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Review

Advances and Challenges in Zeolite-Based Catalysts for the Selective Catalytic Oxidation of Ammonia

by
Xiaoxin Chen
†,
Jun Huang
† and
Guoju Yang
*
State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, China
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Catalysts 2025, 15(3), 204; https://doi.org/10.3390/catal15030204
Submission received: 11 January 2025 / Revised: 10 February 2025 / Accepted: 18 February 2025 / Published: 21 February 2025
(This article belongs to the Special Issue Synthesis and Catalytic Applications of Advanced Porous Materials)
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Abstract

:
Ammonia (NH3) emissions from mobile sources pose significant environmental challenges, contributing to air pollution, ecosystem degradation, and climate change. The selective catalytic oxidation of NH3 (NH3-SCO) offers a sustainable solution by converting NH3 into nitrogen and water, yet designing catalysts that balance high efficiency, selectivity, and stability under operational conditions remains a critical challenge. This review provides a comprehensive overview of zeolite-based catalysts, renowned for their high surface area, tunable pore structures, and exceptional hydrothermal stability, which make them ideal for NH3-SCO applications. The review synthesizes recent advancements in catalyst design, emphasizing innovative architecture, the role of zeolite frameworks in active site dispersion, and strategies for optimizing catalytic architectures. Key insights include an enhanced understanding of NH3-SCO reaction mechanisms, progress in mitigating catalyst deactivation caused by poisoning and sintering, and the development of bimetallic and core-shell catalysts to improve performance and durability. Current limitations, including the sensitivity of catalysts to operational environments and scalability issues, are critically analyzed, and potential strategies for overcoming these barriers are proposed. This review highlights the state-of-the-art in zeolite-based NH3-SCO catalysis, offering valuable insights into the fundamental and applied aspects of catalyst design. The findings presented here provide a roadmap for future innovations in environmental catalysis, paving the way for more efficient and robust solutions to ammonia emission control.

1. Introduction

Ammonia (NH3) plays a crucial role in global industry, primarily as a key component in fertilizer production, which supports agricultural productivity and food security. Beyond agriculture, ammonia is increasingly being explored as a sustainable energy carrier due to its high hydrogen content and carbon-free combustion potential [1]. Given its versatility in various industrial applications, including refrigeration, water purification, and fuel, ammonia production remains a critical area of research and innovation to meet the growing global demand while ensuring environmental sustainability. However, NH3 emissions have become a critical environmental issue, originating primarily from industrial processes, agricultural activities, and mobile sources. In industrial sectors, particularly in fertilizer production and wastewater treatment, substantial quantities of NH3 are released into the atmosphere. Additionally, mobile sources contribute to ammonia pollution through ammonia slip from NH3-SCR systems, designed to reduce nitrogen oxide emissions in diesel engines. This uncontrolled release of NH3 presents significant risks to air quality and ecosystems. NH3 readily reacts with acidic compounds in the atmosphere to form fine particulate matter (PM2.5), a known contributor to respiratory and cardiovascular health issues [2]. Furthermore, ammonia deposition into soil and water bodies promotes eutrophication, adversely affecting biodiversity and water quality [3]. These environmental impacts underscore the urgent need to mitigate NH3 emissions.
Selective Catalytic Oxidation of ammonia (NH3-SCO) has emerged as a promising strategy to address NH3 emissions. The NH3-SCO process directly converts NH3 into benign products, primarily nitrogen (N2) and water (H2O), under optimal conditions. This approach provides a more direct and efficient method for eliminating excess NH3 from both mobile and industrial sources. However, one of the major challenges in SCO is achieving high selectivity for N2 while avoiding the formation of undesirable by-products such as nitrogen oxides (NO and NO2, collectively referred to as NOx) or nitrous oxide (N2O). Additionally, maintaining high catalytic activity at low temperatures is critical for applications like vehicle exhaust aftertreatment. Therefore, the development of efficient NH3-SCO catalysts that ensure complete and selective NH3 conversion remains a key challenge.
Several catalyst types have been investigated for the NH3-SCO reaction, each with distinct characteristics. Transition metal oxides such as CuO, V2O5, Co3O4, and MnOx are widely used due to their high activity in promoting NH3 oxidation at moderate temperatures [4,5,6]. However, these catalysts often suffer from limited thermal stability and vulnerability to deactivation by sulfur and water, particularly under the demanding conditions typical of mobile and industrial applications [3]. Noble metal catalysts, including Ag, Pt, and Pd, exhibit excellent low-temperature activity but face challenges related to the over-oxidation of NH3, which leads to the formation of NOx and N2O as undesired by-products [7,8]. Moreover, their high cost and sensitivity to sulfur poisoning significantly limit their lifespan and feasibility in large-scale applications. Perovskite-based catalysts offer tunable redox properties and high oxygen mobility, but their performance is constrained by low surface area and stability issues, which hinder their effectiveness under varying operational conditions [9].
Given these limitations, there is an increasing demand for catalysts that combine high activity, excellent selectivity for N2, resistance to deactivation, and robust stability across a wide range of operating conditions. Zeolite-based catalysts have emerged as highly promising candidates for NH3-SCO, offering several advantages over traditional catalysts. Their unique features—such as high surface area, well-defined pore structures, and superior hydrothermal stability—make them particularly well-suited for catalyzing reactions under challenging conditions [10]. Moreover, the ability to incorporate transition metals into zeolite frameworks enhances their catalytic potential, allowing for the precise tuning of active sites to promote selective NH3 oxidation [3]. Consequently, zeolites provide a versatile and effective solution to the challenges faced by existing NH3-SCO catalysts, particularly in improving selectivity toward N2 and ensuring long-term stability in industrial environments.
Despite these advances, several challenges remain in the development of zeolite-based NH3-SCO catalysts. Catalyst deactivation, particularly in the presence of sulfur and water, remains a major concern as these species can poison active sites or lead to the sintering of metal particles. Additionally, maintaining high catalytic efficiency at low temperatures, which is critical for applications such as vehicle exhaust aftertreatment, continues to be a significant hurdle. Furthermore, the underlying mechanisms governing NH3 oxidation on zeolites are not yet fully understood, particularly regarding the role of metal active sites and how the zeolite’s pore structure influences reaction pathways.
Given the pressing need to reduce NH3 emissions and the promising yet underdeveloped potential of zeolite-based catalysts, this review aims to provide a comprehensive analysis of the latest advancements in the field of NH3-SCO using zeolite materials. We will examine key aspects such as catalyst design, the role of metal active sites, reaction mechanisms, and strategies to overcome current challenges, including catalyst deactivation and low-temperature performance. By addressing these issues, this review aims to offer valuable insights that can guide future research and development in environmental catalysis.

