- Email: [email protected]
The role of CuO modified La0·7Sr0·3FeO3 perovskite on intermediate-temperature partial oxidation of methane via chemical looping scheme
international journal of hydrogen energy xxx (xxxx) xxx
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
The role of CuO modified La0·7Sr0·3FeO3 perovskite on intermediate-temperature partial oxidation of methane via chemical looping scheme Rongjiang Zhang a,b,c,d, Yan Cao a,b,c, Haibin Li a,b,c, Zengli Zhao a,b,c, Kun Zhao a,b,c,*, Liqun Jiang a,b,c,** a
Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, China CAS Key Laboratory of Renewable Energy, China c Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, China d University of Chinese Academy of Sciences, Beijing, 100049, China b
highlights The CLPO temperature was reduced to 750 C with CuO modified La0$7Sr0$3FeO3. Interaction between CuO and La0$7Sr0$3FeO3 was the key point in oxygen carriers. CuO modification promoted methane dissociation and activation of lattice oxygen. 10CuO/La0$7Sr0$3FeO3 exhibited good reactivity and regenerability on CLPO process.
article info
abstract
Article history:
Synthesis gas production via chemical-looping partial oxidation (CLPO) of methane reduces
Received 11 October 2019
operating cost and likely avoids carbon dioxide emissions. Previous studies exhibit La0$7-
Received in revised form
Sr0$3FeO3 (LSF) a good high-temperature oxygen carrier, but not being applicable in
4 December 2019
intermediate-temperature CLPO processes. This work proposes a CuO modified LSF (xCuO/
Accepted 13 December 2019
LSF, x ¼ 2,5,10,15) for enhancing the reactivity of oxygen carriers at relatively low tem-
Available online xxx
peratures. Characterization methods, including N2 physisorption, ICP-OES, H2-TPR, XRD
Keywords:
determine the best candidate under 750 C is 10CuO/LSF. The methane conversion rate of
Chemical-looping partial oxidation
10CuO/LSF is more than 3 times compared to that of unmodified LSF and the concentration
(CLPO)
of syngas increases by 375% with the H2/CO molar ratio of about 2.5. The regenerability of
Methane
10CuO/LSF proves to be good. This study provides a promising and simple way to lower the
Syngas
operating temperature of the CLPO process, likely leading a considerable energy saving.
and XPS, are implemented to determine properties of xCuO/LSF. Fixed-bed experiments
Perovskite
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Copper oxide Oxygen carrier
* Corresponding author. Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, China. ** Corresponding author. CAS Key Laboratory of Renewable Energy, China. E-mail addresses: [email protected] (K. Zhao), [email protected] (L. Jiang). https://doi.org/10.1016/j.ijhydene.2019.12.082 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Zhang R et al., The role of CuO modified La0$7Sr0$3FeO3 perovskite on intermediate-temperature partial oxidation of methane via chemical looping scheme, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.12.082
2
international journal of hydrogen energy xxx (xxxx) xxx
Introduction Methane, usually found in natural gas, shale gas [1], clathrates [2] and even landfills and biomass gas [3], is one of the most abundant hydrocarbons on earth. Methane can be a fuel to generate electricity, a chemical feedstock for upgrading to value-added chemicals [4], or utilized for domestic heating and cooking. Its cleanness and convenience make it be attracted in great interests. One of the important chemical routes of methane is the conversion of methane to syngas which can further produce a wide range of chemicals [5]. The methane-to-syngas involves the most mature steam methane reforming processes (SRM), and further dry reforming (DRM) and recent partial oxidation (POM) [6e8]. POM has aroused extensive attentions, because of its relatively low energy consumption and the ideal H2/CO ratio which is more suitable and feasible for subsequent industrial processes. However, POM demands the supply of pure oxygen, making it complicated and costly [9]. To overcome this contradiction, the partial oxidation of methane (CLPO) in a chemical-looping scheme, one of the most promising techniques, has been proposed [10,11]. Certain metal oxides will be applied in CLPO as oxygen carriers to transfer oxygen between two reactors, oxidizer to gain oxygen from air and reducer to release oxygen to methane toward syngas, as shown in Fig. 1. It avoids the complicated and costly step of the air separation, and most significantly carbon dioxide can be ready for the sequestration in an economic way [12]. Simultaneously, the operational safety could be greatly improved owing to the isolation of methane and pure oxygen. However, CLPO is a complex and challengeable redox reaction process involving adsorption and dissociation of methane, formation and detachment of
Fig. 1 e Schematic diagram of chemical-looping partial oxidation of methane.
oxygenated intermediates as well as evolution and migration of lattice oxygen in oxygen carriers. Therefore, the performance of oxygen carriers is one of important factors affecting the CLPO process [10,13,14]. Transition metal oxides had been easily studied in CLPO process, such as Ni, Cu, Fe, Co, and Mn oxides [15e17]. Different inert supports, such as ZrO2, TiO2, SiO2, Al2O3, NiAl2O4 and MgAl2O4 [16,18e20], were also used to enhance the mechanical strength, reactivity, anti-coking ability, and regenerability. Ni-based oxygen carriers exhibit better reactivity than Cu-, Fe-, and Mn-, but their anti-coking ability should be further improved [21]. Ce-based oxygen carriers have widely been studied and proven to be feasible in partial oxidation of methane [22]. Some transition metals were found to be promoters to form mixed-metal oxides with Ce, such as CeeZr, CeeFe, CeeCu, CeeMn, CeeAl2O3, to increase its reactivity, stability and selectivity [23e27]. So far, CeeFeeO presented the best performance among these oxides above [28]. Perovskite-type oxides with a general formula of ABO3 (where A site is usually alkaline earth or rare earth metals cation and B is transition metal cations) have enormous potential to be oxygen carriers owing to their excellent thermal stability, oxygen mobility, redox cyclability and catalytic activity [29]. The role of A site metal cations is usually to stabilize the structure of oxides, while the activity of oxides is mainly determined by B site metal cations. LaFeO3 is one of the most representatives of perovskite-type oxygen carriers [30]. To acquire better reactivity, selectivity, stability and anti-coking ability, A and B metal cations have sometimes been substituted by certain alkaline and transition metal cations, respectively. Thus, a series of complex perovskite-type oxides have been synthesized and investigated in chemical-looping oxidation of methane, for example, La1xSrxMO3 (M ¼ Mn, Ni, Fe) [31e33], LaFe1xNixO3 [34], LaFe1xCoxO3 [35]. Some double-perovskite type oxygen carriers such as La1-xMxNiO4 (M ¼ Ca or Sr) have also received widespread attention [36]. Most those oxygen carriers were applied ideally at relatively high temperatures of 850e1000 C because of the drastically decrease of their reactivity when the temperature was lower than 800 C. For instance, La0$7Sr0$3FeO3 exhibited a good reactivity with methane at 850 C, achieving the methane conversion rate of up to 80% and the syngas selectivity upon around 90%. But when the reaction was conducted at 750 C, methane conversion rate dropped sharply below 10% and the syngas selectivity was also greatly reduced, likely attributing to the loss of catalytic effect on the dissociation of methane at a relatively low temperature. The well-performed La0$7Sr0$3FeO3 oxygen carrier at a higher-temperature demands its improvement at an intermediate temperature for energy saving purpose. This study proposed the CuO-modified La0$7Sr0$3FeO3 as an oxygen carrier candidate to improve its performance in the intermediate-temperature POM in a chemical looping scheme, inspired by previous theoretical calculations on the feasible abstraction of hydrogen from CeH bond in CH4 molecule by chemisorbed oxygen species or lattice oxygen in CuO [37]. It’s hypothesized that the combination of CuO and La0$7Sr0$3FeO3 can simultaneously affects the partial oxidation of methane, where CuO can lower the activation energy barrier of methane
Please cite this article as: Zhang R et al., The role of CuO modified La0$7Sr0$3FeO3 perovskite on intermediate-temperature partial oxidation of methane via chemical looping scheme, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.12.082
international journal of hydrogen energy xxx (xxxx) xxx
dissociation and La0$7Sr0$3FeO3 provides transportable lattice oxygen for partial oxidation reaction. In this study, five CuO modified La0$7Sr0$3FeO3 oxygen carriers with different content of CuO were prepared by solgel and consequent impregnation method. A series of characterization techniques including N2 physical adsorptiondesorption, ICP-OES, H2-TPR, XRD and XPS were implemented to determine physical and chemical properties of xCuO/LSF oxygen carriers. The interaction between CuO and La0$7Sr0$3FeO3 was confirmed by experiments. Subsequent fixed-bed reaction indicated that the methane conversion rate of 10CuO/LSF was more than 3 times compared to that of unmodified LSF and the concentration of syngas increased by 375% with the n(H2)/n(CO) of about 2.5. The reaction mechanism was analyzed finally.
3
Emission Spectrometer (ICP-OES). The sample was digested in an aqua regia solution before measurement.
H2-tempreture programmed reduction (H2-TPR) Reduction reactivity of the fresh samples were studied using H2-temperature programmed reduction (H2-TPR) (Quantachrome ChemStar™). Each sample was weighted 100 mg ± 5 mg and placed in a U-shape quartz tube. The sample was first pretreated at 300 C for 1 h under the Ar flow of 30 ml/min and then cooled down to the ambient temperature. Reduction step was conducted under hydrogen gas mixture flow (10 vol% H2 in He) of 30 ml/min, at a heating rate of 10 C/min until 900 C. Hydrogen consumption was measured by a thermal conductivity detector (TCD).
X-ray diffraction (XRD)
Experiment Synthesis of oxygen carriers A modified sol-gel method (the Pechini method) was used for the preparation La0$7Sr0$3FeO3 in a pure perovskite structure [38]. Stoichiometric amounts of La(NO3)3$6H2O, Sr(NO3)2 and Fe(NO3)3$9H2O were weighted and dissolved in deionized water under constant stirring at 60 C to form a nitrate solution. EDTA, citric acid and aqueous ammonia were mixed under continuous stirring at 60 C at a molar ratio of total metal ions, EDTA and citric acid at 1:1:1.5. The prepared nitrate solution was gradually dropped into the EDTA-citricNH3$H2O solution, which was mixed, stirred and heated to 80 C until a gel was formed. The gel was then dried at 120 C for 24 h followed by the calcination at 900 C for 4 h using a heating ramp at 10 C/min until the La0$7Sr0$3FeO3 perovskite powder was obtain. It was labeled as LSF. Respective amounts of Cu(NO3)3$xH2O were weighted and mixed with La0$7Sr0$3FeO3 in 10 ml deionized water. The mixtures were placed in an ultrasonic bath for 2 h, and then quickly dried in air and further calcined at 900 C in an oven for 2 h to obtain the CuO-modified LSF. The ratios of CuO and LSF were theoretically controlled at 2 wt%, 5 wt%, 10 wt% and 15 wt% CuO, respectively. They were labeled as 2CuO/LSF, 5CuO/LSF, 10CuO/LSF and 15CuO/LSF, respectively.
X-ray diffraction (XRD) was carried out to identify the structural features and compositions of fresh, reduced and after redox cycles samples using PANalytical X’ Pert Pro MPD with Cu Ka (l ¼ 0.154 nm). The operating voltage and current were 40 kV and 40 mA, respectively. The 2 theta degree ranged from 5 to 80 at a scanning rate of 10 /min.
