Enhanced Low-Temperature Activity and Hydrothermal Stability of Ce-Mn Oxide-Modified Cu-SSZ-39 Catalysts for NH₃-SCR of NO_x

Abstract

Cu-SSZ-39 zeolite with an AEI structure exhibits excellent hydrothermal stability and can be a potential alternative to Cu-SSZ-13 zeolite SCR catalysts for NO_x removal in diesel vehicles. However, the inferior low-temperature performance of Cu-SSZ-39 leads to substantial NO_x emissions during the cold-start period, impeding its practical application. In this study, Ce-Mn oxide-modified Cu-SSZ-39 catalysts (CeMnO_x/Cu-SSZ-39) and references (CeO₂/Cu-SSZ-39 and MnO_x/Cu-SSZ-39) were prepared by the ion-exchange of Cu ions followed by impregnation of the oxide precursors, with the aim of enhancing the NH₃-SCR performance at low temperatures. The modified catalysts exhibited improved low-temperature activity and hydrothermal stability compared to the unmodified counterpart. In particular, CeMnO_x/Cu-SSZ-39 showed the highest activity among the three catalysts and achieved NO_x conversions above 90% within the temperature range of 180 °C to 600 °C, even after undergoing hydrothermal aging at 800 °C. Experimental results indicated that the synergistic effect between Ce and Mn in CeMnO_x improves the redox properties and acidity of the catalyst due to the presence of Ce³⁺, Mn⁴⁺, and abundant adsorbed oxygen species, which facilitate low-temperature SCR reactions. Furthermore, the interaction of CeMnO_x with Cu-SSZ-39 stabilizes the zeolite framework and hinders the agglomeration of Cu species during the hydrothermal aging process, contributing to its exceptional hydrothermal stability. The kinetics and NO oxidation experiments demonstrated that CeMnO_x provides access to fast SCR reaction pathways by oxidizing NO to NO₂, resulting in a significant increase in low-temperature activity. This study provides novel guidelines for the design and preparation of Cu-SSZ-39 zeolite with outstanding SCR performance over a wide temperature range.

1. Introduction

Nitrogen oxides (NO_x), emitted in particular by mobile sources, are one of the major precursors of urban air pollution, such as haze and ozone [1]. Currently, selective catalytic reduction (SCR) of NO_x with ammonia (NH₃) has been proven to be one of the most efficient technologies for NO_x purification [2]. Cu-based small-pore zeolite SCR catalysts (especially Cu-SSZ-13 zeolite with a CHA structure) have been widely adopted in diesel engines owing to their wide active temperature window and hydrothermal stability [3,4,5]. In practical applications, the periodically increased exhaust temperature required to stimulate diesel particulate filter (DPF) regeneration necessitates SCR catalysts with high hydrothermal stability [6]. Nevertheless, the Cu-SSZ-13 catalyst exhibits a considerable decrease in deNO_x activity after hydrothermal aging at 800 °C, particularly for Al-rich Cu-SSZ-13 catalysts [7,8]. In recent years, many other small-pore zeolites with variable stable framework structures have attracted extensive attention in an attempt to increase hydrothermal stability of the catalysts, including LTA [9,10], KFI [11], AFX [12,13], RTH [14], and AEI [15,16]. In view of its catalytic performance and the availability of synthetic precursors, Cu-SSZ-39 zeolite with an AEI structure is considered to be an excellent substitute for Cu-SSZ-13, possessing a bright future in industrial applications [17,18,19,20,21]. However, Cu-SSZ-39 catalysts frequently exhibit insufficient activity at low exhaust temperatures, leading to a critical problem: up to 80% of NO_x emissions occur during the cold-start period before the SCR catalysts achieve the light-off temperature [5]. Therefore, optimizing the SCR performance (especially the low-temperature activity) of Cu-SSZ-39 catalysts is urgently needed.

Recently, more and more researchers have concentrated on enhancing the SCR performance of Cu-SSZ-39 catalysts, and two strategies have been proposed. One is modulating Cu²⁺ ions, which is the fundamental approach to optimize the SCR performance since the Cu²⁺ ions are the active sites of Cu-SSZ-39 [22]. Fu et al. increased the Cu content in Cu-SSZ-39, which led to a significant improvement in low-temperature SCR activity by endowing the catalyst with a large amount of highly stable, isolated Cu²⁺ ions [23]. Additionally, the framework tetrahedral Al atoms were stabilized by the Cu²⁺ ions located on the face of the double six-membered ring (d6r) units of the AEI framework, thereby enhancing their hydrothermal stability. Du and colleagues regulated Cu²⁺ ions by varying the precursors during the synthesis of Na-SSZ-39, and the ZSM-5 precursor provided more Cu(OH)⁺ species as well as a higher Si/Al ratio in the obtained Cu-SSZ-39, which guaranteed superior low-temperature activity and hydrothermal stability in comparison to the three other catalysts prepared using Y zeolite, Beta zeolite, colloidal silica, and sodium aluminate precursors, respectively [24]. Distinctively, Du et al. discovered that a hydrothermal aging treatment (HTA) allowed some of the Cu²⁺ ions near d6r to transform into CuO species, which promoted the production of crucial nitrate intermediates in SCR reactions, resulting in boosted low-temperature activity [18]. Similarly, Chen et al. observed a comparable phenomenon in the phosphorus-poisoned Cu-SSZ-39 catalyst, where some of the deactivated Cu²⁺ ions were recovered due to the partial decomposition of Cu-P species after an HTA [19].