2. Fundamentals of NH3-SCO Reactions

The NH3-SCO reactions involve the oxidation of NH3 into N2 or NOx through a series of redox reactions on the catalyst surface [8]. The ideal reaction pathway converts NH3 to N2 and H2O, as shown in Equation (1).
4NH3 + 3O2 → 2N2 + 6H2O
2NH3 + 2O2 → N2O + 3H2O
4NH3 + 5O2 → 4NO + 6H2O
This pathway is environmentally benign, as N2 and H2O are non-polluting end-products. However, depending on the catalyst and reaction conditions, alternative and less desirable pathways may occur, leading to the formation of N2O or NOx. For example, incomplete oxidation can result in the formation of N2O, a potent greenhouse gas, as shown in Equation (2).
Excessive oxidation can lead to the formation of NOx, which contributes to air pollution and acid rain (Equation (3)). Controlling the reaction pathway is therefore crucial to minimize harmful emissions and ensure selective production of nitrogen.
Selectivity towards N2 is of paramount importance in NH3-SCO reactions because of its inert and non-toxic nature. Achieving high selectivity requires precise control of the catalytic environment to suppress the formation of undesirable by-products such as N2O and NOx, which are environmentally damaging. NOx compounds are major contributors to smog and respiratory issues, while N2O is a potent greenhouse gas with a global warming potential substantially higher than that of CO2.
One of the main challenges in achieving high selectivity to N2 is the competition between different oxidation pathways. In particular, the formation of NOx is thermodynamically favorable at higher temperatures, making it difficult to avoid under harsh conditions. This challenge is compounded by the variability in catalytic performance across different materials. For example, Cu-exchanged zeolites can achieve high N2 selectivity under optimal conditions but may still produce NOx or N2O under suboptimal conditions. Therefore, a deep understanding of the catalyst’s active sites and the specific reaction conditions is essential to optimize selectivity.
To achieve high selectivity for N2 in NH3-SCO reactions, a profound understanding of catalytic materials and their active sites is imperative. Zeolite-based catalysts have emerged as a cornerstone in this field due to their unique structural and chemical properties, which make them particularly suitable for controlling reaction pathways.

3. Key Characteristics of Zeolites for NH3-SCO Catalysis

Zeolites are crystalline aluminosilicate materials characterized by their well-defined, three-dimensional microporous structures. These materials are widely used in catalysis due to their high surface area, tunable pore architecture, and the ability to incorporate active sites within their framework. Zeolites such as ZSM-5, Y, and SSZ-13 have emerged as prime candidates for NH3-SCO applications owing to their exceptional structural properties and stability [11,12,13,14].
The aluminosilicate framework consists of SiO4 and AlO4 tetrahedra linked by oxygen atoms, forming negatively charged sites that can host cations, such as protons or transition metals. These metal cations, including Cu2+ and Fe3+, facilitate redox reactions, reducing undesired by-products like NOx or N2O while enhancing selectivity toward N2 and water [15]. Additionally, the microporous structure ensures efficient dispersion of active sites and reactants, which is crucial for catalytic performance [11,16].
A critical property of zeolites for NH3-SCO is their hydrothermal stability. This robust stability allows zeolites to maintain structural integrity and catalytic activity under high temperatures and humid conditions, such as those encountered in exhaust treatment systems. For instance, zeolites like SSZ-13 resist deactivation caused by water vapor and sintering, which are common challenges in long-term catalytic applications. These characteristics make zeolites versatile and durable frameworks for hosting metal species in NH3-SCO.

