English

• Nitrous oxide (N₂O), as one of the greenhouse gases that were limited by the Kyoto protocol, has a global warming potential (GWP) of 310 and lifetime of 120 years in the atmosphere. In addition, N₂O is also harmful to the ozone layer; if disposed to the atmosphere, it may cause great harm to our ecological environment. The production of nitric and adipic acid contributes mostly to the anthropogenic source of N₂O. The decomposition of N₂O to non-toxic and harmless N₂ and O₂ is a simple and practical method to reduce the N₂O emission. Various catalysts were reported for the N₂O decomposition, including supported noble metals^[1-3], ion-exchanged zeolites^[4-7], and transition metal oxides^[8-10], among them, the Co₃O₄-based catalysts have been widely studied^[11-16]. It was also observed in our previous work that the Co₃O₄ catalyst prepared by precipitation was highly active in the decomposition of N₂O^[17].

  To enhance the performance of Co₃O₄-based catalysts in N₂O decomposition, in this work, Co₃O₄ was hydrothermally synthesized by using hexadecyl trimethyl ammonium bromide (CTAB) as a template. It was reported that with the increase of CTAB concentration in aqueous solution, CTAB may be present in the form of monomers, pre-micelles, spherical, rod, and layered micelles; the first and second critical micelle concentrations (CMC) were 0.00089 and 0.021 mol/L, respectively, as measured by Liu et al^[18]. If the CTAB concentration was higher than the second CMC, rod micelles were formed and cobaltosic oxide precursors were then produced. The Co₃O₄ catalysts were then prepared by calcining the cobaltosic oxide precursors, which was further modified with K by impregnation with K₂CO₃ solution and used in the decomposition of N₂O under the atmosphere containing oxygen and steam. The effect of preparation parameters such as CTAB concentration, molar ratio of CTAB to cobalt, molar ratio of urea to cobalt, and K modification on the catalytic activity of Co₃O₄ was then investigated.

  1.   Experimental

  1.1   Catalyst preparation

  Appropriate amounts of Co(NO₃)₂·6H₂O, urea, and CTAB was added into 50 mL deionized water; the mixture was stirred under ultrasonic irradiation for 10 min and then transferred to a 100 mL Teflon-sealed autoclave. The autoclave was then heated to 120 ℃ under rotation and held at this temperature for 6 h. The precipitant obtained was washed with deionized water, dried at 60 ℃ for 6 h, and then calcinated at 400 ℃ for 2 h. In this way, three series of cobaltosic oxides were prepared: the first one is denoted as Co₃O₄(CTAB-x), which is obtained with a CTAB/cobalt molar ratio of 1, a urea/cobalt molar ratio of 4, and a CTAB concentration of x (x = 0, 0.01, 0.03, 0.04, 0.05, and 0.06, in mol/L); the second one is expressed as Co₃O₄(CTAB/Co-y), prepared with 0.05 mol/L CTAB solution, a urea/cobalt molar ratio of 4, and a CTAB/cobalt molar ratio of y (y = 0.83, 1, 1.1, 1.3); the third one is expressed as Co₃O₄(urea/Co-z), synthesized with 0.05 mol/L CTAB solution, a CTAB/cobalt molar ratio of 1, and a urea/cobalt molar ratio of z (z = 3, 4, 5).

  To prepare the K-modified Co₃O₄ catalyst with a K/Co molar ratio of 0.02 (0.02K/Co₃O₄), the Co₃O₄ catalyst prepared by using 0.05 mol/L CTAB solution, with a CTAB to cobalt molar ratio of 1 and a urea to cobalt molar ratio of 4 was then impregnated with K₂CO₃ solution at room temperature for 24 h, dried at 60 ℃ for 12 h, and then calcined at 400 ℃ for 2 h.

