Remarkable Enhancement of the Activity and Hydrothermal Stability of a CeO2-Based NH3-SCR Catalyst by Sn Modification

Ying Zhu; Jingjing Liu; Guangzhi He; Shaohua Xie; Wenpo Shan; Zhihua Lian; Fudong Liu; Hong He

doi:10.1016/j.eng.2024.02.011

Engineering ›› 2025, Vol. 48 ›› Issue (5) :148 -158. DOI: 10.1016/j.eng.2024.02.011

Research Green Chemical Engineering—Article

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Remarkable Enhancement of the Activity and Hydrothermal Stability of a CeO₂-Based NH₃-SCR Catalyst by Sn Modification

Ying Zhu ^a^,^d^,^#
, Jingjing Liu ^c^,^#
, Guangzhi He ^c
, Shaohua Xie ^e
, Wenpo Shan ^a^,^b^,^*
, Zhihua Lian ^a
, Fudong Liu ^e
, Hong He ^a^,^c^,^d^,^*

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^aCenter for Excellence in Regional Atmospheric Environment & Key Laboratory of Urban Pollutant Conversion, Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 361021, China

^bZhejiang Key Laboratory of Pollution Control for Port-Petrochemical Industry, Ningbo Urban Environment Observation and Research Station, Institute of Urban Environment, Chinese Academy of Sciences, Ningbo 315800, China

^cState Key Joint Laboratory of Environment Simulation and Pollution Control, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China

^dUniversity of Chinese Academy of Sciences, Beijing 100049, China

^eDepartment of Civil, Environmental and Construction Engineering, Catalysis Cluster for Renewable Energy and Chemical Transformations, NanoScience Technology Center, University of Central Florida, Orlando, FL 32816, USA

E⁃mail addresses: wpshan@iue.ac.cn (W. Shan),

honghe@rcees.ac.cn (H. He)

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Abstract

Catalytic activity and hydrothermal stability are both crucial for the application of the selective catalytic reduction of NO_x with NH₃ (NH₃-SCR) catalyst in diesel vehicles. In this study, a tin (Sn)-modified Ce–Nb mixed-oxide catalyst was synthesized as an NH₃-SCR catalyst for NO_x emission control. After the introduction of Sn, both the NH₃-SCR activity and the hydrothermal stability of the catalyst were remarkably promoted. Even after hydrothermal aging at 1000 °C, the developed Ce₁Sn₂Nb₁O_x catalyst achieved more than 90% NO_x conversion at 325–500 °C. Various methods, including N₂-physisorption, X-ray diffraction, in-situ high-temperature X-ray diffraction, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray absorption fine-structure spectroscopy, temperature-programmed reduction of hydrogen, temperature-programmed desorption of ammonia, and density functional theory calculations were used to investigate the promotional effects induced by the Sn species. The characterization results showed that the addition of Sn not only promoted the formation of the Ce–Nb active phase but also improved its thermal stability, contributing to the excellent NH₃-SCR performance and hydrothermal stability. This study provides an excellent sintering-resistance catalyst for the application of diesel engine NO_x emission control.

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Keywords

SnO₂ / CeO₂-based catalyst / NH₃-SCR / Hydrothermal stability / NO_x emission control

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Ying Zhu, Jingjing Liu, Guangzhi He, Shaohua Xie, Wenpo Shan, Zhihua Lian, Fudong Liu, Hong He. Remarkable Enhancement of the Activity and Hydrothermal Stability of a CeO₂-Based NH₃-SCR Catalyst by Sn Modification. Engineering, 2025, 48(5): 148-158 DOI:10.1016/j.eng.2024.02.011

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1. Introduction

Nitrogen oxides (NO and NO₂, jointly referred to as NO_x) emitted by diesel vehicles induce a series of environmental pollution problems, including acid rain, haze, and photochemical smog [1], [2], [3]. The selective catalytic reduction of NO_x with NH₃ (NH₃-SCR) is the most dominant technology for NO_x removal from the exhaust of diesel vehicles [1], [4]. However, to meet Chinese VI and European VI diesel emission standards, an NH₃-SCR catalytic converter is generally combined with an upstream diesel particulate filter (DPF) [5]. The active regeneration of the DPF typically exposes the NH₃-SCR catalyst to high temperatures of up to 750–800 °C in the presence of water vapor [6]. Therefore, NH₃-SCR catalysts for the purification of NO_x in diesel exhaust should simultaneously possess excellent catalytic performance and superior hydrothermal stability [7].