In addition to modulating Cu²⁺ ions, the introduction of heteroatoms is another effective way to improve the SCR performance of Cu-SSZ-39 catalysts [25,26]. Transition and rare-earth metals with variable oxidation states are commonly employed to modify Cu-SSZ-39 catalysts due to their outstanding redox properties and reactivity. For instance, Y³⁺ ions were introduced to induce more Cu²⁺ ions to locate in the stable ion-exchange (d6r) sites of SSZ-39 zeolite, thus inhibiting the deactivation of Cu-SSZ-39 catalysts even during hydrothermal aging at 900 °C [27]. Furthermore, Y³⁺ ions could serve as a sacrificial agent for SO₂ poisoning to protect active Cu²⁺ ions from inactivation in harsh conditions [28]. Notably, Wang et al. demonstrated that incorporating Ce or Mn into Cu-SSZ-39 catalysts (i.e., CeCu-SSZ-39 or MnCu-SSZ-39) has a beneficial effect on their low-temperature activity and hydrothermal stability [29]. This was achieved through the formation of highly dispersed active CuO crystallites in the inner channel and the enhancement of acidity. Based on the reaction mechanisms, they proposed a fast SCR mechanism in addition to the Langmuir–Hinshelwood and Eley–Rideal mechanisms, which contributed to the improved low-temperature activity of the MnCu-SSZ-39 catalyst after hydrothermal aging at 850 °C. Moreover, NO₂ was discovered to have a stimulating impact on the SCR activity of Cu-SSZ-39 [30]. Consequently, this provides us with the possibility to enhance the low-temperature activity by utilizing the fast SCR pathway for Cu-SSZ-39, while this is not applicable to Cu-SSZ-13 [31,32].

Inspired by previous studies on optimizing the SCR performance of small-pore zeolite catalysts and developing highly effective oxide catalysts [33,34,35,36,37,38,39,40], we selected Ce-Mn oxide to modify the Cu-SSZ-39 catalyst, not only because it possesses excellent low-temperature SCR activity [41], but also because it provides access to the fast SCR reaction pathway via NO₂ generation [42]. Herein, Ce-Mn oxide-modified Cu-SSZ-39 catalysts (CeMnO_x/Cu-SSZ-39) as well as references (CeO₂/Cu-SSZ-39 and MnO_x/Cu-SSZ-39) are prepared, and the activity and hydrothermal aging performance of pure and oxide-modified Cu-SSZ-39 catalysts are evaluated for SCR reactions. Particularly, the interaction of oxides with Cu-SSZ-39 and the synergetic effect between Ce and Mn in Ce-Mn oxides are systematically investigated and the possible mechanisms of increased low-temperature activity and hydrothermal stability are speculated on.

2. Results and Discussion

2.1. Structural and Morphological Characterizations

Figure 1a shows the X-ray diffraction (XRD) patterns of the obtained Cu-SSZ-39, CeO₂/Cu-SSZ-39, CeMnO_x/Cu-SSZ-39, and MnO_x/Cu-SSZ-39 catalysts, which present the characteristic diffraction peaks of SSZ-39 zeolites with AEI topology [15,23]. The structure of Cu-SSZ-39 did not change after supporting Ce/Mn or Ce-Mn oxides, while the intensity of peaks belonging to SSZ-39 dropped. Moreover, no peak was ascribed to Ce or Mn oxides, indicating that the supported oxides might be evenly distributed. After hydrothermal aging at 800 °C (Figure 1b), a negligible change in AEI structure was observed for all the catalysts, suggesting the hydrothermal stability of SSZ-39 zeolite. The N₂ adsorption–desorption results revealed that all the catalysts displayed the typical isotherms of microporous materials (Figure S1).

Table 1. Textural properties and quantitative analysis of Cu-SSZ-39, CeO₂/Cu-SSZ-39, CeMnO_x/Cu-SSZ-39, and MnO_x/Cu-SSZ-39 catalysts before and after hydrothermal aging.

Samples	Surface Area (m2∙g−1)	Pore Volume (cm3∙g−1)	Pore Size (nm)	Si/Al Ratio a	Cu a (wt.%)
Cu-SSZ-39	664.8	0.28	1.66	8.7	1.6
CeO2/Cu-SSZ-39	560.5	0.27	1.91	8.8	1.5
CeMnOx/Cu-SSZ-39	568.0	0.26	1.84	8.4	1.6
MnOx/Cu-SSZ-39	585.1	0.24	1.66	8.6	1.6
Cu-SSZ-39-A	613.8	0.26	1.70	8.5	1.6
CeO2/Cu-SSZ-39-A	555.9	0.27	1.91	8.6	1.5
CeMnOx/Cu-SSZ-39-A	547.5	0.24	1.74	8.6	1.7
MnOx/Cu-SSZ-39-A	551.8	0.22	1.57	8.4	1.7

^a Derived from EDS mappings.

illustrates that the BET surface areas and pore volumes of the Cu-SSZ-39 catalyst decreased slightly due to oxide supporting and hydrothermal aging, which could be attributed to the loaded oxide covering some micropores of the catalysts. On the other hand, loaded oxides with different textural properties can contribute to the mesopores, leading to the fact that a higher specific surface area does not necessarily correspond to a larger pore volume. The SEM images and corresponding EDS mappings depicted that the fresh and hydrothermally aged samples consisted of aggregated crystals with rectangular morphology and a size of ~700 nm, and highly dispersed Cu, Ce, and Mn species were observed on the catalysts (Figure 2 and Figure S2). Table 1 also summarizes the quantified Si/Al ratios and Cu contents of the catalysts. Compared with Cu-SSZ-39, there was no significant change in the Si/Al ratio and Cu content after supporting oxides or hydrothermal aging.

