1. Introduction

NO_x and CO, which are mainly generated by the burning of fossil fuels in stationary and mobile sources, have resulted in a mass of air pollution problems and destruction of the ozone layer. Thus, the elimination of NO_x and CO is extremely important. Selective catalytic reduction of NO_x by NH₃ (NH₃-SCR) is a mainstream de-NO_x technology for cleaning exhausts from diesel engines and power plants^[1-3]. At present, V₂O₅-WO₃/TiO₂ catalyst has been commercialized and widely used in these de-NO_x processes. However, the biotoxicity of V₂O₅, weak resistance to SO₂ deactivation, and the narrow operation temperature window have driven people to develop more efficient and environmentally benign catalysts with lower costs^[4,5]. At present, CO removing mainly depends on oxidation. Obtaining high-efficiency to remove simultaneously NO and CO holds promising industrial prospects. However, it is still of great necessity to design high-performance catalysts to accomplish this process successfully.

So far, quite several kinds of catalysts have shown considerable reaction performance in conducting NH₃-SCR and CO oxidation together^[6-9]. Ceria-based catalysts, usually with excellent oxygen storage ability, have drawn a lot of attention^[10,11]. It was reported that the NH₃-SCR performance on catalysts containing CeO₂ can be improved by doping the CeO₂ matrix with a secondary cation, or loading other metal oxides onto it^[12]. Yang et al. synthesized MnO_x-CeO₂ catalysts with several methods, and found that a MnO_x(0.3)-CeO₂ catalyst fabricated with citric acid and calcined at 650 °C exhibited the optimum reaction performance, which achieves nearly 100% NO conversion at 120 °C due to the presence of highly dispersed MnO_x species, as well as more abundant active oxygen species^[13]. Liu et al. prepared Ce-Sn mixed oxide catalysts for NH₃-SCR and found that the interaction between Ce and Sn improved the redox properties and the surface Lewis acidity, which can effectively activate NH₃ molecules, thus enhancing the NH₃-SCR performance^[14]. Moreover, Liu et al. found that a CeO₂-SnO₂ catalyst fabricated by co-precipitation displayed high activity in 250-400 °C with over 90 % NO conversion and good SO₂ tolerance, due to the high dispersion of SnO₂ and the formation of a solid solution structure^[15]. By introducing acid oxides (W or Nb) into CeO₂-SnO₂ composite oxides, the acidity can be further increased to improve the NH₃-SCR performance, and the tolerance to water vapor and sulfur deactivation^[16-19].

In our former studies, SnO₂/Beta catalysts with varied SnO₂ contents were synthesized for NO_x-SCR by propylene based on the monolayer dispersion theory. On a 15% SnO₂/Beta sample, which is near the monolayer dispersion capacity, SnO₂ is in a sub-monolayer form, thereby forming more surface defects, active O₂^- and acid sites, which are beneficial to the C₃H₆-SCR performance^[20]. Moreover, we also investigated the influence of doping reducible (Ce⁴⁺ and Cu²⁺) or acidic (In³⁺ and W⁶⁺) oxides into the SnO₂ matrix. The results have illustrated that all the modified catalysts can form more abundant facile oxygen and acidic sites favorable for NH₃-SCR, due to the generation of solid solutions. The samples doped by Ce⁴⁺ or Cu²⁺ have a good balance between reactive oxygen and acidic sites, thus displaying very good catalytic performance^[21].

In addition, research on using Ce and Sn oxides in CO oxidation is also quite extensive. Liu et al. prepared a CuO/CeO₂ catalyst adopting an OH-rich Ce(OH)_x support, which exhibited excellent low-temperature CO oxidation activity. The reason is attributed to the enhancement of the dispersion of CuO_x on CeO₂ by the abundant OH species, thus increasing the CuO_x-CeO₂ interface and surface vacancies and promoting oxygen activation and migration^[22]. Sun et al. employed SnO₂ to modify CuMnO_x for low-temperature CO oxidation, finding that a suitable quantity of SnO₂ can improve the formation of more amorphous oxides, increase active oxygen content, and favor the dispersing of Cu and Mn. Among them, the CuMnSn8 catalyst achieved full CO conversion below 55 °C and 85% humidity^[23]. The Boronin team prepared Pd/CeO₂-SnO₂ catalysts with different Ce/Sn ratios and found that when the ratio was below 3/1, these catalysts maintained high thermal stability and excellent low-temperature CO oxidation activity even after calcination up to 1,000 °C^[24]. To further investigate the modifying effect of tin ions on the fluorite crystal structure of cerium dioxide, they synthesized complex, highly defective composite oxide catalysts, PtCeC and PtCeSnC, stabilized within a carbon matrix. Through experimental and density functional theory (DFT) calculations, it was found that the modification of the CeO₂ fluorite lattice by tin induces a large number of mobile active oxygen species, thereby significantly enhancing the low-temperature CO oxidation activity^[25].

Although some researchers have studied the simple CeO₂-SnO₂ mixed oxides and their solid solutions for catalytic NO_x-SCR and CO oxidation, Ce₂Sn₂O₇ pyrochlore-based catalysts have not yet been investigated^[26]. An A₂B₂O₇ pyrochlore compound usually possesses good thermal stability and intrinsic oxygen vacancies, which can facilitate the creation of plentiful active surface oxygen and acidic sites^[27-30]. As mentioned above, these two types of reactive sites are crucial to getting high-efficiency NH₃-SCR and CO oxidation catalysts. Therefore, it is worthwhile to study further the catalysis chemistry of A₂B₂O₇ pyrochlores in NH₃-SCR reaction and CO oxidation.

