MgO modified perovskite type oxides for chemical-looping steam reforming of methane

JOURNAL OF FUEL CHEMISTRY AND TECHNOLOGY Volume 44, Issue 6, June 2016 Online English edition of the Chinese language journal Cite this article as: J Fuel Chem Technol, 2016, 44(6), 680688

RESEARCH PAPER

CaO/MgO modified perovskite type oxides for chemical-looping steam reforming of methane ZHAO Kun1,2, HE Fang1,*, HUANG Zhen1, WEI Guo-qiang1, ZHENG An-qing1, LI Hai-bin1, ZHAO Zeng-li1 1

Key Laboratory of Renewable Energy, Chinese Academy of Sciences; Guangdong Key Laboratory of New and Renewable Energy Research

and Development; Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640,China; 2

University of Chinese Academy of Sciences, Beijing 100049, China

Abstract:

Chemical-looping steam methane reforming (CL-SMR) is a novel method proposed on the base of chemical looping

combustion (CLC) technology. In the CL-SMR scheme, methane is partially oxidized to syngas (H2/CO=2.0) by the lattice oxygen in reformer reactor in the absence of gaseous oxidant, and then the reduced oxygen carrier is oxidized by steam to produce hydrogen in steam reactor. The use of perovskite type oxide LaFeO3 as an oxygen carrier in CL-SMR was studied. While the basicity of CaO/MgO modified oxygen carriers, LaFeO3-CaO and LaFeO3-MgO, were also synthesized aiming to increase specific surface area, thermostability, and resistance to coke formation. The synthesized oxides were characterized by X-ray diffraction (XRD), H2-temperature-programmed reduction (H2-TPR), Brunauer-Emmett-Teller (BET) surface area and X-ray photoelectron spectroscopy (XPS). Three oxygen carriers exhibited high active and selective for syngas production from methane, and maintained perovskite type over cyclic redox operations. The LF-CaO sample is the best candidate for the CL-SMR of the three samples judging from the reactivity, selectivity, and resistance to carbon formation. It showed good regenerability during 5 redox reactions. Keywords: chemical-looping; methane reforming; perovskite; CaO/MgO modified; redox

Methane, which is the main component of natural gas, is used directly or indirectly to produce value-added chemicals and fuels such as ethylene, methanol, gasoline, and diesel on account of its abundance storage. A hot strategy for elaboration of methane is to convert it into syngas (H2+CO), which, in turn, can be used as feedstock to produce liquid fuels or other chemicals through Fischer-Tropsch synthesis. For the traditional steam reforming of CH4 (SMR), syngas with a high H2 to CO molar ratio are produced, not being optimal for a number of desired products. The necessary downstream refinement is required to yield a syngas with suitable H2 to CO molar ratio of 2.0. Moreover, the stainless steel reactors with high temperature resistance used in this process are costly. On the other hand, hydrogen is an important feedstock for petroleum and chemical industry as a clean energy with high quality. Hydrogen production from fossil fuels such as natural gas or coal is the most competitive technology in the long run. Based on these, chemical-looping steam methane reforming (CL-SMR) is getting more and more attentions[1,2]. CL-SMR is a novel

technology to produce syngas and hydrogen respectively, as shown in Figure 1. In the CL-SMR scheme, methane is partially oxidized to syngas (H2/CO (molar ratio) =2.0) by the lattice oxygen of the oxygen carrier in the reformer reactor, and then the reduced oxygen carrier is re-oxidized by steam to produce hydrogen in the steam reactor. Syngas and hydrogen can be respectively produced via two steps, and the net reactions of CL-SMR can be illustrated as follows: Methane reduction: MexOy+CH4→MexOy–1+CO+2H2 (1) Steam oxidation: MexOy–1+H2O→MexOy+H2 (2) Net reactions: CH4+H2O→CO+3H2 (3) The effectiveness of CL-SMR is largely affected by performance of the oxygen carrier which plays an important role to transport oxygen and heat between the reformer and the steam reactor. The selection of oxygen carrier is very crucial. Among the various metal oxygen carriers, perovskite-type oxides have received increasing interest due to its high thermal stability, good mechanical properties, as well as decent activity.