2.2. SCR Performance

NO_x conversion and N₂ selectivity as a function of reaction temperature between 150 and 600 °C in the Cu-SSZ-39, CeO₂/Cu-SSZ-39, CeMnO_x/Cu-SSZ-39, and MnO_x/Cu-SSZ-39 catalysts are shown in Figure 3a and Figure S3a. Cu-SSZ-39 showed a so-called “seagull shape” curve resulting from its low Cu loading [18], while MnO_x/Cu-SSZ-39 presented an evident decline in NO_x conversion over 400 °C. The presence of CeO₂ and CeMnO_x significantly benefitted the low-temperature NO_x reduction performance of the catalysts, particularly for CeMnO_x/Cu-SSZ-39, whose active temperature window is from 180 to 600 °C. The higher catalytic performance of CeMnO_x/Cu-SSZ-39 in comparison with those of CeO₂/Cu-SSZ-39 and MnO_x/Cu-SSZ-39 indicated the existence of synergetic effects between Ce and Mn in CeMnO_x in improving the low-temperature activity of Cu-SSZ-39 [18,41], which is consolidated by varying Mn/(Ce + Mn) ratios (Figure S4). After hydrothermal aging at 800 °C, NO_x conversions of the Cu-SSZ-39-A, CeO₂/Cu-SSZ-39-A, and MnO_x/Cu-SSZ-39-A catalysts declined, and their active temperature window became narrow (Figure 3b). As for Cu-SSZ-39-A, the NO_x conversion decreased to ~90% and remained within the temperature range of 300–600 °C. Therefore, the SCR activity deteriorated seriously after the hydrothermal aging treatment. In contrast, CeO₂/Cu-SSZ-39-A and MnO_x/Cu-SSZ-39-A still showed similarly high deNO_x activity as fresh counterparts despite a slight narrowing in active temperature windows (225–600 °C and 200–400 °C, respectively). Excitingly, CeMnO_x/Cu-SSZ-39-A even exhibited a slight increase in low-temperature activity in comparison with CeMnO_x/Cu-SSZ-39, presumably due to the interaction of CeMnO_x and the aggregated CuO species which facilitated the low-temperature SCR performance [18]. The active temperature window of CeMnO_x/Cu-SSZ-39-A was broadened to 175–600 °C, suggesting CeMnO_x/Cu-SSZ-39 is a promising catalyst in a wide temperature window. In addition, the N₂ selectivity of all the catalysts maintained levels above 95% even after hydrothermal aging (Figure S3). Therefore, the above results indicated the enhanced low-temperature activity and hydrothermal stability of Ce-Mn oxide-modified Cu-SSZ-39 catalysts.

2.3. Structural-Activity Relationships

In order to clarify how CeMnO_x enhances the SCR performance of Cu-SSZ-39 catalyst, the catalysts were comprehensively characterized as below. First, X-ray photoelectron spectroscopy (XPS) was employed to investigate the chemical state of surface elements of the catalysts, and the results are summarized in Figure 4 and

Table 2. Cu 2p_3/2, O 1s, Ce 3d, and Mn 2p_3/2 XPS analysis of the catalysts.

Sample	Cu (%)		O (%)			Ce (%)		Mn (%)
	Cu2+	CuO	Osur	Oads	Olat	Ce3+	Ce4+	Mn2+	Mn3+	Mn4+
Cu-SSZ-39	64.6	35.4	31.3	43.9	24.8	-	-	-	-	-
CeO2/Cu-SSZ-39	64	36	17.3	54.6	28.1	18.7	81.3	-	-	-
CeMnOx/Cu-SSZ-39	71	29	18.7	61	20.3	22.2	77.8	25.3	33	41.7
MnOx/Cu-SSZ-39	63.6	36.4	18	52.8	29.2	-	-	30.5	34.5	35
Cu-SSZ-39-A	38.8	61.2	30.5	36.3	33.2	-	-	-	-	-
CeO2/Cu-SSZ-39-A	42.4	57.6	16.6	51.8	31.6	18.5	81.5	-	-	-
CeMnOx/Cu-SSZ-39-A	48.5	51.5	16.6	58.7	24.7	21.6	78.4	27	35.1	37.9
MnOx/Cu-SSZ-39-A	44.6	55.4	17.6	47.4	35	-	-	34.5	39.1	26.4

. As shown in Figure 4(a1), each Cu 2p_3/2 XPS spectrum of the fresh catalysts can be deconvoluted into two peaks at ~935.8 and ~933.4 eV, which are attributed to Cu²⁺ and CuO species [43,44]. After the addition of Mn/Ce or Ce-Mn oxides, the peaks of Cu²⁺ and CuO species in the oxide-modified Cu-SSZ-39 catalysts all shift towards lower binding energies due to electron transfer from the Mn/Ce oxides to Cu species because of the higher electronegativity of Cu²⁺ ions [43]. After the hydrothermal treatment, the Cu²⁺ ratios in the aged catalysts decreased due to the aggregation of isolated Cu species to surface CuO species (Figure 4(a2) and Table 2). Notably, the binding energies of the Cu species in CeO₂/Cu-SSZ-39-A, CeMnO_x/Cu-SSZ-39-A, and MnO_x/Cu-SSZ-39-A catalysts are all lower than that in Cu-SSZ-39-A, suggesting that there still exist strong interactions between the supported oxides and Cu species in Cu-SSZ-39 even after hydrothermal aging. Furthermore, CeMnO_x/Cu-SSZ-39-A maintained the highest amount of Cu²⁺ ions among the oxide-modified catalysts after hydrothermal aging (Table 2), which revealed its superior SCR activity (Figure 3b). This also indicated CeMnO_x could effectively stabilize Cu-SSZ-39 zeolite and interact with the generated CuO species, contributing to its excellent hydrothermal resistance [27].

