English

• 0.   Introduction

  As is known, methane is difficult to be oxidized due to the strongest C-H bond among hydrocarbons, however, it has been studied extensively over the past decades, and the catalytic combustion is still a challenging technology that can convert methane into carbon dioxide and water at relatively low temperature^[1-4]. Especially, ventilation air methane (VAM) is much more difficult to be oxidized for its low concentration (0.1%~1.0% (V/V)) and fast flow rate^[5]. Up to now, about 60%~70% methane is emitted to the atmosphere through VAM all over the world, particularly, in China, about 85%~90% of the total coal mine methane emissions mainly comes from VAM each year^[6-7]. Moreover, the warming potential of methane is higher than that of carbon dioxide. Therefore, the efforts to remove methane from VAM and limit the emission of toxic gases become rather important^[8].

  Accordingly, catalysts for methane combustion undergo very demanding conditions: they must resist thermal, mechanical shocks and exhibit high activity. Noble metal-based catalysts, such as supported Pd and Pt catalysts, are well-known to be high activity at low temperature. Unfortunately, they are limited in industrial applications due to their high cost and low thermal stability^[9-11]. Alternatively, oxide catalysts such as perovskites, hexaaluminates and transition metal oxides catalysts are the promising combustion catalysts with a low cost and relatively high thermal stability in methane combustion^[12-15]. Compared to other oxide catalysts, cobalt oxide (Co₃O₄) is regarded as the most efficient catalyst, and many researchers identified that the activity for methane combustion follows the order: Co₃O₄ > CuO >NiO > Fe₂O₃ > Mn₂O₃ > Cr₂O₃^[16-18].

  Generally, the morphologies and the crystal planes of Co₃O₄ nanocrystals have promoting performances in methane combustion. Chen et al.^[19] reported that the Co₃O₄ catalysts containing {111} planes exhibit the higher catalytic activity than the {100} planes for the methane combustion, which confirms the effect of crystal planes on the catalytic performance. Moreover, some researches show that the high Co³⁺/Co²⁺ ratio is favorable to the methane combustion, because it can facilitate desorption of oxygen and in turn assist methane combustion^[20-21]. Furthermore, the adsorbed oxygen species play an important role in catalytic oxidation reactions^[22-24]. For example, Fei et al.^[25] suggested that the Co₃O₄ nanotubes have higher catalytic activity than the Co₃O₄ nanoparticles in the methane combustion, because the nanotubes Co₃O₄ possess much more adsorbed oxygen.

  In the current work, a Co₃O₄ rectangular prism catalyst was prepared by a two-step method, and its catalytic activity was evaluated by the temperature of methane combustion in VAM. The relationship between the structure and catalytic performance of the above-mentioned catalyst is well studied based on plenty of characterizations such as PXRD, SEM, HRTEM, N₂ adsorption-desorption, H₂-TPR, and XPS.

  1.   Experimental

  1.1   Materials

  All the reagents were obtained from commercial sources and used without further purification. Cobalt(Ⅱ) acetate tetrahydrate (99.5%) was purchased from Sinopharm Chemical Reagent Co., Ltd. Oxalic acid (99.0%) was purchased from Tianjin Chemical Reagent Co., Ltd. Hexamethylenetetramine (99.0%) was purchased from Tianjin Beichen Founder Reagent Factory. All used gases were high pure gases (99.99%).

  1.2   Preparation of the catalysts

  In a typical synthesis procedure, 20 mmol of Cobalt(Ⅱ) acetate tetrahydrate was dissolved in 100 mL of distilled water at 40 ℃. Then 10 mL of an aqueous solution containing 20 mmol of hexamethylenetetramine and 4 mmol of oxalic acid was added dropwise under stirring within two minutes. The pink turbidity solution was refluxed at 95 ℃ for 6 h and a light pink Co-based precursor was obtained. The Co₃O₄ was made by heating the precursor at 350 ℃ in air for 2 h.

  1.3   Characterization of the catalyst

  The phase purity and crystal structure of the catalysts were examined on a Rigaku Mini Flex Ⅱ benchtop X-ray diffractometer using Cu Kα radiation (30 kV, 15 mA, λ=0.154 18 nm) in the 2θ range of 10°~80° with a step size of 0.01° and a scanning rate of 8°·min^-1.

  Morphologies of the samples were observed by SEM (Hitachi, SU8010, 3 kV). The HRTEM measurement was carried out with FEI Tecnai G2 F20 S-Twin equipment operated at an accelerating voltage of 200 kV. The catalyst powder was ultrasonically dispersed in ethanol and dropped onto a copper grid coated with amorphous carbon film, then dried in air.

