Research Progress on Zeolite-Type High-Temperature NH₃-SCR Catalysts

Abstract

Gas turbines operate at exhaust gas temperatures exceeding 500 °C. Vanadium-based catalysts encounter challenges in NH₃-SCR denitrification due to vanadium volatilization and titanium dioxide support phase transition at high temperatures. This restricts the effective denitrification temperature range to 300~400 °C, falling short of gas turbine denitrification requirements. Zeolite-supported catalysts, known for their high specific surface area, abundant acid sites, and stable framework structure, demonstrate superior catalytic activity and hydrothermal stability at high temperatures. This review synthesizes recent advancements in high-temperature catalysts utilizing ZSM-5, Beta, SSZ-13, and SAPO-34 zeolites as supports. It elucidates the interaction mechanisms between active components (e.g., transition metals Fe, Cu, W, rare earth elements) and zeolite supports. Furthermore, it examines variations in denitrification performance through the lens of the high-temperature NH₃-SCR reaction mechanism, offering valuable insights for high-temperature denitrification catalyst development.

1. Introduction

As a core flexible power generation technology supporting the integration of high proportions of renewable energy, gas turbines play a key role in the global energy transition [1,2,3]. However, their high-temperature exhaust gases (typically > 500 °C) make nitrogen oxide (NO_x) emissions reduction a major bottleneck constraining their compliant operation [4]. Unlike traditional coal-fired flue gas, gas turbine flue gas has extremely low particulate matter and SO₂ content [5,6], enabling its denitrification system to adopt a simpler, lower-cost high-temperature configuration without needing to address traditional challenges such as catalyst blockage and ammonium bisulfate poisoning [7,8]. However, this high-temperature advantage imposes extremely stringent requirements on the denitrification catalyst itself. Commercially available V₂O₅-WO₃(MoO₃)/TiO₂ catalysts rapidly deactivate at temperatures exceeding 500 °C due to issues such as the volatilization of the active component vanadium, crystal structure transformation of the TiO₂ support, and particle sintering, making them unsuitable for direct application [9,10,11,12,13,14]. Based on actual operating data from gas turbines, flue gas temperatures typically fluctuate between 500~650 °C (reaching up to 680 °C under variable load conditions) [15,16,17,18,19], while commercial vanadium-based catalysts cannot operate stably above 500 °C [20,21]. Therefore, developing a highly efficient and stable NO_x reduction catalyst covering the 500~650 °C temperature window is critical to achieving deep NO_x purification in gas turbines.

To address this challenge, researchers have extensively explored various metal oxide catalysts. For instance, Zhang et al. [22] introduced Co oxide-modified vanadium-based catalysts, leveraging their enhanced surface acidity and redox capabilities to achieve over 90% NO_x conversion rates within the 300~550 °C range. The W-Zr-O_x/TiO₂ catalyst developed by Chen et al. [23] formed solid hyperacid sites on the TiO₂ surface, increasing acid and oxygen vacancy concentrations to maintain high activity between 330~600 °C. Leveraging CeO₂’s superior oxygen storage capacity, our group developed a Ce-W/Ti-Si catalyst exhibiting over 80% NO conversion efficiency across 225~560 °C [24]. Although the aforementioned strategies of modifying active components to optimize acidity and redox capabilities demonstrate significant effectiveness in the high-temperature range, none have overcome the inherent structural defect of metal oxide catalyst systems—insufficient thermodynamic stability of the support [25,26,27]. Whether TiO₂, Al₂O₃, or other common metal oxide supports, severe sintering and phase transformations occur above 600 °C. This structural collapse of the support leads to a drastic decrease in specific surface area, pore blockage, and agglomeration and deactivation of the loaded active components [28]. Therefore, although modifying the active component can optimize catalytic performance to some extent [29,30,31,32], the thermal stability of the support itself fundamentally limits the upper temperature limit of the entire catalyst system, preventing it from meeting the stable operating requirements of the 500~650 °C full operating window in gas turbines.

Given the inherent limitations of metal oxide supports, research focus inevitably shifts toward thermally stable zeolite materials [33,34,35]. Unlike amorphous or semi-crystalline supports such as TiO₂, the crystalline framework of zeolites (e.g., the MFI structure in ZSM-5) exhibits exceptional thermal stability, withstanding temperatures exceeding 800 °C [36]. This fundamentally resolves core challenges associated with support sintering and phase transitions. Simultaneously, their vast specific surface area (300~1000 m²/g) and well-ordered pore systems not only effectively stabilize and disperse metal active sites while inhibiting sintering but also optimize mass transfer efficiency for reactant molecules [37]. In recent years, zeolite catalysts loaded with active sites such as Cu, Fe, Ce, and W (e.g., ZSM-5 [38,39], Beta [40,41], SSZ-13 [42,43,44], SAPO-34 [45,46]) have emerged as a frontier research direction in high-temperature denitrification. However, significant bottlenecks persist in this field: First, the structure-activity relationships between different active components (single-metal/multi-metal) and various zeolite supports remain unclear [47,48,49]. Second, research on reaction mechanisms and deactivation mechanisms in the ultra-high-temperature zone above 600 °C remains relatively weak [50,51]. Furthermore, existing studies predominantly focus on performance optimization at the laboratory scale, lacking systematic evaluation of the large-scale production costs, hydrothermal stability, and lifecycle of zeolites with diverse structures. This severely limits their industrial applicability [52]. The ambiguity surrounding these critical scientific issues significantly hinders the rational design and development of next-generation high-temperature catalysts for gas turbine applications.

Several reviews have explored NH₃-SCR catalysts, including zeolite-based systems [53,54,55]. However, most studies have focused on low-temperature applications, single zeolite types, or fundamental theories. This has resulted in a gap in systematic analysis and comparison of multiple zeolite frameworks under high-temperature conditions (>500 °C), particularly lacking in-depth exploration of the interactions between framework stability, active site engineering, and industrial feasibility.

This review aims to fill this gap by critically synthesizing research advances in zeolite-based high-temperature NH₃-SCR catalysts from 2018 to 2025. We focus on extensively studied and promising supports—ZSM-5, Beta, SSZ-13, and SAPO-34—while exploring emerging materials such as SSZ-16 (AFX). This paper systematically analyzes advances in single-metal (Cu, Fe) and composite metal-modified zeolite catalysts. Unlike previous studies, we emphasize: (1) elucidating structure-activity relationships and reaction mechanisms specific to high-temperature operating conditions; (2) critically evaluating performance trade-offs related to framework topology, hydrothermal stability, and synthesis costs; (3) Providing forward-looking perspectives on challenges and opportunities for commercial application through discussions integrating advanced characterization, machine learning-assisted design, and green synthesis routes. This paper aims to offer comprehensive references and valuable insights for researchers engaged in the rational design and development of next-generation high-temperature denitration catalysts for gas turbines.

2. Structural and Functional Comparison of Zeolite Support

The selection of zeolite supports in high-temperature “NH₃-Selective Catalytic Reduction (NH₃-SCR)” denitrification technology is fundamentally determined by their combined structural and chemical properties, which directly govern the catalyst’s catalytic activity and durability [56]. Unlike traditional V₂O₅-WO₃(MoO₃)/TiO₂ catalysts constrained by thermal stability [57], zeolite-based catalyst systems enable tunable architectural design through framework topology, pore geometry, and acidic site engineering [58].

Table 1. Structural parameters of the zeolites with different topologies [59].

Topology	Name	Maximum Diameter (Å)	Channel System	Ring Member	Synthesis Cost	Hydrothermal Stability
MFI	ZSM-5	5.1 × 5.5	3-dimensional	10/6/5/4	Medium	Low
		5.3 × 5.6
BEA	Beta	6.6 × 6.7	3-dimensional	12/6/5/4	Medium	Medium
		5.6 × 5.6
CHA	SSZ-13	3.8 × 3.8	3-dimensional	8/6/4	High	Excellent
	SSZ-62
	SAPO-34
	SAPO-47
AFX	SSZ-16	3.4 × 3.6	3-dimensional	8/6/4	High	Excellent
	SAPO-56
AEI	SSZ-39	3.8 × 3.8	3-dimensional	8/6/4	High	Excellent
	SAPO-18
LTA	UZM-9	4.1 × 4.1	3-dimensional	8/6/4	High	Good
	SAPO-42

summarizes the structural parameters of zeolites with different topologies [59].

MFI zeolites (represented by ZSM-5) find extensive applications in catalysis, adsorption, and separation due to their three-dimensional cross-linked pore structure and tunable acidity [60,61]. However, their poor hydrothermal stability and high-temperature metal agglomeration issues limit their use under harsh conditions [62]. BEA-type zeolites (represented by Beta) exhibit outstanding mass transfer capabilities due to their three-dimensional 12-membered ring macroporous structure and high specific surface area. However, their complex structure (highly defective, two polymorphic structures) adversely affects hydrothermal stability [63]. CHA-type zeolites (e.g., SSZ-13, SAPO-34) exhibit outstanding hydrothermal stability and good catalytic performance due to their cage-like topology [64,65,66]. However, their widespread application is constrained by the high cost of template agents (e.g., N,N,N-trimethyl-1-adamantammonium hydroxide) and complex synthesis processes [67,68,69]. The three-dimensional octahedral ring dual-channel system of AFX-type zeolites (such as SSZ-16) effectively balances the diffusion and selectivity of small molecules [70]. However, the high cost of template agents (such as amantadine derivatives) and potential channel blockage issues severely hinder their large-scale practical application [71,72]. AEI zeolites (e.g., SSZ-39) exhibit outstanding catalytic performance due to their unique three-dimensional channels [73], yet high synthesis costs (N,N-dimethylpiperidinium templates) similarly restrict their adoption [74]. LTA zeolites (e.g., 4A) remain irreplaceable in small-molecule sieving and ion exchange [37], but their narrow octahedral ring window and low thermal stability limit catalytic functionality [75].

