Fabrication of Mn–CeO_x/CNTs catalysts by a redox method and their performance in low-temperature NO reduction with NH₃

Abstract

Highly active Mn–CeO_x/CNTs catalysts were first fabricated by a novel redox method, and a formation mechanism was proposed. The as-obtained catalyst possessed an amorphous structure, and high Ce³⁺/(Ce³⁺ + Ce⁴⁺) and O_α/(O_α + O_β) ratios, which endowed it with 52.2–98.4% NO conversion at a weight hourly space velocity of 210 000 ml g_cat⁻¹ h⁻¹.

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Nitrogen oxide (NO_x) has led to many environmental issues, such as acid rain, photochemical smog, ozone depletion and greenhouse effects.^1,2 Selective catalytic reduction (SCR) of NO with NH₃ is considered as the optimum clean technology for elimination of the NO from stationary sources.^3,4 However, as a core of the commercial SCR catalyst, V₂O₅–WO₃(MoO₃)/TiO₂ only shows better SCR activity at a high working temperature window of 300–400 °C.^5,6 In addition, due to the deactivation of SO₂ and ash, this kind of catalyst should be located downstream of the desulfurizer and electrostatic precipitator, and the flue gas temperature at this point is usually below 200 °C.^2,7 Therefore, it is necessary to develop a catalyst with excellent SCR activity below 200 °C.

Based on their unique one-dimensional tubular, electronic, physical and chemical characteristics,^8–10 carbon nanotubes (CNTs) used as support material has obtained a series of outstanding SCR catalysts, including MnO_x/CNTs,^5,7,8 CeO_x/CNTs,^11,12 and VO_x/CNTs.¹³ However, the above CNTs-based catalysts still require high operating temperature window (200–300 °C), indicating the importance of development of high efficient SCR catalyst below 200 °C.

Recently, Mn–CeO_x/CNTs catalysts have been applied in SCR reaction and exhibited the desirable catalytic activity.^1,14–16 However, the preparation methods of these catalysts still have some shortcomings, which was ascribed to the high-pressure hydrothermal reaction, high-temperature calcination treatment or unclear formation mechanism. Given this, a novel one-step redox method was adopted to prepare the highly active Mn–CeO_x/CNTs catalyst, and its synthesis reaction mechanism was proposed. As shown in Fig. S1,† the preparation procedure was listed below: Ce³⁺ ions were firstly absorbed on the surface of CNTs by the electrostatic force, followed by hydrolyzation to Ce(OH)₃ and HCl. After that, Ce(OH)₃ was transformed to Ce₂O₃ and CeO₂ by KMnO₄. Meanwhile, the KMnO₄ was reduced into MnO₂. Finally, the as-prepared catalysts were applied in low-temperature NO reduction with NH₃ and showed excellent catalytic activity at 80–180 °C.

The NO conversion as a function of temperature for CNTs-based catalysts was displayed in Fig. 1. The NO conversion of Mn–CeO_x/CNTs catalysts prepared via redox method was higher than that of Mn–CeO_x/CTNs-MI catalyst fabricated by modified impregnation method, and could reach 74–96% at 140 °C at a high space velocity of 210 000 ml/g_cat h. It was worth noting that the 6% Mn–CeO_x/CNTs catalyst presented the first-rate SCR activity in the tested temperature range, and 98.4% NO conversion was obtained at 180 °C.

Fig. 1 NO conversion as a function of temperature for CNTs-based catalysts. Reaction conditions: [NH₃] = [NO] = 500 ppm, [O₂] = 5%, N₂ as balance gas, WHSV = 210 000 ml g_cat⁻¹ h⁻¹, 0.2 g catalyst.

The H₂-TPR profiles of 6% Mn–CeO_x/CNTs and Mn–CeO_x/CNTs-MI were given in Fig. S2.† Clearly, the catalysts showed two reduction peaks. The former peak located at 150–250 °C should be attributed to the reduction of MnO₂ and Mn₂O₃ to Mn₃O₄.¹⁷ The latter peak located between 350–450 °C was possibly ascribed to the overlap of the reduction peaks based on Mn₃O₄ and CeO₂.¹⁸ It should be noted that the 6% Mn–CeO_x/CNTs catalyst exhibited better low-temperature reducibility, which could be due to highly-dispersed amorphous Mn–Ce mixed oxide catalysts.