  1.2   N₂O decomposition reaction

  The decomposition of N₂O was carried out in a fixed-bed reactor; 0.5 g catalyst was used for each test. Unless otherwise stated, the reactant feed consisted of 1% N₂O and balanced argon, with a total flow rate of 150 mL/min. The effluent stream was analyzed by a gas chromatography (GC-920, Shanghai Haixin) equipped with a Porapak Q column and a thermal conductivity detector (TCD). In the case of oxygen atmosphere, the feed contained 2% O₂ besides 1% N₂O, whereas in the case of oxygen and steam atmosphere, the feed bore 8.2% H₂O, besides 1% N₂O and 2% O₂.

  The initial activity was expressed by the conversion of N₂O measured after reaction at given temperature for 30 min. To test the catalytic stability of 0.02 K/Co₃O₄, the reaction was conducted at 400 ℃ for 50 h.

  1.3   Catalyst characterization

  X-ray diffraction (XRD) patterns were recorded on a Shimadzu powder diffractometer (XRD-6100) with Cu Kα radiation and graphite monochromator at 40 kV and 30 mA.

  Specific surface area was measured by nitrogen physisorption apparatus (NOVA3000, Quantachrome) and calculated by BET equation. Prior to the measurement, the catalyst samples were pre-treated at 300 ℃ for 2 h to remove any impurities; the nitrogen adsorption and desorption were performed at -196 ℃ and room temperature, respectively.

  Scanning electron microscopy (SEM) images were taken on Hitachi S-4800; the catalyst samples were coated by using an ion sputter with platinum (E-1045, Hitachi) in order to improve their electric conductivity.

  X-ray photoelectron spectra (XPS) of cobalt element were recorded in an ESCALAB250 spectrometer using Al Kα radiation with pass energy of 20 eV. The charging effect was corrected by referencing C 1s peak centered at 284.6 eV.

  Temperature-programmed reduction by hydrogen (H₂-TPR) was carried out on a chemical adsorption instrument (PCA-1200, Beijing Builder). Prior to each measurement, 80 mg catalyst was pre-treated in argon at 400 ℃ for 30 min and then cooled naturally to room temperature. Subsequently, the catalyst was exposed to 10% H₂/Ar with a flow rate of 20 mL/min and heated from room temperature to 800 ℃ with a ramp of 10 ℃/min. The hydrogen consumption was detected by a TCD.

  Temperature programmed desorption of oxygen (O₂-TPD) were also performed on the PCA-1200 chemical adsorption apparatus. 100 mg catalyst was first exposed to O₂ at 120 ℃ for 30 min; after cooling to ambient temperature, it was then heated in helium from room temperature to 700 ℃ at a ramp of 10 ℃/min, during which the desorbed oxygen was measured by the TCD.

  2.   Results and discussion

  2.1   Catalytic activity of Co₃O₄ prepared by changing the CTAB concentration

  A series of Co₃O₄ catalysts were first synthesized by changing the concentration of CTAB solution. As revealed by the XRD patterns shown in Figure 1, the Co₃O₄ catalysts have a spinel structure.

  Figure 1

  

  Figure 1.  XRD patterns of various Co₃O₄ catalysts synthesized by changing the CTAB concentrations

  a: Co₃O₄(CTAB-0); b: Co₃O₄(CTAB-0.01); c: Co₃O₄(CTAB-0.03); d: Co₃O₄(CTAB-0.05); e: Co₃O₄(CTAB-0.06)

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  Table 1 illustrates that their BET surface area increases with an increase in CTAB concentration till 0.05 mol/L and then decreases with a further increase in CTAB concentration (e.g. Co₃O₄(CTAB-0.06)). The SEM images shown in Figure 2 suggest that Co₃O₄ prepared without using CTAB is in irregular particles. Irregular particles still appear with a low CTAB concentration (e.g. ≤ 0.01 mol/L). However, the Co₃O₄ catalysts prepared with 0.03-0.06 mol/L CTAB solution take the rod morphology.

