Methane reforming with carbon dioxide to synthesis gas over Co-doped Ni-based magnetoplumbite catalysts

Applied Catalysis A: General 250 (2003) 13–23

Methane reforming with carbon dioxide to synthesis gas over Co-doped Ni-based magnetoplumbite catalysts JunXia Wang, Yan Liu, TieXin Cheng, WenXing Li, YingLi Bi, KaiJi Zhen∗ College of Chemistry, Jilin University, Changchun 130023, PR China Received 8 July 2002; received in revised form 16 November 2002; accepted 24 January 2003

Abstract A series of Co-doped magnetoplumbite LaNix Co1−x Al11 O19+δ catalysts were prepared and characterized by XRD, UV-DRS, TPR, and DTA–TG techniques. The experimental results showed that the series of catalysts possessed the pure magnetoplumbite structure and exhibited similar reduction stability values. At x ≤ 0.375, the activities of the catalysts apparently increased with the increase of x-value, while over 0.375 ≤ x ≤ 1.0, the activities of the catalysts remained almost constant. Thus, it can be concluded that the content of Ni that is between 0.375 ≤ x ≤ 0.50, is suitable for the titled reaction. © 2003 Elsevier B.V. All rights reserved. Keywords: Magnetoplumbite LaNix Co1−x Al11 O19+δ ; Methane; CO2 -reforming; Synthesis gas

1. Introduction Catalytic reforming of CH4 with CO2 to produce synthesis gas has gained a growing interest in the last decades, in plans for the comprehensive utilization of natural gas and CO2 , which are substances intimately related to the energy resources and the environment [1,2]. Numerous supported metal catalysts including Ni and noble metals (Ru, Rh, Pd, Pt, and Ir) [3–17] have been tested for this reaction. The main problems encountered in this reaction are rapid deactivation of the catalysts caused by carbon deposition and sintering of the metallic active component. In general, the noble metal-based catalysts can be used under conditions with lower carbon deposition. However, from a viewpoint of economics, it is desirable to employ non-precious metals. Supported Ni-based catalysts are ∗ Corresponding author. Tel.: +86-431-8922331-2356; fax: +86-431-8949334. E-mail address: [email protected] (K. Zhen).

known as typical catalysts for this reaction. However, carbon deposition and sintering of the metallic nickel often occur, causing deactivation and blockage of the catalysts [18–22]. Therefore, it is extremely important to improve the state of Ni-based catalysts and to decrease the loading amount of Ni in order to overcome these disadvantages. The present authors [23,24] have reported a series of Ni-based hexaaluminate oxides, and have investigated their structure and catalytic activities for the titled reaction. Hexaaluminate LaNiAl11 O19+δ showed excellent catalytic activities and good reducibility, because of high dispersion of Ni ions in the hexaaluminate lattice. But there are no reports on the influence of the substitution of other transition metals for part of nickel ions on the structure and the catalytic properties. In this study, the Ni-based magnetoplumbite oxides doped with Co were successfully prepared to investigate the influence of the amount of Ni ions in the hexaaluminate lattice as well as the influence of the Co amount on their structures and the catalytic

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properties for this reaction. The experimental results showed that the amount of Ni, within a limited range, had no apparent influence on the catalytic activities. This phenomenon was contrary to the facts over the traditional supported Ni-based catalysts over which the activities of the catalysts reached a maximum with increasing loading of the active component, and then decreased with further increase of the nickel loading. Possible explanations for the obtained results are given.

Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were carried out in air on a Perkin-Elmer TGA 7 thermogravimetric analyzer and a Perkin-Elmer DTA 1700 differential thermal analyzer, respectively, at a heating rate of 20 ◦ C/min. TEM images of deposited carbon were taken by means of a HITACHI-8100IV electron microscope operated at 200 kV. The samples were dispersed by ultrasonic waves in an aqueous surfactant solution before being mounted on a Cu grid. 2.3. Catalytic activity test