X-ray photoelectron spectroscopy (XPS) X-ray photoelectron spectroscopy (XPS, ThermoFisher Scientific ESCALAB 250Xi) was used to obtain valence states of metal ions and different oxygen species of oxygen carriers. The X-ray source was a monochromatic Al Ka (1486.6 eV photons). The operating voltage and current were 20 kV and 10 mA, respectively.
Reactivity test Fixed bed reactor, made of quartz tube, was used in tests of the partial oxidation methane using prepared oxygen carriers, as shown in Fig. 2. The fresh oxygen carrier of 2.0 g was placed in the middle of fixed bed reactor, in which the thermocouple was inserted and its tip was close to the sample so as to detect the sample temperature accurately. Prior to the methane conversion, the oxygen carrier was heated at a heating rate of 10 C/ min in a 100 ml/min Ar flow until 750 C, and followed by 50 ml/ min CH4 mixture (40 vol% CH4 in N2) flow into reactor. The gaseous products were collected using gas sampling bags and subject to analysis using an Agilent 7890 A gas chromatography.
Characterizations N2 physical adsorption-desorption N2 physisorption was applied to obtain the specific surface area and total pore volume using Quadrasorb SI-MP-10. Each sample was degassed at 300 C for more than 12 h before its measurement. The BET method was used for calculating the surface area of each sample, while the amount of adsorbed nitrogen at maximum p/p0 was used for total pore volume.
Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES) The actual CuO content of fresh samples was measured using PerkinElmer OPTIMA 8000 Inductively Coupled Plasma Optical
Fig. 2 e Schematic diagram of fixed bed reactor.
Please cite this article as: Zhang R et al., The role of CuO modified La0$7Sr0$3FeO3 perovskite on intermediate-temperature partial oxidation of methane via chemical looping scheme, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.12.082
4
international journal of hydrogen energy xxx (xxxx) xxx
The conversion rate of CH4, the molar ratios of H2/CO and CO/CO2 are defined as following equations (1)e(3): CH4 conv ¼
MCH4;in MCH4;out MCH4;in
(1)
nðH2 Þ = nðCOÞ ¼ MH2 = MCO
(2)
nðCOÞ = nðCO2 Þ ¼ MCO = MCO2
(3)
where MCH4;in refers to the mole of introduced CH4, and MCH4;out , MH2 , MCO and MCO2 refer to the mole of CH4, H2, CO and CO2 in the exhaust gas. In terms of the redox cycle procedure, 50 ml/min Air was injected into the reactor for oxidizing the reduced oxygen carrier to its original state. Between each reduction and oxidation stages, 100 ml/min Ar was purged into the reactor for 20 min to prevent the mixing of combustible gas and oxygen.
Results and discussion Characterization of fresh oxygen carriers Fig. 3 showed the XRD patterns of the freshly prepared CuO modified LSF (xCuO/LSF). It was noticeable that the diffraction pattern of the LSF corresponded to an orthorhombic perovskite phase (PDF 01-089-1269) and no peaks to La2O3, SrO and Fe2O3, indicating a pure perovskite crystal structure of LSF in the freshly prepared CuO modified LSF. The characteristic
diffraction peaks of CuO (PDF 01-089-5895) could be observed in the freshly prepared CuO modified LSF oxygen carriers, revealing the occurrence of the pure CuO crystal on LSF surface. In addition, the intensity of the CuO diffraction peak was related to the content of copper doping (xCuO/LSF, x ¼ 2,5,10,15), and becomes stronger as the copper content increased. Table 1 showed physical properties of the freshly prepared CuO modified LSF, such as the specific surface area, the total pore volume and the actual weight percentage of CuO. The specific surface area of the LSF perovskite alone was approximately 6 m2/g (with a corresponding pore volume of about 15 103 cm3/g), which was consistent to those reported by open publications. A decrease in both the specific surface area and total pore volume of 2CuO/LSF was observed compared to those of the pure LSF perovskite, very likely attributing to the CuO coverage on the surface of the LSF perovskite. For xCuO/ LSF (x ¼ 2,5), their specific surface area remained almost unchanged as the CuO content increased, while the total pore volume kept increasing, likely attributing to the growth of CuO on LSF perovskite surface form a larger pore size structure. The further increase of copper doping amount to 15 wt% decreased its specific surface area and total pore volume, corresponding to the slight aggregation of CuO. Overall, the copper doping seemed less significantly changed the specific surface area and the total pore volume of the CuO-modified LSF, assuming less likely affected their reactivity. Actual CuO content in the CuO-modified LSF was also listed in Table 1. The H2-TPR profiles of prepared samples were shown in Fig. 4. H2-TPR results showed their reducibility and the interaction between CuO and LSF. The pure LSF exhibited two distinct reduction peaks ranging from 300 to 400 C and 750e900 C, attributing well to two different kinds of oxygen species, adsorbed oxygen and lattice oxygen existing in the LSF oxygen carrier [32,39]. The H2 consumption of first reduction peak could be corresponded to the adsorbed oxygen, while the second to the lattice oxygen. This seemed not change after the copper doping, only the first peak shifting toward lower temperature ranging within 280e300 C was observed, implying the improvement of adsorbed oxygen reactivity. Meanwhile, the intensity of the first reduction peak
Table 1 e Physical properties of prepared oxygen carriers. Oxygen carrier
LSF 2CuO/LSF 5CuO/LSF 10CuO/ LSF 15CuO/ LSF
Fig. 3 e XRD spectra of fresh oxygen carriers with different CuO content.
Surface areaa (m2/g)
Total pore volumeb ( 103 cm3/g)
6.681 5.667 5.543 5.757
15.42 11.66 14.97 17.03
e 2 5 10
e 1.75 4.45 8.65
5.336
14.61
15
13.08
Amount of copper oxide (weight %) Theoretical Actualc
Note: a Calculated from the adsorption isotherm by using BET method. b Calculated from the amount of adsorbate at the maximum p/p0. c Measured by ICP-OES.
Please cite this article as: Zhang R et al., The role of CuO modified La0$7Sr0$3FeO3 perovskite on intermediate-temperature partial oxidation of methane via chemical looping scheme, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.12.082
international journal of hydrogen energy xxx (xxxx) xxx
Fig. 4 e H2-TPR curves of oxygen carriers.
constantly increased with the increase of CuO content, indicating a strong correlation between the doped CuO and the first reduction peaks in xCuO/LSF (x ¼ 2,5,10,15) oxygen carriers. The shift of the doped CuO reduction peak to the higher temperature compared to that of the pure CuO implied that the interaction between CuO and LSF perovskite. As for 15CuO/LSF, only a smaller and broader peak was within 320 C and 420 C, implying a decrease of adsorbed oxygen and weakness of the interaction between CuO and LSF perovskite by the excessive copper doping. Present study revealed the increase of the content of adsorbed oxygen decreased the selectivity to syngas because the adsorbed oxygen was more prone to completely oxidize CH4 to CO2 and H2O. Thus, it was important to determine an appropriate amount of copper doping that can benefit both the reactivity of oxygen carriers and the selectivity to syngas. There were no significant changes in the second reduction peak of oxygen carriers after copper doping, indicating less correlation between the doped CuO and the lattice oxygen.
Reactivity of oxygen carriers in chemical-looping partial oxidation The methane conversion on the oxygen carriers followed three main reaction pathways: (I) methane complete oxidation CH4/CO2þH2O, (II) methane partial oxidation CH4/CO þ H2 and (III) methane cracking CH4/C þ H2. The CH4 conv corresponds to methane reactivity on the oxygen carriers. The n(H2)/n(CO) can reflect the respective proportion of reaction pathway (II) and (III). The higher the n(H2)/n(CO) is, the more the methane cracking instead of partially oxidized toward syngas. The n(CO)/n(CO2) reflects the respective proportion of (I) and (II). The higher n(CO)/n(CO2) is, the more methane is partially oxidized instead of complete oxidization. Overall, CH4 conv, n(H2)/n(CO) and n(CO)/n(CO2) are three
5
major parameters to evaluate performance of oxygen carriers, as shown in Fig. 5. Referring to Fig. 5(a), (c), all five oxygen carriers showed fairly high reactivity at the initial reaction stage (within 0e8 min) and the n(CO)/n(CO2) was quite close to 0, implying the complete oxidization of CH4 toward CO2 and H2O by adsorbed oxygen. This was also accompanied by drastic decrease of the CH4 conv on account of the rapidly exhausted adsorbed oxygen. Within 8 min of reaction, the CH4 conv of LSF and 2CuO/LSF remained at a considerably low level and continuously decreased. The CH4 conv of 5CuO/LSF started to increase at 16 min. But the CH4 conv of xCuO/LSF (x ¼ 10,15) increased immediately after the initial stage. It can be concluded that the reactivity between CH4 and oxygen carriers increased as the CuO content increased, and the 15CuO/ LSF exhibited a highest methane conversion rate. The copper doping had a significant promotion on the methane conversion over the LSF perovskite oxygen carriers. However, excessive CuO would negatively affected on the methane partial oxidation. On the other hand, the production of syngas in the n(H2)/n(CO) within 2 and 3 was desirable for the subsequent chemical synthesis process. Fig. 5(b), (c) showed that the n(H2)/n(CO) of xCuO/LSF (x ¼ 10,15) were within 2 and 3 where the reaction was in a tie window of 12e22 min and 8e18 min, respectively. For the 10CuO/LSF, its n(CO)/n(CO2) was lower at 12 min but rapidly increased over time; yet for the 15CuO/LSF, its n(CO)/n(CO2) was kept at a low level within 8e10 min and the increment of its n(CO)/n(CO2) was lower than 10CuO/LSF. Thus, 15CuO/LSF produced more CO2 instead of syngas compared to 10CuO/LSF. This also coincided with the conclusion derived from H2-TPR tests in that 15CuO/LSF contained more adsorbed oxygen than 10CuO/ LSF. In addition, the 15CuO/LSF also produced more CO2 than 10CuO/LSF at the initial reaction stage, owing to the higher adsorbed oxygen in 15CuO/LSF. More CO2 meant more energy input for the following separation, also reduced the selectivity of syngas. Therefore, 15CuO/LSF produced less high-quality syngas than 10CuO/LSF. Considering both parameters of preferably higher the methane conversion rate and the selectivity of syngas, 10CuO/LSF seemed the best performed in both parameters in this study. Consequently, it was utilized in the subsequent successive redox cycle. The crystal phase changes of five oxygen carriers of methane reduction in the fixed bed after 30 min were shown in Fig. 6. Both LSF and 2CuO/LSF showed no structural change compared to their original states, corresponding to their extremely low CH4 conversion rate. For the xCuO/LSF (x ¼ 5,10,15), new crystal phases were detected in all these three samples after their reduction, which involved La2CuO4 (PDF 00-038-0709) and Copper (PDF 03-065-9026). The existence of La2CuO4 clearly exhibited the interaction between Cu and LSF. The combination of La and Cu caused left Sr and Fe to form SrFeO3-x, which introduced more oxygen defects and vacancies in its bulk to improve the lattice oxygen mobility and accordingly to enhance the reactivity between CH4 and the oxygen carriers. Simultaneously, the main crystal structures of xCuO/ LSF (x ¼ 5,10,15) could still keep its orthorhombic phase, owing to the strong skeleton formed by La3þ and Sr2þ ions, which showed a higher structural stability and regenerability.
