Highly Engineered Cr-In/H-SSZ-39 Catalyst for Enhanced Performance in CH₄-SCR of NO_x

Abstract

The selective catalytic reduction of NO_x with CH₄ (CH₄-SCR) holds the potential to simultaneously abate harmful NO_x and CH₄ greenhouse gases. In this study, a series of bimetallic M-In/H-SSZ-39 catalysts (where M represents Cr, Co, Ce, and Fe) were prepared via an ion exchange method and subsequently evaluated for their CH₄-SCR activity. The influences of the preparation parameters, including the metal ion concentration and calcination temperature, as well as the operating conditions, such as the CH₄/NO ratio, O₂ concentration, water vapor content, and gas hourly space velocity (GHSV), on the catalytic activity of the optimal Cr-In/H-SSZ-39 catalyst were meticulously examined. The results revealed that the Cr-In/H-SSZ-39 catalyst exhibited peak CH₄-SCR catalytic performance when the Cr(NO₃)₃ concentration was 0.0075 M, the In(NO₃)₃ concentration was 0.066 M, and the calcination temperature was 500 °C. Under optimal operating conditions, namely GHSV of 10,000 h⁻¹, 400 ppm NO, 800 ppm CH₄, 15 vol% O₂, and 6 vol% H₂O, the NO_x conversion rate reached 93.4%. To shed light on the excellent performance of Cr-In/H-SSZ-39 under humid conditions, a comparative analysis of the crystalline phase, chemical composition, pore structure, surface chemical state, surface acidity, and redox properties of Cr-In/H-SSZ-39 and In/H-SSZ-39 was conducted. The characterization results indicated that the incorporation of Cr into In/H-SSZ-39 enhanced its acidity and also facilitated the generation of InO⁺ active species, which promoted the oxidation of NO and the activation of CH₄, respectively. A synergistic effect was observed between Cr and In species, which significantly improved the redox properties of the catalyst. Consequently, the activated CH₄ could further interact with InO⁺ to produce carbon-containing intermediates such as HCOO⁻, which ultimately reacted with nitrate-based intermediates to yield N₂, CO₂, and H₂O.

1. Introduction

Climate change and atmospheric pollution constitute pressing global challenges confronting humanity in contemporary times [1]. As a consequence, increasingly stringent emission restrictions are driving improvements in both engine technology and fuel quality around the world. Compared to diesel, liquefied natural gas (LNG) has shown significant advantages in reducing emissions such as particulate matter (PM), sulfur oxides (SO_x), and carbon dioxide (CO₂). However, LNG engines may produce higher methane (CH₄) emissions and also have nitrogen oxide (NO_x) emission issues [2,3]. CH₄ is a greenhouse gas with high global warming potential (GWP) that is approximately 25 times (100-year horizon) greater than that of CO₂ [4]. NO_x is a prominent constituent of atmospheric pollution, posing a substantial threat to the environment and human health [5,6,7]. Therefore, the advancement of technologies focused on the simultaneous removal of NO_x and CH₄ is crucial for future progress. The selective catalytic reduction of NO_x using CH₄ (CH₄-SCR) as a reductant holds great promise for environmental protection and energy conservation [8,9,10].

Rational catalyst design is of great significance in ensuring an efficient and durable CH₄-SCR reaction. For a considerable period of time, efforts have been devoted to metal-exchanged medium- or large-pore zeolites characterized by their ten-membered or twelve-membered rings (MR) [9,10,11,12,13,14,15,16,17,18]. However, these catalysts either had limited catalytic activity or were susceptible to framework collapse under high-temperature and high-humidity conditions [10]. In-exchanged zeolite beta (In/H-Beta) exhibited remarkable deNO_x efficiency of ~97.6% under dry CH₄-SCR conditions. Nonetheless, once water vapor was introduced, the catalytic activity of In/H-Beta was drastically diminished [19,20]. InO⁺ species tend to combine with H₂O, forming $\mathrm{I}\mathrm{n}{\left(\mathrm{O}\mathrm{H}\right)}_{3-\mathrm{z}}^{\mathrm{z}+}$ species, which are unable to activate CH₄ [18]. Moreover, prolonged exposure to H₂O at elevated temperatures may induce the irreversible structural collapse of the zeolite framework, further compromising its catalytic performance [21]. Therefore, inhibiting the conversion of active centers and selecting a hydrothermally stable zeolite support are essential in mitigating catalyst deactivation caused by water vapor.