Received: 07-Jan-2016; Revised: 07-Mar-2016. Foundation item: Support by National Natural Science Foundation of China (51406208, 51406214) and the Science & Technology Research Project of Guangdong Province (2013B050800008). *Corresponding author. E-mail: hefang@ ms.giec.ac.cn. Copyright  2016, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.

ZHAO Kun et al / Journal of Fuel Chemistry and Technology, 2016, 44(6): 680688

temperature-programmed reduction (H2-TPR), Brunauer-Emmett-Teller (BET) surface area and X-ray photoelectron spectroscopy (XPS). Then the reactivity of them was investigated in a fixed-bed reactor.

1 1.1

Fig. 1

CL-SMR for syngas and hydrogen production

Perovskite-type complex metal oxides are a series of mixed oxides with general formula ABO3, where A is usually a lanthanide ion and/or alkaline earth metal and B is a transition metal ion. There is a wide range modeling space of perovskite-type metal oxides due to its wide range of choices of A/B site metals. Li et al[3] used perovskite-type metal oxides such as La0.8Sr0.2FeO3 in CLR, and showed that CH4 can be partially oxidized to CO and H2 by the lattice oxygen. Then the reduced La0.8Sr0.2FeO3–δ could be re-oxidized by air to recover the lattice oxygen. Various perovskite-type metal oxides AFeO3 (A = La, Nd, Sm, Eu) have also been used for CLR[4,5] by the same research group. Results showed that they have good recycling capacity in terms of releasing and regaining lattice oxygen. Magnus et al[6] used four perovskite materials, three metal-oxide materials and four metal-oxide mixtures as oxygen carriers for chemical-looping applications, they found that LaxSr1–xFeO3–δ perovskites provided very high selectivity towards CO/H2 (molar ratio) and should be well suited for chemical-looping reforming. Substituting of La for Sr was found to increase the oxygen capacity of these materials, but reduced the selectivity towards CO/H2 (molar ratio) and the reactivity with CH4. Dai et al[7] compared LaFeO3, La0.8Sr0.2FeO3 and La0.8Sr0.2Fe0.9Co0.1O3 perovskite oxides as oxygen carrier for partial oxidation of methane. Methane was oxidized to syngas with high selectivity by oxygen species of perovskite oxides in the absence of gaseous oxygen, LaFeO3 and La0.8Sr0.2FeO3 exhibited excellent structural stability and continuous oxygen supply. But according to our previous work[8], perovskite-type oxides had a few disadvantages such as small specific surface area and weak resistance to carbon deposition. While doping basic metal oxides on it, these problems could be effectively resolved. In this work, LaFeO3 (LF) perovskite-type metal oxide, the basicity of CaO/MgO modified oxygen carriers, LaFeO3-CaO (LF-CaO) and LaFeO3-MgO (LF-MgO) were synthesized by co-precipitation method. The properties of the oxides were characterized by X-ray diffraction (XRD), H2

Experimental Synthesis of perovskite-type oxides

Perovskite-type oxides were prepared by coprecipitation method. For LaFeO3 the required amounts of La(NO3)3∙6H2O (AR 99%, Aladdin) and Fe(NO3)3∙9H2O (AR 99%, Aladdin) were weighed at a desired stoichiometric ratio and put into a beaker. Deionized water was added to make an aqueous solution A of the mixed nitrates. The co-precipitator (NH4)2CO3-NH4OH was dissolved in deionized water combined with ammonia water to make solution B which was slowly dripped into solution A to form sediment under a water bath at 50°C. The sediment was allowed to settle for 2 h and filtered. Then the sediment was dried overnight in a convection oven at 80°C. Finally, the as-prepared precursor was calcined at 400°C for 2 h and then heated to 850°C for 5 h. For LF-CaO/MgO, the CaO/MgO (AR 98%, Aladdin) was weighed at a desired stoichiometric ratio and dissolved in deionized water to form slurry. Then the excess precipitator (NH4)2CO3 (AR 40%, China) and nitrate solution (La(NO3)3∙6H2O and Fe(NO3)3∙9H2O) were added to CaO/MgO slurry in parallel flow with continuous stirring at 50°C. Next, the sediment was washed twice by deionized water and absolute ethyl alcohol respectively. The as-prepared precursor was calcined at 400°C for 2 h and then heated to 850°C and kept for 5 h. 1.2