Many types of catalysts have been applied to the NH₃-SCR reaction, including oxides and zeolites based on transition metals and rare earth metals. At present, the dominant NH₃-SCR catalyst for diesel vehicles is copper (Cu)-SSZ-13, a type of Cu-based small-pore zeolite with excellent catalytic activity and hydrothermal stability [8], [9], [10]. However, this type of catalyst cannot withstand hydrothermal aging at temperatures above 800 °C [11], [12], [13]. Among metal oxide catalysts, CeO₂-based catalysts have been widely studied due to their unique redox ability, oxygen-storage capability, and acid–base properties [14], [15]. Nevertheless, pure CeO₂ exhibits inferior NH₃-SCR performance and poor thermal stability. Therefore, a series of additives have been introduced under the guidance of a high-dispersion close-coupling (HDCC) principle for the development of highly efficient NH₃-SCR catalysts that involves the close coupling of redox and acid sites [1]. Niobium (Nb) compounds have been widely investigated as promoters for cerium oxide in the NH₃-SCR reaction because the formation of Nb–OH and Nb=O not only increases the number of Brønsted and Lewis acid sites [16], [17], [18], [19] but also helps to increase the number of oxygen vacancies in the catalysts [19], [20]. In a previous study by our group, Ding et al. [20] found that the introduction of Nb improved the activity and hydrothermal stability of CeZrO_x. However, the catalytic activity was less than 90% across the whole temperature range when the hydrothermal temperature increased to 850 °C, indicating that the role of Nb in promoting the hydrothermal stability of CeO₂-based catalysts was still unsatisfactory. Therefore, the hydrothermal durability of CeO₂-based catalysts still urgently needs to be enhanced.

SnO₂ is an n-type semiconductor with a tetragonal structure and a high melting point of 1630 °C [21]. It has been widely used in catalytic materials because it possesses a large number of intrinsic defects [22], [23]. Leite et al. [24], [25] and Weber et al. [26] reported that the formation of an M–Sn solid solution (M = Y, La, Ce, or Nb) granted SnO₂ excellent sintering resistance. Xu et al. [21], [27] and Zeng et al. [28] synthesized the above materials, which exhibited excellent thermal stability, and applied them to the field of carbon monoxide (CO) and methane (CH₄) oxidation. Recently, we found that tin (Sn) stabilized a cerium–tungsten (Ce–W) oxide catalyst, and the obtained Ce–W–Sn ternary metal oxide catalyst realized more than 90% NO_x conversion at 300–500 °C, even after severe hydrothermal aging at 1000 °C [29]. However, the promotional effects of the addition of Sn on NH₃-SCR performance and hydrothermal stability still require further investigation, and the universality of the stability-promotion effect of Sn needs to be verified with other CeO₂-based catalysts.

In this study, a prominent Ce–Sn–Nb ternary metal oxide catalyst with excellent NH₃-SCR activity and superior hydrothermal stability was synthesized with the addition of Sn to a Ce–Nb oxide catalyst. The promotional effects of the Sn were systemically investigated using various methods, including N₂-physisorption, X-ray diffraction (XRD), in-situ high-temperature X-ray diffraction (HTXRD), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), X-ray absorption fine-structure (XAFS) spectroscopy, temperature-programmed reduction of hydrogen (H₂-TPR), temperature-programmed desorption of ammonia (NH₃-TPD), in-situ diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS), and density functional theory (DFT) calculations. Sn is demonstrated to be an effective additive to improve the hydrothermal stability of a Ce–Sn–Nb catalyst.

2. Experimental section

2.1. Catalyst preparation and hydrothermal aging treatment

The Ce–Sn–Nb mixed oxide catalyst (denoted as Ce₁Sn_aNb₁O_x, where a = 0.5, 1.0, 1.5, 2.0, 2.5, and 3.0) was synthesized by means of a co-precipitation method using NH₃·H₂O (25.0%–28.0%; Sinopharm Chemical Reagent Co., Ltd., China) as the precipitator. The required amounts of Ce(NO₃)₃·6H₂O (≥ 99.0%; Sinopharm Chemical Reagent Co., Ltd.), SnCl₄·5H₂O (98.0%; Shanghai Aladdin Biochemical Technology Co., Ltd., China), and C₁₀H₅NbO₂₀ (98.0%; Shanghai Macklin Biochemical Technology Co., Ltd., China) were dissolved together in deionized water. The details of the catalyst preparation are provided in Appendix A.

The selected samples, fresh Ce₁Nb₁O_x, and Ce₁Sn₂Nb₁O_x catalysts were hydrothermally aged at 800 °C for 16 h, 900 °C for 16 h, and 1000 °C for 3 h, respectively, in a feed gas of 10 volume percent (vol%) H₂O/air condition to test the hydrothermal stability. The obtained samples were denoted as Ce₁Nb₁O_x-T and Ce₁Sn₂Nb₁O_x-T, where T represented the hydrothermal aging temperatures in °C.