Please cite this article as: Zhang R et al., The role of CuO modified La0$7Sr0$3FeO3 perovskite on intermediate-temperature partial oxidation of methane via chemical looping scheme, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.12.082
6
international journal of hydrogen energy xxx (xxxx) xxx
Fig. 5 e Reactivity performance of different oxygen carriers under 750 C. (a)CH4 conv; (b)n(H2)/n(CO); (c)n(CO)/n(CO2).
Fig. 6 e XRD spectra of reduced oxygen carriers with different CuO content.
The cyclic performance of oxygen carrier is essential for chemical-looping process. The successive redox cycle was performed to evaluate the durability of 10CuO/LSF which was determined to be the best candidate among the synthesized samples. Each cycle involved a reduction stage and an oxidation stage and both were conducted at 750 C. The initial test with 10CuO/LSF in the fixed bed helped in determining the setting of the reduction time to be 22 min to keep the appropriate n(H2)/n(CO), and the time setting of the oxidation stage was 20 min. Fig. 7 showed the evolvement of CO2, CO and H2 at every reduction stage to identify the cyclic performance of 10CuO/LSF, using that of LSF in the slashed area as a reference. The sample 10CuO/LSF showed a favorable cyclic stability within ten cycles. Its syngas yield decreased only slightly from 1st to 6th cycle and kept steady then. The average methane conversion rate of 10CuO/LSF was 21.6% in ten cycles, almost 3 times higher than that of the pure LSF of 7.1%. The average concentration of syngas increased by 375% compared to that of unmodified LSF. Significantly, the n(H2)/ n(CO) of 10CuO/LSF in ten cycles was maintained at about 2.5, while the n(H2)/n(CO) of LSF was close to 4, indicating a positive effect of the copper doping on the methane partial oxidation to syngas at intermediate temperature. Fig. 8 showed the comparison of XRD patterns between fresh and used 10CuO/LSF after ten cycles. The diffraction peak positions of LSF perovskite and CuO were basically unchanged, indicating the crystalline structure of the used 10CuO/LSF
Please cite this article as: Zhang R et al., The role of CuO modified La0$7Sr0$3FeO3 perovskite on intermediate-temperature partial oxidation of methane via chemical looping scheme, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.12.082
international journal of hydrogen energy xxx (xxxx) xxx
7
Fig. 7 e Cyclic performance of 10CuO/LSF in ten redox cycles.
recovered to its original state. This result proved good regenerability of 10CuO/LSF according to XRD results of perovskite oxygen carriers reported in other literature [40]. However, the peak intensities of LSF perovskite and CuO decreased, mainly because some copper atoms incorporated into LSF perovskite crystalline, decreasing the crystallinity of LSF perovskite. The reducing of the methane conversion rate might be caused by the loss of surface CuO.
respectively. The binding energy at around 528.8 eV could be labeled as OI, which is associated with metal oxides (lattice oxygen). The binding energy peak at around 531.2 eV is probably from metal carbonate species. The binding energy peak at around 533.7 eV, which is attributed to metal hydroxide or adsorbed molecular water on surface, is usually
Evolution of various elements in oxygen carriers All XPS spectra were calibrated by C1s standard binding energy 248.8 eV. The XPS spectra of O1s for xCuO/LSF (x ¼ 2,5,10,15) are shown in Fig. 9. Each O1s spectrum was fitted by three peaks and labeled as OI, OII and OIII,
Fig. 8 e XRD spectra of 10CuO/LSF: fresh and after cycling.
Fig. 9 e O1s spectra of fresh xCuO/LSF (x ¼ 2,5,10,15) oxygen carriers.
Please cite this article as: Zhang R et al., The role of CuO modified La0$7Sr0$3FeO3 perovskite on intermediate-temperature partial oxidation of methane via chemical looping scheme, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.12.082
8
international journal of hydrogen energy xxx (xxxx) xxx
classified as adsorbed oxygen [41]. As reported in literature, OI was related to the partial oxidation of methane, while OII and OIII were more likely involved in the complete oxidation of methane [40]. Table 2 shows fractions of different oxygen species in xCuO/LSF (x ¼ 2,5,10,15). When the value of x is increased from 2 to 10, the corresponding lattice oxygen contents in the oxygen carriers were 51.70%, 47.80%, 44.43% and 40.59%, respectively. The increase of CuO content gradually decreased the proportion of lattice oxygen and increased that of adsorbed oxygen, which is consistent to results of H2TPR and fixed bed experiments. As shown in Fig. 10, the sample exhibited a lower OI peak and higher OII peak after the reduction stage, indicating the consumption of lattice oxygen and the formation of metal carbonates. The OI peak can recover its original state after reoxidation by air. Changes in chemical state of various metal elements (Cu, La, Sr and Fe) at different reaction stages are displayed in Fig. 11. The was a strong Cu2þ satellite peak shown in Fig. 11(a), implying presence of Cu(II) in all three stages (fresh, reduced and after cycled) in the form of CuO in fresh and after cycled samples and La2CuO4 in reduced sample. Simultaneously, the Cu2p3/2 binding energy of the reduced sample shifts to lower position (from 934.0 eV to 933.1 eV), indicating that a fraction of Cu(II) is reduced to Cu. The intensity of Cu2p peak became smaller after many cycles, probably attributing to the migration of CuO phase from the LSF surface into its inner part and explaining the decreased of the reactivity of 10CuO/LSF after several cycles. In Fig. 11(b), La3d region had well separated spin-orbit components, each of which can be further split by multiplet splitting. The magnitude of the multiplet splitting and intensity ratio of each multiplet-split component were chemically diagnostic. The comparison of DE and the relative intensity of La3d5/2 doublet peak revealed that La in both the fresh and 10Cu/LSF after cycled mainly occurred in the form of a metal oxide (DE ¼ 4.4 eV), but La2(CO3)3 (DE ¼ 3.5 eV) appeared in reduced sample. Reaction with methane made carbon adsorption on lanthanum to form surface carbonates. Sr3d region has well resolved spin-orbit components (DE ¼ 1.76 eV). When its multiplet chemical states presented, apparent resolution of these two spin-orbit components may vary depending on concentrations. Strontium in LSF phase and strontium carbonate are represented by blue and red color, respectively. The ratio of strontium in LSF and SrCO3 can be determined by peak areas of different colors. The fresh sample surface contains 45.77% strontium in LSF and 54.23% SrCO3. After methane reduction, strontium in LSF and SrCO3 turned to be 25.49% and 74.51%. These two different kinds
Table 2 e Different oxygen species for the xCuO/LSF (x ¼ 2,5,10,15) measured from XPS. Oxygen carriers
2CuO/LSF 5CuO/LSF 10CuO/LSF 15CuO/LSF
Peaks areas (%) OI (metal oxides)
OII (metal carbonates)
OIII (metal hydroxides)
51.70 47.80 44.43 40.59
45.27 47.90 50.69 56.01
3.03 4.30 4.88 3.40
Fig. 10 e O1s spectra of 10CuO/LSF at different stages.
of strontium returned to their original states, containing 42.53% and 57.47% after many redox cycles. This was a clear message on carbon adsorption on strontium to form more strontium carbonates in the process of reacting with methane. The XPS spectra of Fe2p at different stages are shown in Fig. 11(d). Fe(III) compounds are always high-spin, leading to complex multiplet and having satellite features. Fitting Fe2p spectra of Fe3þ (EB ¼ 710.8 eV) and Fe4þ (EB ¼ 713.1 eV) revealed their proportion were 61.91% and 38.09%, respectively. A doping of Sr in A site made the partial transformation of Fe3þ to Fe4þ in B site, resulting in an increase of the oxygen vacancies and the enhancement of the lattice oxygen mobility. After reduction, the proportion of Fe2þ, Fe3þ and Fe4þ changed to be 20.48%, 41.45% and 38.07%, respectively. Methane reduces a part of Fe3þ/Fe4þ to Fe2þ, but it is difficult to reduce Fe2þ to lower valences. Simultaneously, the existence of La2CuO4 phase resulted in more SrFeO3-x phase as described above, causing an increase in the Fe4þ content and thus further produced more oxygen vacancies and improved the oxygen mobility. After many redox cycles, the chemical states of Fe recovered to its original state where the proportion of Fe3þ and Fe4þ was found to be 60.40% and 39.60%, respectively, showing a superior cyclic performance.
Reaction mechanism The mechanism on promotion effect of copper oxide is shown in Fig. 12. Copper oxide acted as active center to considerably facilitate the dissociation of methane in the initial stage. The
Please cite this article as: Zhang R et al., The role of CuO modified La0$7Sr0$3FeO3 perovskite on intermediate-temperature partial oxidation of methane via chemical looping scheme, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.12.082
international journal of hydrogen energy xxx (xxxx) xxx
9
Fig. 11 e Various metal elements spectra of 10CuO/LSF at different stages. (a)Cu2p; (b)La3d; (c)Sr3d; (d)Fe2p.
Fig. 12 e Schematic diagram of reaction mechanism in various stages.
Please cite this article as: Zhang R et al., The role of CuO modified La0$7Sr0$3FeO3 perovskite on intermediate-temperature partial oxidation of methane via chemical looping scheme, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.12.082
10
international journal of hydrogen energy xxx (xxxx) xxx
first-principles calculations in literature [42] revealed that the activation barrier of CeH bonds of methane was diminished to 60.5 kJ/mol on the high-energy CuO (010) surface and to 76.6 kJ/mol on the most stable CuO (111) surface compared to ~440 kJ/mol without any surfaces. Even on a clean Cu (111) surface, the activation energy barrier can be as low as 169.8 kJ/ mol. Copper and its oxide could promote the dissociation of methane. Simultaneously, the interaction between copper and LSF makes the formation of La2CuO4 and SrFeO3-x which contained more oxygen vacancies and further improved the reactivity of oxygen carriers. Dissociated methane was partially oxidized by the lattice oxygen to generate syngas. As the reaction proceeded, the formation of lanthanum and strontium carbonates was observed on the surface of oxygen carrier. The increasing coverage of these carbonates resulted in gradual declining in oxidizing capacity of oxygen carrier and increasing the share of methane thermal-cracking. Meanwhile, the lattice oxygen in the LSF bulk was constantly lost, and a portion of Fe3þ/Fe4þ in the oxygen carrier was reduced to Fe2þ. The combination of the dissociation of methane by CuO and the migration of LSF lattice oxygen highly improved the reactivity and selectivity of oxygen carrier.