4. Zeolite Supported Metal Catalysts for NH3-SCO

4.1. Effects of Zeolite Frameworks on Metal Catalysts

The zeolite framework plays a crucial role in stabilizing and enhancing the performance of supported metal active sites in NH3-SCO catalysis. Its well-defined microporous structure provides a highly organized environment that facilitates the uniform dispersion of metal species and minimizes their mobility. This prevents sintering and aggregation, even during high-temperature operations, ensuring prolonged catalytic activity [11].
The acidic properties of the zeolite framework, governed by the Si/Al ratio, significantly influence catalytic behavior. Higher acidity, as observed in zeolites like SSZ-13, promotes NH3 adsorption, forming NH4+ species that act as reservoirs for subsequent redox reactions [17]. Conversely, lower acidity facilitates the formation of well-dispersed metallic nanoparticles, which are particularly effective at low operating temperatures [11,18].
Beyond structural and acidic properties, the interaction between the zeolite framework and active metal species determines the catalyst’s overall efficiency. The unique ability of zeolites to host a range of metal species, including noble metals such as Pt and Ag and non-noble metals such as Cu and Fe, enhances their catalytic versatility [19,20]. As shown in Figure 1, the zeolite support enhances catalyst stability by providing resistance to deactivation, improving hydrothermal stability, stabilizing metal sites, and offering adjustable acidity. The metal active sites facilitate key catalytic functions, including controlling reaction pathways, enabling low-temperature oxidation, enhancing NOx reduction, and improving redox properties. The synergy between the zeolite framework and metal species optimizes NH3 conversion and selectivity, making metal/zeolite catalysts highly effective for NH3-SCO applications.

4.2. Noble Metal-Based Zeolite Catalysts

Noble metals, such as Ag, Pt, and Pd, have been widely investigated as catalysts for NH3-SCO due to their exceptional catalytic activity and stability in various oxidation reactions. When these metals are dispersed as NPs on zeolite supports, they can significantly enhance the selectivity and activity of NH3 oxidation, particularly under low-temperature conditions, where conventional metal oxide catalysts often show limited performance [21].
Silver (Ag)-supported zeolites have shown promising activity for NH3-SCO due to the ability of Ag NPs to promote the selective oxidation of ammonia to N2 [22]. Their excellent redox properties allow the efficient activation of molecular oxygen (O2), which is critical for oxidizing NH3 into N2. For instance, Ag/SSZ-13 exhibits high activity at temperatures as low as 175 °C due to the uniform dispersion of Ag NPs on the zeolite surface [11]. To enhance catalyst durability, Wang et al. encapsulated Ag NPs within a hollow zeolite framework, where the unique hollow architecture of ZSM-5 effectively inhibited the sintering or leaching of Ag particles during NH3 oxidation [16]. Additionally, zeolite membranes demonstrated superior separation efficiency for SO2, preventing its diffusion into the hollow cavities. As a result, the Ag NPs were safeguarded against SO2 poisoning, exhibiting remarkable resistance to both SO2 and H2O.
Ag NPs can interact with the acidic sites of the zeolite framework, which enhances the adsorption of NH3 and promotes its activation. The small size and high dispersion of Ag NPs also play a crucial role in ensuring that the active sites are fully accessible to reactants, further improving the catalytic efficiency of the system [23]. Studies reveal that CHA-type zeolites support the formation of Ag NPs at elevated temperatures (400 °C), significantly enhancing catalytic activity, while RHO-type zeolites predominantly stabilize inactive Ag clusters, highlighting the critical influence of framework topology [24]. In AgY zeolites, metallic Ag NPs formed via H2 reduction drive NH3 oxidation, whereas Ag+ species facilitate NO reduction via the i-SCR mechanism. The presence of water in reaction feeds promotes the dynamic interconversion between Ag0 NPs and Ag+ species, slightly impacting performance but highlighting the importance of Ag speciation in NH3-SCO [13].
Platinum is another noble metal that has been extensively studied in NH3-SCO reactions. Pt/zeolite catalysts, such as Pt/ZSM-5 [25], have demonstrated exceptional catalytic activity and durability under harsh conditions, such as high temperatures and the presence of water vapor. Pt NPs are highly efficient in activating both NH3 and O2, enabling the complete oxidation of NH3 at relatively low temperatures (200–300 °C). The strong interaction between Pt nanoparticles and zeolite supports enhances catalyst stability by preventing metal agglomeration, even under severe operating conditions. Additionally, the combination of Pt’s catalytic properties and the acidic nature of zeolites promotes efficient NH3 activation, ensuring high conversion rates and excellent selectivity toward N2 [26].
Noble metal catalysts such as Pd, supported on metal oxides and molecular sieves, exhibit good catalytic performance at low temperature in NH3-SCO system. Research has shown that at 350 °C, 5.51wt% Pd/SSZ-13 in the anhyrous NH3-SCO system can achieve 92% NH3 conversion and 73% N2 selectivity [27]. In addition, it has been reported that Pd/Y (SAR = 2.6) shows considerable N2 selectivity in the reaction process. DRIFTS studies have found that N2H4 intermediates appear on Pd/Y catalyst at 250 °C, indicating that ammonia oxidation has different parallel pathways and the mechanism is complex [28,29,30].
In summary, noble metal NPs supported on zeolites, particularly Ag, Pt, and Au, offer significant advantages in NH3-SCO due to their superior redox properties, stability, and ability to selectively produce N2 over harmful byproducts. The strong interaction between the metal NPs and the zeolite framework plays a key role in enhancing catalytic performance.

