Jointly published by Kluwer Academic Publishers, Dordrecht and Akadémiai Kiadó, Budapest
React.Kinet.Catal.Lett. Vol. 71, No. 2, 263-271 (2000)
RKCL3762 DEEP OXIDATION OF METHANE OVER MANGANESE OXIDE MODIFIED BY Mg, Ca, Sr AND Ba ADDITIVES Xiang Wang* and You-chang Xie Institute of Physical Chemistry, Peking University, Beijing, 100871, China Received August 2, 2000 Accepted September 25, 2000
Abstract Manganese oxide catalysts modified by Mg, Ca, Sr and Ba additives were studied for methane deep oxidation. The Ba promoted sample is the most effective one for this reaction among all the catalysts. The catalysts were examined by BET, XRD and H2-TPR techniques. It is speculated that the formation of some more active oxygen species and the formation of basic sites from the addition of alkaline earth metal oxides are responsible for the improvement of the inherent CH4 oxidation activity of the modified catalysts. Keywords: Methane deep oxidation, manganese oxide catalysts, Mg, Ca, Sr and Ba additives
INTRODUCTION CH4, the main component of natural gas, is an attractive fuel due to its high H and low C content, and its widespread distribution around the world [1-3]. Furthermore, CH4 is reported to be a much stronger greenhouse gas than CO2 [1]. Though the emission of CH4 is not restricted now, it is expected to be regulated in the future. Deep oxidation of CH4 over suitable catalysts is an effective approach to utilize it as energy source, and prevent it from emitting into the atmosphere to pollute the environment. Presently, supported precious metals such as Pt, Pd, Rh or their combinations are well known active catalysts for this reaction, among which the Pd catalyst is the most active for CH4 oxidation [4,5]. In fact, precious metals are currently the only kind of commonly ____________________________________
*Corresponding author. Current address: Department of Chemical Engineering, Towne Building, Room 311A, University of Pennsylvania, Philadelphia, PA 19014, USA. Email:
[email protected]. 0133-1736/2000/US$ 12.00. © Akadémiai Kiadó, Budapest. All rights reserved.
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used catalysts in real processes. However, due to the limited source and high price of precious metals, much attention was paid to composite base metal oxides over recent years, expecting to achieve some cheaper catalysts with rival performance to precious metals. Though quite a lot of work has already been done in this area, much is needed to carry out to obtain an applicable catalyst with the desired properties. Manganese oxides are active oxidation catalysts [6-9]. However, manganese oxides prepared by general methods have low specific surface areas [9,10], which may restrict their activity, especially at high hydrocarbon conversion levels, the diffusion-control region. An effective method has been found by our group to prepare a high specific surface area MnO2 (265 m2/g) that shows remarkable activity toward CO oxidation; this catalyst sinters easily with the increase of reaction temperature, accompanying the fast drop of its activity. Work by Xie et al. shows that monolayer dispersed metal oxides can stabilize the surface area of a support and promote its reaction performance [11-14]. Therefore, to add some components to the high surface area MnO2 precursor to obtain some catalysts with better thermal stability and activity is our objective to carry out this work. Basic sites are suggested to be effective for CH4 activation [15]. Lin et al. found that alkaline earth metal oxides as additives can increase the basicity of alumina support, with Ba modified sample having the strongest basicity [16]. The same modification effects may occur on MnO2, thus forming catalysts containing basic sites. Therefore, in this work, Mg, Ca, Sr and Ba oxides are employed to modify the MnO2 precursor. EXPERIMENTAL The MnO2 precursor with high specific surface area (265 m2/g) was prepared by the redox reaction between KMnO4 and Mn(NO3)2 solution [17]. MMn0.10 with M/Mn=0.10 (mol ratio, M=Mg, Ca, Sr and Ba) were prepared by impregnating the corresponding nitrate solution to the MnO2 precursor, followed by drying at 100oC and calcination in static air atmosphere at 600oC for 4~6 hours. MnOx catalyst was achieved by calcination of MnO2 precursor under the same conditions. The specific surface areas were measured by nitrogen sorption at 77 K with ST-30 instrument. XRD patterns were recorded on a BD-90 X-ray diffractometer with CuKα radiation of 40 kV x 20 mA and Ni filter. The scan step is 0.1o with a preset counting time of 4 s. H2-TPR experiments were carried out with H2/N2 (5.1%) gas mixture as the reductant, the temperature being increased from room temperature to 800oC with a ramp of 10oC/min. A TCD was used to measure the H2 consumption.