  Table 1

  

  Table 1.  BET surface area of various Co₃O₄ catalysts synthesized by changing the CTAB concentration

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  Catalyst	BET surface area A/(m2·g-1)
  Co3O4(CTAB-0)	21.1
  Co3O4(CTAB-0.01)	42.2
  Co3O4(CTAB-0.03)	47.2
  Co3O4(CTAB-0.05)	50.3
  Co3O4(CTAB-0.06)	27.8

  Figure 2

  

  Figure 2.  SEM images of various Co₃O₄ catalysts synthesized by changing the CTAB concentration

  (a): Co₃O₄(CTAB-0); (b): Co₃O₄(CTAB-0.01); (c): Co₃O₄(CTAB-0.03); (d): Co₃O₄(CTAB-0.05); (e): Co₃O₄(CTAB-0.06)

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  The conversion of N₂O over various Co₃O₄ catalysts is displayed in Figure 3, which indicates that the Co₃O₄(CTAB-0.05) catalyst is most active in N₂O decomposition. The reaction rate constants (k) can be derived by assuming the first order kinetics^{[19, 20]}:

  Figure 3

  

  Figure 3.  N₂O conversions over various Co₃O₄ catalysts synthesized by changing the CTAB concentration

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  $ x = 1-\exp \left( {-k{V_{{\rm{bed}}}}/F} \right) $

  (1)

  where x is the N₂O conversion, V_bed is the catalyst bed volume, and F is the volumetric reactant flow rate.

  The calculated rate coefficient (k), apparent activation energy (E_a) and pre-exponential factor (A) are then given in Table 2. Obviously, the Co₃O₄(CTAB-0.05) catalyst with higher activity has a lower apparent activation energy. Meanwhile, the values of E_a and A change in the same direction, that is, lnA = 3.53 + 0.159E_a, indicating that there is a dynamic compensation effect between E_a and A.

  Table 2

  

  Table 2.  Kinetic data for N₂O decomposition over various Co₃O₄ catalysts synthesized by changing the CTAB concentration

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  Catalyst	k/s-1					Ea/(kJ·mol-1)	lnA
  	300 ℃	325 ℃	350 ℃	375 ℃	400 ℃
  Co3O4(CTAB-0)	-	-	1.96	3.77	6.55	84.2	16.9
  Co3O4(CTAB-0.01)	0.75	0.91	2.45	4.02	6.55	74.5	15.2
  Co3O4(CTAB-0.03)	1.01	1.87	3.53	5.70	7.30	65.4	13.8
  Co3O4(CTAB-0.05)	1.33	2.45	4.40	7.50	11.50	69.8	14.9
  Co3O4(CTAB-0.06)	0.75	1.42	2.86	5.39	7.71	77.1	15.9

  Concerning the N₂O decomposition mechanism, it was reported that N₂O was first adsorbed on the catalyst surface and N-O bond was then broken; after that, two adsorbed oxygen atoms reacted to produce one oxygen molecule, which was then desorbed from the catalyst surface^[21]. The reaction cycle over Co₃O₄ may follow the formula:

  $ {{\rm{N}}_2}{\rm{O}} + {\rm{C}}{{\rm{o}}^{2 + }} \to {{\rm{N}}_2} + {\rm{C}}{{\rm{o}}^{3 + }}-{{\rm{O}}^-} $

  (2)

  $ 2{\rm{C}}{{\rm{o}}^{3 + }}-{{\rm{O}}^-} \to {{\rm{O}}_2} + 2{\rm{C}}{{\rm{o}}^{2 + }} $

  (3)

  where the desorption of oxygen species may be the controlling step.

