Selective catalytic reduction of NOx with NH3 over Mn-Ce mixed oxide catalyst at low temperatures

doi:10.1016/j.cattod.2013.06.009

Catalysis Today

Volume 216, 1 November 2013, Pages 76-81

https://doi.org/10.1016/j.cattod.2013.06.009 Get rights and content

Highlights

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Catalyst for the selective reduction of NO_x with NH₃ at low temperatures.
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Mn-Ce mixed oxide prepared by the surfactant-template (ST) method is more active.
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More reactive NH₃ species formed on Mn-Ce catalyst prepared by the ST method.
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NO_x adsorption and activation was promoted on Mn-Ce catalyst prepared by the ST method.

Abstract

A series of Mn-Ce mixed oxide catalysts with different molar ratio of Mn/Ce were prepared by the surfactant-template (ST) method and conventional co-precipitation (CP) method for the selective catalytic reduction of NO_x with ammonia (NH₃-SCR) at low temperatures. Catalyst preparation method exerts significant influence on the catalytic performance, and Mn-Ce mixed oxide catalyst prepared by ST method is more active than that prepared by CP method. The best Mn-Ce mixed oxide catalyst prepared by ST method yielded nearly 100% NO_x conversion in the temperature range of 100–200 °C. Moreover, its resistance against H₂O and SO₂ is higher. The high surface area of Mn-Ce mixed oxide catalyst prepared by the ST method could contribute to the adsorption and activation of NH₃ as well as NO_x, thus promoting the NO_x reduction to proceed.

Graphical abstract

Introduction

Nitrogen oxides (NO, NO₂, and N₂O), a major source for air pollution, can cause a variety of environmentally harmful effects such as photochemical smog, acid rain and ozone depletion [1], [2]. Selective catalytic reduction of NO_x with NH₃ (NH₃-SCR) has been widely used for the removal of NO_x and the typical commercial catalyst for this process is V₂O₅-WO₃(MoO₃)/TiO₂ [3], [4]. This type of catalysts are efficient at relatively high operating temperatures (300–400 °C), which makes it necessary to locate SCR unit upstream of the electrostatic precipitator. However, this renders these catalysts susceptible to deactivation from the high concentration of dust and SO₂. Therefore, there is a great interest in developing a highly active catalyst for the low temperature SCR(<250 °C) that works downstream of the electrostatic precipitator and desulfurization, which could also lead to improved economics for the SCR process [5].

Great efforts have been made to develop the efficient low-temperature SCR catalyst and some transition metal-oxides and mixed-oxide catalysts are found to be active [6], [7], [8], [9]. Mn-containing catalysts, such as MnO_x/TiO₂ and MnO_x, exhibited relatively high activity for the low temperature NH₃-SCR [10], [11]. Cr-MnO_x mixed-oxide catalyst is highly active in the range of 120–220 °C [8]. Fe_0.5Mn_0.5TiO_x with the molar ratio of Fe/Mn = 1:1 also showed high activity, over which NO_x was totally eliminated at 175 °C at GHSV = 50,000 h⁻¹ [12]. Ce-based NH₃-SCR catalyst has also attracted attention due to the high oxygen storage capacity and excellent redox property of CeO₂. Shan et al. [13] reported that a novel Ce-W mixed oxide catalyst exhibited excellent NH₃-SCR performance.

Recently, Mn-Ce mixed oxide catalyst has been studied and it appeared to be promising for the low temperature SCR process [14]. The catalyst previously reported was prepared by co-precipitation or citric acid method [14], [15]. It is well known that the catalytic performance of NH₃-SCR catalyst depends strongly on the method of preparation [16], [17]. For example, a CeTiO_x catalyst prepared by a homogenous precipitation method showed higher activity and a wider activity temperature window than CeO₂/TiO₂ prepared by a sol–gel method [18]. However, to the best of our knowledge, no investigation on the Mn-Ce mixed oxide prepared by the surfactant-template method is reported yet. Therefore, it is necessary to carry out the comparison of the activity and characterization of Mn-Ce mixed oxide catalyst prepared by the different methods. In the present study, we investigated the effect of preparation method on the catalytic performance of Mn-Ce mixed oxide catalyst in detail and found that the catalyst prepared by the surfactant-template method is more active. The effects of H₂O and SO₂ on the activity of Mn-Ce mixed oxide catalyst were also studied. On the basis of in situ DRIFTS experiments, the mechanistic cause of the higher performance of Mn-Ce mixed oxide catalyst was elucidated.

Section snippets

Surfactant-template method

A series of MnO_x-CeO₂ catalysts with different ratio of Mn/Ce were prepared by the surfactant-template method using hexadecyl trimethyl ammonium bromide(CTAB) as the template. Appropriate amounts of 0.5 M cerium chloride, 0.5 M manganese chloride and CTAB (CTAB/(Mn + Ce) = 0.8) were mixed and stirred at room temperature for 15 min, then ammonia solution was added slowly to the above solution under vigorous stirring until pH is ca. 11. After stirring at room temperature for 2 h, the obtained suspension

Effect of preparation method

Fig. 1 showed the catalytic activities of MnCe catalysts prepared by the two different methods in the absence of H₂O and SO₂. It is evident that the catalyst preparation method exerts significant influence on the catalytic performance. For the MnCe(CP) catalyst, Mn₁Ce₉(CP) exhibited the lowest activity. Increasing the ratio of Mn/Ce results in the increase of the activity and the highest activity was obtained over Mn₅Ce₅(CP). In contrast, MnCe(ST) catalyst showed higher activity than MnCe (CP)

Conclusions

Mn-Ce mixed oxide catalysts prepared by the two different preparation methods have been evaluated for the reduction of NO_x with NH₃ at low temperatures. Nearly 100% de-NO_x efficiency was obtained over the MnCe catalyst prepared by the surface template preparation method in the temperature range of 100–200 °C. Even in the co-presence of H₂O and SO₂, it still showed higher activity than that prepared by the co-precipitation method. The promoted activity of MnCe (ST) catalyst could be attributed to

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (20907003, 21077009) and the National High-Tech Research and Development (863) Program of China (2010AA065003).

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Selective catalytic reduction of NOx with NH3 over Mn-Ce mixed oxide catalyst at low temperatures

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Surfactant-template method

Effect of preparation method

Conclusions

Acknowledgements

Appl. Catal. B

Appl. Catal. B

Appl. Catal. B

Appl. Catal. B

J. Catal.

Catal. Today

Appl. Catal. A

Appl. Catal. B

Appl. Catal. B

J. Catal.

Appl. Catal. B

Catal. Commun.

Chem. Eng. J.

Appl. Catal. B

Selective catalytic reduction of NO_x with NH₃ over Mn-Ce mixed oxide catalyst at low temperatures