Journal of Natural Gas Chemistry 12(2003)228 232 Catalytic Combustion of Methane over MnO x /ZrO 2 -Al 2 O 3 Catalysts Xiufeng Xu 1, Yanfei Pan 2, Yanxia Liu 1, Zhanghuai Suo 1, Shixue Qi 1, Lidun An 1 1.Institute of Applied Catalysis, Yantai University, Yantai 264005, China; 2.Analysis Center, Yantai University, Yantai 264005, China [Manuscript received December 03, 2003; revised December 15, 2003] Abstract: MnO x/al 2O 3 and MnO x/zro 2-Al 2O 3 catalysts were prepared by incipient wetness impregnation of Mn(CH 3COO) 2 on the corresponding supports, followed by the characterization using X-ray diffraction (XRD), temperature programmed reduction (TPR) and BET surface area techniques. The result shows the BET surface area of ZrO 2-Al 2O 3 is lower than that of Al 2O 3 due to the loading of ZrO 2. However the resulted MnO x/zro 2-Al 2O 3 catalyst exhibits higher activity for methane combustion than MnO x/al 2O 3, because the addition of ZrO 2 onto Al 2O 3 is beneficial for the dispersion of Mn species and the improvement of the lattice oxygen activity in MnO x, subsequently the activation of methane during combustion. The optimum loading of Zr in MnO x/zro 2-Al 2O 3 is in the range of 5% 10% correlated with the calcination temperatures of catalyst supports. Key words: MnO x/al 2O 3, MnO x/zro 2-Al 2O 3, X-ray diffraction, temperature programmed reduction, catalytic combustion, methane 1. Introduction The thermal combustion of methane results in a large amount of pollutants due to its low utilization rate and rather high combustion temperature. The catalytic combustion of methane can lower the reaction temperature, improve the energy transforming efficiency and decrease the NO x formation. Several series of catalysts [1 5] have been investigated, such as transition alumina supported catalysts. Al 2 O 3 has been extensively used as the catalyst support because of its high surface area for the dispersion of active species. During the recent years, many researchers have modified alumina by other oxides to improve its properties, especially those at high reaction temperatures [6 11]. In addition, the introduction of other oxides into alumina may change the dispersion of active phases through their interaction with modified Al 2 O 3, and subsequently change the catalytic activity of the resulted catalysts. In the present study, the catalysts of MnO x /Al 2 O 3 and MnO x /ZrO 2 -Al 2 O 3 were prepared using Mn(CH 3 COO) 2 4H 2 O as the precursor, and characterized by performing X-ray diffraction (XRD) and temperature programmed reduction (TPR). The catalytic performances of the prepared catalysts for methane combustion were evaluated in relation with the physicochemical properties of the catalytic materials. 2. Experimental 2.1. The preparation of MnO x /Al 2 O 3 and MnO x /ZrO 2 -Al 2 O 3 The ZrO 2 -Al 2 O 3 supports, with 5% 15% of Zr Corresponding author. Tel/Fax: (0535)6902233; E-mail: xxf@ytu.edu.cn. This work was financially supported by Shandong Provincial Department of Science and Technology (project number: 981206403) and the State Key Laboratory of Coal Conversion at Institute of Coal Chemistry of CAS (2002-2003).
