Catalytic performance of cerium iron complex oxides for partial oxidation of methane to synthesis gas

JOURNAL OF RARE EARTHS, Vol. 26, No. 5, Oct. 2008, p. 705

Catalytic performance of cerium iron complex oxides for partial oxidation of methane to synthesis gas LI Kongzhai (), WANG Hua ( ), WEI Yonggang ( ), LIU Mingchun (  ) (Faculty of Materials and Metallurgy Engineering, Kunming University of Science and Technology, Kunming 650093, China) Received 1 September 2007; revised 3 October 2007

Abstract: The cerium iron complex oxides oxygen carrier was prepared by the co-precipitation method. The reactions between methane and lattice oxygen from the complex oxides were investigated in a fixed micro-reactor system. The reduced oxygen carrier could be re-oxidized by air and its initial state could be restored. The characterizations of the oxygen carriers were studied using XRD, O2-TPD, and H2-TPR. The results showed that the bulk lattice oxygen of CeO2-Fe2O3 was found to be suitable for the partial oxidation of methane to synthesis gas. There were two kinds of oxygen species on the oxygen carrier: the stronger oxygen species that was responsible for the complete oxidation of methane, and the weaker oxygen species (bulk lattice oxygen) that was responsible for the selective oxidation of methane to CO and H2 at a higher temperature. Then, the lost bulk lattice oxygen could be selectively supplemented by air re-oxidation at an appropriate reaction condition. CeFeO3 appeared on the oxygen carrier after 10 successive redox cycles, however, it was not bad for the selectivity of CO and H2. Keywords: methane; cerium iron complex oxides; lattice oxygen; selective oxidation; syngas; cycles; rare earths

Catalytic partial oxidation of methane (POM) to synthesis gas is an attractive topic in the field of natural gas application. However, this technology has not been commercialized owing to some problems, such as the rapid temperature increase of the catalyst and the risk of explosion owing to premixing of CH4/O2 within the ignition and explosion limits. To avoid these problems, some researchers[1] proposed a novel two-step process for the production of synthesis gas from methane in the absence of gaseous oxidant. In this process, a suitable oxygen storage compound (OSC) was circulated between two reactors. In one reactor, methane was oxidized to synthesis gas by the lattice oxygen of OSC, and in the other, the reduced OSC was re-oxidized by air, water, or carbon dioxide. This technology enables us to obtain better target production selectivity and avoid the risk of explosion, and meanwhile, the cost can be grandly cut since there is no requirement for pure oxygen supply. The key of this process is the preparation of an oxygen carrier, which has high oxygen storage capacity and excellent performance in oxidization-reduction reaction. Complex oxide materials have been receiving tremendous attention because of their diverse application in automotive three-way converter (TWC). Cerium oxide belongs to fluorite-type oxides, which has great oxygen storage capabil-

ity[1–3]. According to our previous researches, ceria-based complex oxides that have lattice oxygen with high activity and selectivity are ideal materials for the partial oxidation of methane to synthesis gas in the absence of molecular oxygen. In this study, the partial oxidation methane and redox properties of CeO2-Fe2O3 oxygen carrier prepared via coprecipitation are studied. The CeO2-Fe2O3 complex oxides were characterized by XRD, O2-TPD, and H2-TPR and some meaningful results were obtained.

1 Experimental 1.1 Oxygen carrier preparation Cerium iron complex oxide was prepared by co-precipitation. A solution of NaOH was gradually added into the mixed aqueous solution of Ce (NO3)2·6H2O and Fe (NO3)3·9H2O with the Ce: Fe molar ratio of 1:1 with continuous stirring. As the pH was increased to 7–8, a solution of 10 ammonia was gradually added to the mixture till pH=10. Subsequently, the precipitate was filtered and washed with distilled water. The resulting mixture was dried at 110 °C for 24 h and calcined under ambient air at 800 °C for 6 h, and then the CeO2-Fe2O3 sample was obtained.

