5)
where moc is the mass of oxygen carriers loaded into the reactor, nH2 , nCO and nCO2 are the moles of H2, CO and CO2 produced, respectively. nCH4;i and nCH4;o are the moles of methane fed to the reactor and methane left the reactor separately.
3
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Table 1 The lattice parameter of LaMnO3 and La1-xMnFexO3/LaMn1-xFexO3 oxygen carriers. Cell volume (Å3)
Samples
Lattice structure
Crystal size (nm)
Lattice constant (Å) a
b
c
LaMnO3 La0.95MnFe0.05O3 La0.9MnFe0.1O3 La0.85MnFe0.15O3 La0.8MnFe0.2O3 LaMn0.95Fe0.05O3 LaMn0.9Fe0.1O3 LaMn0.85Fe0.15O3 LaMn0.8Fe0.2O3
Hexagonal Hexagonal Hexagonal Hexagonal Hexagonal Hexagonal Hexagonal Hexagonal Hexagonal
29.464 24.056 29.199 20.175 19.587 21.469 33.475 30.440 24.755
5.505 5.490 5.489 5.484 5.481 5.494 5.491 5.489 5.490
5.505 5.490 5.489 5.484 5.481 5.494 5.491 5.489 5.490
13.297 13.294 13.290 13.303 13.307 13.341 13.339 13.335 13.310
348.97 347.06 346.80 346.46 346.18 348.69 348.30 347.94 347.40
Table 2 Surface element compositions of LaMnO3, La0.85MnFe0.15O3 and LaMn0.9Fe0.1O3 samples. Samples LaMnO3 La0.85MnFe0.15O3 LaMn0.9Fe0.1O3
Surface element composition Mn4þ/Mn3þ
Osurf/Olatt
0.60 0.87 0.82
0.68 0.96 0.89
3. Results Fig. 1A and B shows the XRD patterns of the samples, all the diffraction peaks of the La1-xMnFexO3/LaMn1-xFexO3 perovskites oxides match well with the patterns of LaMnO3. Weak peaks for Mn3O4 at about 2θ ¼ 28.9� , 36� and 60.1� are detected in the La0.8MnFe0.2O3 sample because the Mn cations don’t completely enter the LaMnO3 structure. Detailed analysis of the crystalline size, lattice constant and cell volume of the oxygen carriers are presented in Table 1. It can be seen that all oxygen carriers generate single-phase hexagonal structure. There is no obvious difference on the crystal sizes of the samples. The crystal sizes of the La1-xMnFexO3 (19.587–29.199 nm) series samples are smaller than that of pure LaMnO3 (29.464 nm). La0.85MnFe0.15O3 and La0.8MnFe0.2O3 own smaller crystal size and weaker diffraction peak intensity among all samples. It is found that the LaMn0.9Fe0.1O3 sample presents the stron gest peak intensity and the highest crystallinity. As shown in Table 1, the cell volumes of La1-xMnFexO3 (346.18–347.06 Å3) are smaller than that of LaMnO3 (348.97 Å3). It can be attributed that La3þ (1.17 Å) with a larger ionic radius is partially substituted with smaller Fe3þ (0.62 Å), leading to the contraction of cell volumes. The cell volumes of the LaMn1-xFexO3 samples (347.40–348.69 Å3) are slightly smaller than that of pure LaMnO3 (348.97 Å3), which are sequentially decreased with Fe content. These phenomena can be attributed that there is no obvious difference in ionic radius between Mn3þ (0.645 Å) and Fe3þ (0.62 Å), which causes lattice shrinkage and cell volumes reduction [31,32]. Raman spectroscopy technique is commonly used to investigate the structure of the materials. This is due to the fact that Raman spectros copy can characterize the structure of amorphous oxides and crystalline oxides. As shown in Fig. 1C and D, Raman spectrum of the hexagonal structure LaMnO3 exhibits a peak at ~660 cm 1 [33]. The peak at ~660 cm 1 can be attributed to the extension of Mn–O in MnO6 units [34]. The peak intensity of La1-xMnFexO3/LaMn1-xFexO3 oxygen carrier at ~660 cm 1 is slightly higher than that of LaMnO3, indicating that the addition of Fe leads to an increase in Jahn-Teller (JT) distortion [35]. As a result, the lattice distortion of the La1-xMnFexO3/LaMn1-xFexO3 oxygen carrier leads to more oxygen vacancies. The Mn 2p and O 1s XPS spectra of LaMnO3, La0.85MnFe0.15O3 and LaMn0.9Fe0.1O3 samples are shown in Fig. 1E and F. Table 2 lists the corresponding quantitative analysis results. From Fig. 1E, it can be seen that the spectra of Mn 2p in each sample are fitted by four peaks. The signals at ~641.6 eV and ~653.2 eV are ascribed to Mn3þ, whereas the
Fig. 2. H2-TPR profiles of LaMnO3 (a), La0.95MnFe0.05O3 (b), La0.9MnFe0.1O3 (c), La0.85MnFe0.15O3 (d), La0.8MnFe0.2O3 (e), LaMn0.95Fe0.05O3 (f), LaMn0.9 Fe0.1O3 (g), LaMn0.85Fe0.15O3 (h) and LaMn0.8Fe0.2O3 (i).
peaks at ~643.1 eV and ~654.7 eV are assigned to Mn4þ [36]. As listed in Table 2, with the doping of Fe, the molar ratio of Mn4þ/Mn3þ in creases gradually, the molar ratio of Mn4þ/Mn3þ increases following the order as: LaMnO3 (0.6) < LaMn0.9Fe0.1O3 (0.82) < La0.85MnFe0.15O3 (0.87). La0.85MnFe0.15O3 (0.87) owns the largest molar ratio of Mn4þ/Mn3þ. It is reported that larger Mn4þ/Mn3þ molar ratio is related to lattice defects [37], because part of Mn3þ oxidized into Mn4þ ions on the surfaces of the oxygen carriers gave rise to the formation of struc tural defects [22]. Lattice distortion occurs when Fe ions are added to pure LaMnO3, forming oxygen vacancies. Compared with LaMn0.9 Fe0.1O3, La0.85MnFe0.15O3 oxygen carrier possesses a large degree of lattice distortion and more oxygen vacancies. This demonstrates that La at A-site position in LaMnO3 is more conducive to the formation of Mn4þ after being partially substituted by Fe (15%). Fig. 1F displays the XPS signals from the O1s element of samples. The higher binding energy peak at ~531.0 eV is ascribed to surface oxygen (Osurf) from the car bonate or hydroxyl species. The lower binding energy peak at ~529.4 eV is attributed to lattice oxygen (Olatt) from the bulk side of the perovskite oxide [38]. It is well known that surface adsorbed oxygen holds better mobility than lattice oxygen, as well as surface oxygen is generally considered to be more oxidative activity [39]. It can be seen that the molar ratio of Osurf/Olatt gradually increases with the loading of Fe. The Osurf/Olatt molar ratio of the oxygen carrier could reflect oxygen vacancy concentration to some extent [39]. The Osurf/Olatt molar ratio of La0.85MnFe0.15O3 (0.96) is larger than that of LaMn0.9Fe0.1O3 (0.89), confirming that La0.85MnFe0.15O3 possesses higher oxygen vacancy concentration. Three samples exhibit an increasing Osurf/Olatt molar ratio as following: LaMnO3 (0.68) < LaMn0.9Fe0.1O3 (0.89) < 4
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Table 3 Quantitative results of reduction species over the LaMnO3 and La1-xMnFexO3/LaMn1-xFexO3 samples. Samples
α LaMnO3 La0.95MnFe0.05O3 La0.9MnFe0.1O3 La0.85MnFe0.15O3 La0.8MnFe0.2O3 LaMn0.95Fe0.05O3 LaMn0.9Fe0.1O3 LaMn0.85Fe0.15O3 LaMn0.8Fe0.2O3
Total H2 consumption (mmol g-1)
H2 consumption (mmol g-1) 0.075 0.047 0.050 0.059 0.089 0.070 0.024 0.038 0.050
β
γ 1.070 1.081 0.605 0.559 0.625 0.951 – 0.933 0.720
1.145 1.128 1.165 1.213 1.561 1.021 0.942 0.971 1.167
– – 0.510 0.595 0.847 – 0.918 – 0.397
Peak temperature (oC)
α
β
γ
321 293 278 270 281 316 313 311 312
442 430 447 419 420 424 – 434 431
– – 464 460 485 – 476 – 491
Fig. 3. CH4-TPR profiles over LaMnO3 (A), La0.95MnFe0.05O3 (B), La0.9MnFe0.1O3 (C), La0.85MnFe0.15O3 (D), La0.8MnFe0.2O3 (E), LaMn0.95Fe0.05O3 (F), LaMn0.9 Fe0.1O3 (G), LaMn0.85Fe0.15O3 (H) and LaMn0.8Fe0.2O3 (I).
La0.85MnFe0.15O3 (0.96). The H2-TPR are in order to investigate the reducibility of the oxygen carriers, and the results are shown in Fig. 2. Table 3 summarizes the peak temperatures and the calculated H2 consumption amount of the reduc tion peaks. It is reported that the reduction process of Mn-based
perovskite can be divided into two steps [40]. The low temperature reduction peaks labeled as α (270–321 � C) are due to the removal of a small quantity of surface adsorbed oxygen species. In the high-temperature stage, the β peaks located at 419–476 � C and γ peaks located at 460–491 � C are attributed to the removal of the 5
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Fig. 4. (A–I) CH4 isothermal reaction at 850 � C over LaMnO3 (A), La0.95MnFe0.05O3 (B), La0.9MnFe0.1O3 (C), La0.85MnFe0.15O3 (D), La0.8MnFe0.2O3 (E), LaMn0.95 Fe0.05O3 (F), LaMn0.9Fe0.1O3 (G), LaMn0.85Fe0.15O3 (H) and LaMn0.8Fe0.2O3 (I) oxygen carriers. (J–L) The CO selectivity (J), H2/CO ratio (K) and syngas productivity (L) over the oxygen carriers during CH4 isothermal reaction at 850 � C.
non-stoichiometric excess oxygen contained in the lattice and the reduction of Mn4þ to Mn3þ, respectively [41]. As shown in Table 3, when Fe ions are added to LaMnO3, the lowtemperature reduction peaks move towards lower temperature. This indicates that the introduction of Fe could weaken the Mn–O bonds located near the iron, thereby promoting the mobility of lattice oxygen [42]. LaMn0.9Fe0.1O3 shows the smallest hydrogen consumption amount of α peak, indicating a lower concentration of surface oxygen on the LaMn0.9Fe0.1O3 samples. However, the low temperature reduction peaks of the LaMn1-xFexO3 samples shift a small amplitude to lower tempera ture. La0.85MnFe0.15O3 exhibits the lowest peak temperatures in the
reduction peak temperatures of all samples. A broad reduction peak (between 350 and 600 � C) of La0.9MnFe0.1O3, La0.85MnFe0.15O3, La0.8MnFe0.2O3 and LaMn0.8Fe0.2O3 differentiates into a medium tem perature reduction peak (β) and a high temperature reduction peak (γ) during the reduction process. As the increase of Fe contents, the total hydrogen consumption of the oxygen carriers increases. The reducibility of the La1-xMnFexO3 samples is better than that of LaMn1-xFexO3. The total hydrogen consumption of La0.9MnFe0.1O3 (1.165 mmol g 1), La0.85MnFe0.15O3 (1.213 mmol g 1), La0.8MnFe0.2O3 (1.561 mmol g 1) and LaMn0.8Fe0.2O3 (1.167 mmol g 1) are higher than that of LaMnO3 (1.145 mmol g 1), indicating better reducibility. The total hydrogen 6
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Fig. 5. H2 contents (A1, B1) and productivity (A2, B2) of LaMnO3 (a), La0.95MnFe0.05O3 (b), La0.9MnFe0.1O3 (c), La0.85MnFe0.15O3 (d), La0.8MnFe0.2O3 (e), LaMn0.95Fe0.05O3 (f), LaMn0.9Fe0.1O3 (g), LaMn0.85Fe0.15O3 (h) and LaMn0.8Fe0.2O3 (i) samples during the water splitting reaction at 850 � C.
