Preparation of double perovskite-type oxide ... - Science Direct

JOURNAL OF RARE EARTHS, Vol. 34, No. 10, Oct. 2016, P. 1032

Preparation of double perovskite-type oxide LaSrFeCoO6 for chemical looping steam methane reforming to produce syngas and hydrogen ZHAO Kun (赵坤)1,2,3, SHEN Yang (沈阳)3, HE Fang (何方)1,*, HUANG Zhen (黄振)1, WEI Guoqiang (魏国强)1, ZHENG Anqing (郑安庆)1, LI Haibin (李海滨)1, ZHAO Zengli (赵增立)1 (1. Key Laboratory of Renewable Energy, Chinese Academy of Sciences, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, China; 2. Guangdong Key Laboratory of New and Renewable Energy Research and Development, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, China; 3. University of Chinese Academy of Sciences, Beijing 100049, China) Received 8 March 2016; revised 18 May 2016

Abstract: Double-perovskite type oxide LaSrFeCoO6 was used as oxygen carrier for chemical looping steam methane reforming (CL-SMR) due to its unique structure and reactivity. Solid-phase, amorphous alloy, sol-gel and micro-emulsion methods were used to prepare the LaSrFeCoO6 samples, and the as-prepared samples were characterized by means of X-ray diffraction (XRD), hydrogen temperature-programmed reduction (H2-TPR), X-ray photoelectron spectroscopy (XPS), Brunauer-Emmett-Teller (BET) surface area. Results showed that the samples made by the four different methods exhibited pure crystalline perovskite structure. The ordered double perovskite LaSrFeCoO6 was regarded as a regular arrangement of alternating FeO6 and CoO6 corner-shared octahedra, with La and Sr cations occupying the voids in between the octahedral. Because the La3+ and Sr2+ ions in A-site did not take part in reaction, the TPR patterns showed the reductive properties of the B-site metals. The reduction peaks at low temperature revealed the reduction of adsorbed oxygen on surface and combined with the reduction of Co3+ to Co2+ and to Co0, while the reduction of Fe3+ to Fe2+ and the partial reduction of Fe2+ to Fe0 occurred at higher temperatures. From the point of view of the oxygen-donation ability, resistance to carbon formation, as well as hydrogen generation capacity, the sample made by micro-emulsion method exhibited the best reactivity. Its redox reactivity was very stable in ten successive cycles without deactivation. Compared to the single perovskite-type oxides LaFeO3 and LaCoO3, the double perovskite LaSrFeCoO6 exhibited better syngas and hydrogen generation capacity. Keywords: double-perovskite; CL-SMR; micro-emulsion; oxygen species; redox; rare earths

As a novel technology, chemical looping steam methane reforming (CL-SMR) technology has been drawing wide attention for the conversion and utilization of methane. In CL-SMR technology, methane is partially oxidized to syngas by the lattice oxygen of the oxygen carrier in the reformer reactor, and the reduced oxygen carrier is oxidized by steam to recover lattice oxygen and simultaneously to produce hydrogen in the steam reactor. Usually, an air reactor is needed since the oxygen species in oxygen carrier cannot be completely recovered by steam oxidation[1,2]. Syngas and hydrogen can be simultaneously obtained through two steps: Methane reduction: MxOy+CH4→MxOy–δ1–δ2+(2H2+CO) (1) Steam oxidation: MxOy–δ1–δ2+H2O→MxOy–δ1+H2 (2) Air oxidation: MxOy–δ1+O2→MxOy (3) where MxOy is an oxygen carrier, MxOy–δ1 and MxOy–δ1–δ2 are the corresponding reduced oxygen carrier. Compared to the traditional partial oxidation of CH4, the syngas