Figure 4(b1,b2) shows the O 1s XPS spectra of the catalysts. The spectra could be fitted into three peaks, corresponding to the oxygen in the adsorbed hydroxyl group on the surface (O_sur), chemisorbed surface oxygen (O_ads), and lattice oxygen (O_lat), respectively [45]. A new peak centered at ~530 eV appeared in CeO₂/Cu-SSZ-39 and CeMnO_x/Cu-SSZ-39, which is associated with lattice oxygen of CeO₂ [34]. The calculated O_ads concentrations in CeO₂/Cu-SSZ-39, CeMnO_x/Cu-SSZ-39, and MnO_x/Cu-SSZ-39 were higher than that in Cu-SSZ-39, which facilitated an increase in the mobility of oxygen and the redox ability, and thus could accelerate low-temperature SCR reactions [37,46]. The increased O_ads species could be attributed to the addition of CeO₂, MnO_x, or CeMnO_x with excellent redox properties, which provided more vacancies for O_ads. Furthermore, CeMnO_x/Cu-SSZ-39, with the largest amount of O_ads, exhibited the highest low-temperature activity among the oxide-modified Cu-SSZ-39 catalysts (Figure 3a). However, the amount of O_sur decreased with the addition of oxides, indicating the hydroxyl group as the Brønsted acid site was covered by the supported oxides. After the hydrothermal aging treatment, the ratios of O_ads species decreased due to partial sintering of the oxides.

As for the Ce 3d spectra, eight peaks were detected (Figure 4c), marked as u′ (903.5–904.0 eV) and v′ (885.1–885.5 eV) representing Ce³⁺, and with others marked as u‴ (916.7–917.0 eV), u″ (907.6–908.1 eV), u (900.5–901.2 eV), v‴ (898.1–898.5 eV), v″ (888.7–889.0 eV), and v (882.1–882.4 eV), attributed to Ce⁴⁺ [37]. Clearly, the surface Ce³⁺ ratio of CeMnO_x/Cu-SSZ-39 was higher than that of CeO₂/Cu-SSZ-39, and a similar trend was maintained even after hydrothermal aging. This may be a result of the redox cycles between Mnⁿ⁺ and Ceⁿ⁺ as followed by Mn³⁺ + Ce⁴⁺ ↔ Mn⁴⁺ + Ce³⁺ [38]. The presence of Ce³⁺ may cause an environment with imbalanced charges, resulting in an increase in oxygen vacancies. This, in turn, enhances the activation of surface oxygen species and improves the SCR activity [47,48].

The Mn 2p_3/2 spectra in Figure 4d were fitted into three peaks, corresponding to Mn⁴⁺ species (644.0–644.5 eV), Mn³⁺ species (642.3–642.7 eV), and Mn²⁺ species (640.9–641.6 eV), respectively [37]. As shown in Table 2, Mn⁴⁺ species are the dominant species in the catalysts, especially in CeMnO_x/Cu-SSZ-39 and CeMnO_x/Cu-SSZ-39-A. The higher ratio of Mn⁴⁺ species in CeMnO_x/Cu-SSZ-39 and CeMnO_x/Cu-SSZ-39-A indicates that the synergistic effect between Ce and Mn promoted the formation of Mn⁴⁺ species [45,49]. According to previous publications [38,50], the higher surface content of Mn⁴⁺ species can improve the catalytic activity at low temperatures, which is consistent with the SCR performance of the catalysts.

To sum up, the XPS analysis indicated that the synergistic effect between Ce and Mn resulted in an increase in active Cu²⁺ sites, O_ads, Ce³⁺, and Mn⁴⁺ species in Ce-Mn oxide-modified Cu-SSZ-39 catalysts with different Mn/(Ce + Mn) ratios (Figure S5 and Table S1). These factors contributed to the improved redox property and low-temperature SCR activity.

To investigate the redox property of the catalysts, which is intimately related to their SCR performance, a temperature programmed reduction of hydrogen (H₂-TPR) was carried out, and the results are shown in Figure 5. Three peaks are observed in the H₂-TPR profile of Cu-SSZ-39 (Figure 5a). The peak at ~230 °C is ascribed to the reduction of tri-coordinated Cu²⁺ to Cu⁺, i.e., [Cu(OH)]⁺ ions that are located in the window of eight-membered rings balanced with one framework negative charge (denoted as ZCuOH) [43]. As the XPS results confirmed the existence of CuO species in Cu-SSZ-39 (Figure 4(a1)), the broad peak ranging from 300 to 500 °C is attributed to the reduction of CuO and Cu²⁺ ions located in the window of six-membered rings and coordinated with 2Al sites (denoted as Z₂Cu) [28]. The peak above 500 °C is assigned to the reduction of Cu⁺ ions to Cu⁰ [51]. After supporting the oxides, the H₂-TPR profiles became more complicated, and some new H₂ consumption peaks were detected. In the case of CeO₂/Cu-SSZ-39, the emergence of broad reduction peaks centered at ~350 and 670 °C is ascribed to the reduction of CeO₂ (Ce⁴⁺ to Ce³⁺) in a mixed oxide phase [34,52]. For MnO_x/Cu-SSZ-39, two prominent peaks at ~300 and 650 °C were detected, which could be assigned to the reduction of MnO_x (Mn⁴⁺ to Mn³⁺) and Mn₂O₃ (Mn³⁺ to Mn²⁺), respectively [38,53]. In the H₂-TPR profile of CeMnO_x/Cu-SSZ-39, the reduction peaks of ZCuOH, MnO_x, Mn₂O₃, and CeO₂ shifted to lower temperatures, suggesting the promoted redox property after supporting CeMnO_x on Cu-SSZ-39. A similar phenomenon was observed in Ce-Mn oxide-modified Cu-SSZ-39 catalysts with different Mn/(Ce + Mn) ratios (Figure S6). These results indicate that the interaction of CeMnO_x with Cu-SSZ-39 and the synergistic effect between Ce and Mn in CeMnO_x improve the redox property, and thus contribute to the enhanced SCR activity.