The comparative analysis above indicates that no single zeolite support excels in all aspects. These performance differences fundamentally stem from the unique framework constraints of each support and the synergistic interaction between the active components and the support [76,77]. Based on this, the following section will delve into modification strategies for various zeolite supports (ZSM-5, Beta, SSZ-13, SAPO-34, etc.). It summarizes the interactions between active components—such as transition metals (Fe, Cu, W) and rare earth elements—and different support types, while exploring their NH₃-SCR reaction mechanisms. This analysis aims to provide insights for developing high-temperature denitrification catalysts.

3. ZSM-5 Support

Among various zeolite supports, ZSM-5 (MFI topology) has garnered significant attention due to its unique pore structure, strong acidity, high specific surface area, and excellent thermal stability [37]. Its relatively simple synthesis process and low cost further position it as a commercially promising SCR catalyst support. This section critically reviews research progress on single-metal and multi-metal modified ZSM-5 catalysts, focusing on design principles, catalytic performance, and reaction mechanisms under high-temperature conditions.

3.1. Single Metal-Doped ZSM-5 Catalyst

Among various metal-modified ZSM-5 catalysts, Fe is the most extensively studied and effective active metal in high-temperature NH₃-SCR systems due to its excellent redox properties and ability to form highly active sites [38,39,78,79,80,81,82,83,84,85,86,87].

Fe/ZSM-5 catalysts represent one of the earliest zeolite-type denitrification catalysts studied. Long and Ma et al. [78,79] reported nearly simultaneously in 1999 that Fe/ZSM-5 catalysts exhibit high NH₃-SCR reaction activity. Long found that Fe/ZSM-5 catalysts achieve nearly 100% NO conversion within the 400~550 °C temperature range (Figure 1). Subsequently, Long et al. [80] conducted detailed studies on the NH₃-SCR reaction mechanism of Fe/ZSM-5 catalysts. They proposed that the NH₃-SCR reaction on Fe/ZSM-5 catalysts follows the “Eley-Rideal (E-R)” reaction model (Figure 2). In the gas phase, NH₃ molecules adsorb and activate at Brønsted acid sites to form NH₄⁺, while NO molecules oxidize on Fe³⁺ to form NO₂. Subsequently, adsorbed NH₃ reacts with gas-phase NO₂ to generate the intermediate NO₂ [NH₄⁺]₂, which then reacts with gas-phase NO to yield the final products N₂ and H₂O.

The silica–alumina ratio of different zeolites and the preparation method of catalysts significantly influence NH₃-SCR activity. Brandenberger et al. [81] used NH₄-ZSM-5 zeolite with a Si/Al ratio of 14 as a support and found that Fe/ZSM-5 catalysts prepared via liquid ion exchange exhibited NO conversion rates exceeding 80% within the 350~650 °C range. Qi et al. [82] investigated the effects of SiO₂/Al₂O₃ ratios (11, 25, 40) and iron sources (FeCl₂ and FeCl₃) on catalyst performance. They demonstrated that catalysts prepared using FeCl₂ as the active component exhibited higher activity than those using FeCl₃. When FeCl₂ was the iron source, the Fe/ZSM-5 catalyst exhibited optimal activity at a Si/Al ratio of 11 and an iron loading of 2.5 wt%, achieving NO conversion rates exceeding 80% between 320~550 °C. The study revealed that SCR activity decreases with increasing Si/Al ratio in the ZSM-5 support. This is attributed to the higher Brønsted acidity of ZSM-5 zeolites with lower Si/Al ratios. Strong acidity promotes NH₃ adsorption, thereby enhancing the activity of Fe/ZSM-5 catalysts.

Regarding catalyst preparation methods, Shi et al. [83,84] investigated the effects of liquid ion exchange, solid ion exchange, and isovolumetric impregnation on the denitrification performance and mechanism of Fe/ZSM-5 catalysts. The study revealed that Fe/ZSM-5 catalysts prepared via isovolumetric impregnation exhibited optimal denitrification activity, achieving a maximum NO_x conversion rate of 100% and maintaining over 90% conversion efficiency even at 550 °C. Wang et al. [38,39] investigated the effects of equal-volume impregnation and aqueous solution ion exchange on the NO_x removal performance of Fe/ZSM-5 catalysts. Results indicated that the Fe/ZSM-5 catalyst with 1.0 wt% Fe content prepared by equal-volume impregnation exhibited NO_x conversion rates exceeding 80% between 425~680 °C (Figure 3a). At elevated temperatures, this catalyst exhibits enhanced strength of strong acidic sites with decreasing Fe loading (Figure 3b) and reduced non-selective oxidation of NH₃ (Figure 3c), thereby demonstrating excellent high-temperature NO_x removal performance. DRIFT testing confirmed the coexistence of “Langmuir-Hinshelwood (L-H)” and E-R reaction pathways. However, DRIFT results reveal that for Fe/ZSM-5 catalysts prepared via aqueous solution ion exchange, the primary SCR reaction pathway is the E-R mechanism. This arises from the reaction mechanism between adsorbed NO and gaseous NH₃, as depicted in Equations (1)–(3) and Figure 4 (where S represents the zeolite structure) [39]. This indicates that the reaction mechanism of the Fe/ZSM-5 catalyst is closely related to its preparation method.

[S − Fe³⁺ = O] + NO → [S − Fe³⁺ = O][NO]

(1)

[S − Fe³⁺ = O][NO] + NH₃ → N₂ + H₂O + [S − Fe²⁺ − OH]

(2)

[S − Fe²⁺ − OH] + 1/4 O₂ → 1/2 H₂O + [S − Fe³⁺ = O]

(3)

3.2. Composite Metal-Doped ZSM-5 Catalyst

To further enhance high-temperature activity and hydrothermal stability, introducing a second or third metal to generate synergistic effects has proven to be a highly effective strategy, typically yielding superior performance compared to single-metal catalysts.

CeO₂ possesses high oxygen storage and redox capacity. As an active component, it can synergistically interact with other transition metals to enhance the catalyst’s denitrification performance. Chen et al. [85] investigated Ce- or La-modified Fe/ZSM-5 catalysts, finding that Ce-modified Fe/ZSM-5 exhibits a broader active temperature window. Long et al. [86] observed that Ce-Fe/ZSM-5 catalysts prepared via ion exchange achieved NO conversion rates exceeding 90% within the 370~600 °C range. Wang et al. [87] employed Fe(NO₃)₃·9H₂O, Ce(NO₃)₃·6H₂O, and ZSM-5 zeolite (SiO₂/Al₂O₃ = 27) as raw materials to prepare a series of Ce-Fe/ZSM-5 catalysts via an isovolumetric impregnation method. Their study revealed that Fe₄/ZSM-5 catalysts with 4 wt% Fe exhibited severe ammonia oxidation at high temperatures. However, loading a certain amount of Ce increased the strength and quantity of surface acids, particularly Lewis acid sites, thereby reducing the proportion of adsorbed oxygen on the surface. exhibited severe ammonia oxidation at elevated temperatures. Loading Ce increased both the strength and quantity of surface acids, particularly Lewis acid sites, while enhancing the proportion of adsorbed oxygen. The synergistic interaction between Ce and Fe improved the catalyst’s high-temperature redox capacity, suppressing NH₃ oxidation to NO and boosting high-temperature activity. At 550 °C, the NO_x conversion rate of the Fe₄/ZSM-5 catalyst was only 77.11%. When Ce was loaded at 1 wt%, the Ce₁-Fe₄/ZSM-5 catalyst achieved a NO_x conversion rate exceeding 95% across the 400~550 °C range. After hydrothermal aging, the NO_x conversion rate remained approximately 90% within the 450~550 °C range, demonstrating good hydrothermal stability.

WO₃ is an excellent solid acid commonly used as an active component in SCR applications. ZrO₂ is an acid-base amphoteric oxide with favorable redox properties. Consequently, some researchers have employed both WO₃ and ZrO₂ as active components to enhance the NH₃-SCR performance of catalysts. Chen et al. [23] discovered that WO₃ and ZrO₂ form a solid hyperacid (W-Zr-O_x). The interaction between WO₃ and ZrO₂ significantly enhances the catalyst’s surface acidity and redox properties, thereby effectively improving its high-temperature activity. Feng et al. [88] investigated the effects of three preparation methods—sol–gel, impregnation, and grinding—on W-Zr/ZSM-5 catalyst performance using ZSM-5 as a support. Results indicated that the sol–gel-prepared catalyst exhibited optimal activity. attributed to greater surface enrichment of W and Zr elements. This promotes the formation of acidic sites on the catalyst surface, while the interaction between WO₃ and ZrO₂ creates oxygen vacancies (Figure 5), enhancing the catalyst’s redox properties. After calcination at 700 °C for 2 h, the W-Zr/ZSM-5 catalyst (15 wt% WO₃ and 1 wt% ZrO₂) exhibited NO conversion rates exceeding 90% within the 350~630 °C range. In situ infrared studies revealed that NO removal on the W-Zr/ZSM-5 catalyst follows the L-H reaction mechanism, with amide species (-NH₂), monodentate nitrite, NH₄⁺, surface-adsorbed NH₃ species, and ad-NO₂ species were identified as key reaction intermediates at elevated temperatures. Specifically, -NH₂ and adsorbed NH₃ reacted with ad-NO₂ and ad-NO to form N₂ and H₂O, respectively. NH₄⁺ reacted with ad-NO₂ to produce NH₄NO₂, which ultimately decomposed into N₂ and H₂O. Additionally, the ad-NO₂ species participates in the “rapid SCR” reaction, effectively promoting the reaction process. The reaction mechanism of the catalyst at high temperatures is shown in Equations (4)–(13).