  The Brunauer-Emmett-Teller (BET) surface area and pore size distribution of the catalyst were measured with a Micromeritics TriStar Ⅱ 3020 instrument using adsorption of N₂ at 77 K. Before each adsorption experiment, the catalyst was heated at 200 ℃ under vacuum for 3 h. Barrett-Joyner-Halenda (BJH) method was used to calculate the pore size distribution from desorption branch of the isotherm.

  The X-ray photoelectron spectroscopy (XPS) test was performed on an ESCALAB 220i-XL spectrometer by using Al Kα (1 486.6 eV) as the X-ray source. The equipment base pressure was 3×10^-5 Pa, and the sample was characterized at room temperature. Detailed spectra were recorded for the region of Co2p and O1s photo-electrons with a 0.1 eV step. Analysis was performed by the XPS Peak Fit software, and charging effects were corrected by adjusting binding energy (B.E.) of C1s (284.6 eV).

  The H₂-temperature programmed reduction (H₂-TPR) was analyzed with a Micromeritics AutoChem Ⅱ 2920 instrument. Prior to H₂-TPR experiment, 50 mg catalyst was purged in flowing Ar at 200 ℃ for 1 h with a total flow rate of 30 mL·min^-1, then cooled down to 50 ℃ in Ar flow. The reduction process was carried out in the temperature range of 50~900 ℃ in H₂/Ar (V_H2/V_Ar=10%, 30 mL·min^-1). The hydrogen consumption was estimated from the area under the peak after taking the thermal conductivity detector response into consideration. Calibration of thermal conductivity detector (TCD) signal has been done with an Ag₂O standard (Merck, reagent grade). The data processing has been done by using Origin Pro 8.0 program, which allows the deconvolution of the temperature-programmed reduction (TPR) peaks in well-defined Gaussian-shaped components.

  1.4   Catalytic activity measurement

  The methane combustion on Co₃O₄ rectangular prism catalyst was carried out at atmospheric pressure in a conventional flow system using a fixed-bed quartz micro-reactor (length=400 mm, inner diameter=6 mm). A gas mixture consisted of CH₄, O₂ and N₂ (V_CH4:V_O2:V_N2=1:20:79) was introduced into the quartz micro-reactor at a total flow rate of 40 mL·g^-1·h^-1 corresponding to a gas hourly space velocity (GHSV) of 16 000 mL·g^-1·h^-1. When GHSV changed from 16 000 to 112 500 mL·g^-1·h^-1, the total flow rate was varied from 40 to 150 mL·g^-1 ·h^-1. According to different GHSVs, 80~150 mg of catalysts (20~40 mesh) were loaded in the quartz tube micro-reactor, respectively. Prior to each measurement, the catalyst was pretreated at 200 ℃ for 1 h with a nitrogen flow of 30 mL·min^-1. Activity data were obtained at steady state condition from 200 to 450 ℃ while increasing the temperature by 50 ℃.The effluent gases were analyzed online with a gas chromatograph (ZHONGKEHUIFEN GC-6890A) equipped with a TDX-01 column and a thermal conductivity detector. The methane conversion (X_CH4) was calculated according to the following equation: X_CH4=(X₀-X_T)/X₀×100%, where X₀ refers to the volumetric concentration of methane in the feed and X_T corresponds to the concentration of methane at the given temperature. In all tests, CO₂ and H₂O were the only detected products in the exhaust stream during reaction, and CO was not found in the effluent gases, implying the conversion of methane to carbon dioxide. For comparison, commercial Co₃O₄ was also investigated.

  2.   Results and discussion

  2.1   Structural and morphological analysis

  Fig. 1a presents the PXRD patterns of as-prepared CoC₂O₄·2H₂O precursors. The presence of peaks at 2θ=18.7°, 22.7°, 30.1°, 35.0°, 37.6°, 40.4°, 43.3°, 47.3°, 48.4°, and 51.1° could be assigned to the (202), (004), (400), (022), (206), (315), (224), (602), (026) and (130) planes of CoC₂O₄·2H₂O (PDF No.48-1068). In the Fig. 1b, the diffraction peaks of Co₃O₄ rectangular prism at 19.0°, 31.2°, 36.6°, 38.5°, 44.8°, 55.7°, 59.4° and 65.3° could be assigned to the (111), (220), (311), (222), (400), (422), (511) and (440) planes of the spinel phase Co₃O₄ (PDF No.42-1467). In the case of the Co₃O₄ sample, no other peaks can be detected for impurities, which indicate that the sample consists of pure Co₃O₄ phase.