with molar ratio of H2/CO close to 2:1 can be obtained in CL-SMR and then can be used as a feed gas for direct methanol or Fischer-Tropsch synthesis. Then in the steam oxidation step, pure hydrogen can be obtained just by cooling the H2/steam mixture exiting the steam reactor, without requirement of additional gas treatments such as reforming or shifting and separation processes. In the CL-SMR technology, the most important factor is the properties of oxygen carrier. High methane conversion, high syngas selectivity, high sintering resistance and good activity for water splitting to produce hydrogen are four dominating requirements for oxygen carrier. Various metal oxides such as Fe-based oxides, Ni-based oxides, Cu-based oxides, Mn-based oxides, Ce-based oxides, and complex mixed metal oxides have been investigated as oxygen carriers for this process in previous researches[3–10]. As a special mixed-metal oxide, perovskite-type metal oxides have attracted more and more attentions due to the high reactivity and thermal stability[11–14]. Perovskite-type metal oxides are a series of mixed-metal oxides with general formula ABO3,

Foundation item: Project supported by the National Natural Science Foundation of China (51406208, 51406214) and Science & Technology Research Project of Guangdong Province (2013B050800008, 2015A010106009) * Corresponding author: HE Fang (E-mail: [email protected]; Tel.: +86-20-87057721) DOI: 10.1016/S1002-0721(16)60131-X

ZHAO Kun et al., Preparation of double perovskite-type oxide LaSrFeCoO6 for chemical looping steam methane …

where A is usually a lanthanide ion and/or alkaline earth metal and B is a transition metal ion. The catalytic activity of ABO3 is mainly determined by the B site metal, while A site is non-catalyzed but affects the oxygen vacancy and valence states of B site elements[15]. Samples of LaFeO3[16,17], La1–xSrxFeO3[18], LaFe1–xCoxO3[19], La1–xSrxCo1–yFeyO3[20] and LaFe1–xNixO3[21] have been used for CH4 oxidation. Moreover, perovskite-type oxides can serve not only as lattice oxygen carriers but also as catalysts for CH4 activation[22]. Therefore, more varied perovskite-type oxides are exploited to satisfy the requests for oxygen carriers in CL-SMR. Double perovskite-type oxide is a type of special perovskites with formula A’A’’B’B’’O6, where B’–O6 and B’’ –O6 octahedras are arranged alternately by the way of corner-sharing[23]. Differing from the single perovskite-type oxide, the catalytic activities of double perovskite-type oxides can be effectively improved through the synergy and coordination of the metals in special structure of B’–O–B’’. Hu and the co-workers[24–27] investigated double perovskite-type oxides La2CuNiO6, La2CuMnO6, LaSrFeCoO6, and La2CoAlO6 for methane combustion. They found that the synergistic effect of B and B’ metals could effectively enhance the catalytic activity. Zheng et al.[28] prepared LaSrFeMo1–x CoxO6 for methane catalytic combustion and also demonstrated the synergistic effect of Fe3+ with Mo2+ and Co2+. Therefore, the different configuration and different exchange interaction between A’/A’’ and B’/B’’ metals in double perovskite-type oxide may provide an extensive modeling space for researches. Due to the special structure, double perovskite- type oxides are widely used as magnetic materials[29], superconducting materials[30], and catalytic materials[31,32]. All the researches confirmed the excellent performance of the double perovskite-type oxides in catalysis and oxidation. But as far as we know, there is no report regarding the catalytic performance of double perovskites-type oxides for CL-SMR technology, especially from the points of both chemical reforming of methane and water splitting. Also the cyclic reactivity of double perovskite- type oxide for oxygen delivery has no research. In this work, double perovskite-type metal oxide LaSrFeCoO6 was used as oxygen carrier for the CLSMR process. The properties of the oxides were characterized by means of X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) surface area, H2 temperature-programmed reduction (H2-TPR) techniques, and X-ray photoelectron spectroscopy (XPS). Then the reactivities of LaSrFeCoO6 were investigated in a fixed-bed reactor. Moreover, the behaviors of single perovskitetype oxides LaFeO3 and LaCoO3 were used to make comparison with LaSrFeCoO6.

