Low-temperature selective catalytic reduction of NO with NH3 over ordered mesoporous MnxCo3 − xO4 catalyst

doi:10.1016/j.catcom.2015.01.022

Catalysis Communications

Volume 62, 5 March 2015, Pages 107-111

https://doi.org/10.1016/j.catcom.2015.01.022 Get rights and content

Highlights

•
The mesoporous MnCo₂O₄ catalyst was synthesized by nanocasting method.
•
The mesoporous catalyst was first used in SCR reaction.
•
The MnCo₂O₄ catalyst showed much better SCR activity than the MnO₂ catalyst.
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Catalytic activities depend on the strong interaction of manganese and cobalt oxide species.

Abstract

The ordered mesoporous materials (MnO₂, Co₃O₄ and MnCo₂O₄) were successfully synthesized by the nanocasting method and tested for the selective catalytic reduction (SCR) of NO with NH₃. The MnCo₂O₄ catalyst had higher N₂ selectivity, more extensive operating-temperature window, and high SO₂ tolerance. The TEM results suggested that the special porous structure provided a larger surface area to adsorb and activate reaction gases. The H₂-TPR and NH₃-TPD results demonstrated that the MnCo₂O₄ catalyst possessed a more powerful reducibility and stronger acid strength.

Graphical abstract

Introduction

Nitrogen oxides (NO_x) derived from power plant and vehicle exhausts have resulted in serious atmospheric environment pollutions nowadays, such as ozone depletion, photochemical smog and acid rain [1], [2]. Selective catalytic reduction (SCR) of NO_x with NH₃ is an effective technology to reduce NO_x emissions [3], [4]. Nowadays V₂O₅–WO₃/TiO₂ is the most widely used catalyst. However, there are some inevitable problems that remain in this catalyst, such as the toxicity of vanadium, SO₂ oxidation to SO₃, and the need to operate at high temperatures (300–400 °C). Thus, it is urgent to develop low-temperature (LT) SCR catalysts, which could be located downstream of the electrostatic precipitator and desulfurization.

Recently, it has been reported that transition metal oxides (e.g. Cr₂O₃, Co₃O₄, CuO, or MnO₂) show good activity for LT-SCR reaction [5], [6], [7]. Among all metal oxides studied, manganese and cobalt containing catalysts are low cost, environmentally friendly and relatively highly active for NO reduction. However, they are very sensitive to the presence of SO₂ in the exhaust, and the temperature window is narrow and not suitable to practical application. Appropriate combinations of metal oxides may exhibit higher activity and thermal stability than the single oxides [8], [9], [10], [11]. The traditional nanoparticles could agglomerate easily during the catalytic process, resulting in the decline of catalytic activity. Compared to the nanoparticles, the ordered mesoporous materials with large surface-to-volume ratios and uniform porous structure exhibit many interesting properties in energy conversion and storage, catalysis, sensing and magnetic devices because they possess d-shell electrons confined to nanosized walls, redox active internal surfaces, and connected pore networks [12], [13].

Herein, in this paper, a highly ordered mesoporous MnCo₂O₄ was synthesized via the nanocasting method. The synthesis of mesoporous metal oxides via the nanocasting route can deliver materials with highly ordered pore structures and highly crystalline walls, as well as good thermal stability. The MnCo₂O₄ catalyst exhibited ultra-high low temperature catalytic activity for cleaning NO with NH₃ from 100 to 300 °C. The major reasons were studied.

Section snippets

Synthesis of samples

The porous silica template with cubic (Ia3d) pore symmetry (KIT-6) was synthesized according to the previous literature [14]. In a typical synthesis process, 1.0 g KIT-6 was suspended in 70 mL dry n-hexane, then stirred for 3 h, and 1.0 mL aqueous solution containing 2.3 mmol Mn(NO₃)₂·4H₂O and 4.6 mmol Co(NO₃)₂·6H₂O were added with vigorous stirring. The mixture was stirred overnight, filtered and dried at 40 °C, then calcined in a muffle furnace at 450 °C for 4 h with a heating rate of 1 °C/min.

Results and discussion

The XRD patterns of the sample were shown in Fig. 1. The diffraction peaks of single Co₃O₄ and MnO₂ could be ascribed to cubic spinel phase Co₃O₄ (JCPDS 74-2120) and pyrolusite-MnO₂ (JCPDS 81-2261). For the MnCo₂O₄ sample, the diffraction peaks that belonged to the face-centered-cubic (fcc) space group (Fd-3m(227); JCPDS 23-1237) could be observed. According to the Scherrer Formula, the grain sizes of Co₃O₄, MnO₂ and MnCo₂O₄ were 12.4, 11.1 and 8.9 nm, respectively. From low-angle XRD patterns

Conclusions

In summary, we have successfully fabricated mesoporous MnCo₂O₄ with a spinel structure via the nanocasting route, for the low temperature NH₃-SCR. Compared to the single metal oxide, MnO₂ and Co₃O₄, the MnCo₂O₄ catalyst exhibits an enhanced NH₃-SCR activity, high N₂ selectivity, wide operating temperature window, and improved SO₂ tolerance. The outstanding NH₃-SCR activity of the MnCo₂O₄ catalyst can be attributed to the strong interaction of manganese and cobalt oxide species as well as the

Acknowledgment

This study was financially supported by the National Natural Science Foundation of China (21377061 and 81270041), the Asia Research Center in Nankai University (AS1326), the Natural Science Foundation of Tianjin (12JCQNJC05800), and the Key Technologies R&D Program of Tianjin (13ZCZDSF00300).

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Short communicationLow-temperature selective catalytic reduction of NO with NH3 over ordered mesoporous MnxCo3 − xO4 catalyst

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Synthesis of samples

Results and discussion

Conclusions

Acknowledgment

J. Catal.

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Microporous Mesoporous Mater.

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Short communication
Low-temperature selective catalytic reduction of NO with NH₃ over ordered mesoporous Mn_xCo_3 − xO₄ catalyst

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