consumption decreases in the order of La0.8MnFe0.2O3 > La0.85Mn Fe0.15O3 > LaMn0.8Fe0.2O3 > La0.9MnFe0.1O3 > LaMnO3. Fig. 3 shows the CH4-TPR profiles over LaMnO3, La1-xMnFexO3 and LaMn1-xFexO3 samples. The reduction process of the samples can be divided into two phases. In the first stage, CH4 is converted to CO2 and H2O by surface oxygen when the temperature ranged from 500 to 700 � C. In the second stage, CH4 is mainly converted to CO and H2 by lattice oxygen at temperatures above 700 � C [43]. In the low temperature stage (500–700 � C), for pure LaMnO3, both CO2 and H2O are at a low level. After doping of Fe to LaMnO3, both CO2 and H2O increase, and the higher content of Fe, the more amount of CO2 and H2O, indicating that the amount of active oxygen on the surface of the samples increases after the introduction of iron. Compared with LaMn1-xFexO3, the series of La1-xMnFexO3 samples present higher activity for methane oxidation. In the high temperature stage (>700 � C), the bulk oxygen diffuses to the surface of the oxygen carriers and reacts with methane, both CO and H2 produce, while CO2 and H2O decrease. This suggests that the mobility of lattice oxygen plays a crucial role in the partial oxidation of methane. La–Mn–Fe–O mixed oxides produce higher CO and H2, exhibiting higher CO selectivity than pure LaMnO3. In conclusion, La–Mn–Fe–O mixed oxides behave higher reducibility than pure LaMnO3 due to the syner gistic effect of Mn and Fe [44]. As shown in Fig. 3, the intensities of CO and H2 are mainly produced in the range of 800–900 � C. Further, when the isothermal reaction temperature is lower than 850 � C, the reactivity of oxygen carrier is poor, and carbon deposition occurs when the isothermal reaction temperature is higher than 850 � C (see Fig. S3). Thus, 850 � C is chosen as the optimal reaction temperature for the isothermal reactions to guarantee a higher reactivity and a resistance to carbon deposition. As shown in Fig. 4(A-I), H2, CO and CO2 are the main gases produced during CH4 isothermal reaction at 850 � C and the process is endothermic (Fig. S1A and Table S1). At the initial stage, higher contents of CO2 produce. This is due to the complete oxidation between CH4 and surface oxygen of the oxygen carrier. The amount of CO2 over La1-xMnFexO3 samples is higher than that over LaMn1-xFexO3, indicating that there is
more surface adsorbed oxygen over the La1-xMnFexO3 oxygen carriers. After that, the contents of CO2 decrease gradually, while the H2 and CO begin to increase. This stage can be attributed to the partial oxidation of methane with bulk oxygen after the surface oxygen is gradually consumed. It can be seen from Fig. 4J, the CO selectivity of LaMnO3 and LaMn0.95Fe0.05O3 stabilizes at ~87% after 600 s. However, the CO selectivity of La0.9MnFe0.1O3, La0.85MnFe0.15O3, La0.8MnFe0.2O3, LaMn0.9Fe0.1O3 and LaMn0.8Fe0.2O3 reaches to ~99%. La0.85MnFe0.15O3 shows the higher CO selectivity than the other samples. It is known that the selectivity of CO is determined by the relative content of surface oxygen and lattice oxygen [45]. It has been speculated from the amount of CO2 that there are more surface oxygen on the La0.85MnFe0.15O3, therefore the La0.85MnFe0.15O3 sample should also own more lattice oxygen. Eventually, with the gradual consumption of lattice oxygen, the contents of CO and H2 decrease slowly. The contents of CO and H2 over La1-xMnFexO3/LaMn1-xFexO3 samples are much higher than the pure LaMnO3 sample, indicating that the addition of Fe improves the pro duction of syngas. As seen in Fig. 4K, the H2/CO ratios of LaMnO3, LaMn0.95Fe0.05O3 and LaMn0.85Fe0.15O3 gradually decrease to a lower value (~1.75) as the reaction proceeded. However, the H2/CO ratios of the other oxygen carriers, especially La0.85MnFe0.15O3, keeps stable at the ideal ratio of 2.0 (1.93–2.07) during the whole methane partial oxidation process. As shown in Fig. 4L, the introduction of Fe to oxygen carriers enhances the syngas productivity of pure LaMnO3 (2.61 mmol g 1) and the La0.85Mn0.15O3 sample exhibits the highest syngas pro ductivity (3.78 mmol g 1) among all oxygen carriers. Fig. 5 shows the contents and productivity of H2 during the water splitting reaction at 850 � C. The process is also endothermic and the heat analysis of reaction is shown in the supplemental material (Fig. S1B and Table S1). It can be seen in Fig. 5A1 and B1, both H2 and a small amount of CO can be detected over the pure LaMnO3 samples. While only H2 can be detected and no carbon-containing gas observed over La1-xMnFexO3/ LaMn1-xFexO3 samples during the water splitting reaction, indicating that there is no carbon deposition over the La1-xMnFexO3/LaMn1-xFexO3 samples during the CH4 oxidation [46]. In the partial oxidation of 7
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Fig. 6. The evolved amounts of products gas during 20 successive CH4 reduction (A1, B1 and C1) and water splitting (A2, B2 and C2) cycles at 850 � C over LaMnO3, La0.85MnFe0.15O3 and LaMn0.9Fe0.1O3. H2 productivity (A3, B3 and C3) over the oxygen carriers during the 20 cycles of water splitting at 850 � C.
methane, the amount of carbon deposition is calculated from eq. (4). The coke is formed by methane cracking, and the amount of coke could be calculated directly from the amount of carbons gas (eg. CO or CO2) during water splitting step [47]. As shown in Fig. S2, only the CH4 consumption over LaMnO3 is greater than the total production of CO and CO2, indicating that a small amount of carbon deposition is formed. This is consistent with the experimental phenomenon in Fig. 5A1 and B1. At the initial stage, the content of H2 increases rapidly with the introduc tion of steam. The hydrogen content of the samples reaches to a maximum immediately. And then the hydrogen contents maintain at a high value for 600–700 s. After that, the hydrogen contents gradually decrease to zero owing to the restoration of lattice oxygen and surface oxygen [48]. The whole process lasts for ~1700 s. Fig. 5A2 and B2 show that the H2 productivity of pure LaMnO3 is 1.55 mmol g 1, while the H2 productivities of La0.85MnFe0.15O3 and LaMn0.9Fe0.1O3 are 1.76 and 1.64 mmol g 1, respectively, which are higher than that of the pure LaMnO3. This phenomenon indicates that the introduction of a proper amount of Fe could improve the H2 productivity of pure LaMnO3, and when the La (A site) is substituted by Fe for 15%, the La0.85MnFe0.15O3 sample achieves the highest H2 productivity. The gas evolution of LaMnO3, LaMn0.85Fe0.15O3 and LaMn0.9Fe0.1O3 oxygen carriers during 20 successive redox cycles at 850 � C are shown in Fig. 6. In the first cycle, CO2 content reaches to a high value rapidly in
the initial stage of methane reduction, but decreases afterwards. With the consumption of surface oxygen, lattice oxygen starts to react with the CH4, leading to the production of syngas (CO and H2). For all the samples, the amount of the product gas keeps at a high value during the first redox, after that the gas content decreases slightly with the cycle number. Compared with pure LaMnO3, the CO2, H2 and CO contents of the La0.85MnFe0.15O3 and LaMn0.9Fe0.1O3 samples are relatively higher in the reduction step, indicating that the content of reactive oxygen species increases with the doping of Fe. After that, steam is introduced, leading to the production of H2 [49]. It can be seen from Fig. 6A2, B2 and C2 that the H2 contents of LaMn0.85Fe0.15O3 and LaMn0.9Fe0.1O3 are higher than that of LaMnO3 in the water splitting step. Additionally, the quantitative analysis results of three samples in the oxidation step are shown in Fig. 6A3, B3 and C3. It is noted that the contents of H2 decrease with the number of cycles in the redox process, and the average hydrogen productivity increases as follows: LaMnO3 < LaMn0.9Fe0.1O3 < La0.85MnFe0.15O3. It can be found that the evident decline in the gas content during the first cycle, and then the gases contents maintain at a relatively stable level, suggesting that the content of reactive oxygen species in fresh samples is higher than that of samples after the cyclic reaction. The H2 productivity of La0.85Mn0.15O3 (0.55 mmol g 1) is still higher than that of LaMnO3 (0.48 mmol g 1) and LaMn0.9Fe0.1O3 (0.49 mmol g 1) after 20 cycles, demonstrating that La0.85MnFe0.15O3 exhibits 8
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Table 4 Details of lattice parameter and cell volume of LaMnO3, La0.85MnFe0.15O3 and LaMn0.9Fe0.1O3 composites after 20 redox cycles at 850 � C. Cell volume (Å3)
Samples
Lattice structure
Crystal size (nm)
Lattice constant (Å) a
b
c
LaMnO3 La0.85MnFe0.15O3 LaMn0.9Fe0.1O3
Orthorhombic Orthorhombic Orthorhombic
37.954 29.056 34.093
5.535 5.541 5.530
5.744 5.682 5.692
7.694 7.686 7.724
244.62 241.96 243.13
Fig. 7. XRD pattern (A), Raman spectrum (B) and XPS spectra of Mn 2p (C) and O 1s (D) for LaMnO3 (a), La0.85MnFe0.15O3 (b) and LaMn0.9Fe0.1O3 (c) after 20 redox cycles at 850 � C.
an outstanding structural and thermal stability. It is interesting that there is no carbon deposition detected among the three samples during the 20 successive redox cycles. Fig. 7A presents the XRD patterns of LaMnO3, La0.85MnFe0.15O3 and LaMn0.9Fe0.1O3 oxygen carriers after 20 redox cycles at 850 � C. It can be observed that La0.85MnFe0.15O3 shows a single-phase perovskite struc ture, while a new phase of La2O3 is detected in the cycled LaMnO3 and LaMn0.9Fe0.1O3 oxygen carriers. This phenomenon suggests that the La0.85MnFe0.15O3 exhibits higher thermal stability and structural sta bility than the other two samples after redox cycles. In addition, the diffraction peak intensity of the La0.85MnFe0.15O3 sample is weak, indicating lower crystallinity [50]. The lattice parameter and cell vol ume of LaMnO3, La0.85MnFe0.15O3 and LaMn0.9Fe0.1O3 composites after 20 redox cycles at 850 � C are listed in Table 4. It can be seen that the crystal structure of the cycled samples belongs to orthorhombic system, where the cell volume decreases. La0.85MnFe0.15O3 (241.96 Å3) pos sesses a smaller unit cell volume than LaMnO3 (244.62 Å3) and LaMn0.9Fe0.1O3 (243.13 Å3). As shown in Table 4, after cycled, the crystal size of all the samples increased. Compared with LaMnO3 (37.954 nm) and LaMn0.9Fe0.1O3 (34.093 nm), LaMn0.85Fe0.15O3 (29.056 nm) shows a smaller grain size, indicating a better thermal stability. The Raman spectra of the LaMnO3, La0.85MnFe0.15O3 and LaMn0.9 Fe0.1O3 oxygen carriers after 20 redox cycles at 850 � C are displayed in
Table 5 Surface atomic ratio of the LaMnO3, La0.85MnFe0.15O3 and LaMn0.9Fe0.1O3 ox ygen carriers after 20 redox cycles at 850 � C measured by XPS. Samples
Surface element composition Mn4þ/Mn3þ
LaMnO3 La0.85MnFe0.15O3 LaMn0.9Fe0.1O3
0.39 0.79 0.72
Osurf/Olatt 0.43 0.73 0.71
Fig. 7B. Three major Raman modes are centered on ~275 cm 1, ~478 cm 1 and ~604 cm 1. Compared with the fresh samples, the cycled samples show extra Raman peaks near 275 cm 1 and 478 cm 1 which can be related to the local JT distortions [51]. Moreover, it is observed that the position and width of two most prominent Raman peaks (~478 cm 1 and ~604 cm 1) match well with the orthorhombic structure [52]. The peak intensity of La0.85MnFe0.15O3 and LaMn0.9Fe0.1O3 at 604 cm 1 is slightly weaker than LaMnO3, which is mainly related to a disordered perovskite structure [51]. Compared with LaMn0.9Fe0.1O3, the Raman peak intensity of La0.85MnFe0.15O3 at 604 cm 1 is weaker. Fig. 7C and D shows the XPS spectra of Mn 2p and O 1s for the oxygen carriers after 20 redox cycles at 850 � C. The molar ratios of Mn4þ/Mn3þ and Osurf/Olatt calculated by XPS spectra are listed in Table 5. Compared 9
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Fig. 8. (A) H2-TPR profiles of LaMnO3 (a), La0.85MnFe0.15O3 (b) and LaMn0.9Fe0.1O3 (c) after 20 redox cycles at 850 � C. (B) CH4-TPR profiles over LaMnO3 (B1), La0.85MnFe0.15O3 (B2) and LaMn0.9Fe0.1O3 (B3) after 20 redox cycles at 850 � C.