2.2. Catalytic activity measurement

NH₃-SCR activity tests of the as-obtained catalysts were carried out at atmospheric pressure in a fixed-bed flow reactor system, which was equipped with a quartz tube with an inner diameter of 6 mm. The reaction conditions were controlled as follows: 500 ppm NO, 500 ppm NH₃, 5 vol% O₂, 5 vol% H₂O, and N₂ balance, with a total flow rate of 500 mL·min⁻¹ and a gas hourly space velocity (GHSV) of 100 000 h^–1. Water vapor was fed into the airflow by adding N₂ to the water in a thermostatic water bath. All the gas paths were covered with heating tape and aluminum foil to reduce heat loss and ensure that the reactants were completely vaporized. Reactant gases were adjusted using mass flowmeters before entering the reactor. The concentrations of the reactants and products were continuously monitored using a gas analyzer (Antaris IGS, Thermo Fisher Scientific, USA), which was equipped with a heated, low-volume multiple-path gas cell (2 m). Fourier-transform infrared (FTIR) spectra were collected when the NH₃-SCR reaction reached a steady state, and the NO_x conversion and N₂ selectivity were calculated using the following formulas:

(1)

α_{{N O}_{x}} = (1 - \frac{{[{N O}_{x}]}_{o u t}}{{[{N O}_{x}]}_{i n}}) \times 100 %

(2)

S_{N_{2}} = [1 - \frac{2 {[N_{2} O]}_{o u t}}{({[{N O}_{x}]}_{i n} - {[{N O}_{x}]}_{o u t}) + ({[{N H}_{3}]}_{i n} - {[{N H}_{3}]}_{o u t})}] \times 100 %

where α_{NO_x} referred to the conversion of NO_x; S_N₂ indicated the selectivity of N₂; [NO_x]_in/out (x = 1 or 2) and [NH₃]_in/out referred to the concentration of NO_x and NH₃ in the inlet and outlet, respectively; [N₂O]_out indicated the N₂O concentration in the outlet.

2.3. Catalyst characterization

The catalysts were characterized by means of N₂-physisorption (BELSORP MAX, MicrotracBEL Corp., Japan), XRD (X’Pert PRO, PANalytical, the Netherlands), in-situ HTXRD (D8 Advance, Bruker, Germany), HRTEM (JEM-3200F, JEOL, Japan), XPS (AXIS Supra, Kratos Analytical Inc., UK), XAFS spectroscopy (7-BM Quick X-ray Absorption and Scattering (QAS) of the National Synchrotron Light Source II, Brookhaven National Laboratory, USA), H₂-TPR (Autochem II 2920, Micromeritics, USA), and NH₃-TPD (Autochem II 2920), as described in detail in Appendix A.

2.4. DFT calculations

Spin-polarized periodic DFT calculations were performed using the Perdew–Burke–Ernzerhof (PBE) functional [30], as implemented in the Vienna ab initio simulation package (VASP) [31]. Details are provided in Appendix A.

3. Results and discussion

3.1. NH₃-SCR catalytic activity

The NO_x conversion and N₂ selectivity over the Ce₁Sn_aNb₁O_x catalysts are shown in Fig. S1 in Appendix A. After considering all the conditions, the Ce₁Sn₂Nb₁O_x catalyst with a Ce:Sn:Nb molar ratio of 1:2:1 was selected and tested for hydrothermal stability in this study. The NH₃-SCR activities over the ternary Ce₁Sn₂Nb₁O_x, unary (CeO₂, SnO₂, and Nb₂O₅), and binary (Ce₁Nb₁O_x, Ce₁Sn₂O_x, and Sn₂Nb₁O_x) catalysts at a GHSV of 100 000 h^–1 are shown in Fig. 1(a). All the unary catalysts exhibited very low NH₃-SCR activities, while the binary catalysts showed obviously improved activities. In fact, the Ce₁Nb₁O_x catalyst already exhibited high NH₃-SCR activity over a wide temperature range, indicating that the coupling of Ce and Nb oxides has synergistic effects for the NH₃-SCR. After the addition of Sn, the low-temperature activity of the Ce₁Nb₁O_x catalyst was further enhanced.

Interestingly, in addition to improving the NH₃-SCR activity, the addition of Sn remarkably enhanced the hydrothermal stability of the catalyst. After hydrothermal aging at an extremely high temperature of 1000 °C, almost 100% NO_x conversion was still obtained at 325–500 °C with the Ce₁Sn₂Nb₁O_x catalyst (Fig. 1(b)), accompanied by high N₂ selectivity during the NH₃-SCR reaction (Fig. S2 in Appendix A). On the other hand, hydrothermal aging induced a sharp decrease in the NH₃-SCR activity of Ce₁Nb₁O_x, with almost no NO_x conversion after aging at 900 and 1000 °C, indicating relatively low hydrothermal stability without the Sn doping. Therefore, the extraordinary hydrothermal stability of the Ce₁Sn₂Nb₁O_x catalyst was determined to be due to the addition of Sn. For comparison, a commercial Cu-SSZ-13 catalyst was aged at 800 °C for 16 h under the same conditions as the Ce₁Sn₂Nb₁O_x, and the NH₃-SCR activity was determined. The results showed that the Ce₁Sn₂Nb₁O_x catalyst possesses much better hydrothermal stability than the Cu-SSZ-13 catalyst, which has been commercially applied to diesel vehicles worldwide [32].