1 Experimental

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1.1 Synthesis of the double perovskite-type oxides The chemical reactions and physical changes during the preparation process are different for different preparation methods. The preparation methods would largely affect the properties of oxides, consequently to affect the oxidation reactivities. In this paper, solid-phase method, amorphous alloy method, sol-gel method and micro-emulsion method were used for the preparation of double perovskite-type oxide LaSrFeCoO6. The required amounts of La(NO3)3·6H2O, N2O6Sr, CoN2O6·6H2O and Fe(NO3)3·9H2O were weighed at a desired stoichiometric ratio and dissolved in deionized water to get a mixed nitrates solution. 1.1.1 Solid-phase method The mixed nitrates were put into the ball mill to be grinded for 4 h. After that, the resulting solution was suction filtrated and dried at 110 ºC for 24 h. The powdered precursor was then thermally decomposed at 500 ºC for 2 h and calcined at 1000 ºC for 6 h. The resulting product was ground to obtain the fresh double perovskite-type oxides of LaSrFeCoO6, which was named sample SP. 1.1.2 Amorphous alloy method Slightly excessive ammonium bicarbonate was added in the mixed nitrates solution. All the mixed solutions were added into DTPA for pyro-condensation 30 min. The obtained viscous solution was then dried at 110 ºC for 24 h. Finally, the precursor was thermally decomposed at 500 ºC for 2 h and calcined at 1000 ºC for 6 h. The resulting product was ground to obtain the fresh double perovskite-type oxides of LaSrFeCoO6, which was named sample AA. 1.1.3 Sol-gel method Glycine was added into the solution of the mixed nitrates to reach a glycine/nitrates molar ratio of 1.05. The resulting solution was allowed to evaporate by stirring in a 100 mL beaker at 70 ºC until a viscous gel was obtained. The gel was then thermally decomposed at 500 ºC for 2 h and calcined at 1000 ºC for 6 h. The resulting product was ground to obtain the fresh double perovskite-type oxides of LaSrFeCoO6, which was named sample SG. 1.1.4 Micro-emulsion method Surfactant Tritonx-100, cosurfactant n-butyl alcohol and oil phase cyclohexane were added to make a mixture of solution A. Meanwhile, the same ratio of surfactant Tritonx-100, cosurfactant n-butyl alcohol and oil phase cyclohexane were added into (NH4)2CO3-NH4OH to make a mixture of solution B. After that, the solution B was slowly dripped into solution A to form sediment under a water bath at 50 ºC. The sediment was allowed to settle for 2 h and filtered. Then the sediment was dried overnight in a convection oven at 110 °C. Finally, the as-prepared precursor was thermally decomposed at 500 ºC for 2 h and calcined at 1000 ºC for 6 h. The resulting

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JOURNAL OF RARE EARTHS, Vol. 34, No. 10, Oct. 2016

product was ground to obtain the fresh double perovskite-type oxides of LaSrFeCoO6, which was named sample ME.

H2 selectivity (%) =

1.2 Characterization The crystal phases of the oxides were identified by XRD in a Japan Science D/max-R diffractometer with Cu Kα radiation (λ=0.15406 nm), operating voltage of 40 kV and current of 40 mA, and the diffraction angle (2θ) was scanned from 10º to 80º. The hydrogen-temperature programmed reduction (H2-TPR) experiments were conducted in 5.0 vol.% H2 balanced with helium at a flow rate of 60 mL/min from room temperature to 800 ºC with a heating rate of 10 ºC/min. X-ray photoelectron spectroscopy (XPS) was used to probe the near-surface composition of the oxides. The equipment was Thermo Fisher Scientific Inc with an Al Kα X-ray source at an operating voltage of 20 kV and a current of 10 mA, under the conditions of 20 and 100 eV pass energy for the survey spectra and the single element spectra. The specific surface areas and average pore diameter of the prepared perovskites were measured at liquid nitrogen temperature by BET method (SI-MP-10/PoreMaster 33). 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[33]. The fixedbed 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 the methane conversion step, 2.0 g of the oxides (particle size 80–100 mesh) were placed on the baffle under atmospheric pressure at 850 ºC. The flow rate of the feed gas was set at 50 mL/min in which methane was 40.0 vol.% and nitrogen gas used as balance gas. The product gases out of the reactor were collected with gas bags and analyzed by a gas chromatograph (Shimadzu GC-2010 plus). 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 ºC using a micro pump was introduced into the reactor for 20 min with N2 as carrier gas (50 mL/min). The flow rate of the water was controlled at 0.2 mL/min. The CH4 conversion, CO selectivity and H2 selectivity were calculated as follows: moles of methane consumed Methane conversion (%) = ×100% moles of methane introduced (4) moles of CO produced CO selectivity (%) = × 100% moles of CO and CO2 produced (5)

