Preparation and characterization of Ce1-xFexO2 complex oxides and its catalytic activity for methane selective oxidation

Preparation and characterization of Ce1-xFexO2 complex oxides and its catalytic activity for methane selective oxidation

JOURNAL OF RARE EARTHS, Vol. 26, No. 2, Apr. 2008, p. 245 Preparation and characterization of Ce1-xFexO2 complex oxides and its catalytic activity fo...

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JOURNAL OF RARE EARTHS, Vol. 26, No. 2, Apr. 2008, p. 245

Preparation and characterization of Ce1-xFexO2 complex oxides and its catalytic activity for methane selective oxidation LI Kongzhai (李孔斋), WANG Hua (王 华), WEI Yonggang (魏永刚), LIU Mingchun (刘明春) (Faculty of Materials and Metallurgy Engineering, Kunming University of Science and Technology, Kunming 650093, China) Received 15 August 2007; revised 25 December 2007

Abstract: A series of Ce1-xFexO2 (x=0, 0.2, 0.4, 0.6, 0.8, 1) complex oxide catalysts were prepared using the coprecipitation method. The catalysts were characterized by means of XRD and H2-TPR. The reactions between methane and lattice oxygen from the complex oxides were investigated. The characteristic results revealed that the combination of Ce and Fe oxide in the catalysts could lower the temperature necessary to reduce the cerium oxide. The catalytic activity for selective CH4 oxidation was strongly influenced by dropped Fe species. Adding the appropriate amount of Fe2O3 to CeO2 could promote the action between CH4 and CeO2. Dispersed Fe2O3 first returned to the original state and would then virtually form the Fe species on the catalyst, which could be considered as the active site for selective CH4 oxidation. The appearance of carbon formation was significant and the oxidation of carbon appeared to be the rate-determining step; the amounts of surface reducible oxygen species in CeO2 were also relevant to the activity. Among all the catalysts, Ce0.6Fe0.4O2 exhibited the best activity, which converted 94.52% of CH4 at 900 °C. Keywords: Ce1-xFexO2 complex oxides; H2-TPR; lattice oxygen; methane selective oxidation; rare earths

The chemical utilization of methane to produce basic chemicals is one of the desirable goals in the current chemical industry. Generally, the indirect transformation of methane via synthesis gas is the most valuable process. Recently, the direct oxidation of methane to synthesis gas by gas–solid reactions, using the lattice oxygen of metal oxides, in the absence of gaseous oxidant has been raised[1]. In this process a suitable Oxygen Storage Compound (OSC) is circulated between two reactors. In one reactor, methane is oxidized to synthesis gas by the lattice oxygen of OSC, and in the other, the reduced OSC is reoxidized by air, water or carbon dioxide. This technology has many advantages when compared with the Partial Oxidation of Methane (POM). First, it can avoid the risk of explosion because of the premixed CH4/O2 mixture within the ignition and explosion limits. Second, the selectivity of the desired product can be enhanced, because the product is not easy to be deeply oxidized in the absence of molecular oxygen. Third, it can save the oxygen supply by the cryogenic distillation of air needs additional investment and operational expense, because which does not need using pure oxygen. In the past few years, manganese, perovskite materials, cerium oxides, and cerium containing

mixed oxides have been studied[2-8]; however, these materials still have a very big disparity when compared to the industrial application. It is also of primary importance that the design of the oxygen storage compound be able to convert methane into synthesis gas by the use of lattice oxygen. CeO2 has elevated oxygen storage capacity and redox properties, and it has been widely used as an essential part in various industrial heterogeneous catalysts and in automotive Three-Way Converters (TWC)[9] It is well known that the lattice oxygen mobility and concomitant oxide ion conductivity in cerium oxide can be increased by the substitution of another metal ion for cerium. Because the ceria shows much improved properties under doping, a lot of ceria-based systems have been investigated. It has been proved that the lower valence ions in ceria influence the energetic properties by lowering the activation energy for oxygen migration[10]. Given the effects that trivalent ions and smaller sizes have on the structure and properties, there is considerable scientific interest in introducing M3+ (e.g., Fe3+) ions into the ceria lattice[11]. In this article, ferric oxide and ceria were incorporated with the aim of increasing the oxygen storage capacity and the oxygen mobility in the oxygen storage compound for