FeO_x-based catalysts show excellent N₂ selectivity and anti-deactivation to sulfur in medium-temperature for NO_x-SCR reactions. Compared with single Fe₂O₃, the low temperature catalytic performance can be promoted by introducing secondary metal oxides, due to synergistic interaction between FeO_x and other components^[31,32]. For example, Han et al. found that a Fe₂O₃-CeO₂@Al₂O₃ displayed over 90% NO conversion and 98% N₂ selectivity in 250-430 °C, and with superior SO₂ resistance. The strong Fe₂O₃-CeO₂ interaction produces more oxygen defects and reactive oxygen sites to accelerate the redox cycle. SO₂ will preferentially sulfate CeO₂ to prevent the deactivation of Fe₂O₃, thus keeping the high reactivity of NH₄⁺ and NH₂ species^[33].

The performance of A₂B₂O₇ catalysts is closely related to their sub-crystalline phase structure, which is determined by the ratio of the elements A and B (r_A/r_B). At the theoretical boundary value of r_A/r_B, A₂B₂O₇ paramorphs, with identical elemental compositions but exhibiting two crystalline phases, can be fabricated by regulating the temperature, pressure, or other factors^[34,35]. In this work, ‘paramorphism effect’ refers to the phenomenon that two materials have identical chemical compositions (Ce/Sn = 1:1) but different crystal topologies, such as ordered pyrochlore Ce₂Sn₂O₇ vs. biphasic Ce₂Sn₂O_8+x solid solution compounds in this study, which exhibit significantly varied catalytic performance. This concept is used as a direct comparison of structure–activity relationships, as well as a structural regulation strategy to design efficient catalysts.

Targeting to obtain high-performance NH₃-SCR and CO oxidation catalysts, Ce₂Sn₂O₇ pyrochlore and Ce₂Sn₂O_8+x solid solution, two paramorphs supports, were thus intentionally prepared by a hydrothermal method. 5% FeO_x was loaded onto them to prepare catalysts for elimination of NO and CO together. It was revealed that the Fe/Ce₂Sn₂O₇ pyrochlore catalyst displays better reaction performance than the Fe/Ce₂Sn₂O_8+x solid solution catalyst. By using multiple characterization techniques and DFT calculations, the paramorphism effect of the Ce-Sn complex oxides, and their interactions with FeO_x, were investigated. In addition, the reactive intermediate differences on the catalysts were explored with in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS).

2. Experimental Section

2.1 Catalyst preparation

2.1.1 Preparation of the supports

The Ce₂Sn₂O₇ pyrochlore and Ce₂Sn₂O_8+x solid solution supports as well as individual CeO₂ and SnO₂ were acquired by a hydrothermal method. For Ce₂Sn₂O₇ and Ce₂Sn₂O_8+x preparation, Ce(NO₃)₃·6H₂O (AR) was used as the Ce source, while SnCl₄·5H₂O (AR) and Na₂SnO₃·3H₂O (AR) were used as Sn sources, respectively, with the Ce/Sn molar ratio controlled at 1:1. With the synthesis of Ce₂Sn₂O₇ as an example here, equal moles of Ce(NO₃)₃·6H₂O and Na₂SnO₃·3H₂O were dissolved in 50 mL deionized (DDI) water under magnetic stirring separately, and then mixed together. After stirring for 10 minutes, 5 mol/L NaOH solution was dripped into the mixed solution with continuous stirring until the pH was around 12. Subsequently, the mixture was stirred for additional 4 h at room temperature, which was then transferred into a Teflon-sealed autoclave of 150 mL capacity and aged at 180 °C for 24 h. Afterwards, the precipitate was vacuum filtered and washed with DDI water for several times until the filtrate was Na⁺ free, with total dissolved solids less than 20 ppm. The powder was dried at 110 °C for 8 h, and then calcined at 550 °C for 4 h in air with a ramp of 2 °C min^-1. For the preparation of the Ce₂Sn₂O_8+x solid solution support, the same procedure was adopted except that SnCl₄·5H₂O was used as the Sn precursor. For comparison purposes, single CeO₂ and SnO₂ were also synthesized following the same method by using Ce(NO₃)₃·6H₂O and SnCl₄·5H₂O as the precursors.

2.1.2 Preparation of the supported FeO_x catalysts

The supported FeO_x catalysts were manufactured by a wet impregnation method. The required amount of Fe(NO₃)₃·9H₂O (AR) was dissolved first in 20 mL DDI water and followed by vigorous stirring for 10 min. After this, a certain mass of Ce₂Sn₂O₇ and Ce₂Sn₂O_8+x supports as well as CeO₂ and SnO₂ were added into the solution to keep a FeO_x mass fraction of 5 wt%. The mixtures were magnetically stirred at room temperature for 4 h to make Fe species sufficiently disperse on the surface of the supports. Subsequently, they were moved to an 80 °C water bath with continuous stirring to evaporate the excess water. After drying further in an oven at 110 °C overnight in air, the samples were grinded and calcined in a muffle room at 550 °C in air for 4 h to obtain the final catalysts, which were named Fe/Ce₂Sn₂O₇, Fe/Ce₂Sn₂O_8+x, Fe/CeO₂, and Fe/SnO₂. The elemental composition of each catalyst was confirmed with ICP analysis (Table 1).

