The opportunities and challenges of iron-zeolite as NH3-SCR catalyst in purification of vehicle exhaust

doi:10.1016/j.apcata.2020.117865

Applied Catalysis A: General

Volume 607, 25 October 2020, 117865

https://doi.org/10.1016/j.apcata.2020.117865 Get rights and content

Highlights

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Fe-zeolite catalyst with superiority in high-temperature NH₃-SCR performance is an important class of material for NO reduction in diesel vehicle.
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The highlights and challenges about the application of Fe-zeolite SCR catalysts and the origins of some peculiar properties that differ from Cu-zeolite are reviewed.
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Some feasible strategies in tailoring/designing Fe-zeolite catalyst to improve activity, stability and toxic tolerance are discussed.
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The construction methods and potential advantages of (Fe + Cu)-zeolites for SCR application is addressed.
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The reaction mechanisms of NH₃-SCR reaction and Fe species nature over Fe-zeolites are concluded.

Abstract

Selective catalytic reduction of NO_x with NH₃ over zeolite catalysts is recognized as the most practical technology used in mobile exhaust gases to transform NO_x into N₂. Despite a comprehensive structural-activity insight into Cu-zeolite SCR catalyst, Fe-zeolite, which is also popular in the automotive catalysis community, is necessary to be thoroughly investigated on micro-mechanism. This review summarizes the highlights and challenges in the application of Fe-zeolite, and simultaneously analyzes the reasons of some peculiar properties, such as the NH₃ inhibition effect at low temperatures, the low N₂O production and the sensitivity to NO₂/NO_x ratio in the feed. In addition, some feasible strategies are classified and summarized subsequently to provide suggestions for the improvement of low-temperature activity, the enhancement of resistance to hydrothermal aging and sulfur poisoning, the construction of controlled structure and the combination of Fe- and Cu- zeolites. Finally, the reaction mechanisms of NH₃-SCR over Fe-zeolites are concluded. After establishing a comprehensive overview of Fe-zeolite SCR catalysts, some directions that remain to be thoroughly explored on Fe-zeolite are put forward to assist the future catalyst design and investigation of NH₃-SCR.

Graphical abstract

Introduction

Environmental harmful compounds, such as unburnt hydrocarbons, oxides of sulfur, particulate matter and carbon oxide, are the target components to be removed from automotive exhaust gas. Among them, NO_x emissions from the combustion of fuels have long been considered to be deleterious atmospheric pollutant, which causes an adverse effect on both the environment and human health [1]. Considerable research interests on heterogeneous catalytic technologies have been focused on the catalytic removal of NO_x. The selective catalytic reduction of NO_x by NH₃ (NH₃-SCR) is a commercially available NO_x abatement technology that reduces the NO_x to N₂ since the 1970s [2]. Many endeavors have been made to reduce air pollution from transportation by means of modifying the combustion system and upgrading the aftertreatment technology [3]. Nowadays, the utilization of compression ignition engine based on diesel/biodiesel fuels blends is carried out to boost high thermal efficiency and to achieve the sustainable use of energy [4]. Because of the variation of combustion system to lean-burn condition, the noble-metal based three-way catalysts used in car emissions (gasoline engines) and vanadia-based catalysts applied in stationary source are no longer applicable to diesel engine. Although the lean combustion environment is a very attractive solution to diminish consumption and CO₂ emission, it poses great challenges to NO_x elimination in oxygen-rich conditions, which is a daunting task [5]. Catalyst plays a decisive role in SCR technology, making the need to modify existing catalysts or develop novel materials urgent. On the one hand, the real conditions for diesel engine are far from being constant, leading to the demand for stable and high-efficiency catalysts in aftertreatment system. A wide active temperature window between 150 and 650℃ as well as the exclusive selectivity to N₂ are needed. In addition, high stability is desired to overcome the harsh steam environment [6]. On the other hand, with increasingly stringent emission norms introduced, typically for the Euro 6 standard regulation in 2014, the NO_x emission limit is set to 0.08 g/km, which is 55.6 % less than the Euro 5 in 2009 [7]. As such, the above factors necessitate the development of harmless zeolites as alternative catalysts in SCR.

Metal-exchanged zeolites outperform vanadia-based catalysts in both SCR activity across a wide temperature range and thermal stability [8]. In view of vehicle accelerations operating and occasional high temperature events (soot combustion and desulfation), considerable investigations are needed for Fe-zeolite catalysts with superiority in high-temperature performance. In the early 1990s, ZSM-5 loaded Cu or Fe as SCR catalysts with hydrocarbon or NH₃ as reductants received much attention on account of the comparable or even better performance than the commercialized V₂O₅/WO₃-TiO₂ [9,10]. At temperatures typical of diesel exhaust, Fe-ZSM-5 showed higher hydrothermal stability than Cu-ZSM-5 [[11], [12], [13], [14]]. Nonetheless, the inferior low-temperature activity of Fe-ZSM-5 hampered its further applications [15]. Consequently, beta zeolite involving Cu or Fe became the major research objective because of their enhanced durability [16,17]. Iron containing catalysts based on ZSM-5 and beta zeolites were compared by He et al. [18] with respect to activity, hydrothermal stability and hydrocarbon coking resistance. It was found that Fe/Beta-zeolite performed better in every respect. However, whether it is ZSM-5 with medium pores or beta with large pores, there is a common problem that hydrocarbon can easily poison the zeolite by strong adsorption. Furthermore, the greatly exothermic combustion of hydrocarbons may degrade the zeolite topology. Moreover, the lack of breakthrough in improving the hydrothermal stability drives further exploration towards small-pore zeolites. Chabazite (CHA) zeolites with small pores were expected to be less susceptible to deactivation by poisoning and aging due to the specific properties from small opening. Cu-exchanged CHA zeolites (SAPO-34 and SSZ-13) were developed and commercially applied for lean-NO_x NH₃-SCR in diesel vehicles in around 2010 [19,20]. After several years, Fe/SSZ-13 with excellent high-temperature activity was reported by Gao et al. [21]. Our unpublished works on Fe-SSZ-13 have dealt with the evolution of Fe species with calcination temperature and the improvement of hydrothermal stability by Zr doping, respectively. Subsequently, Fe-SAPO-34 was also prepared, showing outstanding high-temperature deNO_x activity and high resistance towards hydrothermal treatment [22,23]. Besides, alternative zeolite, for instance Fe-exchanged high-silica LTA, was developed and demonstrated to possess superior hydrothermal stability than Fe-SSZ-13 in NH₃-SCR [24]. With the aim to perfect the SCR performance and lifetime, novel catalysts will always be the subject of academic community.