4.3. Non-Noble Metal-Based Zeolite Catalysts

While noble metal-based zeolite catalysts excel in catalytic activity and stability, their high cost and limited availability have spurred interest in alternative materials. Non-noble metals, such as copper (Cu) and iron (Fe), offer a cost-effective and environmentally sustainable solution with strong redox capabilities and proven effectiveness in NH3-SCO. Zeolite-supported Cu and Fe catalysts have shown considerable promise in NH3-SCO applications [14,31,32]. The reaction predominantly follows the internal selective catalytic reduction (i-SCR) mechanism, involving the oxidation of NH3 to NOx at oxide sites and the subsequent reduction of NOx to N2 at cation centers [14]. This mechanism ensures high N2 selectivity. The catalytic performance of these non-noble metal catalysts is strongly influenced by the zeolite framework, metal loading, and preparation methods.
Among these catalysts, Cu-based materials are widely employed due to their exceptional redox properties and catalytic efficiency [33]. The redox behavior and spatial distribution of Cu species within the zeolites are critical determinants of catalytic performance. Active sites, such as isolated Cu+ and Cu2+ ions and Cu dimers, drive NH3 oxidation and NO reduction via redox cycling [15]. However, excessive Cu loading can lead to the formation of CuOx clusters, which enhance NH3 conversion but compromise N2 selectivity [31].
Previous Studies by Gang et al. [34,35] and Kwak et al. [36] highlighted catalytic inefficiencies of Cu/Y zeolites above 300 °C, including high N2O emissions and instability after hydrothermal treatment. Wang et al. [37] noted a significant T50 increase from 300 °C to 500 °C following the hydrothermal aging of Cu/Y. In contrast, Góra-Marek et al. [38] demonstrated that Ag -containing USY zeolites (Si/Al = 2.5) exhibited superior NH3-SCO performance, with high N2 selectivity and water resistance. Further investigation into high-silica USY zeolites is necessary to advance Cu-USY systems.
Wang et al. [37] ranked Cu-based zeolites for NH3-SCO, with Cu/ZSM-5, Cu/Beta, and Cu/MCM-49 outperforming others due to their small channel pores, which maintained >95% N2 selectivity up to 500 °C. In contrast, larger-pore zeolites produced significant NOx above 400 °C. Incorporation of mesoporosity into ZSM-5 through NaOH or NaOH/TPAOH treatments has been shown to enhance acidity, reducibility, and catalytic efficiency for both NH3-SCO and NH3-SCR [39].
Cu-based CHA-type catalysts, such as Cu-SSZ-13 and Cu-SAPO-34, have garnered significant attention for NH3-SCO due to their exceptional catalytic activity, high N2 selectivity, and remarkable hydrothermal stability [14]. Their small-pore structure (0.38 nm) effectively suppresses dealumination and stabilizes Cu species, enabling sustained catalytic performance under high-temperature and steam-rich conditions. In Cu-SSZ-13 catalysts, Cu species exist as monomeric cations (Cu+ and Cu2+) or aggregated forms such as CuOx, while Cu-SAPO-34 features a coexistence of these species irrespective of Cu loading [40]. The hydrothermal treatment of Cu-CHA facilitates CuO cluster integration into Cu-CHA, leading to a higher dispersion of Cu. A uniform distribution of Cu species, without obstructing the narrow zeolite pores, ensures excellent catalytic activity and N2 selectivity. Ammonia oxidation over Cu-based catalysts operates via a well-documented redox mechanism [41].
Yu et al. [42] studied Cu-containing SAPO-34 catalysts prepared by ion exchange and impregnation. Their findings revealed that the ion-exchanged SAPO-34 predominantly contained isolated Cu2+ species, while CuOx was the primary species in the impregnated samples. CuOx was identified as the active site for NH3 oxidation in Cu/SAPO-34 [33]. Zhang et al. [15] reported a one-pot synthesized Cu-SSZ-13 catalyst for NH3-SCO and found that dilute HNO3 post-treatment eliminated excess Cu species, increased Cu2+ ions, and improved their spatial distribution, enhancing NH3 oxidation. Guo et al. [14] demonstrated that CuOx/SSZ-13 exhibited superior NH3 activity and N2 selectivity compared to CuOx/SAPO-34 below 350 °C under both dry and wet conditions. Hydrothermal aging significantly reduced the activity of CuOx/SAPO-34 but had a minimal effect on CuOx/SSZ-13, which retained its high performance due to abundant acid sites and robust redox properties. Han et al. [31] identified superoxo species (O2−) on CuO nanoclusters as critical active sites for NH3 oxidation, particularly below 250 °C. However, the exact nature of the oxygen species responsible for NH3 oxidation remains unclear. However, the exact nature of the oxygen species responsible for NH3 oxidation remains unclear, and further studies are needed to elucidate these mechanisms. CHA-type zeolites exhibit superior tolerance to water vapor, maintaining high activity and stability compared to larger-pore zeolites such as Cu-Y, which are prone to deactivation under humid conditions. Their hydrothermal stability, even after exposure to temperatures exceeding 700 °C, underscores their suitability for practical applications [18].
Fe-based zeolites are also highly promising for NH3-SCO. In Fe-based systems, the role of oxygen activation is critical, as the formation of superoxide and peroxide species accelerates the overall reaction kinetics and enhances the conversion of NH3 to N2 [25,43]. Active oxygen species attack NH3 adsorbed on the Fe sites, leading to the formation of N2 while minimizing the production of harmful by-products. Góra-Marek et al. [44] reported that Fe-loaded mesostructured ZSM-5 (0.68–0.92 wt.%) achieved outstanding catalytic activity and N2 selectivity below 450 °C, attributed to its mesoporosity introduced via post-synthetic modification or direct synthesis. This structure minimized iron oxide aggregation and preserved isolated Fe3+ ions within the zeolite framework. Introducing mesoporosity in zeolite frameworks improves mass transport, reduces diffusion limitations, and enhances catalytic activity by increasing the accessibility of active sites. However, Fe-zeolites are more sensitive to water vapor, which can lead to partial deactivation through competitive adsorption on the active sites [43]. This highlights the need for further improvements in their hydrothermal stability.
Future research should focus on optimizing the distribution and coordination of Cu and Fe species within the zeolite to enhance activity and selectivity. Additionally, investigations under realistic exhaust conditions, including the presence of COx, SOx, and water, are necessary to ensure the practical applicability of these catalysts. Such advancements will enable the development of robust Cu- and Fe-zeolite catalysts for industrial-scale NH3-SCO applications.