moles of H2 produced moles of methane introduced × 2

×100%

(6) For the successive CL-SMR cycles, the methane conversion step and the steam oxidation step were introduced as above. After the steam oxidation step, dry air (50 mL/min) was introduced into the reactor to guarantee the oxygen carrier recovery its oxygen species completely.

2 Results and discussion 2.1 Characterization of the oxygen carrier

2.1.1 XRD The fresh, reduced, and regenerated LaSrFeCoO6 double-perovskites were examined by XRD to identify the crystalline phases formed as shown in Fig. 1. It is observed from Fig. 1(a) that the XRD patterns of the fresh synthesized oxides with four different preparation methods are in good agreement with JCPDS (Joint Committee on Powder Diffraction Standards) card 01-075-0541, which confirms the formation of the desired perovskite phases. The ordered double perovskites LaSrFeCoO6 may be regarded as a regular arrangement of alternating FeO6 and CoO6 corner-shared octahedra, with La and Sr cations occupying the voids in between the octahedralm[34]. However, the peak splitting of the characteristic peaks at about 2θ=32°, 46°, 58° are observed for the samples of SP, AA, and SG, indicating that a mixed crystal phases are formed in these samples. While the sample ME exhibits a monocrystalline perovskite structure with cubic symmetry, with higher crystallinity and purity than the other three samples. Fig. 1(b) shows the reduced samples after CH4 reduction. Obvious structural changes of the double-perovskite occurred after exposure to methane reducing atmosphere. In order to discuss the reduced samples in detail, the XRD pattern of the reduced sample made by solid-phase method was taken for an example as shown in Fig. 1(c). Although the characteristic peaks of La2O3, SrO, Fe2O3, FeO, CoO, Co0 and Fe0 are observed in the XRD pattern, the main structure of perovskite still exists with some impurity phases. As mentioned above, A site metals do not act as the catalyst, while B site metals are the main active component. The presence of Fe2O3 and FeO may indicate the reduction of Fe3+ to Fe2+/Fe0. While the reduction of Co3+ to Co2+/Co0 are confirmed by the presence of CoO and Co0. The La3+ and Sr2+ maintain their valence states and appear in the form of La2O3 and SrO. After the oxidation reaction with steam and air, the regenerated oxides recover double-perovskite structures as shown in Fig. 1(d), which means that the oxides show good regenerability. Compared to the fresh samples, the intensity of the main characteristic peaks of the regener-


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Fig. 1 XRD patterns of LaSrFeCoO6 double-perovskites (a) Fresh (★–perovskite, ☆–La2SrOx impurity phase); (b) Reduced; (c) Reduced sample of SP (★–perovskite, ☆–La2SrOx impurity phase, e–La2O3, f–FeO, g–Fe2O3, h–SrO, j–CoO, k–Fe0; l–Co0); (d) Regenerated

ated sample all increase after a cyclic redox reaction, indicating the optimization of crystallinity. 2.1.2 H2-TPR The H2-TPR patterns of LaSrFeCoO6 double-perovskites are shown in Fig. 2. Because the La3+ and Sr2+ ions in A-site do not take part in reaction, the TPR patterns provide the reductive messages of B-site metals. It can be seen that multi-peaks are presented for the four samples in the TPR patterns. For the samples SP and SG, there are four reduction peaks presented in the TPR patterns. While for the samples ME and AA, only three reduction peaks are presented in their TPR curves. As reported in