  Figure 4 shows the XPS spectra of various Co₃O₄ catalysts synthesized by changing the CTAB concentration. The binding energy of Co³⁺ 2p_3/2 electrons on Co₃O₄(CTAB-0), Co₃O₄(CTAB-0.01), and Co₃O₄(CTAB-0.03) is about 782.2 eV, whereas that on Co₃O₄(CTAB-0.05) and Co₃O₄(CTAB-0.06) shifts about 0.1-0.2 eV to a lower value, indicating the Co³⁺-O^- bond in Co₃O₄(CTAB-0.05) and Co₃O₄(CTAB-0.06) is weakened. As suggested by the second step of above mechanism, the weakened Co³⁺-O^- bond in the (CTAB-0.05) and Co₃O₄(CTAB-0.06) catalysts is beneficial to the removal of oxygen species and the whole reaction cycle.

  Figure 4

  

  Figure 4.  XPS spectra of various Co₃O₄ catalysts synthesized by changing the CTAB concentration

  a: Co₃O₄(CTAB-0); b: Co₃O₄(CTAB-0.01); c: Co₃O₄(CTAB-0.03); d: Co₃O₄(CTAB-0.05); e: Co₃O₄(CTAB-0.06)

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  To extend the second step in above mechanism, it is assumed that two adsorbed oxygen atoms (Co³⁺-O^-) produced by the dissocation of N₂O molecules react with each other, forming one oxygen molecule O₂^2- (i.e. Co³⁺-O-O-Co³⁺), which is then desorbed from the catalyst surface to regenerate the active Co²⁺ species:

  $ 2{\rm{C}}{{\rm{o}}^{3 + }}-{{\rm{O}}^-} \to {\rm{C}}{{\rm{o}}^{3 + }}-{\rm{O}} - {\rm{O}} - {\rm{C}}{{\rm{o}}^3} \to {{\rm{O}}_2} + 2{\rm{C}}{{\rm{o}}^{2 + }} $

  (4)

  O₂-TPD is an effective method to characterize the interaction between oxygen species and catalyst surface. As shown in Figure 5, the oxygen desorbed in the O₂-TPD profiles at 100-270 ℃ is assigned to the oxygen adsorbed on catalysts surface, whereas that desorbed at 330-420 ℃ is from the surface lattice oxygen. Obviously, the peak temperatures for oxygen desorption on Co₃O₄(CTAB-0.05) are 221.8 and 375.6 ℃, which are lower than those on other catalysts, indicating that the oxygen species on Co₃O₄(CTAB-0.05) are more easily desorbed, consistent with its higher activity.

  Figure 5

  

  Figure 5.  O₂-TPD profiles of various Co₃O₄ catalysts synthesized by changing the CTAB concentration

  a: Co₃O₄(CTAB-0); b: Co₃O₄(CTAB-0.01); c: Co₃O₄(CTAB-0.03); d: Co₃O₄(CTAB-0.05); e: Co₃O₄(CTAB-0.06)

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  2.2   Catalytic activity of Co₃O₄ prepared by changing the molar ratios of CTAB/cobalt and urea/cobalt

  The effect of CTAB/cobalt and urea/cobalt molar ratios on the catalytic activity of Co₃O₄ was also considered. By changing the CTAB/cobalt molar ratio, it was found that the surface areas of Co₃O₄(CTAB/Co-0.83), Co₃O₄(CTAB/Co-1), Co₃O₄(CTAB/Co-1.1), and Co₃O₄(CTAB/Co-1.3) measured by nitrogen sorption are 40.3, 50.3, 38.8, and 36.7 m²/g, respectively. Meanwhile, by changing the urea/cobalt molar ratio, Co₃O₄(urea/Co-3), Co₃O₄(urea/Co-4), and Co₃O₄(urea/Co-5) have the surface areas of 42.1, 50.3, and 43.6 m²/g, respectively. The XRD results reveal that they all have the spinel structure.

  Figures 6 and 7 illustrate that the Co₃O₄ catalyst with a CTAB/Co molar ratio of 1 and a urea/Co molar ratio of 4 is most active, consistent with its high specific surface area. In the O₂-TPD profiles shown in Figures 8 and 9.The oxygen desorbed at 100-270 ℃ is ascribed to the surface adsorbed oxygen, and that at 330-420 ℃ is assigned to the surface lattice oxygen. Obviously, the peak temperatures for the desorption of oxygen species on Co₃O₄ (CTAB/Co = 1 and urea/Co = 4) are 221.8 and 375.6 ℃, much lower than those on other catalysts.