In the last few decades, a wide number of reviews have covered subjects about previous achievements in NH₃-SCR. Recent advances in NO_x emission control on Cu-zeolite catalysts were summarized by Beale et al. [25] and Wang et al. [26], giving a comprehensive microstructure insight and far-reaching application prospect. It should be noted that Fe-zeolite is considered for commercial application because of its advantage in the fast-SCR reaction. Moreover, Fe-zeolite was shown to achieve higher NO_x conversion under real-world transient condition. Thereby, the intention of this paper is to review the characteristic properties of Fe-zeolite in comparison to Cu-zeolite for SCR reaction. Specifically, its highlights of excellent high-temperature activity, low N₂O selectivity, sensitivity to NO₂ and resistant to chemical poisoning as well as its drawbacks of inconvenience in preparation, low-temperature NH₃ inhibition effect and hydrothermal deactivation are listed and analyzed. Also, the solutions to optimize the performance of Fe-zeolite catalysts in different aspects are systematically summarized. Finally, the reaction mechanisms of NH₃-SCR over Fe-zeolites are discussed. These characteristics are directly related to the on-road application of iron-based catalysts, which guide the direction of physical and chemical modification.

Section snippets

Opportunities and highlights of Fe-zeolite for NH₃-SCR application

The chemistry of Fe-zeolite in NH₃-SCR is different from that of Cu-zeolite in some ways. The strong oxophilicity of iron creates distinctive interaction with zeolite framework and hydroxyl groups. Therefore, the coordination of Fe is closely related to the catalyst composition and reaction condition. Moreover, the charge property of Feⁿ⁺ counterion and the redox of Fe²⁺/Fe³⁺ ion pair differ from that of Cuⁿ⁺ and Cu⁺/Cu²⁺. Furthermore, there is a difference between Cu- and Fe-zeolite in the

The challenges of Fe-zeolite in the application of NH₃-SCR reaction

Given that performance defects offer the breakthrough for catalysts innovation, in this section, we focus on several challenges of Fe-zeolite catalysts when used for lean burn vehicles exhaust purification.

The strategies to improve low-temperature activity

NO_x produced during cold start accounts for a great part of the total NO_x emissions. It pushes forward the demand for prospective catalysts with enhanced low-temperature activity. A number of strategies are proposed from different aspects with regard to Fe-zeolite: 1) To expand the types of zeolite support that can be used for SCR reactions. Hitherto, zeolites with different framework structures and compositions have been used as supports for iron exchange. Recently, an innovative Fe-UZM-35

The reaction mechanisms of NH₃-SCR over Fe-zeolites

On the mechanism of NH₃-SCR reaction, it is a consensus that the coexistence of acid circle and redox circle is requisite for both vanadium and zeolite catalysts. Nevertheless, investigations have derived various discussions on identification of reactive acid sites and active metal sites and on activation pathway of reactive molecules, causing the uncertainty in current understanding on reaction intermediate. Concerning catalytic cycle of Cu-zeolite, abundant information is experimentally and

Conclusions and perspectives

With the popularity of zeolite catalysts in deNO_x process of mobile source exhaust, extraordinary successes have been achieved in application of Fe and Cu modifying zeolite. A more sophisticated SCR system catalyzed by Cu-zeolite has been shown in previous studies. However, many details of Fe-zeolite system are still unknown, remaining to be clarified. In this review, the Fe-zeolite catalysts for NH₃-SCR of NO_x are evaluated based on the relative advantages and disadvantages in comparison to

Declaration of Competing Interest

The authors report no declarations of interest.

Acknowledgment

The authors gratefully acknowledge ﬁnancial support from the China Postdoctoral Science Foundation (2019M662630).

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The opportunities and challenges of iron-zeolite as NH3-SCR catalyst in purification of vehicle exhaust

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Opportunities and highlights of Fe-zeolite for NH3-SCR application

The challenges of Fe-zeolite in the application of NH3-SCR reaction

The strategies to improve low-temperature activity

The reaction mechanisms of NH3-SCR over Fe-zeolites

Conclusions and perspectives

Declaration of Competing Interest

Acknowledgment

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Catal. Today

The opportunities and challenges of iron-zeolite as NH₃-SCR catalyst in purification of vehicle exhaust

Opportunities and highlights of Fe-zeolite for NH₃-SCR application

The challenges of Fe-zeolite in the application of NH₃-SCR reaction

The reaction mechanisms of NH₃-SCR over Fe-zeolites