  Figure 1

  

  Figure 1.  PXRD patterns of CoC₂O₄·H₂O precursors (a) and Co₃O₄ catalysts (b)

  下载: 全尺寸图片 幻灯片

  In order to have a better understanding of the morphological and structural, a detailed microscopy investigation by SEM and high-resolution analysis are performed on both CoC₂O₄·2H₂O precursors and Co₃O₄ catalysts. Fig. 2 displays SEM imags of the as-prepared CoC₂O₄·2H₂O at 95 ℃ and the corresponding Co₃O₄ products after calcination. As shown in Fig. 2a, most CoC₂O₄·2H₂O are uniform rectangular prisms with smooth surfaces, and the size distribution is in the range of 2~5 μm (Fig. 2b). Fig. 2(c, d) show the SEM images of the prepared Co₃O₄ catalysts after calcination at 350 ℃. It is found that most of the Co₃O₄ catalysts well maintain the rectangular prism shape. Fig. 3 shows the HRTEM images of Co₃O₄ catalysts. Seen in Fig. 3a, the Co₃O₄ catalyst is formed by the accumulation of small particles of 10~20 nm, ultrasonication results in the breakdown of rectan-gular prisms into nanoparticles. Their lattice fringes are clear (Fig. 3b), which are attributed to (220) planes with a lattice space of 0.278 nm. The dominant exposed plane of Co₃O₄ rectangular prisms is {111} planes, which is the plane normal to the set of (220) planes.

  Figure 2

  

  Figure 2.  SEM images of (a, b) CoC₂O₄·H₂O precursors and (c, d) Co₃O₄ catalysts

  下载: 全尺寸图片 幻灯片

  Figure 3

  

  Figure 3.  HRTEM images for Co₃O₄ catalysts

  下载: 全尺寸图片 幻灯片

  2.2   Specific surface area and pore diameter distribution

  To further investigate the specific surface areas and the porous nature of the CoC₂O₄·2H₂O precursors and Co₃O₄ catalysts, Nitrogen adsorption-desorption isotherms of the two samples are shown in Fig. 4, and the insets illustrate the corresponding Barrett-Joyner-Halenda (BJH) pore size distribution plots. For Co₃O₄ catalysts, nitrogen adsorption experiment has given a typical type-Ⅳ isotherm with a distinct hysteresis loop observed in the relative pressure (P/P₀) range of 0.7~1.0, which is the characteristic of mesoporous materials. The BET surface area for the CoC₂O₄·2H₂O precursors and Co₃O₄ catalysts are found to be about 4 and 45 m²·g^-1, respectively. The increase of BET surface area may result from the decomposition of the CoC₂O₄·2H₂O precursors. Moreover, the BET surface area of the commercial Co₃O₄ catalyst is measured to be 1 m²·g^-1. According to the BJH plot calculated from the nitrogen isotherm, the average pore diameter of Co₃O₄ catalysts is about 14 nm, which indicated that the sample contains mesoscale pores.

  Figure 4

  

  Figure 4.  N₂ adsorption-desorption isotherms of Co₃O₄ catalysts and CoC₂O₄·H₂O precursors, and pore diameter distribution (inset) of Co₃O₄

  下载: 全尺寸图片 幻灯片

  2.3   X-ray photoelectron spectroscopy (XPS)

  XPS analysis was performed in order to gain the binding energy and the percentages of surface atoms. O1s and Co2p photoelectron spectra for the rectangular prism Co₃O₄ are shown in Fig. 5. As indicated in Fig. 5a, three peaks have been observed. The peak at ~529.7 eV (O^2-) is attributed to lattice oxygen species (O_lat) of the catalyst. And the peaks at ~531.3 (O^-) and ~533.0 eV (O₂^-) are attributed to the adsorption oxygen species (O_ads)^[26]. According to literatures, higher relative concentration ratio of O_lat/O_ads is previously found to be preferable for methane combustion. The ratio of the peak intensities of the surface-adsorbed oxygen species to lattice oxygen is 1.24, thus high activity for methane combustion can be obtained^[27-28].