NH₃(g) → NH₄⁺(a) (Brønsted acid sites)

(4)

O₂ + 2* → 2 − O*

(5)

NH₃(g) + * → -NH₃*

(6)

-NH₃* + -O* → -NH₂* + -OH*

(7)

NO(g) + -O* → -NO₂*

(8)

NO(g) + * → -NO*

(9)

-NH₂* + -NO₂* → N₂ + H₂O + -O*

(10)

-NH₃* + -NO* → N₂ + H₂O + -H*

(11)

NH₄⁺ + -NO₂* → NH₄NO₂→ N₂ + 2H₂O

(12)

2-NH₃* + -NO₂* + -NO* → 2N₂ + 3H₂O

(13)

Due to the excellent NO conversion rate of W-Zr/ZSM-5 catalysts at high temperatures and the ability of rare earth element doping to enhance SCR activity, Feng et al. [89] prepared Sm-W-Zr/ZSM-5 catalysts using the sol–gel method. Results indicate that the W-Zr/ZSM-5 catalyst loaded with 1.0 wt% Sm achieves complete NO conversion within the 400~600 °C range and maintains NO conversion rates exceeding 90% between 380~640 °C (Figure 6). The introduction of Sm species suppressed agglomeration of the active components (as indicated by the red arrows in Figure 7), thereby promoting dispersion of the active components on the catalyst surface and slightly improving the NH₃-SCR performance at high temperatures. Furthermore, the Sm-W-Zr/ZSM-5 catalyst exhibited excellent regeneration capability and could be regenerated through heat treatment at 350 °C.

Another critical factor affecting catalyst activity at high temperatures is the overoxidation of NH₃. Liu et al. [90] discovered that the ion-exchange-prepared Fe-Ni-W/ZSM-5 ternary metal catalyst possesses Brønsted acid sites and extra-framework metal exchange sites. These active sites exhibit high activation barriers for the NH₂ → NH conversion during NH₃ oxidation, significantly suppressing NH₃ overoxidation and thereby improving high-temperature NH₃-SCR performance. Simultaneously, metal loading significantly influenced SCR performance (Figure 8a), with the catalyst containing 0.5 wt% Fe, 0.5 wt% Ni, and 0.25 wt% W exhibiting optimal activity, achieving 90% NO_x conversion efficiency between 480~750 °C. After hydrothermal aging at 800 °C, the NO conversion rate slightly decreased (Figure 8b). In situ DRIFT results revealed potential pathways for NH₃ oxidation. Initially, hydrogen is extracted from NH₃ adsorbed on the catalyst surface. As temperature increases, the reduced binding energy of metal-oxygen bonds facilitates gradual dehydrogenation induced by active lattice oxygen in the metal oxide. Subsequently, oxidation products form in the presence of lattice/gas-phase oxygen, as depicted by Reactions (14)–(19).

NH₃ → -NH₂ + *H

(14)

-NH₂ + O → -NH + OH

(15)

-NH + O → -N + OH

(16)

-NH + O → HNO → *H + NO

(17)

-N + O → NO

(18)

-N + NO → N₂O

(19)

In summary, for high-temperature NH₃-SCR applications, ZSM-5 (MFI structure) zeolite catalysts with Fe as the active center exhibit superior high-temperature activity and hydrothermal stability, making them one of the most promising systems. Research indicates that its catalytic performance strongly depends on the iron source precursor and preparation method. Both factors significantly influence catalytic activity and reaction mechanisms (E-R or L-H mechanisms) by regulating the morphology of active Fe species (isolated ions, oligomeric Fe_xO_y clusters, or oxide particles). Furthermore, introducing secondary metal components such as Ce, W, or Zr can generate synergistic effects. These effects optimize surface acidity, enhance redox capabilities, improve metal dispersion, and suppress excessive NH₃ oxidation, thereby further improving the catalyst’s high-temperature catalytic performance and hydrothermal stability.

4. Beta Support

Compared to ZSM-5 with an MFI structure, Beta zeolite (BEA topology) demonstrates significant advantages in high-temperature NH₃-SCR catalysis due to its unique three-dimensional twelve-membered ring cross-linked pore system, higher specific surface area, and abundant acid site distribution [91,92,93]. Its larger pore dimensions provide a more favorable spatial environment for reactant diffusion and intermediate formation, making it particularly suitable for reactions involving macromolecules or requiring rapid mass transfer. Although its hydrothermal stability is generally considered slightly lower than that of mesoporous zeolites (e.g., CHA), Beta-based catalysts can maintain outstanding catalytic activity and stability in the high-temperature range through rational metal modification and synthesis optimization [41]. This section systematically reviews research progress on single-metal and multi-metal catalysts supported on Beta zeolite, focusing on their catalytic performance, hydrothermal stability, and reaction mechanisms under high-temperature conditions.

4.1. Single Metal-Doped Beta Catalyst

Among various metal modification strategies for Beta zeolites, Fe has been the most extensively and deeply studied as an active center. Liu et al. [91] reported early on that Fe/Beta (SiO₂/Al₂O₃ = 25) catalysts prepared via the impregnation method could achieve NO conversion rates exceeding 95% within the 320~550 °C range (Figure 9). Subsequent work by Frey et al. [92] also demonstrated that Fe/Beta catalysts prepared via impregnation exhibit exceptionally high NH₃-SCR reactivity. Zhu et al. [93] employed the same preparation method to further investigate the influence of different Si/Al ratios (Si/Al = 9 and Si/Al = 19) on the NH₃-SCR performance of Fe/Beta catalysts. The study revealed that Fe/Beta catalysts with lower Si/Al ratios exhibit higher SCR activity. This is attributed to the increased acidity of zeolites with lower Si/Al ratios, which facilitates greater ammonia adsorption and activation on the catalyst surface. Furthermore, compared to Fe/Beta (Si/Al = 19), the Fe/Beta (Si/Al = 9) catalyst exhibited less aluminum leaching during hydrothermal aging, demonstrating superior hydrothermal stability.

In addition to the impregnation method, Fe/Beta catalysts can also be prepared via ion exchange. Ma et al. [94] compared the two methods and found that Fe/Beta catalysts prepared by ion exchange exhibited optimal activity, achieving nearly 100% NO_x conversion rates between 350~550 °C. The ion exchange method is more conducive than the impregnation method for generating more iron ions at ion exchange sites. Furthermore, the introduction of iron material does not significantly affect the surface area and pore volume of the ion exchange method samples, whereas the specific surface area and pore volume of Fe/Beta prepared by the impregnation method are relatively significantly reduced.

Liu et al. [95] also prepared Fe/Beta catalysts via ion exchange. They found that the catalytic activity of Fe/Beta increased with rising Fe loading, with the catalyst at 0.52 wt% Fe loading exhibiting outstanding performance, achieving NO conversion rates exceeding 90% across the 310~600 °C range. Kinetics experiments revealed that isolated Fe³⁺ species serve as the active sites for the NH₃-SCR reaction, while oligomers act as the active sites for NH₃ oxidation on the Fe/Beta catalyst. UV-vis results indicated that the Fe-0.52 sample contained a higher concentration of isolated Fe³⁺, thereby conferring the catalyst with enhanced catalytic activity. Additionally, NO_x conversion rates exhibit a slight decrease at elevated temperatures, particularly for the Fe-0.52 sample, where conversion rates drop by nearly 10% between 500 and 600 °C. This decline stems from excessive Fe loading, which increases oligomeric Fe_xO_y clusters. These clusters catalyze NH₃ oxidation, thereby impairing the catalyst’s high-temperature activity.

Shi et al. [96] discovered that the presence of water vapor significantly enhances the NH₃-SCR performance of Fe-BEA catalysts. Under steam conditions, the Fe-BEA catalyst (Si/Al ≈ 20) exhibits NO_x conversion rates exceeding 80% across the 200~640 °C temperature range, demonstrating outstanding catalytic activity and N₂ selectivity surpassing 90% (Figure 10). Studies indicate that due to the abundance of Lewis acid sites on the Fe-BEA surface, the addition of water transforms these Lewis acid sites into Brønsted acid sites. This facilitates the adsorption of more NO₃⁻/NO₂⁻ and NH₄⁺ species onto the surface, thereby enhancing the catalyst’s SCR activity. Furthermore, in situ DRIFTS studies confirm that both the L-H and E-R pathways coexist when water is present in the reaction feed stream.

The research group led by Klukowski [97] deduced the detailed SCR reaction mechanism of Fe/Beta catalysts (Figure 11). DRIFTS and NO_x-TPD analyses revealed that NO molecules adsorb on the catalyst surface to form Fe³⁺-NO species, which then react with adsorbed NH₃. DRIFTS and NH₃-TPD confirmed that NH₃ primarily adsorbs on the Beta zeolite support. XANES results indicated that NH₃ can also adsorb on Fe³⁺ sites. On the Fe/Beta catalyst, NO and NH₃ adsorb simultaneously on adjacent Fe³⁺ sites. NH₃ undergoes multiple adsorption–desorption cycles on the zeolite support before ultimately adsorbing onto Fe³⁺ sites. The binding of NH₃ to Fe³⁺ sites causes partial reduction of these sites, followed by oxidation cycles via O₂. Nevertheless, the reaction pathway involving isolated Fe³⁺ sites cannot be entirely ruled out.