2. Experimental 2.1. Catalyst preparation Magnetoplumbite oxides, LaNix Co1−x Al11 O19+δ were prepared via La(NO3 )3 ·6H2 O, Ni(NO3 )2 ·6H2 O, Co(NO3 )2 ·6H2 O and Al(NO3 )3 ·9H2 O; polyethylene glycolisopropyl alcohol solution was used as dispersion reagent. The mixture was evaporated to dryness at 80 ◦ C, then stored in an oven to remove polyethylene glycol and to decompose the nitrates. After being ground into fine powder, the sample was calcined at 400 ◦ C for 2 h and at 1250 ◦ C for 5 h in air. 2.2. Catalyst characterization The XRD patterns of the samples, scanned on a Shimadzu XD-3A diffractometer using Cu K␣ radiation at 30 kV and 20 mA, were used to identify the phase of the calcined catalysts. The reducibility of the catalysts was characterized by temperature programmed reduction technique (TPR); here 0.1 g of catalyst was embedded in a quartz tube with an inner diameter of 8 mm. Before reduction, the samples were purged and then the reactor was heated from room temperature to 1200 ◦ C at a heating rate of 20 ◦ C/min in a 10% H2 /Ar mixture gas flow of 30 ml/min. The temperature was measured by using a thermocouple located in the catalyst bed. The effluent gases were analyzed using a gas chromatograph (Shimadzu GC-8A) equipped with a thermal conductivity detector (TCD). Coordination environment of the Ni2+ ions in the samples was detected on an ultraviolet–visible (UV–Vis) spectrophotometer (Shimadzu UV-1601PC) with 1 nm resolution over a range of 200–800 nm.

Catalytic activities were tested in a fixed-bed quartz reactor under atmosphere pressure. The catalysts were pre-reduced in H2 flow for 30 min at 900 ◦ C before each test. The optimal reaction conditions were: reaction temperature range 650–800 ◦ C; CH4 :CO2 = 1:1, total flow rate of 40 ml/min, amount of catalysts 0.2 g (0.25 ml and 40–60 mesh). 3. Results and discussion 3.1. Catalytic properties As shown in Fig. 1, for LaNix Co1−x Al11 O19+δ , the CH4 and CO2 conversions are clearly increased with the rising of reaction temperature. However, at 0.0 ≤ x ≤ 0.375, the CH4 and CO2 conversions are increased with the increase of x-value at the same reaction temperature. On the contrary, at 0.375 ≤ x ≤ 1.0, their catalytic activities became close to each other with the increase of x-value at the same reaction temperature, and the CH4 and CO2 conversions over all catalysts reached 93 and 96% at 800 ◦ C, respectively. This phenomenon was contrary to that which occurred on the traditional supported Ni catalysts, where the activities of the catalysts reached a maximum with increasing loading of the active component, but was often very low at higher nickel loading. Assuming that the mentioned results are due to the active species of nickel highly dispersed into the magnetoplumbite lattice, we could decrease the amount of Ni in by adding Co ions to the lattice, in order to enhance the catalytic properties due to increase of resistance to the active metallic particle sintering and carbon deposition. In order to know the effect of space velocity on the catalytic activity for this reaction over this series

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Fig. 1. Temperature dependence of catalytic activities over LaNix Co1−x Al11 O19+δ . Catalyst: 0.2 g; CH4 :CO2 = 1:1; GHSV: 9600 h−1 .

of magnetoplumbite oxide, we measured the change of catalytic activities over the LaNi0.5 Co0.5 Al11 O19+δ with space velocity for this reaction at 800 ◦ C as shown in Fig. 2. The increase in the space velocity resulted in a slow decrease in the conversions of CH4 and CO2 . At 9600 h−1 , the conversions of CH4 and CO2 were the highest: 92 and 95%, respectively. It was noted that at low space velocity a thermodynamic equilibrium of this reaction may be reached [25].

3.2. Characterization of the catalysts 3.2.1. Crystal structure and stability of the catalysts The X-ray diffraction patterns of the LaNix Co1−x Al11 O19+δ are given in Fig. 3a. It is noted that the series of magnetoplumbite oxides exhibit the same crystalline structure, namely, the same diffraction peak (2θ) position. Their characteristic diffraction

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Fig. 2. Effect of GHSV on Catalytic activities over LaNi0.5 Co0.5 Al11 O19+δ . Catalyst: 0.2 g; CH4 :CO2 = 1:1; reaction temperature: 800 ◦ C.

peaks are at 35.9, 33.8, and 31.9◦ , respectively, meaning that the LaNix Co1−x Al11 O19+δ are of pure magnetoplumbite-type structure [26–28]. No NiO, CoO, NiAl2 O4 and CoAl2 O4 phases are detected, indicating that both Ni2+ and Co2+ ions are well incorporated into the lattice. Our experiment verified

that the complex oxides with this kind of structure were extremely stable. As shown in Fig. 3b, the positions and intensities of the diffraction peaks of the samples reduced with H2 at 900 ◦ C for 2 h are unchanged but with an additional small peak at 44.5◦ which is contributed by the (1 1 1) plane of metal-

J. Wang et al. / Applied Catalysis A: General 250 (2003) 13–23

lic nickel microcrystal. Besides, we also determined the crystal structure of the used catalysts shown in Fig. 3c. Our results indicate that, after 2 h usage, this series of complex oxides still keep the same structure as that of the fresh ones. Comparing Fig. 3b and c, one can verify the structural stability of this series of complex oxides.