In the NH₃-SCR field, the Cu-SSZ-39 zeolite (AEI topology), featuring cage-like channels with 8MR pore openings of 3.8 Å × 3.8 Å, has been highlighted for its exceptional hydrothermal stability and commendable catalytic activity [21,22,23,24,25]. Du et al. [21] conducted an in-depth investigation into the channel structure of a Cu-SSZ-39 catalyst, revealing its superior activity under humid conditions compared to its Cu-SSZ-13 counterpart. Even after undergoing high-temperature hydrothermal aging at 850 °C for 16 h, Cu-SSZ-39′s high deNO_x activity was not significantly degraded. This stability was attributed to its more tortuous channel structure in comparison to SSZ-13, which effectively mitigated aluminum leaching and copper species aggregation under hydrothermal conditions. In some recent studies, SSZ-39 zeolites have also emerged as promising supports for CH₄-SCR catalysts [26,27,28,29]. The Co-SSZ-39 catalyst synthesized by Li et al. [26] demonstrated a wider operating temperature window, higher peak NO_x conversion, and more robust anti-poisoning performance compared to its Co-SSZ-13 counterpart in the CH₄-SCR reaction. Detailed characterization of the catalyst revealed that Co²⁺ at the ion exchange sites served as the active species responsible for the catalytic activity in the CH₄-SCR reaction. Typically, indium-exchanged zeolites are capable of catalyzing the CH₄-SCR reaction more efficiently, making In species the preferred active components in many reported catalysts [10]. For instance, An et al. [27] reported that In/SSZ-39 exhibited superior catalytic activity compared to Cu-, Co-, and Fe/SSZ-39 catalysts. The low-temperature (400–450 °C) CH₄-SCR activity of In/SSZ-39 was significantly enhanced with increasing indium loading within the studied range. The 3In/SSZ-39 catalyst demonstrated high NO removal efficiency of 90% and CH₄ selectivity of 74.2% at a reaction temperature of 400 °C and a low CH₄/NO ratio of 1. High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) images revealed that In species were highly dispersed on the SSZ-39 zeolite, which facilitated moderate CH₄ activation and thereby enhanced the CH₄-SCR activity at low temperatures. More importantly, the indium species loaded within the zeolite have been reported to synergize with various introduced second metals (or metal oxides), thereby significantly enhancing the activity and/or stability of the catalyst [18,28,30,31,32,33,34,35]. For instance, Chen et al. [28] constructed an In-Co₃O₄/H-SSZ-39(OA) catalyst through mild acid etching and ion exchange, which demonstrated improved stability under harsh operating conditions and achieved NO removal efficiency of ~80% at ~600 °C under a GHSV of 24,000 h⁻¹. A small amount of Co₃O₄ nanoparticles on the zeolite surface greatly enhanced the catalytic activity by promoting CH₄ conversion and enabling the greater storage of stable N_xO_y species at high temperatures.

In this study, a series of bimetallic M-In/H-SSZ-39 (M = Cr, Ce, Co, Fe) catalysts were prepared via a liquid-state ion exchange method, with the preparation conditions of Cr-In/H-SSZ-39—the catalyst exhibiting the highest catalytic activity—being optimized. The effects of various operating conditions on the catalytic activity of the Cr-In/H-SSZ-39 catalyst were systematically investigated. Furthermore, a comparative analysis of the crystalline phase, chemical composition, pore structure, surface chemical state, surface acidity, and redox properties of Cr-In/H-SSZ-39 and the monometallic In/H-SSZ-39 control was conducted to elucidate the underlying mechanisms responsible for the excellent catalytic activity.

2. Results and Discussion

2.1. Screening of Bimetallic Catalysts

Various M-In/H-SSZ-39 catalysts were prepared by introducing Cr, Ce, Co, and Fe as the second metals, and their catalytic performance was assessed, as shown in Figure 1. It is evident from Figure 1a that the incorporation of Ce, Co, and Fe species failed to effectively enhance the CH₄-SCR activity compared with the In/H-SSZ-39 catalyst. Conversely, the incorporation of Cr species significantly enhanced the catalytic performance, achieving NO_x conversion of 55% at 575 °C, further escalating to 67.8% at 620 °C. Figure 1b shows that the CH₄ conversion of all four catalysts increased as the temperature rose. Notably, the Cr-In/H-SSZ-39 catalyst exhibited superior CH₄ conversion at >500 °C, achieving CH₄ conversion of 60.1% at 600 °C. The CH₄ conversion profiles of the other three catalysts were relatively similar. Methane had two possible conversion pathways: (1) partial oxidation, which further participated in NO_x reduction reactions, and (2) direct catalytic combustion. The significantly different NO_x conversion and similar CH₄ conversion rates reflected the catalysts’ varying abilities to guide methane toward the desired NO_x reduction pathway, as indicated by the CH₄ selectivity (Figure 1c). It was apparent that the incorporation of Ce, Co, and Fe as secondary metals resulted in a decrease in CH₄ selectivity under hydrothermal conditions compared to In/H-SSZ-39. However, incorporating Cr had a beneficial effect, with Cr-In/H-SSZ-39 demonstrating the highest CH₄ selectivity of 61.4% at 550 °C.

2.2. Effects of Preparation Conditions

2.2.1. Effects of Cr Concentration

Maintaining the In(NO₃)₃ concentration at 0.066 M, the catalytic performance of the xCr-In/H-SSZ-39 (x = 0, 0.0025, 0.005, 0.0075, and 0.01 M Cr(NO₃)₃) catalysts was compared, as shown in Figure 2. With the concentration of Cr(NO₃)₃ increased from 0 to 0.01 M, the catalytic activity roughly exhibited an initial enhancement, followed by a gradual decline (Figure 2a). When the Cr(NO₃)₃ concentration was 0.0075 M, the resulting catalyst exhibited the highest NO_x conversion of 67.8% at 605 °C. Figure 2b shows the CH₄ conversion profiles of the Cr-In/H-SSZ-39 catalysts. The Cr-containing samples all exhibited enhanced CH₄ conversion compared to the monometallic In/H-SSZ-39. Figure 2c shows that the introduction of Cr ions at varying concentrations enhanced the CH₄ selectivity of the catalyst. Notably, the CH₄ selectivity of 0.0075Cr-In/H-SSZ-39 was the highest at 600 °C, corresponding to its highest deNO_x activity. A mixed solution of 0.0075 M Cr(NO₃)₃ and 0.066 M In(NO₃)₃ was therefore used for ion exchange in subsequent experiments.