4.4. Bimetallic Zeolite Catalysts

While noble and non-noble metal-based catalysts have demonstrated significant potential in NH3-SCO applications, their performance is often limited by trade-offs between activity, selectivity, and stability. To address these challenges, researchers have turned to bimetallic systems, which combine two active metal species to achieve enhanced catalytic performance through synergistic effects, as shown in Figure 2.
An effective NH3-SCO catalyst system should simultaneously possess excellent NH3 oxidation and SCR activity. Recent advancements highlight the potential of bimetallic catalysts, which integrate two active metal species within or on the surface of zeolites, to achieve superior catalytic performance through synergistic effects in NH3-SCO. Bimetallic systems, such as Cu/Fe, Ag/Cu, and Pt/Fe zeolites, leverage the distinct redox capabilities of Cu or Fe and the oxygen activation properties of Pt or Ag to enhance NH3 conversion rates and improve selectivity toward N2 formation. In these systems, Cu or Fe sites facilitate the reduction of NOx to N2, while Pt or Ag promotes oxygen and NH3 activation, leading to more efficient NH3 oxidation [25].
Several studies have demonstrated the advantages of such dual-metal systems. Shrestha et al. [45] examined dual-layer Pt/Al2O3 and Cu-SSZ-13 monolithic catalysts, which enhanced NH3 oxidation but exhibited high N2O selectivity, limiting their practical application. Similarly, Dhillon et al. [46] proposed a hybrid configuration with a mixed Pt/Al2O3-Cu-SSZ-13 bottom layer and Cu-SSZ-13 top layer. While this design improved activity, it achieved suboptimal N2 yields (below 80%) within the temperature range of 623–773 K.
In specific examples, Pt/Fe-ZSM-5 [12] has outperformed single-metal catalysts due to enhanced dispersion of active metal sites and a balanced redox environment, contributing to superior catalytic efficiency. Additionally, our recent work on Cu/ZSM-5@AgS-1 [23] illustrates the advantages of core-shell structures. The S-1 shell in this system suppresses the migration of Ag to the Cu/ZSM-5 core, preserving Cu2+ active sites crucial for catalysis. Moreover, the S-1 shell prevents the excessive formation of Ag+ and maintains higher levels of metallic Ag (Ag0), which are critical for effective O2 activation and NH3 dehydrogenation. Bimetallic catalysts present a promising avenue for optimizing NH3-SCO performance by leveraging the complementary properties of multiple metal species.
Bimetallic PtCu catalysts supported on SSZ-13 have recently emerged as a promising solution to address the trade-off between catalytic activity and selectivity in NH3-SCO. The incorporation of Cu into the Pt/SSZ-13 catalyst enhances N2 selectivity by shifting the reaction pathway from the NH mechanism to a dual mechanism involving NH and internal selective catalytic reduction (i-SCR) [20]. This shift is facilitated by the electronic interaction between Pt and Cu in the alloy structure, which renders Pt electron-rich and enables the rapid consumption of in situ-formed NO via the i-SCR pathway. Isolated Cu2+ species, highly dispersed within the zeolite framework, play a critical role in suppressing NOx formation while promoting N2 selectivity. The structural integrity and catalytic efficiency of the PtCu alloy have been confirmed using advanced characterization techniques such as XRD and XPS, demonstrating its ability to achieve up to 87% N2 selectivity at 250 °C. The exploration of alloy engineering, as demonstrated by PtCu systems, offers a clear path forward for designing robust and efficient catalysts for practical applications.
Bimetallic catalysts present a promising avenue for optimizing NH3-SCO performance by leveraging the complementary properties of multiple metal species. Future research should focus on fine-tuning the interactions between the metal species and the zeolite framework to further enhance activity, selectivity, and stability under industrially relevant conditions.