Conclusion In summary, this work studied the reactivity improvement of La0$7Sr0$3FeO3 (LSF) with the copper modification in an intermediate-temperature partial oxidation of methane via a chemical-looping scheme. Results indicated that CuO crystal grew on the surface of as-prepared CuO modified LSF, which still kept its intact perovskite structure. The H2-TPR experiment confirmed the occurrence of interaction between CuO and LSF, and CuO could activate the adsorbed oxygen in LSF phase. The most suitable CuO doping content was found 10% that was confirmed in view of both the conversion rate and syngas selectivity of methane in the fixed bed experiment. Fixed-bed reaction indicated that the methane conversion rate of 10CuO/LSF was more than 3 times compared to that of unmodified LSF and the concentration of syngas increased by 375% with the n(H2)/n(CO) of about 2.5. The increase in methane conversion rate attributes to the function of CuO for lowering the activation energy barrier of CeH bond in CH4 molecule. Thus, the improvement of the reactivity of at a relatively low temperature. Given the ideal H2/CO ratio is 2, results further exhibited the CuO modified LSF helping in the domination of the partial oxidation of methane and elimination of methane cracking. This was mainly attributed to the formation of La2CuO4, which induced more oxygen vacancies and promoted the oxygen mobility. Further studies confirmed the good cyclic performance of the prepared 10CuO/LSF over 10 redox cycles. Several redox cycles only slightly decreased reactivity of 10CuO/LSF as a result of the CuO migration from surface to interior of LSF crystal. The Cumodification method provided a promising and simple pathway on the preparation of oxygen carrier to decrease the reaction temperature of partial oxidation of methane via a chemical-looping scheme.
Acknowledgement The financial support of National Key Research and Development Program of China [2017YFE0105500] is gratefully acknowledged. This work was also supported by the National Natural Science Foundation of China [51876205] and the Pearl River S&T Nova Program of Guangzhou [201906010092].
references
[1] Wahlen M. The global methane cycle. Annu Rev Earth Planet Sci 1993;21:407e26. [2] Barone G, Chianese E. Hydrates of natural gases and small molecules: structures, properties, and exploitation perspectives. ChemSusChem 2009;2:992e1008. [3] Salkuyeh YK, Saville BA, MacLean HL. Techno-economic analysis and life cycle assessment of hydrogen production from different biomass gasification processes. Int J Hydrogen Energy 2018;43:9514e28. [4] Lunsford JH. Catalytic conversion of methane to more useful chemicals and fuels: a challenge for the 21st century. Catal Today 2000;63:165e74. [5] Sonal, Ahmad E, Upadhyayula S, Pant KK. Biomass-derived CO2 rich syngas conversion to higher hydrocarbon via Fischer-Tropsch process Chao over Fe-Co bimetallic catalyst. Int J Hydrogen Energy 2019;44:27741e8. [6] Shah YT, Gardner TH. Dry reforming of hydrocarbon feedstocks. Catal Rev Sci Eng 2014;56:476e536. [7] Sokolovskii VD, Coville NJ, Parmaliana A, Eskendirov I, Makoa M. Methane partial oxidation. Challenge and perspective. Catal Today 1998;42:191e5. [8] Xu JG, Froment GF. Methane steam reforming, methanation and water-gas shift .1. Intrinsic kinetics. AIChE J 1989;35:88e96. [9] Sengodan S, Lan R, Humphreys J, Du DW, Xu W, Wang HT, et al. Advances in reforming and partial oxidation of hydrocarbons for hydrogen production and fuel cell applications. Renew Sustain Energy Rev 2018;82:761e80. [10] Zeng L, Cheng Z, Fan JA, Fan LS, Gong JL. Metal oxide redox chemistry for chemical looping processes. Nat Rev Chem 2018;2:349e64. [11] Luo SW, Zeng L, Fan LS. Chemical looping technology: oxygen carrier characteristics. In: Prausnitz JM, editor. Annual review of chemical and biomolecular engineering, vol. 6. Palo Alto: Annual Reviews; 2015. p. 53e75. [12] Voldsund M, Jordal K, Anantharaman R. Hydrogen production with CO2 capture. Int J Hydrogen Energy 2016;41:4969e92. [13] Tang MC, Xu L, Fan MH. Progress in oxygen carrier development of methane-based chemical-looping reforming: a review. Appl Energy 2015;151:143e56. [14] Cheng Z, Qin L, Fan JA, Fan LS. New insight into the development of oxygen carrier materials for chemical looping systems. Engineering 2018;4:343e51. € rje. Integrated [15] Zafar Qamar, Mattisson Tobias, Gevert Bo hydrogen and power production with CO2 capture using chemical-looping reforming-redox reactivity of particles of CuO, Mn2O3, NiO, and Fe2O3 using SiO2 as a support. Ind Eng Chem Res 2005;44:3485e96. nez J, Garcı´a-Labiano F, Abad A, [16] Diego LFD, Ortiz M, Ada n P. Synthesis gas generation by chemical-looping Gaya reforming in a batch fluidized bed reactor using Ni-based oxygen carriers. Chem Eng J 2008;144:289e98.
Please cite this article as: Zhang R et al., The role of CuO modified La0$7Sr0$3FeO3 perovskite on intermediate-temperature partial oxidation of methane via chemical looping scheme, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.12.082
international journal of hydrogen energy xxx (xxxx) xxx
n M, Lyngfelt A, Mattisson T. Synthesis gas generation [17] Ryde by chemical-looping reforming in a continuously operating laboratory reactor. Fuel 2006;85:1631e41. [18] Johansson M, Mattisson T, Lyngfelt A, Abad A. Using continuous and pulse experiments to compare two promising nickel-based oxygen carriers for use in chemicallooping technologies. Fuel 2008;87:988e1001. [19] Antzara A, Heracleous E, Silvester L, Bukur DB, Lemonidou AA. Activity study of NiO-based oxygen carriers in chemical looping steam methane reforming. Catal Today 2016;272:32e41. [20] Hafizi A, Rahimpour MR, Heravi M. Experimental investigation of improved calcium-based CO2 sorbent and Co3O4/SiO2 oxygen carrier for clean production of hydrogen in sorption-enhanced chemical looping reforming. Int J Hydrogen Energy 2019;44:17863e77. [21] Li DL, Nakagawa Y, Tomishige K. Methane reforming to synthesis gas over Ni catalysts modified with noble metals. Appl Catal A-Gen. 2011;408:1e24. [22] Zhu Xing, Wang Hua, Wei Yonggang, Li Kongzhai, Cheng X. Hydrogen and syngas production from two-step steam reforming of methane using CeO2 as oxygen carrier. J Nat Gas Chem 2010;28:281e6. [23] He F, Wei Y, Li H, Wang H, He F, Wei Y, et al. Synthesis gas generation by chemical-looping reforming using Ce-based oxygen carriers modified with Fe, Cu, and Mn oxides. Energy Fuel 2009;23:2095e102. [24] Kongzhai LI, Wang H, Wei Y, Liu M. Preparation and characterization of Ce1-xFexO2 complex oxides and its catalytic activity for methane selective oxidation. J Rare Earths 2008;26:245e9. [25] Li K, Hua W, Wei Y, Yan D. Syngas production from methane and air via a redox process using CeeFe mixed oxides as oxygen carriers. Appl Catal B Environ 2010;97:361e72. [26] Xing Z, Wei Y, Hua W, Li K. CeeFe oxygen carriers for chemical-looping steam methane reforming. Int J Hydrogen Energy 2013;38:4492e501. [27] Zheng Y, Li K, Hua W, Xing Z, Wei Y, Min Z, et al. Enhanced activity of CeO2eZrO2 solid solutions for chemical-looping reforming of methane via tuning the macroporous structure. Energy Fuel 2016;30. acs.energyfuels.5b02151. [28] Wei Y, Wang H, Li K. Ce-Fe-O mixed oxide as oxygen carrier for the direct partial oxidation of methane to syngas. J Rare Earths 2010;28:560e5. [29] O’Connell M, Norman AK, Hu¨ttermann CF, Morris MA. Catalytic oxidation over lanthanum-transition metal perovskite materials. Catal Today 1999;47:123e32. [30] Ciambelli P, Cimino S, De Rossi S, Lisi L, Minelli G, Porta P, et al. AFeO(3) (A ¼ La, Nd, Sm) and LaFe1-xMgxO3 perovskites as methane combustion and CO oxidation catalysts:
[31]
[32]
[33]
[34]
[35]
[36]
[37]
[38]
[39]
[40]
[41]
[42]
11
structural, redox and catalytic properties. Appl Catal B Environ 2001;29:239e50. Zhao K, He F, Huang Z, Zheng AQ, Li HB, Zhao ZL. La1xSrxFeO3 perovskites as oxygen carriers for the partial oxidation of methane to syngas. Chin J Catal 2014;35:1196e205. He F, Li X, Zhao K, Huang Z, Wei G, Li H. The use of La1xSrxFeO3 perovskite-type oxides as oxygen carriers in chemical-looping reforming of methane. Fuel 2013;108:465e73. Li RJ, Yu CC, Ji WJ, Shen SK. Methane oxidation to synthesis gas using lattice oxygen in La1xSrxFeO3 perovskite oxides instead of molecular oxygen. Stud Surf Sci Catal 2004;147:199e204. Yang S, Zhao K, Fang H, Li H. The structure-reactivity relationships of using three-dimensionally ordered macroporous LaFe 1x Ni x O 3 perovskites for chemicallooping steam methane reforming. J Energy Inst 2018;92(2):239e46. S1743967117309078. Zhao K, Fang H, Zhen H, Wei G, Zheng A, Li H, et al. Perovskite-type oxides LaFe 1 x Co x O 3 for chemical looping steam methane reforming to syngas and hydrogen co-production. Appl Energy 2016;168:193e203. Melo VRM, Medeiros R, Braga RM, Macedo HP, Ruiz JAC, Moure GT, et al. Study of the reactivity of Double-perovskite type oxide La1-xMxNiO4 (M ¼ Ca or Sr) for chemical looping hydrogen production. Int J Hydrogen Energy 2018;43:1406e14. Niu TC, Jiang Z, Zhu YG, Zhou GW, van Spronsen MA, Tenney SA, et al. Oxygen-promoted methane activation on copper. J Phys Chem B 2018;122:855e63. Chen Y, Galinsky N, Wang Z, Li F. Investigation of perovskite supported composite oxides for chemical looping conversion of syngas. Fuel 2014;134:521e30. Zheng Y, Li K, Wang H, Tian D, Wang Y, Zhu X, et al. Designed oxygen carriers from macroporous LaFeO 3 supported CeO 2 for chemical-looping reforming of methane. Appl Catal B Environ 2017;202:51e63. Zhao K, Zheng AQ, Li HB, He F, Huang Z, Wei GQ, et al. Exploration of the mechanism of chemical looping steam methane reforming using double perovskite-type oxides La1.6Sr0.4FeCoO6. Appl Catal B Environ 2017;219:672e82. Ding HR, Xu YQ, Luo C, Wang QY, Shen C, Xu JX, et al. A novel composite perovskite-based material for chemical-looping steam methane reforming to hydrogen and syngas. Energy Convers Manag 2018;171:12e9. Varghese JJ, Trinh QT, Mushrif SH. Insights into the synergistic role of metalelattice oxygen site pairs in fourcentered CeH bond activation of methane: the case of CuO. Catal Sci Technol 2016;6:3984e96.