Foundation item: Project supported by the National Natural Science Foundation of China (50574046) and National Natural Science Foundation of Major Research Projects (90610035), Natural Science Foundation of Yunnan Province (2004E0058Q), High School Doctoral Subject Special Science and Research Foundation of Ministry of Education (20040674005) Corresponding author: WANG Hua (E-mail: [email protected]; Tel.: +86-871-5153405)

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1.2 Oxygen carrier activity tests The reaction between CH4 and the oxygen carrier was carried out in a continuous flow fixed-bed reactor system under atmospheric pressure. An amount of 600 mg of oxygen carrier was placed in a quartz tube with 10 mm inner diameter. Prior to the catalytic reactions, the catalysts were heated at 300 °C for 2 h in air, and then N2 flow to the reactor was carried out at 400 °C for 1 h. The temperature programmed reaction was performed at a rate of 15 °C/min. The gas flow rate of the reaction was controlled by a mass flow controller at a specific flow rate of 10 ml/min. The purity of N2 and CH4 was 99.99%. The reactant and the product components were analyzed online by a gas chromatograph (GC112A, produced by Shanghai Precision & Scientific Instrument Co.) equipped with a thermal conductivity detector (TCD). Argon was employed as a carrier gas. CH4 conversion (XCH4), CO selectivity (SCO), and H2 selectivity (SH2) were calculated based on the analysis results of GC. 1.3 Redox performance tests The regeneration of oxygen carrier: After the methane oxidation reaction proceeded for a while, the purging of CH4 into the reactor was stopped, while the purging of pure N2 was continued for 30 min; then, the pressed air was introduced to regenerate the reduced sample. The reaction proceeded till no H2, CH4, CO, and CO2 appeared. Again, pure N2 was purged for 30 min over the regenerated sample, and its redox performance was evaluated. 1.4 Oxygen carrier characterization The XRD experiments were performed on an X-ray diffractometer (Rigaku, 3015) using Cu Kα radiation. The X-ray tube was operated at 35 kV and 20 mA. The X-ray diffractogram was recorded at 8°/min in the range of 4°≤ 2θ≤80°. The O2-TPD experiment was performed on TPR Win v 1.50 (produced by Quanta Chrome Instruments Co.) under a flow of pure He (75 ml/min) over 100 mg catalyst at a heating rate of 10 °C/min. H2-TPR measurement was carried out on the same device and under the same condition as O2-TPD.

JOURNAL OF RARE EARTHS, Vol. 26, No. 5, Oct. 2008

The CO and H2 selectivity obviously enhances with the temperature increase, and SCO and SH2 reach their steady state levels at about 96% and 97%, respectively, when the temperature is higher than 800 °C. For methane conversion, it grows with the increase of temperature after a period of slight decrease. It indicates that there is an oxygen species with strong oxidization property, which can cause the complete oxidation of methane to CO2 and H2O. Also, there is another oxygen species that can selectively oxidize CH4 to CO and H2 when the temperature is high enough. Combining with the TPR (Fig.7) profiles of fresh oxygen carriers and the analysis in Refs.[4,5], we think that the first oxygen species should be adsorption oxygen including weak adsorption molecular oxygen and surface lattice oxygen, and the second oxygen species is bulk lattice oxygen, which has very strong selectivity but is only active under the condition of high temperature. In the beginning of the reaction, the adsorption oxygen completely oxidizes methane by consuming itself; meanwhile, the release of bulk lattice oxygen is slow under low temperature, and thus, the CH4 conversion decreases owing to the lack of oxygen. When the temperature is higher than 725 °C, the release of bulk lattice oxygen is accelerated and its activity becomes stronger, thus, the CH4 conversion increases under this condition, and as the bulk lattice oxygen shifts to be dominant in the reaction, the selectivity of H2 and CO is enhanced obviously. 2.1.2 Continuous flow reaction at fixed temperature The continuous flow reaction between CH4 and CeO2-Fe2O3 was proceeded at 820, 850, and 880 °C, respectively. The results are shown in Figs. 2, 3, and 4. Fig.2 shows that the selectivity of CO and H2 increases remarkably with the proceeding of the reaction. Then, both reach a steady state level at about 93%, while the conversion of CH4 decreases for a period before becoming stable. By calculation, it can be found that the value of n(H2)/n(CO) increases from 1.63 to 2.06 with the reaction time from the

2 Results and discussion 2.1 Evaluation on the activity of the selective oxidation of CH4 2.1.1 Temperature programmed reaction The reaction proceeds at atmospheric pressure and the result is shown in Fig.1. It can be seen that H2 and CO appear at 650 and 700 °C, respectively; prior to this, the main products are CO2 and H2O.