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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this redox cycle process. To study the effect of the atomic ratio of Ce to Fe on the activity of OSC in the catalytic performance, a series of Ce1-xFexO2 (x=0, 0.2, 0.4, 0.6, 0.8, 1) complex oxides were prepared by coprecipitation. In combination with the characterization methods of XRD and H2-TPR, the reactions between methane and lattice oxygen, coming from the complex oxides, were investigated in a fixed microreactor system.

1 Experimental 1.1 Catalyst preparation Ce1-xFexO2 (x=0, 0.2, 0.4, 0.6, 0.8, 1) complex oxides series were prepared by coprecipitation. The starting materials Ce (NO3)3·6H2O and Fe (NO3)3·9H2O were mixed according to the desired mole ratio. The blended solution was sufficiently mixed in a magnetic stirrer and heated at 70 °C. A solution of 10% ammonia was gradually added to the mixture, with stirring. When the pH value was increased to 7–8 and 10–11, the resulting solution was maintained at 70 °C with continuous stirring for 1 h, respectively. Subsequently, the precipitate was filtered and washed with distilled water and ethanol after a two-hour settlement. The resulting mixture was dried at 110 °C for 24 h after natural drying overnight, and some massive objects with different colors were obtained. These massive objects were subjected to decomposition at 300 °C for 2 h and were later ground into powder. These powders were calcined under ambient air at 800 °C for 6 h, and Ce-Fe-O OSC samples were obtained. 1.2 Catalyst characterization The powder X-Ray Diffraction (XRD) experiments were performed on a Japan Science D/max-R diffractometer using Cu Ka radiation (λ=0.15406 nm). The X-ray tube was operated at 40 kV and 40 mA. The X-ray diffraction was recorded at 0.01°intervals in the range of 10°≤2θ≤80° with 3 s count accumulation per step. The identification of the phase was made with the help of JCPDS cards (Joint Committee on Powder Diffraction Standards). Temperature-Programmed Reduction (TPR) experiments were performed on TPR Win v 1.50 (produced by Quanta Chrome Instruments Co.) under a flow of a 10% H2/He mixture (75 ml/min), over 100 mg catalysts, using a heating rate of 10 °C/min.

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and then pure N2 was allowed to flow into the reactor at 400 °C for an hour. Methane (99.99% purity) oxidation reaction was performed in the temperature range of 600–900 °C at a heating rate of 15 °C/min. The total gas flow rate of the reaction mixture was controlled by a mass flow controller at a specific flow rate of 10 ml/min. Reactant and product components were analyzed online by a gas chromatograph (GC112A, produced by Shanghai Precision & Scientific Instruments Co.) equipped with a thermal conductivity detector (TCD) and an active carbon column (2 m). Argon was employed as a carrier gas. CH4 conversion, CO selectivity, and H2 selectivity were calculated based on the analysis results of GC.

2 Results and discussion 2.1 Catalyst characterization The XRD results of the CeO2, Fe2O3, and CeO2-Fe2O3 oxygen storage compound with different Ce/Fe mole ratios are shown in Fig.1. It can be seen that the diffraction of pure CeO2 presents a very strong peak at 2θ =28.5°, and other weak peaks characteristic of fluorite CeO2[12]. To study the interaction between Fe2O3 and CeO2, the mean parameter of the ceria lattice has been calculated on the basis of the XRD peak of crystal plane. The lattice parameter of Ce0.8Fe0.2O2 (0.5392 nm) is a little smaller than that of pure CeO2 (0.5411 nm). It suggests that Fe3+ has been incorporated into the ceria lattice and results in a contraction of the cell parameter. However, from Fig.1 it can be seen that very weak α-Fe2O3 characteristic peaks appear in Ce0.8Fe0.2O2 and the peaks are intensified with the contents of α-Fe2O3 increasing. Therefore, the authors speculate that only a small part of Fe3+ has incorporated into the ceria lattice to form solid solutions and the rest are left on the surface of the mixed oxide. Compared to the pure CeO2 and α-Fe2O3, the characteristic CeO2 and α-Fe2O3 peaks of the CeO2-Fe2O3 catalysts become weaker and wider. One of the reasons is