Fig. 8B shows the CH4-TPR profiles of LaMnO3, La0.85MnFe0.15O3 and LaMn0.9Fe0.1O3 oxygen carriers after 20 redox cycles. Compared with fresh samples, the contents of H2 and CO produced in CH4-TPR process of LaMnO3, La0.85MnFe0.15O3 and LaMn0.9Fe0.1O3 samples are reduced after 20 redox cycles. La0.85MnFe0.15O3 and LaMn0.9Fe0.1O3 still own higher H2 and CO contents and lower CO2 and H2O contents than that of LaMnO3, exhibiting higher CO selectivity.
Table 6 Quantitative results of reduction species over the LaMnO3, La0.85MnFe0.15O3 and LaMn0.9Fe0.1O3 samples after 20 redox cycles at 850 � C. Samples
LaMnO3 La0.85MnFe0.15O3 LaMn0.9Fe0.1O3
H2 consumption (mmol g 1)
α
β
0.006 0.001 0.010
0.032 0.087 0.029
Total H2 consumption (mmol g-1)
0.038 0.088 0.039
Peak temperature (oC)
α
β
333.5 337.4 346.1
458.1 483.1 449.9
4. Discussion The main products of H2 and CO during the CH4 partial oxidation process can be directly used for methanol production and FischerTropsch synthesis when the H2/CO molar ratio is 2.0. However, car bon deposition often formed during the CH4 partial oxidation reaction, leading to the H2/CO molar ratio is much higher than 2.0. The formation of carbon deposition would reduce the catalytic activities. Ma et al. [54] reported that during the CH4 partial oxidation process over NiO/Al2O3 spherical catalyst, the carbon deposition resulted in the decrease of CH4 conversion within the initial 4 h of the reaction. Alvarez-Ganvan et al. [55] proved that the deactivation of Ni/ZrO2 catalyst was mainly related to the formation of carbon deposition during the partial oxidation of methane to syngas and/or hydrogen, which resulted in a lower CH4 conversion and CO selectivity. Fakeeha et al. [56] demonstrated that a decrease in methane conversion of Ni-800/(ZrO2 þ Al2O3) can be attributed that carbon deposition blocked the active sites. In addition, the presence of carbon deposition would also reduce the purity of produced hydrogen in the water splitting step. Zhao et al. [57] presented that the La2FeCoO6 produced not only a large amount of H2 but also a large amount of CO and CO2 during water splitting step, which was derived from gasification of coke and proved that methane was cracked during methane oxidation. Hafizi and Rahimpour et al. [58] found that the conversion of carbon deposition during the oxidation process reduced the purity of hydrogen produced in the process (about 98.5%). Our research confirmed that only pure hydrogen was produced
with that of the fresh oxygen carriers, the Mn4þ/Mn3þ and Osurf/Olatt molar ratios of the cycled oxygen carriers are reduced, which can be attributed to the decrease of oxygen vacancy concentration [53]. It can be seen that the molar ratio of Mn4þ/Mn3þ for La0.85MnFe0.15O3 is 0.79, which is higher than that of LaMnO3 (0.39) and LaMn0.9Fe0.1O3 (0.72). Similar to the molar ratio of Mn4þ/Mn3þ, La0.85MnFe0.15O3 (0.73) ex hibits a higher molar ratio of Osurf/Olatt than LaMnO3 (0.43) and LaMn0.9Fe0.1O3 (0.71), indicating higher oxygen vacancy concentration. The molar ratios of Mn4þ/Mn3þ and Osurf/Olatt are increased as follows: LaMnO3 < LaMn0.9Fe0.1O3 < La0.85MnFe0.15O3. Fig. 8A shows the reduction peaks of LaMnO3, La0.85MnFe0.15O3 and LaMn0.9Fe0.1O3 oxygen carriers after 20 times of isothermal reaction at 850 � C. Table 6 lists the peak temperatures and the calculated H2 con sumption amount of the reduction peaks. La0.85MnFe0.15O3 exhibits a broad β peak instead of two well-defined β and γ peaks after 20 isothermal cycles. Compared with fresh samples, the total hydrogen consumption of LaMnO3 (0.038 mmol g 1), La0.85MnFe0.15O3 (0.088 mmol g 1) and LaMn0.9Fe0.1O3 (0.039 mmol g 1) samples decreased after cyclic reaction. La0.85MnFe0.15O3 still possesses higher hydrogen consumption among three oxygen carriers. The α peaks of three samples all move to higher temperatures. The β peak of La0.85MnFe0.15O3 moves to higher temperature to 483.1 � C in three samples. 10
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in the water splitting stage and no carbon-containing gas was detected. Only a small amount of CO was detected during the regeneration of the LaMnO3 oxygen carrier (as shown in Fig. 5). However, no carbon-containing gas was produced over La1-xMnFexO3/LaMn1-xFexO3 oxygen carriers during the reaction with water vapor and even no car bon deposition formed after 20 successive cycles (see Fig. 6A2, B2 and C2). It can be concluded that the introduction of a small amount of Fe to the LaMnO3 oxygen carrier can prevent the carbon deposition [59], due to the interaction of Fe and Mn and the abundant oxygen vacancies.
[10] M. Nomura, J. Membr. Sci. 240 (2004) 221–226. [11] K. Mazloomi, C. Gomes, Renew. Sustain. Energy Rev. 16 (2012) 3024–3033. [12] L. Jiang, X. Yuan, G. Zeng, Z. Wu, L. Jie, X. Chen, L. Leng, W. Hui, W. Hou, Appl. Catal. B Environ. 221 (2018), 725-725. [13] Z. Fan, P. Zhao, N. Meng, J. Maddy, Int. J. Hydrogen Energy 41 (2016) 14535–14552. [14] K. Zhao, A. Zheng, H. Li, H. Fang, H. Zhen, G. Wei, S. Yang, Z. Zhao, Appl. Catal. B Environ. 219 (2017) 672–682. [15] A. Hafizi, M.R. Rahimpour, S. Hassanajili, Appl. Energy 165 (2016) 685–694. [16] C. Dueso, M. Ortiz, A. Abad, F. García-Labiano, L.F.D. Diego, P. Gay� an, J. Ad� anez, Chem. Eng. J. 188 (2012) 142–154. [17] M. Ortiz, P. Gay� an, L.F.D. Diego, F. García-Labiano, A. Abad, M.A. Pans, J. Ad� anez, J. Power Sources 196 (2011) 4370–4381. [18] M. Ryd�en, P. Ramos, Fuel Process. Technol. 96 (2012) 27–36. [19] H. Zhao, G. Lei, X. Zou, Appl. Energy 157 (2015) 408–415. [20] L. Lin, Y. Song, J. Bo, K. Wang, Z. Qian, Energy 131 (2017) 58–66. [21] M.J. Tyler, W.G. Hardin, D. Sheng, K.P. Johnston, K.J. Stevenson, Nat. Mater. 13 (2014) 726–732. [22] J. Chen, M. Shen, X. Wang, Q.I. Gongshin, J. Wang, L.I. Wei, Appl. Catal. B Environ. 134 (2013) 251–257. [23] T. Nitadori, T. Ichiki, M. Misono, T. Nitadori, T. Ichiki, M. Misono, Bull. Chem. Soc. Jpn. 61 (1988) 621–626. [24] S. Ponce, M.A. Pe~ na, J.L.G. Fierro, Appl. Catal. B Environ. 24 (2000) 193–205. [25] A. Giroirfendler, A. Baylet, L. Retailleau, A. Boreave, G.Z. Lu, Y. Guo, Y.L. Guo, C. H. Zhang, Appl. Catal. B Environ. 148–149 (2014) 490–498. [26] M. Al-Mamun, Sustain. Energy Fuels 1 (2017) 1013–1017. [27] G. Landi, P.S. Barbato, A.D. Benedetto, G. Russo, R. Pirone, Appl. Catal. B Environ. 134–135 (2013) 110–122. [28] X. Zhu, K. Li, L.M. Neal, F. Li, ACS Catal. 8 (2018) 8213–8236. [29] Q. Imtiaz, D. Hosseini, C.R. Müller, Energy Technol. 1 (2013) 633–647. [30] H. Fang, Y. Wei, H. Li, W. Hua, Energy Fuels 23 (2009) 2095–2102. [31] J.C. Jr, R. Vrba, K. Castkova, J. Cihlar, Int. J. Hydrogen Energy 42 (2017) 19920–19934. [32] S. Jauhar, M. Dhiman, S. Bansal, S. Singhal, J. Sol. Gel Sci. Technol. 75 (2015) 124–133. [33] L. Martı ́n-Carr� on, A. De Andr�es, M.J. Martı ́nez-Lope, J. Alloy. Comp. 323–324 (2001) 494–497. [34] L. Wang, H. Xie, X. Wang, G. Zhang, Y. Guo, Y. Guo, G. Lu, Chin. J. Catal. 38 (2017) 1406–1412. [35] C. Xie, S. Lei, J. Zhao, L. Yang, S. Zhou, Y. Dan, Appl. Surf. Sci. 351 (2015) 188–192. [36] C. Zhang, W. Chao, W. Zhan, Y. Guo, G. Yun, G. Lu, A. Baylet, A. Giroir-Fendler, Appl. Catal. B Environ. 129 (2013) 509–516. [37] Q. Yu, C. Wang, X. Li, Z. Li, L. Wang, Q. Zhang, G. Wu, Z. Li, Fuel 239 (2019) 1240–1245. [38] H.S. Lim, M. Lee, D. Kang, J.W. Lee, Int. J. Hydrogen Energy 43 (2018) 20580–20590. [39] Y. Wang, S. Aghamohammadi, D. Li, K. Li, R. Farrauto, Appl. Catal. B Environ. 244 (2019) 438–447. [40] Z. Sarshar, F. Kleitz, S. Kaliaguine, Energy Environ. Sci. 4 (2011) 4258–4269. [41] L. Wang, C. Wang, H. Xie, W. Zhan, Y. Guo, Y. Guo, Catal. Today 327 (2019) 190–195. [42] L.C. Wang, Q. Liu, X.S. Huang, Y.M. Liu, Y. Cao, K.N. Fan, Appl. Catal. B Environ. 88 (2009) 204–212. [43] Y. Zheng, K. Li, W. Hua, T. Dong, Y. Wang, Z. Xing, Y. Wei, Z. Min, Y. Luo, Appl. Catal. B Environ. 202 (2017) 51–63. [44] X. Zhu, Y. Wei, W. Hua, K. Li, Int. J. Hydrogen Energy 38 (2013) 4492–4501. [45] Y. Zheng, K. Li, W. Hua, Z. Xing, Y. Wei, Z. Min, Y. Wang, Energy Fuels 30 (2016) 638–647. [46] R.D. Solunke, G.T. Veser, Ind. Eng. Chem. Res. 49 (2010) 11037–11044. [47] J. Zhang, V. Haribal, F. Li, Sci. Adv. 3 (2017), e1701184. [48] K. Zhao, H. Fang, H. Zhen, G. Wei, A. Zheng, H. Li, Z. Zhao, Appl. Energy 168 (2016) 193–203. [49] C.L. Muhich, B.W. Evanko, K.C. Weston, L. Paul, L. Xinhua, M. Janna, C. B. Musgrave, A.W. Weimer, Science 341 (2013) 540–542. [50] H. Xuehui, N. Pengju, P. Hongyun, S. Xiaohui, J. Solid State Chem. 265 (2018) 218–226. [51] F. Teng, W. Han, S. Liang, B. Gaugeu, R. Zong, Y. Zhu, J. Catal. 250 (2007) 1–11. [52] A. Dubey, V.G. Sathe, J. Phys. Condens. Matter 19 (2007) 346232–346266. [53] Z.X. Wei, L. Wei, L. Gong, Y. Wang, C.W. Hu, J. Hazard Mater. 177 (2010) 554–559. [54] Y. Ma, Y. Ma, Z. Zhao, X. Hu, Z. Ye, J. Yao, C.E. Buckley, D. Dong, Renew. Energy 138 (2019) 1010–1017. [55] C. Alvarez-Galvan, M. Melian, L. Ruiz-Matas, J.L. Eslava, R.M. Navarro, M. Ahmadi, B. Roldan Cuenya, J.L.G. Fierro, Front. in Chem. 7 (2019) 104–120. [56] A.H. Fakeeha, Y. Arafat, A.A. Ibrahim, H. Shaikh, H. Atia, A.E. Abasaeed, U. Armbruster, A.S. Al-Fatesh, Processes 7 (2019) 141–157. [57] K. Zhao, L. Li, A. Zheng, H. Zhen, H. Fang, S. Yang, G. Wei, H. Li, Z. Zhao, Appl. Energy 197 (2017) 393–404. [58] A. Hafizi, M. Rahimpour, Catalysts 8 (2018) 27–42. [59] D. Hosseini, F. Donat, S.M. Kim, L. Bernard, A.M. Kierzkowska, C.R. Müller, ACS Appl. Energy Mater. 1 (2018) 1294–1303.