  Figure 5

  

  Figure 5.  XPS spectra of O1s (a) and Co2p (b) for Co₃O₄ catalysts

  下载: 全尺寸图片 幻灯片

  Two sharp peaks at 795.2 and 780.0 eV correspond to the Co2p_1/2 and Co2p_3/2 spin-orbit-split doublet peak of Co₃O₄ spinel, respectively. There is an energy difference of approximately 15.2 eV between them. The same chemical information can be obtained by analyzing the Co2p_1/2 and Co2p_3/2 spectra. Therefore, only Co2p_3/2 peaks in Fig. 5b are fitted and de-convoluted into two peaks at 781.8 and 779.8 eV, which are attributed to Co²⁺ and Co^{3+ [28-29]}, respectively. Co₃O₄, containing a Co³⁺/Co²⁺ couple, is favorable to methane combustion. Moreover, the main oxidation state of Co in the Co₃O₄ rectangular prism is Co³⁺ (the ratio of the peak intensities of Co³⁺ to Co²⁺ is 2.87), accordingly, higher oxidation state of Co species was previously found to be preferable for oxidation reactions over the Co containing catalysts^{[19, 30]}.

  2.4   H₂ temperature-programmed reduction (H₂-TPR)

  To investigate the reducibility of the Co species in Co₃O₄ rectangular prism and commercial Co₃O₄ catalysts, H₂-TPR experiments are carried out. The reduction profiles of the samples have been displayed in Fig. 6. In this case, the Co₃O₄ rectangular prism contains two reduction peaks, the first peak is at 256 ℃, which is associated with the reduction of Co₃O₄ to CoO, and the second broad peak at 351 ℃ is correspond to the reduction of CoO to Co. However, the commercial Co₃O₄ catalysts have a wide reduction peak centered at 460 ℃. The results show that the performance of Co₃O₄ rectangular prism is better than that of commercial Co₃O₄ catalysts. Accordingly, the same oxide species, which has the lower reduction temperature, owing the easier activation of bond metal-oxygen (Co-O)^[31-32].

  Figure 6

  

  Figure 6.  H₂-TPR profiles of Co₃O₄ rectangular prism and commercial Co₃O₄ catalysts

  下载: 全尺寸图片 幻灯片

  2.5   Catalytic performance for methane oxidation

  The activity in methane complete oxidation was expressed in terms of methane conversion with respect to the reaction temperature for Co₃O₄. Accordingly, the catalytic activity of methane oxidation is evaluated by the light-off temperature (T_10%), the half-conversion temperature (T_50%) and the total conversion temperature (T_90%), representing the temperature of methane conversions at 10%, 50% and 90%, respectively. The T_10%, T_50% and T_90% over the rectangular prism Co₃O₄ catalyst are 236, 322 and 428 ℃ at 16 000 mL·g^-1·h^-1, respectively (Fig. 7a). However, the T_10% over commercial Co₃O₄ catalyst is 378 ℃, in addition, the highest methane conversion merely reached to 18.06% at 450 ℃. Moreover, the methane conversion over the rectangular prism Co₃O₄ catalyst increased with increasing temperature from 200 ~450 ℃. To convert low-concentration methane effectively by catalytic oxidation in practical application, GHSV is a critical parameter. Thus, the effects of GHSVs on methane conversion over the Co₃O₄ rectangular prism catalyst were studied. A general change is observed from Fig. 7a, that is, methane conversion rates decrease as increasing GHSVs. When the GHSVs are 30 000, 60 000, 90 000 and 112 500 mL·g^-1·h^-1, the methane conversions are decreased 4.16%, 8.54%, 15.89% and 20.85% compared to 16 000 mL·g^-1·h^-1 at 450 ℃, respectively. In general, a high GHSV has provided a short residence time and frequent contacts between catalyst and reaction gases, which leads to the decrease in the methane conversions. This result displays that enough contact time is necessary for enhancing the catalytic activity. As seen in Fig. 7b, the catalytic stability of the rectangular prism Co₃O₄ catalyst was examined at different GHSVs. After the samples were operated at 450 ℃ for 20 h under different GHSVs, the methane conversion rates only have slight decreases. For comparison, when the GHSV was 16 000 mL·g^-1·h^-1, the catalytic stability of the commercial Co₃O₄ catalyst was tested. It can be seen that the methane conversion rates keep at approximately 16% within running stable for 20 h.