Characterization

The crystal phases of the oxides were identified by XRD in a Japan Science D/max-R diffractometer with Cu K radiation (λ=0.15406 nm), operating voltage of 40 kV and current of 40 mA, and the diffraction angle (2θ) was scanned from 10°to 80°. The specific surface areas and average pore diameter of the prepared perovskites were measured at liquid nitrogen temperature by Brunauer-Emmett-Teller (BET) method (SI-MP-10/PoreMaster 33). The hydrogen-temperature programmed reduction (H2-TPR) experiments were conducted in 5.0% H2 balanced with helium at a flow rate of 60 mL/min from room temperature to 900°C with a heating rate of 10°C/min. X-ray photoelectron spectroscopy (XPS) was used to probe the near-surface composition of the oxides. The equipment was Shimadzu Amicus with a Al K radiation source (hv=1486.8 eV) at an

ZHAO Kun et al / Journal of Fuel Chemistry and Technology, 2016, 44(6): 680688

operating voltage of 15 kV and a current of 10 mA, a 500 m spot area, and a chamber with a base pressure 2×10–10 kPa. 1.3

Reactivity tests

CL-SMR reactivity evaluation was carried out in a fixed-bed quartz reactor under atmospheric pressure, which was reported in our previous work[9]. The fixed-bed reactor was a quartz tube with a baffle in the middle part that only allows gas to get through. The quartz tube was 30 mm in diameter and 700 mm in length. For methane conversion step, 2.0 g of the oxides (particle size 80–100 mesh) were placed on the baffle for each test. Prior to the reaction, the oxygen carrier was purged in N2 at 300°C for 30 min and then was heated up to 850°C at 10°C/min. A mixture of 40.0% (volume ratio) methane with nitrogen as balance was used as feed gas of 50 mL/min. The product gases out of the reactor were collected every 1.0–2.0 min and analyzed by gas chromatography (Shimadzu GC-2010 plus gas chromatograph). When the methane conversion step finished, pure N2 (50 mL/min) was fed to the reactor for 30 min to avoid mixing of gases arising during the two steps. Then the steam generated by injecting demineralized water in an electric furnace at 400°C using a micro pump was introduced into the reactor for 20 min with N2 as carrier gas (50 mL/min). The flow rate of the water was controlled at 0.2 mL/min.

2

Results and discussion

2.1 2.1.1

Characterization of the oxygen carrier XRD

The structures of perovskite-type oxides of fresh and used were examined by XRD to identify the crystalline phases formed, as shown in Figure 2. The XRD patterns of three fresh oxides are in good agreement with JCPDS (Joint Committee on Powder Diffraction Standards) card 01-075-0541, which

Fig. 2

confirms the formation of the desired monophase crystalline perovskite phases. Compared with LF, the strongest characteristic peaks of LF-CaO and LF-MgO show a slight broad and shift towards higher direction, corresponding to the lattice contraction. This indicates that doped CaO or MgO would restrain the increase of the crystal size of LF oxide during thermal treatment, which may be attributed to the formation of Fe-Ca or Fe-Mg solid solution[10]. Besides, the XRD patterns of LF-CaO exhibits CaO characteristic peaks at 2θ of 32.194°, 37.345°, 53.843°, 64.136°, 67.357° (JCPDS File Card No.01-077-2010). While MgO characteristic peaks emerged at 2θ of 36.889°, 42.856°, 62.217°(JCPDS File Card No. 01-071-1176) for LF-MgO. A small peak at about 30º cannot be found on the basis of the currently available database. It is likely to be attributed to the incomplete solid-state reactions[11]. No other impurity crystalline phase existed. Figure 2(b) shows the XRD patterns of the three oxides after a cycle. The XRD patterns of regeneration samples are in agreement with the patterns of fresh samples, indicating that lattice oxygen of the carrier can be recovered to the original state after cyclic reaction. 2.1.2