After the hydrothermal aging treatment, the reduction peaks of ZCuOH species almost disappeared in the catalysts, while the peaks of CuO species became more pronounced due to the generation of CuO species with the deconstruction of the zeolite framework (Figure 5b). Particularly for CeMnO_x/Cu-SSZ-39-A, the reduction peak of CuO species shifted to lower temperatures, suggesting that the interaction between CeMnO_x and CuO promoted the redox property and, thus, maintained the excellent SCR activity even after hydrothermal aging (Figure 3b). Additionally, this interaction effectively prevented the aggregation of CuO as well as the further exacerbation of the zeolite collapse.

The acidity of the catalysts also played a crucial role in the analysis of NH₃ adsorption and activation during the SCR reaction [54]. Temperature programmed desorption of NH₃ (NH₃-TPD) was conducted to determine the acid strength of the catalysts, and the results are illustrated in Figure 6. The curves of the fresh catalysts show similar features with three desorption peaks (Figure 6a). The literature suggests that the peak at approximately 220 °C (Peak α) is attributed to the weakly adsorbed NH₃, mainly terminal hydroxyl species. The medium-temperature peak (Peak β) is ascribed to the NH₃ adsorbed on the strong Lewis acid sites created by the metal ions at around 305 °C. Finally, the high-temperature desorption peak (Peak γ) above 480 °C is attributed to the NH₃ desorption from the Brønsted acid sites [17,25,29,55]. To quantify the number of acid sites, the acidity of fresh and aged catalysts was calculated and is summarized in

Table 3. Quantification of NH₃ desorption amount based on NH₃-TPD results.

Sample	NH3 Desorption Amount (μmol·gcat−1)
	Weak (Peak α)	Medium (Peak β)	Strong (Peak γ)	Total
Cu-SSZ-39	55.3	110.6	624.4	790.3
CeO2/Cu-SSZ-39	46.1	138.1	473.7	657.9
CeMnOx/Cu-SSZ-39	60.5	204.2	491.6	756.3
MnOx/Cu-SSZ-39	153.1	145.7	429.9	728.7
Cu-SSZ-39-A	18.6	121.0	170.6	310.2
CeO2/Cu-SSZ-39-A	26.4	201.9	227.5	455.8
CeMnOx/Cu-SSZ-39-A	34.5	222.7	236.1	493.3
MnOx/Cu-SSZ-39-A	87.8	294.0	177.2	559.0

. The total acid amounts in CeO₂/Cu-SSZ-39, CeMnO_x/Cu-SSZ-39, and MnO_x/Cu-SSZ-39 are lower than that in Cu-SSZ-39, which is probably due to the acid sites being partially covered by the supported oxides, consistent with XPS results. Notably, the oxide-modified Cu-SSZ-39 catalysts had larger sums of weak and medium acidity compared with Cu-SSZ-39, which indicated that the existence of Ce/Mn or Ce-Mn oxide in Cu-SSZ-39 could promote the adsorption and activation of NH₃ at low and medium temperatures, thus facilitating the SCR reaction [56].

Compared to the fresh catalysts, the desorption temperatures of the peaks over the aged ones shifted towards lower values and the NH₃ desorption amount significantly decreased (Figure 6b and Table 3), suggesting that both the strength and amount of acid sites are affected by the hydrothermal aging treatment. The decrease in Peaks α and γ can be attributed to the collapse of the zeolite framework and the loss of active sites; in contrast, the increased intensity in Peak β indicates that the additional Lewis acid sites arise from the transformation of Cu²⁺ ions and the aggregation of oxides during hydrothermal aging [4]. Interestingly, CeO₂/Cu-SSZ-39-A, CeMnO_x/Cu-SSZ-39-A, and MnO_x/Cu-SSZ-39-A catalysts exhibited a slightly lower decrease in Peaks α and γ compared to Cu-SSZ-39-A. The lesser disruptive effect on the acid sites can be attributed to the interaction of the supported oxides with Cu-SSZ-39, which results in their higher activity and hydrothermal stability, as indicated in the SCR performance of the aged catalysts (Figure 3b).

In order to reveal the stabilizing effect of CeMnO_x on the zeolite framework of Cu-SSZ-39, ²⁹Si, and ²⁷Al, MAS NMR spectra were measured to determine the local Si and Al environment in fresh and hydrothermally aged Cu-SSZ-39 and CeMnO_x/Cu-SSZ-39 catalysts. As illustrated in Figure 7a, the chemical shifts at −101, −106, and −112 ppm were assigned to Si (4Si), Si (3Si, 1Al), and Si (2Si, 2Al), respectively. As shown in

Table 4. The deconvolution results of ²⁹Si and ²⁷Al MAS NMR spectra.