2.2 Catalyst characterization and activity tests

The detailed information including the used instrument types and models, the test parameters, the experimental processes and DFT calculations are presented in the Supplementary material file.

3. Results and Discussion

3.1 Structure properties

3.1.1 X-ray diffraction (XRD) and Raman analysis

XRD patterns were collected to analyze the crystal structures. As demonstrated in Figure 1A, the typical diffraction peaks assigned to CeO₂ with a cubic fluorite phase, and SnO₂ with a tetragonal rutile phase are observed for individual CeO₂ and SnO₂, respectively^[19,36]. It is mentioned that the Ce₂Sn₂O_8+x support shows three diffraction peaks at 28.66° (111), 47.82° (220), 56.84° (311) related to cubic CeO₂, and two strong peaks at 26.24° (110), 51.30° (211) attributed to rutile SnO₂, indicating the coexistence of a CeO₂-based cubic solid solution where Sn⁴⁺ substitutes part of the Ce⁴⁺ in the CeO₂ matrix, and a SnO₂-based tetragonal solid solution where Ce⁴⁺ substitutes part of the Sn⁴⁺ in the SnO₂ matrix. This suggests that Ce₂Sn₂O_8+x is a biphasic solid solution system. Interestingly, the peak positions attributed to CeO₂ of the Ce₂Sn₂O_8+x support are distinctly higher than that of pure CeO₂ (28.40°, 47.32°, 56.26°), testifying that the Sn⁴⁺ with a smaller ionic radius (0.069 nm with a coordination number of 6) could be embedded into the lattice of CeO₂ to form mainly a cubic CeO₂-based solid solution, thus leading to unit lattice contraction^[17,19]. However, a small portion of Ce⁴⁺ with a larger ionic radius (0.087 nm) could also be doped into the matrix of SnO₂ to generate a tetragonal SnO₂-based solid solution, thus resulting in lattice expansion. This is well supported by the moving of the 2q peaks to a lower angle (51.30°) in comparison with pure SnO₂^[18,21]. For the Ce₂Sn₂O₇ support, the peaks at 29.10°, 33.64°, 48.70° and 57.84° are respectively assigned to the (222), (400), (440) and (622) planes of Ce₂Sn₂O₇ pyrochlore (PDF#48-0640) with a space group of Fd-3m^[37-39]. The lattice parameters in Table 1 have also testified that a pyrochlore phase has been formed in Ce₂Sn₂O₇. After loading FeO_x, Figure 1B displays no typical diffraction peaks assigned to FeO_x, but the initial diffraction peaks of the supports become lower, which implies that FeO_x is highly dispersed on the supports in an amorphous state or small clusters.

Display Full Size

Figure 1. XRD results of the supports (A) and the catalysts (B); Raman spectra of the supports (C) and the catalysts (D). XRD: X-ray diffraction.

Raman spectra are exhibited in Figure 1C for the catalysts. Pure SnO₂ shows three bands at 475, 633 and 774 cm^-1, which are attributable to the E_g, A_1g and B_2g vibrations of the tetragonal rutile SnO₂ phase^[21]. Pure CeO₂ exhibits a distinct peak at 463 cm^-1 ascribed to the characteristic F_2g mode of the symmetric vibration of Ce-O bonds^[19]. In contrast, two distinctive Raman peaks are observed for the Ce₂Sn₂O_8+x support at 461 and 631 cm^-1, which are respectively assigned to the F_2g mode of the CeO₂ fluorite phase and the A_1g mode of rutile SnO₂. Compared with pure CeO₂ and SnO₂, the two peaks reveal shifting to lower wavenumbers and broadening, testifying again to the doping of Sn⁴⁺ into the CeO₂ lattice and the doping of Ce⁴⁺ into the SnO₂ lattice, which could create more lattice disorder and defects^[40]. Whereas, the Raman spectroscopy of the Ce₂Sn₂O₇ support detects 6 bands at 281, 355, 391, 461, 558, and 701 cm^-1, which are typical for a pyrochlore structure^{[35,38,41-43]}. Specifically, the largest band at 461 cm^-1 is attributed to the F_2g mode of Ce-O stretching vibration. Other three F_2g modes at 355, 391 and 701 cm^-1 are assigned to Ce-O/Sn-O stretching and O-Sn-O bending vibration in sequence. The peak at 281 cm^-1 belongs to the Sn-O vibration of the E_g mode, and the peak at 558 cm^-1 belongs to the A_1g mode of O-Sn-O bending vibration in the SnO₆ octahedra. All these characteristic Raman bands have strongly proven that a disordered pyrochlore phase is formed in Ce₂Sn₂O₇. Figure 1D shows the Raman spectra of the FeO_x catalysts. Compared with the corresponding supports, except that the intensity of all the peaks is weakened, no peak related to FeO_x can be detected, confirming that the FeO_x is highly dispersed, well consistent with the XRD measurements.

