Elsevier

Applied Catalysis A: General

Volume 506, 5 October 2015, Pages 246-253
Applied Catalysis A: General

Potential synergic effect between MOR and BEA zeolites in NOx SCR with methane: A dual bed design approach

https://doi.org/10.1016/j.apcata.2015.09.005Get rights and content

Highlights

  • MOR and BEA zeolite catalysts containing Pd and Ce were prepared in a similar way.

  • Characterisation evidenced different metal species in each catalyst.

  • Catalyst exhibited different NO oxidation and NOx CH4–SCR performances.

  • Dual bed configuration revealed a potential synergic effect for NOx CH4–SCR.

Abstract

The selective catalytic reduction of NOx with methane (NOx CH4–SCR) under lean conditions was investigated with catalysts based on two different zeolite structures (MOR and BEA) containing Pd and Ce. The catalytic performance for NO oxidation to NO2 reaction, considered an important first key step in the NOx CH4–SCR mechanism, was also assessed.

Pd(0.3)Ce(2)–HBEA was found to be very active for NO oxidation but exhibits poor activity for NOx CH4–SCR. Conversely, Pd(0.3)Ce(2)–HMOR presents modest activity for NO oxidation, compared to Pd(0.3)Ce(2)–HBEA, but exhibits mild activity for NOx CH4–SCR reaction. Characterisation by H2–TPR, DRS UV–vis, TEM/EDS and FTIR-CO allowed the identification of palladium stabilised as Pd2+ ions in exchange positions in both monometallic and bimetallic MOR based catalysts, whereas, in BEA catalysts, it is presented as PdO clusters. Cerium is stabilised in Pd(0.3)Ce(2)–HMOR as small CeO2 particles, whereas, in Pd(0.3)Ce(2)–HBEA, it is present as large clusters. Catalysts were also tested in dual bed configuration, in which Pd(0.3)Ce(2)–HBEA was placed as first layer and Pd(0.3)Ce(2)–HMOR as second layer in the catalytic bed. The catalytic performance was significantly improved (higher NOx conversion into N2 and higher CH4 selectivity to SCR reaction), when compared to the catalytic performance of each catalyst individually, suggesting the existence of a synergic effect. This synergy is explained by the complementary roles that each catalyst play in HC–SCR mechanism.

Introduction

The increasing concerns about air pollution control have recently resulted in the publication, by different countries, of legislation that establishes more stringent maxima of allowed emission values for several pollutants (including nitrogen oxides, hydrocarbons and particulate matter), particularly in the road transportation sector (mobile sources) [1]. It is known that the importance of natural gas vehicles is increasing worldwide [2]. One possible after-treatment solution that simultaneously removes NOx and HC from the exhaust gases of these vehicles, working in lean-burn conditions, is the NOx selective catalytic reduction with hydrocarbons (HC–SCR) over zeolite-based catalysts containing metals [3], [4], [5]. Despite several decades of studies, this technology still lacks some technological improvement in order to be successfully implemented as a commercial solution.

Pd-zeolites are known to be active for NOx SCR using methane, under lean conditions, as a reductant [6], [7], [8]. The catalytic performance of these catalysts can be improved by considering bimetallic formulations, such as PdCo-zeolites [9], [10] and PdCe-zeolites [11], [12]. Despite being active and selective catalysts, it has been reported in literature that PdCo-zeolites [13] suffer from deactivation over time, but the same seems not to happen with PdCe-zeolites, which activity has been reported to be constant over the time [14]. Hence, the further understanding of the role of Ce in these catalysts might be important to move forward towards a commercial application of this type of after-treatment system.