2.2.2. Effects of Calcination Temperature

The calcination temperature exerts a significant influence on the specific surface area of a catalyst and the chemical state of the active species [20]. To investigate the effects of the calcination temperature on the deNO_x activity of the catalysts, Cr-In/H-SSZ-39 samples were calcined at different temperatures (400 °C, 450 °C, 500 °C, 550 °C, and 600 °C) and designated as Cr-In/H-SSZ-39-x, where x represents the calcination temperature. As shown in Figure 3a, the catalytic activity of the Cr-In/H-SSZ-39 catalysts initially increased and then declined with the rise in the calcination temperature. The catalysts calcined at 500 °C demonstrated the optimal CH₄-SCR deNO_x performance, high CH₄ conversion (Figure 3b), and high CH₄ selectivity (Figure 3c), indicating that 500 °C was the optimal calcination temperature for the Cr-In/H-SSZ-39 catalyst.

2.3. Effects of Reaction Conditions

2.3.1. CH₄/NO Ratio

The CH₄-SCR process removes NO through the reaction of methane, nitric oxide, and oxygen, generating harmless nitrogen, water, and low-GWP carbon dioxide. The CH₄/NO ratio is a key indicator in assessing the effectiveness of the reaction. Figure 4a illustrates the catalytic activity of the Cr-In/H-SSZ-39 catalysts under various CH₄/NO ratios. At a CH₄/NO ratio of 1.0, Cr-In/H-SSZ-39 exhibited excellent catalytic activity in the high-temperature range, achieving a peak NO_x conversion of 59.6% at 600 °C. By progressively elevating the CH₄ concentration in the feed gas, the catalyst’s activity at <600 °C was continuously enhanced. A reasonable explanation is that the higher CH₄ initial concentration at a fixed NO_x concentration allowed more CH₄ to be involved in NO_x reduction at low temperatures, where CH₄ activation was relatively difficult. At a CH₄/NO ratio of 1.5, the Cr-In/H-SSZ-39 catalyst demonstrated its peak NO_x conversion of 67.8% at 600 °C. When the CH₄/NO ratio was further increased to 2, the catalyst’s peak NO_x conversion reached 76.1% at 600 °C. However, an excessively high concentration of CH₄ could not be fully converted over the catalyst, leading to the direct emission of high-GWP CH₄. It is evident from Figure 4b that the CH₄ conversion decreased as the CH₄/NO ratio increased. Moreover, as shown in Figure 4c, a decrease in the CH₄ selectivity at >600 °C was observed as the CH₄/NO ratio increased, which might be attributed to the fact that a major portion of CH₄ was catalytically combusted at high temperatures. At CH₄/NO ratios of 1.5 and 2, the CH₄ selectivity at 500 and 550 °C was similar, with a higher supply of CH₄ naturally leading to enhanced deNO_x activity.

2.3.2. O₂ Concentration

In CH₄-SCR, inadequate O₂ concentrations might result in incomplete reactions between CH₄ and NO_x, whereas excessive ones might prompt the overcombustion of CH₄. Figure 5a depicts the NO_x conversion profile of the Cr-In/H-SSZ-39 catalyst under different O₂ concentrations. The catalyst exhibited optimal deNO_x performance at an O₂ concentration of 10 vol%, with stable catalytic performance exceeding 60% above 550 °C. Specifically, at 600 °C, the deNO_x performance remained stable above 60%, achieving peak catalytic performance of 80.6%. However, as the O₂ concentration was further increased to 20 vol%, the catalytic performance of the catalyst declined. As observed in Figure 5b, CH₄ conversion increased with the rise in the O₂ concentration, primarily due to the enhanced oxidation kinetics of CH₄ under oxygen-rich conditions. Specifically, the highest CH₄ conversion occurred at 20 vol% O₂, followed by comparable CH₄ conversion at 15 vol% and 10 vol% O₂. As shown in Figure 5c, the CH₄ selectivity exhibited a trend similar to that of the NO_x removal efficiency with respect to the O₂ concentration. This correlation can be attributed to the fact that, at an O₂ concentration of 20 vol%, CH₄ more readily reacts with oxygen, thereby reducing its selectivity toward NO_x.

2.3.3. Gaseous Hourly Space Velocity

The influence of the GHSV on the deNO_x performance of the Cr-In/H-SSZ-39 catalyst was also investigated, as shown in Figure 6. It could be found that the NO_x conversion gradually increased with the reduction in GHSV from 30,000 h⁻¹ to 10,000 h⁻¹ (Figure 6a). At a GHSV of 16,000 h⁻¹, the catalyst achieved the peak catalytic activity at 600 °C, with an NO_x conversion rate of 86.1%. When the GHSV was further decreased to 10,000 h⁻¹, the catalyst demonstrated its highest deNO_x activity at a slightly lower temperature of 590 °C, with remarkable NO_x conversion of 93.4%. A decreasing GHSV led to an increase in CH₄ conversion (Figure 6b). CH₄ was more effectively reacted within the temperature range of 400–650 °C at a lower GHSV. Furthermore, Figure 6c reveals that the CH₄ selectivity remained high at a GHSV of 10,000 h⁻¹, indicating that CH₄ predominantly participated in NO_x reduction.