The specific surface areas of Ce₂Sn₂O₇ and Ce₂Sn₂O_8+x supports are close to each other, which are much higher than those of the individual oxides. After loading FeO_x and being subjected to the second time calcination, all the surface areas are slightly decreased in comparison with the supports (Table 1).

3.1.2 Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) analysis

TEM, HRTEM and energy dispersive spectroscopy (EDS)-mapping were employed to analyze the samples (Figure 2). The TEM pictures in Figure 2A1,A2,A3,A4 depict that all the samples are composed of irregular grains with non-uniform sizes. The size distribution profiles in Figure 2B1,B2,B3,B4 indicate that after loading FeO_x, the mean particle sizes of the samples become slightly smaller than the respective supports. As demonstrated in Figure 2C1,C2,C3,C4, the HRTEM images of the two Ce₂Sn₂O₇ catalysts observe the lattice fringe with spacings of 0.187 and 0.305 nm, which can be attributed to the (440) and (222) facets of the cubic Ce₂Sn₂O₇ pyrochlore phase^[37]. In contrast, the images of the two Ce₂Sn₂O_8+x samples observe the lattice fringes of both CeO₂ and SnO₂, confirming that both CeO₂-based and SnO₂-based solid solutions are present together, in agreement with the XRD and Raman results. Notably, no lattice fringes related to FeO_x species are detected, confirming that FeO_x is finely dispersed on the supports without forming detectable crystalline domains.

Display Full Size

Figure 2. TEM pictures (A1-A4), particle size distribution profiles (B1-B4) and HRTEM pictures (C1-C4) and of the catalysts. TEM: Transmission electron microscopy.

As shown in Figure 3, the EDS-mapping images of Fe/Ce₂Sn₂O₇ and Fe/Ce₂Sn₂O_8+x catalysts demonstrate that Fe, Sn, Ce and O elements are uniformly distributed, testifying that pure phase Ce₂Sn₂O₇ pyrochlore and Ce₂Sn₂O_8+x solid solution are formed respectively in the catalysts. In addition, the loaded FeO_x is also dispersed evenly on the supports, well consistent with the XRD, Raman and HRTEM tests.

Display Full Size

Figure 3. EDS-mapping images of Fe/Ce₂Sn₂O_8+x and Fe/Ce₂Sn₂O₇ catalysts. EDS: energy dispersive spectroscopy.

3.2 Catalytic performance

The performance of all catalysts was evaluated for the simultaneous removal of NO and CO. As depicted in Figure S1, individual CeO₂ and SnO₂ exhibit low NO and CO conversion. After loading FeO_x, Fe/CeO₂ and Fe/SnO₂ have only mildly improved reaction performance. In contrast, the performance on the Ce₂Sn₂O₇ pyrochlore and Ce₂Sn₂O_8+x solid solution supports is evidently higher, achieving a maximum NO conversion above 85% around 350 °C. As manifested in Figure 4A, after supporting FeO_x, the NH₃-SCR performance is significantly improved in comparison with the Ce-Sn supports at the low temperature region. On the optimal Fe/Ce₂Sn₂O₇ catalyst, a NO conversion of > 90% is maintained within 275-400 °C, while 100% CO oxidation is achieved at 200 °C. The activity for the simultaneous removal of NO and CO is significantly improved compared to that of the Ce₂Sn₂O₇ support. For the Ce₂Sn₂O_8+x sample, after loading FeO_x, the activity of the Fe/Ce₂Sn₂O_8+x shows a milder improvement. Figure 4B displays that in the whole tested temperature region, both Fe/Ce₂Sn₂O₇ and Fe/Ce₂Sn₂O_8+x, and the corresponding supports exhibit high N₂ selectivity, maintaining > 90% within 100-400 °C.

Display Full Size

Figure 4. NH₃-SCR performance tests. (A) NO conversion; (B) N₂ selectivity; (C) CO conversion; (D) Arrhenius plots; (E) NO conversion rates (R_w and R_s) at 115 °C; (F) stability tests of Fe/Ce₂Sn₂O₇ and Fe/Ce₂Sn₂O_8+x catalysts with H₂O and/or SO₂ at 300 °C; (Reaction conditions: 10 mg catalyst, [NH₃] = [NO] = 500 ppm, [CO] = 2,000 ppm, [O₂] = 5 %, N₂ balance, GHSV = 600,000 h^-1). NH₃-SCR: NH₃-selective catalytic reduction; GHSV: gas hourly space velocity.

To elucidate the intrinsic activity change of the catalysts, the Arrhenius plots of Fe/Ce₂Sn₂O₇, Fe/Ce₂Sn₂O_8+x and the corresponding supports are plotted in Figure 4C. In a testing process, the NO conversion is always controlled below 10% to minimize the effect of diffusion and heat transfer. The NO conversion rates normalized by catalyst weight (R_w) and surface area (R_s) were achieved from 100 to 145 °C by adjusting the catalyst mass. The activation energies extracted from Figure 4D are listed in Table S1, which indicates that Fe/Ce₂Sn₂O₇ has the lowest activation energy (33 kJ/mol), among all the catalysts, significantly lower than that of Fe/Ce₂Sn₂O_8+x (46 kJ/mol). This indicates that Fe/Ce₂Sn₂O₇ possesses the strongest ability to activate NO molecules, which is attributed to its higher density of oxygen vacancies and active oxygen species derived from the pyrochlore structure. The R_w and R_s collected at 115 °C on the catalysts are particularly compared in Figure 4E. It is apparent that with the addition of FeO_x, the intrinsic activity on both Fe/Ce₂Sn₂O₇ and Fe/Ce₂Sn₂O_8+x are enhanced in comparison with the supports. This indicates that the dispersed FeO_x interacts with the supports to create more reactive sites, thus improving the NH₃-SCR performance. Figure S2 shows the R_S for Fe/Ce₂Sn₂O₇ and Fe/Ce₂Sn₂O_8+x at 200-300 °C. It can be clearly seen that the R_S of Fe/Ce₂Sn₂O₇ are significantly higher than that of the Fe/Ce₂Sn₂O_8+x, which strongly proves the impact of the paramorphism effect.