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Catalytic tests were carried out in a U-shaped fixed-bed microreactor (I.D. = 6 mm) with a continuous downflow. Typically, 0.2 mL 40-60 mesh catalysts were used for activity evaluation. The volume composition of the feed gas is CH4 1.5%, O2 18% and balanced by high purity N2. The total feed flow rate was 70 mL/min corresponding to a gas hour space velocity (GHSV) 21,000 h-1. The reactants and products were analyzed with an on-line 1102G GC equipped with a TCD on Porapak Q column for CH4 and CO2, and on molecule sieve 5A column for CH4 and CO. Before injection, the reaction at each temperature over the catalysts was stabilized for at least 30 min. The flow rate of the carrier gas He (99.99%) was 30 mL/min. Methane conversion was calculated by the change of its peak area before and after the catalyst bed. RESULTS AND DISCUSSION The light-off curves of CH4 over the catalysts are shown in Fig. 1. T10, T50 and T98, the temperatures corresponding to 10%, 50% and 98% CH4 conversion, are listed in Table 1. In this work, T98 is regarded as the CH4 complete conversion temperature.
Fig. 1. CH4 conversion vs. reaction temperature over the catalysts
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At low CH4 conversion (600
24 350 430 600
24 330 410 540
The BET surface areas of the samples are listed in Table 1. The modified samples have similar surface areas to the unmodified one, except MgMn0.10; thus it is suggested that surface area does not play an important role for the alteration of the reaction performance of the modified samples. Among all the modified samples, only BaMn0.10 shows higher activity than MnOx in the whole CH4 conversion region. Therefore, it was subjected to a durability test, with the results shown in Fig. 2. No deactivation was observed during a 600 min run, indicating the catalyst is stable. The effect of feed flow on the reaction performance of BaMn0.10 was also measured. As shown in Fig. 3, with the increase of GHSV from 10,000 h-1 to 80,000h-1, CH4 conversion decreases from 100% gradually to 75%. In the region of 10,000h-1 to 40,000h-1, CH4 can be converted completely into CO2. To clarify the modification effects of alkaline earth metal oxides on MnO2 precursor, XRD was used to identify the bulk phase composition of the samples (Fig. 4). As reported previously, MnO2 precursor mainly consists of microcrystalline α-MnO2, which shows several weak and wide diffraction peaks [18]. This is consistent with its very high surface area. MnOx, which was
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prepared by direct calcination of MnO2 precursor, displays sharp diffraction peaks belonging to α-Mn2O3 (Bixbyte). This demonstrates the desorption of part of the lattice oxygen from MnO2 during the calcination, accompanying the decrease of the valence state of Mn from 4+ to 3+. Two small β-MnO2 (Pyrolusite) diffraction peaks are still observed, as labeled with “o” in Fig. 4. MgMn0.10 shows also the diffraction peaks of α-Mn2O3 and β-MnO2 with a similar intensity to those of MnOx; besides these peaks, several unidentified peaks are observed which cannot be assigned to MgO. This may imply the formation of some new composite oxide phases between Mg and Mn during the calcination. Compared with MnOx, the curves of CaMn0.10, SrMn0.10 and BaMn0.10 show much weaker diffraction peaks of α-Mn2O3; β-MnO2 cannot be detected any more. As shown in Table 1, the BET surface areas of these samples are similar to MnOx, which exclude the possibility of impeding the crystallization of manganese oxide by the additives, though this phenomenon is widely observed in supported catalysts [11,13]. In CaMn0.10, a new phase, CaMn3O7 is detected; in BaMn0.10, two new phases, BaMn8O16 and BaMnO3 were formed, as labeled by “´ DQG ³■”, respectively, in Fig. 4. In SrMn0.10, besides the weak peak of α-Mn2O3, quite a lot of diffraction peaks are present, which complicate the assignment of them. However, this seems to indicate that the low intensity of the diffraction peaks of α-Mn2O3 in these samples is due to its low amount, that is caused by the formation of new phases between Ca, Sr, Ba and manganese oxide. These new-formed phases may help to improve the inherent activity of the catalysts.
