Elsevier

Catalysis Today

Volume 258, Part 1, 1 December 2015, Pages 183-189
Catalysis Today

Combined catalytic systems for enhanced low-temperature NOx abatement

https://doi.org/10.1016/j.cattod.2015.05.023Get rights and content

Highlights

  • Strong synergistic effect is found for [(Mn–Ce)/CeO2–ZrO2 + zeolite] mixture.

  • Fast SCR activity of zeolite is a prerequisite of the synergistic effect.

  • Standard SCR activity of zeolite plays a minor role in the observed synergy.

  • Two-stage bifunctional mechanism is proposed to explain the observed effect.

  • However further development of the mechanism is required.

Abstract

NH3-DeNOx performances of the combined catalysts (CombiCats) prepared by mechanical mixing of RedOx component (CeO2–ZrO2 or Mn–Ce/CeO2–ZrO2) and zeolite component (Fe-Beta, H-Beta, USY, etc.) were studied in details. By comparing DeNOx performances of the CombiCats and their individual components a pronounced synergistic effect was revealed, as evidenced by the improvement of DeNOx activities and selectivities far above performances of the individual components. Significant improvement was observed within 150–250 °C temperature range. Detailed study of a possible origin of the synergistic effect suggested that the improvement of NH3-DeNOx activity can be attributed to the bifunctional mechanism comprising two main stages:(1)2NO+O22NO2overRedOxcomponent(2)NO+NO2+2NH32N2+3H2Ooverzeolitecomponent

Remarkably, the synergistic effect was attained by mixing RedOx component with H-Beta possessing negligible activity in Standard SCR but active in Fast SCR. Nevertheless, further research is required for revealing overall reaction network and understanding reaction mechanism responsible for the observed synergy.

Introduction

Selective catalytic reduction of nitrogen oxides by ammonia (NH3-SCR) is of theoretical and practical interest for abatement of NOx emission from automotive (diesel engines) and stationary (power plants) sources [1], [2]. Recently, NH3-SCR can be achieved by using catalytic systems based on Fe-Beta or Cu-Beta. However, Fe-Beta or Cu-Beta zeolites have two main drawbacks: (1) insufficient NOx conversion at “cold-start” condition (150–250 °C) and (2) NH3-slip problem due to incomplete conversion or exhaust temperature upswings. Many studies have been devoted to the development of metal oxide (Mn, Ce, Cu and Fe), supported metals, or bimetallic catalysts for low-temperature SCR applications [3], [4], [5]. Dual-bed catalysts were also considered as promising systems due to their high activity within wide temperature range [6].

Another promising solution can be provided by combined catalysts (CombiCats) comprising SCR and RedOx functions. This approach was firstly applied by Misono et al. [7], [8]. It was shown that physical mixing of Mn2O3 with M-zeolites results in the activity improvement. They proposed a bifunctional mechanism of C3H6-SCR over the physical mixture of Mn2O3 or CeO2 and M-zeolite (M – various metals). The observed activity improvement was explained as follows: Mn2O3 oxidizes NO to NO2 and M-zeolite is responsible for the reaction between NO2 and propylene and the decomposition of intermediates to N2. Our recent study [9] indicated that such bifunctional mechanism also can be successfully applied in NH3-SCR process. The basic idea was based on the fact discovered by Kasaoka et al. [10] that the NH3-SCR of NO/NO2 mixtures (Eq. (1)) is much faster than the NH3-SCR of NO alone (Eq. (2)):NO + NO2 + 2NH3  2N2 + 3H2O – Fast SCR4NH3 + 4NO + O2  4N2 + 6H2O – Standard SCR

