The opportunities and challenges of iron-zeolite as NH3-SCR catalyst in purification of vehicle exhaust
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, NOx 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 NOx. The selective catalytic reduction of NOx by NH3 (NH3-SCR) is a commercially available NOx abatement technology that reduces the NOx 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catalysts used in car emissions (gasoline engines) and vanadia-based catalysts applied in stationary source are no longer applicable to diesel engine.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 NOx 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 NOx 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-NOx 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 deNOx 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 NOx 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 NOx 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
NOx produced during cold start accounts for a great part of the total NOx 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 deNOx 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 NOx 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).
References (162)
- et al.
Catal. Today
(1992) - et al.
Chem. Eng. J.
(2020) - et al.
Appl. Catal. B-Environ.
(2002) - et al.
J. Catal.
(2014) - et al.
Appl. Catal. B-Environ.
(2004) - et al.
J. Catal.
(1999) - et al.
Catal. Today
(1998) - et al.
Catal. Commun.
(2016) - et al.
Appl. Catal. B-Environ.
(1993) - et al.
J. Catal.
(1996)
J. Catal.
Stud. Surf. Sci. Catal.
Appl. Catal. A-Gen.
J. Catal.
Appl. Catal. B-Environ.
Appl. Catal. B-Environ.
Appl. Catal. B-Environ.
J. Catal.
Catal. Today
Chem. Eng. Sci.
J. Catal.
Catal. Today
J. Catal.
Catal. Today
Catal. Today
Catal. Today
Appl. Catal. B-Environ.
J. Ind. Eng. Chem.
Appl. Catal. B-Environ.
Appl. Catal. B-Environ.
J. Catal.
J. Catal.
Catal. Today
Appl. Catal. B-Environ.
Appl. Catal. B-Environ.
Appl. Catal. A-Gen.
J. Catal.
J. Catal.
Appl. Catal. B-Environ.
Catal. Today
Appl. Catal. B-Environ.
Appl. Catal. B-Environ.
Appl. Catal. B-Environ.
Appl. Catal. B-Environ.
Appl. Catal. B-Environ.
J. Catal.
Appl. Catal. B-Environ.
Appl. Catal. B-Environ.
Appl. Catal. B-Environ.
Catal. Today
Cited by (57)
Selective catalytic oxidation of ammonia to nitrogen on mixed and dual-layer monolithic catalysts
2024, Journal of Environmental Chemical EngineeringMicrowave-associated chemistry in environmental catalysis for air pollution remediation: A review
2023, Chemical Engineering Journal