Please cite this article as: Zhang R et al., The role of CuO modified La0$7Sr0$3FeO3 perovskite on intermediate-temperature partial oxidation of methane via chemical looping scheme, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.12.082
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
The role of CuO modified La0·7Sr0·3FeO3 perovskite on intermediate-temperature partial oxidation of methane via chemical looping scheme Rongjiang Zhang a,b,c,d, Yan Cao a,b,c, Haibin Li a,b,c, Zengli Zhao a,b,c, Kun Zhao a,b,c,*, Liqun Jiang a,b,c,** a
Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, China CAS Key Laboratory of Renewable Energy, China c Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, China d University of Chinese Academy of Sciences, Beijing, 100049, China b
highlights The CLPO temperature was reduced to 750 C with CuO modified La0$7Sr0$3FeO3. Interaction between CuO and La0$7Sr0$3FeO3 was the key point in oxygen carriers. CuO modification promoted methane dissociation and activation of lattice oxygen. 10CuO/La0$7Sr0$3FeO3 exhibited good reactivity and regenerability on CLPO process.
article info
abstract
Article history:
Synthesis gas production via chemical-looping partial oxidation (CLPO) of methane reduces
Received 11 October 2019
operating cost and likely avoids carbon dioxide emissions. Previous studies exhibit La0$7-
Received in revised form
Sr0$3FeO3 (LSF) a good high-temperature oxygen carrier, but not being applicable in
4 December 2019
intermediate-temperature CLPO processes. This work proposes a CuO modified LSF (xCuO/
Accepted 13 December 2019
LSF, x ¼ 2,5,10,15) for enhancing the reactivity of oxygen carriers at relatively low tem-
Available online xxx
peratures. Characterization methods, including N2 physisorption, ICP-OES, H2-TPR, XRD
Keywords:
determine the best candidate under 750 C is 10CuO/LSF. The methane conversion rate of
Chemical-looping partial oxidation
10CuO/LSF is more than 3 times compared to that of unmodified LSF and the concentration
(CLPO)
of syngas increases by 375% with the H2/CO molar ratio of about 2.5. The regenerability of
Methane
10CuO/LSF proves to be good. This study provides a promising and simple way to lower the
Syngas
operating temperature of the CLPO process, likely leading a considerable energy saving.
and XPS, are implemented to determine properties of xCuO/LSF. Fixed-bed experiments
Perovskite
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Copper oxide Oxygen carrier
* Corresponding author. Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, China. ** Corresponding author. CAS Key Laboratory of Renewable Energy, China. E-mail addresses: [email protected] (K. Zhao), [email protected] (L. Jiang). https://doi.org/10.1016/j.ijhydene.2019.12.082 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Zhang R et al., The role of CuO modified La0$7Sr0$3FeO3 perovskite on intermediate-temperature partial oxidation of methane via chemical looping scheme, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.12.082
2
international journal of hydrogen energy xxx (xxxx) xxx
Introduction Methane, usually found in natural gas, shale gas [1], clathrates [2] and even landfills and biomass gas [3], is one of the most abundant hydrocarbons on earth. Methane can be a fuel to generate electricity, a chemical feedstock for upgrading to value-added chemicals [4], or utilized for domestic heating and cooking. Its cleanness and convenience make it be attracted in great interests. One of the important chemical routes of methane is the conversion of methane to syngas which can further produce a wide range of chemicals [5]. The methane-to-syngas involves the most mature steam methane reforming processes (SRM), and further dry reforming (DRM) and recent partial oxidation (POM) [6e8]. POM has aroused extensive attentions, because of its relatively low energy consumption and the ideal H2/CO ratio which is more suitable and feasible for subsequent industrial processes. However, POM demands the supply of pure oxygen, making it complicated and costly [9]. To overcome this contradiction, the partial oxidation of methane (CLPO) in a chemical-looping scheme, one of the most promising techniques, has been proposed [10,11]. Certain metal oxides will be applied in CLPO as oxygen carriers to transfer oxygen between two reactors, oxidizer to gain oxygen from air and reducer to release oxygen to methane toward syngas, as shown in Fig. 1. It avoids the complicated and costly step of the air separation, and most significantly carbon dioxide can be ready for the sequestration in an economic way [12]. Simultaneously, the operational safety could be greatly improved owing to the isolation of methane and pure oxygen. However, CLPO is a complex and challengeable redox reaction process involving adsorption and dissociation of methane, formation and detachment of
Fig. 1 e Schematic diagram of chemical-looping partial oxidation of methane.
oxygenated intermediates as well as evolution and migration of lattice oxygen in oxygen carriers. Therefore, the performance of oxygen carriers is one of important factors affecting the CLPO process [10,13,14]. Transition metal oxides had been easily studied in CLPO process, such as Ni, Cu, Fe, Co, and Mn oxides [15e17]. Different inert supports, such as ZrO2, TiO2, SiO2, Al2O3, NiAl2O4 and MgAl2O4 [16,18e20], were also used to enhance the mechanical strength, reactivity, anti-coking ability, and regenerability. Ni-based oxygen carriers exhibit better reactivity than Cu-, Fe-, and Mn-, but their anti-coking ability should be further improved [21]. Ce-based oxygen carriers have widely been studied and proven to be feasible in partial oxidation of methane [22]. Some transition metals were found to be promoters to form mixed-metal oxides with Ce, such as CeeZr, CeeFe, CeeCu, CeeMn, CeeAl2O3, to increase its reactivity, stability and selectivity [23e27]. So far, CeeFeeO presented the best performance among these oxides above [28]. Perovskite-type oxides with a general formula of ABO3 (where A site is usually alkaline earth or rare earth metals cation and B is transition metal cations) have enormous potential to be oxygen carriers owing to their excellent thermal stability, oxygen mobility, redox cyclability and catalytic activity [29]. The role of A site metal cations is usually to stabilize the structure of oxides, while the activity of oxides is mainly determined by B site metal cations. LaFeO3 is one of the most representatives of perovskite-type oxygen carriers [30]. To acquire better reactivity, selectivity, stability and anti-coking ability, A and B metal cations have sometimes been substituted by certain alkaline and transition metal cations, respectively. Thus, a series of complex perovskite-type oxides have been synthesized and investigated in chemical-looping oxidation of methane, for example, La1xSrxMO3 (M ¼ Mn, Ni, Fe) [31e33], LaFe1xNixO3 [34], LaFe1xCoxO3 [35]. Some double-perovskite type oxygen carriers such as La1-xMxNiO4 (M ¼ Ca or Sr) have also received widespread attention [36]. Most those oxygen carriers were applied ideally at relatively high temperatures of 850e1000 C because of the drastically decrease of their reactivity when the temperature was lower than 800 C. For instance, La0$7Sr0$3FeO3 exhibited a good reactivity with methane at 850 C, achieving the methane conversion rate of up to 80% and the syngas selectivity upon around 90%. But when the reaction was conducted at 750 C, methane conversion rate dropped sharply below 10% and the syngas selectivity was also greatly reduced, likely attributing to the loss of catalytic effect on the dissociation of methane at a relatively low temperature. The well-performed La0$7Sr0$3FeO3 oxygen carrier at a higher-temperature demands its improvement at an intermediate temperature for energy saving purpose. This study proposed the CuO-modified La0$7Sr0$3FeO3 as an oxygen carrier candidate to improve its performance in the intermediate-temperature POM in a chemical looping scheme, inspired by previous theoretical calculations on the feasible abstraction of hydrogen from CeH bond in CH4 molecule by chemisorbed oxygen species or lattice oxygen in CuO [37]. It’s hypothesized that the combination of CuO and La0$7Sr0$3FeO3 can simultaneously affects the partial oxidation of methane, where CuO can lower the activation energy barrier of methane
Please cite this article as: Zhang R et al., The role of CuO modified La0$7Sr0$3FeO3 perovskite on intermediate-temperature partial oxidation of methane via chemical looping scheme, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.12.082
international journal of hydrogen energy xxx (xxxx) xxx
dissociation and La0$7Sr0$3FeO3 provides transportable lattice oxygen for partial oxidation reaction. In this study, five CuO modified La0$7Sr0$3FeO3 oxygen carriers with different content of CuO were prepared by solgel and consequent impregnation method. A series of characterization techniques including N2 physical adsorptiondesorption, ICP-OES, H2-TPR, XRD and XPS were implemented to determine physical and chemical properties of xCuO/LSF oxygen carriers. The interaction between CuO and La0$7Sr0$3FeO3 was confirmed by experiments. Subsequent fixed-bed reaction indicated that the methane conversion rate of 10CuO/LSF was more than 3 times compared to that of unmodified LSF and the concentration of syngas increased by 375% with the n(H2)/n(CO) of about 2.5. The reaction mechanism was analyzed finally.
3
Emission Spectrometer (ICP-OES). The sample was digested in an aqua regia solution before measurement.
H2-tempreture programmed reduction (H2-TPR) Reduction reactivity of the fresh samples were studied using H2-temperature programmed reduction (H2-TPR) (Quantachrome ChemStar™). Each sample was weighted 100 mg ± 5 mg and placed in a U-shape quartz tube. The sample was first pretreated at 300 C for 1 h under the Ar flow of 30 ml/min and then cooled down to the ambient temperature. Reduction step was conducted under hydrogen gas mixture flow (10 vol% H2 in He) of 30 ml/min, at a heating rate of 10 C/min until 900 C. Hydrogen consumption was measured by a thermal conductivity detector (TCD).
X-ray diffraction (XRD)
Experiment Synthesis of oxygen carriers A modified sol-gel method (the Pechini method) was used for the preparation La0$7Sr0$3FeO3 in a pure perovskite structure [38]. Stoichiometric amounts of La(NO3)3$6H2O, Sr(NO3)2 and Fe(NO3)3$9H2O were weighted and dissolved in deionized water under constant stirring at 60 C to form a nitrate solution. EDTA, citric acid and aqueous ammonia were mixed under continuous stirring at 60 C at a molar ratio of total metal ions, EDTA and citric acid at 1:1:1.5. The prepared nitrate solution was gradually dropped into the EDTA-citricNH3$H2O solution, which was mixed, stirred and heated to 80 C until a gel was formed. The gel was then dried at 120 C for 24 h followed by the calcination at 900 C for 4 h using a heating ramp at 10 C/min until the La0$7Sr0$3FeO3 perovskite powder was obtain. It was labeled as LSF. Respective amounts of Cu(NO3)3$xH2O were weighted and mixed with La0$7Sr0$3FeO3 in 10 ml deionized water. The mixtures were placed in an ultrasonic bath for 2 h, and then quickly dried in air and further calcined at 900 C in an oven for 2 h to obtain the CuO-modified LSF. The ratios of CuO and LSF were theoretically controlled at 2 wt%, 5 wt%, 10 wt% and 15 wt% CuO, respectively. They were labeled as 2CuO/LSF, 5CuO/LSF, 10CuO/LSF and 15CuO/LSF, respectively.
X-ray diffraction (XRD) was carried out to identify the structural features and compositions of fresh, reduced and after redox cycles samples using PANalytical X’ Pert Pro MPD with Cu Ka (l ¼ 0.154 nm). The operating voltage and current were 40 kV and 40 mA, respectively. The 2 theta degree ranged from 5 to 80 at a scanning rate of 10 /min.