Fig.1 Effects of temperature on catalytic performance of CeO2Fe2O3 oxygen carrier (1) SH2 ; (2) SCO; (3) XCH4

LI K Z et al., Catalytic performance of cerium iron complex oxides for partial oxidation of methane to synthesis gas

Fig.2 Result of reaction between methane and CeO2-Fe2O3 oxygen carrier at 820 °C (1) SH2; (2) SCO; (3) XCH4

Fig.3 Result of reaction between methane and CeO2-Fe2O3 oxygen carrier at 850 °C (1) SH2; (2) SCO; (3) XCH4

30th second to the 240th second, and then it maintains around 2 till the 540th second. This indicates that the adsorption oxygen can completely oxidize methane to CO2 and H2O at the early stage of the reaction. After that, the inside bulk lattice oxygen releases from the oxygen carrier and oxidizes methane selectively to CO and H2. The high rate of adsorption oxygen consumption and the lower release rate of bulk lattice oxygen together account for the decrease of methane conversion at the early stage of the reaction. When the adsorption oxygen is completely consumed, a steady methane conversion is obtained. The result of the reaction at 850 °C is illustrated in Fig.3. Comparing with the results at 820 °C, it is seen that the selectivity of H2 and CO varies in a similar tendency. However, the methane conversion increases again after a period of stable state between the 240th second and the 540th second. This may be because the Fe2O3 can be reduced to iron with the consumption of lattice oxygen and can act as a catalyst for methane decomposition. Fig.4 shows the results of the methane oxidation reaction at 880 °C. At the early stage, the selectivities of both H2 and

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Fig.4 Result of reaction between methane and CeO2-Fe2O3 oxygen carrier at 880 °C (1) SH2; (2) SCO; (3) XCH4

CO are very low; however, they increase quickly with the proceeding reaction. At the same time, the methane conversion drops drastically to a steady state of about 54.5%. It demonstrates that the adsorption oxygen on the oxygen carrier can be quickly consumed and the bulk lattice oxygen can be released rapidly to oxidize methane to H2 and CO with strong activity and selectivity under the high temperature condition (e.g., 880 °C). In the above-mentioned analysis, we can find that the oxygen carrier shows its high selectivity only after the adsorption oxygen is completely consumed. Therefore, we believe that the adsorption oxygen functions to completely oxidize methane while the bulk lattice oxygen functions to selectively oxidize methane. This conclusion is in accordance with that of Ref. [6]. 2.2 Redox cyclic property of CeO2-Fe2O3 oxygen carrier Since the bulk lattice oxygen can be preferentially regenerated at high temperature[7], we can enhance the selectivity of the oxygen carrier by controlling the regeneration time. According to our preliminary research, we can assure the experiment condition: after the methane oxidation reaction proceeds for 480 s, the air (flow rate, 30 ml/min) is introduced into the reactor to regenerate the reduced oxygen carrier, and this process lasts for 5 min. Table 1 shows the effects of successive cycles on the catalytic performance of CeO2-Fe2O3 oxygen carrier. The CH4 conversion, CO and H2 selectivity, and the value of n(H2)/n(CO) are all average values of the period of 30–480 s. Table 1 shows that with the increase of the redox circle number, the methane conversion decreases slightly, while the selectivity of CO and H2 increases obviously. Compared with the H2-TPR results of the different pretreated oxygen carrier in Fig.7, it can be found that the above mentioned results must be related to the decrease of adsorption oxygen