Sample	Si/Al Ratio	Proportion of Si (nSi, mAl) (%)			Tetra-Coordinated Al	Penta-Coordinated Al	Hexa-Coordinated Al
		Si (4Si)	Si (3Si, 1Al)	Si (2Si, 2Al)
Cu-SSZ-39	9.3	63.4	30.4	6.2	91.7	-	8.3
CeMnOx/Cu-SSZ-39	9.1	62.5	31	6.5	96.2	-	3.8
Cu-SSZ-39-A	25.3	87.7	8.8	3.5	67.1	22.8	10.1
CeMnOx/Cu-SSZ-39-A	16.5	81.2	13.3	5.5	87.7	10.5	1.8

, the = Si/Al ratios calculated using ²⁹Si MAS NMR were in good agreement with those of Cu-SSZ-39 and CeMnO_x/Cu-SSZ-39 catalysts derived from EDS mappings (Table 1). However, the hydrothermal aging treatment resulted in an increase in Si/Al ratios (calculated using NMR), suggesting significant dealumination of the zeolite framework. Notably, CeMnO_x/Cu-SSZ-39-A exhibited a much lower increase in the Si/Al ratio compared to Cu-SSZ-39-A, indicating its higher hydrothermal stability. The ²⁷Al MAS NMR spectra (Figure 7b) showed features at 58 and 0 ppm, which corresponded to tetra-coordinated framework Al and hexa-coordinated extra-framework Al, respectively [23]. After hydrothermal aging, the feature at 33 ppm, arising from the penta-coordinated extra-framework Al, emerged, indicating that Al ions detached from the framework [43]. In order to quantify differently coordinated Al species, ²⁷Al MAS NMR spectra were deconvoluted, and the results are also listed in Table 4. It is worth noting that CeMnO_x/Cu-SSZ-39-A preserved a greater amount of tetrahedrally coordinated framework Al than Cu-SSZ-39-A, suggesting that it possesses a higher hydrothermal stability. Combining the results of NMR and SCR activity, the supported CeMnO_x on Cu-SSZ-39 could effectively inhibit the dealumination from the zeolite framework and thus guarantee the excellent hydrothermal stability and SCR activity of CeMnO_x/Cu-SSZ-39-A.

2.4. Reaction Kinetics and Possible Mechanisms

To further investigate the essential influence of CeMnO_x on the SCR performance of Cu-SSZ-39, CeMnO_x/Cu-SSZ-39 and Cu-SSZ-39 were measured in an SCR reaction under kinetics-controlled differential conditions (Figure S7), and the Arrhenius plots are shown in Figure 8. The apparent activation energies (E_a) depend on the reaction mechanisms of the catalyst being employed, and the E_a of Cu-SSZ-39 was 50.2 kJ·mol⁻¹, which is comparable to that reported in previous studies [30]. Distinctively, the E_a of CeMnO_x/Cu-SSZ-39 (17.1 kJ·mol⁻¹) was much lower than that of Cu-SSZ-39, suggesting that Ce-Mn oxide could decrease the energy barriers and accelerate the SCR reaction rate, especially at low temperatures [38]. Moreover, the huge difference in E_a values indicates that the SCR reaction in CeMnO_x/Cu-SSZ-39 probably proceeded through another pathway, i.e., fast SCR [55]. The literature suggests that the formation of NO₂ from NO oxidation is a crucial process for low-temperature activity since NO₂ can initiate fast SCR reactions [20]. Consequently, NO oxidation experiments were conducted, and the results are shown in Figure 9. The NO conversion of Cu-SSZ-39 was low and there was almost no change above 200 °C, indicating that NO was hardly oxidized into NO₂, which rules out the possibility that the formation of NO₂ in Cu-SSZ-39 catalysts facilitated NO_x conversion through the fast SCR pathway. Notably, NO oxidation in CeMnO_x/Cu-SSZ-39 was much higher than that in Cu-SSZ-39 and increased rapidly with increasing temperatures, due to the improved redox property derived from CeMnO_x, which was confirmed by the XPS and H₂-TPR results (Figure 4 and Figure 5). Based on the literature, the generated NO₂ from NO oxidation participates in fast SCR reactions, and its reaction rate is higher than that of the standard SCR reaction (Figure S7); thus, the low-temperature SCR performance is improved [29,30].

To corroborate this speculation, the performance of Cu-SSZ-39 under fast SCR feed conditions was measured. As shown in Figure 10, the presence of NO₂ in the feed increased the NO_x conversions at low temperatures for Cu-SSZ-39 in comparison with under the standard SCR condition (Figure S8), confirming that both NO and NO₂ took part in the fast SCR reactions. This improved low-temperature activity is derived from the promoting effect of NO₂ on the SCR performance of Cu-SSZ-39 catalysts [30,55]. For CeMnO_x/Cu-SSZ-39, further enhancement of NO and NO₂ conversions was observed compared with Cu-SSZ-39 at low temperatures (Figure 10 and Figure S8), which could be basically attributed to NO oxidation and the low-temperature SCR activity of CeMnO_x. Notably, the NO₂ conversion of CeMnO_x/Cu-SSZ-39 did not increase with temperature as in the case of Cu-SSZ-39, and it had a minimum value at 175 °C. According to previous research, this decreased NO₂ conversion is probably due to the reduction of NH₄NO₃ by NO to generate NO₂ [32,34]. As such, the catalytic effect of CeMnO_x on NO oxidation provides a further impetus for accessing the fast SCR pathway of Cu-SSZ-39 [53].