The reaction stability and the capability to tolerate SO₂ and H₂O deactivation were tested for Fe/Ce₂Sn₂O₇ and Fe/Ce₂Sn₂O_8+x. As displayed in Figure 4F, for both catalysts, the presence of SO₂ and/or H₂O imposes only recoverable deactivation, testifying that SO₂ and H₂O compete merely for the active sites with the reactants, but have no permanent damage to the surface structure. However, it can be seen that Fe/Ce₂Sn₂O₇ has better water resistance than Fe/Ce₂Sn₂O_8+x. The long-term (50-100 hours) stability tests shown in Figure S3, show that under SO₂-only conditions, both catalysts show stable NO conversion over 50 h with only slight decreases. However, upon H₂O introduction, Fe/Ce₂Sn₂O₇ exhibits much less activity loss than Fe/Ce₂Sn₂O_8+x, confirming its superior resistance to both H₂O and SO₂. The pyrochlore structure of Ce₂Sn₂O₇ plays a key role in sulfur and water vapor poisoning. First, its higher oxygen vacancy density promotes SO₂ oxidation to surface sulfates, sacrificially protecting FeO_x sites. Second, the flexible pyrochlore lattice and Ce³⁺/Ce⁴⁺ redox cycles allow reversible sulfate decomposition, explaining the recoverable deactivation. Third, the exclusive Brønsted acid sites (NH₄⁺) are less competitive with H₂O than Lewis acid sites, enhancing its water resistance. Considering all these issues, it is concluded that Fe/Ce₂Sn₂O₇ is the optimal catalyst in this work, which shows promising application potential.

3.3 H₂-TPR and O₂-TPD analysis

The redox and oxygen property is usually a key factor to determine the NH₃-SCR performance^[10]. Thus, H₂-TPR and O₂-TPD were adopted to analyze the catalysts as well as the supports. Figure 5A exhibits the H₂-TPR profiles and Table S2 presents the respective quantitative results. For all the samples, there are mainly two reduction peaks present in 150-300 °C and 450-600 °C. The first peak is assigned to the reduction of chemisorbed oxygen, and the second peak is ascribed to the reduction of Sn⁴⁺ to metallic Sn^0[17,28,44]. Notably, the H₂ consumption of the low-temperature peaks of the two Ce₂Sn₂O₇ samples is clearly higher than that of the two Ce₂Sn₂O_8+x samples (Table S2), demonstrating that more chemisorbed oxygen species promoting the NH₃-SCR performance at lower temperature are formed on the pyrochlore samples. After loading FeO_x onto the Ce₂Sn₂O₇ support, the H₂ consumption of the low temperature peak is further improved, and the peak position moves to a lower temperature. This illustrates that the interaction of FeO_x and the Ce₂Sn₂O₇ support can promote the generation of more active oxygen sites. However, loading FeO_x onto the Ce₂Sn₂O_8+x solid solution support can suppress slightly the chemisorbed oxygen amount. As a result, Fe/Ce₂Sn₂O₇ owns a richer amount of chemisorbed oxygen species than Fe/Ce₂Sn₂O_8+x, thereby showing better catalytic performance.

Display Full Size

Figure 5. H₂-TPR (A) and O₂-TPD (B) of the catalysts and corresponding supports.

The O₂-TPD curves are displayed in Figure 5B, which can be classified into three types. The α peak below 250 °C is related to the desorption of loosely chemisorbed oxygen species. The β peak between 250 and 550 °C is ascribed to the release of chemisorbed surface oxygen species with medium strength. And the γ peak over 550 °C might be assigned to the release of surface lattice oxygen, which could have limited contribution to the reaction^[45]. As quantified in Table S3, the desorbed oxygen amount of Fe/Ce₂Sn₂O₇ corresponding to α + β peaks is higher than that over Fe/Ce₂Sn₂O_8+x, being well consistent with the H₂-TPR results. Indeed, the H₂-TPR and O₂-TPD results have demonstrated that the number of surface-active oxygen sites might be a vital factor to influence the catalytic performance over the two supported FeO_x catalysts.