4.2. Composite Metal-Doped Beta Catalyst

To further enhance the high-temperature activity of Fe/Beta catalysts, some researchers have attempted to incorporate small amounts of rare earth elements into Fe/Beta catalysts. Zhang et al. [41] prepared Ce-Fe/Beta catalysts using H-Beta (Si/Al = 25), FeCl₂, and Ce(NO₃)₃ as raw materials via an isovolumetric impregnation method. The study revealed that at a Ce doping level of 0.81%, the activity window of the Ce-Fe/Beta catalyst expanded to 311~683 °C (Figure 12a). Ce doping promoted the adsorption and activation of NO_x species, enhanced the adsorption capacity for NH₃ and NO, and increased the formation of reaction intermediates, thereby improving the NO_x removal activity of the Fe/Beta catalyst. After 24 h of hydrothermal aging at 750 °C (Figure 12b), the Fe/Beta catalyst exhibited the most significant performance decline, with NO_x conversion decreasing by approximately 12%. The Ce-Fe/Beta catalyst showed only a 3% reduction in NO_x conversion, indicating that Ce incorporation enhances the catalyst’s hydrothermal stability. In situ DRIFTS experiments revealed coexistence of the L-H and E-R pathways in the SCR reaction, with the mechanism depicted in Figure 13. First, NO coordinated with hydroxyl groups forms HNO₂ species, which react with NH₃ at Lewis acid sites to generate intermediate NH₄NO₂. These intermediates readily decompose into N₂ and H₂O. Second, NH₄⁺ at Brønsted acidic sites reacts with HNO₂ species to form NH₄NO₂, which subsequently decomposes into N₂ and H₂O. Third, NH₃ adsorbed at Lewis acid sites reacts with adsorbed oxygen to form surface amine species -NH₂ and -OH. The -NH₂ species reacts with gaseous NO to form NH₂NO, an unstable species that readily decomposes into N₂ and H₂O. Finally, NH₃ at Lewis acid sites reacts with NO₃⁻ species and -OH to form NH₄NO₃, which subsequently decomposes into N₂ and H₂O. The unstable NH₄NO₃ reacts with adsorbed NO to form NH₄NO₂ and NO₂, where NO₂ can further participate in reactions with NH₃ species, thereby achieving rapid SCR.

Sultana et al. [98] investigated the effects of different rare earth (La, Pr, Gd) modifications on Fe/Beta catalysts. The study found that the Gd-modified Gd-Fe/Beta catalyst exhibited optimal SCR activity, achieving NO_x conversion rates exceeding 95% within the 450~600 °C range (Figure 14). Moreover, the catalyst retained high hydrothermal stability even after hydrothermal aging at 800 °C. The incorporation of rare earth ions reduced the agglomeration of active iron species and stabilized the zeolite structure, thereby enhancing both SCR activity and hydrothermal stability. However, different methods for introducing rare earth cations into the zeolite matrix exerted varying effects on NO_x reduction activity. Therefore, Asima Sultana et al. incorporated Gd into Fe/Beta via co-ion exchange (CIE), subsequent ion exchange (SIE), impregnation (IMP), and physical mixing (PM) methods. The effects of different preparation methods on the activity of Gd-Fe/Beta catalysts were investigated. Figure 15 shows the SCR activity of these catalysts at 600 °C. The impregnation-prepared catalyst exhibited the highest activity, while the ion-exchange-prepared catalyst showed relatively lower activity. This activity difference likely stems from varying exchange amounts of Gd and Fe within the zeolite. In ion exchange, precise control of rare earth metal loading is challenging due to steric hindrance limitations in the liquid phase, where extensive hydrated metal cations form.

In summary, Fe/Beta catalysts exhibit superior NH₃-SCR reactivity and stability compared to Fe/ZSM-5. Employing ion exchange techniques with Beta zeolites featuring relatively low Si/Al ratios as supports, and utilizing Fe as the primary active component supplemented with rare earth elements such as Ce and Gd, enhances the catalyst’s high-temperature catalytic activity and hydrothermal stability. However, the highly complex structure of Beta zeolites (e.g., high defect density and presence of two polymorphic structures) adversely affects hydrothermal stability. Furthermore, the active sites and reaction mechanisms for NH₃-SCR on Fe/Beta catalysts require further confirmation, making it an unsuitable choice for fundamental research.

5. SSZ-13 Support

SSZ-13 (with CHA topology) is a mesoporous zeolite composed of silicon, aluminum, and oxygen, garnering significant attention due to its unique octahedral cage structure. Compared to macroporous zeolites such as MFI and BEA, zeolites with CHA structures not only possess high specific surface areas and abundant acid sites, but their uniform small pore diameters also confer exceptional molecular shape-selective catalytic capabilities. More importantly, the CHA framework exhibits exceptional stability under high-temperature hydrothermal conditions, effectively suppressing de-alumination and structural collapse. This property demonstrates significant application potential for high-temperature NH₃-SCR applications. This section systematically reviews recent research advances on single-metal and multi-metal modified SSZ-13 catalysts, focusing on the interaction between active metal species and the CHA framework, as well as the underlying catalytic reaction mechanisms.

5.1. Single Metal-Doped SSZ-13 Catalyst

Among single-metal-doped SSZ-13 catalysts, Fe and Cu represent the two most extensively studied and representative active sites, each exhibiting distinct advantages and challenges across different temperature ranges and reaction environments.

Fe/SSZ-13 catalysts have garnered significant attention for their outstanding high-temperature activity. However, the effective construction of their active sites remains a major challenge. Since the ionic diameter of hydrated Fe³⁺ (9 Å) [99] is significantly larger than the pore opening size of the CHA structure (3.8 Å) [59], conventional ion exchange struggles to introduce it into the channels. Therefore, iron precursors are typically maintained in the Fe²⁺ state during exchange under an inert atmosphere (e.g., N₂) to enhance exchange efficiency [100]. Gao et al. [101] successfully synthesized Fe/SSZ-13 catalysts using this strategy. At an Fe loading of 1.2 wt%, the catalyst achieved over 80% NO conversion within the 350~550 °C temperature range. The study further indicates that different types of active iron species dominate at distinct temperature ranges: isolated Fe³⁺ ions serve as the active sites for low-temperature SCR reactions, while oligomeric iron clusters significantly contribute to high-temperature catalytic activity. Liu et al. [102] investigated the effects of calcination temperature on iron species distribution and catalytic performance in Fe/SSZ-13 catalysts through systematic characterization and kinetic analysis. Results indicate that while elevated calcination temperatures weaken low-temperature activity, they enhance high-temperature catalytic performance. This occurs because higher calcination temperatures induce slight aggregation of isolated Fe³⁺ ions into dimers or oligomeric iron clusters, thereby boosting high-temperature activity (achieving over 90% NO conversion within the 380~575 °C range). Additionally, Chen et al. [103] discovered that treating SSZ-13 zeolite with HNO₃ releases Brønsted acid sites partially covered by Fe_xO_y species and removes some framework aluminum to form strong Lewis acid sites, further optimizing catalytic performance. Although the aforementioned methods enhance the activity of Fe/SSZ-13 catalysts, their preparation processes remain relatively complex, and their high-temperature hydrothermal stability still requires improvement. These factors limit their large-scale industrial application to some extent.

In addition to Fe-based SSZ-13 catalysts, Cu-based systems have also become a research focus due to their broader active temperature window and superior catalytic performance. Early studies by Kwak et al. [104] demonstrated that Cu/SSZ-13 exhibits significantly higher activity than Cu/ZSM-5 and Cu/Beta molecular sieves, maintaining over 80% NO_x conversion efficiency between 200~550 °C. Its performance is highly dependent on the morphology and distribution of Cu species. Research indicates that preparation methods decisively influence the state of Cu species. For instance, Han et al. [105] discovered that catalysts synthesized via hydrothermal methods exhibit more stable Cu²⁺ sites within the hexagonal ring structure compared to those prepared by impregnation or ion exchange. These catalysts also demonstrate stronger adsorption capacities for NH₃ and NO, resulting in superior and stable catalytic activity across the 215~600 °C temperature range. Furthermore, the choice of copper precursor directly governs Cu²⁺ placement and catalytic behavior. Wang et al. [106] systematically compared the effects of different copper sources (Cu(NO₃)₂, CuSO₄, CuCl₂, and Cu(CH₃COO)₂). They found that when using Cu(NO₃)₂ and CuSO₄, Cu²⁺ predominantly distributed in octahedral sites; while Cu²⁺ preferentially localized in the hexagonal ring with CuCl₂ and Cu(CH₃COO)₂, the latter facilitating Cu²⁺/Cu⁺ redox cycling due to stronger charge displacement. All catalysts achieved over 90% NO conversion and near-100% N₂ selectivity across the 200~680 °C temperature range (Figure 16). Notably, hydrothermal treatment induces Cu species restructuring, promoting CuO formation and enhancing NO_x adsorption. This paradoxically boosts high-temperature activity while shifting the reaction pathway from the L-H mechanism to the E-R mechanism (Figure 17).