H2-TPR

The reduction peak area in H2-TPR profile, shown in Figure 3, partially reflects reactivity of the oxygen carriers. Two reduction peaks emerged for LF oxide, which were a small reduction peak at 515°C and a broad reduction peak at 702°C, respectively. The positions and area percentages of all the peaks are summarized in Table 1. It was known that two types of oxygen species may exist in the perovskite-typed oxide ascribed to the different metal ion reduction. The  peak, usually observed at lower temperature, represents the desorption of oxygen adsorbed on the catalyst surface. The  peak, characterized by higher onset temperature, strictly depends on the nature of the B ion, being correlated to its partial reduction to a lower oxidation state[12].

XRD patterns of oxygen carriers of (a) fresh and (b) regenerated : orthorhombic phase; : La2O3; : CaO; : MgO

ZHAO Kun et al / Journal of Fuel Chemistry and Technology, 2016, 44(6): 680688

2.1.3

Fig. 3 Table 1

Hydrogen-temperature programmed reduction profiles Position and area percentage of the major reduction peaks for the three samples

Sample

Peak position t/°C

LF

515

3.3

702

96.7

403

41.8

545

8.3

640

49.9

LF-CaO

LF-MgO

Fractional peak area /%

435

2.9

560

45.4

780

51.7

Investigations[13] suggest that the presence of the  peak and its onset temperature can be adopted as a qualitative description of catalyst reducibility and oxygen mobility within the oxide. A broad  peak at 702°C emerged for LF oxide, accounting for 96.7% of the total hydrogen consumption (see Table 1). Its broad peaking temperature spans from 540 to 820°C should be ascribed to the reduction of Fe 4+ and Fe3+ to Fe2+ and/or metallic iron. It means that the perovskite-type LF oxide shows good activity in a reduced atmosphere, while the H2-TPR patterns of the basic metal doping oxides has an obvious difference. The reduction of LF-CaO/MgO oxides is evidenced by three peaks, indicating an increase of surface adsorbed oxygen. The three peaks are associated with reduction of Fe2O3 to Fe3O4 and then to Fe0[14]. Meanwhile, the hydrogen reduction peak for Fe3O4→Fe0 process can be divided into two peaks with process of Fe3O4→FeO→Fe0 due to interaction between Ca/Mg and Fe species. Meanwhile the hydrogen consumption peaks of LF-CaO shift to lower temperature compared with LF. The reduction peaks reached the maximum values at 403, 545 and 640°C, respectively. This means that CaO modification make it easier to be reduced. While the reduction pattern of LF-MgO is evidenced by three peaks at 435, 560 and 780°C. The last broad peak emerged at the highest temperature confirms the high stability of LF-MgO under reducing condition.

Specific surface area

The pore volume and specific surface area of these perovskite-type oxides were determined using the BET measurement method and shown in Table 2. The doping of basic metal has an obvious influence on the specific surface area which is about six times higher for LF-CaO or LF-MgO than that for LF. It significantly increases when CaO or MgO was incorporated with the perovskite oxide. The specific surface area of perovskite-type oxides made by different preparation method is very different. It was reported that perovskite LaFeO3 synthesized using sol-gel method give relatively higher surface area of 16.5 m2/g[15], while the sample prepared by coprecipitation has lower surface area. Even so, the surface area of LF-CaO/MgO presented here is much higher than that, which is beneficial to its performance for catalytic and other applications. 2.1.4