3.4 EPR and DFT calculation study on the surface vacancies

EPR was employed to certify the existence of oxygen vacancies and evaluate their concentration on Fe/Ce₂Sn₂O₇ and Fe/Ce₂Sn₂O_8+x catalysts. The EPR spectra in Figure 6A show a signal at g = 2.003 for both catalysts, which can be assigned to surface oxygen vacancies^[2,46]. Furthermore, the signal intensity of Fe/Ce₂Sn₂O₇ is distinctly higher than that on Fe/Ce₂Sn₂O_8+x, indicating the formation of more oxygen vacancies on Fe/Ce₂Sn₂O₇. It is well agreed that the surface oxygen vacancies can facilitate the chemisorption of gas phase O₂ to create surface-active oxygen species^[47-49]. A catalyst with a richer amount of oxygen defects usually possesses more surface active oxygen species. This is in good agreement with the H₂-TPR and O₂-TPD findings that Fe/Ce₂Sn₂O₇ owns more abundant active oxygen sites than Fe/Ce₂Sn₂O_8+x, thus showing better NH₃-SCR and CO oxidation performance.

Display Full Size

Figure 6. (A) EPR spectra and (B) DFT calculation results and models for oxygen vacancy formation energies of Fe/Ce₂Sn₂O₇ and Fe/Ce₂Sn₂O_8+x catalysts. EPR: electron paramagnetic resonance; DFT: density functional theory.

To further understand the surface defects property on the two paramorphic catalysts, a DFT method was adopted to calculate the oxygen vacancy formation energies of Fe/Ce₂Sn₂O₇ and Fe/Ce₂Sn₂O_8+x. Considering the XRD and HRTEM results, the (222) planes on the Ce₂Sn₂O₇ pyrochlore and the (111) planes on the Ce₂Sn₂O_8+x solid solution are employed to construct DFT models, as shown in Figure 6B. The calculations are performed via the Vienna Ab initio Simulation Package (VASP) code. Apparently, the formation energy of oxygen vacancies on the Fe/Ce₂Sn₂O₇ catalyst is lower than that on Fe/Ce₂Sn₂O_8+x, illustrating that it is easier to form oxygen vacancies on the Fe/Ce₂Sn₂O₇ catalyst. This is in good accordance with the EPR, H₂-TPR and O₂-TPD results, validating that the surface oxygen defects and active oxygen species are important for the reaction, which could be one of the key factors in deciding the catalytic performance.

3.5 Surface acid property

The adsorbing and activating of NH₃ on the acid sites is a crucial step in NH₃-SCR reaction. Thus, NH₃-TPD was performed for the catalysts and the respective supports. As shown in Figure S4, the main NH₃ desorption peak is detected below 300 °C for all the samples, which can be ascribed to the release of NH₃ adsorbed on weak acid sites^[50,51]. Notably, the Fe/Ce₂Sn₂O₇ and Fe/Ce₂Sn₂O_8+x catalysts show similar NH₃ desorption behavior to the corresponding supports, but the temperature shifts to a lower region. Moreover, the addition of FeO_x improves also the NH₃ desorption amounts for both samples (Table 2). Interestingly, Fe/Ce₂Sn₂O₇ possesses obviously more surface acidic sites than the Fe/Ce₂Sn₂O_8+x catalyst. Taking into account the reaction performance, the abundance of surface acidity could be another critical element to decide the reaction performance.

To elucidate further the NH₃ adsorption behavior, in-situ DRIFTS spectra of NH₃ adsorbing at 50 °C on the catalysts and corresponding supports were collected in Figure 7. After the introduction of 1,000 ppm NH₃/Ar for 30 min, multiple peaks belonging to NH₃ adsorption are detected. The peaks at 1,158, 1,201, 1,287 and 1,559 cm^-1 are assignable to NH₃ attached on Lewis acidic sites (L-NH₃). And the peaks at 3,389 and 3,243 cm^-1 are respectively assigned to the stretching vibration of N-H^[17,52]. Interestingly, two special bands at 1,442 and 1,680 cm^-1, which are respectively assigned to NH₄⁺ species attached to Brønsted acidic sites (B-NH₄⁺)^[53,54], are only detected on the two Ce₂Sn₂O₇ samples. These B-NH₄⁺ peaks are obviously not observed on the two Ce₂Sn₂O_8+x samples. The ordered A₂B₂O₇ pyrochlore lattice contains intrinsic O′ sites that are easily hydroxylated to form M–OH^[55]. The strong Lewis acidity of Sn⁴⁺ polarizes adjacent O–H bonds, enhancing their proton-donating ability to NH₃ (forming NH₄⁺). Additionally, the high density of oxygen vacancies (EPR/DFT) provides unsaturated metal sites that facilitate H₂O dissociation, generating more surface hydroxyls, some of which act as Brønsted acids. The biphasic solid solution lacks these structural features, explaining the absence of NH₄⁺ in DRIFTS. It was reported that the presence of Brønsted acidic sites is favorable to enhance NH₃-SCR performance at medium and low temperature^[56]. Indeed, this could be an important reason that Fe/Ce₂Sn₂O₇ exhibits better reaction performance than Fe/Ce₂Sn₂O_8+x. It is mentioned here that loading FeO_x onto the supports can increase markedly the quantities of both Lewis and Brønsted acidic sites, which are beneficial to the reaction performance.

Display Full Size

Figure 7. In-situ DRIFTS test of NH₃ adsorption at 50 °C for 30 min on Ce₂Sn₂O_8+x, Ce₂Sn₂O₇, Fe/ Ce₂Sn₂O_8+x and Fe/Ce₂Sn₂O₇ catalysts. DRIFTS: diffuse reflectance infrared Fourier transform spectroscopy.