  Figure 7

  

  Figure 7.  (a) Methane conversion curves of Co₃O₄ catalysts and commercial Co₃O₄ catalysts; (b) Long-term stability of samples at 450 ℃ under different GHSVs conditions

  下载: 全尺寸图片 幻灯片

  3.   Conclusions

  In summary, a novel Co₃O₄ rectangular prism with excellent activity and good stability towards the methane combustion has been synthesized through a two-steps method. The superior catalytic activity can be attributed to the following reasons: firstly, the Co₃O₄ rectangular prism dominantly exposed the {111} crystal planes, which confirmed the effect of crystal planes on the methane combustion performance; Secondly, high surface Co³⁺ content and high content surface adsorbed oxygen both play crucial roles in the methane catalytic oxidation. Owing to its simplicity of synthesis, low cost and excellent methane combustion performance, the novel Co₃O₄ rectangular prism could be a very important and promising heterogeneous catalyst.

• 1. [1]

     O'malley A, Hodnett B K. Catal. Today, 1999, 54(1):31-38 doi: 10.1016/S0920-5861(99)00166-2

  2. [2]

     任晓光, 葛秀涛.无机化学学报, 2012, 28(4):823-828 http://www.wjhxxb.cn/wjhxxbcn/ch/reader/view_abstract.aspx?flag=1&file_no=20120428&journal_id=wjhxxbcnREN Xiao-Guang, GE Xiu-Tao. Chinese J. Inorg. Chem., 2012, 28(4):823-828 http://www.wjhxxb.cn/wjhxxbcn/ch/reader/view_abstract.aspx?flag=1&file_no=20120428&journal_id=wjhxxbcn

  3. [3]

     Kirchnerova J, Alifanti M, Delmon B. Appl. Catal., A, 2002, 231(1/2):65-80 http://www.sciencedirect.com/science/article/pii/S0926860X01009036

  4. [4]

     陈永东, 朱艺, 王健礼, 等.无机化学学报, 2009, 25(10):1771-1778 doi: 10.3321/j.issn:1001-4861.2009.10.012CHEN Yong-Dong, ZHU Yi, WANG Jian-Li, et al. Chinese J. Inorg. Chem., 2009, 25(10):1771-1778 doi: 10.3321/j.issn:1001-4861.2009.10.012

  5. [5]

     Guo T Y, Du J P, Wu J T, et al. Chem. Eng. J., 2016, 306:745-753 doi: 10.1016/j.cej.2016.08.020

  6. [6]

     Ouyang S, Xu S, Song N, et al. Fuel, 2013, 113:420-425 doi: 10.1016/j.fuel.2013.06.004

  7. [7]

     Cheng Y P, Wang L, Zhang X L. Int. J. Greenhouse Gas Control, 2011, 5(1):157-166 doi: 10.1016/j.ijggc.2010.07.007

  8. [8]

     Guo T Y, Du J P, Li J P. J. Mater. Sci., 2016, 51(24):10917-10925 doi: 10.1007/s10853-016-0303-z

  9. [9]

     Lovón-Quintana J J, Santos J B O, Lovón A S P, et al. J. Mol. Catal. A:Chem., 2016, 411:117-127 doi: 10.1016/j.molcata.2015.08.001

  10. [10]

      Ruiz J A C, Oliveira E C, Fraga M A, et al. Catal. Commun., 2012, 25:1-6 doi: 10.1016/j.catcom.2012.04.006

  11. [11]

      Chen C, Yeh Y H, Cargnello M, et al. ACS Catal., 2014, 4(11):3902-3909 doi: 10.1021/cs501146u

  12. [12]

      Cao X X, Zhou R, Rui N, et al. Catal. Today, 2017, 297:219-227 doi: 10.1016/j.cattod.2017.01.042

  13. [13]

      缪建文, 范以宁, 金永漱, 等.无机化学学报, 2003, 19(12):1361-1365 doi: 10.3321/j.issn:1001-4861.2003.12.019MIU Jian-Wen, FAN Yi-Ning, JIN Yong-Shu, et al. Chinese J. Inorg. Chem., 2003, 19(12):1361-1365 doi: 10.3321/j.issn:1001-4861.2003.12.019

  14. [14]

      Hu R S, Ding R R, Chen J, et al. Catal. Commun., 2012, 21:38-41 doi: 10.1016/j.catcom.2012.01.008

  15. [15]

      郑建东, 葛秀涛.无机化学学报, 2013, 29(6):1307-1311 http://www.wjhxxb.cn/wjhxxbcn/ch/reader/view_abstract.aspx?flag=1&file_no=20130631&journal_id=wjhxxbcnZHENG Jian-Dong, GE Xiu-Tao. Chinese J. Inorg. Chem., 2013, 29(6):1307-1311 http://www.wjhxxb.cn/wjhxxbcn/ch/reader/view_abstract.aspx?flag=1&file_no=20130631&journal_id=wjhxxbcn