In summary, by optimizing preparation strategies and precursor selection, the chemical environment and spatial distribution of Cu species within SSZ-13 can be precisely controlled, yielding Cu/SSZ-13 catalysts that combine a broad active window with high hydrothermal stability. However, further in situ characterization is required to elucidate the reaction mechanism and species dynamics evolution.

5.2. Composite Metal-Doped SSZ-13 Catalyst

The high-temperature activity and hydrothermal stability of single metal modified SSZ-13 catalysts remain limited. Consequently, introducing a second metal to form bifunctional active sites has emerged as an effective modification strategy [107,108,109]. Depending on the core function of the second metal, its primary roles can be categorized into two types: first, stabilizing the zeolite framework and suppressing sintering of the active components [110,111,112,113]; second, introducing new active sites and synergistically broadening the catalytic temperature window [114,115,116,117,118]. It is worth noting that multiple metal modifiers can simultaneously possess both functions, thereby synergistically enhancing the overall performance of the catalyst.

Among these, Zr serves as a typical representative for stabilizing the framework and suppressing sintering. Research by Chen et al. [110] indicates that introducing Zr significantly enhances the hydrothermal stability of Fe/SSZ-13 catalysts. After harsh hydrothermal aging at 750 °C, the activity of unmodified Fe/SSZ-13 declined sharply, whereas Zr-Fe/SSZ-13 (especially at a Zr loading of 0.31 wt%) maintained over 90% NO conversion efficiency within the 430~575 °C range. Characterization analyses indicate that Zr introduction optimizes Fe species dispersion, effectively suppressing migration and sintering during hydrothermal aging, thereby stabilizing the catalyst structure. Furthermore, in situ DRIFTS studies reveal that the SCR reaction on Fe/SSZ-13 and Zr-Fe/SSZ-13 catalysts proceeds via three pathways (Figure 18). Zr doping did not alter the SCR reaction pathways. It not only promoted Fe dispersion and the formation of more active sites but also enhanced NH₃ and NO adsorption and the formation of reaction intermediates (such as -NH₂, -HNO₂, and -NO₂), thereby positively promoting the reaction.

In addition to the aforementioned Zr, Ti and Zn are also typical elements dedicated to stabilizing the structure. Liu et al. [111] found that post-treatment modification of Cu/SSZ-13 with TiCl₄ solution resulted in an optimized Ti-Cu/SSZ-13 catalyst capable of achieving over 90% NO_x conversion efficiency between 250~640 °C, while exhibiting excellent hydrothermal stability. The incorporation of Ti increases TiO₂ content within the zeolite, where highly dispersed TiO₂ facilitates high-temperature denitrification reactions. Moreover, Ti partially protects the zeolite framework during high-temperature hydrothermal processes, conferring exceptional hydrothermal stability. Particularly noteworthy is the behavior of zinc. Although Zn is considered toxic in certain scenarios, Xu et al. [112] discovered that incorporating Zn into the Cu/SSZ-13 system via a one-pot method yields a high-performance catalyst. It achieves over 90% NO_x conversion between 200~600 °C and exhibits significantly superior hydrothermal stability compared to conventional Cu/SSZ-13 (Figure 19). Mechanism studies (Figure 20) indicate that Zn formation creates more stable complexes such as [Zn-OH]-Z and [Cu-O-Zn]-Z. These complexes act as “anchor points,” effectively suppressing Cu²⁺ ion migration and sintering during high-temperature hydrothermal aging, thereby stabilizing the catalytic structure.

In addition to transition metal doping, rare earth element doping exhibits similar effects. Addressing the insufficient hydrothermal stability of high-aluminum Cu/SSZ-13 catalysts, Zhao et al. [113] proposed modifying the Cu/SSZ-13 catalyst by introducing different rare earth ions (Ce, La, Sm, Y, Yb). Results indicate that among all tested rare earth elements, Y exhibits the most pronounced effect. Y effectively stabilizes the alumina framework of the molecular sieve, protects Brønsted acid sites, and promotes the preferential occupation of Cu²⁺ ions at highly stable hexagonal ring sites. This significantly enhances the catalyst’s NH₃-SCR activity and stability after harsh hydrothermal aging at 800 °C.

Unlike the metals used for structural stabilization mentioned above, another class of research focuses on synergistic effects achieved by introducing metals with variable valence states. This approach creates new high-temperature active sites, thereby further expanding the temperature window. In this regard, Fe and Nb demonstrate significant potential. Fe species not only offer low cost but also introduce highly effective high-temperature active sites into the system. Studies [114,115,116] demonstrate that incorporating Fe—whether via the one-pot method or ion exchange—significantly enhances the catalyst’s high-temperature performance, enabling it to maintain over 90% NO conversion across a broad temperature range of 200~625 °C (Figure 21). Research further indicates that H₂O presence may enhance high-temperature activity by suppressing non-selective NH₃ oxidation. This performance boost stems from synergistic and complementary active sites: isolated Cu²⁺ species dominate low-temperature SCR reactions, while introduced monomeric Fe³⁺ species serve as highly efficient high-temperature active centers. This synergistic effect significantly broadens the operational temperature window of Fe-Cu/SSZ-13 compared to single Cu systems. However, it is important to note that Fe loading has an optimal value; excessive doping leads to destruction of the zeolite structure and formation of inactive Fe₂O₃ clusters, resulting in performance degradation.

Nb exhibits multiple valence states and demonstrates significant potential similar to Fe in providing new active sites and synergistic effects. Studies by Wang et al. [117] and Liu et al. [118] both confirm that introducing Nb significantly broadens the activity window of Cu/SSZ-13. For instance, after loading 0.7 wt% Nb, the temperature window for the Nb_0.7-Cu₂/SSZ-13 catalyst achieving >90% NO_x conversion expanded from 275~580 °C to 200~625 °C; while Nb modification of the catalyst with the optimal Cu loading (2.8 wt%) substantially enhanced its high-temperature performance (Figure 22b), extending the active window to 700 °C. This enhancement primarily stems from two mechanisms: First, the formation of Nb=O significantly increases the number of Lewis and Brønsted acid sites on the catalyst surface, enhancing NH₃ adsorption capacity; Second, Nb introduction optimized Cu species distribution, increasing the number of active Cu²⁺ sites and thereby enhancing overall redox capability. In situ DRIFTS studies confirmed that both Cu/SSZ-13 and Nb-Cu/SSZ-13 catalysts follow the L-H mechanism (Figure 23).

In summary, modifying SSZ-13 with secondary metals—whether for structural stabilization (e.g., Zr, Ti, Zn, Sm) or synergistic enhancement (e.g., Fe, Nb)—significantly improves its high-temperature catalytic performance and hydrothermal stability. These strategies offer diverse and viable pathways for designing efficient SCR catalysts tailored to the extreme operating conditions of gas turbines.

6. SAPO-34 Support

As discussed in Section 5, SAPO-34 with its CHA topology also exhibits core advantages such as high hydrothermal stability and shape-selective catalysis [119,120,121,122]. However, its unique silicon-phosphorus-aluminum (SAPO) framework endows it with physicochemical properties distinct from SSZ-13, most notably milder framework acidity. This section focuses on research progress concerning SAPO-34-based catalysts, emphasizing the structure and reaction mechanisms of single-metal active sites represented by Cu and Fe. It also analyzes the synergistic optimization mechanisms of composite metal doping on their activity distribution, redox performance, and hydrothermal stability.

6.1. Single Metal-Doped SAPO-34 Catalyst

Currently, research on single-metal-doped SAPO-34 catalysts has primarily focused on Cu-based systems, with most employing ion exchange methods for preparation. Although Fe-based zeolites typically exhibit excellent catalytic activity and hydrothermal stability at high temperatures, SAPO-34, being a mesoporous zeolite similar to SSZ-13, presents significant steric hindrance to Fe³⁺ ion incorporation due to its pore structure. This hindrance makes it difficult to effectively load iron active sites using conventional ion exchange methods. To address this, some researchers have developed a one-pot hydrothermal synthesis strategy to directly incorporate Fe species into the SAPO-34 framework. This method not only offers a simple process with good reproducibility but also allows flexible control over the loading amount of active components, making it more conducive to constructing efficient SCR catalysts with a wide temperature window. For instance, the one-pot Fe/SAPO-34 catalyst (Fe loading: 1.03 wt%) synthesized by Andonova et al. [123] achieved over 80% NO_x conversion within the 300~600 °C range. Notably, during hydrothermal aging, migration of Fe species formed more stable active sites, further enhancing catalytic activity.

However, due to the relatively complex synthesis process of Fe/SAPO-34, related research remains far less extensive than that on Cu-based systems. Although Cu/SAPO-34 catalysts exhibit high activity and N₂ selectivity in the low-to-medium temperature range (e.g., around 200 °C), their high-temperature performance often declines significantly due to excessive NH₃ oxidation. As reported by Wang et al. [124], Cu/SAPO-34 achieves a NO conversion rate of 95% at 200 °C, but its activity continuously declines with increasing temperature, reaching only about 60% at 600 °C.

To improve the high-temperature performance of Cu/SAPO-34, researchers have explored various preparation methods. Zhang et al. [125] successfully synthesized a Cu/SAPO-34 catalyst with over 90% activity across 260~575 °C using solid-state ion exchange. Tang et al. [126] achieved NO conversion rates exceeding 80% between 200~600 °C for Cu/SAPO-34 obtained via a one-pot method. Wang et al. [127] employed a one-step hydrothermal synthesis to prepare Cu/SAPO-34 on cordierite support. At a space velocity of 12,000 h⁻¹, it exhibited nearly 100% NO conversion between 300~670 °C, demonstrating outstanding high-temperature activity and hydrothermal stability. This indicates that optimizing preparation methods can effectively broaden the operating temperature window and enhance high-temperature sintering resistance. Zhang et al. [125] investigated the reaction mechanism on Cu/SAPO-34 catalysts, revealing that the L-H mechanism dominates on these catalysts (Figure 24).