  Figure 6

  

  Figure 6.  N₂O conversions over various Co₃O₄ catalysts synthesized by changing the CTAB/cobalt molar ratio

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  Figure 7

  

  Figure 7.  N₂O conversions over various Co₃O₄ catalysts synthesized by changing urea/cobalt molar ratio

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  Figure 8

  

  Figure 8.  O₂-TPD profiles of various Co₃O₄ catalysts synthesized by changing the CTAB/cobalt molar ratio

  a: Co₃O₄(CTAB/Co-0.83); b: Co₃O₄(CTAB/Co-1); c: Co₃O₄(CTAB/Co-1.1); d: Co₃O₄(CTAB/Co-1.3)

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  Figure 9

  

  Figure 9.  O₂-TPD profiles of various Co₃O₄ catalysts synthesized by changing urea/cobalt molar ratio

  a: Co₃O₄(urea/Co-3); b: Co₃O₄(urea/Co-4); c: Co₃O₄(urea/Co-5)

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  2.3   Catalytic activity and stability of K-modified Co₃O₄ catalyst in the presence of oxygen and steam

  Above results illustrate that the Co₃O₄ catalyst synthesized by using 0.05 mol/L CTAB solution, with a CTAB/cobalt molar ratio of 1, and a urea/cobalt molar ratio of 4, exhibits high activity in N₂O decomposition. To further enhance its catalytic activity, it was then modified by impregnation with K₂CO₃ solution. The K-modified 0.02 K/Co₃O₄ also shows the spinel structure and no crystallites of potassium species are observed. In comparison with K-free Co₃O₄, the surface area of 0.02 K/Co₃O₄ is decreased from 50.3 to 29.4 m²/g.

  The cobalt XPS spectra shown in Figure 10 indicate that K-free Co₃O₄ has a Co²⁺/Co³⁺ molar ratio of 1.65, whereas K-modified 0.02 K/Co₃O₄ has a Co²⁺/Co³⁺ molar ratio of 1.69. For N₂O decomposition, more active Co²⁺ species on 0.02 K/Co₃O₄ are beneficial to the adsorption and activation of N₂O molecules. In addition, the binding energy of Co³⁺ 2p_3/2 electrons on K-free Co₃O₄ is 782.1 eV, about 0.3 eV lower than that on 0.02 K/Co₃O₄, suggesting a weaker Co³⁺-O^- bond on the K-modified 0.02 K/Co₃O₄ catalyst.

  Figure 10

  

  Figure 10.  Cobalt XPS spectra of the K-free and K-modified Co₃O₄ catalysts a: Co₃O₄; b: 0.02K/Co₃O₄

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  Figure 11 depicts the H₂-TPR spectra of Co₃O₄ and 0.02K/Co₃O₄ catalysts.

  Figure 11

  

  Figure 11.  H₂-TPR profiles of the K-free and K-modified Co₃O₄ catalysts

  a: Co₃O₄; b: 0.02K/Co₃O₄

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  The low temperature peaks at 230-360 ℃ are ascribed to the reduction of Co³⁺ to Co²⁺, whereas the high temperature peaks at 360-540 ℃ are assigned to the reduction of Co²⁺ to metallic cobalt. For K-free Co₃O₄, the reduction of Co³⁺ to Co²⁺ appears at 338.7 ℃, whereas for K-modified 0.02 K/Co₃O₄, it is shifted to 307.0 ℃. It means that the Co³⁺-O^- species on 0.02 K/Co₃O₄ is more easily reduced, consistent with the XPS result, which may contribute to the high activity of K-modified 0.02 K/Co₃O₄ (Figure 12).