  16. [16]

      Xu X L, Han H, Liu J J, et al. J. Rare Earths, 2014, 32(2):159-169 doi: 10.1016/S1002-0721(14)60046-6

  17. [17]

      Zavyalova U, Scholz P, Ondruschka B. Appl. Catal., A, 2007, 323:226-233 doi: 10.1016/j.apcata.2007.02.021

  18. [18]

      Hu L H, Peng Q, Li Y D. J. Am. Chem. Soc., 2008, 130(48):16136-16137 doi: 10.1021/ja806400e

  19. [19]

      Chen Z P, Wang S, Liu W G, et al. Appl. Catal., A, 2016, 525:94-102 doi: 10.1016/j.apcata.2016.07.009

  20. [20]

      Zou G C, Chen M X, Shangguan W F. Catal. Commun., 2014, 51:68-71 doi: 10.1016/j.catcom.2014.03.028

  21. [21]

      Wang Q, Peng Y, Fu J, et al. Appl. Catal., B, 2015, 168:42-50

  22. [22]

      Chen Z P, Wang S, Ding Y, et al. Appl. Catal., A, 2017, 532:95-104 doi: 10.1016/j.apcata.2016.12.021

  23. [23]

      Wang F G, Zhang L J, Xu L L, et al. Fuel, 2017, 203:419-429 doi: 10.1016/j.fuel.2017.04.140

  24. [24]

      Huang H, Dai Q G, Wang X Y. Appl. Catal., B, 2014, 158:96-105 http://www.sciencedirect.com/science/article/pii/S0926337314000824

  25. [25]

      Fei Z Y, He S C, Li L, et al. Chem. Commun., 2012, 48(6):853-855 doi: 10.1039/C1CC15976C

  26. [26]

      Yang N T, Ni S L, Sun Y H, et al. Mol. Catal., 2018, 452:28-35 doi: 10.1016/j.mcat.2018.03.016

  27. [27]

      Petitto S C, Marsh E M, Carson G A, et al. J. Mol. Catal., A, 2008, 281(1/2):49-58 http://www.sciencedirect.com/science/article/pii/S1381116907005067

  28. [28]

      Wei Y C, Ren X, Ma H M, et al. Chem. Commun., 2018, 54(12):1533-1536 doi: 10.1039/C7CC08423D

  29. [29]

      Yang H, Yang W H, Lv K L, et al. Microporous Mesoporous Mater., 2018, 255:36-43 doi: 10.1016/j.micromeso.2017.07.016

  30. [30]

      Li Y, Shen W J. Chem. Soc. Rev., 2014, 43(5):1543-1574 doi: 10.1039/C3CS60296F

  31. [31]

      Zhu Z Z, Lu G Z, Zhang Z G, et al. ACS Catal., 2013, 3(6):1154-1164 doi: 10.1021/cs400068v

  32. [32]

      Yan Q Y, Li X Y, Zhao Q D, et al. J. Hazard. Mater., 2012, 209:385-391 http://www.sciencedirect.com/science/article/pii/S0304389412000702

• 

  Figure 1  PXRD patterns of CoC₂O₄·H₂O precursors (a) and Co₃O₄ catalysts (b)

  下载: 全尺寸图片 幻灯片
  

  Figure 2  SEM images of (a, b) CoC₂O₄·H₂O precursors and (c, d) Co₃O₄ catalysts

  下载: 全尺寸图片 幻灯片
  

  Figure 3  HRTEM images for Co₃O₄ catalysts

  下载: 全尺寸图片 幻灯片
  

  Figure 4  N₂ adsorption-desorption isotherms of Co₃O₄ catalysts and CoC₂O₄·H₂O precursors, and pore diameter distribution (inset) of Co₃O₄

  下载: 全尺寸图片 幻灯片
  

  Figure 5  XPS spectra of O1s (a) and Co2p (b) for Co₃O₄ catalysts

  下载: 全尺寸图片 幻灯片
  

  Figure 6  H₂-TPR profiles of Co₃O₄ rectangular prism and commercial Co₃O₄ catalysts

  下载: 全尺寸图片 幻灯片
  

  Figure 7  (a) Methane conversion curves of Co₃O₄ catalysts and commercial Co₃O₄ catalysts; (b) Long-term stability of samples at 450 ℃ under different GHSVs conditions

  下载: 全尺寸图片 幻灯片
•