5. Conclusions LaMnO3 and a series of La1-xMnFexO3/LaMn1-xFexO3 (x ¼ 0.05, 0.1, 0.15, 0.2) perovskite-type oxygen carriers were prepared by sol-gel method and applied in CLRM processes. La1-xMnFexO3/LaMn1-xFexO3 oxygen carriers possessed stronger redox performance for syngas and H2 production during CLRM process than pure LaMnO3. It was found that no carbon deposition occurred over La0.85MnFe0.15O3 in the redox pro cess. The Raman and XPS characterization results further confirmed that the substitution of Fe ions into the LaMnO3 lattice resulted in lattice distortion and more oxygen vacancies, and the oxygen vacancy con centration was higher in La0.85MnFe0.15O3 oxygen carrier. During the CLRM process, the doping of Fe increased the productivity of syngas, and no carbon deposition was generated in the whole process. The CO selectivity (about 99%), syngas productivity (3.78 mmol g 1) and H2 productivity (1.76 mmol g 1) over La0.85MnFe0.15O3 were the highest, and its H2/CO molar ratio was stably maintained at the ideal ratio of 2.0 (1.93–2.07). No carbon deposition was detected in LaMnO3, La0.85Mn Fe0.15O3 and LaMn0.9Fe0.1O3 during 20 successive redox processes, while La0.85MnFe0.15O3 exhibited better reactivity and stability. In addition, La0.85MnFe0.15O3 exhibits the optimal performance and su perior resistance to carbon deposition, producing high purity of syngas and hydrogen in CLRM. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work has been financially supported by the National Natural Science Foundation of China (Project Nos. 21706108). The Yunnan Applied Basic Research Projects (No. 2018FD032), and the Analysis and Testing Center of Kunming University of Science and Technology. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227505. References [1] [2] [3] [4] [5] [6] [7] [8] [9]
H. Balat, E. Kırtay, Int. J. Hydrogen Energy 35 (2010) 7416–7426. J.R. Bartels, M.B. Pate, N.K. Olson, Int. J. Hydrogen Energy 35 (2010) 8371–8384. S. Xie, S. Yuan, P. Liao, M. Jia, Y. Wang, Water Res. 86 (2015) 74–81. S. Dutta, J. Ind. Eng. Chem. 20 (2014) 1148–1156. M. Ni, M.K.H. Leung, D.Y.C. Leung, K. Sumathy, Renew. Sustain. Energy Rev. 11 (2007) 401–425. A. Steinfeld, Sol. Energy 78 (2005) 603–615. S. Oros-Ruiz, R. Zanella, R. Lopez, A. Hernandez-Gordillo, R. Gomez, J. Hazard Mater. 263 (2013) 2–10. G. Cipriani, V. Di Dio, F. Genduso, D. La Cascia, R. Liga, R. Miceli, G.R. Galluzzo, Int. J. Hydrogen Energy 39 (2014) 8482–8494. Y. Zhang, I. Angelidaki, Water Res. 81 (2015) 188–195.
11
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Evaluation of Fe substitution in perovskite LaMnO3 for the production of high purity syngas and hydrogen Yajing Wang a, Yane Zheng a, b, *, Yuhao Wang b, c, Hua Wang b, c, Xing Zhu b, c, Yonggang Wei b, c, Yaming Wang a, Lihong Jiang a, Zhiyuan Yang a, Kongzhai Li b, c, ** a b c
Faculty of Chemical Engineering, Kunming University of Science and Technology, Kunming, 650093, China State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization, Kunming University of Science and Technology, Kunming, 650093, China Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming, 650093, China
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� The doping of Fe to LaMnO3 promotes the production of syngas and pure hydrogen. � La–Mn–Fe–O oxygen carriers possess higher reactivity and ideal H2/CO molar ratio. � La0.85MnFe0.15O3 exhibits the optimal performance during 20 redox cycles.
A R T I C L E I N F O
A B S T R A C T
Keywords: Chemical looping reforming of methane Fe ion substitution LaMnO3 Syngas Hydrogen Carbon deposition
Chemical looping reforming of methane (CLRM) technology is a new approach for syngas and pure hydrogen generation. During the whole reaction, methane is first partially oxidized to syngas by the lattice oxygen of the oxygen carrier, and then the lattice oxygen of the reduced oxygen carrier is recovered by thermal water splitting, with pure hydrogen produced. LaMnO3 is used as an oxygen carrier for the CLRM. It obtains a high resistance to carbon deposition, while a lower CH4 conversion due to the unstable structure. Fe ion substitution in LaMnO3 can be used to improve its reactivity and thermal stability. La1-xMnFexO3/LaMn1-xFexO3 (x ¼ 0.05, 0.1, 0.15, 0.2) oxygen carriers own higher reactivity and better resistance to carbon deposition, producing high purity syngas and hydrogen. La0.85MnFe0.15O3 sample exhibits the highest CO selectivity (~99%), syngas productivity (3.78 mmol g 1) and H2 productivity (1.76 mmol g 1) due to the higher oxygen vacancy concentration, and its H2/CO molar ratio is maintained at the ideal ratio of 2.0 (1.93–2.07) during the whole process. Notably, no carbon deposition is generated over LaMnO3, La0.85MnFe0.15O3 and LaMn0.9Fe0.1O3 oxygen carriers even after 20 redox cycles, and La0.85MnFe0.15O3 exhibits superior resistance to carbon deposition due to the better structural and thermal stability.
* Corresponding author. Faculty of Chemical Engineering, Kunming University of Science and Technology, Kunming, 650093, China. ** Corresponding author. Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming, 650093, China. E-mail addresses: [email protected] (Y. Zheng), [email protected] (K. Li). https://doi.org/10.1016/j.jpowsour.2019.227505 Received 23 August 2019; Received in revised form 22 November 2019; Accepted 25 November 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Yajing Wang, Journal of Power Sources, https://doi.org/10.1016/j.jpowsour.2019.227505
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1. Introduction
oxidation [25]. Moreover, LaMnO3 also exhibits high reactivity in chemical looping combustion (CLC). Since partial substitution A or B sites of the perovskite oxides can result in the formation of crystal lattice defects and thus affect the catalytic activity, the reactivity of LaM nO3-based perovskite can be adjusted by doping metal cations like Fe and Cu at A or B site [26,27]. It has been reported that replacing cations in CaMnO3 with iron in chemical looping with oxygen uncoupling (CLOU) and CLC processes can remarkably improve the oxygen absorption-release properties and stability of perovskite type oxygen carriers [28,29]. He et al. [30] found that the Fe doped on MgAl2O4 exhibited a high reactivity and redox ability in chemical-looping pro cesses. Therefore, the combination of perovskite LaMnO3 with Fe may create a promising oxygen carrier for CLRM technology to produce both syngas and hydrogen. In this work, LaMnO3 and a series of La1-xMnFexO3/LaMn1-xFexO3 (x ¼ 0.05, 0.1, 0.15, 0.2) perovskite-type oxygen carriers were prepared by sol-gel method. Moreover, the performances of oxygen carriers in the CLRM process were investigated. The substitution of Fe could improve the activity of pure LaMnO3 and the influences of Fe content on the performances of La1-xMnFexO3/LaMn1-xFexO3 were mainly studied. The La0.85MnFe0.15O3 achieved higher selectivity, productivity, and ideal H2/CO ratio with no carbon deposition. The superior carbon resistance led to the generation of pure syngas and hydrogen. In addition, we also explored the reactivity and stability of oxygen carriers for CLRM in successive redox cycles.
Fossil fuels, as the major source of energy today, present serious economic and environmental problems, such as limited resources, high prices and the generation of greenhouse gas (CO2) after combustion [1]. Therefore, the exploration of clean energy has become a key issue. Hydrogen energy plays a vital role due to its low cost and cleanliness. It can also provide energy for hydrogen fuel cell vehicles and portable electronic devices [2–4]. At present, most of the H2 industrially gener ated comes from fossil fuel gasification/reforming, but it refers to a lot of gas separation and purification steps and high energy consumption [5, 6]. Therefore, a natural, sustainable and clean technology of water decomposition is developed to produce hydrogen [7]. In recent years, there are several techniques to produce hydrogen from water decom position such as electrochemical, photocatalytic and thermal decom position of water [8–10]. The hydrogen production through the electrochemical process is simple and pollution-free, however, its low conversion efficiency and high power consumption result in high cost and limited application [11]. During the photocatalytic process, the search and optimization of high-efficiency photo-catalysts has become a major issue [12]. The traditional thermal decomposition of water con tains only one-step, while this process requires quite high temperatures (above 2500 K) and is also subject to the effective separation of hydrogen and oxygen [13]. Therefore, we urgently need to find a new type of technology to achieve more economical, efficient and clean hydrogen. Chemical looping reforming of methane (CLRM) as a novel two-step process, couples the partial oxidation of CH4 to syngas and hydrogen production by water splitting. In the first step of the CLRM process, syngas was generated, which can be used for the production of industrial chemicals. Meanwhile, the pure hydrogen produced in the second step can be applied to fuel cells or synthetic ammonia, etc. [14]. The CLRM process can be expressed as follows: � (r.1) sCH4 þ Mex Oy →Mex Oy s þ s CO þ 2H2 Mex Oy s þ sH2 O→Mex Oy þ sH2
2. Experimental 2.1. Oxygen carriers preparation In this paper, LaMnO3 and La1-xMnFexO3/LaMn1-xFexO3 series oxy gen carriers were prepared by sol-gel method. The chemicals La (NO3)3⋅6H2O, Mn(NO3)2 (50 wt % in H2O), Fe(NO3)3⋅9H2O and C6H8O7⋅H2O of analytical grade were purchased from Aladdin. At room temperature, the appropriate amount of La(NO3)3⋅6H2O, Mn(NO3)2 (50 wt % in H2O), Fe(NO3)3⋅9H2O and C6H8O7⋅H2O (CA) dissolved in deionized water to form a solution, in which the molar ratio of total metal ions to citric acid was 2:1. In other words, the molar ratios of components in LaMnO3 and La1-xMnFexO3/LaMn1-xFexO3 series oxygen carriers were La/Mn/CA ¼ 1:1:1 and (La þ Fe)/Mn/CA ¼ 1:1:1 or La/ (Mn þ Fe)/CA ¼ 1:1:1, respectively. The solution then reacted in water bath at 70 � C for heating and stirring for 4 h until the gel was formed. After that, the gel was dried in an oven at 120 � C for 24 h. Eventually, the obtained precursor was calcined at 300 � C for 2h and then at 800 � C for 2 h. The particle sizes of the oxygen carriers were in the range of 20–40 mesh.