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JOURNAL OF RARE EARTHS, Vol. 26, No. 5, Oct. 2008

Table 1 Effects of successive cycles on the catalytic performance of CeO2-Fe2O3 oxygen carrier Number of cycle

XCH4 /%

SCO/%

SH2/ %

n(H2 )/n(CO)

1

57.54

88.10

86.29

1.84

5

54.58

96.72

95.90

1.97

10

52.62

98.83

98.64

1.99

on the complex oxides. This decrease can be explained from two aspects. First, it may be because of the excellent control of regeneration time that enables the reduced oxygen carrier to be regenerated selectively with bulk lattice oxygen. This make the adsorption oxygen on the oxygen carrier become quite slight. On the other hand, the formation of CeFeO3 may change the oxygen storage capacity of the oxygen carrier, which can also lead to the decrease of adsorption oxygen. Therefore, we can conclude that the main reason for the higher selectivity of oxygen carrier after successive cycles is the decrease of adsorption oxygen, and this also causes the decrease of the total oxygen content, which accounts for the slight decline of CH4 conversion. 2.3 Material characterizations 2.3.1 Results of XRD From Fig.5, it can be seen that there is no separation of Fe2O3 phase for the fresh oxygen carrier. The absence of Fe2O3 diffraction peaks can be described by the following reasons. The Fe2O3 species are present in a highly dispersed state. The crystal size of Fe2O3 is very small to be detected. Additionally, the Fe2O3 species form a solid solution with CeO2. The above two states should be coexistent. The lattice structure of the oxygen carrier changes greatly after treating with methane. Besides the diffraction peak of CeO2 becoming weaker, the Fe and Ce are also detected. This demonstrates that CeO2 and Fe2O3 together supply a large amount of lattice oxygen, which takes part in the reaction of the par-

Fig.5 XRD patterns of CeO2-Fe2O3 oxygen carrier after different treatments

tial oxidation of methane. After five cycles, the dominant lattice of oxygen carrier is still CeO2, however, Fe2O3 also appears. This phenomenon may be owing to the accumulation of high dispersed Fe2O3, which results from the successive redox cycles. After ten redox cycles, the oxygen carrier becomes quite different from the fresh one. Besides, there appears CeFeO3, and the CeO2 diffraction peaks become considerably wider and weaker. For the appearance of CeFeO3, this can be explained from two aspects. First, the continuous redox process may contribute to the grain refining of complex oxides and change the surface physical chemistry property of the material, which will promote the inter-dispersion between CeO2 and Fe2O3 and lead to the formation of CeFeO3. Second, some researches[8,9] indicate that the Fe species in CeFeO3 is mainly Fe2+. In this experiment, we always introduce N2 to dispel the CH4 in the reactor to avoid explosion in the interval of switching CH4 with air, and at the same time, several Fe species in the oxygen carrier are in their low valence (e.g. FeO); the N2 here functions as protective gas for the formation of CeFeO3. 2.3.2 O2-TPD and H2-TPR results Fig.6 shows the O2TPD profiles of fresh sample. It is seen that there are three obvious peaks of oxygen desorption appearing at 195, 345, and 548 °C, respectively. Generally, there are two kinds of oxygen species on the oxides, adsorption oxygen in surface and bulk lattice oxygen[10]. According to Ref.[11], the desorption peak appearing at 195 °C should be ascribed to the weak adsorption molecular oxygen; the desorption peak appearing at 345 °C should be ascribed to the chemically adsorbed oxygen on the surface; the desorption peak appearing at 548 °C should be ascribed to the chemically adsorbed oxygen in the oxygen vacancy. Thus, we can conclude that there are large amounts of adsorption oxygen on the fresh oxygen carrier, which can emigrate quickly at lower temperature. The adsorption oxygen can cause the complete oxidation of methane, which is the reason that the selectivity of CO and H2 is low in the initial stage of the reaction. Meanwhile, we notice that no desorption peak appears at higher temperature. It indicates that the bulk lattice of fresh