3.6 NO_x storage property

Given that the activation of NO_x molecules on a catalyst is vital during the NH₃-SCR process, the NO_x storage capacity was evaluated by performing NO + O₂-TPD experiments. Notably, the effluent was monitored by a gas analyzer, thus the NO and NO₂ curves in Figure 8 reflect the real NO and NO₂ concentrations. All the samples show two desorption peaks in both NO and NO₂ profiles. The peak below 200 °C is assigned to the release of weakly adsorbed NO_x and unstable nitrates; and the peak above 200 °C is ascribed to the release of more strongly attached NO_x with better thermal stability^[57]. In comparison with the supports, it is apparent that the NO_x desorption amount increases after loading FeO_x, testifying the increase of the NO_x storage capacity (Table S4). Moreover, Fe/Ce₂Sn₂O₇ catalyst exhibits lower desorption temperature than the support. Particularly, the high temperature peak moves significantly from 300 to 209 °C. This indicates that besides the capacity improvement, the adsorbed NOx becomes also more facile. Whereas, for Fe/Ce₂Sn₂O_8+x catalyst, adding FeO_x increases both peaks to higher temperature evidently, especially the high temperature peak. This testifies that on Fe/Ce₂Sn₂O_8+x, though the amount of adsorbed NO_x is larger than that of the support, it becomes less active. According to the literatures, the presence of uncoordinated electrons in the NO molecule makes it preferentially adsorbed on the catalyst surface to form nitrate intermediates^[58]. In addition, the surface active oxygen species can not only improve the formation of surface nitrates, but also promote the breakdown of nitrates or nitrites via weakening N-O bonds^[59,60]. As demonstrated above, Fe/Ce₂Sn₂O₇ owns more abundant active oxygen sites than Fe/Ce₂Sn₂O_8+x, thus forming more reactive adsorbed NO_x species. In other words, although the total NO_x capacity of Fe/Ce₂Sn₂O_8+x is higher than that of Fe/Ce₂Sn₂O₇, the adsorbed NO_x species is more stable. As a result, Fe/Ce₂Sn₂O₇ displays higher NH₃-SCR reaction performance than Fe/Ce₂Sn₂O_8+x.

Display Full Size

Figure 8. NO+O₂-TPD profiles of Ce₂Sn₂O₇ (A1), Fe/Ce₂Sn₂O₇ (A2), Ce₂Sn₂O_8+x (B1) and Fe/Ce₂Sn₂O_8+x (B2) catalysts.

3.7 Surface chemical state analyzed by X-ray photoelectron spectroscopy (XPS)

XPS tests were performed to investigate the valence states over the catalysts and corresponding supports. The C 1s spectra of the samples are demonstrated in Figure S5. Apart from the C−C bond peak at 284.8 eV of the carbon standard, the other two peaks at 286.4 and 288.5 eV are respectively assigned to C−O and C=O bands of CO₃^2- species in sequence^[61]. This suggests that surface carbonate species are formed on the samples and contribute to the O 1s signal. Based on this, the O 1s spectra are closely analyzed for the catalysts and displayed in Figure 9A, which can be fitted into four peaks. The peaks at ~529.7, ~530.8, 532.0 and 533.0 eV are assigned to surface lattice oxygen (O^2-), O₂^2-, OH^-/CO₃^2- and O₂^- species in sequence^[27]. As quantified in Table 3, the amount of chemical adsorbed oxygen (O₂^2-+O₂^-) on Fe/Ce₂Sn₂O₇ is obviously higher than that on Fe/Ce₂Sn₂O_8+x, matching well with the H₂-TPR and O₂-TPD, EPR and DFT calculation results. In Figure 9B, the Fe 2p XPS spectra prove that on both Fe/Ce₂Sn₂O₇ and Fe/Ce₂Sn₂O_8+x, the Fe³⁺ peak at 716.1 eV and Fe²⁺ peak at 712.3 eV are detected, but the Fe³⁺ ions are denominated on the surfaces^[62]. As shown in Table 3, the Fe³⁺ percentage/amount on both catalysts is very close, indicating that it is not the main reason to discriminate their NH₃-SCR and CO oxidation performance, though the presence of FeO_x sites is important for the reaction.

Display Full Size

Figure 9. XPS of O 1s (A) and Fe 2p (B) on the catalysts. XPS: X-ray photoelectron spectroscopy.

The Ce 3d XPS signals of the catalysts and respective supports are displayed in Figure S6A, which can all be fitted into eight peaks. The six bands denoted as v, v″, v‴, u, u″ and u‴ are associated with surface Ce⁴⁺, and the other two peaks marked as u′ and v′ are related to Ce^3+[18]. Either forming Ce₂Sn₂O₇ pyrochlore or Ce₂Sn₂O_8+x solid solution, both Ce³⁺ and Ce⁴⁺ are present on the surface, but obviously in different amounts. The Ce³⁺/Ce_total percentages are calculated in Table 3 for all the samples via the fitted peak areas. It is apparent that Fe/Ce₂Sn₂O₇ possesses much more surface Ce³⁺ cations than Fe/Ce₂Sn₂O_8+x, which could be favorable to promote the chemisorption of O₂ molecules to generate active surface oxygen sites^[49,63].