3.2. Textural properties

3.2.1. N₂ physisorption analysis

The Brunauer–Emmett–Teller (BET) surface area (S_BET) and total pore volume (V_total) of the fresh and aged Ce₁Nb₁O_x and Ce₁Sn₂Nb₁O_x catalysts were determined by means of nitrogen physisorption analysis; the results are listed in Table 1. The introduction of Sn resulted in some changes in the textural properties of the catalysts, with the Ce₁Sn₂Nb₁O_x catalyst exhibiting a higher specific surface area and total pore volume in comparison with the Ce₁Nb₁O_x catalyst. After hydrothermal aging, it was evident that a significant decrease in S_BET and V_total occurred for both of the Ce₁Nb₁O_x and Ce₁Sn₂Nb₁O_x catalysts. However, even when the hydrothermal aging temperature was increased to 1000 °C, Ce₁Sn₂Nb₁O_x-1000 still possessed an S_BET value of 16.2 m²·g⁻¹ and a V_total value of 0.10 cm³·g⁻¹, which were much higher than those of Ce₁Nb₁O_x-1000 (3.6 m²·g⁻¹ and 0.01 cm³·g⁻¹, respectively). This finding indicates that the formation of sintered particles of Ce₁Nb₁O_x during the hydrothermal aging was inhibited by the addition of Sn, which would be beneficial to the NH₃-SCR reaction.

3.2.2. XRD analysis

To investigate the phase composition, we examined the XRD patterns of the catalysts (Fig. 2), which suggested that the Ce₁Nb₁O_x catalyst is a polycrystalline oxide (Fig. 2(a)) consisting of a cubic CeO₂ (c-CeO₂) phase (Powder Diffraction File (PDF) 43-1002), an orthorhombic Nb₂O₅ (o-Nb₂O₅) phase (PDF 30-0873), and a tetragonal NbO₂ (t-NbO₂) phase (PDF 34-0898). A transition from the orthorhombic Nb₂O₅ phase to the monoclinic Nb₂O₅ (m-Nb₂O₅) phase (PDF 74-0298) appeared for the Ce₁Nb₁O_x-800 catalyst. As the hydrothermal aging temperature increased to 900 °C, aside from significant sintering, a phase transition from Nb₂O₅ and NbO₂ to Ce_0.333(NbO₃) (PDF 70-4920) and CeNb₅O₁₄ (PDF 20-0267) was observed. When the hydrothermal aging temperature was further increased to 1000 °C, crystallization of the Ce_0.333(NbO₃) phase was clearly suppressed, while a new CeNbO_4.08 (PDF 53-0754) phase and CeNbO_4.25 (PDF 53-0755) phase appeared. As shown in Fig. 2(b), with the addition of Sn to the Ce₁Nb₁O_x catalyst, a c-CeO₂ phase and a tetragonal SnO₂ (t-SnO₂) phase were observed in the Ce₁Sn₂Nb₁O_x samples, while no characteristic diffraction peaks attributed to Nb species were detected, suggesting that the Nb species may exist in an amorphous or highly dispersed state, or may enter a lattice of Ce or Sn to form a solid solution. After hydrothermal aging at 800 °C, little change was observed. Upon further increasing the hydrothermal aging temperature, however, a new CeNbO_4.25 phase with diffraction peaks at 29.4°, 31.4°, 43.5°, 46.6°, 53.9°, and 54.7° appeared. This phase became dominant on the catalyst that suffered from at an aging temperature of 1000 °C, indicating the occurrence of phase separation. In comparison with Ce₁Nb₁O_x, the crystal phase transition of the Ce₁Sn₂Nb₁O_x catalyst was clearly inhibited by the Sn doping and the stability of its structure during hydrothermal aging was improved, which may account for the excellent hydrothermal stability of the Ce₁Sn₂Nb₁O_x catalyst.

To further investigate the structure of the Ce₁Sn₂Nb₁O_x catalyst, Rietveld refinement was used to calculate the lattice parameters and the mean crystallite size of each phase (Fig. 3; Table S1 and Fig. S3 in Appendix A). For the binary Ce₁Sn₂O_x catalyst, the cell volume of c-CeO₂ (V_CeO₂) was less than that of the pure CeO₂ phase, while the cell volume of t-SnO₂ (V_SnO₂) was larger than that of the pure SnO₂ phase, as listed in Table S1. Since the ionic radii of Ce⁴⁺, Sn⁴⁺, and Nb⁵⁺ are 0.87, 0.69, and 0.64 Å, respectively, the lattice contraction of c-CeO₂ and the lattice expansion of t-SnO₂ may be due to the partial incorporation of each other, resulting in the formation of c-Ce(Sn)O₂ and t-Sn(Ce)O₂ solid solutions, in line with previous studies [27], [28]. Nevertheless, the peak of SnO₂(110) shifted toward a higher angle for the Ce₁Sn₂Nb₁O_x catalyst (Fig. S3(b)). At the same time, the V_SnO₂ of the Ce₁Sn₂Nb₁O_x catalyst decreased compared with that of the Ce₁Sn₂O_x, whereas V_CeO₂ hardly changed. This may be caused by the incorporation of Nb⁵⁺ into the lattice of t-SnO₂ to form a Sn–Nb solid solution structure. When the hydrothermal aging temperature was increased to 900 °C, the m-CeNbO_4.25 structure was observed. It was also noted that lattice expansion of c-CeO₂ occurred, which may be caused by the movement of some Sn species from the inner lattice to the surface and a transition from Ce⁴⁺ to Ce³⁺. Combining these findings with later XPS results, we deduced that part of the source of Ce³⁺ may be the formation of the CeNbO_4.25 phase. Leite et al. [24], [25] revealed that the particle growth of SnO₂ can be controlled by the addition of CeO₂ and Nb₂O₅, resulting in high thermal stability. In the present work, the formation of Ce–Sn and Sn–Nb metastable solid solutions, as mentioned above, was beneficial for inhibiting the particle growth of SnO₂ and stabilizing the active surface Ce and Nb species for the NH₃-SCR reaction.