  Figure 12

  

  Figure 12.  Activity of K-modified Co₃O₄ catalyst in N₂O decomposition under different reaction atmospheres

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  Figure 12 illustrates that when oxygen and steam are added into reaction feed, the conversion of N₂O over 0.02 K/Co₃O₄ is decreased evidently. Table 3 gives the calculated rate coefficient (k), apparent activation energy (E_a) and pre-exponential factor (A). Obviously, oxygen and steam lead to a decrease in the reaction rate and an increase in the apparent activation energy; meanwhile, E_a and A values change in the same direction, viz., lnA = 6.95 + 0.144E_a, indicating that there is a dynamic compensation effect between E_a and A.

  Table 3

  

  Table 3.  Kinetic data of N₂O decomposition over 0.02K/Co₃O₄ under different reaction atmospheres

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  Reaction feed	k/s-1				Ea /(kJ·mol-1)	lnA
  	250 ℃	275 ℃	300 ℃	325 ℃
  1%N2O + Ar	2.76	7.50	13.09	20.10	68.2	16.8
  1%N2O + 2%O2 + Ar	1.78	5.39	9.90	14.04	71.4	17.2
  1%N2O + 2%O2 + 8.2%H2O + Ar	0.52	1.42	3.19	6.73	88.5	19.7

  Figure 13 shows the catalytic stability of 0.02 K/Co₃O₄. Over the K-modified 0.02K/Co₃O₄ catalyst, the N₂O conversion remains over 91% at 400 ℃ after conducting the reaction for 50 h in the presence of oxygen and steam, which is much more superior to the performance of Co₃O₄ prepared by precipitation reported in our previous work^[17].

  Figure 13

  

  Figure 13.  Stability of the 0.02 K/Co₃O₄ catalyst in N₂O decomposition at 400 ℃ under the atmosphere of 1%N₂O, 2%O₂, 8.2%H₂O, and balanced argon

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  3.   Conclusions

  Cobaltosic oxide precursors were hydrothermally synthesized by using CTAB as the template and the Co₃O₄ catalysts were then prepared by calcining the cobaltosic oxide precursors, which was further modified by impregnation with K₂CO₃ solution and used in the decomposition of N₂O; the effect of various preparation parameters on the catalytic activity of Co₃O₄ in N₂O decomposition was then investigated.

  The results indicated that the Co₃O₄ catalyst prepared by using 0.05 mol/L CTAB solution, with a CTAB to cobalt molar ratio of 1 and a urea to cobalt molar ratio of 4, exhibits high activity in N₂O decomposition. The catalytic performance of Co₃O₄ can be further enhanced by modifying with K. Over the 0.02 K/Co₃O₄ catalyst, the N₂O conversion remains over 91% at 400 ℃ after conducting the reaction for 50 h in the presence of oxygen and steam, which is much more superior to the performance of Co₃O₄ prepared by precipitation reported in our previous work.

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  Figure 1  XRD patterns of various Co₃O₄ catalysts synthesized by changing the CTAB concentrations

  a: Co₃O₄(CTAB-0); b: Co₃O₄(CTAB-0.01); c: Co₃O₄(CTAB-0.03); d: Co₃O₄(CTAB-0.05); e: Co₃O₄(CTAB-0.06)

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  Figure 2  SEM images of various Co₃O₄ catalysts synthesized by changing the CTAB concentration

  (a): Co₃O₄(CTAB-0); (b): Co₃O₄(CTAB-0.01); (c): Co₃O₄(CTAB-0.03); (d): Co₃O₄(CTAB-0.05); (e): Co₃O₄(CTAB-0.06)

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  Figure 3  N₂O conversions over various Co₃O₄ catalysts synthesized by changing the CTAB concentration

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  Figure 4  XPS spectra of various Co₃O₄ catalysts synthesized by changing the CTAB concentration

  a: Co₃O₄(CTAB-0); b: Co₃O₄(CTAB-0.01); c: Co₃O₄(CTAB-0.03); d: Co₃O₄(CTAB-0.05); e: Co₃O₄(CTAB-0.06)