1.3 Catalytic activity tests The selective oxidation of CH4 was carried out in a continuous flow fixed-bed reactor system under atmospheric pressure. An amount of 1.8 g of catalyst was placed in a quartz tube with 19 mm inside diameter. Prior to the catalytic reactions, the catalysts were heated in air at 300 °C for 2 h,

Fig.1 XRD patterns of CeO2-Fe2O3 oxygen storage compound

LI K Z et al., Preparation and characterization of Ce1-xFexO2 complex oxides and its catalytic activity for methane …

that the contents of CeO2 and Fe2O3 are less than the pure samples, and the other is, the combination of Ce and Fe oxide in the catalysts can reduce the α-Fe2O3 and CeO2 crystallite size[13]. Therefore, CeO2-Fe2O3 catalysts should exhibit better catalytic activity than the pure one. This is in accordance with the observation of the catalytic activity experiment and the result of TPR investigation (Fig.2). The temperature programmed reduction (TPR) technique was applied to determine the type of Fe2O3 and CeO2 species that were present in the catalysts. Fig.2 shows the H2-TPR profiles of the Ce1-xFexO2 (x=0, 0.2, 0.4, 0.6, 0.8, 1) catalyst. There are three reduction peaks at about 145, 340 and above 900 °C, of pure CeO2 catalyst. As reported in Ref.[14], the first peak must belong to the reduction of weak adsorption molecular oxygen; the second one should characterize the reduction profile of surface CeO2 and the third one should be attributed to the reduction of bulk CeO2. αFe2O3 has two reduction peaks at low temperature about 635 °C and high temperature with a broad feature between 670 and 900 °C. Compared to the typical hydrogen reduction profiles of iron oxide present two stage reductions[15]. First,

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Fe2O3 converts to Fe3O4 at low temperature and second Fe3O4 converts to Fe at high temperature. However, the four kinds of CeO2-Fe2O3 catalysts show three reduction peaks, α (about 500 °C), β (between 550 and 750 °C), γ (>750 °C), respectively. Comparing with the TPR profiles of pure CeO2 and Fe2O3, it is clear that the three peaks should ascribe to the reduction of dispersed superficial α-Fe2O3, and CeO2 combines with the bulk α-Fe2O3 and bulk CeO2 species. When the content of CeO2 is very slight, the γ peak is covered by the β peak. Furthermore, it is obvious that the relative intensity of α peaks of all four Ce1-xFexO2 catalysts are much more pronounced, as the content of Fe2O3 increases and the temperatures at which the peaks appear are much lower than the pure α-Fe2O3. Compared with pure CeO2, two obvious β peaks can be seen in Ce0.8Fe0.2O2 and Ce0.6Fe0.4O2 catalyst’s TPR profiles, meanwhile, the high temperature γ peaks are much stronger and appear much earlier (the peak temperature maximum drops from 900℃ to 790 and 845 °C, respectively). These results indicate that there is a strong interaction between the Ce and Fe species. 2.2 Correlation between Ce/Fe mole ratios and catalytic activity In the present study, the catalytic activities of CeO2, Fe2O3, and CeO2-Fe2O3 catalysts for selective CH4 oxidation were performed. To study the catalytic performance, the se-