3. Materials and Methods

3.1. Preparation

Na-SSZ-39 (Si/Al = 8.7) was synthesized by the inter-zeolite transformations of FAU zeolite according to procedures described in the literature. Y (Si/Al = 3.1) and N,N-dimethyl-3,5-dimethylpiperidinium hydroxide (DMDMPOH) were used as a precursor and an organic structure directing agent (OSDA), respectively [19]. Typically, 15.2 mg NaOH was dissolved in 35.6 g deionized water; then, 10.1 mL DMDMPOH was added and stirred for 30 min. After that, 20.1 mL Na₂SiO₃ and 1.0 g Y zeolite were added to the above mixture with continuous stirring for 2 h, and the final molar composition of the mixture was 1 SiO₂: 0.015 Al₂O₃: 0.559 NaOH: 0.116 OSDA: 24.1 H₂O. The final mixture was transferred to a 100 mL Teflon-lined stainless-steel autoclave and crystallized at 140 °C for 24 h. The as-prepared zeolite was then centrifuged, washed, and dried at 105 °C overnight, followed by calcination at 575 °C for 8 h in air. Subsequently, the resulting Na-SSZ-39 zeolite was ion-exchanged with (NH₄)₂SO₄ and CuSO₄ solutions. After drying at 105 °C overnight and calcining at 575 °C for 8 h, the Cu-SSZ-39 catalyst was obtained.

Ce-Mn oxide-modified Cu-SSZ-39 was prepared via the impregnation method by using Ce(NO₃)₃ and Mn(NO₃)₂ as precursors. The total component of Ce-Mn composite oxide was 5 wt.% and the molar ratio of Mn/(Ce + Mn) was 0.3. For more detail, Cu-SSZ-39 was slowly added to the Ce(NO₃)₃ and Mn(NO₃)₂ mixed solution under ultrasonic conditions. The amount of mixed solution was equal to the pore volumes of Cu-SSZ-39 zeolite. And then, the mixture was dried overnight at 105 °C. After being calcined at 550 °C for 4 h in muffle, the modified catalysts were obtained and labeled as CeMnO_x/Cu-SSZ-39. Additionally, Ce-Mn oxide-modified Cu-SSZ-39 catalysts with different Mn/(Ce + Mn) ratios [Mn/(Ce + Mn) = 0.2, 0.4, and 0.5] were also prepared to further demonstrate the synergetic effects between Ce and Mn in Ce-Mn oxides. For comparison, CeO₂- and MnO_x-modified Cu-SSZ-39 catalysts were prepared in the same way (denoted as CeO₂/Cu-SSZ-39 and MnO_x/Cu-SSZ-39) and used as references.

Hydrothermal aging of the catalysts was carried out in a flowing wet-air stream containing 10 vol.% H₂O and held at 800 °C for 16 h. The obtained aged catalysts were denoted as Cu-SSZ-39-A, CeMnO_x/Cu-SSZ-39-A, CeO₂/Cu-SSZ-39-A, and MnO_x/Cu-SSZ-39-A, respectively.

3.2. Characterizations

XRD measurements were performed on Smartlab SE X-ray diffractometer (Rigaku, Tokyo, Japan) with Cu Kα radiation. The diffraction patterns were taken in the 2θ range of 5–50°. The surface area and pore structure were analyzed using N₂ adsorption/desorption measurements obtained with a Micromeritics ASAP 2020 instrument (Micromeritics, Norcross, GA, USA). The samples were degassed at 300 °C for 8 h before analysis. A GeminiSEM 300 microscope (Carl Zeiss, Oberkochen, Germany) was used to perform scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS). X-ray photoelectron spectroscopy (XPS) was performed on an AXIS Supra instrument (Shimadzu, Kyoto, Japan), calibrating binding energies using a C 1s peak at 284.8 eV. A Bruker AVANCE NEO 600 WB (Bruker, Saarbrucken, Germany) was used to conduct magnetic angle spinning nuclear magnetic resonance (MAS NMR) experiments. The resonance frequencies of ²⁹Si and ²⁷Al are 119.23 and 156.38 MHz, respectively. To record ²⁹Si MAS NMR spectra, a 4 mm MAS probe was used with a spinning frequency of 10 kHz, a recycle delay of 2 s, and a π/2 pulse length of 1.67 μs. For ²⁷Al MAS NMR spectra, a 4 mm MAS probe was used with a spinning frequency of 15 kHz, a pulse delay of 20 s, and a π/2 pulse length of 1.67 μs.

Temperature programmed reduction with H₂ (H₂-TPR) experiments were performed using a XIANQUAN TP-5080D chemisorption analyzer (XIANQUAN, Tianjin, China). The sample (100 mg) was treated at 500 °C for an hour in an O₂ flow before conducting TPR in 5 vol.% H₂/N₂ at a flow rate of 20 mL∙min⁻¹. To perform the temperature programmed desorption with NH₃ (NH₃-TPD) experiments, a mass spectrometer (OmniStar GSD 320, Pfeiffer, Germany) was used. Before the experiments, the samples were pretreated at 500 °C for 30 min in 10 vol.% O₂/He and cooled to 100 °C. NH₃ was adsorbed in 0.4 vol.% NH₃/He until saturation. Then, the samples were purified by purging with He to clean the surface. The TPD experiments involved increasing the temperature from 100 to 800 °C at a ramp rate of 10 °C·min⁻¹ under He flow.