(r.2)
In the methane partial oxidation step (see formula. 1), methane re acts with metal oxygen carriers (MexOy) to generate syngas (CO þ H2). During the oxygen carrier regeneration step, H2O as the reactants not only supply oxygen species for the reduced metal oxygen carrier (MexOys) but also provide pure H2 [12,13]. Moreover, the acquisition of pure hydrogen from water splitting eliminates the separation and purification process, reducing the cost of hydrogen production [15]. At present, the suitable oxygen carriers is a key challenge for improving CLRM technology. A suitable oxygen carrier should own high activity, rich oxygen storage capacity, good thermal stability and negligible carbon deposition. Over the optimal oxygen carrier, methane can be converted to H2 and CO with the appropriate molar ratio of H2/ CO (2.0), which can be directly applied to the Fischer-Tropsch synthesis reaction. What’s more, during the second step, the reduced oxygen carriers could still exhibit high reactivity in the presence of H2O [16–18]. Additionally, the required oxygen carriers should also possess low cost and environmental friendliness [19]. The perovskite type oxides own rich oxygen vacancy, good oxygen mobility, high oxygen storage capacity and thermal stability, which are of great significance for the oxygen carriers in the CLRM process [20]. In general, the perovskite type oxides are present in the form of ABO3, where A can be a lanthanide or alkaline earth element, which is located at the center of the dodecahedral structure. B is a transition metal element, which is coordinated with oxygen in the octahedral structure [21,22]. Nitadori et al. [23] found that the activity of ABO3 principally depended on the B site, such as Mn and Co as B sites exhibiting high activity for the oxidation of hydrocarbons and CO [24]. LaMnO3-based perovskite oxides have received extensive attention on CH4 catalytic
2.2. Physical and chemical characterization of oxygen carriers The phase compositions of oxygen carriers were determined by X-ray diffraction (XRD, Rigaku Miniflex 600) using Cu-Kα radiation (λ ¼ 1.54056 Å). XRD patterns were acquired at a scan speed of 1 deg min 1 and a scan range of 20-80� , respectively. The X-ray diffractometer operated at a voltage of 40 kV and a tube current of 15 mA. Raman spectra were acquired on a Thermo Fisher DXRxi Raman spectrometer using an Arþ laser with an incident light wavelength of 532 nm. The laser output power for oxygen carrier testing was 7.0 mw, and the exposure time and scanning range in this process were 80 s and 100-1800 cm 1, respectively. X-ray photoelectron spectroscopy (XPS) was to analyze the surface composition and chemical state of the oxygen carriers. XPS character ization was operated on the PHI5500 X-ray photoelectron spectrometer, which used Al Kα as the radiation source. The internal standard cor rected binding energy of C1s peak on oxygen carrier was 284.8 eV. H2 temperature programmed reduction (H2-TPR) experiment was conducted on the Chembet Pulsar TPR/TPD instrument produced by 2
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Fig. 1. The X-ray diffraction patterns (A, B) and Raman spectra (C, D) of LaMnO3 (a), La0.95MnFe0.05O3 (b), La0.9MnFe0.1O3 (c), La0.85MnFe0.15O3 (d), La0.8MnFe0.2O3 (e), LaMn0.95Fe0.05O3 (f), LaMn0.9Fe0.1O3 (g), LaMn0.85Fe0.15O3 (h) and LaMn0.8Fe0.2O3 (i). Mn 2p (E) and O 1s (F) XPS spectra of LaMnO3 (a), La0.85MnFe0.15O3 (b) and LaMn0.9Fe0.1O3 (c) samples.
Quantanchrome Company. The concentration of exhaust gases was detected via a TCD detector. 50 mg oxygen carriers were purged for 30 min with a helium flow rate of 20 mL min 1 at 120 � C. Volumetric flow is cited under normal conditions for temperature and pressure (NTP), viz. 25 � C, 1 atm. Then it was raised from room temperature to 900 � C at a heating rate of 10 � C min 1 in flowing 10% H2/Ar mixture (20 mL min 1) (NTP). CH4 temperature programmed reduction (CH4-TPR) microactivity tests were carried out in a reaction tube and the exhaust gases were analysed using an online mass spectrometer (QGA, manufactured by Hiden Analytical Co., England). 50 mg oxygen carriers were weighed and put into a fixed bed reactor. After purged with Ar, 5% CH4/Ar mixed gas with a flow rate of 30 mL min 1 (NTP) was introduced, and then the reaction temperature was increased from room temperature to 900 � C at a heating rate of 10 � C min 1.
from Messer Company. The compositions of the gases were analysed using an on-line gas analyzer (GASBOARD-3100, Wuhan cubic opto electronic Co., China) and a gas chromatograph (Agilent 7890A GC System, manufactured by Agilent Co.). The CO selectivity, syngas productivity, H2/CO mole ratio and H2 productivity were calculated throughout the redox process as follows. In the methane reduction step:
2.3. Reactivity tests
Coke productivity ¼
CO selectivityð%Þ ¼
nCH4;i
Syngas productivity ¼
H2
nCO � 100 nCH4;o
(eq.1)
nCO þ nH2 mOC
(eq.2)
� nH CO mole ratio ¼ 2 nCO nCH4;i
(eq.3) nCO
nCO2 mOC
nCH4;o
(eq.4)
In the water splitting step:
Both CH4 reduction and water splitting reaction were carried out in a tubular fixed bed reactor. 1.8 g oxygen carriers with a particle size be tween 20 and 40 mesh were loaded in the middle of the reaction furnace. Firstly, the furnace temperature was increased from room temperature to 850 � C at a heating rate of 10 � C min 1 with introduction of high purity N2 (99.99%). CH4 reduction reaction began when the gas was switched to a 5% CH4/N2 mixture. After 30 min, N2 was introduced to purge the gas. And then steam was brought into the reaction tube to start the water splitting reaction. In the CH4 reduction reaction, the gas flow rate was 150 mL min 1 (NTP). The gases used above were purchased
H2 productivity ¼
nH2 mOC
(eq.5)
where moc is the mass of oxygen carriers loaded into the reactor, nH2 , nCO and nCO2 are the moles of H2, CO and CO2 produced, respectively. nCH4;i and nCH4;o are the moles of methane fed to the reactor and methane left the reactor separately.
3
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Table 1 The lattice parameter of LaMnO3 and La1-xMnFexO3/LaMn1-xFexO3 oxygen carriers. Cell volume (Å3)
Samples
Lattice structure
Crystal size (nm)
Lattice constant (Å) a
b
c
LaMnO3 La0.95MnFe0.05O3 La0.9MnFe0.1O3 La0.85MnFe0.15O3 La0.8MnFe0.2O3 LaMn0.95Fe0.05O3 LaMn0.9Fe0.1O3 LaMn0.85Fe0.15O3 LaMn0.8Fe0.2O3
Hexagonal Hexagonal Hexagonal Hexagonal Hexagonal Hexagonal Hexagonal Hexagonal Hexagonal
29.464 24.056 29.199 20.175 19.587 21.469 33.475 30.440 24.755
5.505 5.490 5.489 5.484 5.481 5.494 5.491 5.489 5.490
5.505 5.490 5.489 5.484 5.481 5.494 5.491 5.489 5.490
13.297 13.294 13.290 13.303 13.307 13.341 13.339 13.335 13.310
348.97 347.06 346.80 346.46 346.18 348.69 348.30 347.94 347.40
Table 2 Surface element compositions of LaMnO3, La0.85MnFe0.15O3 and LaMn0.9Fe0.1O3 samples. Samples LaMnO3 La0.85MnFe0.15O3 LaMn0.9Fe0.1O3
Surface element composition Mn4þ/Mn3þ
Osurf/Olatt
0.60 0.87 0.82
0.68 0.96 0.89
3. Results Fig. 1A and B shows the XRD patterns of the samples, all the diffraction peaks of the La1-xMnFexO3/LaMn1-xFexO3 perovskites oxides match well with the patterns of LaMnO3. Weak peaks for Mn3O4 at about 2θ ¼ 28.9� , 36� and 60.1� are detected in the La0.8MnFe0.2O3 sample because the Mn cations don’t completely enter the LaMnO3 structure. Detailed analysis of the crystalline size, lattice constant and cell volume of the oxygen carriers are presented in Table 1. It can be seen that all oxygen carriers generate single-phase hexagonal structure. There is no obvious difference on the crystal sizes of the samples. The crystal sizes of the La1-xMnFexO3 (19.587–29.199 nm) series samples are smaller than that of pure LaMnO3 (29.464 nm). La0.85MnFe0.15O3 and La0.8MnFe0.2O3 own smaller crystal size and weaker diffraction peak intensity among all samples. It is found that the LaMn0.9Fe0.1O3 sample presents the stron gest peak intensity and the highest crystallinity. As shown in Table 1, the cell volumes of La1-xMnFexO3 (346.18–347.06 Å3) are smaller than that of LaMnO3 (348.97 Å3). It can be attributed that La3þ (1.17 Å) with a larger ionic radius is partially substituted with smaller Fe3þ (0.62 Å), leading to the contraction of cell volumes. The cell volumes of the LaMn1-xFexO3 samples (347.40–348.69 Å3) are slightly smaller than that of pure LaMnO3 (348.97 Å3), which are sequentially decreased with Fe content. These phenomena can be attributed that there is no obvious difference in ionic radius between Mn3þ (0.645 Å) and Fe3þ (0.62 Å), which causes lattice shrinkage and cell volumes reduction [31,32]. Raman spectroscopy technique is commonly used to investigate the structure of the materials. This is due to the fact that Raman spectros copy can characterize the structure of amorphous oxides and crystalline oxides. As shown in Fig. 1C and D, Raman spectrum of the hexagonal structure LaMnO3 exhibits a peak at ~660 cm 1 [33]. The peak at ~660 cm 1 can be attributed to the extension of Mn–O in MnO6 units [34]. The peak intensity of La1-xMnFexO3/LaMn1-xFexO3 oxygen carrier at ~660 cm 1 is slightly higher than that of LaMnO3, indicating that the addition of Fe leads to an increase in Jahn-Teller (JT) distortion [35]. As a result, the lattice distortion of the La1-xMnFexO3/LaMn1-xFexO3 oxygen carrier leads to more oxygen vacancies. The Mn 2p and O 1s XPS spectra of LaMnO3, La0.85MnFe0.15O3 and LaMn0.9Fe0.1O3 samples are shown in Fig. 1E and F. Table 2 lists the corresponding quantitative analysis results. From Fig. 1E, it can be seen that the spectra of Mn 2p in each sample are fitted by four peaks. The signals at ~641.6 eV and ~653.2 eV are ascribed to Mn3þ, whereas the
Fig. 2. H2-TPR profiles of LaMnO3 (a), La0.95MnFe0.05O3 (b), La0.9MnFe0.1O3 (c), La0.85MnFe0.15O3 (d), La0.8MnFe0.2O3 (e), LaMn0.95Fe0.05O3 (f), LaMn0.9 Fe0.1O3 (g), LaMn0.85Fe0.15O3 (h) and LaMn0.8Fe0.2O3 (i).