3.2.3. In-situ HTXRD analysis

An in-situ HTXRD experiment was carried out to investigate the phase transition and thermal stability of the catalysts (Fig. S4 in Appendix A); the most pronounced region of crystal phase transition (22.8°–34.4°) is shown in Fig. 4. When the temperature reached 850 °C, the CeNbO_4.08 (42.9°) and Nb₂O₅ (24.3°) phases were observed for the Ce₁Nb₁O_x catalyst. Upon further increasing the temperature to 900, 950, and 1000 °C, the new CeNb₃O₉ (23.1°, 25.7°, 27.9°, 31.3°, 32.6°, 46.4°, 48.3°, 52.4°, and 67.8°) and Nb₂O₅ (74.5°) phases were detected. A large number of Ce–Nb crystal phases were formed by solid–solid reactions accompanied by the release of oxygen (e.g., 2CeO₂ + 3Nb₂O₅ → 6Ce_1/3NbO₃ + 1/2O₂, 2/3CeO₂ + 1/3CeNb₃O₉ → CeNbO₄ + 1/6O₂, t-CeNbO₄ + δ/2O₂ → m-CeNbO₄₊_δ) [33], [34], which blocked the redox cycle of the Ce species through the process of oxygen release/storage, restraining the high-temperature performance [29]. For the Ce₁Sn₂Nb₁O_x catalyst, however, there was no crystalline phase change up to 950 °C. Nb₂O₅ with different crystal types (PDF 18-0911, 22-1196, and 32-0711) formed at 1000 °C and then completely disappeared when the temperature cooled down to room temperature. Meanwhile, a new CeNbO_4.25 crystal phase was observed at room temperature. The Ce–Nb crystal phase did not form in the Ce₁Sn₂Nb₁O_x catalyst throughout the entire high-temperature experimental environment, indicating that the redox cycle of the catalyst was not blocked under such harsh heat treatment. Therefore, Sn-doping contributes to maintaining the redox cycle under high-temperature aging by preserving the dispersion of Ce–Nb species.

3.2.4. HRTEM analysis

HRTEM was used to explore the morphologies of the obtained catalysts (Fig. S5 in Appendix A). As shown in Figs. S5(a) and (e), the particle sizes of Ce₁Sn₂Nb₁O_x were clearly much smaller than those of Ce₁Nb₁O_x. With an increase in the hydrothermal aging temperature, the particle sizes of Ce₁Nb₁O_x grew rapidly, while only mild particle growth occurred for Ce₁Sn₂Nb₁O_x, even after hydrothermal aging at 1000 °C for 3 h (Figs. S5(b–d) and (f–h)). This finding indicated that the addition of Sn inhibited the grain growth of the Ce₁Nb₁O_x catalyst through the formation of Ce–Sn and Sn–Nb solid solutions.

The energy dispersive X-ray spectroscopy (EDS) mapping of Ce, Nb, and oxygen (O)—with and without Sn—for the catalysts is illustrated in Fig. 5, Fig. 6. The Ce and Nb species of the Ce₁Nb₁O_x catalyst visibly agglomerated after hydrothermal aging at 800 °C. When the hydrothermal aging temperature was increased further, this agglomeration became more serious. However, notably, the introduction of Sn restrained the agglomeration of Ce and Nb species in Ce₁Sn₂Nb₁O_x to some extent, even at 1000 °C, accompanied by the high dispersion of Sn species. These results further verified that the addition of Sn would promote the sintering resistance of a catalyst during hydrothermal aging.