2.3.4. Water Vapor Content

The tolerance of the Cr-In/H-SSZ-39 catalyst to water vapor was examined (Figure 7). In the absence of water vapor, the catalyst achieved high NO_x conversion of 86% at 580 °C (Figure 7a). Peak NO_x conversion of 80.6% was observed at 600 °C when 6 vol% water vapor was introduced. As the water content was further increased to 10 vol% and 15 vol%, the peak NO_x conversion declined to 70.6% and 52%, respectively. These findings underscore the robust hydrothermal stability of the Cr-In/H-SSZ-39 catalyst. Figure 7b shows that CH₄ conversion declined with increasing water content, indicating that water vapor inhibited CH₄ conversion. At water vapor content of 10 vol%, the CH₄ conversion of the catalyst was 51.3% at 600 °C, a value between those observed at 6 vol% and 15 vol% water vapor content. Figure 7c shows the direct correlation between the CH₄ selectivity and catalytic performance at temperatures below 550 °C. When the water vapor content was 6 vol%, the CH₄ selectivity at 600 °C exceeded that observed without water vapor, indicating that water vapor might exert a stronger inhibitory effect on CH₄ combustion.

2.4. Cyclic and Stability Testing

The results of the long-term operation test and three consecutive TPSR tests of the Cr-In/H-SSZ-39 catalyst are presented in Figure 8. At a constant temperature of 600 °C, the NO_x conversion of the catalyst decreased from 70% to 59% after 1 h of exposure to hydrothermal conditions (Figure 8a). Following this initial decline, the conversion exhibited a gradual and steady decrease over time. In the presence of 10 vol% H₂O, the catalytic activity remained above 50% for up to 5 h and above 40% for up to 12 h. These results demonstrate the robust stability of the Cr-In/H-SSZ-39 catalyst under high-temperature hydrothermal conditions. As shown in Figure 8b, the Cr-In/H-SSZ-39 catalyst retained a high level of catalytic activity, achieving NO removal efficiency of 58% and 51% in the second and third cycles, respectively. Compared with the In/H-Beta catalyst [20], the Cr-In/H-SSZ-39 catalyst exhibited higher stability after multiple cycles. The N₂ selectivity, as an essential indictor, was assessed, and the results are presented in Figure 8c. It can be seen that the N₂ selectivity exceeds 99%, with almost no N₂O being produced.

2.5. Characterization and Analysis of the Cr-In/H-SSZ-39 Catalyst

2.5.1. Analysis of Composition and Texture Properties

Figure 9 shows the PXRD patterns of the H-SSZ-39, In/H-SSZ-39, and Cr-In/H-SSZ-39 samples. The characteristic diffraction peaks (9.5°, 10.7°, 13.0°, 16.2°, 17.0°, 17.3°, 20.8°, and 24.1°) of each sample aligned well with the simulated AEI structure, indicating that the SSZ-39 zeolite remained intact after incorporating In and Cr species. For zeolite-based deNO_x catalysts, maintaining a stable crystalline structure is crucial to ensure the distribution of metal species in their exchangeable sites. The micromorphologies of the In/H-SSZ-39 and Cr-In/H-SSZ-39 catalysts were observed by SEM, as shown in Figure 10. There was no discernible difference in the micromorphologies of the two samples, both consisting of aggregated cuboid crystals with smooth surfaces. The surface area and pore structure of the catalyst influence the dispersion and accessibility of the active species. As shown in Figure 11 and

Table 1. Textural properties of In/H-SSZ-39 and Cr-In/H-SSZ-39 samples.

Sample	SBET a	Vtotal b	Vmicro c
	(m2 g−1)	(cm3 g−1)	(cm3 g−1)
H-SSZ-39	754.5	0.293	0.269
In/H-SSZ-39	685.5	0.271	0.243
Cr-In/H-SSZ-39	676.7	0.267	0.240

^a BET surface area obtained from N₂ adsorption isotherm in the relative pressure range of 0.05–0.30. ^b Total pore volume calculated as the amount of N₂ adsorbed at P/P₀ = 0.90. ^c Micropore volume calculated via t-plot method.

, the Cr-In/H-SSZ-39 and In/H-SSZ-39 catalysts presented micropore-dominant pore structure, evidenced by the type I N₂ adsorption–desorption isotherms (Figure 11) and steep N₂ adsorption behavior at P/P₀ < 0.01. The preserved intrinsic microporosity characteristics demonstrated that the introduction of Cr/In species did not cause significant disruption to the ordered framework structure of the SSZ-39 zeolite, in line with the PXRD patterns and SEM observations.

To determine the elemental compositions of the catalysts, the ICP-OES technique was employed, as summarized in

Table 2. Compositional analysis of H-SSZ-39, In/H-SSZ-39, and Cr-In/H-SSZ-39 samples.