XPS

The chemical states of metals and oxygen in the samples are given by fitting the XPS curves as shown in Figure 4, and the elemental compositions and relative proportions were calculated (Table 3). The XPS patterns of Fe show a double-peaked spectrum with Fe 2p3/2 and Fe 2p1/2. The Fe 2p3/2 contributes to low bind energy and Fe 2p1/2 contributes to high bind energy, because the electron density of Fe3+ is smaller than that of Fe2+. The XPS patterns of La are similar with the form of trivalent La 3d. The standard La pattern reveals two clear La 3d3/2 and La 3d5/2 core level binds at 850 and 834 eV, respectively. When the complex oxides formed by La, satellite peak phenomenon appeared with four spectral peaks, double peaks for La 3d3/2 and La 3d5/2 separately in Figure 4. The splitting of characteristic peaks of La 3d can be explained by an electronic unbalance in the perovskite lattice compensated by the transfer of 2p valence electrons, which coordinated with La and O occurred due to the electron ionization of La 3d[16]. Meanwhile, it is notable that the binding energy of Fe 2p and La 3d in LF-CaO is the lowest, which means it has the highest electron density. The higher the electron density, the smaller the granularity of active species, consequently leading to the stronger resistance to carbon deposition. Table 2

Specific surface area of perovskites

Oxygen carriers

LF

LF-CaO

LF-MgO

Specific surface area

3.5

20.3

21.7

28.4

3.7

11.2

A/(m2g–1) Average pore diameter d/nm

ZHAO Kun et al / Journal of Fuel Chemistry and Technology, 2016, 44(6): 680688

Fig. 4

La 3d (a) and Fe 2p (b) XPS spectra of the three samples

Fig. 5

O 1s XPS patterns of three samples

Figure 5 shows O 1s spectrum of perovskite-type oxides. The envelope curve of three fitting peaks is overlapped with original curve wonderfully. The three overlapped peaks at ca. 528.3–529 (denoted as OI), 530.5–530.9 (denoted as OII) and 531.5–531.7 eV (denoted as OIII), are assigned to the peaks of lattice oxygen, chemisorbed oxygen and physical adsorbed oxygen, respectively. The lattice oxygen is conducive to the partial oxidation of CH4. The adsorbed oxygen, relating to the defect oxides or surface molecular oxygen with low coordination, is beneficial to the complete oxidation of CH 4. The relative percentages of different oxygen species are shown in Table 3. Compared with LF, the LF-CaO and LF-MgO contain more adsorbed oxygen species with higher binding energy or less negative charge. The more abundant adsorbed oxygen (OII+OIII) is partially associated with the higher specific surface area of samples. On the other hand, the electrons lying on 3d orbitals in Fe atoms exist at a high spin state due to low coordination and low symmetry, which weakens the interaction of Fe–O. The weakness of Fe–O is easily affected by the doping of CaO/MgO, thus favors the transformation from lattice oxygen into chemisorbed oxygen and formation of oxygen vacancy[17]. 2.2

Reactivity tests

The reactivity of perovskite-type oxides were evaluated using a fixed-bed reactor. The compositions of gaseous products as a function of time are shown in Figure 6. The gas contents variation follows the same trend. The unreacted methane in the outlet gas decreases quickly and reached below 10% after 6 min

due to the consumption of lattice oxygen and adsorbed oxygen. The CO2 fraction decreases rapidly and closes to zero after 5 min, whereas the fractions of target products (CO and H2) increase continuously as the reaction proceeds. It corresponds to our previous studies[9] that the surface oxygen contributes to the complete oxidation of methane to CO2 and H2O at beginning of the reaction, and the lattice oxygen is usually prone to CH4 partial oxidation to H2 and CO in the mid and later stages. The higher CO2 concentration at initial 3 min also demonstrates that there are more adsorbed oxygen existed in La-CaO/MgO, which are corresponded to the XPS results. Figure 7 represents the typical kinetic curves for methane conversion, CO and H2 selectivity, and H2/CO molar ratio during reactions. The CH4 conversion, CO selectivity and H2 selectivity are calculated as follows: Methane conversion (%)= moles of methane consumed ×100% (4) moles of methane introduced CO selectivity (%)= moles of CO produced ×100% (5) moles of CO and CO2 produced H2 selectivity (%)= moles of H2 produced ×100% (6) moles of methane introduced×2 The three samples show a high methane conversion which increases rapidly as the reaction proceeds and reaches above 80% after 6 min. LF-MgO gives the highest methane conversion, while that of LF is the lowest. The oxidation of methane to syngas (H2+CO) or CO2+H2O and the decomposition of methane to H2+C are two decisive factors for methane conversion.