X-ray photoelectron spectroscopy (XPS) X-ray photoelectron spectroscopy (XPS, ThermoFisher Scientific ESCALAB 250Xi) was used to obtain valence states of metal ions and different oxygen species of oxygen carriers. The X-ray source was a monochromatic Al Ka (1486.6 eV photons). The operating voltage and current were 20 kV and 10 mA, respectively.
Reactivity test Fixed bed reactor, made of quartz tube, was used in tests of the partial oxidation methane using prepared oxygen carriers, as shown in Fig. 2. The fresh oxygen carrier of 2.0 g was placed in the middle of fixed bed reactor, in which the thermocouple was inserted and its tip was close to the sample so as to detect the sample temperature accurately. Prior to the methane conversion, the oxygen carrier was heated at a heating rate of 10 C/ min in a 100 ml/min Ar flow until 750 C, and followed by 50 ml/ min CH4 mixture (40 vol% CH4 in N2) flow into reactor. The gaseous products were collected using gas sampling bags and subject to analysis using an Agilent 7890 A gas chromatography.
Characterizations N2 physical adsorption-desorption N2 physisorption was applied to obtain the specific surface area and total pore volume using Quadrasorb SI-MP-10. Each sample was degassed at 300 C for more than 12 h before its measurement. The BET method was used for calculating the surface area of each sample, while the amount of adsorbed nitrogen at maximum p/p0 was used for total pore volume.
Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES) The actual CuO content of fresh samples was measured using PerkinElmer OPTIMA 8000 Inductively Coupled Plasma Optical
Fig. 2 e Schematic diagram of fixed bed reactor.
Please cite this article as: Zhang R et al., The role of CuO modified La0$7Sr0$3FeO3 perovskite on intermediate-temperature partial oxidation of methane via chemical looping scheme, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.12.082
4
international journal of hydrogen energy xxx (xxxx) xxx
The conversion rate of CH4, the molar ratios of H2/CO and CO/CO2 are defined as following equations (1)e(3): CH4 conv ¼
MCH4;in MCH4;out MCH4;in
(1)
nðH2 Þ = nðCOÞ ¼ MH2 = MCO
(2)
nðCOÞ = nðCO2 Þ ¼ MCO = MCO2
(3)
where MCH4;in refers to the mole of introduced CH4, and MCH4;out , MH2 , MCO and MCO2 refer to the mole of CH4, H2, CO and CO2 in the exhaust gas. In terms of the redox cycle procedure, 50 ml/min Air was injected into the reactor for oxidizing the reduced oxygen carrier to its original state. Between each reduction and oxidation stages, 100 ml/min Ar was purged into the reactor for 20 min to prevent the mixing of combustible gas and oxygen.
Results and discussion Characterization of fresh oxygen carriers Fig. 3 showed the XRD patterns of the freshly prepared CuO modified LSF (xCuO/LSF). It was noticeable that the diffraction pattern of the LSF corresponded to an orthorhombic perovskite phase (PDF 01-089-1269) and no peaks to La2O3, SrO and Fe2O3, indicating a pure perovskite crystal structure of LSF in the freshly prepared CuO modified LSF. The characteristic
diffraction peaks of CuO (PDF 01-089-5895) could be observed in the freshly prepared CuO modified LSF oxygen carriers, revealing the occurrence of the pure CuO crystal on LSF surface. In addition, the intensity of the CuO diffraction peak was related to the content of copper doping (xCuO/LSF, x ¼ 2,5,10,15), and becomes stronger as the copper content increased. Table 1 showed physical properties of the freshly prepared CuO modified LSF, such as the specific surface area, the total pore volume and the actual weight percentage of CuO. The specific surface area of the LSF perovskite alone was approximately 6 m2/g (with a corresponding pore volume of about 15 103 cm3/g), which was consistent to those reported by open publications. A decrease in both the specific surface area and total pore volume of 2CuO/LSF was observed compared to those of the pure LSF perovskite, very likely attributing to the CuO coverage on the surface of the LSF perovskite. For xCuO/ LSF (x ¼ 2,5), their specific surface area remained almost unchanged as the CuO content increased, while the total pore volume kept increasing, likely attributing to the growth of CuO on LSF perovskite surface form a larger pore size structure. The further increase of copper doping amount to 15 wt% decreased its specific surface area and total pore volume, corresponding to the slight aggregation of CuO. Overall, the copper doping seemed less significantly changed the specific surface area and the total pore volume of the CuO-modified LSF, assuming less likely affected their reactivity. Actual CuO content in the CuO-modified LSF was also listed in Table 1. The H2-TPR profiles of prepared samples were shown in Fig. 4. H2-TPR results showed their reducibility and the interaction between CuO and LSF. The pure LSF exhibited two distinct reduction peaks ranging from 300 to 400 C and 750e900 C, attributing well to two different kinds of oxygen species, adsorbed oxygen and lattice oxygen existing in the LSF oxygen carrier [32,39]. The H2 consumption of first reduction peak could be corresponded to the adsorbed oxygen, while the second to the lattice oxygen. This seemed not change after the copper doping, only the first peak shifting toward lower temperature ranging within 280e300 C was observed, implying the improvement of adsorbed oxygen reactivity. Meanwhile, the intensity of the first reduction peak
Table 1 e Physical properties of prepared oxygen carriers. Oxygen carrier
LSF 2CuO/LSF 5CuO/LSF 10CuO/ LSF 15CuO/ LSF
Fig. 3 e XRD spectra of fresh oxygen carriers with different CuO content.
Surface areaa (m2/g)
Total pore volumeb ( 103 cm3/g)
6.681 5.667 5.543 5.757
15.42 11.66 14.97 17.03
e 2 5 10
e 1.75 4.45 8.65
5.336
14.61
15
13.08
Amount of copper oxide (weight %) Theoretical Actualc
Note: a Calculated from the adsorption isotherm by using BET method. b Calculated from the amount of adsorbate at the maximum p/p0. c Measured by ICP-OES.
Please cite this article as: Zhang R et al., The role of CuO modified La0$7Sr0$3FeO3 perovskite on intermediate-temperature partial oxidation of methane via chemical looping scheme, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.12.082
international journal of hydrogen energy xxx (xxxx) xxx
Fig. 4 e H2-TPR curves of oxygen carriers.
constantly increased with the increase of CuO content, indicating a strong correlation between the doped CuO and the first reduction peaks in xCuO/LSF (x ¼ 2,5,10,15) oxygen carriers. The shift of the doped CuO reduction peak to the higher temperature compared to that of the pure CuO implied that the interaction between CuO and LSF perovskite. As for 15CuO/LSF, only a smaller and broader peak was within 320 C and 420 C, implying a decrease of adsorbed oxygen and weakness of the interaction between CuO and LSF perovskite by the excessive copper doping. Present study revealed the increase of the content of adsorbed oxygen decreased the selectivity to syngas because the adsorbed oxygen was more prone to completely oxidize CH4 to CO2 and H2O. Thus, it was important to determine an appropriate amount of copper doping that can benefit both the reactivity of oxygen carriers and the selectivity to syngas. There were no significant changes in the second reduction peak of oxygen carriers after copper doping, indicating less correlation between the doped CuO and the lattice oxygen.
Reactivity of oxygen carriers in chemical-looping partial oxidation The methane conversion on the oxygen carriers followed three main reaction pathways: (I) methane complete oxidation CH4/CO2þH2O, (II) methane partial oxidation CH4/CO þ H2 and (III) methane cracking CH4/C þ H2. The CH4 conv corresponds to methane reactivity on the oxygen carriers. The n(H2)/n(CO) can reflect the respective proportion of reaction pathway (II) and (III). The higher the n(H2)/n(CO) is, the more the methane cracking instead of partially oxidized toward syngas. The n(CO)/n(CO2) reflects the respective proportion of (I) and (II). The higher n(CO)/n(CO2) is, the more methane is partially oxidized instead of complete oxidization. Overall, CH4 conv, n(H2)/n(CO) and n(CO)/n(CO2) are three
5
major parameters to evaluate performance of oxygen carriers, as shown in Fig. 5. Referring to Fig. 5(a), (c), all five oxygen carriers showed fairly high reactivity at the initial reaction stage (within 0e8 min) and the n(CO)/n(CO2) was quite close to 0, implying the complete oxidization of CH4 toward CO2 and H2O by adsorbed oxygen. This was also accompanied by drastic decrease of the CH4 conv on account of the rapidly exhausted adsorbed oxygen. Within 8 min of reaction, the CH4 conv of LSF and 2CuO/LSF remained at a considerably low level and continuously decreased. The CH4 conv of 5CuO/LSF started to increase at 16 min. But the CH4 conv of xCuO/LSF (x ¼ 10,15) increased immediately after the initial stage. It can be concluded that the reactivity between CH4 and oxygen carriers increased as the CuO content increased, and the 15CuO/ LSF exhibited a highest methane conversion rate. The copper doping had a significant promotion on the methane conversion over the LSF perovskite oxygen carriers. However, excessive CuO would negatively affected on the methane partial oxidation. On the other hand, the production of syngas in the n(H2)/n(CO) within 2 and 3 was desirable for the subsequent chemical synthesis process. Fig. 5(b), (c) showed that the n(H2)/n(CO) of xCuO/LSF (x ¼ 10,15) were within 2 and 3 where the reaction was in a tie window of 12e22 min and 8e18 min, respectively. For the 10CuO/LSF, its n(CO)/n(CO2) was lower at 12 min but rapidly increased over time; yet for the 15CuO/LSF, its n(CO)/n(CO2) was kept at a low level within 8e10 min and the increment of its n(CO)/n(CO2) was lower than 10CuO/LSF. Thus, 15CuO/LSF produced more CO2 instead of syngas compared to 10CuO/LSF. This also coincided with the conclusion derived from H2-TPR tests in that 15CuO/LSF contained more adsorbed oxygen than 10CuO/ LSF. In addition, the 15CuO/LSF also produced more CO2 than 10CuO/LSF at the initial reaction stage, owing to the higher adsorbed oxygen in 15CuO/LSF. More CO2 meant more energy input for the following separation, also reduced the selectivity of syngas. Therefore, 15CuO/LSF produced less high-quality syngas than 10CuO/LSF. Considering both parameters of preferably higher the methane conversion rate and the selectivity of syngas, 10CuO/LSF seemed the best performed in both parameters in this study. Consequently, it was utilized in the subsequent successive redox cycle. The crystal phase changes of five oxygen carriers of methane reduction in the fixed bed after 30 min were shown in Fig. 6. Both LSF and 2CuO/LSF showed no structural change compared to their original states, corresponding to their extremely low CH4 conversion rate. For the xCuO/LSF (x ¼ 5,10,15), new crystal phases were detected in all these three samples after their reduction, which involved La2CuO4 (PDF 00-038-0709) and Copper (PDF 03-065-9026). The existence of La2CuO4 clearly exhibited the interaction between Cu and LSF. The combination of La and Cu caused left Sr and Fe to form SrFeO3-x, which introduced more oxygen defects and vacancies in its bulk to improve the lattice oxygen mobility and accordingly to enhance the reactivity between CH4 and the oxygen carriers. Simultaneously, the main crystal structures of xCuO/ LSF (x ¼ 5,10,15) could still keep its orthorhombic phase, owing to the strong skeleton formed by La3þ and Sr2þ ions, which showed a higher structural stability and regenerability.