Sample	In Content	Cr Content	Al Content	Si Content	Si/Al
	(wt%)	(wt%)	(wt%)	(wt%)
H-SSZ-39	/	/	2.44	39.41	16
In/H-SSZ-39	5.5	/	2.45	40.33	16
Cr-In/H-SSZ-39	4.7	0.075	2.90	43.06	15

. The Cr content in Cr-In/H-SSZ-39 was 0.075 wt%, indicating that the Cr ions (0.0075 M) in the ion exchange solution migrated into the zeolites. In/H-SSZ-39 exhibited In content of 5.5 wt%, whereas the In content of Cr-In/H-SSZ-39 slightly decreased to 4.7 wt%, suggesting that In and Cr ions might compete for zeolites’ finite exchangeable sites during the ion exchange process.

2.5.2. Analysis of Chemical States and Redox Properties

Figure 12a presents the In 3d_5/2 XPS spectra of the In/H-SSZ-39 and Cr-In/H-SSZ-39 catalysts, and the according calculations are summarized in

Table 3. Surface element compositions of In/H-SSZ-39 and Cr-In/H-SSZ-39 samples.

Sample	${\mathbf{InO}}^{+}/({\mathbf{InO}}^{+}+{\mathbf{In}}_2{\mathbf{O}}_3+\mathbf{I}\mathbf{n}{\left(\mathbf{O}\mathbf{H}\right)}_{3-\mathbf{z}}^{\mathbf{z}+})$	Oβ/(Oα + Oβ + Oγ)	Cr3+/(Cr3+ + Cr6+)
In/H-SSZ-39	0.49	0.36	/
Cr-In/H-SSZ-39	0.54	0.40	0.58
Used Cr-In/H-SSZ-39	0.54	0.40	0.57

. Spectral deconvolution allowed for the identification of three indium species: $\mathrm{I}\mathrm{n}{\left(\mathrm{O}\mathrm{H}\right)}_{3-\mathrm{z}}^{\mathrm{z}+}$ (~445.6 eV), InO⁺ (~444.5 eV), and In₂O₃ (~443.9 eV) [29]. Furthermore, a discernible shift in the In 3d_5/2 spectra toward higher binding energies was observed for Cr-In/H-SSZ-39 compared to In/H-SSZ-39, which could be explained by the interaction between Cr and In species. Higher binding energy is indicative of more oxidative surface chemistry, which promotes stronger interactions with SCR reactant species and, consequently, enhances NO_x conversion. Table 3 shows that the Cr-In/H-SSZ-39 catalyst demonstrated a larger proportion of InO⁺ species (InO⁺/($\mathrm{I}\mathrm{n}{\left(\mathrm{O}\mathrm{H}\right)}_{3-\mathrm{z}}^{\mathrm{z}+}$ + InO⁺ + In₂O₃) = 0.54) compared to In/H-SSZ-39 (0.49). Given that InO⁺ is widely recognized as the primary active site in In-exchanged zeolite CH₄-SCR catalysts [18,30,33,34], more InO⁺ species might also contribute to the exceptional deNO_x activity of Cr-In/H-SSZ-39. Figure 12b shows the O 1s XPS spectra of the two catalysts, which were deconvoluted into three distinct peaks, i.e., oxygen adsorbed on hydroxyl groups (O_γ, 532.4–533.1 eV), surface oxygen species (O_β, 532.1–532.6 eV), and lattice oxygen (O_α, 531.0–531.5 eV) [20,36,37,38]. Notably, the O_β content of Cr-In/H-SSZ-39 reached 40.4%, representing a 3.9% increase compared to that of In/H-SSZ-39 (36.5%). Introducing Cr species into the In/H-SSZ-39 catalyst led to an increase in the O_β content. Considering that surface oxygen is more involved in complete oxidation reactions, the higher surface oxygen content of Cr-In/H-SSZ-39 promotes CH₄ activation [19], thereby enhancing its catalytic activity.

The redox properties of the two catalysts were investigated by the H₂-TPR method, as depicted in Figure 13. Cr-In/H-SSZ-39 exhibited a reduction peak at 222 °C, attributable to the reduction of Cr⁶⁺ to Cr³⁺ [39]. Additionally, a prominent reduction peak was observed at 310 °C/329 °C, which corresponded to the reduction of In³⁺ [30]. Notably, the reduction peak of InO⁺ in the Cr-In/H-SSZ-39 catalyst shifted to a higher temperature in comparison to that of the In/H-SSZ-39 catalyst. This finding suggests a synergistic interaction between Cr and In species [40].

The acidity of the two catalysts was investigated by NH₃-TPD, and the results are shown in Figure 14 and

Table 4. The strength and quantity of surface acid sites of In/H-SSZ-39 and Cr-In/H-SSZ-39 samples based on NH₃-TPD measurements.