Our study demonstrated [9] that high NOx SCR performance at Treact. < 250 °C can be provided by the combined catalysts comprising a RedOx component (possessing high activity in NO oxidation) and zeolite component (highly effective in the Fast SCR). For [CeO2–ZrO2 + Fe-Beta] and [Mn/CeO2–ZrO2 + Fe-Beta] compositions a pronounced synergistic effect was observed and the catalytic activity of the CombiCat significantly exceeds activities of the individual components. It was suggested that the SCR process over CombiCats proceeds according to the bifunctional pathway, comprising (1) NO to NO2 oxidation over RedOx component and (2) Fast SCR reaction over zeolite component:2NO + O2  2NO2 – on RedOx componentNO + NO2 + 2NH3  2N2 + 3H2O – on zeolite component

Salazar et al. [11] further explored this principle for SCR promoting. Intense synergistic effect was observed when combination of Fe-ZSM-5 with Mn-oxide or binary Mn–X (X = Ce, Cr, Cu) mixed oxides were used as the catalysts for the selective reduction of NO with NH3. Experimental data obtained by Salazar et al. indicated that the bifunctional model of a sequential mechanism of NO2 formation followed by Fast SCR may indeed operate, although a direct proof is difficult, since other processes may also be involved.

In this study we attempted to obtain additional evidences for bifunctional NH3-SCR mechanism over CombiCats. The main idea was to discriminate Fast SCR and Standard SCR contributions of the zeolite component to the overall mechanism. For this purpose a series of Beta with various Fe loading including “Fe-free” sample was used for the CombiCat preparation. This approach was based on the observation that Fe cations play an important role both in Standard SCR [12] and in Fast SCR [13], [14]. However, our recent study indicated that even residual amount of Fe (<0.02 wt%) is sufficient for Fast SCR reaction over BETA zeolite [15], while its Standard SCR activity is essentially ceased. These results are in a good agreement with the data obtained by Grünert et al. [14]. Thus, reducing the Fe loading to ultra-low level, we attempted to minimize Standard SCR activity of Beta component leaving its significant Fast SCR activity to clearly distinguish Fast SCR contributions into overall synergistic effect. In addition to that, usage of zeolite USY and amorphous aluminosilicate, having no activity in Standard and Fast SCR, allowed us to obtain evidences of the crucial role of Fast SCR reaction in NH3-SCR mechanism over CombiCats.

CeO2–ZrO2 and Mn–Ce/CeO2–ZrO2 demonstrated sufficient NO to NO2 oxidation activity were used as the RedOx components.

Section snippets

Catalyst preparation

Parent Fe-Beta (Si/Al  12), NH4-Beta (Si/Al  12.5), USY (Si/Al  40) are commercially available zeolites. NH4-Beta was converted into the H-form by calcination in airflow at 550 °C for 4 h. SiO2–Al2O3 (Si/Al  2.1) was provided by “Sasol”. “Fe-free” Beta (Si/Al  12.5) was synthesized via hydrothermal synthesis described in detail by Doronkin et al. [12]. All zeolites and SiO2–Al2O3 powders were calcined at 550 °C for 4 h in airflow.

Chemical compositions of Beta zeolites were determined by inductively

FE-SEM

The morphologies of the Fe-Beta zeolite, before and after mixing with Ce–Zr are shown in Fig. 1A and B, respectively. In both figures one can easily distinguish relatively large zeolite microcrystals of regular shape. In the case of mechanical mixture (Fig. 1B) each zeolite microcrystal is surrounded by CeO2–ZrO2 mixed oxide particles of smaller size. This observation evidences that zeolite and oxide component are evenly distributed in the CombiCat, and such microstructure insures tight contact

Conclusions

The data obtained in this study provide several evidences in favor of bifunctional mechanism suggested for explanation of synergistic effect observed for combined catalysts prepared by mechanical mixing zeolite and RedOx components:

  • 1.

    It is evident, that Fast SCR activity of zeolite component is a prerequisite of synergistic effect, while Standard SCR activity plays a minor role in the overall performance of CombiCat. Thus, despite essentially negligible Standard SCR activity of H-Beta component,

Acknowledgements

The support of Russian Foundation for Basic Research (Grant: 15-03-07802 A) is gratefully acknowledged. A. Mytareva and D. Krivoruchenko are grateful to Haldor Topsøe A/S for financial support in the framework of Ph.D. student support programme.

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