6.2. Composite Metal-Doped SAPO-34 Catalyst

To enhance the high-temperature NH₃-SCR performance of Cu/SAPO-34 catalysts, introducing a second metal to construct bifunctional active sites has become a key strategy. Studies indicate that doping with metals such as Fe and Ce can significantly modulate the distribution of catalytic active sites, surface acidity, and redox capabilities, thereby synergistically enhancing both broad-temperature catalytic activity and hydrothermal stability.

Zhao et al. [128] incorporated Fe into Cu/SAPO-34 via impregnation. They found that appropriate Fe loading promotes the transformation of bulk CuO into isolated Cu²⁺ sites while introducing additional isolated Fe³⁺ active sites. This enables Fe-Cu/SAPO-34 to maintain NO_x conversion rates above 80% across the 200~550 °C range. The Cu-Fe/SAPO-34 catalyst prepared by ion exchange by Doan et al. [129] exhibited NO_x conversion rates exceeding 90% across the broad temperature range of 250~600 °C, maintaining a 95% removal rate even at 600 °C (Figure 25). The coexistence of Fe and Cu not only enhances the surface acidity of the catalyst and promotes NH₃ adsorption but also provides more isolated Cu²⁺ and oligomeric Fe_xO_y species, thereby broadening the active temperature window.

On the other hand, the introduction of the rare earth element Ce has also been proven to be an effective approach for enhancing high-temperature performance. Wang et al. [130] found that Ce modification significantly enhances the NO removal capacity of Cu/SAPO-34 in the high-temperature range of 375~570 °C. Mechanistic studies indicate that Ce doping increases the surface acidity, Cu⁺/Cu²⁺ ratio, and NO adsorption-oxidation capability of the catalyst. Mao et al. [131] further investigated Ce’s impact on hydrothermal stability, noting that Ce introduction helps stabilize the CHA framework, maintain medium-to-weak acidic sites, and suppress Cu species migration and agglomeration. Optimal Ce loading (e.g., Cu/Ce mass ratio of 4:5) effectively mitigates activity decline and structural damage caused by hydrothermal aging at 750 °C.

However, current research on SAPO-34 catalysts remains predominantly focused on the low-to-medium temperature range, with limited studies reported for the high-temperature range. Moreover, the reaction mechanism of SAPO-34 catalysts under high-temperature conditions remains unclear. Consequently, SAPO-34 supports are not an optimal choice for high-temperature denitrification applications.

7. Other Support

The preceding sections have systematically explored the application of mainstream zeolite supports such as ZSM-5, Beta, SSZ-13, and SAPO-34 in high-temperature NH₃-SCR systems. To broaden the catalytic material portfolio, researchers continue to develop zeolites featuring novel topologies, unique acidities, or improved economic viability to overcome existing performance limitations. This chapter summarizes several emerging or niche zeolite materials with development potential beyond the aforementioned mainstream supports (as shown in

Table 2. Preparation conditions and SCR performance of other emerging zeolite supported catalysts.

Catalyst	Preparation Method	Reaction Condition	Denitrification Activity	Refs.
Cu/SSZ-16	ion-exchange	[NO] = [NH3] = 600 ppm, [O2] = 6%, [H2O] = 5%, GHSV = 400,000 h−1	(300~670 °C) > 95%	[132]
Cu/SSZ-16	ion-exchange	[NO] = [NH3] = 500 ppm, [O2] = 5%, [H2O] = 5%, GHSV = 100,000 h−1	(175~600 °C) > 90%	[133]
Cu/UZM-9	ion-exchange	[NO] = [NH3] = 500 ppm, [O2] = 5%, [H2O] = 10%, GHSV = 100,000 h−1	(250~700 °C) > 90%	[134]
Cu/SSZ-39	ion-exchange	[NO] = [NH3] = 500 ppm, [O2] = 5%, [H2O] = 5%, GHSV = 190,000 h−1	(250~550 °C) > 90%	[135]
Cu/SSZ-39	ion-exchange	[NO] = [NH3] = 500 ppm, [O2] = 5%, [H2O] = 5%, GHSV = 200,000 h−1	(200~550 °C) > 90%	[136]
Cu/ZJM-7	ion-exchange	[NO] = [NH3] = 500 ppm, [O2] = 5%, [H2O] = 5%, GHSV = 80,000 h−1	(200~550 °C) > 90%	[137]
Cu/SUZ-4	ion-exchange	[NO] = [NH3] = 500 ppm, [O2] = 5%, [H2O] = 5%, GHSV = 200,000 h−1	(250~550 °C) > 90%	[138]
Cu/LTA	ion-exchange	[NO] = [NH3] = 500 ppm, [O2] = 5%, [H2O] = 10%, GHSV = 100,000 h−1	(220~600 °C) > 90%	[139]
Cu/ERI	ion-exchange	[NO] = [NH3] = 500 ppm, [O2] = 5%, [H2O] = 3%, GHSV = 50,000 h−1	(250~600 °C) > 90%	[140]
Fe/SSZ-39	one-pot	[NO] = 50 ppm, [NH3] = 60 ppm, [O2] = 10%, [H2O] = 10%	(330~550 °C) > 90%	[141]
Fe/UZM-35	one-pot	[NO] = [NH3] = 500 ppm, [O2] = 5%, [H2O] = 10%, GHSV = 100,000 h−1	(250~600 °C) > 90%	[142]
Fe/MCM-56	ion-exchange	[NO] = [NH3] = 2500 ppm, [O2] = 2.5%	(430~600 °C) > 80%	[143]
Cu/SAPO-18	incipient wetness impregnation	[NO] = 1000 ppm, [NH3] = 1100 ppm, [O2] = 5%, [H2O] = 10%, GHSV = 30,000 h−1	(230~575 °C) > 80%	[144]
Ce-Cu/SAPO-18	ion-exchange	[NO] = [NH3] = 500 ppm, [O2] = 14%, [H2O] = 5%, GHSV = 130,000 h−1	(200~600 °C) > 90%	[145]
Cu/SAPO-44	ion-exchange	[NO] = [NH3] = 500 ppm, [O2] = 5.3%, [H2O] = 10%, GHSV = 100,000 h−1	(200~550 °C) > 90%	[146]
Cu/SAPO-47	ion-exchange	[NO] = [NH3] = 500 ppm, [O2] = 5%, [H2O] = 5%, GHSV = 100,000 h−1	(250~600 °C) > 90%	[147]
Cu/SAPO-18/34	incipient wetness impregnation	[NO] = 1000 ppm, [NH3] = 1100 ppm, [O2] = 5%, [H2O] = 10%, GHSV = 30,000 h−1	(200~575 °C) > 90%	[148]

). Although related research remains in its developmental stage, these materials have demonstrated unique advantages in aspects such as activity windows, hydrothermal stability, or synthesis costs, offering new avenues for designing high-temperature SCR catalysts.

As shown in Table 2, small-pore zeolites such as SSZ-16 (AFX) and UZM-9 (LTA) demonstrate outstanding potential in high-temperature SCR. Among these, the performance of the Cu/SSZ-16 catalyst is particularly noteworthy. Research by Chen et al. [132] systematically investigated the impact of Cu loading on SSZ-16 performance. The study revealed that at a Cu loading of 2.21 wt% (Cu_0.091/SSZ-16), the catalyst achieves > 95% NO conversion within the 300~670 °C temperature window. Notably, even samples with low Cu loading maintained 90% activity at the extreme high temperature of 700 °C (Figure 26a), with nearly all samples exhibiting N₂ selectivity close to 100% (Figure 26b). This outstanding high-temperature performance stems from its unique resistance to Cu species sintering (inhibiting the conversion of [CuOH]⁺ to CuO_x) and relatively weak NH₃ adsorption strength, effectively suppressing non-selective NH₃ oxidation. However, the hydrothermal stability of the catalyst is critical for practical applications. After aging at 650 °C and 750 °C [133], the Cu/SSZ-16 catalyst exhibited minimal activity loss due to the retention of more active Cu²⁺-2Z species; However, when the calcination temperature reached 850 °C, the AFX framework collapsed. The interaction between active Cu species and the framework weakened, leading to their conversion into inactive CuO_x and Cu(AlO₂)₂ species, resulting in a sharp decline in activity (Figure 27).

Compared to SSZ-16, UZM-9 also exhibits a broad activity window. The Cu/UZM-9 (2.90 wt%) catalyst prepared by Wei et al. [134] achieved NO conversion rates >90% within the 250~700 °C range, reaching 100% between 300 and 600 °C (Figure 28a). However, the formation of the byproduct N₂O increases significantly at high Cu loadings (Figure 28b), posing challenges to its N₂ selectivity.

Among the SAPO family members excluding SAPO-34, SAPO-18 with an AEI topology has drawn attention due to its similarity to the CHA structure [144]. However, single Cu modification exhibits limited high-temperature performance, prompting researchers to frequently employ rare earth element doping for optimization. Research by Han [145] and Gao [149] et al. indicates that among various rare earth elements, Ce exhibits the most pronounced modification effect. The Ce-Cu/SAPO-18 catalyst prepared by introducing 1.24 wt% Ce achieves over 90% NO conversion within the 200~600 °C range, with N₂ selectivity approaching 100% (Figure 29). Mechanistic studies reveal that Ce doping promotes the migration of active Cu²⁺ species to energetically favorable positions, thereby enhancing catalytic activity. Concurrently, Ce mitigates framework hydrolysis during hydrothermal aging, significantly improving catalyst durability. In situ DRIFTS results indicate that reactions on this catalyst follow a mixed L-H and E-R mechanism, with the lower-energy L-H pathway dominating.