Sample	Peak I		Peak II		Peak III		Qtotal
	T (°C)	Q (mmol g−1)	T (°C)	Q (mmol g−1)	T (°C)	Q (mmol g−1)	(mmol g−1)
In/H-SSZ-39	178	1.098	477	0.186	546	0.529	1.813
Cr-In/H-SSZ-39	180	1.236	471	0.195	540	0.587	2.018
Used Cr-In/H-SSZ-39	182	1.145	470	0.184	538	0.552	1.881

. The NH₃-TPD profiles were deconvoluted into three distinct peaks: peak I at ~180 °C, attributed to surface hydroxyl groups and weak Lewis acid sites (LAS); peak II at ~470 °C, associated with strong LAS; and peak III at ~540 °C, linked to strong Brønsted acids. Notably, Cr-In/H-SSZ-39 demonstrated 11.3% higher total acidity (2.018 mmol g⁻¹ vs. 1.813 mmol g⁻¹) than its In/H-SSZ-39 counterpart. This observation aligns well with the trend observed in the CH₄-SCR performance, thereby reinforcing the pivotal role of acidic sites in enhancing catalytic activity [29].

2.6. Characterization of the Cr-In/H-SSZ-39 Catalyst After Reaction

2.6.1. Analysis of Chemical States

Figure 15a and Table 4 present a comparison of the Cr 2p spectra for the Cr-In/H-SSZ-39 catalyst before and after the reaction. XPS spectral deconvolution resolved characteristic Cr³⁺ (2p_3/2 at 571.8 eV, 2p_1/2 at 588.1 eV) and Cr⁶⁺ (2p_3/2 at 577.5 eV, 2p_1/2 at 586.8 eV) oxidation states. Cr³⁺ played a pivotal role in generating oxygen vacancies on the catalyst surface. Oxygen vacancies serve as active sites that promote reactant activation via interacting with NO₂ to generate surface-adsorbed nitrite ($\mathrm{N}{\mathrm{O}}_2^{-}$) and nitrate ($\mathrm{N}{\mathrm{O}}_3^{-}$) intermediates, ultimately contributing to the enhanced catalytic performance [41]. The Cr³⁺/Cr⁶⁺ ratios of the fresh and used Cr-In/H-SSZ-39 catalysts were 0.58 and 0.57, respectively, indicating that the catalyst retained a considerable number of Cr³⁺ active sites, which were essential for its effective catalytic performance. Figure 15b shows the In 3d_5/2 spectra of the fresh and used Cr-In/H-SSZ-39 catalysts, which could be deconvoluted into three distinct indium species: $\mathrm{I}\mathrm{n}{\left(\mathrm{O}\mathrm{H}\right)}_{3-\mathrm{z}}^{\mathrm{z}+}$, InO⁺, and In₂O₃. The calculated InO⁺/(InO⁺ + In₂O₃ + $\mathrm{I}\mathrm{n}{\left(\mathrm{O}\mathrm{H}\right)}_{3-\mathrm{z}}^{\mathrm{z}+}$) ratio of the used Cr-In/H-SSZ-39 catalyst remained stable at 0.53, almost identical to the value of the fresh one (0.54). Additionally, as shown in Figure 15c, the content of O_β was almost unchanged after the reaction under humid conditions, decreasing slightly from 40.4% to 40.2%. The consistent Cr³⁺/Cr⁶⁺, InO⁺/(InO⁺ + In₂O₃ + $\mathrm{I}\mathrm{n}{\left(\mathrm{O}\mathrm{H}\right)}_{3-\mathrm{z}}^{\mathrm{z}+}$), and O_β ratios explain the excellent cyclability and hydrothermal stability of the Cr-In/H-SSZ-39 catalyst.

2.6.2. Analysis of Acidity

Figure 16 shows the NH₃-TPD profiles of the fresh and used Cr-In/H-SSZ-39 catalysts, which could be deconvoluted into three NH₃ desorption peaks. The NH₃ desorption from the used Cr-In/H-SSZ-39 catalyst at the three temperature ranges was slightly lower than that of the fresh catalyst, indicating the reduced acidity of the catalyst after the reaction (Table 4). The minor loss of acidity after the reaction might also have contributed to the retention of its strong deNO_x performance.

2.7. Analysis of Water Resistance Mechanism

Based on our previous study of the CH₄-SCR deNO_x mechanism over In/H-Beta [38], BAS could induce a small amount of In₂O₃ on the zeolite surface to generate InO⁺ active sites; CH₄ is mainly adsorbed on InO⁺ sites, which provide the activated oxygen (O*) necessary for CH₄ activation. Combining this with the present research results, the CH₄-SCR deNO_x mechanism over the Cr-In/H-SSZ-39 catalyst is as follows:

• It could be seen from PXRD that the catalyst maintained its crystalline properties to a great extent after introducing Cr and In species;
• The NH₃-TPD and XPS results showed that the incorporation of Cr into In/H-SSZ-39 increased the number of BAS and generated more InO⁺ species, thus promoting NO oxidation and CH₄ activation;
• A strong interaction between Cr and In species was found from the XPS and H₂-TPR results; introducing Cr species increased the redox properties of the catalyst, thus promoting NO oxidation.

3. Experimental Section

3.1. Catalyst Preparation

3.1.1. Chemicals

H-SSZ-39 zeolite with an Si/Al ratio of 16 was provided by China Catalyst Holding Co., Ltd. (Zibo, China). Indium(III) nitrate hydrate (In(NO₃)₃·xH₂O, 99.9% metal basis, indium content of 28–37%) was acquired from Shanghai Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). Chromium(III) nitrate nonahydrate (Cr(NO₃)₃·9H₂O, 99.95% metal basis), cerium(III) nitrate hexahydrate (Ce(NO₃)₃·6H₂O, 99.5% metal basis), cobalt(II) nitrate hexahydrate (Co(NO₃)₂·6H₂O, 99.99% metal basis), and iron(III) nitrate nonahydrate (Fe(NO₃)₃·9H₂O, 99.9% metal basis) were bought from Shanghai Macklin Biochemical Technology Co., Ltd. (Shanghai, China). All reagents were used as received, without further purification. Home-made ultrapure water (18.25 MΩ·cm) was used throughout the entire experiment.