Fig.6 O2-TPD profile of fresh CeO2-Fe2O3 oxygen carrier

LI K Z et al., Catalytic performance of cerium iron complex oxides for partial oxidation of methane to synthesis gas

sample cannot release at high temperature. This is not in accordance with the results of TPR. The reasonable explanation is that the bulk lattice of oxygen is extremely stable in inert gas, but it can show comparative activity and selectivity under reducing condition. Fig.7 depicts the H2-TPR profiles of fresh sample and the sample after ten successive cycles. There are five reduction peaks in the TPR profiles of fresh oxygen carrier. The first peak appears at 192 °C, which is ascribed to the reaction of weak adsorption molecular oxygen. The peak at 330 °C, which has a broad reduction temperature region can be ascribed to the reaction of chemically adsorbed oxygen. The two sharp peaks appearing from 400 to 550 °C should be ascribed to the reduction of lattice oxygen. Generally, lattice oxygen consists of surface lattice oxygen and bulk lattice oxygen[12]. Surface lattice oxygen can contact directly with H2 and it can be first reduced at low temperature. Judging from the temperatures of the two peaks appearing, the peak at 430 °C should be ascribed to the reduction of surface ceria[13] and the peak at 500 °C should be ascribed to the reduction of surface ferric oxide[14]. Comparing with the H2-TPR profiles of pure CeO2[15,16], it is found that the reduction temperature of the surface lattice oxygen becomes lower. This indicates that there must be a strong interaction between the highly dispersed Fe2O3 and CeO2, which can accelerate the fluidity of surface O2-. With the temperature increasing, the bulk lattice oxygen of the oxygen carrier is reduced (the reduction peak appears at about 800 °C). The TPR profile of the oxygen carrier after ten redox cycles has only two obvious peaks. The first one appears at 450 °C and the second at 750 °C. Obviously, there is absolutely no weak adsorption molecular oxygen on this sample; also, the surface lattice oxygen becomes very little, and therefore, the bulk lattice oxygen is the main oxygen species. This well explains why this kind of carrier has shown high selectivity in Table 1, and it proves that the bulk lattice oxygen indeed is high selectivity oxygen species. Two main reasons may account for the disappearance of adsorption

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oxygen and the drastic decrease of surface lattice oxygen: First, it may relate to the cycle process that is chosen; and second, it may be related to the formation of CeFeO3. The surface lattice oxygen is comprised of several species, such as OH–, O–, O2–, and O2–, etc[17]. The formation of CeFeO3 is accompanied with the transformation from Fe3+ to Fe2+ [8,9]. The transferred electrons probably come from the surface lattice oxygen, such as O–, O2–, or O2– etc. Then, the surface lattice oxygen that loses electrons transforms to weak adsorption molecule oxygen, which can emigrate from the oxygen carrier soon in N2 flow, which is shown in TPR as the decrease of surface lattice oxygen. As observed from Table 1, it can be seen that the methane conversion slightly decreases while the selectivity of CO and H2 slightly increases with the number of redox cycles increasing from five to ten. This should be related to the formation of CeFeO3.

3 Conclusion CeO2-Fe2O3 oxygen carrier was prepared by the co-precipitation method. Some of the Fe2O3 species were present in a highly dispersed state and others formed a solid solution with CeO2. The O2-TPD and H2-TPR characteristic results revealed that there were two kinds of oxygen species on the fresh oxygen carrier: adsorption oxygen and bulk lattice oxygen. The adsorption oxygen could cause the complete oxidation of methane but the bulk lattice oxygen was responsible for methane selective oxidation into CO and H2 at high temperature. Successive cycle experiment indicated that the CeO2-Fe2O3 complex oxide had good circulation property. The selectivity of oxygen carrier could be effectively enhanced by choosing the appropriate redox cycle process. Some CeFeO3 appeared in the oxygen carrier after ten cycles, and the selectivity of CO and H2 increased slightly and reached 98.83 and 98.64 , respectively, when compared with the sample after five cycles.

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Fig 7 H2-TPR profiles of CeO2-Fe2O3 oxygen carrier after different treatments

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