The surface area, pore volume and average pore diameter of acid-treated CNTs as well as Mn–CeO_x/CNTs catalysts were listed in Table S1.† The results revealed that the 6% Mn–CeO_x/CNTs catalyst has the smallest surface area and pore volume but the optimum SCR activity, implying that the surface area was not a crucial factor for catalytic activity. To compare the change of surface area and pore structure of the two catalysts, the N₂ adsorption–desorption and pore size distribution (inset) curves were illustrated in Fig. 2. Based on the classification of the IUPAC,¹⁹ the typical type-IV curves with the type-H3 hysteresis loops could be observed in Fig. 2, indicating the existence of mesopore. In addition, the pore size of the two catalysts showed a narrow distribution centered at 3.7 nm, further demonstrating the mesopore structure of the catalysts. Besides, part of the pore size for 6% Mn–CeO_x/CNTs was more than 3.7 nm (inset of Fig. 2), and these big pores would influence the surface area. This conclusion was good agreement with the results of Table S1.†

Fig. 2 N₂ adsorption–desorption isotherms of (a) 6% Mn–CeO_x/CNTs and (b) Mn–CeO_x/CNTs-MI.

The morphologies of acid-treated CNTs and CNTs-based catalysts were shown in Fig. 3. It was found from Fig. 3a and b that the external walls of the acid-treated CNTs was clear, and then, was covered with nanoflake-like metal oxides after introduction of the Mn–Ce mixed oxide catalysts, suggesting that metal oxide catalysts have been successfully supported on CNTs. Additionally, the apparent lattice fringe ascribing to the manganese oxides or cerium oxides was not observed from HRTEM of 6% Mn–CeO_x/CNTs (Fig. 3c), implying amorphous Mn–Ce mixed oxide catalysts. Moreover, Fig. 3c revealed that the nanoflake metal oxide catalysts closely adhered to the surface of CNTs, which was advantageous for the stability of the catalyst morphology. As for Mn–CeO_x/CNTs-MI catalyst, metal oxides catalysts attached on the surface of CNTs with a smaller contact area (Fig. 3d), and thus, easily desquamate from CNTs. Furthermore, the Mn, Ce, O and C elements could be detected by EDX spectrum (Fig. 3e), confirming the existence of Mn–Ce mixed oxide catalysts.

Fig. 3 TEM and HRTEM of CNTs-based catalysts: (a) acid-treated CNTs; (b and c) 6% Mn–CeO_x/CNTs; (d) Mn–CeO_x/CNTs-MI; (e) EDX spectrum of 6% Mn–CeO_x/CNTs from the annular region of Fig. 3b.

The chemical states of the metal oxides and the content of the elements in the near surface region were evaluated by XPS, and the results were listed in Fig. 4. The XPS full spectrum (Fig. 4A) displayed the signals of Mn, Ce, C and O, indicating the presence of Mn, Ce, C and O elements. The Mn 2p spectrum (Fig. 4B) exhibited two peaks attributing to Mn 2p^1/2 and Mn 2p^3/2, which were centered at the binding energies of 653.85 eV and 641.95 eV, respectively. Besides, the energy separation of 11.9 eV between Mn 2p^1/2 and Mn 2p^3/2 was in line with previous studies for MnO₂.¹⁷

Fig. 4 (A) XPS full spectrum and (B) Mn 2p spectrum of 6% Mn–CeO_x/CNTs catalyst; (C) Ce 3d and (D) O 1s spectra for (a) 6% Mn–CeO_x/CNTs and (b) Mn–CeO_x/CNTs-MI.