3.3. Composition and distribution

XPS measurements were carried out to clarify the surface compositions of fresh and hydrothermally aged Ce₁Nb₁O_x and Ce₁Sn₂Nb₁O_x catalysts (Fig. 7; Table S2 and Figs. S6 and S7 in Appendix A). As shown in Fig. 7, the Ce/(Ce + Sn + Nb) molar ratio decreased with an increase in the aging temperature. Moreover, the Nb/(Ce + Sn + Nb) ratio first increased and then gradually decreased. Surface enrichment of Nb species was observed after hydrothermal aging at 800 °C. When the hydrothermal aging temperature was further increased to 900 and 1000 °C, Ostwald ripening occurred in the Nb species [24], [25], [35]. During the Ostwald ripening process, dispersed Ce and Nb species may be in close contact and exist on the SnO₂ surface under a humid 1000 °C atmosphere, serving as redox–acid coupled active sites (Ce–Nb active sites) for the NH₃-SCR reaction, which might be responsible for the anti-sintering property of the Ce₁Sn₂Nb₁O_x catalyst [29].

To further clarify the effect of Sn doping on hydrothermal stability, XAFS measurements were conducted to investigate the valance states of the Ce and Nb species and the local structure around the Nb species. The ratio of Ce³⁺/Ce calculated from the X-ray absorption near-edge structure (XANES) spectra showed the same trend as the XPS results, confirming that the Sn doping and hydrothermal aging treatment increased the value of Ce³⁺/Ce (Figs. S8(a) and S9 in Appendix A). As shown in Fig. S8(b), the XANES results indicated that the coordination structures of the Nb species in the Ce₁Nb₁O_x catalyst were transformed by severe hydrothermal aging, while those in the Ce₁Sn₂Nb₁O_x catalyst remained stable. Therefore, the addition of Sn stabilized the coordination structures of the Nb species in the Ce₁Sn₂Nb₁O_x catalyst.

In order to further investigate the local structure around the Nb atoms, we examined the magnitude of the k³-weighted Fourier-transform spectra (with a k range from 3.0 to 13.5 Å⁻¹) for Nb₂O₅, Ce₁Nb₁O_x, Ce₁Nb₁O_x-900, Ce₁Sn₂Nb₁O_x, and Ce₁Sn₂Nb₁O_x-900 (Fig. 8). There are two well-separated main peaks between 0.8–2.3 and 2.3–3.9 Å, respectively [36]. In the first coordination shell, the peaks in this region are ascribed to backscattering by the different oxygen atoms. The peaks found in the region of 2.3–3.9 Å are mainly due to the backscattering of Nb neighbors [37], [38]. The spectra of the Ce₁Nb₁O_x catalyst showed a significant change after hydrothermal aging, indicating that both the first and the second coordination shells were severely influenced by the high-temperature hydrothermal aging treatment. The introduction of Sn also caused a change in the first and second coordination shells. However, for the Ce₁Sn₂Nb₁O_x catalyst, the spectra remained virtually unchanged after hydrothermal aging, proving once again that the doping with Sn stabilized the local structure around the Nb atoms to resist the hydrothermal aging.

3.4. Redox ability and surface species

An H₂-TPR experiment was conducted to investigate the reducibility and oxygen mobility of all the obtained catalysts. As shown in Fig. 9(a), two H₂ consumption peaks of CeO₂ were observed at about 517 and 771 °C; these were assigned to the reduction of the surface Ce⁴⁺ to Ce³⁺ and of the bulk Ce⁴⁺ to Ce³⁺, respectively [39], [40], [41]. Two main peaks were observed for SnO₂ at 200–400 and 400–900 °C, which were assigned to the consumption of surface oxygen species or partial Sn⁴⁺ to the Sn²⁺ state, and Sn²⁺ or Sn⁴⁺ to the metal Sn, respectively [39]. For the Nb₂O₅ sample, the reduction peaks at 476 and 861 °C were attributed to the reduction of surface and bulk Nb⁵⁺ to Nb⁴⁺, respectively [17], [20], [42]. For the Ce₁Nb₁O_x catalyst, two reduction peaks were observed, centered at about 588 and 714 °C, respectively. The reduction peak at 588 °C was assigned to the reduction of surface Ce⁴⁺ to Ce³⁺, while the peak at 714 °C was ascribed to the reduction of bulk CeO₂ and Nb₂O₅ [20]. After the addition of Sn, the reduction peaks of the surface and bulk oxygen species did not shift significantly. However, it is worth noting that the intensities of the reduction peaks for Ce₁Sn₂Nb₁O_x were obviously higher than those for Ce₁Nb₁O_x, indicating that Ce₁Sn₂Nb₁O_x possessed many more redox sites than Ce₁Nb₁O_x, in accordance with the XPS results.

The H₂-TPR profiles of hydrothermally aged Ce₁Nb₁O_x catalysts are shown in Fig. 9(b). With an increase in the hydrothermal aging temperature below 900 °C, the reduction peaks shifted to higher reduction temperatures, and the integration of the H₂-TPR curve gradually decreased. Unexpectedly, after hydrothermal aging at 1000 °C for 3 h, the reduction peaks exhibited a shift to a lower temperature. According to the HRTEM results, the Ce and Nb species agglomerated remarkably in the catalyst after hydrothermal aging at 1000 °C, which severely weakened the interaction between them. Therefore, the H₂ reduction temperature of Ce₁Nb₁O_x-1000 was close to those of pure CeO₂ and Nb₂O₅—that is, shifted to a lower temperature. As shown in Fig. 9(c), with an increase in the hydrothermal aging temperature, the reduction peaks of Ce₁Sn₂Nb₁O_x gradually shifted to higher reduction temperatures, but the shapes of the peaks were maintained.