3.1.2. Preparation of Monometallic In/H-SSZ-39 Catalyst

The In/H-SSZ-39 catalyst was prepared via a liquid-state ion exchange method. First, 100 mL of 0.066 M In(NO₃)₃ aqueous solution was prepared by dissolving In(NO₃)₃·xH₂O in ultrapure water. Subsequently, 3 g of H-SSZ-39 zeolite was dispersed into the In(NO₃)₃ solution. The resultant mixture was uniformly agitated at a constant temperature of 85 °C for 8 h using a magnetic stirrer. The solid was recovered by centrifugation, thoroughly washed five times with deionized water, and then dried in an oven at 80 °C for 12 h. Lastly, the dried sample was calcined in a muffle furnace at 500 °C for 3 h, after which it was sealed and stored for future use.

3.1.3. Preparation of Bimetallic M-In/H-SSZ-39 Catalyst

The preparation process of the M-In/H-SSZ-39 catalyst was similar to that of In/H-SSZ-39, except that a mixed aqueous solution of In(NO₃)₃ and the second metal M precursor (Cr(NO₃)₃·9H₂O, Ce(NO₃)₃·6H₂O, Co(NO₃)₂·6H₂O, and Fe(NO₃)₃·9H₂O) was used. Variations were introduced into the synthesis process to optimize the properties of the Cr-In/H-SSZ-39 catalyst. For Cr content optimization, different amounts of Cr(NO₃)₃·9H₂O (0.11 g, 0.21 g, 0.32 g, and 0.43 g) were dissolved in 100 mL of 0.066 M In(NO₃)₃ aqueous solution during the initial step. To determine the optimal calcination temperature, the materials were calcined at 450 °C, 500 °C, 550 °C, and 600 °C in the final step.

3.2. Catalytic Activity Measurement

The activity evaluation experiments were conducted at atmospheric pressure using the temperature-programmed surface reaction (TPSR) method. A certain mass of catalyst particles (40–60 mesh) was first loaded in a fixed-bed quartz reactor. The reaction gas mixture composed of 400 ppm NO, 600 ppm CH₄, 10 vol% O₂, and 6 vol% H₂O with an Ar balance at a total flow rate of 100 mL min⁻¹ was introduced into the reactor, corresponding to a GHSV of ~21,000 h⁻¹ for 0.1 g of catalyst particles. After establishing NO_x adsorption–desorption equilibrium at 100 °C, a programmed temperature ramp was initiated, gradually raising the reactor temperature from 100 °C to 675 °C at a heating rate of 5 °C min⁻¹. The concentrations of NO_x were monitored by a nitrogen oxide analyzer (Teledyne Model T200H, Teledyne Monitor Labs, Inc., Centennial, CO, USA), while the CH₄, CO, and CO₂ concentrations were analyzed by a gas chromatograph (Fuli GC9790II, China) equipped with a Porapak-Q column (Agilent, Santa Clara, CA, USA) and a flame ionization detector (FID). A gas chromatograph (Agilent 7890B) was used to detect the N₂ content; the detectors were TCD and ECD detectors; a 5A molecular sieve column was used; and the carrier gases were N₂ and He. The CH₄-SCR activity of the catalyst was assessed using the following parameters: NO_x removal efficiency (η), CH₄ conversion (γ), CH₄ selectivity (α), and N₂ selectivity (S_N2). These were calculated using Equations (1), (2), (3), and (4), respectively:

$$\mathrm{\eta }={\displaystyle \frac{\mathrm{c}{({\mathrm{N}\mathrm{O}}_{\mathrm{x}})}_{\mathrm{in}}-\mathrm{c}{({\mathrm{N}\mathrm{O}}_{\mathrm{x}})}_{\mathrm{o}\mathrm{u}\mathrm{t}}}{\mathrm{c}{({\mathrm{N}\mathrm{O}}_{\mathrm{x}})}_{\mathrm{i}\mathrm{n}}}}\times 100\%$$

(1)

$$\mathrm{\gamma }={\displaystyle \frac{\mathrm{c}{({\mathrm{C}\mathrm{H}}_4)}_{\mathrm{i}\mathrm{n}}-\mathrm{c}{({\mathrm{C}\mathrm{H}}_4)}_{\mathrm{o}\mathrm{u}\mathrm{t}}}{\mathrm{c}{({\mathrm{C}\mathrm{H}}_4)}_{\mathrm{i}\mathrm{n}}}}\times 100\%$$

(2)

$$\mathsf{\alpha }={\displaystyle \frac{0.5(\mathrm{c}{({\mathrm{N}\mathrm{O}}_{\mathrm{x}})}_{\mathrm{in}}-\mathrm{c}{({\mathrm{N}\mathrm{O}}_{\mathrm{x}})}_{\mathrm{o}\mathrm{u}\mathrm{t}})}{\mathrm{c}{({\mathrm{C}\mathrm{H}}_4)}_{\mathrm{i}\mathrm{n}}-\mathrm{c}{({\mathrm{C}\mathrm{H}}_4)}_{\mathrm{o}\mathrm{u}\mathrm{t}}}}\times 100\%$$