peaks at ~643.1 eV and ~654.7 eV are assigned to Mn4þ [36]. As listed in Table 2, with the doping of Fe, the molar ratio of Mn4þ/Mn3þ in creases gradually, the molar ratio of Mn4þ/Mn3þ increases following the order as: LaMnO3 (0.6) < LaMn0.9Fe0.1O3 (0.82) < La0.85MnFe0.15O3 (0.87). La0.85MnFe0.15O3 (0.87) owns the largest molar ratio of Mn4þ/Mn3þ. It is reported that larger Mn4þ/Mn3þ molar ratio is related to lattice defects [37], because part of Mn3þ oxidized into Mn4þ ions on the surfaces of the oxygen carriers gave rise to the formation of struc tural defects [22]. Lattice distortion occurs when Fe ions are added to pure LaMnO3, forming oxygen vacancies. Compared with LaMn0.9 Fe0.1O3, La0.85MnFe0.15O3 oxygen carrier possesses a large degree of lattice distortion and more oxygen vacancies. This demonstrates that La at A-site position in LaMnO3 is more conducive to the formation of Mn4þ after being partially substituted by Fe (15%). Fig. 1F displays the XPS signals from the O1s element of samples. The higher binding energy peak at ~531.0 eV is ascribed to surface oxygen (Osurf) from the car bonate or hydroxyl species. The lower binding energy peak at ~529.4 eV is attributed to lattice oxygen (Olatt) from the bulk side of the perovskite oxide [38]. It is well known that surface adsorbed oxygen holds better mobility than lattice oxygen, as well as surface oxygen is generally considered to be more oxidative activity [39]. It can be seen that the molar ratio of Osurf/Olatt gradually increases with the loading of Fe. The Osurf/Olatt molar ratio of the oxygen carrier could reflect oxygen vacancy concentration to some extent [39]. The Osurf/Olatt molar ratio of La0.85MnFe0.15O3 (0.96) is larger than that of LaMn0.9Fe0.1O3 (0.89), confirming that La0.85MnFe0.15O3 possesses higher oxygen vacancy concentration. Three samples exhibit an increasing Osurf/Olatt molar ratio as following: LaMnO3 (0.68) < LaMn0.9Fe0.1O3 (0.89) < 4
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Table 3 Quantitative results of reduction species over the LaMnO3 and La1-xMnFexO3/LaMn1-xFexO3 samples. Samples
α LaMnO3 La0.95MnFe0.05O3 La0.9MnFe0.1O3 La0.85MnFe0.15O3 La0.8MnFe0.2O3 LaMn0.95Fe0.05O3 LaMn0.9Fe0.1O3 LaMn0.85Fe0.15O3 LaMn0.8Fe0.2O3
Total H2 consumption (mmol g-1)
H2 consumption (mmol g-1) 0.075 0.047 0.050 0.059 0.089 0.070 0.024 0.038 0.050
β
γ 1.070 1.081 0.605 0.559 0.625 0.951 – 0.933 0.720
1.145 1.128 1.165 1.213 1.561 1.021 0.942 0.971 1.167
– – 0.510 0.595 0.847 – 0.918 – 0.397
Peak temperature (oC)
α
β
γ
321 293 278 270 281 316 313 311 312
442 430 447 419 420 424 – 434 431
– – 464 460 485 – 476 – 491
Fig. 3. CH4-TPR profiles over LaMnO3 (A), La0.95MnFe0.05O3 (B), La0.9MnFe0.1O3 (C), La0.85MnFe0.15O3 (D), La0.8MnFe0.2O3 (E), LaMn0.95Fe0.05O3 (F), LaMn0.9 Fe0.1O3 (G), LaMn0.85Fe0.15O3 (H) and LaMn0.8Fe0.2O3 (I).
La0.85MnFe0.15O3 (0.96). The H2-TPR are in order to investigate the reducibility of the oxygen carriers, and the results are shown in Fig. 2. Table 3 summarizes the peak temperatures and the calculated H2 consumption amount of the reduc tion peaks. It is reported that the reduction process of Mn-based
perovskite can be divided into two steps [40]. The low temperature reduction peaks labeled as α (270–321 � C) are due to the removal of a small quantity of surface adsorbed oxygen species. In the high-temperature stage, the β peaks located at 419–476 � C and γ peaks located at 460–491 � C are attributed to the removal of the 5
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Fig. 4. (A–I) CH4 isothermal reaction at 850 � C over LaMnO3 (A), La0.95MnFe0.05O3 (B), La0.9MnFe0.1O3 (C), La0.85MnFe0.15O3 (D), La0.8MnFe0.2O3 (E), LaMn0.95 Fe0.05O3 (F), LaMn0.9Fe0.1O3 (G), LaMn0.85Fe0.15O3 (H) and LaMn0.8Fe0.2O3 (I) oxygen carriers. (J–L) The CO selectivity (J), H2/CO ratio (K) and syngas productivity (L) over the oxygen carriers during CH4 isothermal reaction at 850 � C.
non-stoichiometric excess oxygen contained in the lattice and the reduction of Mn4þ to Mn3þ, respectively [41]. As shown in Table 3, when Fe ions are added to LaMnO3, the lowtemperature reduction peaks move towards lower temperature. This indicates that the introduction of Fe could weaken the Mn–O bonds located near the iron, thereby promoting the mobility of lattice oxygen [42]. LaMn0.9Fe0.1O3 shows the smallest hydrogen consumption amount of α peak, indicating a lower concentration of surface oxygen on the LaMn0.9Fe0.1O3 samples. However, the low temperature reduction peaks of the LaMn1-xFexO3 samples shift a small amplitude to lower tempera ture. La0.85MnFe0.15O3 exhibits the lowest peak temperatures in the
reduction peak temperatures of all samples. A broad reduction peak (between 350 and 600 � C) of La0.9MnFe0.1O3, La0.85MnFe0.15O3, La0.8MnFe0.2O3 and LaMn0.8Fe0.2O3 differentiates into a medium tem perature reduction peak (β) and a high temperature reduction peak (γ) during the reduction process. As the increase of Fe contents, the total hydrogen consumption of the oxygen carriers increases. The reducibility of the La1-xMnFexO3 samples is better than that of LaMn1-xFexO3. The total hydrogen consumption of La0.9MnFe0.1O3 (1.165 mmol g 1), La0.85MnFe0.15O3 (1.213 mmol g 1), La0.8MnFe0.2O3 (1.561 mmol g 1) and LaMn0.8Fe0.2O3 (1.167 mmol g 1) are higher than that of LaMnO3 (1.145 mmol g 1), indicating better reducibility. The total hydrogen 6
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Fig. 5. H2 contents (A1, B1) and productivity (A2, B2) of LaMnO3 (a), La0.95MnFe0.05O3 (b), La0.9MnFe0.1O3 (c), La0.85MnFe0.15O3 (d), La0.8MnFe0.2O3 (e), LaMn0.95Fe0.05O3 (f), LaMn0.9Fe0.1O3 (g), LaMn0.85Fe0.15O3 (h) and LaMn0.8Fe0.2O3 (i) samples during the water splitting reaction at 850 � C.
consumption decreases in the order of La0.8MnFe0.2O3 > La0.85Mn Fe0.15O3 > LaMn0.8Fe0.2O3 > La0.9MnFe0.1O3 > LaMnO3. Fig. 3 shows the CH4-TPR profiles over LaMnO3, La1-xMnFexO3 and LaMn1-xFexO3 samples. The reduction process of the samples can be divided into two phases. In the first stage, CH4 is converted to CO2 and H2O by surface oxygen when the temperature ranged from 500 to 700 � C. In the second stage, CH4 is mainly converted to CO and H2 by lattice oxygen at temperatures above 700 � C [43]. In the low temperature stage (500–700 � C), for pure LaMnO3, both CO2 and H2O are at a low level. After doping of Fe to LaMnO3, both CO2 and H2O increase, and the higher content of Fe, the more amount of CO2 and H2O, indicating that the amount of active oxygen on the surface of the samples increases after the introduction of iron. Compared with LaMn1-xFexO3, the series of La1-xMnFexO3 samples present higher activity for methane oxidation. In the high temperature stage (>700 � C), the bulk oxygen diffuses to the surface of the oxygen carriers and reacts with methane, both CO and H2 produce, while CO2 and H2O decrease. This suggests that the mobility of lattice oxygen plays a crucial role in the partial oxidation of methane. La–Mn–Fe–O mixed oxides produce higher CO and H2, exhibiting higher CO selectivity than pure LaMnO3. In conclusion, La–Mn–Fe–O mixed oxides behave higher reducibility than pure LaMnO3 due to the syner gistic effect of Mn and Fe [44]. As shown in Fig. 3, the intensities of CO and H2 are mainly produced in the range of 800–900 � C. Further, when the isothermal reaction temperature is lower than 850 � C, the reactivity of oxygen carrier is poor, and carbon deposition occurs when the isothermal reaction temperature is higher than 850 � C (see Fig. S3). Thus, 850 � C is chosen as the optimal reaction temperature for the isothermal reactions to guarantee a higher reactivity and a resistance to carbon deposition. As shown in Fig. 4(A-I), H2, CO and CO2 are the main gases produced during CH4 isothermal reaction at 850 � C and the process is endothermic (Fig. S1A and Table S1). At the initial stage, higher contents of CO2 produce. This is due to the complete oxidation between CH4 and surface oxygen of the oxygen carrier. The amount of CO2 over La1-xMnFexO3 samples is higher than that over LaMn1-xFexO3, indicating that there is
more surface adsorbed oxygen over the La1-xMnFexO3 oxygen carriers. After that, the contents of CO2 decrease gradually, while the H2 and CO begin to increase. This stage can be attributed to the partial oxidation of methane with bulk oxygen after the surface oxygen is gradually consumed. It can be seen from Fig. 4J, the CO selectivity of LaMnO3 and LaMn0.95Fe0.05O3 stabilizes at ~87% after 600 s. However, the CO selectivity of La0.9MnFe0.1O3, La0.85MnFe0.15O3, La0.8MnFe0.2O3, LaMn0.9Fe0.1O3 and LaMn0.8Fe0.2O3 reaches to ~99%. La0.85MnFe0.15O3 shows the higher CO selectivity than the other samples. It is known that the selectivity of CO is determined by the relative content of surface oxygen and lattice oxygen [45]. It has been speculated from the amount of CO2 that there are more surface oxygen on the La0.85MnFe0.15O3, therefore the La0.85MnFe0.15O3 sample should also own more lattice oxygen. Eventually, with the gradual consumption of lattice oxygen, the contents of CO and H2 decrease slowly. The contents of CO and H2 over La1-xMnFexO3/LaMn1-xFexO3 samples are much higher than the pure LaMnO3 sample, indicating that the addition of Fe improves the pro duction of syngas. As seen in Fig. 4K, the H2/CO ratios of LaMnO3, LaMn0.95Fe0.05O3 and LaMn0.85Fe0.15O3 gradually decrease to a lower value (~1.75) as the reaction proceeded. However, the H2/CO ratios of the other oxygen carriers, especially La0.85MnFe0.15O3, keeps stable at the ideal ratio of 2.0 (1.93–2.07) during the whole methane partial oxidation process. As shown in Fig. 4L, the introduction of Fe to oxygen carriers enhances the syngas productivity of pure LaMnO3 (2.61 mmol g 1) and the La0.85Mn0.15O3 sample exhibits the highest syngas pro ductivity (3.78 mmol g 1) among all oxygen carriers. Fig. 5 shows the contents and productivity of H2 during the water splitting reaction at 850 � C. The process is also endothermic and the heat analysis of reaction is shown in the supplemental material (Fig. S1B and Table S1). It can be seen in Fig. 5A1 and B1, both H2 and a small amount of CO can be detected over the pure LaMnO3 samples. While only H2 can be detected and no carbon-containing gas observed over La1-xMnFexO3/ LaMn1-xFexO3 samples during the water splitting reaction, indicating that there is no carbon deposition over the La1-xMnFexO3/LaMn1-xFexO3 samples during the CH4 oxidation [46]. In the partial oxidation of 7
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Fig. 6. The evolved amounts of products gas during 20 successive CH4 reduction (A1, B1 and C1) and water splitting (A2, B2 and C2) cycles at 850 � C over LaMnO3, La0.85MnFe0.15O3 and LaMn0.9Fe0.1O3. H2 productivity (A3, B3 and C3) over the oxygen carriers during the 20 cycles of water splitting at 850 � C.