Please cite this article as: Zhang R et al., The role of CuO modified La0$7Sr0$3FeO3 perovskite on intermediate-temperature partial oxidation of methane via chemical looping scheme, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.12.082
6
international journal of hydrogen energy xxx (xxxx) xxx
Fig. 5 e Reactivity performance of different oxygen carriers under 750 C. (a)CH4 conv; (b)n(H2)/n(CO); (c)n(CO)/n(CO2).
Fig. 6 e XRD spectra of reduced oxygen carriers with different CuO content.
The cyclic performance of oxygen carrier is essential for chemical-looping process. The successive redox cycle was performed to evaluate the durability of 10CuO/LSF which was determined to be the best candidate among the synthesized samples. Each cycle involved a reduction stage and an oxidation stage and both were conducted at 750 C. The initial test with 10CuO/LSF in the fixed bed helped in determining the setting of the reduction time to be 22 min to keep the appropriate n(H2)/n(CO), and the time setting of the oxidation stage was 20 min. Fig. 7 showed the evolvement of CO2, CO and H2 at every reduction stage to identify the cyclic performance of 10CuO/LSF, using that of LSF in the slashed area as a reference. The sample 10CuO/LSF showed a favorable cyclic stability within ten cycles. Its syngas yield decreased only slightly from 1st to 6th cycle and kept steady then. The average methane conversion rate of 10CuO/LSF was 21.6% in ten cycles, almost 3 times higher than that of the pure LSF of 7.1%. The average concentration of syngas increased by 375% compared to that of unmodified LSF. Significantly, the n(H2)/ n(CO) of 10CuO/LSF in ten cycles was maintained at about 2.5, while the n(H2)/n(CO) of LSF was close to 4, indicating a positive effect of the copper doping on the methane partial oxidation to syngas at intermediate temperature. Fig. 8 showed the comparison of XRD patterns between fresh and used 10CuO/LSF after ten cycles. The diffraction peak positions of LSF perovskite and CuO were basically unchanged, indicating the crystalline structure of the used 10CuO/LSF
Please cite this article as: Zhang R et al., The role of CuO modified La0$7Sr0$3FeO3 perovskite on intermediate-temperature partial oxidation of methane via chemical looping scheme, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.12.082
international journal of hydrogen energy xxx (xxxx) xxx
7
Fig. 7 e Cyclic performance of 10CuO/LSF in ten redox cycles.
recovered to its original state. This result proved good regenerability of 10CuO/LSF according to XRD results of perovskite oxygen carriers reported in other literature [40]. However, the peak intensities of LSF perovskite and CuO decreased, mainly because some copper atoms incorporated into LSF perovskite crystalline, decreasing the crystallinity of LSF perovskite. The reducing of the methane conversion rate might be caused by the loss of surface CuO.
respectively. The binding energy at around 528.8 eV could be labeled as OI, which is associated with metal oxides (lattice oxygen). The binding energy peak at around 531.2 eV is probably from metal carbonate species. The binding energy peak at around 533.7 eV, which is attributed to metal hydroxide or adsorbed molecular water on surface, is usually
Evolution of various elements in oxygen carriers All XPS spectra were calibrated by C1s standard binding energy 248.8 eV. The XPS spectra of O1s for xCuO/LSF (x ¼ 2,5,10,15) are shown in Fig. 9. Each O1s spectrum was fitted by three peaks and labeled as OI, OII and OIII,
Fig. 8 e XRD spectra of 10CuO/LSF: fresh and after cycling.
Fig. 9 e O1s spectra of fresh xCuO/LSF (x ¼ 2,5,10,15) oxygen carriers.
Please cite this article as: Zhang R et al., The role of CuO modified La0$7Sr0$3FeO3 perovskite on intermediate-temperature partial oxidation of methane via chemical looping scheme, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.12.082
8
international journal of hydrogen energy xxx (xxxx) xxx
classified as adsorbed oxygen [41]. As reported in literature, OI was related to the partial oxidation of methane, while OII and OIII were more likely involved in the complete oxidation of methane [40]. Table 2 shows fractions of different oxygen species in xCuO/LSF (x ¼ 2,5,10,15). When the value of x is increased from 2 to 10, the corresponding lattice oxygen contents in the oxygen carriers were 51.70%, 47.80%, 44.43% and 40.59%, respectively. The increase of CuO content gradually decreased the proportion of lattice oxygen and increased that of adsorbed oxygen, which is consistent to results of H2TPR and fixed bed experiments. As shown in Fig. 10, the sample exhibited a lower OI peak and higher OII peak after the reduction stage, indicating the consumption of lattice oxygen and the formation of metal carbonates. The OI peak can recover its original state after reoxidation by air. Changes in chemical state of various metal elements (Cu, La, Sr and Fe) at different reaction stages are displayed in Fig. 11. The was a strong Cu2þ satellite peak shown in Fig. 11(a), implying presence of Cu(II) in all three stages (fresh, reduced and after cycled) in the form of CuO in fresh and after cycled samples and La2CuO4 in reduced sample. Simultaneously, the Cu2p3/2 binding energy of the reduced sample shifts to lower position (from 934.0 eV to 933.1 eV), indicating that a fraction of Cu(II) is reduced to Cu. The intensity of Cu2p peak became smaller after many cycles, probably attributing to the migration of CuO phase from the LSF surface into its inner part and explaining the decreased of the reactivity of 10CuO/LSF after several cycles. In Fig. 11(b), La3d region had well separated spin-orbit components, each of which can be further split by multiplet splitting. The magnitude of the multiplet splitting and intensity ratio of each multiplet-split component were chemically diagnostic. The comparison of DE and the relative intensity of La3d5/2 doublet peak revealed that La in both the fresh and 10Cu/LSF after cycled mainly occurred in the form of a metal oxide (DE ¼ 4.4 eV), but La2(CO3)3 (DE ¼ 3.5 eV) appeared in reduced sample. Reaction with methane made carbon adsorption on lanthanum to form surface carbonates. Sr3d region has well resolved spin-orbit components (DE ¼ 1.76 eV). When its multiplet chemical states presented, apparent resolution of these two spin-orbit components may vary depending on concentrations. Strontium in LSF phase and strontium carbonate are represented by blue and red color, respectively. The ratio of strontium in LSF and SrCO3 can be determined by peak areas of different colors. The fresh sample surface contains 45.77% strontium in LSF and 54.23% SrCO3. After methane reduction, strontium in LSF and SrCO3 turned to be 25.49% and 74.51%. These two different kinds
Table 2 e Different oxygen species for the xCuO/LSF (x ¼ 2,5,10,15) measured from XPS. Oxygen carriers
2CuO/LSF 5CuO/LSF 10CuO/LSF 15CuO/LSF
Peaks areas (%) OI (metal oxides)
OII (metal carbonates)
OIII (metal hydroxides)
51.70 47.80 44.43 40.59
45.27 47.90 50.69 56.01
3.03 4.30 4.88 3.40
Fig. 10 e O1s spectra of 10CuO/LSF at different stages.
of strontium returned to their original states, containing 42.53% and 57.47% after many redox cycles. This was a clear message on carbon adsorption on strontium to form more strontium carbonates in the process of reacting with methane. The XPS spectra of Fe2p at different stages are shown in Fig. 11(d). Fe(III) compounds are always high-spin, leading to complex multiplet and having satellite features. Fitting Fe2p spectra of Fe3þ (EB ¼ 710.8 eV) and Fe4þ (EB ¼ 713.1 eV) revealed their proportion were 61.91% and 38.09%, respectively. A doping of Sr in A site made the partial transformation of Fe3þ to Fe4þ in B site, resulting in an increase of the oxygen vacancies and the enhancement of the lattice oxygen mobility. After reduction, the proportion of Fe2þ, Fe3þ and Fe4þ changed to be 20.48%, 41.45% and 38.07%, respectively. Methane reduces a part of Fe3þ/Fe4þ to Fe2þ, but it is difficult to reduce Fe2þ to lower valences. Simultaneously, the existence of La2CuO4 phase resulted in more SrFeO3-x phase as described above, causing an increase in the Fe4þ content and thus further produced more oxygen vacancies and improved the oxygen mobility. After many redox cycles, the chemical states of Fe recovered to its original state where the proportion of Fe3þ and Fe4þ was found to be 60.40% and 39.60%, respectively, showing a superior cyclic performance.
Reaction mechanism The mechanism on promotion effect of copper oxide is shown in Fig. 12. Copper oxide acted as active center to considerably facilitate the dissociation of methane in the initial stage. The
Please cite this article as: Zhang R et al., The role of CuO modified La0$7Sr0$3FeO3 perovskite on intermediate-temperature partial oxidation of methane via chemical looping scheme, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.12.082
international journal of hydrogen energy xxx (xxxx) xxx
9
Fig. 11 e Various metal elements spectra of 10CuO/LSF at different stages. (a)Cu2p; (b)La3d; (c)Sr3d; (d)Fe2p.
Fig. 12 e Schematic diagram of reaction mechanism in various stages.
Please cite this article as: Zhang R et al., The role of CuO modified La0$7Sr0$3FeO3 perovskite on intermediate-temperature partial oxidation of methane via chemical looping scheme, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.12.082
10
international journal of hydrogen energy xxx (xxxx) xxx
first-principles calculations in literature [42] revealed that the activation barrier of CeH bonds of methane was diminished to 60.5 kJ/mol on the high-energy CuO (010) surface and to 76.6 kJ/mol on the most stable CuO (111) surface compared to ~440 kJ/mol without any surfaces. Even on a clean Cu (111) surface, the activation energy barrier can be as low as 169.8 kJ/ mol. Copper and its oxide could promote the dissociation of methane. Simultaneously, the interaction between copper and LSF makes the formation of La2CuO4 and SrFeO3-x which contained more oxygen vacancies and further improved the reactivity of oxygen carriers. Dissociated methane was partially oxidized by the lattice oxygen to generate syngas. As the reaction proceeded, the formation of lanthanum and strontium carbonates was observed on the surface of oxygen carrier. The increasing coverage of these carbonates resulted in gradual declining in oxidizing capacity of oxygen carrier and increasing the share of methane thermal-cracking. Meanwhile, the lattice oxygen in the LSF bulk was constantly lost, and a portion of Fe3þ/Fe4þ in the oxygen carrier was reduced to Fe2þ. The combination of the dissociation of methane by CuO and the migration of LSF lattice oxygen highly improved the reactivity and selectivity of oxygen carrier.