Fig. 2 H2-TPR profiles

previous paper[18], there are two kinds of oxygen species, called α and β, existing in the perovskite-type oxides. Usually, α oxygen at low temperature represents the non-stoichiometric oxygen; while the β oxygen is associated with the lattice oxygen of B-site cation with high oxidative state or with oxygen specie occupying inner vacancies. The oxygen species in the double-perovskite oxide are similar to these. For the sample SP, four reduction peaks emerge at 358, 519, 620 and 763 ºC, respectively. It is known that Co is an element with unstable valence states and is more easily to be reduced than Fe ion. Therefore, the reduction peaks at 358 and 519 ºC may be attributed to the reduction of adsorbed oxygen on surface and combined with the reduction of Co3+ to Co2+ and to Co0. While the two reduction peaks at higher temperatures (620 and 763 ºC), may be assigned to the reduction of Fe3+ to Fe2+, and the partial reduction of Fe2+ to Fe0, which is induced by the presence of Co0 particles. The reduction peaks of sample SG have similar profiles, but the first three peaks emerge earlier, indicating that it can be reduced more easily than the sample SP. But the last reduction peak, which is corresponding to the reduction of Fe2+ to Fe0, emerges at the highest temperature of 928 ºC. That means it is difficult to get deep reduction for the sample SG. While the TPR profiles of sample AA

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and ME have two or three peaks, indicating the simultaneously reductions of adsorbed oxygen with Co3+ to Co2+ and/or Fe3+ to Fe2+ and Fe2+ to Fe0, with better reducibility. The hydrogen consumptions as shown in Table 1 are calculated based on the peak areas. The H2 consumption of sample SP is the most with four reduction peaks, with the highest value of 31.2 mmol per gram oxygen carrier. The sample ME exhibits the second-highest H2 consumption of 23.4 mmol per gram oxygen carrier. But the TPR pattern of sample ME has the broadest reduction area at high temperature, which is beneficial to the CH4 partial oxidation to obtain the target product of syngas. 2.1.3 XPS As analyzed above, there are two kinds of oxygen in the double perovskite-type oxide. One is the adsorbed oxygen, with high binding energy (higher than 530.0 eV) that is related to the concentration of oxygen vacancies. Another is the lattice oxygen with low binding energy (lower than 530.0 eV) which is related to the reactivity of metal oxide[35,36]. Therefore the chemical state O in the investigated four samples are given by fitting the XPS curves as shown in Fig. 3, and the relative proportion was calculated on the basis of XPS data (Table 2). Fig. 3 shows that the peak height of OI species is lower than that of OII for the samples SP and AA, while that is op-

posite for the samples SG and ME. As shown in Table 2, the OI species in sample ME is the highest with a composition value of 40.1%. As we known, the lattice oxygen (OI) is conducive to the partial oxidation of CH4, while the adsorbed oxygen (OII) is beneficial to the complete oxidation of CH4. Therefore, the higher content of OI in sample ME is more beneficial to the syngas production by CH4 partial oxidation.

Table 1 H2 consumption of the four samples in TPR tests

2.2 Reactivity tests

Isothermal methane oxidation tests were conducted in a fixed-bed reactor. The typical kinetic curves of methane conversion, H2/CO molar ratio, CO selectivity and H2 selectivity towards the reaction of methane at 850 ºC for the LaSrFeCoO6 are shown in Fig. 4. It can be seen that CH4 conversion increases as the reaction proceeds. As for the samples of SP and ME, CH4 conversions increase gently and reach about 50% and 80% after 8 min, respectively. While in the case of the samples AA and SG, the conversions increase rapidly at the initial stage of Table 2 Relative proportion of O1s for the samples measured by XPS LaSrFeCoO6

Surface compositions of O/mol.% OI

OII

SP

31.39

68.61 73.4

AA

26.6

Sample

SP

AA

SG

ME

SG

39.8

60.2

H2 consumption/(mmol/g oxygen carrier)