methane, the amount of carbon deposition is calculated from eq. (4). The coke is formed by methane cracking, and the amount of coke could be calculated directly from the amount of carbons gas (eg. CO or CO2) during water splitting step [47]. As shown in Fig. S2, only the CH4 consumption over LaMnO3 is greater than the total production of CO and CO2, indicating that a small amount of carbon deposition is formed. This is consistent with the experimental phenomenon in Fig. 5A1 and B1. At the initial stage, the content of H2 increases rapidly with the introduc tion of steam. The hydrogen content of the samples reaches to a maximum immediately. And then the hydrogen contents maintain at a high value for 600–700 s. After that, the hydrogen contents gradually decrease to zero owing to the restoration of lattice oxygen and surface oxygen [48]. The whole process lasts for ~1700 s. Fig. 5A2 and B2 show that the H2 productivity of pure LaMnO3 is 1.55 mmol g 1, while the H2 productivities of La0.85MnFe0.15O3 and LaMn0.9Fe0.1O3 are 1.76 and 1.64 mmol g 1, respectively, which are higher than that of the pure LaMnO3. This phenomenon indicates that the introduction of a proper amount of Fe could improve the H2 productivity of pure LaMnO3, and when the La (A site) is substituted by Fe for 15%, the La0.85MnFe0.15O3 sample achieves the highest H2 productivity. The gas evolution of LaMnO3, LaMn0.85Fe0.15O3 and LaMn0.9Fe0.1O3 oxygen carriers during 20 successive redox cycles at 850 � C are shown in Fig. 6. In the first cycle, CO2 content reaches to a high value rapidly in
the initial stage of methane reduction, but decreases afterwards. With the consumption of surface oxygen, lattice oxygen starts to react with the CH4, leading to the production of syngas (CO and H2). For all the samples, the amount of the product gas keeps at a high value during the first redox, after that the gas content decreases slightly with the cycle number. Compared with pure LaMnO3, the CO2, H2 and CO contents of the La0.85MnFe0.15O3 and LaMn0.9Fe0.1O3 samples are relatively higher in the reduction step, indicating that the content of reactive oxygen species increases with the doping of Fe. After that, steam is introduced, leading to the production of H2 [49]. It can be seen from Fig. 6A2, B2 and C2 that the H2 contents of LaMn0.85Fe0.15O3 and LaMn0.9Fe0.1O3 are higher than that of LaMnO3 in the water splitting step. Additionally, the quantitative analysis results of three samples in the oxidation step are shown in Fig. 6A3, B3 and C3. It is noted that the contents of H2 decrease with the number of cycles in the redox process, and the average hydrogen productivity increases as follows: LaMnO3 < LaMn0.9Fe0.1O3 < La0.85MnFe0.15O3. It can be found that the evident decline in the gas content during the first cycle, and then the gases contents maintain at a relatively stable level, suggesting that the content of reactive oxygen species in fresh samples is higher than that of samples after the cyclic reaction. The H2 productivity of La0.85Mn0.15O3 (0.55 mmol g 1) is still higher than that of LaMnO3 (0.48 mmol g 1) and LaMn0.9Fe0.1O3 (0.49 mmol g 1) after 20 cycles, demonstrating that La0.85MnFe0.15O3 exhibits 8
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Table 4 Details of lattice parameter and cell volume of LaMnO3, La0.85MnFe0.15O3 and LaMn0.9Fe0.1O3 composites after 20 redox cycles at 850 � C. Cell volume (Å3)
Samples
Lattice structure
Crystal size (nm)
Lattice constant (Å) a
b
c
LaMnO3 La0.85MnFe0.15O3 LaMn0.9Fe0.1O3
Orthorhombic Orthorhombic Orthorhombic
37.954 29.056 34.093
5.535 5.541 5.530
5.744 5.682 5.692
7.694 7.686 7.724
244.62 241.96 243.13
Fig. 7. XRD pattern (A), Raman spectrum (B) and XPS spectra of Mn 2p (C) and O 1s (D) for LaMnO3 (a), La0.85MnFe0.15O3 (b) and LaMn0.9Fe0.1O3 (c) after 20 redox cycles at 850 � C.
an outstanding structural and thermal stability. It is interesting that there is no carbon deposition detected among the three samples during the 20 successive redox cycles. Fig. 7A presents the XRD patterns of LaMnO3, La0.85MnFe0.15O3 and LaMn0.9Fe0.1O3 oxygen carriers after 20 redox cycles at 850 � C. It can be observed that La0.85MnFe0.15O3 shows a single-phase perovskite struc ture, while a new phase of La2O3 is detected in the cycled LaMnO3 and LaMn0.9Fe0.1O3 oxygen carriers. This phenomenon suggests that the La0.85MnFe0.15O3 exhibits higher thermal stability and structural sta bility than the other two samples after redox cycles. In addition, the diffraction peak intensity of the La0.85MnFe0.15O3 sample is weak, indicating lower crystallinity [50]. The lattice parameter and cell vol ume of LaMnO3, La0.85MnFe0.15O3 and LaMn0.9Fe0.1O3 composites after 20 redox cycles at 850 � C are listed in Table 4. It can be seen that the crystal structure of the cycled samples belongs to orthorhombic system, where the cell volume decreases. La0.85MnFe0.15O3 (241.96 Å3) pos sesses a smaller unit cell volume than LaMnO3 (244.62 Å3) and LaMn0.9Fe0.1O3 (243.13 Å3). As shown in Table 4, after cycled, the crystal size of all the samples increased. Compared with LaMnO3 (37.954 nm) and LaMn0.9Fe0.1O3 (34.093 nm), LaMn0.85Fe0.15O3 (29.056 nm) shows a smaller grain size, indicating a better thermal stability. The Raman spectra of the LaMnO3, La0.85MnFe0.15O3 and LaMn0.9 Fe0.1O3 oxygen carriers after 20 redox cycles at 850 � C are displayed in
Table 5 Surface atomic ratio of the LaMnO3, La0.85MnFe0.15O3 and LaMn0.9Fe0.1O3 ox ygen carriers after 20 redox cycles at 850 � C measured by XPS. Samples
Surface element composition Mn4þ/Mn3þ
LaMnO3 La0.85MnFe0.15O3 LaMn0.9Fe0.1O3
0.39 0.79 0.72
Osurf/Olatt 0.43 0.73 0.71
Fig. 7B. Three major Raman modes are centered on ~275 cm 1, ~478 cm 1 and ~604 cm 1. Compared with the fresh samples, the cycled samples show extra Raman peaks near 275 cm 1 and 478 cm 1 which can be related to the local JT distortions [51]. Moreover, it is observed that the position and width of two most prominent Raman peaks (~478 cm 1 and ~604 cm 1) match well with the orthorhombic structure [52]. The peak intensity of La0.85MnFe0.15O3 and LaMn0.9Fe0.1O3 at 604 cm 1 is slightly weaker than LaMnO3, which is mainly related to a disordered perovskite structure [51]. Compared with LaMn0.9Fe0.1O3, the Raman peak intensity of La0.85MnFe0.15O3 at 604 cm 1 is weaker. Fig. 7C and D shows the XPS spectra of Mn 2p and O 1s for the oxygen carriers after 20 redox cycles at 850 � C. The molar ratios of Mn4þ/Mn3þ and Osurf/Olatt calculated by XPS spectra are listed in Table 5. Compared 9
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Fig. 8. (A) H2-TPR profiles of LaMnO3 (a), La0.85MnFe0.15O3 (b) and LaMn0.9Fe0.1O3 (c) after 20 redox cycles at 850 � C. (B) CH4-TPR profiles over LaMnO3 (B1), La0.85MnFe0.15O3 (B2) and LaMn0.9Fe0.1O3 (B3) after 20 redox cycles at 850 � C.
Fig. 8B shows the CH4-TPR profiles of LaMnO3, La0.85MnFe0.15O3 and LaMn0.9Fe0.1O3 oxygen carriers after 20 redox cycles. Compared with fresh samples, the contents of H2 and CO produced in CH4-TPR process of LaMnO3, La0.85MnFe0.15O3 and LaMn0.9Fe0.1O3 samples are reduced after 20 redox cycles. La0.85MnFe0.15O3 and LaMn0.9Fe0.1O3 still own higher H2 and CO contents and lower CO2 and H2O contents than that of LaMnO3, exhibiting higher CO selectivity.
Table 6 Quantitative results of reduction species over the LaMnO3, La0.85MnFe0.15O3 and LaMn0.9Fe0.1O3 samples after 20 redox cycles at 850 � C. Samples
LaMnO3 La0.85MnFe0.15O3 LaMn0.9Fe0.1O3
H2 consumption (mmol g 1)
α
β
0.006 0.001 0.010
0.032 0.087 0.029
Total H2 consumption (mmol g-1)
0.038 0.088 0.039
Peak temperature (oC)
α
β
333.5 337.4 346.1
458.1 483.1 449.9
4. Discussion The main products of H2 and CO during the CH4 partial oxidation process can be directly used for methanol production and FischerTropsch synthesis when the H2/CO molar ratio is 2.0. However, car bon deposition often formed during the CH4 partial oxidation reaction, leading to the H2/CO molar ratio is much higher than 2.0. The formation of carbon deposition would reduce the catalytic activities. Ma et al. [54] reported that during the CH4 partial oxidation process over NiO/Al2O3 spherical catalyst, the carbon deposition resulted in the decrease of CH4 conversion within the initial 4 h of the reaction. Alvarez-Ganvan et al. [55] proved that the deactivation of Ni/ZrO2 catalyst was mainly related to the formation of carbon deposition during the partial oxidation of methane to syngas and/or hydrogen, which resulted in a lower CH4 conversion and CO selectivity. Fakeeha et al. [56] demonstrated that a decrease in methane conversion of Ni-800/(ZrO2 þ Al2O3) can be attributed that carbon deposition blocked the active sites. In addition, the presence of carbon deposition would also reduce the purity of produced hydrogen in the water splitting step. Zhao et al. [57] presented that the La2FeCoO6 produced not only a large amount of H2 but also a large amount of CO and CO2 during water splitting step, which was derived from gasification of coke and proved that methane was cracked during methane oxidation. Hafizi and Rahimpour et al. [58] found that the conversion of carbon deposition during the oxidation process reduced the purity of hydrogen produced in the process (about 98.5%). Our research confirmed that only pure hydrogen was produced
with that of the fresh oxygen carriers, the Mn4þ/Mn3þ and Osurf/Olatt molar ratios of the cycled oxygen carriers are reduced, which can be attributed to the decrease of oxygen vacancy concentration [53]. It can be seen that the molar ratio of Mn4þ/Mn3þ for La0.85MnFe0.15O3 is 0.79, which is higher than that of LaMnO3 (0.39) and LaMn0.9Fe0.1O3 (0.72). Similar to the molar ratio of Mn4þ/Mn3þ, La0.85MnFe0.15O3 (0.73) ex hibits a higher molar ratio of Osurf/Olatt than LaMnO3 (0.43) and LaMn0.9Fe0.1O3 (0.71), indicating higher oxygen vacancy concentration. The molar ratios of Mn4þ/Mn3þ and Osurf/Olatt are increased as follows: LaMnO3 < LaMn0.9Fe0.1O3 < La0.85MnFe0.15O3. Fig. 8A shows the reduction peaks of LaMnO3, La0.85MnFe0.15O3 and LaMn0.9Fe0.1O3 oxygen carriers after 20 times of isothermal reaction at 850 � C. Table 6 lists the peak temperatures and the calculated H2 con sumption amount of the reduction peaks. La0.85MnFe0.15O3 exhibits a broad β peak instead of two well-defined β and γ peaks after 20 isothermal cycles. Compared with fresh samples, the total hydrogen consumption of LaMnO3 (0.038 mmol g 1), La0.85MnFe0.15O3 (0.088 mmol g 1) and LaMn0.9Fe0.1O3 (0.039 mmol g 1) samples decreased after cyclic reaction. La0.85MnFe0.15O3 still possesses higher hydrogen consumption among three oxygen carriers. The α peaks of three samples all move to higher temperatures. The β peak of La0.85MnFe0.15O3 moves to higher temperature to 483.1 � C in three samples. 10
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in the water splitting stage and no carbon-containing gas was detected. Only a small amount of CO was detected during the regeneration of the LaMnO3 oxygen carrier (as shown in Fig. 5). However, no carbon-containing gas was produced over La1-xMnFexO3/LaMn1-xFexO3 oxygen carriers during the reaction with water vapor and even no car bon deposition formed after 20 successive cycles (see Fig. 6A2, B2 and C2). It can be concluded that the introduction of a small amount of Fe to the LaMnO3 oxygen carrier can prevent the carbon deposition [59], due to the interaction of Fe and Mn and the abundant oxygen vacancies.
