Simultaneous improvement of ammonia mediated NOx SCR and soot oxidation for enhanced SCR-on-Filter application

https://doi.org/10.1016/j.apcata.2020.117538Get rights and content

Highlights

  • SCR and selective soot oxidation catalysts were physically mixed.

  • Simultaneous improvement in the soot oxidation and NOx conversion was obtained.

  • Soot oxidation temperature was lowered by more than 150 °C.

  • Standard SCR was partially transformed to Fast SCR.

Abstract

The integration of NOx reduction and catalytic soot oxidation was investigated for the SCRoF (Selective Catalytic Reduction on Filter) applications. By physically mixing a commercial SCR catalyst (either Fe-ZSM-5 and Cu-ZSM-5) with a soot oxidation catalyst (K/CeO2-PrO2), it was possible to lower the soot oxidation temperature by more than 150 degrees and, by optimizing the catalysts mass ratio in the mixture, NOx conversion simultaneously increased, because NO oxidation induced a fast SCR reaction pathway, unlike during standard SCR. Such an improvement in NOx conversion was more pronounced with the Fe-ZSM-5 than with the Cu-ZSM-5 zeolite, as the latter was more sensitive to the NO2/NOx ratio. In order to make the soot oxidation catalyst inactive towards ammonia oxidation, poisoning of the surface acid sites with 3.0 wt.% K2CO3 (corresponding to only 1.0 wt.% K) was performed. In the soot oxidation and SCR catalysts physical mixture, the soot was oxidized mainly by O2 and the contribution of NO2 to oxidation was negligible, as NO2 itself was a key reactant in the (kinetically much faster) SCR reaction.

Introduction

Diesel engines inherently have higher thermodynamic efficiency due to lean operation, which make them be preferred over petrol-based internal combustion engines in long haul transport, locomotives, work machines, etc. Unfortunately, diesel engines have higher NOx and soot emissions that are difficult to remove from the exhaust due to low temperature and net oxidizing conditions [[1], [2], [3]]. Due to the harmfulness of the exhaust gases, even more stringent emission limits are implemented with the latest Euro 6d, which is expected to come into force in 2020. To stay below threshold limits, especially those of NOx and particulate matter (PM) emissions, aftertreatment of exhaust gasses is necessary [4,5]. Current aftertreatment systems are usually complex, expensive and require several successive reaction steps and monolith bricks. The main components are, typically, a diesel oxidation catalyst (DOC) for both NO and hydrocarbon oxidation, a catalyzed or non-catalyzed diesel particulate filter (DPF) for PM removal and a component for selective catalytic reduction (SCR) of NOx [[5], [6], [7], [8]]. NOx SCR is mediated by a reductant, which is commonly ammonia (NH3) obtained from the decomposition of urea in aqueous solution, and Cu- or Fe-zeolites are currently the most efficient catalysts [[9], [10], [11], [12], [13]].

The main reactions occurring in aftertreatment systems are:2NO + 2NH3 + 1/2O2 → 2N2 + 3H2ONO + NO2 + 2NH3 → 2N2 + 3H2O6NO2+ 8NH3 → 7N2 + 12H2ONO + 1/2O2 → NO2NH3 + O2 → xN2O + xNO + xN2C + O2 → xCO + xCO2C + NO2 + O2 → xCO + xCO2 + NO

where R1, R2 and R3 are known as standard SCR, fast SCR and NO2 SCR, respectively, and R4, R5, R6 and R7 represent the oxidation of NO, the non-selective oxidation of ammonia, the oxidation of soot with oxygen and the NOx-assisted oxidation of soot, respectively. One method to reduce complexity and cost and to improve efficiency of aftertreatment systems is to integrate the DPF and the NOx SCR into a single device, which is called a SCR on Filter (SCRoF) device. In a SCRoF, the SCR catalyst is washcoated on the pores of a monolith and the channels are plugged on alternating ends, to force exhaust gases through the wall, thereby performing simultaneously SCR and soot filtration. By this way, both size and cost are reduced and, if it is in close-coupled position, higher operating temperatures and more efficient performance can be achieved [11,[13], [14], [15], [16], [17], [18], [19]]. Several experimental and modeling studies has proven that passive soot oxidation is inhibited on SCRoF, as the fast SCR reaction (R2) is consuming the produced NO2, which becomes unavailable for soot oxidation [11,[13], [14], [15], [16], [17], [18], [19]]. As soot accumulates during filtration, both resistance to the flow and pressure drop increase and the filter has to be regenerated by an active method whereby fuel is injected to increase the temperature above 600 °C to oxidize all the soot [11,[13], [14], [15], [16], [17], [18], [19]]. Such regeneration is usually performed less frequently (or completely avoided) in DPF, as they are typically coated with a Pt-based NO oxidation catalyst, as NO2 can passively oxidize soot at much lower temperature (<400 °C) as compared to O2 [1]. The high temperatures reached during regeneration can easily damage the filter and deactivate the SCR catalyst. For this reason, typically Fe and Cu zeolites are used for SCRoF application, as they present high hydrothermal stability and, to some extent, can withstand the harsh conditions of regeneration [11,[13], [14], [15], [16], [17], [18], [19]].