5. Mechanistic Pathways in NH3-SCO on Zeolite Catalysts

The catalytic performance of zeolite-supported metal catalysts in NH3-SCO is intrinsically linked to the underlying reaction mechanisms they facilitate. Understanding these mechanisms is crucial for optimizing catalyst design and reaction conditions to enhance selectivity and minimize by-products.
The NH3-SCO over zeolite catalysts proceeds via several pathways involving interactions between NH3, metal active sites, and surface oxygen species. The main mechanisms for zeolite catalysts include the imide mechanism, hydrazine mechanism, and i-SCR mechanism [47,48]. These pathways are highly temperature-dependent, influencing both reaction selectivity and by-product formation. At low temperatures (below ~140 °C), the reaction predominantly follows the imide or hydrazine mechanisms, leading to higher N2O formation. As the temperature increases (~200–300 °C), the i-SCR mechanism becomes dominant, reducing N2O production and enhancing N2 selectivity. The transitional temperature range of 200–300 °C marks the coexistence of multiple mechanisms, achieving a balance between activity and selectivity [49].
In the imide mechanism, ammonia undergoes stepwise dehydrogenation to form -NH and -NH2 intermediates on metal-exchanged sites of zeolites. These intermediates react with surface oxygen species to produce N2. This mechanism is particularly active at lower temperatures [49]. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) studies at 200 °C indicated that with the introduction of O2, the NH2* band decreases, and the NH* peak and HNO* peak appear, indicating the formation of key intermediates in imide mechanism [11].
The hydrazine mechanism involves the formation of hydrazine (N2H4) as an intermediate. In this pathway, NH3 is first converted to NH2 groups, which combine to form N2H4, which is further dehydrogenated to N2 and H2O. This mechanism is prominent under conditions of high NH3 concentration or limited oxygen availability, which favor N2O as a by-product. For instance, AgCu/ZSM-5 [23] has shown only the N2H4* absorption band in the DRIFTS spectra without the presence of NH* and HNO* absorption bands. The intensity of the N2H4* band decreases upon O2 introduction, indicating that the N2H4 intermediate reacts with active O to form N2 and N2O.
The i-SCR mechanism is the most widely reported pathway for zeolite-based NH3-SCO catalysts, particularly for containing Cu-exchanged zeolites such as Cu-SSZ-13 [18,41]. In this pathway, NH3 is first oxidized to NOx over active metal sites. Subsequently, the NOx is reduced back to N2 by residual NH3. The stepwise reaction ensures high N2 selectivity while minimizing the formation of N2O and NOx. Wang et al. [37]. reported DRIFTS evidence showing the presence of intermediate species, including nitrate and NH4+, on Cu-containing zeolites (e.g., Cu/Beta and Cu/SSZ-13). These intermediates interact in the i-SCR mechanism, leading to efficient NH3 conversion and N2 formation.
Understanding the mechanisms governing NH3-SCO on zeolite catalysts provides a solid foundation for optimizing their performance. However, translating this mechanistic understanding into practical applications requires addressing several critical challenges that impact catalyst efficiency, selectivity, and durability.

6. Challenges and Strategies in Zeolite-Based NH3-SCO Catalysts

Zeolite-based NH3-SCO catalysts face several critical challenges that limit their efficiency and practical applicability, as depicted in Figure 3. These include deactivation caused by poisoning, reduced hydrothermal stability under harsh conditions, high production costs, undesirable NOx formation, and suboptimal performance at low temperatures. Addressing these limitations requires a combination of innovative materials design, advanced synthesis techniques, and operational strategies to enhance catalyst durability, activity, and cost-effectiveness. This section examines these challenges in detail and discusses potential strategies to overcome them, paving the way for the development of next-generation NH3-SCO catalysts.

6.1. Poisoning and Resistance Strategies

Zeolite-based NH3-SCO catalysts face significant challenges from poisoning by sulfur oxides (SOx), water vapor, and phosphorus compounds. Sulfur poisoning occurs when SO2 reacts with active metal centers, forming surface sulfates that block catalytic sites and reduce efficiency. Water vapor competes with NH3 for adsorption sites, disrupting ammonia oxidation. Phosphorus poisoning, primarily from phosphate species found in exhaust streams (e.g., from engine lubricant additives), is another critical issue. Phosphate compounds form strongly bonded complexes on active metal sites, leading to blocked catalytic sites, reduced surface acidity, and altered redox properties. These combined effects significantly degrade catalyst performance over time.
To mitigate these poisoning effects, researchers have developed several strategies. Surface modifications, such as the creation of core-shell structures, are effective in reducing exposure to poisoning agents. Encapsulating active metals like Ag NPs within hollow zeolite frameworks protects the active sites from SO2 and H2O without compromising catalytic activity. Co-doping with elements such as cerium (Ce) and Cu enhances resistance to sulfur and phosphorus poisoning by promoting the formation of oxygen vacancies. These vacancies prevent the accumulation of sulfates and phosphates on active sites, maintaining high catalytic efficiency and N2 selectivity under real-world operating conditions. Additionally, applying hydrophobic coatings to zeolite catalysts has been shown to reduce the interaction between phosphorus-containing species and the catalyst surface, further enhancing resistance to P-poisoning.

6.2. Hydrothermal Stability and Enhancement Strategies

The hydrothermal stability of zeolite-based catalysts, particularly the active metal sites, is a significant challenge. High-temperature and high-humidity conditions common in exhaust systems can cause dealumination of the zeolite framework and migration or aggregation of metal species, leading to reduced dispersion and catalytic activity. Cu- and Fe-based catalysts are especially susceptible to hydrothermal aging, which diminishes their long-term durability.
Several strategies have been developed to enhance hydrothermal stability. Encapsulating active metals such as Pt and Cu within the zeolite framework effectively prevents sintering and maintains metal dispersion at elevated temperatures [23,31]. The use of sintering-resistant materials, such as cerium oxide (CeO2) and zirconium oxide (ZrO2), establishes strong metal-support interactions that inhibit metal particle agglomeration [6,50]. Additionally, constructing hydrophobic shells around metal-zeolite catalysts has shown great potential in improving hydrothermal stability. Hydrophobic shells reduce the interaction between the zeolite framework and water molecules, thereby minimizing dealumination and preserving the integrity of the zeolite structure under high-humidity conditions [23].
Composite catalysts that combine noble and base metals, such as Ag/CeSnOx, further enhance stability by leveraging synergistic interactions between metals [51]. These approaches collectively address the adverse effects of high-temperature and high-humidity environments, ensuring sustained catalytic performance in industrial applications.