The Ce 3d spectra (Fig. 4C) of the two catalysts could be fitted into the eight peaks. The peaks denote as u1 (882.4 eV), u3 (888.7 eV), u4 (898.1 eV), v1 (901 eV), v3 (907.4 eV), and v4 (916.4 eV) was indexed to Ce⁴⁺ species, while u2 (885.3 eV) and v2 (903.67 eV) was assigned to the Ce³⁺ species, implying the coexistence of Ce³⁺ and Ce⁴⁺ species.^2,20 Moreover, Table S2† showed that the ratio of Ce³⁺/(Ce³⁺ + Ce⁴⁺) for 6% MnO_x/CNTs (0.161) was higher than that of MnO_x/CNTs-MI (0.119), which could create more chemisorbed oxygen and enhance the oxygen mobility and, thus, improve the SCR activity.²¹

The O 1s spectra (Fig. 4D) of the two catalysts were deconvoluted by the peak-fitting process. The peaks centered at 529.4 eV were attributed to the lattice oxygen (O_β), and the binding energies at 529.4–534 eV were ascribed to the surface oxygen (O_α).¹⁷ Table S2† displayed that the O_α/(O_α + O_β) ratio of 6% Mn–CeO_x/CNTs (55.5%) was higher than that of Mn–CeO_x/CNTs-MI (51.8%). Owing to its high mobility, the O_α was more active than that of O_β in the oxidation reaction. Therefore, the high O_α/(O_α + O_β) ratio was conducive to NO oxidation to NO₂, and thereby, accelerated the “fast SCR” reaction.²²

The XRD patterns of CNTs-based samples were illustrated in Fig. 5. It was observed that all samples presented the typical graphite peaks at 26.3°, 42.8°, 54.3° and 77.7°.²³ After being supported by Mn–Ce mixed oxide catalysts, the peak intensity of graphite declined, implying an interaction between Mn–Ce mixed oxides and CNTs.^7,16 The Mn–CeO_x/CNTs-MI catalyst exhibited three typical peaks at 28.6°, 47.5° and 56.3°, which was assigned to CeO₂ (PDF#34-0394). For Mn–CeO_x/CNTs catalysts, the weak diffraction peak at 30.3° was indexed to Ce₂O₃ (PDF#44-1086), which was related to the XPS results (Fig. 4C). It should be noted that the typical peaks of MnO₂ for Mn–CeO_x/CNTs was absence, suggesting its amorphous structure.² This conclusion was consistent with the HRTEM results (Fig. 3c).

Fig. 5 XRD patterns of (a) acid-treated CNTs; (b) 2% Mn–CeO_x/CNTs; (c) 4% Mn–CeO_x/CNTs; (d) 6% Mn–CeO_x/CNTs; (e) 8% Mn–CeO_x/CNTs and (f) Mn–CeO_x/CNTs-MI.

According to the XRD and XPS results as well as the generation of the Cl₂ in the preparation process, the formation mechanism of the Mn–CeO_x/CNTs catalyst was proposed as follows: the CeCl₃ was firstly hydrolyzed into Ce(OH)₃ and HCl. Then, the HCl was oxidized into Cl₂ by KMnO₄, which could accelerate the hydrolyzation reaction. Meanwhile, part of Ce(OH)₃ was transformed into Ce₂O₃, the other part was oxidized into CeO₂ by KMnO₄, respectively. At the same time, the KMnO₄ was reduced into MnO₂. Finally, the Mn–CeO_x/CNTs catalysts were obtained. The relevant reaction equations were listed below:

Conclusions

A novel redox route was employed to prepare the Mn–CeO_x/CNTs catalysts, and the as-prepared catalysts exhibited the excellent SCR activity at 80–180 °C. Thereinto, the 6% Mn–CeO_x/CNTs catalyst exhibited the first-rate low-temperature SCR activity, which benefited from its amorphous phase, better low-temperature reducibility, higher Ce³⁺/(Ce³⁺ + Ce⁴⁺) and O_α/(O_α + O_β) ratios. The clear formation mechanism was helpful for synthesization of the highly efficient Mn–CeO_x/CNTs catalyst.

Acknowledgements

This work was supported by Scientific and Technological Innovation Project of Fujian Province (Grant no. 2012H6008).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental section, the relative concentration ratios of Ce and O, N₂ adsorption–desorption isotherms curves, BET and pore volume data. See DOI: 10.1039/c5ra01129a

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