31.2

16.9

12.5

23.4

ME

40.1

55.9

Fig. 3 O1s XPS patterns of four samples


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Fig. 4 Catalytic performance of oxygen carriers for methane selective oxidation (a) CH4 conversion; (b) H2/CO ratio; (c) H2 selectivity; (d ) CO selectivity

reaction, and reach to level off at about 70% after 8 min. It is noted that methane conversion can be contributed to two factors in CL-SMR. One is the oxidation of CH4 with oxygen carrier; another is a negative factor of methane decomposition to H2 and C. It can be seen from Fig. 4(b) that the H2/CO molar ratios reach about 2.0 at the initial stage of reaction from the 4th to 12th minute. After 12th minute, the H2/CO molar ratios increase continuously. Among the four samples, the H2/CO molar ratio of sample AA shows the fastest increasing and gets far exceeding 2.0. That means the high methane conversion of sample AA is largely attributed to methane cracking. Therefore, the reduction time should be strictly controlled to prevent the methane cracking. Then the H2 and CO selectivity have similar profiles, which increase continuously from the initial stage of reaction and stabilize at 6th minute, which reach at about 55% of H2 selectivity and 100% of CO selectivity. But the H2 selectivity of sample SP exhibits a stable trend around 70% from the beginning to the end. After CH4 reduction stage, the oxygen vacancies are formed due to the removal of [O] in the oxides. When the steam is introduced into the reactor, hydrogen can be generated immediately as the oxygen vacancies of the oxides are recovered. Fig. 5 shows the gaseous products during the steam oxidation step. It can be seen that the hydrogen fraction increases as soon as the reaction begin,

then gets the maximum value a few minutes after the reaction starts. As the reaction proceeds, the H2 fractions decrease sharply and close to zero after 20th minute. The maximum hydrogen fraction for sample AP, AA, SG and ME is 30%, 22%, 18%, and 38%, respectively. The sample ME shows the largest hydrogen generation capacity. Moreover, a small quantity of CO and CO2 exist in the gaseous products, which are aroused by the carbon gasification. But the fraction of CO and CO2 are very small and can be eliminated through controlling the reaction time of methane reduction. In conclusion, the solid-phase method is a simple method with low cost. But due to the calcination at high temperature, the intermediate compounds are formed quickly to cause grain coarsening; therefore the activity of the SP sample in the successive reactions is weakened. The obtained oxide has a low specific surface area, as shown in Table 3. Moreover, impurity phases are easily formed in the solid-phase preparation process. Amorphous alloy method is a new method developed in recent years. In this method, amorphous state compounds are used as precursor. The double perovskite-type oxide can be obtained just by thermal treatment of the precursor. But the sample AA is not suitable for CL-SMR process with excess adsorbed oxygen. The particle size and structure of the sample can be controlled in the molecular scale for the sol-gel and mico-emulsion methods. The

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Fig. 5 Gaseous products in steam oxidation step Table 3 BET surface area of the samples Oxygen carriers

SP

AA

SG

ME

Specific surface area/(m2/g)

3.25

4.50

4.55

3.99

oxides made by these two methods are usually exhibiting higher catalytic activity. As the experiments shows, the sample ME exhibits the best reactivity for CH4 partial reduction and hydrogen generation. Then the successive redox reactions were carried out on the sample ME to examine its redox reactivity. In order to prevent the excessive carbon deposition, the reaction time for the methane conversion was controlled for 12 min and the water splitting step for 15 min. Then 10 min air was introduced to make sure the complete oxidation of the reduced oxygen carriers. The CH4 conversion,

CO and H2 selectivity and H2/CO molar ratio in reduction period during ten redox cycles are shown in Fig. 6(a), and then the H2 generation capacity and H2 concentration in the steam oxidation step are shown in Fig. 6(b). It can be seen that the reactivity of sample ME in ten cycles are stable with few variation. The H2/CO molar ratio is close to 2:1 during ten cycles, and the average CH4 conversion is 78%, CO selectivity is 87% and H2 selectivity is 44%. While in the steam oxidation stage, the H2 generation capacity is about 98 mL per gram oxygen carrier, and the H2 concentration is high up to 96 vol.%. 2.3

Comparison of double perovskite-type oxide with single perovskite-type oxide

In order to determine the reactivity of double perovskite-

Fig. 6 Catalytic performance of sample ME in ten cycles (a) Methane reduction stage; (b) Steam oxidation stage