Beyond introducing secondary metals, constructing co-existing zeolites represents another innovative approach to optimizing SAPO catalyst performance. While pure-phase Cu/SAPO-34 exhibits excellent hydrothermal stability, its high-temperature SCR activity remains suboptimal. Conversely, pure-phase Cu/SAPO-18 demonstrates distinct reaction characteristics due to its lower Brønsted acid site density and strength, yet its overall performance still holds room for improvement. To simultaneously overcome the limitations of single-phase materials, Zhang et al. [148] innovatively synthesized a Cu/SAPO-18/34 symbiotic zeolite. Characterization and performance testing revealed that this symbiotic structure successfully integrates the advantages of both individual phases, achieving synergistic enhancement: its NH₃-SCR activity and hydrothermal stability significantly outperform those of either Cu/SAPO-18 or Cu/SAPO-34 alone. Specifically, this catalyst exhibits NO conversion rates exceeding 90% across the 200~575 °C temperature range and maintains over 80% NO conversion even after severe hydrothermal aging at 800 °C. The mechanism behind its enhanced performance can be attributed to: the symbiotic structure provides a higher number and intensity of acid sites, which not only promotes the generation of more active isolated copper ions (Cu²⁺ and Cu⁺) and suppresses the formation of inert CuO_x clusters, but also facilitates the adsorption and activation of reactant molecules (NH₃ and NO). In situ DRIFTS studies reveal the generation of multiple intermediate species including -NH₂, NH₄⁺, NO₃⁻, and NO₂⁻ during the reaction. Unlike Cu/SAPO-34, which follows solely the L-H mechanism, the Cu/SAPO-18/34 co-supported zeolite, similar to Cu/SAPO-18, follows a mixed L-H and E-R reaction mechanism. Due to the lower reaction barrier, the L-H mechanism dominates. This work provides a novel approach for designing and developing a new generation of high-performance SAPO-based SCR catalysts.

In summary, non-mainstream zeolite supports such as SSZ-16, UZM-9, and SAPO-18 demonstrate significant application potential in high-temperature NH₃-SCR systems. This chapter’s discussion indicates that these materials typically exhibit unique performance advantages in specific aspects: For instance, SSZ-16’s distinctive pore architecture endows Cu species with exceptional sintering resistance and high N₂ selectivity, enabling efficient denitrification across an ultra-wide temperature window (300~700 °C). UZM-9 similarly exhibits broad activity ranges, though its N₂O generation under high loading requires urgent resolution. SAPO-18 modified with rare earth elements like Ce achieves synergistic improvements in both activity and hydrothermal stability, with more in-depth mechanistic studies. However, it must be clearly recognized that these “other” supports still face significant common challenges in transitioning from laboratory research to industrial application. First, their synthesis costs are often higher, with more expensive templates or more complex processes, directly limiting the feasibility of large-scale production. Second, for most materials, their long-term hydrothermal stability has not been systematically and fully evaluated, which is a critical test in the actual operating environment of gas turbines. Finally, relevant research is currently largely confined to ideal laboratory conditions, lacking performance verification under pilot-scale or actual flue gas conditions.

8. Comparison of Catalytic Characteristics and Mechanisms for Fe/Cu-Based Zeolite Catalysts with Doping Modification in High-Temperature NH₃-SCR

To more clearly elucidate the regulatory effects of different metallic elements,

Table 3. Effect of different metal doping on the performance of Fe/Cu-Based zeolite catalysts.

Primary Active Metals	Doped Modifying Elements	Reaction Mechanism	Advantages After Doping	Core Limitations	Typical Zeolite Support	References
Fe	—	E-R or L-H	—	Severe oxidation of NH3 at high temperatures, with poor activity above 600 °C	ZSM-5, Beta, SSZ-13, SAPO-34	[38,39,91,92,93,94,95,96,100,101,102,123]
	Zr	E-R	Enhance the dispersion of iron species, increase acidic sites, promote the formation of intermediates, and improve hydrothermal stability	The preparation method is complex and may clog the channels	SSZ-13	[110]
	Ce	E-R or L-H	Promote NOx adsorption and activation, enhancing high-temperature SCR activity and hydrothermal stability	Severe oxidation of NH3 at high temperatures and poor hydrothermal stability	ZSM-5, Beta	[41,85,86,87]
	Gd	E-R or L-H	Inhibit the agglomeration of iron species and enhance the stability of the zeolite framework	The high cost of the rare earth element Gd	Beta	[98]
	Ni, W	E-R or L-H	Significantly inhibits the oxidation of NH3	The preparation method is complex	ZSM-5	[90]
Cu	—	E-R or L-H	—	Poor high-temperature activity	ZSM-5, Beta, SSZ-13, SAPO-34	[104,124,125,126]
	—	E-R or L-H	—	The preparation method is complex and costly, and it produces the byproduct N2O	SSZ-16, UZM-9	[132,133,134]
	Fe	E-R or L-H	Significantly enhanced high-temperature activity	The preparation method is complex	SSZ-13, SAPO-34	[114,128,129]
	Nb	L-H	The formed Nb=O can increase acidity, optimize the species distribution of Cu, and enhance redox capability	The optimal load capacity of Nb is critical, but it comes at a high cost	SSZ-13	[117,118]
	Zn	E-R or L-H	Inhibit Cu2+ migration and sintering to enhance hydrothermal stability	Zn may be toxic under certain conditions, and its framework may collapse at extremely high temperatures	SSZ-13	[112]
	Ti	E-R or L-H	Protect the molecular sieve framework to enhance hydrothermal stability	Ti modification is costly and requires post-processing	SSZ-13	[111]
	Ce	E-R and L-H	Enhance high-temperature performance and inhibit backbone hydrolysis	The preparation method is complex	SAPO-18	[145]
	Y	E-R or L-H	Stabilize the molecular sieve framework and protect Brønsted acid sites	The cost of the rare earth element Y is high	SSZ-13	[113]

systematically summarizes the influence of various metal doping on the performance of Fe/Cu-based zeolite catalysts.

9. Outlook and Future Research Directions

Despite demonstrating significant potential in high-temperature NH₃-SCR applications, zeolite-based catalysts face substantial challenges in transitioning from laboratory research to commercial gas turbine deployment. Based on the systematic analysis presented in this review, future research must achieve breakthroughs across four dimensions—mechanism understanding, design methodologies, engineering validation, and sustainability—to bridge the gap between fundamental research and industrial demands.

9.1. Deepening Mechanistic Understanding Through Ultra-High-Temperature In Situ Characterization

Currently, research on reaction mechanisms at temperatures above 600 °C remains fragmented, particularly regarding the dynamic evolution of active sites and the competitive mechanisms between primary and secondary reactions. Future studies should leverage advanced in situ/operational characterization techniques to capture real-time structural changes:

For iron-based zeolites (such as Fe/ZSM-5, Fe/SSZ-13), in situ X-ray absorption spectroscopy (XAS) should be employed to track the valence state transition (Fe³⁺↔Fe²⁺) of iron species and the agglomeration behavior of oligomeric Fe_xO_y clusters under high-temperature (500~700 °C) and high-steam-vapor conditions. This will clarify why isolated Fe³⁺ sites remain stable at 750 °C, while oligomeric clusters readily sinter [38,102].

For copper-based zeolites (e.g., Cu/SSZ-13, Cu/SAPO-34), it is necessary to combine in situ diffuse reflect Fourier transform infrared spectroscopy (DRIFTS) with mass spectrometry (MS) to identify key intermediates (e.g., -NH₂, NO₂⁻, NH₄⁺) and quantify their conversion rates to clarify whether the reaction at 600 °C primarily follows the L-H mechanism or the E-R mechanism [106,125].

For composite metal modification systems (e.g., Ce-Fe/Beta, Zr-Cu/SSZ-13), in situ electron paramagnetic resonance (EPR) can characterize intermetallic synergistic effects (e.g., Ce³⁺/Ce⁴⁺ redox cycling promoting Fe³⁺ dispersion) and their influence on suppressing NH₃ overoxidation [41,110].