To decrease the frequency of filter regeneration, the NO2/NOx ratio should be adjusted above the ideal value of 0.5 required for SCR. The rationale is that part of the excess NO2 will be consumed by the accumulated soot, the NO2/NOx ratio self-regulating back to 0.5 [11,13,14]. Another idea is to concentrate the SCR catalyst in the downstream part of the monolith, with NOx being available for soot oxidation in the inlet side [11,18]. Such partial solutions, however, do not solve the problem of soot accumulation and regeneration, merely delaying with it.

Here, a novel physical mixture of an SCR catalyst and soot oxidation catalyst is proposed as a solution to soot accumulation. So far, some physical mixtures of quite different catalysts have been applied for NOx reduction in various and quite innovative settings, such as Ag/Al2O3 combined with Sn/Al2O3 or Zn-ZSM-5 for hydrocarbon-SCR (HC-SCR) [20,21], Pt/Al2O3 and Cu-Zn-Al water-gas shift to generate in situ hydrogen for NOx reduction [22], combination of Fe- and Cu- zeolites to widen the SCR window [12,23] and combination of Lean NOx Trap (LNT) and SCR catalysts for in situ ammonia generation and utilization in the so-called “urealess passive SCR” [24,25]. Another concept introduced in 1997 by Misono et al. [26] is to combine a catalyst for NO oxidation (R4) with an SCR catalyst and transform the reaction pathway from standard SCR (R1) to fast SCR (R2) whereby higher NOx conversion can be achieved. A similar concept was later investigated in more detail by the research groups of Stakheev et al. [[27], [28], [29]] and Salazar et al. [30,31], where mainly Mn was the NO oxidation catalyst. One important finding, emphasized even in the references [[26], [27], [28], [29], [30], [31]], was that NOx conversion was enhanced only at low temperatures, and decreased significantly above 300 °C. The reason was that the oxidative component oxidized not only NO, but also NH3 (the reductant) producing high amounts of N2O and thus, as ammonia was depleted, the SCR reaction could not proceed. For this reason, we tailored the soot oxidation catalyst specifically to be selectively oxidative towards both soot and NO, to simultaneously improve soot oxidation and NOx conversion by transforming the reaction pathway from standard to fast SCR. An innovative solution was found to prevent ammonia oxidation, whereby the catalyst was impregnated with a reasonably small amount of potassium (ca. 8.0 wt%) [32]. Potassium selectively poisoned the acid sites and the catalyst became passive towards ammonia oxidation, while simultaneously soot oxidation was improved.

The aim of this work is to investigate the integration of soot oxidation and NOx SCR by a two-component selective catalytic system and to investigate the interaction between them. Particularly, the novel solution proposed here is a physical mixture of two different catalysts, namely a SCR catalyst and a soot oxidation catalyst, in order to achieve the combined effect. As SCR catalyst, either Fe- or Cu-ZSM-5 are used, as they are also widely utilized in practical applications and they are well characterized from both chemical and engineering point of view, as well. As soot oxidation catalyst, CeO2-PrO2 was impregnated with potassium to tailor its reactivity towards the various components, as will be described later. In view of possible applications, a nominal content of 1.0 wt.% potassium was used, i.e. much lower than in our previous work [32].

Section snippets

Catalysts preparation

Fe-ZSM-5 and Cu-ZSM-5 were used as SCR catalysts, since they are widely recognized as state-of-the-art catalyst and they have been characterized in previous works [10,23]. In a typical synthesis, 1.0 g H-ZSM-5 (Alfa-Aesar) with SiO2:Al2O3 ratio 23:1 and specific surface area of 425 m2  g-1 was contacted with a 50.0 mM solution of iron(III) nitrate or copper(II) acetate to obtain Fe-ZSM-5 and Cu-ZSM-5, respectively. The suspension was stirred at room temperature for 24 h to allow ion exchange.

Characterization results

No relevant differences in the XRD patterns of both KCP and CP (Fig. S1) were observed, in agreement with the low potassium loading, which is likely also very dispersed at the surface. The characteristic diffraction peaks of CP correspond to the fluorite cubic ceria structure, indicating that Ce and Pr form a solid solution without potassium insertion in the crystalline lattice. The crystallite size, as calculated according to the Scherrer equation, was ca. 30 nm for both CP and KCP.

The FE-SEM

Conclusions

The soot oxidation on SCRoF was successfully enhanced to address the problem of soot accumulation by the combination of a common SCR catalyst and a soot and NO oxidation catalyst. The soot oxidation on the single component (either Cu-ZSM-5 or Fe-ZSM-5) was significantly inhibited by NO2 consumption in the kinetically much faster SCR reaction and temperatures above 600 °C were required to oxidize all the soot. However, by mixing a soot oxidation catalyst and a common SCR catalyst, the

CRediT authorship contribution statement

Ferenc Martinovic: Writing - review & editing, Conceptualization, Methodology, Investigation, Formal analysis, Writing - original draft. Tahrizi Andana: Methodology, Investigation. Marco Piumetti: Methodology. Marco Armandi: Methodology, Investigation. Barbara Bonelli: Validation, Formal analysis, Writing - review & editing. Fabio Alessandro Deorsola: Conceptualization, Methodology, Formal analysis, Writing - original draft, Writing - review & editing, Visualization. Samir Bensaid:

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was funded through a SINCHEM Grant. SINCHEM is a Joint Doctorate programme selected under the Erasmus Mundus Action 1 Programme (FPA 2013-0037).

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