Fig.2 H2-TPR profiles of Ce1-xFexO2 catalysts

Fig.3 Effects of temperature on catalytic performance of Ce1-xFexO2 catalysts

Fig.4 CO and H2 selectivity as a function of reaction temperature for selective oxidation of CH4

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lectivity of the desired product (CO and H2) and the conversion of CH4 were considered. Fig.3 shows CH4 conversion as a function of reaction temperature over the different catalysts. The selectivity of CO and H2 with respect to the reaction temperature is also presented in Fig.4. From Fig.3, it can be seen that the activity of either CeO2 or Fe2O3 for CH4 oxidation is very low at different temperatures. For ceria, the methane conversion elevates along with an increase in temperature, but corresponding to pure Fe2O3, the methane conversion curve appears as a peak at 600–725 °C. According to the H2-TPR profiles of pure α-Fe2O3 in Fig.2, this peak must correspond to the transformation from Fe2O3 to Fe3O4. Meanwhile, as observed in Fig.4 the selectivity of CO and H2 is less than 8.63% and 9.32%, respectively, in this temperature range. This indicates that the lattice oxygen provided by Fe2O3 is reduced to Fe3O4 mainly participated in the methane complete oxidation. Among the six catalysts, it can be seen that the activity of CeO2-Fe2O3 catalysts for CH4 oxidation is higher than that of pure CeO2 or Fe2O3. The increase of CeO2-Fe2O3 catalytic activity may be caused by the strong interaction between CeO2 and Fe2O3 and this is consistent with the TPR analysis results, as presented in Fig.2. Under these conditions, the CH4 conversion of CeO2-Fe2O3 catalysts as a function of temperature expresses as an S-shaped curve in this study. The methane conversion increases with the reaction temperature after a slight decline before 700 °C. The CH4 conversion decreases mainly because the bulk lattice oxygen in the catalyst cannot be released at the time when the dispersed superficial α-Fe2O3 has been reduced at low temperature. Reviewing the whole reaction process, the catalytic activity of the CeO2-Fe2O3 catalyst depends on the Ce/Fe atomic ratio, and Ce0.6Fe0.4O2 exhibits the best activity, which converts 94.52% of CH4 at 900 °C. Selective oxidation of CH4 using lattice oxygen is one method for the production of synthetic gas, which can be connected directly to the Fischer-Tropsch Synthesis. Hence, the selectivity of the desired product (CO and H2) and the n(H2): n(CO) of the product are considered in this study. With regard to all the catalysts the selectivity of CO and H2 elevates along with the increase in temperature, as shown in Fig.4. It indicates that, in the absence of the gas-phase oxygen, methane first reacts with a strong oxygen species to form CO2 and H2O. After these oxygen species decline, the weaker oxygen species are responsible for methane selective oxidation into CO and H2, at high temperature. This is consistent with the Ref.[7] findings. The selectivity of pure CeO2 and α- Fe2O3 have revealed two extremes, for example, the CO selectivity is 96.87% for the CeO2 catalyst and only 27.33% for the α-Fe2O3 catalyst at 900 °C. The addition of Fe2O3 to the CeO2 catalyst reduces the selectivity at the be-