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3.2.2. Reducibility of magnetoplumbite oxides LaNix Co1−x Al11 O19+␦ The TPR results of the LaNix Co1−x Al11 O19+δ are displayed in Fig. 4. From the profiles, we can find that the LaNiAl11 O19+δ (x = 1) shows only one reduction peak. It begins to be reduced at about 790 ◦ C and centers at 1010 ◦ C; the LaCoAl11 O19+δ (x = 1)

Fig. 3. (a) XRD patterns of the LaNix Co1−x Al11 O19+δ before reduction. (b) XRD patterns of the LaNix Co1−x Al11 O19+δ after reduction. (c) XRD patterns of the LaNix Co1−x Al11 O19+δ after reaction.

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Fig. 3. (Continued ).

also displays only one reduction peak. It begins to be reduced at higher temperature and centers at 1105 ◦ C. Based on the above results, we can conclude that Co2+ ions in the lattice are more difficult to be re-

duced than Ni2+ ions. Nevertheless, the higher the reduction temperature, the stronger the interaction between the metallic ions in the lattice. It is confirmed that Co2+ ions in the lattice interact more

Fig. 4. TPR profiles of the LaNix Co1−x Al11 O19+δ before reaction.

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strongly with other ions than Ni ions do. The profiles also indicate that all the catalysts show two reduction peaks. At 0.125 ≤ x ≤ 0.875, one peak centers at 920 ◦ C and the other peak centers at 1050 ◦ C. It LaNiAl11 O19+δ is compared with LaCoAl11 O19+δ , we see that the reduction peaks at lower temperature are contributed to the reduction of Ni2+ , and the peaks at higher temperature result from the reduction of Co2+ . These similar shapes of the TPR profiles of the catalysts indicated that the valence state of transition metals in LaNix Co1−x Al11 O19+δ was Ni2+ and Co2+ , respectively. At temperatures lower than 750 ◦ C, there are no reduction peaks, meaning that Ni2+ and Co2+ ions all enter the magnetoplumbite oxides lattices, and no other phases exist, which is in agreement with their XRD results. Moreover, at 0.125 ≤ x ≤ 0.875, the intensity of the low temperature peak area is increased with the increase of x-value, meaning that more and more Ni2+ ions in the lattices can be reduced to metallic Ni0 with the increase of x-value, but the change tendencies of the intensity of the high reduction peaks are contrary to this case. From the TPR profiles, we could conclude that only Ni2+ ions could be reduced from the lattice when the reduction temperature was below 900 ◦ C,

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which gave a reasonable explanation at the fact that the active species for the reforming reaction was mainly Ni0 . 3.2.3. Ultraviolet diffusion reflectance spectra of the catalysts The diffuse reflection spectra of the LaNix Co1−x Al11 O19+δ are given in Fig. 5. The bands locate around 370, 580, and 630 nm, respectively, which are characteristics of Ni2+ ions in a spinel phase in which they exist in tetrahedral and octahedral sites according to the partial inversion of the NiAl2 O4 spinel phase [29]. The bands around 370 and 580 nm are attributed to Ni2+ in an octahedral symmetry, while the band around 630 nm is assigned to Ni2+ in a tetrahedral coordination. As demonstrated in the previous section, this series of the compound oxides contain both tetrahedral and octahedral Ni2+ sites. In the meantime, there are no apparent changes in the intensity of these three absorption bands with the changes of x-value, likely due to the facts that Co2+ ions and Ni2+ ions are incorporated into the same types of lattice sites on the one hand, and that Co2+ ions provide similar characteristic absorption bands with Ni2+ ions on the other hand.

Fig. 5. UV-DRS spectra of the LaNix Co1−x Al11 O19+δ before reaction.

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Fig. 6. DTA–TG profile of LaNi0.5 Co0.5 Al11 O19+δ after reaction for 2 h at 800 ◦ C.