9.2. Advancing Rational Catalyst Design Using Density Functional Theory and Machine Learning

Traditional trial-and-error methods are inefficient for optimizing zeolite catalysts; integrating theoretical calculations and artificial intelligence is critical to accelerating development:

Density functional theory (DFT) calculations provide a powerful means to unravel reaction mechanisms and material properties at the atomic scale, offering quantitative insights that can break the reliance on traditional trial-and-error approaches. For instance, the team led by Academician He Hong [150] utilized DFT to elucidate the stabilization mechanism of Yttrium (Y)-modified Cu/SSZ-39 catalysts. Their calculations revealed that Y doping significantly increases the energy barrier for breaking the framework Al-O bonds, thereby effectively suppressing dealumination during hydrothermal aging. To enhance SO₂ tolerance, a research group [151] from Fuzhou University employed DFT computations to investigate ZnO-modified Fe-Cu/SSZ-13. Their study found that SO₂ exhibits an extremely high adsorption energy (−7.62 eV) at the ZnO interface sites, compared to a much lower adsorption energy (−0.65 eV) at the Cu active sites. This indicates a “sacrificial protection” mechanism, where SO₂ molecules are preferentially and strongly chemisorbed at the ZnO sites, effectively shielding the intrinsic Cu active centers. This mechanism, further verified by in situ XAFS and DRIFTS, enables the catalyst to maintain over 90% NO_x conversion under harsh conditions containing 20 ppm SO₂ and 20 vol% H₂O. For Cu/SSZ-13 with CHA topology, European research teams [152] employed the PBE+U functional (with a Hubbard U value of 6 eV for Cu 3d orbitals) combined with Grimme-D3 van der Waals correction. They calculated that the energy barrier for the conversion of Z₂Cu²⁺ to ZCu⁺ is 0.82 eV, and the co-adsorption energy of NO + O₂ is 1.27 eV. This confirms that single Cu⁺ ions dominate the high-temperature reaction. Furthermore, through transition state search using the Climbing Image Nudged Elastic Band (CI-NEB) method, they verified that the formation of NO₃⁻ is the rate-determining step, with an activation energy of 0.89 eV. Furthermore, DFT studies on OFF/ERI intergrown zeolites demonstrated that the unique intergrowth interface leads to a substantially higher Cu²⁺ exchange energy (177.3 kJ/mol) compared to that of individual ERI (91.6 kJ/mol) or OFF (76.3 kJ/mol) structures. This theoretically explains the material’s superior ability to stabilize isolated Cu²⁺ ions and its exceptional hydrothermal stability [153].

While DFT unravels atomic-scale mechanisms for diverse zeolite systems, machine learning (ML) complements this by leveraging experimental data to guide practical synthesis. For instance, A representative study by a South Korean research team focused on Beta (BEA) and ZSM-5 (MFI) zeolites [154]. They constructed a database encompassing over 1918 data points from literature. Using a decision tree model, they achieved high prediction accuracy for NO_x conversion (89% for Beta and 85% for ZSM-5). More importantly, the model extracted directly applicable synthesis guidelines. For example, it identified that high low-temperature catalytic activity for Beta zeolite requires “Si/Al ≤ 10 and Cu loading ≥ 1.0 wt%”, whereas for ZSM-5, the condition is “Fe loading between 2.0 and 8.0 wt% and Cu loading at 4.0 wt%”. These data-driven guidelines significantly accelerate the targeted design of high-performance catalysts. Future efforts should focus on integrating DFT-derived descriptors into ML models and extending these approaches to more complex multi-component systems, ultimately paving the way for intelligent and precise catalyst design.

9.3. Reducing Synthesis Costs and Advancing Green Manufacturing

The high cost of synthesizing zeolites (especially CHA and AFX types) is the primary bottleneck hindering their industrialization. Therefore, green synthesis represents the primary direction for future development [155,156,157]. Key focuses include: developing inexpensive alternatives to template agents [158,159], establishing efficient recovery processes for template agents [160,161], and promoting template-free seed-directed synthesis methods [162,163]. Simultaneously, there is an urgent need to establish efficient spent catalyst recycling systems [164]. This involves recovering precious metals through mild leaching techniques (e.g., chelating agents) and developing direct regeneration technologies (e.g., temperature-controlled calcination) to restore catalyst activity. Building a closed-loop material lifecycle—“design-preparation-use-recycling”—is essential not only for economic viability but also aligns with green chemistry principles.

9.4. Verification of Catalyst Performance Under Actual Gas Turbine Operating Conditions

The ideal testing conditions in laboratories differ significantly from the actual operating environment of gas turbines. Future testing must involve endurance tests lasting hundreds or even thousands of hours under complex flue gas conditions characterized by high water vapor content (10~15% H₂O), wide temperature fluctuations (500~700 °C), and trace amounts of SO₂. This represents not only a necessary step in screening final catalyst candidates but also a critical approach to uncovering actual deactivation mechanisms and optimizing regeneration strategies. Ultimately, pilot-scale demonstrations must be actively advanced. Collaboration with industry is essential to validate mechanical strength, pressure drop, and thermal expansion compatibility within real systems—the final hurdle in assessing industrial viability.

For the purpose of facilitating intuitive comparison and summary, the key characteristics, performance metrics, and application potential of the various zeolite catalysts discussed above are systematically summarized in

Table 4. Comprehensive comparison of zeolite-supported high-temperature NH₃-SCR catalysts.

Zeolite Type (Topology)	Active Metal	Denitrification Activity	Hydrothermal Stability	Key Advantages	Main Limitations	Synthesis Cost	Industrial Potential
ZSM-5 (MFI)	Fe	(425~680 °C) > 80%	Low	Low cost, tunable acidity, mature synthesis	Severe oxidation of NH3 at high temperatures	Low	Medium (for mid-temperature, cost-sensitive scenarios)
	Ce, Fe	(370~600 °C) > 90%	Excellent
	W, Zr	(350~630 °C) > 90%	Moderate
	Sm, W, Zr	(380~640 °C) > 90%	Moderate
	Fe, Ni, W	(480~750 °C) > 90%	Excellent
Beta (BEA)	Fe	(310~600 °C) > 90%	Moderate	3D large pores, favorable mass transfer, high surface area	Structural defects, prone to dealumination at high temperatures	Medium	Medium (requires enhanced high-temp stability)
	Ce, Fe	(311~683 °C) > 80%	Excellent
	Gd, Fe	(450~600 °C) > 90%	Excellent
SSZ-13 (CHA)	Cu	(200~680 °C) > 90%	Excellent	Outstanding hydrothermal stability, superior shape selectivity	Expensive template, high synthesis cost	High	High (Top candidate for high-temperature use, pending cost reduction)
	Zr, Fe	(430~575 °C) > 90%
	Ti, Cu	(250~640 °C) > 90%
	Zn, Cu	(200~600 °C) > 90%
	Fe, Cu	(200~625 °C) > 90%
	Nb, Cu	(200~625 °C) > 90%
SAPO-34 (CHA)	Cu	(300~670 °C) > 90%	Good	Good hydrothermal stability, mild acidity suppresses NH3 oxidation	Activity decline at very high temperatures, complex synthesis control	Medium-High	Medium-High (A viable alternative to SSZ-13)
	Fe, Cu	(250~600 °C) > 90%	Good
SSZ-16 (AFX)	Cu	(300~670 °C) > 90%	Good	Ultra-wide temperature window, high N2 selectivity	Expensive template, potential framework collapse at very high temperature	High	Medium (Potential for ultra-wide temperature scenarios)
UZM-9 (LTA)	Cu	(250~700 °C) > 90%	Good	Broad activity window, seed-assisted synthesis possible	Increased N2O byproduct formation at high Cu loadings	Medium	Medium (require solving N2O selectivity issue)
SAPO-18 (AEI)	Ce, Cu	(200~600 °C) > 90%	Good	High N2 selectivity	Lower Brønsted acidity than SAPO-34	Medium-High	Medium-High (require solving low-cost synthetic routes issue)

.

10. Conclusions

This review systematically summarizes research progress on zeolite-based catalysts in high-temperature NH₃-SCR applications, aiming to address challenges faced by commercial V₂O₅-WO₃/TiO₂ catalysts in gas turbine high-temperature flue gases. Based on the detailed discussion above and the comprehensive comparison in Table 1, the following core conclusions can be drawn:

The framework topology of zeolites fundamentally determines their performance limits and application scenarios. ZSM-5 (MFI) and Beta (BEA) exhibit advantages in mass transfer due to their channel structures, but their potential for long-term high-temperature (>600 °C) operation is limited by framework stability. In contrast, the cage-like structures of SSZ-13/SAPO-34 (CHA) and its analogues (e.g., SSZ-16/AFX) confer exceptional hydrothermal stability, making them the most promising candidates for high-temperature gas turbine denitration—though their synthesis costs pose a major challenge.

Precision engineering of active metals and modifiers is crucial for achieving high performance. This review demonstrates that isolated Fe³⁺ sites serve as highly efficient centers driving activity at elevated temperatures (600~700 °C), while Cu²⁺ species excel in the medium-to-low temperature range. More importantly, the incorporation of modifiers such as rare earth elements (Ce, Gd, Y) and transition metals (Zr, Nb) significantly enhances catalyst performance by modulating acidity, stabilizing the framework, and generating synergistic effects. This broadens the temperature window and improves durability.

To translate these scientific insights into industrial reality, future research and development should adopt a prioritized approach. In the short term, priority should be given to the development of composite systems, such as Cu/SSZ-13 modified with Fe or other metals. These catalysts already demonstrate a broad temperature window and excellent hydrothermal stability, and further optimization could yield commercially viable solutions in the near future. For the medium to long term, the focus should shift towards overcoming the primary bottleneck of the most promising yet expensive materials—specifically, achieving a breakthrough in the green synthesis of AFX-type zeolites (e.g., SSZ-16). Success in developing low-cost, template-free, or recyclable template synthesis routes for these top-tier supports would fundamentally enhance the economic feasibility and sustainability of next-generation high-temperature SCR technologies.

In summary, the successful commercialization of zeolite-based high-temperature SCR catalysts is not achieved through breakthroughs in a single performance metric, but rather through a systematic engineering approach involving multi-objective optimization of “structure-performance-cost.” Future development hinges on synergistic innovation across four domains: mechanism research, rational design, green manufacturing processes, and engineering validation. Through sustained interdisciplinary efforts, zeolite catalysts hold promise as core technological solutions for clean and efficient gas turbine operation, poised to play a pivotal role in the global energy transition.