6.3. High Cost and Cost-Reduction Strategies

The high cost of zeolite-based catalysts arises from both the use of noble metals and the energy-intensive synthesis processes. Traditional hydrothermal methods require precise control of temperature and pressure, contributing to elevated production costs. Additionally, noble metals like Pt and Pd, commonly used for their catalytic efficiency, are expensive and limited in supply.
Cost-reduction strategies include substituting noble metals with transition metals such as Cu, Fe, or Co, which offer comparable catalytic performance at a fraction of the cost. Hybrid catalysts integrating small amounts of noble metals with base metals achieve a balance between cost and efficiency. Innovations in synthesis, such as one-pot methods, dry gel conversion, and mechanochemical processes, simplify production while reducing energy consumption and waste. The use of natural zeolites or precursors further lowers costs, making zeolite-based catalysts more economically viable.

6.4. By-Product Formation and Control Strategies

Controlling selectivity to N2 while minimizing undesired by-products such as NOx and N2O is another significant challenge. NOx formation during NH3 oxidation, particularly at high temperatures, negates the environmental benefits of ammonia removal. Achieving a balance between NOx suppression and N2 selectivity depends on the nature of the active sites and the catalyst’s ability to promote complete NH3 oxidation.
Strategies to control NOx formation include modifying the support materials and incorporating bifunctional catalysts. These approaches enhance the catalyst’s ability to facilitate selective pathways while suppressing undesired reactions. Further research into tailoring the active metal sites and reaction conditions is essential for improving selectivity and performance.

6.5. Limited Low-Temperature Activity and Improvement Strategies

Zeolite-based catalysts often exhibit low activity at temperatures below 200 °C, limiting their effectiveness under cold-start conditions in automotive exhaust systems. Incomplete NH3 conversion and increased N2O formation due to suboptimal reaction pathways are common at these lower temperatures.
Enhancing lattice oxygen mobility is a key strategy for improving low-temperature activity. Ce-based catalysts with facile oxygen exchange between Ce3+ and Ce4+ have shown enhanced NH3 oxidation at reduced temperatures. Additionally, introducing oxygen vacancies in zeolite catalysts, such as Cu-ZSM-5 and Cu-SSZ-13, facilitates oxygen adsorption and activation. The incorporation of oxygen storage materials, such as cerium-zirconium mixed oxides, provides a continuous supply of reactive oxygen species, boosting NH3 conversion and N2 selectivity under low-temperature conditions.
By addressing these challenges through targeted strategies, zeolite-based NH3-SCO catalysts have made significant advancements in durability, efficiency, and cost-effectiveness, paving the way for broader adoption in industrial and environmental applications.

7. Future Directions and Prospects

While existing strategies have addressed many challenges in zeolite-based NH3-SCO catalysis, there remain opportunities to further enhance performance, scalability, and practical applicability. Future advancements will require innovative approaches to catalyst synthesis, a deeper understanding of reaction mechanisms through computational tools, and scalable solutions for industrial implementation.

7.1. Innovative Synthesis Techniques for Enhanced Catalytic Performance

Emerging synthesis approaches for zeolite-based NH3-SCO catalysts focus on the development of core-shell architectures, multi-functional catalysts, and advanced fabrication techniques such as three-dimensional (3D) printing. Core-shell designs, where a stable metal oxide shell protects the active metal core, have demonstrated improved resistance to sintering, sulfur poisoning, and water vapor. For instance, encapsulating Ag NPs within hollow ZSM-5 zeolite shells significantly enhances catalyst stability and sulfur resistance. Multi-functional catalysts incorporating multiple metal species, such as Cu and Ce or Pt and Ag, leverage synergistic interactions between metals to boost catalytic activity, N2 selectivity, and stability.
3D printing technology has emerged as a transformative approach to emission control, offering precise structural customization, high catalyst loading, and improved mass transfer efficiency [52,53,54]. Compared to conventional catalyst fabrication methods, 3D printing enables the direct integration of multiple active components within a single monolithic structure, enhancing durability and catalytic performance. By integrating transition metals like Pt or MnOx into zeolite supports using 3D printing, researchers can optimize catalyst architecture for superior performance under real-world conditions [52,54]. In NH3-SCO applications, 3D-printed catalysts provide a controllable strategy for achieving superior hydrothermal stability, increased resistance to poisoning, and optimized low-temperature activity by tailoring geometric design. Specifically, the application of 3D printing in the preparation of ammonia slip catalysts (ASC) allows for the precise distribution of active metal sites, maximizing NH3 conversion efficiency.

7.2. Computational Tools for Catalyst Design and Optimization

The integration of computational tools, particularly Machine Learning (ML) and Artificial Intelligence (AI), is becoming increasingly vital in catalyst development. These technologies facilitate rapid screening and optimization of catalyst compositions and reaction conditions. ML models can predict effective combinations of metal dopants, support materials, and reaction temperatures to maximize selectivity and minimize NOx production. Additionally, Density Functional Theory (DFT) calculations provide insights into the electronic structures of catalysts, aiding in the design of materials with enhanced oxygen mobility and resistance to deactivation. As computational methods continue to advance, they are expected to drive the discovery of novel catalyst compositions specifically tailored to meet the demands of NH3-SCO applications.