The H₂ consumption of the as-prepared catalysts was calculated by means of the integration of the H₂-TPR peaks (Fig. 9(d)). The H₂ consumption of the fresh catalysts increased greatly after the introduction of Sn. According to the analysis of the Rietveld refinement, the formation of the c-Ce(Sn)O₂ and t-Sn(Ce)O₂ solid solutions caused lattice distortion, which led to the formation of oxygen vacancies and defects on the surface of the Ce₁Sn₂Nb₁O_x catalyst [43]. For Ce₁Nb₁O_x, the redox ability was weakened when the hydrothermal aging temperature was increased. However, it is worth noting that there was no significant change in H₂ consumption with an increase in the hydrothermal aging temperature for Ce₁Sn₂Nb₁O_x. Taken together with the in-situ HTXRD results, these results suggest that Sn doping protects the redox sites of the catalyst from hydrothermal aging treatment, which may be an important reason for the high NH₃-SCR activity being maintained after hydrothermal aging.

3.5. Surface acidity

NH₃-TPD experiments were conducted to characterize the NH₃ adsorption and activation capability of the catalysts (Fig. 10). As shown in Fig. 10(a), three overlapping peaks were detected for each sample. The peaks centered at 169 and 234 °C, corresponding to the ionic NH⁺₄ bound to weak Brønsted acid sites (surface hydroxyls and structural defects). Because it is very difficult to distinguish NH₄⁺ desorption on strong Brønsted acid sites and coordinated NH₃ desorption on Lewis acid sites using NH₃-TPD spectra [44], the peaks centered at 260, 328, 353, and 418 °C were ascribed to strong acid sites. In comparison with Ce₁Nb₁O_x, Ce₁Sn₂Nb₁O_x exhibited an obvious increase in coordinated NH₃ species at a higher temperature, and its total NH₃ desorption clearly increased (Figs. 10(a) and (d)). This result implies that the Sn doping improves the strength and number of acid sites due to the synergetic effects between Sn, Ce, and Nb [45]. In addition, SnO₂ possesses a certain number of acid sites, including Brønsted and Lewis acid sites, which could promote NH₃ adsorption and activation on the surface of the catalyst [46], [47].

Figs. 10(b)–(d) show the NH₃-TPD curves of hydrothermally aged Ce₁Nb₁O_x and hydrothermally aged Ce₁Sn₂Nb₁O_x, as well as the total NH₃ desorption amounts and specific NH₃ desorption amounts normalized by the surface area of the as-prepared catalysts, respectively. With an increase in the hydrothermal aging temperature, the NH₃ desorption of Ce₁Nb₁O_x decreased sharply, with almost no NH₃ desorption being observed for catalysts aged at 900 and 1000 °C. In comparison, the decrease in the NH₃ desorption of Ce₁Sn₂Nb₁O_x as the hydrothermal aging temperature increased was much slower, and Ce₁Sn₂Nb₁O_x-1000 still possessed a certain number of acid sites, even more than Ce₁Nb₁O_x-800, suggesting that the addition of Sn helped retain the strength and number of acid sites of the catalyst during the hydrothermal aging treatment. However, the numbers of acid sites on the Ce₁Sn₂Nb₁O_x catalyst still decreased after hydrothermal aging (Fig. 10(d)), which could be mainly due to the loss of specific surface area. There was no significant change in the numbers of acid sites when normalized to the specific surface area, demonstrating the excellent stability of the acid sites of the Ce₁Sn₂Nb₁O_x catalyst.

3.6. DFT calculations

To study the stabilization of the NbO_x, CeO_x, or NbCeO_x species on SnO₂ surfaces, different optimized structures were derived using the DFT method. The formation energy of the NbO_x, CeO_x, or NbCeO_x species on SnO₂ surfaces (

E_{{NbO}_{x} {/CeO}_{x} {/NbCeO}_{x}}^{f}

) was defined as follows [48]:

(3)

E_{N b O_{x} / C e O_{x} / N b C e O_{x}}^{f} = E_{N b O_{x} / C e O_{x} / N b C e O_{x} - {loaded_SnO}_{2}} - E_{S n O_{2}} - E_{N b / C e} - \frac{x}{2} E_{O_{2}}

where E_{NbO_x}_{/CeO_x}_{/NbCeO_x}_-loaded_{_}_SnO₂, E_SnO₂, E_Nb/Ce, and E_O₂ are the energies of the NbO_x, CeO_x, or NbCeO_x species supported on the substrate, the SnO₂ surface, the bulk Nb/Ce metal, and the gas-phase O₂, respectively. The negative formation energy represents the stabilization of NbO_x, CeO_x, and NbCeO_x species on the SnO₂ surfaces. As shown in Fig. 11(a), the formation energies were –2.71, –8.27, and –12.75 eV for NbO_x, CeO_x, and NbCeO_x on SnO₂, respectively. This result suggests that the NbCeO_x species are much more stable than the other species on the SnO₂ surface. Given the surface molar ratios of the fresh and hydrothermally aged Ce₁Sn₂Nb₁O_x catalysts from the XPS experiments, it was inferred that the dispersed CeO_x and NbO_x species tended to couple with each other to form efficient and stable NbCeO_x species, working as redox–acid coupled active sites on the surface of the SnO₂ to preserve the redox and acid sites during the hydrothermal aging.