(3)

$${\mathrm{S}}_{\mathrm{N}2}={\displaystyle \frac{{2\mathrm{c}\left({\mathrm{N}}_2\right)}_{\mathrm{o}\mathrm{u}\mathrm{t}}}{\mathrm{c}{({\mathrm{N}\mathrm{O}}_{\mathrm{x}})}_{\mathrm{i}\mathrm{n}}-\mathrm{c}{({\mathrm{N}\mathrm{O}}_{\mathrm{x}})}_{\mathrm{out}}}}\times 100\%$$

(4)

where c(NO_x)_in is the input concentration of NO_x, c(NO_x)_out is the output concentration of NO_x, c(CH₄)_in is the input concentration of CH₄, c(CH₄)_out is the output concentration of CH₄, and c(N₂)_out is the concentration of N₂ formed, respectively.

3.3. Catalyst Characterization

Scanning electron microscopy (SEM) observation was conducted using a Thermo Scientific Apreo 2S HiVac (Waltham, MA, USA). To understand the crystalline phases and structures of the catalysts, powder X-ray diffraction (PXRD) patterns were recorded by a Bruker D8 Venture diffractometer (Germany) in the diffraction angle range of 2θ = 5–40° (2° min⁻¹) with Cu Kα radiation (λ = 1.5418 Å) at 40 kV and 40 mA. Inductively coupled plasma optical emission spectrometry (ICP-OES, Agilent 720ES, USA) was used to determine the elemental content of Cr, In, Si, and Al in the bulk catalyst. The specific surface area and pore structure of the catalyst samples were calculated from N₂ adsorption–desorption isotherms measured at 77 K on a Micromeritics ASAP 2460 instrument (Micromeritics, GA, USA). Prior to measurement, the samples were degassed at 300 °C for 6 h to eliminate adsorbed impurities. The surface elemental composition and chemical state of the catalyst were analyzed using X-ray photoelectron spectroscopy (XPS) on an ESCALAB MK-II electron spectrometer (Thermo Scientific, Waltham, MA, USA) with a monochromatic Al Kα radiation source (1486.6 eV). The binding energy values were calibrated using the C 1s peak at 284.8 eV for adventitious carbon. The reducibility of the catalysts was evaluated by temperature-programmed reduction with hydrogen (H₂-TPR) using an AutoChem Ⅱ 2920 instrument (Micromeritics, Norcross, GA, USA). Before the measurement, 50 mg samples (40–60 mesh) were loaded into a U-shaped quartz tube and pretreated at 500 °C in an He gas flow of 30 mL/min for 2 h. After being cooled to 50 °C under the same atmosphere, the samples were exposed to a 10 vol% H₂/Ar mixture at a flow rate of 30 mL/min and heated from 50 °C to 650 °C at a ramp rate of 10 °C/min. Temperature-programmed desorption of ammonia (NH₃-TPD) was conducted using a 5 vol% NH₃/N₂ gas mixture under a similar temperature program to investigate the acidic properties of the samples.

4. Conclusions

In summary, a Cr-In/H-SSZ-39 catalyst with high CH₄-SCR deNO_x performance was successfully prepared, and its preparation conditions were systematically optimized. The effects of the operating conditions were investigated, and the CH₄-SCR deNO_x mechanism over Cr-In/H-SSZ-39 was elucidated. The main findings of this study are as follows:

(1) The Cr-In/H-SSZ-39 catalyst exhibited optimal NO_x conversion of 80% when exchanged with a mixed solution of 0.0075 M Cr(NO₃)₃ and 0.066 M In(NO₃)₃, calcined at 500 °C, and tested under conditions of 400 ppm NO, 600 ppm CH₄, 10 vol% O₂, 6 vol% H₂O, and a GHSV of 21,000 h⁻¹. The catalytic activity of Cr-In/H-SSZ-39 under humid conditions was significantly superior to that of the monometallic In/H-SSZ-39.

(2) Under the optimized operating conditions, the Cr-In/H-SSZ-39 catalyst demonstrated excellent regeneration ability and long-term stability. Specifically, under 10 vol% H₂O content and at 600 °C, the NO_x conversion in four consecutive cycles were 71%, 58%, 51%, and 43%, respectively. The long-term operation test further demonstrated that the NO_x removal efficiency was kept at over 50% for up to 5 h and above 40% for up to 12 h.

(3) The incorporation of Cr into the In/H-SSZ-39 catalyst resulted in an increase in BAS and the production of more InO⁺ active species, which were responsible to the oxidation of NO and the activation of CH₄. A potent synergistic interaction was observed between Cr and In species. The additional Cr³⁺ species significantly enhanced the redox properties of the catalyst, further promoting the oxidation of NO and the activation of CH₄, ultimately leading to an improvement in catalytic activity. This approach holds promise as a feasible strategy to enhance the catalytic performance of In/H-SSZ-39 under harsh high-temperature hydrothermal conditions commonly encountered in practical applications.