7.3. Scaling up Zeolite-Based Catalysts for Industrial Applications

Scaling up zeolite-based NH3-SCO catalysts for industrial use involves addressing challenges related to cost, durability, and system integration. Cost-effective synthesis methods that utilize abundant transition metals, such as Cu and Fe, combined with scalable support materials like SSZ-13 and ZSM-5, are essential for reducing production costs. To ensure long-term viability, catalysts must exhibit robust performance in real-world conditions, where exposure to high temperatures, sulfur compounds, and water vapor is unavoidable. Recent studies highlight the potential of layered catalyst structures, hybrid designs, and 3D-printed composites to enhance durability and maintain activity over extended operation periods.
By overcoming these scalability and durability challenges, zeolite-based NH3-SCO catalysts are poised for widespread adoption in both mobile and stationary applications. This includes their integration into existing selective catalytic reduction (SCR) systems, where they can reduce ammonia slip and improve overall nitrogen selectivity, offering significant environmental and industrial benefits.

8. Conclusions

This review has provided a comprehensive overview of recent advancements in zeolite-based NH3-SCO catalysts, emphasizing their catalytic performance, durability, and potential for reducing ammonia emissions. Key innovations include the development of core-shell architectures and bimetallic catalysts, such as PtCu and Cu/Ag systems, which effectively leverage synergistic effects to enhance N2 selectivity and catalytic stability. Furthermore, the integration of non-noble metals like Cu and Fe into zeolite frameworks has demonstrated cost-effective alternatives with strong redox capabilities. Strategies such as promoting oxygen mobility, employing oxygen storage materials, and utilizing advanced frameworks like SSZ-13 have significantly enhanced low-temperature activity and hydrothermal stability, both critical for real-world applications in mobile and stationary systems.
Despite these advancements, several challenges remain. Catalyst deactivation, driven by poisoning (e.g., sulfur and phosphorus) and sintering, continues to limit long-term performance. The high cost and complexity of noble metal catalysts, combined with scalability issues for advanced designs, further hinder widespread industrial adoption. Additionally, controlling NOx and N2O formation and achieving high activity under low-temperature conditions remain persistent obstacles.
Future research should prioritize the development of poisoning-resistant materials, innovative synthesis methods for scalable and cost-effective catalyst production, and strategies to enhance hydrothermal stability in harsh operating environments. Machine learning and computational modeling hold promise for guiding the rational design of catalysts by predicting optimal compositions and reaction conditions. Finally, addressing the balance between catalytic efficiency, durability, and scalability will be essential to enable the deployment of next-generation NH3-SCO catalysts for industrial-scale applications, paving the way for sustainable and efficient ammonia emission control technologies.

Author Contributions

Conceptualization, G.Y. and X.C.; validation, G.Y., X.C. and J.H.; writing—original draft preparation, X.C. and J.H.; writing—review and editing, G.Y.; supervision, G.Y.; project administration, G.Y.; funding acquisition, G.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Jilin Province Science and Technology Project (Grant SKL202302019), the National Natural Science Foundation of China (Grant 22371088) and FAW Volkswagen China Environmental Protection Foundation Automotive Environmental Protection Innovation Leadership Program. X.C. acknowledges the collaboration under the Sino-French International Research Network (IRN) “Zeolites”.

Data Availability Statement

No new data were created or analyzed in this study.

Conflicts of Interest

The authors declare no conflicts of interest.

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Catalysts 15 00204 g001
Figure 1. Schematic representation of metal/zeolite catalysts for NH3-SCO reactions. The synergy between the zeolite framework and metal species makes metal/zeolite catalysts highly effective for NH3-SCO applications. (Source: The Authors).
Figure 1. Schematic representation of metal/zeolite catalysts for NH3-SCO reactions. The synergy between the zeolite framework and metal species makes metal/zeolite catalysts highly effective for NH3-SCO applications. (Source: The Authors).
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Figure 2. Reaction pathways over bimetallic zeolite catalysts for NH3-SCO: NH3 oxidation on metal nanoparticles (i) and NOx reduction on Cu-exchanged zeolites (ii) (Source: The Authors).
Figure 2. Reaction pathways over bimetallic zeolite catalysts for NH3-SCO: NH3 oxidation on metal nanoparticles (i) and NOx reduction on Cu-exchanged zeolites (ii) (Source: The Authors).
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Figure 3. Challenges in metal/zeolite catalysts for NH3-SCO (Source: The Authors).
Figure 3. Challenges in metal/zeolite catalysts for NH3-SCO (Source: The Authors).
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Chen, X.; Huang, J.; Yang, G. Advances and Challenges in Zeolite-Based Catalysts for the Selective Catalytic Oxidation of Ammonia. Catalysts 2025, 15, 204. https://doi.org/10.3390/catal15030204

AMA Style

Chen X, Huang J, Yang G. Advances and Challenges in Zeolite-Based Catalysts for the Selective Catalytic Oxidation of Ammonia. Catalysts. 2025; 15(3):204. https://doi.org/10.3390/catal15030204

Chicago/Turabian Style

Chen, Xiaoxin, Jun Huang, and Guoju Yang. 2025. "Advances and Challenges in Zeolite-Based Catalysts for the Selective Catalytic Oxidation of Ammonia" Catalysts 15, no. 3: 204. https://doi.org/10.3390/catal15030204

APA Style

Chen, X., Huang, J., & Yang, G. (2025). Advances and Challenges in Zeolite-Based Catalysts for the Selective Catalytic Oxidation of Ammonia. Catalysts, 15(3), 204. https://doi.org/10.3390/catal15030204

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