In addition, the surface energies of SnO₂ and the NbO_x/CeO_x/NbCeO_x-loaded SnO₂ surfaces (

E_{{SnO}_{2}}^{s}

and

E_{{NbO}_{x} {/CeO}_{x} {/NbCeO}_{x} - {loaded_SnO}_{2}}^{s}

) were calculated to determine the effect of surface NbO_x, CeO_x, and NbCeO_x species on the structural stabilization. The

E_{{SnO}_{2}}^{s}

and

E_{{NbO}_{x} {/CeO}_{x} {/NbCeO}_{x} - {loaded_SnO}_{2}}^{s}

are respectively defined as follows [49]:

(4)

E_{S n O_{2}}^{s} = \frac{1}{2 A} [E_{S n O_{2}}^{s l a b} - n E_{S n O_{2}}^{b u l k}]

(5)

E_{N b O_{x} / C e O_{x} / N b C e O_{x} - {loaded_SnO}_{2}}^{s} = \frac{1}{2 A} [E_{N b O_{x} / C e O_{x} / N b C e O_{x} - {loaded_SnO}_{2}}^{s l a b} - n E_{S n O_{2}}^{b u l k} - E_{N b / C e} - \frac{x}{2} E_{O_{2}}]

where A is the surface area of the slab, and

E_{{SnO}_{2}}^{slab}

E_{{SnO}_{2}}^{bulk}

, and

E_{{NbO}_{x} {/CeO}_{x} {/NbCeO}_{x} - {loaded_SnO}_{2}}^{slab}

are the energies of the SnO₂ surface, the SnO₂ unit cell, and the NbO_x, CeO_x, or NbCeO_x species supported on the SnO₂ surface, respectively. From Fig. 11(b), the surface energy of the SnO₂(110) crystal plane was predicted to be 0.10 eV·Å⁻², which decreased to 0.05, 0.01, and 0.01 eV·Å⁻², respectively, when there were NbO_x, CeO_x, and NbCeO_x species on the surfaces; this indicates that the loading of NbO_x, CeO_x, and NbCeO_x species significantly enhances the structural stabilization of the Ce₁Sn₂Nb₁O_x catalyst.

4. Conclusions

In summary, a novel Ce₁Sn₂Nb₁O_x ternary metal oxide catalyst with extraordinary hydrothermal stability was synthesized by means of a co-precipitation method for the NH₃-SCR reaction. Even after being hydrothermally aged at an extremely high temperature of 1000 °C for 3 h, the Ce₁Sn₂Nb₁O_x catalyst still showed high NH₃-SCR activity, with more than 90% NO_x conversion in the range of 325–500 °C. It was shown that the addition of Sn to Ce₁Nb₁O_x resulted in a significant decrease in the particle size, enhancement of the specific surface area, and an increase in both the redox and acid sites, as well as the formation of efficient and stable NbCeO_x active species on the surface of the t-SnO₂ lattice. The formation of NbCeO_x active species on the t-SnO₂ surface contributed to the retention of redox sites and acid sites during hydrothermal aging treatment. In addition, the active species stabilized the structure of the catalyst against sintering. All these factors are responsible for the excellent NH₃-SCR performance and hydrothermal stability of the Ce₁Sn₂Nb₁O_x catalyst.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (52225004 and 22276202), the National Key Research and Development Program of China (2022YFC3701804), and the Youth Innovation Promotion Association of Chinese Academy of Sciences (2019045).

This research used beamline 7-BM (QAS) of the National Synchrotron Light Source II, a US Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under Contract No. DE-SC0012704.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.eng.2024.02.011.

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1. Introduction

2. Experimental section

2.1. Catalyst preparation and hydrothermal aging treatment

2.2. Catalytic activity measurement

2.3. Catalyst characterization

2.4. DFT calculations

3. Results and discussion

3.1. NH3-SCR catalytic activity

3.2. Textural properties

3.2.1. N2 physisorption analysis

3.2.2. XRD analysis

3.2.3. In-situ HTXRD analysis

3.2.4. HRTEM analysis

3.3. Composition and distribution

3.4. Redox ability and surface species

3.5. Surface acidity

3.6. DFT calculations

4. Conclusions

Declaration of competing interest

Acknowledgments

Appendix A. Supplementary data

References

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3.1. NH₃-SCR catalytic activity

3.2.1. N₂ physisorption analysis