Few works can be found in literature, describing the combination of different catalysts in dual catalysts systems as a possible solution for HC-SCR. For instance, Chen, et al. have reported the existence of synergies between different zeolites (namely, Fe-MFI and Fe-FER) in the NOx SCR with iso-butane [15]. Fe-FER presents lower activity NOx SCR because the small pores of FER structure become blocked by nitrogen-containing compounds formed due to the interaction between the hydrocarbons and NOx. However, NO2 is smaller enough to pass through the channels. On the other hand, Fe-MFI deactivates over time due to the formation of a deposit that blocks the sites responsible for the NO oxidation to NO2, which is considered to be a first key step in the HC-SCR mechanism [16]. By mixing Fe-FER with Fe-MFI, the NO2 formed in Fe-FER is able to interact with the active groups of the deposit on Fe-MFI and a resulting enhancement on catalytic performance is foreseen.

Holmgreen, et al., have also studied the use of dual catalysts system for HC-SCR in lean-burn conditions, namely using methane as reductant [17]. They observed an enhancing effect on NOx SCR performance of Pd-supported sulphated zirconia (reduction catalyst), when mixed with an oxidation catalyst, such as Co impregnated on zirconia (with low activity for NOx SCR but high activity for NO oxidation to NO2).

In this work, the catalytic performance of PdCe-zeolites (MOR and BEA) for NOx SCR with methane is compared. The choice of these zeolites structures was based on the facts that (i) MOR is described in literature as a zeolite structure that leads to active Pd and Pd-Ce catalysts for NOx CH4–SCR [12], [13], [18]; (ii) BEA zeolite has been used in a catalyst that exhibited enhanced catalytic performance for NOx CH4-SCR after being exposed to water [9], which is naturally present in real exhaust gases. Different characterisation techniques were used in order to identify the main differences in the metal species stabilised in the zeolite structures. A potential synergic effect on NOx SCR with methane due to the mixture of the different zeolite-based catalysts containing identical Pd and Ce metal loadings is reported for the first time.

Section snippets

Catalysts preparation

Catalysts were prepared from CBV21A zeolite (NH4MOR), with Si/Al = 10 and CP814E zeolite (NH4BEA), with Si/Al = 12.5, supplied by Zeolyst. In order to obtain Pd(0.3)–HMOR and Pd(0.3)–HBEA, 0.3 wt.% of palladium was introduced by ion-exchanging the starting zeolites with a solution with adequate concentration, prepared by dilution of a Pd(NH3)4(NO3)2 aqueous solution (Aldrich, 99.99% purity, 10 wt.%), for 24 h, at room temperature. Afterwards, exchanged samples were recovered by centrifugation and

Temperature programmed reduction under hydrogen (H2–TPR)

Fig. 1 shows the H2–TPR profiles of the studied catalysts. The H2–TPR profiles of both HMOR and HBEA catalysts, obtained by applying the same calcination procedure as the one described in the catalyst preparation section for bimetallic catalysts (after cerium introduction) did not exhibit any reduction process. For both Pd(0.3)–HMOR and Pd(0.3)Ce(2)–HMOR, it is possible to observe the existence of a reduction peak between 80 and 200 °C, which is assigned to the reduction of Pd2+ in exchange

Conclusions

The use of different zeolites (BEA and MOR) in the preparation of bimetallic catalysts, containing similar amounts of metal, by addition of cerium to Pd-based catalyst resulted in two catalysts with completely different catalytic performances. Pd(0.3)Ce(2)–HBEA has shown to be very effective in oxidising NO–NO2 and significantly better than Pd(0.3)Ce(2)–HMOR. This reaction is considered to be a first key step on the NOx SCR mechanism using hydrocarbons (HC-SCR). However, Pd(0.3)Ce(2)–HMOR

Acknowledgements

The authors acknowledge Fundação para a Ciência e a Tecnologia (FCT) – project UID/QUI/00100/2013 and grant SFRH/BD/78639/2011 – and ENGIE for financial support (Project ENGIE/IST/UPMC). The authors also acknowledge Laboratoire Catalyse et Spectrochimie (LCS, ENSICAEN) for making possible the execution of the FTIR experiments presented in this work and, in particular, to Professor Frédéric Thibault-Starzik and Dr. Vladimir L. Zholobenko for the valuable discussions regarding the interpretation

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