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  Figure 5  O₂-TPD profiles of various Co₃O₄ catalysts synthesized by changing the CTAB concentration

  a: Co₃O₄(CTAB-0); b: Co₃O₄(CTAB-0.01); c: Co₃O₄(CTAB-0.03); d: Co₃O₄(CTAB-0.05); e: Co₃O₄(CTAB-0.06)

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  Figure 6  N₂O conversions over various Co₃O₄ catalysts synthesized by changing the CTAB/cobalt molar ratio

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  Figure 7  N₂O conversions over various Co₃O₄ catalysts synthesized by changing urea/cobalt molar ratio

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  Figure 8  O₂-TPD profiles of various Co₃O₄ catalysts synthesized by changing the CTAB/cobalt molar ratio

  a: Co₃O₄(CTAB/Co-0.83); b: Co₃O₄(CTAB/Co-1); c: Co₃O₄(CTAB/Co-1.1); d: Co₃O₄(CTAB/Co-1.3)

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  Figure 9  O₂-TPD profiles of various Co₃O₄ catalysts synthesized by changing urea/cobalt molar ratio

  a: Co₃O₄(urea/Co-3); b: Co₃O₄(urea/Co-4); c: Co₃O₄(urea/Co-5)

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  Figure 10  Cobalt XPS spectra of the K-free and K-modified Co₃O₄ catalysts a: Co₃O₄; b: 0.02K/Co₃O₄

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  Figure 11  H₂-TPR profiles of the K-free and K-modified Co₃O₄ catalysts

  a: Co₃O₄; b: 0.02K/Co₃O₄

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  Figure 12  Activity of K-modified Co₃O₄ catalyst in N₂O decomposition under different reaction atmospheres

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  Figure 13  Stability of the 0.02 K/Co₃O₄ catalyst in N₂O decomposition at 400 ℃ under the atmosphere of 1%N₂O, 2%O₂, 8.2%H₂O, and balanced argon

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  Table 1.  BET surface area of various Co₃O₄ catalysts synthesized by changing the CTAB concentration

  Catalyst	BET surface area A/(m2·g-1)
  Co3O4(CTAB-0)	21.1
  Co3O4(CTAB-0.01)	42.2
  Co3O4(CTAB-0.03)	47.2
  Co3O4(CTAB-0.05)	50.3
  Co3O4(CTAB-0.06)	27.8

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  Table 2.  Kinetic data for N₂O decomposition over various Co₃O₄ catalysts synthesized by changing the CTAB concentration

  Catalyst	k/s-1					Ea/(kJ·mol-1)	lnA
  	300 ℃	325 ℃	350 ℃	375 ℃	400 ℃
  Co3O4(CTAB-0)	-	-	1.96	3.77	6.55	84.2	16.9
  Co3O4(CTAB-0.01)	0.75	0.91	2.45	4.02	6.55	74.5	15.2
  Co3O4(CTAB-0.03)	1.01	1.87	3.53	5.70	7.30	65.4	13.8
  Co3O4(CTAB-0.05)	1.33	2.45	4.40	7.50	11.50	69.8	14.9
  Co3O4(CTAB-0.06)	0.75	1.42	2.86	5.39	7.71	77.1	15.9

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  Table 3.  Kinetic data of N₂O decomposition over 0.02K/Co₃O₄ under different reaction atmospheres

  Reaction feed	k/s-1				Ea /(kJ·mol-1)	lnA
  	250 ℃	275 ℃	300 ℃	325 ℃
  1%N2O + Ar	2.76	7.50	13.09	20.10	68.2	16.8
  1%N2O + 2%O2 + Ar	1.78	5.39	9.90	14.04	71.4	17.2
  1%N2O + 2%O2 + 8.2%H2O + Ar	0.52	1.42	3.19	6.73	88.5	19.7

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