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ginning of the reaction. In the case of Ce0.8Fe0.2O2 and Ce0.6Fe0.4O2 catalysts, the CO and H2 selectivity are with almost no distinction, when compared with pure CeO2 in the later period of the reaction. In the case of Ce0.4Fe0.6O2 and Ce0.2Fe0.8O2 catalysts, they exhibit a much lower selectivity in the whole reaction process. This indicates that there must be a suitable ratio exhibiting a higher degree of interaction between the Ce and Fe species, where the values in parenthesis correspond to the methane conversion. It should be noted that the n (H2):n (CO) of products (according to calculation) approach two, for all the six catalysts despite the fact that the CO selectivity of some catalysts is extremely low, such as, α-Fe2O3 and Ce0.2Fe0.8O2 catalysts, when the temperature is over 800℃. This phenomenon indicates that the methane selective oxidation and deep oxidation occur simultaneously in the later stage of reaction, being under a higher temperature condition (≥800 °C). The selectivity to synthesis gas depends on the degree of reduction and the lattice oxygen performance of different catalysts. 2.3 Interaction between Ce and Fe species On the basis of the foregoing analysis, both XRD, TPR characterization and catalytic activity tests show strong interactions between the Ce and Fe species in CeO2-Fe2O3 catalysts. The XRD peaks (Fig.1) reveal that the catalyst surface is heterogeneous and consists of two distinctly different phases; one as cerium oxide and other as α-Fe2O3. The H2-TPR results show that the interaction affects the reduction temperature of cerium oxide. The drop in Fe2O3 also drastically enhances the conversion of methane in catalytic activity tests. Hence, there is no doubt that the Fe species must take an active part in the conversion. It can also be seen that, the conversions of pure CeO2 and Fe2O3 (Fig.3) are very low and the disparity is not very large. Therefore, it can be considered that both CeO2 and Fe2O3 are not the active sites. On the other hand, there must be another species that can enhance the catalytic activity. The XRD results of the catalysts after reaction are shown in Fig.5. Comparing with the fresh samples the diffraction peak of Fe2O3 disappears, and is replaced by Fe and a carbon iron chemical compound such as Fe3C. Iron is commonly used in catalyzing methane decomposition[16]; this indicates that along with the Fe appearance, the entire reaction system can be inevitably changed. Methane can therefore be activated on the Fe surface and the transformation from methane to carbon or FexC and hydrogen is completed. Activated methane (carbon or FexC) is probably not very mobile and spills over to the catalyst’s surface. Moreover, it is possible that the metal affects the oxygen dynamics of the whole cerium oxide[2]. Under such conditions, reaction xC+CeO2→CeO2-x+xCO can be the rate-determining step. However, this does not

LI K Z et al., Preparation and characterization of Ce1-xFexO2 complex oxides and its catalytic activity for methane …

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References:

Fig.5 XRD patterns of CeO2-Fe2O3 catalysts after reaction (1) Ce0.2Fe0.8O2; (2) Ce0.4Fe0.6O2; (3) Ce0.6Fe0.4O2; (4) Ce0.8Fe0.2O2

mean that the performance will become better if the content of Fe species in the catalyst is a lot more. If there is too much Fe content, the lattice oxygen will not be able to eliminate the massive carbon depositions promptly and the catalytic activity will be reduced. This is the reason why the activity of the Ce0.2Fe0.8O2 catalyst is much lower than the others. On the basis of the analysis given earlier, it can be easily deduced that the dispersed iron that has been obtained by the reduction of dispersed Fe2O3 on the catalyst is the active center of the reaction between methane and CeO2-Fe2O3 catalysts. For CeO2, it is the only oxygen source in the selective oxidation of CH4.

3 Conclusion CeO2-Fe2O3 catalysts show a better catalytic activity in the CH4 selective oxidation reaction when compared to pure CeO2 and α-Fe2O3. The phase cooperation between CeO2 and Fe2O3 was responsible for the enhancement of activity. Dispersed Fe2O3 reacted with methane and then virtually formed the Fe active species on the catalyst, which promotes selective CH4 oxidation. CeO2 mainly provided selective lattice oxygen, which is necessary for the synthesis of gas production. The results indicated that carbon formation appearance was significant and the oxidation of carbon appeared to be the rate-determining step. A very high content of Fe2O3 seemed to be disadvantageous for catalytic activity enhancement and favors the deep oxidation of methane. There was a suitable atom ratio, exhibiting the highest degree of interaction between the Ce and Fe species. Comparison of six types of complex oxide systems made in this study showed that Ce0.6Fe0.4O2 had the best catalytic activity. The methane conversion, CO selectivity, and H2 selectivity were 94.52%, 95.21%, and 95.13%, respectively.

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