3.3. Catalyst Activity and Reaction Kinetics Measurements

NH₃-SCR activity was performed in a fixed-bed reactor with a thermocouple placed inside the catalyst bed (about 0.1 g, 40–60 mesh) and in the temperature range 150–600 °C. The reactant gas conditions were as follows: 500 ppm of NO_x (500 ppm of NO for standard SCR or 250 ppm of NO₂ + 250 ppm of NO for fast SCR), 500 ppm of NH₃, 5.3 vol.% of O₂, and He as the balance gas. The total gas flow rate was maintained at 300 mL∙min⁻¹ and the gas hourly space velocity (GHSV) was 100,000 h⁻¹. The concentrations of NO and NO₂ were monitored using a chemiluminescence NO_x analyzer (Model 42i-HL, Thermo Electron Corporation, Waltham, MA, USA), while N₂O and NH₃ were recorded using a quadrupole mass spectrometer (OmniStar GSD 301, Pfeiffer, Germany). The NO_x conversion and N₂ selectivity were calculated using Equations (1) and (2):

$${\mathrm{NO}}_x \mathrm{conversion}=\frac{{\left[{\mathrm{NO}}_x\right]}_{\mathrm{in}}-{\left[{\mathrm{NO}}_x\right]}_{\mathrm{out}}}{{{[\mathrm{NO}}_x]}_{\mathrm{in}}}\times 100\%$$

(1)

$${\mathrm{N}}_2 \mathrm{selectivity}=\frac{{\left[{\mathrm{NO}}_x\right]}_{\mathrm{in}}+[{{\mathrm{NH}}_3]}_{\mathrm{in}}-{\left[{\mathrm{NO}}_x\right]}_{\mathrm{out}}-{\left[{\mathrm{NH}}_3\right]}_{\mathrm{out}}-2\times {[\mathrm{N}}_2\mathrm{O}]}{{\left[{\mathrm{NO}}_x\right]}_{\mathrm{in}}+{\left[{\mathrm{NH}}_3\right]}_{\mathrm{in}}-{\left[{\mathrm{NO}}_x\right]}_{\mathrm{out}}-{\left[{\mathrm{NH}}_3\right]}_{\mathrm{out}}}\times 100\%$$

(2)

The experiment of NO oxidation was conducted using the same procedure as the NH₃-SCR test. The inlet gas composition was as follows: [NO] = 500 ppm, [O₂] = 5.3 vol.%, balanced by He. We calculated the NO conversion using Equation (3):

$$\mathrm{NO} \mathrm{conversion}=\frac{{\left[\mathrm{NO}\right]}_{\mathrm{in}}-{\left[\mathrm{NO}\right]}_{\mathrm{out}}}{{\left[\mathrm{NO}\right]}_{\mathrm{in}}}\times 100\%$$

(3)

To obtain kinetic data within the kinetic regime and eliminate the effects of mass transfer limitations, we selected particles of samples and a total flow rate of 100–200 mesh and 600 mL∙min⁻¹, respectively. Additionally, we estimated the GHSV to be 800,000 h⁻¹ with a decreased amount of catalyst of 0.045 mL. The apparent activation energy (E_a) was calculated based on the Arrhenius plot of the reaction rate (r) versus 1/T (T is the temperature) when the NO_x conversion was kept below 15% [57].

4. Conclusions

In this study, Ce-Mn oxide-modified Cu-SSZ-39 catalysts (CeMnO_x/Cu-SSZ-39) along with CeO₂/Cu-SSZ-39 and MnO_x/Cu-SSZ-39 references were prepared through the ion-exchange of Cu ions followed by impregnation of the oxide precursors. The supported oxides could improve the low-temperature SCR activity for Cu-SSZ-39. Especially for CeMnO_x/Cu-SSZ-39, the low-temperature SCR activity was considerably improved, achieving above 90% NO_x conversion between 180 and 600 °C. Notably, this superior activity was maintained even after hydrothermal aging at 800 °C, indicating excellent hydrothermal stability. The supported oxides rendered more Cu²⁺, Ce³⁺/Mn⁴⁺, and adsorbed oxygen species in the catalyst, and all of these enhanced the redox properties and acidity, which facilitated the SCR reaction at low temperatures. Furthermore, CeMnO_x/Cu-SSZ-39 took advantage of CeMnO_x, as well as the strong interaction between CeMnO_x and Cu-SSZ-39, resulting in exceptional SCR activity over a wide temperature range. Importantly, the interaction of Cu-SSZ-39 with oxides, especially CeMnO_x, contributed to the remarkable hydrothermal stability of the zeolite by effectively inhibiting the dealumination from the zeolite framework and preventing the aggregation of Cu species during the hydrothermal aging process. The NO oxidation and fast SCR activity measurements confirmed that the catalytic effect of CeMnO_x on NO oxidation provides an impetus for accessing the fast SCR pathway of Cu-SSZ-39. Therefore, CeMnO_x opens up the fast SCR pathway by oxidizing NO to NO₂, which significantly enhances the low-temperature activity. Our research results not only demonstrate the importance of synergy in Ce-Mn oxides, but also provide a new approach for developing a highly effective Cu-SSZ-39 zeolite SCR catalyst over a wide temperature range, which is of great significance for achieving efficient NO_x removal in diesel vehicles.