Journal of Natural Gas Chemistry Vol. 12 No. 4 2003 229 loading in its metallic state, were prepared using the incipient wetness impregnation of Zr(NO 3 ) 4 5H 2 O solution on Al 2 O 3, calcined at 750 or 950 for 2 h. ZrO 2 -Al 2 O 3 was impregnated overnight with Mn(CH 3 COO) 2 4H 2 O solution, followed by drying under vacuum and calcination at 500. The Mn loading was limited to 10% in its metallic state. For comparison, the MnO x /Al 2 O 3 catalyst with the same loading of Mn as that in MnO x /ZrO 2 - Al 2 O 3 was prepared by impregnating Al 2 O 3 with Mn(CH 3 COO) 2 4H 2 O solution. 2.2. Physicochemical analysis The measurements of BET surface areas (A BET ) of Al 2 O 3 and ZrO 2 -Al 2 O 3 were carried out by continuous flowing method with physisorbed N 2 at 77 K using H 2 as a carrier gas. Prior to the BET measurement, the samples were preheated at 200 for 1 h to remove the adsorbed water. TCD was used as detector and the current for the thermal conductor is 150 ma. The BET surface areas were determined using the multipoint method at the relative pressures of nitrogen in the range of 0.06 0.30. The X-ray diffraction (XRD) patterns were obtained at room temperature with a Shimadzu XRD- 6100 powder diffractometer using Cu K α radiation. The diffractometer was operated at 40 kv and 30 ma of tube voltage and current respectively. The spectra were recorded in the 2θ range of 10 o 70 o with a scanning rate of 2 o /min. The temperature programmed reduction was carried out using the following procedure. Catalysts were exposed in N 2 to remove the adsorbed water and other impurities at 500 for 1 h. After the exposed temperature was cooled to room temperature, 5%H 2 95%N 2 was changed to the reduced gases with the temperature raised to 700 at a ramp of 10 /min, and the hydrogen consumption was monitored by a TCD detector. 2.3. The catalytic activity measurements for the methane combustion The catalyst was put into the middle of a horizontal quartz reactor. The reaction temperature was controlled by a K-type thermocouple in the heated zone. The flow rate of reactant gases was controlled by a pressure stabilizer before the reactant gases was introduced into the reactor, and it was recorded by a flow meter at the end of reactor. The composition of gases at the inlet and outlet of the reactor was determined by an on-line GC with a TCD detector. The feed of reactants consists of 0.7%CH 4 and 8.2%O 2 diluted in nitrogen with the gas hourly space velocity (GHSV) of 5000 h 1. 3. Results and discussion 3.1. BET surface area and XRD characterization Table 1 gives the BET surface areas of Al 2 O 3 and ZrO 2 -Al 2 O 3. The specific surface area of ZrO 2 -Al 2 O 3 is lower than that of Al 2 O 3 calcined at the same temperature as that of ZrO 2 -Al 2 O 3. For ZrO 2 -Al 2 O 3 samples with different Zr contents, the surface area decreases with the increase of Zr loading except for 10%ZrO 2 -Al 2 O 3 calcined at 950, which could be ascribed to the occupation of Zr species on the external and internal surface of Al 2 O 3. For the same sample calcined at different temperatures, the surface areas of the sample calcined at 950 becomes lower than that calcined at 750, and this is possibly due to the sintering to some extent at the high calcination temperature. Table 1. The BET surface areas of Al 2 O 3 and ZrO 2 -Al 2 O 3 calcined at the different temperatures A BET /(m 2 /g) Supports Calcined at Calcined at 750 950 Al 2 O 3 135.2 108.0 5%ZrO 2 -Al 2 O 3 122.5 93.1 10%ZrO 2 -Al 2 O 3 119.5 98.2 15%ZrO 2 -Al 2 O 3 119.1 83.7 The XRD patterns of MnO x /Al 2 O 3 and MnO x /ZrO 2 -Al 2 O 3 are shown in Figure 1. Only γ-al 2 O 3, or γ-al 2 O 3 plus ZrO 2 peaks appear on the XRD patterns. As stated previously [2], the decomposition product of pure Mn(CH 3 COO) 2 4H 2 O at 500 was Mn 2 O 3, whereas Mn 2 O 3 could not be observed in the present figures. This is possibly due to the little crystallite of MnO x, which corresponded to the high dispersion of MnO x over the surface of Al 2 O 3 and ZrO 2 -Al 2 O 3. The other reason might be that the 10wt.% loading of Mn could not completely cover the mono-layer of Al 2 O 3 or ZrO 2 -Al 2 O 3 possibly existed.