Fig. 2. Durability test of BaMn0.10 at 540oC with a GHSV 20,000 h-1
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Fig. 3. The reaction performance of BaMn0.10 under different feed flow rates at 540oC
Fig. 4. XRD patterns of the catalysts
● α-Mn2O3 ❍ β-MnO2 ◆ CaMn3O7 ■ BaMnO3 BaMn8O16
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H2-TPR method was used to explore the nature of the catalysts (Fig. 5). Each sample shows two reduction peaks. As to MnOx, our previous results testify that the two reduction peaks correspond to the two reduction steps expressed by Mn2O3→Mn3O4→MnO [18]. However, the reduction of the modified samples is complexed by the additives. Though two peaks are still observed for each sample, the shapes and the H2 consumption ratios between the two peaks are changed. We hesitate to assign the two reduction peaks of the modified samples to the above mentioned two steps again. However, after reduction, the brown color of all the samples changed into green, which is typical for MnO [10,18]. Thus it is concluded that the main reduction product of the samples is probably MnO.
Fig. 5. H2-TPR profiles of the catalysts
The position of the main reduction peak of the modified samples is similar to that of MnOx. However, the peak shape of MgMn0.10 and SrMn0.10 become sharper, indicating that the reduction of the oxygen species was enhanced by Mg and Sr additives. Compared with MnOx, the peak shape of CaMn0.10 and BaMn0.10 changes little, while a small new reduction peak positioned at
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~310oC is observed in the curve of BaMn0.10. It seems that a small amount of more active oxygen species was formed in this sample. In contrast to samples modified by Cu, Ag [19], Co and Ni [18] oxide, the improvement of the activity of oxygen species is not very drastic. However, these samples still show enhanced inherent activity toward CH4 deep oxidation, especially the sample modified by Ba. This seems to suggest that some other reasons should be considered here while explaining the results. It is well known that CH4 is very difficult to be activated since it contains only strong C-H bonds. In the CH4 oxidation or coupling reactions, the activation of C-H bonds may be the rate-determining step. Burch et al. suggest that, to activate CH4 molecules efficiently, some highly basic sites are required [15]. The addition of alkaline earth metal oxides into MnO2 precursor may form some basic sites in the catalysts, thus improving the CH4 activation ability of the modified samples, and leading to the higher activity of the catalysts, especially at low conversion level. At high conversion level, generally, the reaction is greatly affected by mass transportation. The addition of Mg, Ca, Sr and Ba may block the pores and cover part of the manganese oxide active sites. For Mg, Ca and Sr modified samples, though their CH4 activation ability can be enhanced, while this may be offset by the inconvenience for reactant diffusion and the more difficult access to active sites. Thus, they show higher T98 than unmodified MnOx. Barium oxide has the strongest basicity among the additives we used, therefore, it is reasonable to deduce that the BaMn0.10 contains the strongest basic sites, which could show the best ability to activate CH4 among all the samples. In addition, some active oxygen species is formed in this sample, as evidenced by H2-TPR results. These reasons lead BaMn0.10 to display the highest activity among all the catalysts both at high and low conversion levels.
Acknowledgement. The authors thank the National Science Foundation of China for financial support. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9.
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