ZHAO Kun et al / Journal of Fuel Chemistry and Technology, 2016, 44(6): 680688 Table 3

Surface elemental composition and relative proportion for the samples measured by XPS Surface compositions /%

Oxygen carrier

La

Fe

Ca

Mg

OI

OII

OIII

LF

20.2

7.7

–

–

27.3

17.1

27.7

LF-CaO

13.7

5.5

7.8

–

24.8

26

22.2

LF-MgO

15.4

5.4

–

9.6

15.6

27.2

26.7

Fig. 6

Gaseous products in methane reduction step : H2; : CH4; : CO; : CO2

Fig. 7

Catalytic performance of oxygen carriers for methane selective oxidation

For H2/CO molar ratio (Figure 7), LF exhibits higher H2/CO molar ratio far above 2.0. This means the high methane conversion of LF is largely contributed to its decomposition. While LF-CaO has a H2/CO molar ratio more close to the target value of 2.0 in the early stage of reactions, indicating the best reactivity for methane partial oxidation. Kee et al[18] reported that MgO promoted Ni/Al2O3 catalyst forms MgAl2O4 spinel phase, which is stable at high temperature and effectively prevents coke formation. Quincoces et al[19] reported that the prevention of coke formation is related to the basic property of CaO, which favors CO2 adsorption and/or

diminishes the ability of Ni to dissociate methane. Choudhary et al[20] reported that catalysts precoated with MgO and CaO show much higher activity, selectivity and productivity in methane-to-syngas conversion reactions than those prepared using catalyst carriers without any precoating. Most of these studies tested on CH4-CO2 reforming suggest that the alkali metal can promote adsorption of CO2 to react with C, thus to eliminate coke formation. Duane et al[21] synthesized MgO-promoted Fe2O3 hematite oxygen carriers using for methane chemical looping combustion.

ZHAO Kun et al / Journal of Fuel Chemistry and Technology, 2016, 44(6): 680688

Fig. 8

Gaseous products in steam oxidation step : H2; : CO; : CO2

Fig. 9

Catalytic performance of LF-CaO in five successive recycles (a): methane reduction step; (b): steam oxidation step : CH4 conversion; : CO selectivity; : H2 selectivity

Results showed that the oxygen utilization of MgO is near zero for methane reduction, suggesting MgO does not directly participate in the CLC reaction. MgO assists to enhance the oxygen transfer capacity and reactivity of hematite with methane. Also, CaO can reduce acid sites on catalyst surface, leading to the electron rich property of active components. The electron rich property is effective to reduce decomposition rate of CH4 and accelerate oxidation of carbon species[22,23]. CO and H2 selectivity is very low initially, but it rises sharply and reaches a steady value above 90% within the first few minutes. It should be noted that the doping of CaO results in a slight decrease in the H2 selectivity. This is also in accordance with its moderate H2/CO molar ratio. The three samples have a similar CO selectivity nearly 100% after 4 min. After exposure to CH4, [O] species in oxygen carriers have been consumed and oxygen vacancies are formed. When the oxidizing agent steam are introduced, the oxygen vacancies are replenished immediately and simultaneously to produce hydrogen. Figure 8 shows the gaseous products in the steam oxidation step. The hydrogen concentrations increase very fast initially and get the maximum value at about 2.0–3.0 min. The maximum hydrogen concentrations of three samples are 59.8%, 69.6% and 58.7%, respectively. LF-CaO exhibits the highest hydrogen generation capacity. Then as the reaction proceeds, the hydrogen concentrations decrease quickly to below 5% after 10 min. Moreover, a few CO and CO 2 are also

observed in the gaseous products, which are aroused by the undesired reactions of carbon gasification (C+H2O→CO+H2) and water gas shift (CO+H2O→CO2+H2). Based on the above results, LF-CaO is the best candidate for CL-SMR among three samples judging from the reactivity, selectivity, and resistance to carbon formation. Five redox reactions of methane reduction and steam oxidation were conducted on LF-CaO to investigate its catalytic stability. The reaction time for methane conversion was set as 6 min to avoid carbon deposition, and the water splitting step was fixed at 10 min. The results are shown in Figure 9. CH4 conversion, CO and H2 selectivity remain stable during 5 redox cycles. CH4 conversion is about 80%, while CO and H2 selectivity is nearly 100% and 50% respectively. Another important target of H2/CO ratio is also steadily kept at about 2.0, indicating the high quality of syngas is obtained. For steam oxidation the average hydrogen productivity in each cycle is about 2.0–3.0 mmol/g oxygen carrier. To summarize, LF-CaO exhibits a good performance for chemical-looping reforming of methane and successive redox cyclic reactivity.

