Mn–Ce/ZSM5 as a new superior catalyst for NO reduction with NH3

doi:10.1016/j.apcatb.2006.06.003

Applied Catalysis B: Environmental

Volume 73, Issues 1–2, 24 April 2007, Pages 60-64

https://doi.org/10.1016/j.apcatb.2006.06.003 Get rights and content

Abstract

Mn–Ce/ZSM-5 catalyst prepared in an aqueous phase at 423 K exhibits a broad temperature window (517–823 K) for high NO conversions (75–100%) in the process of selective catalytic reduction (SCR) by NH₃ even in the presence of H₂O and SO₂. Both the zeolite matrix and the over-exchanged amounts of manganese and cerium contribute to obtain a complex structure that owns microporous–mesoporous characteristics and specific surface properties.

Introduction

Nowadays, nitrogen oxides emitted by stationary sources contribute up to nearly 48% to total (NO)_x emissions therefore being an important source for air pollution (e.g. ozone depletion, photochemical smog, greenhouse effects and acid rain) [1]. The most effective technology to remove them is the selective catalytic reduction (SCR) of NO by ammonia (4NO + 4NH₃ + O₂ → 4N₂ + 6H₂O). Although, many catalysts have been reported to be active for this reaction continuing efforts have been made in developing new catalysts more active within a broader temperature range [2], [3]. Up to now, there have been two different approaches on the study of this catalytic process: one has focused on the low temperature range and the other on the high temperature range. For the low temperature range, catalysts containing transition metals have been investigated (e.g. MnO_x/Al₂O₃ [4], MnO_x/NaY [5], CuO/TiO₂ [6]). Qi and Yang recently reported Mn–Ce mixed oxides as a catalyst with superior activity for the process of NO reduction by NH₃ at low temperature [7], [8]. For the high temperature range, zeolites containing over-exchanged amounts of different metals (Cu, Ni, Mn, Co and specially Fe) have shown high catalytic performances [9]. The combination between zeolite microstructure and over-exchanged amounts [10] of different forms of metal cations and/or metal oxides is able to give rise to a complex structure that possesses special structural features and specific acid–base characteristics [11], [12]. In particular, the textural and surface characteristics of these materials are able to play an important role in establishing their structure–catalytic activity relationship [13].

Considering together the information regarding the special performances of Mn–Ce mixed oxides for the low temperature range of the SCR process and the high activity for NO reduction, over the high temperature range, of zeolites with over-exchanged amounts of different cations, we prepared, by a specific method, ZSM-5 containing over-exchanged amounts of manganese and cerium. In this work, we report Mn–Ce/ZSM-5 as a new catalyst showing high performances, within a broader temperature window, for the process of SCR of NO by ammonia. The porous and the surface properties of the new structure formed by merging the zeolite matrix and over-exchanged amounts of manganese and cerium have also been evaluated.

Section snippets

Synthesis

Manganese and cerium containing ZSM-5 samples (denoted as MnCeZ1 and MnCeZ2) were prepared in an aqueous phase by improving a previous reported method [14]. The zeolite material, NH4-ZSM-5 (Si/Al = 16.4, S_BET = 400 m² g⁻¹), supplied by Zeolyst International was used in its H⁺ form. One hundred millilitres of aqueous solution of manganese(II) acetylacetonate (0.025 M for MnCeZ1 and 0.04 M for MnCeZ2) was added during a 2.5 h period over 3 g of zeolite dispersed in 50 ml of water, at 423 K, under strong

XRD, XRF and XPS analysis

The XRD patterns of the synthesized samples (not shown) are typical for well-crystallized ZSM-5 structure with high aluminum content. No diffraction peaks attributed either to metal and/or to metal oxide clusters were observed, suggesting that the formed manganese and cerium species are in the nanometer size range and well dispersed.

The bulk and surface chemical compositions of the samples are listed in Table 1. For MnCeZ1 sample, the bulk Mn/Al and Ce/Al ratios are higher than the corresponded

Acknowledgments

We acknowledge financial support from Tokyo Institute of Technology, Japan and Ministry of Education and Research, Romania.

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Mn–Ce/ZSM5 as a new superior catalyst for NO reduction with NH3

Abstract

Introduction

Section snippets

Synthesis

XRD, XRF and XPS analysis

Acknowledgments

Appl. Catal. B

Appl. Catal. B

Appl. Catal. B

J. Catal.

J. Catal.

J. Catal.

Catal. Today

Appl. Catal. A

J. Catal.

J. Catal.

J. Catal.

Micropor. Mesopor. Mater.

J. Mol. Catal. A

Catal. Today

Mn–Ce/ZSM5 as a new superior catalyst for NO reduction with NH₃