[10] M. Nomura, J. Membr. Sci. 240 (2004) 221–226. [11] K. Mazloomi, C. Gomes, Renew. Sustain. Energy Rev. 16 (2012) 3024–3033. [12] L. Jiang, X. Yuan, G. Zeng, Z. Wu, L. Jie, X. Chen, L. Leng, W. Hui, W. Hou, Appl. Catal. B Environ. 221 (2018), 725-725. [13] Z. Fan, P. Zhao, N. Meng, J. Maddy, Int. J. Hydrogen Energy 41 (2016) 14535–14552. [14] K. Zhao, A. Zheng, H. Li, H. Fang, H. Zhen, G. Wei, S. Yang, Z. Zhao, Appl. Catal. B Environ. 219 (2017) 672–682. [15] A. Hafizi, M.R. Rahimpour, S. Hassanajili, Appl. Energy 165 (2016) 685–694. [16] C. Dueso, M. Ortiz, A. Abad, F. García-Labiano, L.F.D. Diego, P. Gay� an, J. Ad� anez, Chem. Eng. J. 188 (2012) 142–154. [17] M. Ortiz, P. Gay� an, L.F.D. Diego, F. García-Labiano, A. Abad, M.A. Pans, J. Ad� anez, J. Power Sources 196 (2011) 4370–4381. [18] M. Ryd�en, P. Ramos, Fuel Process. Technol. 96 (2012) 27–36. [19] H. Zhao, G. Lei, X. Zou, Appl. Energy 157 (2015) 408–415. [20] L. Lin, Y. Song, J. Bo, K. Wang, Z. Qian, Energy 131 (2017) 58–66. [21] M.J. Tyler, W.G. Hardin, D. Sheng, K.P. Johnston, K.J. Stevenson, Nat. Mater. 13 (2014) 726–732. [22] J. Chen, M. Shen, X. Wang, Q.I. Gongshin, J. Wang, L.I. Wei, Appl. Catal. B Environ. 134 (2013) 251–257. [23] T. Nitadori, T. Ichiki, M. Misono, T. Nitadori, T. Ichiki, M. Misono, Bull. Chem. Soc. Jpn. 61 (1988) 621–626. [24] S. Ponce, M.A. Pe~ na, J.L.G. Fierro, Appl. Catal. B Environ. 24 (2000) 193–205. [25] A. Giroirfendler, A. Baylet, L. Retailleau, A. Boreave, G.Z. Lu, Y. Guo, Y.L. Guo, C. H. Zhang, Appl. Catal. B Environ. 148–149 (2014) 490–498. [26] M. Al-Mamun, Sustain. Energy Fuels 1 (2017) 1013–1017. [27] G. Landi, P.S. Barbato, A.D. Benedetto, G. Russo, R. Pirone, Appl. Catal. B Environ. 134–135 (2013) 110–122. [28] X. Zhu, K. Li, L.M. Neal, F. Li, ACS Catal. 8 (2018) 8213–8236. [29] Q. Imtiaz, D. Hosseini, C.R. Müller, Energy Technol. 1 (2013) 633–647. [30] H. Fang, Y. Wei, H. Li, W. Hua, Energy Fuels 23 (2009) 2095–2102. [31] J.C. Jr, R. Vrba, K. Castkova, J. Cihlar, Int. J. Hydrogen Energy 42 (2017) 19920–19934. [32] S. Jauhar, M. Dhiman, S. Bansal, S. Singhal, J. Sol. Gel Sci. Technol. 75 (2015) 124–133. [33] L. Martı ́n-Carr� on, A. De Andr�es, M.J. Martı ́nez-Lope, J. Alloy. Comp. 323–324 (2001) 494–497. [34] L. Wang, H. Xie, X. Wang, G. Zhang, Y. Guo, Y. Guo, G. Lu, Chin. J. Catal. 38 (2017) 1406–1412. [35] C. Xie, S. Lei, J. Zhao, L. Yang, S. Zhou, Y. Dan, Appl. Surf. Sci. 351 (2015) 188–192. [36] C. Zhang, W. Chao, W. Zhan, Y. Guo, G. Yun, G. Lu, A. Baylet, A. Giroir-Fendler, Appl. Catal. B Environ. 129 (2013) 509–516. [37] Q. Yu, C. Wang, X. Li, Z. Li, L. Wang, Q. Zhang, G. Wu, Z. Li, Fuel 239 (2019) 1240–1245. [38] H.S. Lim, M. Lee, D. Kang, J.W. Lee, Int. J. Hydrogen Energy 43 (2018) 20580–20590. [39] Y. Wang, S. Aghamohammadi, D. Li, K. Li, R. Farrauto, Appl. Catal. B Environ. 244 (2019) 438–447. [40] Z. Sarshar, F. Kleitz, S. Kaliaguine, Energy Environ. Sci. 4 (2011) 4258–4269. [41] L. Wang, C. Wang, H. Xie, W. Zhan, Y. Guo, Y. Guo, Catal. Today 327 (2019) 190–195. [42] L.C. Wang, Q. Liu, X.S. Huang, Y.M. Liu, Y. Cao, K.N. Fan, Appl. Catal. B Environ. 88 (2009) 204–212. [43] Y. Zheng, K. Li, W. Hua, T. Dong, Y. Wang, Z. Xing, Y. Wei, Z. Min, Y. Luo, Appl. Catal. B Environ. 202 (2017) 51–63. [44] X. Zhu, Y. Wei, W. Hua, K. Li, Int. J. Hydrogen Energy 38 (2013) 4492–4501. [45] Y. Zheng, K. Li, W. Hua, Z. Xing, Y. Wei, Z. Min, Y. Wang, Energy Fuels 30 (2016) 638–647. [46] R.D. Solunke, G.T. Veser, Ind. Eng. Chem. Res. 49 (2010) 11037–11044. [47] J. Zhang, V. Haribal, F. Li, Sci. Adv. 3 (2017), e1701184. [48] K. Zhao, H. Fang, H. Zhen, G. Wei, A. Zheng, H. Li, Z. Zhao, Appl. Energy 168 (2016) 193–203. [49] C.L. Muhich, B.W. Evanko, K.C. Weston, L. Paul, L. Xinhua, M. Janna, C. B. Musgrave, A.W. Weimer, Science 341 (2013) 540–542. [50] H. Xuehui, N. Pengju, P. Hongyun, S. Xiaohui, J. Solid State Chem. 265 (2018) 218–226. [51] F. Teng, W. Han, S. Liang, B. Gaugeu, R. Zong, Y. Zhu, J. Catal. 250 (2007) 1–11. [52] A. Dubey, V.G. Sathe, J. Phys. Condens. Matter 19 (2007) 346232–346266. [53] Z.X. Wei, L. Wei, L. Gong, Y. Wang, C.W. Hu, J. Hazard Mater. 177 (2010) 554–559. [54] Y. Ma, Y. Ma, Z. Zhao, X. Hu, Z. Ye, J. Yao, C.E. Buckley, D. Dong, Renew. Energy 138 (2019) 1010–1017. [55] C. Alvarez-Galvan, M. Melian, L. Ruiz-Matas, J.L. Eslava, R.M. Navarro, M. Ahmadi, B. Roldan Cuenya, J.L.G. Fierro, Front. in Chem. 7 (2019) 104–120. [56] A.H. Fakeeha, Y. Arafat, A.A. Ibrahim, H. Shaikh, H. Atia, A.E. Abasaeed, U. Armbruster, A.S. Al-Fatesh, Processes 7 (2019) 141–157. [57] K. Zhao, L. Li, A. Zheng, H. Zhen, H. Fang, S. Yang, G. Wei, H. Li, Z. Zhao, Appl. Energy 197 (2017) 393–404. [58] A. Hafizi, M. Rahimpour, Catalysts 8 (2018) 27–42. [59] D. Hosseini, F. Donat, S.M. Kim, L. Bernard, A.M. Kierzkowska, C.R. Müller, ACS Appl. Energy Mater. 1 (2018) 1294–1303.
5. Conclusions LaMnO3 and a series of La1-xMnFexO3/LaMn1-xFexO3 (x ¼ 0.05, 0.1, 0.15, 0.2) perovskite-type oxygen carriers were prepared by sol-gel method and applied in CLRM processes. La1-xMnFexO3/LaMn1-xFexO3 oxygen carriers possessed stronger redox performance for syngas and H2 production during CLRM process than pure LaMnO3. It was found that no carbon deposition occurred over La0.85MnFe0.15O3 in the redox pro cess. The Raman and XPS characterization results further confirmed that the substitution of Fe ions into the LaMnO3 lattice resulted in lattice distortion and more oxygen vacancies, and the oxygen vacancy con centration was higher in La0.85MnFe0.15O3 oxygen carrier. During the CLRM process, the doping of Fe increased the productivity of syngas, and no carbon deposition was generated in the whole process. The CO selectivity (about 99%), syngas productivity (3.78 mmol g 1) and H2 productivity (1.76 mmol g 1) over La0.85MnFe0.15O3 were the highest, and its H2/CO molar ratio was stably maintained at the ideal ratio of 2.0 (1.93–2.07). No carbon deposition was detected in LaMnO3, La0.85Mn Fe0.15O3 and LaMn0.9Fe0.1O3 during 20 successive redox processes, while La0.85MnFe0.15O3 exhibited better reactivity and stability. In addition, La0.85MnFe0.15O3 exhibits the optimal performance and su perior resistance to carbon deposition, producing high purity of syngas and hydrogen in CLRM. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work has been financially supported by the National Natural Science Foundation of China (Project Nos. 21706108). The Yunnan Applied Basic Research Projects (No. 2018FD032), and the Analysis and Testing Center of Kunming University of Science and Technology. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227505. References [1] [2] [3] [4] [5] [6] [7] [8] [9]
H. Balat, E. Kırtay, Int. J. Hydrogen Energy 35 (2010) 7416–7426. J.R. Bartels, M.B. Pate, N.K. Olson, Int. J. Hydrogen Energy 35 (2010) 8371–8384. S. Xie, S. Yuan, P. Liao, M. Jia, Y. Wang, Water Res. 86 (2015) 74–81. S. Dutta, J. Ind. Eng. Chem. 20 (2014) 1148–1156. M. Ni, M.K.H. Leung, D.Y.C. Leung, K. Sumathy, Renew. Sustain. Energy Rev. 11 (2007) 401–425. A. Steinfeld, Sol. Energy 78 (2005) 603–615. S. Oros-Ruiz, R. Zanella, R. Lopez, A. Hernandez-Gordillo, R. Gomez, J. Hazard Mater. 263 (2013) 2–10. G. Cipriani, V. Di Dio, F. Genduso, D. La Cascia, R. Liga, R. Miceli, G.R. Galluzzo, Int. J. Hydrogen Energy 39 (2014) 8482–8494. Y. Zhang, I. Angelidaki, Water Res. 81 (2015) 188–195.
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