Conclusion In summary, this work studied the reactivity improvement of La0$7Sr0$3FeO3 (LSF) with the copper modification in an intermediate-temperature partial oxidation of methane via a chemical-looping scheme. Results indicated that CuO crystal grew on the surface of as-prepared CuO modified LSF, which still kept its intact perovskite structure. The H2-TPR experiment confirmed the occurrence of interaction between CuO and LSF, and CuO could activate the adsorbed oxygen in LSF phase. The most suitable CuO doping content was found 10% that was confirmed in view of both the conversion rate and syngas selectivity of methane in the fixed bed experiment. Fixed-bed reaction indicated that the methane conversion rate of 10CuO/LSF was more than 3 times compared to that of unmodified LSF and the concentration of syngas increased by 375% with the n(H2)/n(CO) of about 2.5. The increase in methane conversion rate attributes to the function of CuO for lowering the activation energy barrier of CeH bond in CH4 molecule. Thus, the improvement of the reactivity of at a relatively low temperature. Given the ideal H2/CO ratio is 2, results further exhibited the CuO modified LSF helping in the domination of the partial oxidation of methane and elimination of methane cracking. This was mainly attributed to the formation of La2CuO4, which induced more oxygen vacancies and promoted the oxygen mobility. Further studies confirmed the good cyclic performance of the prepared 10CuO/LSF over 10 redox cycles. Several redox cycles only slightly decreased reactivity of 10CuO/LSF as a result of the CuO migration from surface to interior of LSF crystal. The Cumodification method provided a promising and simple pathway on the preparation of oxygen carrier to decrease the reaction temperature of partial oxidation of methane via a chemical-looping scheme.
Acknowledgement The financial support of National Key Research and Development Program of China [2017YFE0105500] is gratefully acknowledged. This work was also supported by the National Natural Science Foundation of China [51876205] and the Pearl River S&T Nova Program of Guangzhou [201906010092].
references
[1] Wahlen M. The global methane cycle. Annu Rev Earth Planet Sci 1993;21:407e26. [2] Barone G, Chianese E. Hydrates of natural gases and small molecules: structures, properties, and exploitation perspectives. ChemSusChem 2009;2:992e1008. [3] Salkuyeh YK, Saville BA, MacLean HL. Techno-economic analysis and life cycle assessment of hydrogen production from different biomass gasification processes. Int J Hydrogen Energy 2018;43:9514e28. [4] Lunsford JH. Catalytic conversion of methane to more useful chemicals and fuels: a challenge for the 21st century. Catal Today 2000;63:165e74. [5] Sonal, Ahmad E, Upadhyayula S, Pant KK. Biomass-derived CO2 rich syngas conversion to higher hydrocarbon via Fischer-Tropsch process Chao over Fe-Co bimetallic catalyst. Int J Hydrogen Energy 2019;44:27741e8. [6] Shah YT, Gardner TH. Dry reforming of hydrocarbon feedstocks. Catal Rev Sci Eng 2014;56:476e536. [7] Sokolovskii VD, Coville NJ, Parmaliana A, Eskendirov I, Makoa M. Methane partial oxidation. Challenge and perspective. Catal Today 1998;42:191e5. [8] Xu JG, Froment GF. Methane steam reforming, methanation and water-gas shift .1. Intrinsic kinetics. AIChE J 1989;35:88e96. [9] Sengodan S, Lan R, Humphreys J, Du DW, Xu W, Wang HT, et al. Advances in reforming and partial oxidation of hydrocarbons for hydrogen production and fuel cell applications. Renew Sustain Energy Rev 2018;82:761e80. [10] Zeng L, Cheng Z, Fan JA, Fan LS, Gong JL. Metal oxide redox chemistry for chemical looping processes. Nat Rev Chem 2018;2:349e64. [11] Luo SW, Zeng L, Fan LS. Chemical looping technology: oxygen carrier characteristics. In: Prausnitz JM, editor. Annual review of chemical and biomolecular engineering, vol. 6. Palo Alto: Annual Reviews; 2015. p. 53e75. [12] Voldsund M, Jordal K, Anantharaman R. Hydrogen production with CO2 capture. Int J Hydrogen Energy 2016;41:4969e92. [13] Tang MC, Xu L, Fan MH. Progress in oxygen carrier development of methane-based chemical-looping reforming: a review. Appl Energy 2015;151:143e56. [14] Cheng Z, Qin L, Fan JA, Fan LS. New insight into the development of oxygen carrier materials for chemical looping systems. Engineering 2018;4:343e51. € rje. Integrated [15] Zafar Qamar, Mattisson Tobias, Gevert Bo hydrogen and power production with CO2 capture using chemical-looping reforming-redox reactivity of particles of CuO, Mn2O3, NiO, and Fe2O3 using SiO2 as a support. Ind Eng Chem Res 2005;44:3485e96. nez J, Garcı´a-Labiano F, Abad A, [16] Diego LFD, Ortiz M, Ada n P. Synthesis gas generation by chemical-looping Gaya reforming in a batch fluidized bed reactor using Ni-based oxygen carriers. Chem Eng J 2008;144:289e98.
Please cite this article as: Zhang R et al., The role of CuO modified La0$7Sr0$3FeO3 perovskite on intermediate-temperature partial oxidation of methane via chemical looping scheme, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.12.082
international journal of hydrogen energy xxx (xxxx) xxx
n M, Lyngfelt A, Mattisson T. Synthesis gas generation [17] Ryde by chemical-looping reforming in a continuously operating laboratory reactor. Fuel 2006;85:1631e41. [18] Johansson M, Mattisson T, Lyngfelt A, Abad A. Using continuous and pulse experiments to compare two promising nickel-based oxygen carriers for use in chemicallooping technologies. Fuel 2008;87:988e1001. [19] Antzara A, Heracleous E, Silvester L, Bukur DB, Lemonidou AA. Activity study of NiO-based oxygen carriers in chemical looping steam methane reforming. Catal Today 2016;272:32e41. [20] Hafizi A, Rahimpour MR, Heravi M. Experimental investigation of improved calcium-based CO2 sorbent and Co3O4/SiO2 oxygen carrier for clean production of hydrogen in sorption-enhanced chemical looping reforming. Int J Hydrogen Energy 2019;44:17863e77. [21] Li DL, Nakagawa Y, Tomishige K. Methane reforming to synthesis gas over Ni catalysts modified with noble metals. Appl Catal A-Gen. 2011;408:1e24. [22] Zhu Xing, Wang Hua, Wei Yonggang, Li Kongzhai, Cheng X. Hydrogen and syngas production from two-step steam reforming of methane using CeO2 as oxygen carrier. J Nat Gas Chem 2010;28:281e6. [23] He F, Wei Y, Li H, Wang H, He F, Wei Y, et al. Synthesis gas generation by chemical-looping reforming using Ce-based oxygen carriers modified with Fe, Cu, and Mn oxides. Energy Fuel 2009;23:2095e102. [24] Kongzhai LI, Wang H, Wei Y, Liu M. Preparation and characterization of Ce1-xFexO2 complex oxides and its catalytic activity for methane selective oxidation. J Rare Earths 2008;26:245e9. [25] Li K, Hua W, Wei Y, Yan D. Syngas production from methane and air via a redox process using CeeFe mixed oxides as oxygen carriers. Appl Catal B Environ 2010;97:361e72. [26] Xing Z, Wei Y, Hua W, Li K. CeeFe oxygen carriers for chemical-looping steam methane reforming. Int J Hydrogen Energy 2013;38:4492e501. [27] Zheng Y, Li K, Hua W, Xing Z, Wei Y, Min Z, et al. Enhanced activity of CeO2eZrO2 solid solutions for chemical-looping reforming of methane via tuning the macroporous structure. Energy Fuel 2016;30. acs.energyfuels.5b02151. [28] Wei Y, Wang H, Li K. Ce-Fe-O mixed oxide as oxygen carrier for the direct partial oxidation of methane to syngas. J Rare Earths 2010;28:560e5. [29] O’Connell M, Norman AK, Hu¨ttermann CF, Morris MA. Catalytic oxidation over lanthanum-transition metal perovskite materials. Catal Today 1999;47:123e32. [30] Ciambelli P, Cimino S, De Rossi S, Lisi L, Minelli G, Porta P, et al. AFeO(3) (A ¼ La, Nd, Sm) and LaFe1-xMgxO3 perovskites as methane combustion and CO oxidation catalysts:
[31]
[32]
[33]
[34]
[35]
[36]
[37]
[38]
[39]
[40]
[41]
[42]
11
structural, redox and catalytic properties. Appl Catal B Environ 2001;29:239e50. Zhao K, He F, Huang Z, Zheng AQ, Li HB, Zhao ZL. La1xSrxFeO3 perovskites as oxygen carriers for the partial oxidation of methane to syngas. Chin J Catal 2014;35:1196e205. He F, Li X, Zhao K, Huang Z, Wei G, Li H. The use of La1xSrxFeO3 perovskite-type oxides as oxygen carriers in chemical-looping reforming of methane. Fuel 2013;108:465e73. Li RJ, Yu CC, Ji WJ, Shen SK. Methane oxidation to synthesis gas using lattice oxygen in La1xSrxFeO3 perovskite oxides instead of molecular oxygen. Stud Surf Sci Catal 2004;147:199e204. Yang S, Zhao K, Fang H, Li H. The structure-reactivity relationships of using three-dimensionally ordered macroporous LaFe 1x Ni x O 3 perovskites for chemicallooping steam methane reforming. J Energy Inst 2018;92(2):239e46. S1743967117309078. Zhao K, Fang H, Zhen H, Wei G, Zheng A, Li H, et al. Perovskite-type oxides LaFe 1 x Co x O 3 for chemical looping steam methane reforming to syngas and hydrogen co-production. Appl Energy 2016;168:193e203. Melo VRM, Medeiros R, Braga RM, Macedo HP, Ruiz JAC, Moure GT, et al. Study of the reactivity of Double-perovskite type oxide La1-xMxNiO4 (M ¼ Ca or Sr) for chemical looping hydrogen production. Int J Hydrogen Energy 2018;43:1406e14. Niu TC, Jiang Z, Zhu YG, Zhou GW, van Spronsen MA, Tenney SA, et al. Oxygen-promoted methane activation on copper. J Phys Chem B 2018;122:855e63. Chen Y, Galinsky N, Wang Z, Li F. Investigation of perovskite supported composite oxides for chemical looping conversion of syngas. Fuel 2014;134:521e30. Zheng Y, Li K, Wang H, Tian D, Wang Y, Zhu X, et al. Designed oxygen carriers from macroporous LaFeO 3 supported CeO 2 for chemical-looping reforming of methane. Appl Catal B Environ 2017;202:51e63. Zhao K, Zheng AQ, Li HB, He F, Huang Z, Wei GQ, et al. Exploration of the mechanism of chemical looping steam methane reforming using double perovskite-type oxides La1.6Sr0.4FeCoO6. Appl Catal B Environ 2017;219:672e82. Ding HR, Xu YQ, Luo C, Wang QY, Shen C, Xu JX, et al. A novel composite perovskite-based material for chemical-looping steam methane reforming to hydrogen and syngas. Energy Convers Manag 2018;171:12e9. Varghese JJ, Trinh QT, Mushrif SH. Insights into the synergistic role of metalelattice oxygen site pairs in fourcentered CeH bond activation of methane: the case of CuO. Catal Sci Technol 2016;6:3984e96.
Please cite this article as: Zhang R et al., The role of CuO modified La0$7Sr0$3FeO3 perovskite on intermediate-temperature partial oxidation of methane via chemical looping scheme, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.12.082