230 Xiufeng Xu et al./ Journal of Natural Gas Chemistry Vol. 12 No. 4 2003 Figure 1. XRD patterns of MnO x supported on Al 2 O 3 and ZrO 2 -Al 2 O 3 calcined at (a) 750, (b) 950 (1) MnO x/al 2 O 3, (2) MnO x/5% ZrO 2 -Al 2 O 3, (3) MnO x/10%zro 2 -Al 2 O 3, (4) MnO x/15%zro 2 -Al 2 O 3 3.2. TPR results Figure 2 gives the TPR profiles of MnO x supported on Al 2 O 3 and ZrO 2 -Al 2 O 3 calcined at 750 and 950, respectively. Two peaks of adsorbed hydrogen appeared in the MnO x /Al 2 O 3 catalyst, corresponding to 3Mn 2 O 3 +H 2 2Mn 3 O 4 +H 2 O and Mn 3 O 4 +H 2 3MnO+H 2 O. While for the MnO x /ZrO 2 -Al 2 O 3 catalyst, only one wide peak presents in the TPR profile, indicating the two reduction steps as above occur continuously. It can be deduced that the activity of lattice oxygen in MnO x /ZrO 2 -Al 2 O 3 became higher than that of MnO x /Al 2 O 3, which was originated from the improvement of MnO x dispersion in MnO x /ZrO 2 - Al 2 O 3. Figure 2. TPR profiles of MnO x supported on Al 2 O 3 and ZrO 2 -Al 2 O 3 calcined at (a) 750, (b) 950 (1) MnO x/al 2 O 3, (2) MnO x/5% ZrO 2 -Al 2 O 3, (3) MnO x/10%zro 2 -Al 2 O 3, (4) MnO x/15%zro 2 -Al 2 O 3
Journal of Natural Gas Chemistry Vol. 12 No. 4 2003 231 Among the four catalysts with supports calcined at 750, the consumption of hydrogen in the TPR test of MnO x /5%ZrO 2 -Al 2 O 3 was the largest. For the catalysts with supports calcined at 950, MnO x /10%ZrO 2 -Al 2 O 3 exhibits the most consumption of hydrogen in the TPR test. From these results, a conclusion can be made that the addition of ZrO 2 into Al 2 O 3 increased the active oxygen quantity of MnO x in several resulted catalysts by improving the dispersion state of Mn species. 3.3. Catalytic activity for the methane combustion Figure 3 gives the conversion of methane combustion over MnO x /Al 2 O 3 and MnO x /ZrO 2 -Al 2 O 3 catalysts. It is shown the activities of MnO x /ZrO 2 -Al 2 O 3 catalysts, although having a dependence upon the loading of Zr, are higher than that of MnO x /Al 2 O 3. In terms of the four catalysts with supports calcined at 750, MnO x /5%ZrO 2 -Al 2 O 3 presents the highest activity. For the catalysts using supports calcined at 950, MnO x /10%ZrO 2 -Al 2 O 3 exhibits the best catalytic performance. As mentioned above, in comparison with MnO x /Al 2 O 3, the dispersion of MnO x onto Al 2 O 3 doped with ZrO 2 was improved and resulted in the higher activity of lattice oxygen or the increase in active oxygen quantity in MnO x. The lattice oxygen activity of MnO x was in good agreement with the catalytic performance of the corresponding catalysts, indicating that ZrO 2 was beneficial for the activation of methane during combustion. Further, the optimum loading of Zr is related with the calcination temperatures of catalyst supports. Figure 3. The catalytic performance of MnO x/al 2 O 3 and MnO x/zro 2 -Al 2 O 3 with the supports calcined at (a)750 and (b) 950 4. Conclusions The addition of ZrO 2 in Al 2 O 3 led to a decrease in the BET surface area of ZrO 2 -Al 2 O 3 in comparison with pure Al 2 O 3. However, MnO x /ZrO 2 -Al 2 O 3 catalysts present higher catalytic activity for the methane combustion than that of MnO x /Al 2 O 3, due to the benefit of ZrO 2 for the dispersion of MnO x species on catalytic supports and the increase in the lattice oxygen activity in MnO x. The optimum loading of Zr in MnO x /ZrO 2 -Al 2 O 3 catalysts is in the range of 5% 10% correlated with the calcination temperatures of catalyst supports. References [1] Ciuparu D, Katsikis N, Pfefferle L. Appl Catal A, 2001, 216: 209 [2] Xu X F, Suo Zh H, Qi S X et al. Fenzi Cuihua (Chin J Mol Catal), 2002, 16(2): 131 [3] Xiao T C, Ji S F, Wang H T et al. J Mol Catal A, 2001, 175: 111
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