3.2.4. DTA–TG profiles of LaNix Co1−x Al11 O19+␦ Carbon deposition on the catalysts during CO2 reforming of methane was evaluated under the same reaction conditions, as shown in Figs. 6–8, respectively. The curve in Fig. 6 indicates that the amount of the deposited carbon on the LaNi0.5 Co0.5 Al11 O19+δ after reaction at 800 ◦ C for 2 h is about 3.42%, and its DTA profile exhibits that the deposited carbon is eliminated in a temperature range between 327.0 and 680.0 ◦ C. Fig. 8 gives the information that the amount of the deposited carbon on the LaNiAl11 O19+δ after reaction at 800 ◦ C for 2 h is about 3.10%, and its DTA profile exhibits that the deposited carbon can be burned up in a temperature range between 334.0 and 700 ◦ C. Their difference may be mainly due to the fact that the carbon deposited on the LaNi0.5 Co0.5 Al11 O19+δ is more easily burned up and does not result in the deactivation of the catalyst, which is attributed to the relatively low amount of Ni ions that is beneficial to high dispersion of metallic nickel on the catalyst surface and is more resistant to sintering of the nickel particles. However, for the LaNiAl11 O19+δ , the higher amount of Ni ions results in metallic nickel sintering that stimulates coke formation and more strong interaction between the deposited carbon and nickel particles.

From the results in Fig. 7, it is noted that there are no change of weight in the sample. This phenomenon was not similar to that of the above mentioned Co-doped catalysts. We assumed that the different results were due either to differences of the deposited carbon type, some of which were more difficult to eliminate, or to the fact that no carbon is deposited on the catalyst. 3.2.5. TEM profiles of the used catalysts The deposited carbon is formed via different routes, giving the most common types of carbon: whisker-like carbon, encapsulating carbon, and pyrolytic carbon [30]. The TEM profiles of the catalysts in Figs. 9 and 10 provide clear evidence that in our case the deposited carbon on the catalysts is mainly in the form of whisker carbon during this reaction. As observed in the figures, the whisker carbon possesses the shape of nanotubes and does not cause the deactivation of the catalysts. In the TEM images, encapsulating carbon and pyrolytic carbon are not observed after this reaction. So the increased weight of the catalyst (shown in Figs. 6 and 8) should mainly be from the whisker carbon growth. The outside diameter of the nanotubes on the LaNi0.5 Co0.5 Al11 O19+δ is about 30 nm, and

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Fig. 7. DTA–TG profile of the LaNi0.75 Co0.25 Al11 O19+δ after reaction for 2 h at 800 ◦ C.

Fig. 8. DTA–TG profile of the LaNiAl11 O19+δ after reaction for 2 h at 800 ◦ C.

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Fig. 9. TEM micrograph of the LaNi0.5 Co0.5 Al11 O19+δ after reaction for 2 h at 800 ◦ C.

is dependent on the metallic Ni particle size. Metallic particles at the tips of these primary nanotubes in general exhibit a conical shape. The average size of nickel particles on LaNi0.5 Co0.5 Al11 O19+δ is about 35 nm. However, the diameter of the nanotubes on LaNiAl11 O19+δ is about 35 nm and the average size of metallic Ni particles is about 40 nm. In Fig. 11, we cannot observe any kind of carbon on the used LaNi0.75 Co0.25 Al11 O19+δ . This result is in consistent with the DTA–TG profile (shown in Fig. 7); in the meantime, many nickel particles appear in the TEM images after this reaction, the average size of which is relatively uniform and is about 30 nm. This result identified that the LaNi0.75 Co0.25 Al11 O19+δ catalyst had good coking resistivity for some unknown reasons that should be studied later.

Fig. 11. TEM micrograph of the LaNi0.75 Co0.25 Al11 O19+δ after reaction for 2 h at 800 ◦ C.

4. Conclusion A series of LaNix Co1−x Al11 O19+δ catalysts were prepared. These possessed pure magnetoplumbite-type structure and both Ni2+ ions and Co2+ ions were incorporated into the complex oxide lattice. At x ≤ 0.375, the catalytic activities of the catalysts increased with the increase of x-value. However, at 1.0 ≥ x ≥ 0.375, no effect of x-value on the catalytic properties was revealed, and the CH4 and CO2 conversions remained about 93 and 96% at 800 ◦ C, respectively. Thus, we concluded that only a limited amount of Ni was effective for the titled reaction and the excessive amount of Ni was useless on the one hand, and resulted in accommodation of the active species on the catalyst surface on the other hand. The experimental results showed that a range of 0.50 ≥ x ≥ 0.375 was appropriate for this reaction. The Co-doped catalysts could enhance the catalytic properties in the aspects of resistance to metal sintering and carbon deposition. Acknowledgements The authors gratefully acknowledge the Natural Science Foundation of China (29973012) for financial support of this work. References

Fig. 10. TEM micrograph of the LaNiAl11 O19+δ after reaction for 2 h at 800 ◦ C.

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