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Fig. 7 CH4 conversion (a) and H2/CO molar ratio (b) of three samples

type oxide LaSrFeCOO6, the single perovskite-type oxides LaFeO3 and LaCoO3 were used to make comparison. As shown in Fig. 7(a), CH4 conversion of samples LaFeO3 and LaCoO3 show similar profiles which gain high values at the initial stage and decline dramatically from 2nd to 4th minute, and then increase rapidly thereafter. The high CH4 conversion at the initial stage is attributed to the rapid consumption of surface adsorbed oxygen. But as the adsorption oxygen depleted, the lattice oxygen cannot release as soon as possible to supplement the oxygen vacancies. Finally, the CH4 conversions for samples LaFeO3 and LaCoO3 increase to 95% at the end of reaction (18–20 min). While the CH4 conversion of sample LaSrFeCOO6 has a different profile which increases very gently with inconspicuous change as reaction proceeds, the overall CH4 conversion is maintained at about 80%. That means the lattice oxygen and adsorbed oxygen in LaSrFeCOO6 release simultaneously. Fig. 7(b) shows the H2/CO molar ratio of three samples. It can be seen that the H2/CO molar ratio of sample LaCoO3 increases rapidly after 11 min, indicating the serious methane cracking. While the H2/CO molar ratios of the other two samples keep relatively stable during the whole test. The H2/CO molar ratios from 1 min to 11 min are shown in more detail in the upper left area in Fig. 7(b). The H2/CO molar ratio of LaSrFeCOO6 stays very stable at the ideal value of 2.0 with no methane crack, while that for the samples LaFeO3 and LaCoO3 changes differently. The H2/CO molar ratio of LaFeO3 increases continually as the reaction proceed, while that for LaCoO3 changes fluctuated and with a trend to increase. In steam oxidation stage, the H2 generation capacities of three samples are shown in Fig. 8. Moreover, it should be noted that the hydrogen volume presented here are the authentic hydrogen amounts which are generated by the recovery of oxygen in oxides (for example, LaSrFeCoO6–δ1–δ2+δ2H2O→LaSrFeCoO6–δ1+δ2H2). The parts of hydrogen which are derived from the undesirable reactions of carbon gasification (C+H2O→CO+H2) and water gas shift (CO+H2O→CO2+H2) are deducted. It can be seen that the hydrogen generation capacities of

Fig. 8 H2 generation capacities of three samples

samples LaSrFeCOO6 and LaFeO3 are higher than the sample LaCoO3, with the maximum value of 233 mL for sample LaSrFeCOO6 and 244 mL for sample LaFeO3. The sample LaCoO3 shows the lowest hydrogen generation capacity.

3 Conclusions The double perovskite-type oxide LaSrFeCoO6 were prepared by the solid-phase method, amorphous alloy method, sol-gel method and micro-emulsion method, respectively. The four samples made by different methods were all formed the expected double perovskite-type structure. While the sample ME exhibited a monocrystalline perovskite structure with cubic symmetry, showing higher crystallinity and purity than the other three samples. After exposure to CH4 atmosphere, characteristic peaks of Fe2O3, FeO, CoO and Co0 were present, indicating the reduction of Fe3+ to Fe2+/Fe0 and the reduction of Co3+ to Co2+/Co0. While the La3+ and Sr2+ maintained their valence states and appeared in the form of La2O3 and SrO. Then the H2-TPR tests also demonstrated the reductions of Fe3+ to Fe2+/Fe0 and Co3+ to Co2+/Co0. The H2 consumption of sample SP was the most with four reduction peaks, with the highest value of 31.2 mmol per gram oxygen carrier. The sample ME exhibited the second-highest H2 consumption of 23.4 mmol per gram

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oxygen carrier. But the TPR pattern of sample ME had the broadest reduction area at high temperature, which was beneficial to the CH4 partial oxidation to obtain the target product of syngas. Judging from the points of view of the oxygen-donation ability, resistance to carbon formation, as well as hydrogen generation capacity, the sample ME exhibited the best reactivity. The redox reactivity of sample ME was very stable in ten successive cycles with no decline. Moreover, double perovskite-type LaSrFeCoO6 exhibited a better performance than the single perovskite- type oxides LaFeO3 and LaCoO3.

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