3

Conclusions

Perovskite-type oxide LaFeO3, LaFeO3-CaO and LaFeO3-MgO were prepared by co-precipitation method and used as oxygen carrier in chemical-looping steam methane reforming process to produce syngas and hydrogen.

ZHAO Kun et al / Journal of Fuel Chemistry and Technology, 2016, 44(6): 680688

Physicochemical characterizations reveal that the desired monophase crystalline perovskite phases are formed for the three fresh oxides. The structure of the perovskites is stable and not substantially changed in cyclic reaction. The doping of basic metals has obvious effects on the reactivity of LaFeO 3 oxide. H2-TPR patterns of LF-CaO and LF-MgO are evidenced by three peaks, indicating an increase of surface adsorbed oxygen. The more abundant adsorbed oxygen is partially associated with the higher specific surface area of samples. On the other hand, the weakness of Fe–O is easily affected by the doping of CaO/MgO, thus favors transformation from lattice oxygen into chemisorbed oxygen and formation of oxygen vacancy. Meanwhile, XPS results show that LF-CaO sample has the highest electron density, leading to stronger resistance to carbon deposition. Then the best candidate LF-CaO was used as oxygen carrier in CL-SMR redox cycles to investigate its catalytic stability. In the methane reduction step, CH4 conversion reaches about 80%, while CO and H2 selectivity is nearly 100% and 50% respectively. Another important target of H2/CO ratio is also steadily kept at about 2.0, indicating the high quality of syngas is obtained. For steam oxidation, the average hydrogen productivity in each cycle is about 2.0–3.0 mmol/g oxygen carrier. The reactivity of LF-CaO is quiet stable in 5 redox cycles with a good repeatability.

NiO, Fe2O3 and Mn3O4. Int J Greenhouse Gas Control, 2008, 2(1): 21–36. [7] Dai X P, Yu C C, Wu Q. Comparison of LaFeO3, La0.8Sr0.2FeO3, and La0.8Sr0.2Fe0.9Co0.1O3 perovskite oxides as oxygen carrier for partial oxidation of methane. J Nat Gas Chem, 2008, 17(4): 415–418. [8] Zhao K, He F, Huang Z, Zheng A, Li H B, Zhao Z L. La1–xSrxFeO3 perovskites as oxygen carriers for the partial oxidation of methane to syngas. Chin J Catal, 2014, 35(7): 1196–1205. [9] Zhao K, He F, Huang Z, Zheng A Q, Li H B, Zhao Z L. Three-dimensionally ordered macroporous LaFeO3 perovskites for chemical-looping steam reforming of methane. Int J Hydrogen Energy, 2014, 39(7): 3243–3252. [10] Li K Z, Wang H, Wei Y G, Yan D X. Partial oxidation of methane to syngas with air by lattice oxygen transfer over ZrO2-modified Ce-Fe mixed oxides. Chem Eng J, 2011, 173(2): 574–582. [11] Arya S, Nathan G, Huang Y, Chen Y, Li F. Fe2O3@LaxSr1–xFeO3 core-shell redox catalyst for methane partial oxidation. ChemCatChem, 2014, 6: 790–799. [12] Rossetti I, Forni L. Catalytic flameless combustion of methane over perovskites prepared by flame-hydrolysis. Appl Catal B: Environ, 2001, 33(4): 345–352. [13] He F, Li X A, Zhao K, Huang Z, Wei G Q, Li H B. Catalytic flameless combustion of methane over perovskites prepared by

References

flame-hydrolysis. Fuel, 2013, 108: 465–473. [14] Li K Z, Wang H, Wei Y G, Yan D X. Direct conversion of

[1] Go K S, Son S R, Kim S D, Kang K S, Park C S. Hydrogen production from two-step steam methane reforming in a fluidized bed reactor. Int J Hydrogen Energy, 2009, 34: 1301–1309.

methane to synthesis gas using lattice oxygen of CeO2-Fe2O3 complex oxides. Chem Eng J, 2010, 156(3): 512–518. [15] Priti V G, Rajesh B B. Pure phase LaFeO3 perovskite with improved surface area synthesized using different routes and its

[2] Zhu X, Wang H, Wei Y G, Li K Z, Cheng X M. Hydrogen and syngas production from two-step steam reforming of methane

characterization. Mater Chem Phys, 2010, 119(1/2): 324–329. [16] Ma H Q, Tan X, Zhu H M, Zhang J Y, Zhang L. XPS

using CeO2 as oxygen carrier. J Nat Gas Chem, 2011, 20(3):

Characterization of La1–xCexFeO3 perovskite as high-temperture

281–286.

water-gas shift catalysts. J Chin Rare Earth Soc, 2003, 21(4):

[3] Oxygen Li R J, Yu C C, Dai X P, Shen S K. Partial oxidation of methane to synthesis gas using lattice oxygen instead of molecular. Chin J Catal, 2002, 23(4): 381–387.

445–448. [17] Li X, Zhang H B, Liu X X, Li S J, Zhao M Y. XPS study on O(1s) and Fe(2p) for nanocrystalline composite oxide LaFeO3

[4] Li R J, Yu C C, Zhu G R, Shen S K, Zhang Z X. Partial oxidation of natural gas to synthesis gas using lattice oxygen from AFeO3 (A=La, Nd, Sm, Eu) perovskite. Chem Eng Oil Gas, 2004, 33: 5–8.

with the perovskite structure. Mater Chem Phys, 1994, 38: 355–362. [18] Kee Y K, Hyun S R, Yu T S, Dong J S, Wang L Y, Seung B P. Coke study on MgO-promoted Ni/Al2O3 catalyst in combined

[5] Dai X P, Yu C C, Li R J, Wu Q, Shi K J, Hao Z P. Effect of calcination temperature and reaction conditions on methane partial oxidation using lanthanum-based perovskite as oxygen donor. J Rare Earths, 2008, 26(3): 341–346.

H2O and CO2 reforming of methane for gas to liquid (GTL) process. Appl Catal A: Gen, 2008, 340(2): 183–190. [19] Quincoces C E, Dicundo S, Alvarez A M, Gonzalez M G. Effect of addition of CaO on Ni/Al2O3 catalysts over CO2 reforming of

[6] Magnus R, Anders L, Tobias M, Chen D, Anders H, Erlend B.

methane. Mater Lett, 2001, 50(1): 21–27.

chemical-looping

[20] Choudhary V R, Uphade B S, Mamman A S. Large

reforming;

enhancement in methane-to-syngas conversion activity of

LaxSr1−xFeyCo1−yO3−δ perovskites and mixed-metal oxides of

supported Ni catalysts due to precoating of catalyst supports

Novel

oxygen-carrier

combustion

and

materials

for

chemical-looping

ZHAO Kun et al / Journal of Fuel Chemistry and Technology, 2016, 44(6): 680688 with MgO, CaO or rare-earth oxide. Catal Lett, 1995, 32(3/4): 387–390.

deposition on catalysts during the partial oxidation of methane.

[21] Duane D M, Ranjani S. Fluidized-bed and fixed-bed reactor testing of

[22] HU J B, YU C L, ZHOU X C. Research progress of carbon

methane chemical

Nonferrous Met Sci Eng, 2012, 3(2): 5–11.

looping combustion with

[23] Yu C L, Xu H Y, Ge Q J. Characterization of the metallicphase

MgO-promoted hematite James Poston. Appl Energy, 2015, 146:

of Zn-doped Pt and Pt-Sn catalysts for propane dehydrogenation.

111–121.

J Mol Catal A, 2007, 266: 80–89.