As exhibited in Figure S6B, all the catalysts and corresponding supports have two typical Sn 3d peaks around 486.1-486.9 and 494.4-495.2 eV, which are respectively assigned to the Sn 3d_5/2 and Sn 3d_3/2 spin-orbit peak of Sn⁴⁺ cations. Compared with the two Ce₂Sn₂O_8+x solid solution samples, the binding energy of Sn 3d peaks of the two Ce₂Sn₂O₇ pyrochlore samples shifts evidently to lower values, indicating that Sn⁴⁺ cations in the matrix have higher electron density. Indeed, this could also help the chemisorption of gas phase O₂, thus generating more abundant active surface oxygen anions with negative charge.

3.8 In-situ DRIFTS study on the reaction intermediates

To identify the possible intermediates and reaction pathways, three series of in-situ DRIFTS tests have been performed on both Fe/Ce₂Sn₂O₇ and Fe/Ce₂Sn₂O_8+x, catalysts. Before the experiments, the samples were first processed in a 30 mL/min Ar feed at 400 °C for 1 h to get rid of the surface impurities, and then cooled to 150 °C.

3.8.1 Reaction between NH₃ and pre-adsorbed NO+O₂ species

In this series of tests, a feed mixture of 500 ppm NO + 5vol% O₂ + Ar balance was directed to the catalysts and saturated their surfaces at 150 °C. As demonstrated in Figure 10A1,B1, both Fe/Ce₂Sn₂O₇ and Fe/Ce₂Sn₂O_8+x catalysts exhibit similar NO_x adsorption behavior. In detail, three peaks at 1,556, 1,548 and 1,300 cm^-1 assigned to bidentate nitrate, and other three bands at 1,431, 1,359 and 1,197 cm^-1 belonging to trans-N₂O₂^2-, cis-N₂O₂^2- and bridge nitrite are detected, respectively^[5,14,64]. Notably, the peaks at 1,556 and 1,548 cm^-1 increased obviously with the time extension to 40 min for both catalysts. Afterwards, all the peaks remained unchanged, indicating that the catalyst surfaces were saturated. Notably, Fe/Ce₂Sn₂O_8+x catalyst adsorbed more NO_x than Fe/Ce₂Sn₂O₇, in agreement with the NO + O₂-TPD results.

Display Full Size

Figure 10. In-situ DRIFTS study on Fe/Ce₂Sn₂O_8+x and Fe/Ce₂Sn₂O₇ catalysts at 150 °C; (A1) and (B1) in a 500 ppm of NO + 5% O₂/Ar flow, (A2) and (B2) in a 500 ppm of NH₃/Ar flow after pre-adsorbing NO + O₂. DRIFTS: diffuse reflectance infrared Fourier transform spectroscopy.

Following this step, the saturated catalysts were purged with an Ar flow to get rid of the physically adsorbed NO_x species. Afterwards, a 500 ppm NH₃ + Ar flow was switched to the catalyst. Figure 10A2 displays that on Fe/Ce₂Sn₂O_8+x solid solution catalyst, the bidentate nitrate species at 1,556 and 1,548 cm^-1 gradually decreased and became stable after 30 min, while the peaks corresponding to other nitrate species remained nearly unchanged. In addition, with the increase of the exposing time in NH₃, new peaks at 3,395, 3,250, 1,600 and 1,160 cm^-1 were observed. The bands at 3,395 and 3,250 cm^-1 are related to N-H vibration of NH₃, and the bands at 1,600 and 1,160 cm^-1 belong to NH₃ adsorbed on Lewis acidic sites^[65]. This indicates that bidentate nitrates might be the active intermediates for NH₃-SCR on Fe/Ce₂Sn₂O_8+x.

In contrast, as displayed in Figure 10B2, over the Fe/Ce₂Sn₂O₇ pyrochlore catalyst, the peak of bridge nitrite at 1,204 cm^-1 decreased and disappeared in 5 minutes. At the same time, the intensity of the bidentate nitrates decreased gradually. This indicates that the bridge nitrite could be another active intermediate besides the bidentate nitrates on the Fe/Ce₂Sn₂O₇ catalyst. Notably, a new peak at 1,442 cm^-1 assigned to NH₄⁺ species on the Brønsted acidic sites can be detected on Fe/Ce₂Sn₂O₇, in good accordance with the NH₃ adsorption in-situ DRIFTS results in Figure 7. In brief, these results testified that the bidentate nitrate could be the crucial active surface intermediates for both Fe/Ce₂Sn₂O₇ and Fe/Ce₂Sn₂O_8+x catalysts in NH₃-SCR reaction. The additional bridge nitrite formed on Fe/Ce₂Sn₂O₇ might be a factor to further improve the reaction performance.

3.8.2 Reaction between NO+O₂ and pre-adsorbed NH₃ species

Like the first series of experiments, the catalysts were cleaned with the same procedure and then kept at 150 °C. As exhibited in Figure 11A1,B1, as soon as a 500 ppm NH₃+Ar stream was introduced onto the samples, the adsorbed NH₃ intermediates were generated, which displayed three bands at 1,599, 1,201 and 1,160 cm^-1 belonging to NH₃ attached on Lewis acidic sites, and other three peaks at 3,394, 3,258 and 3,150 cm^-1 attributed to the vibration of N-H bonds for NH₃ molecules^[56,66]. Notably, on the Fe/Ce₂Sn₂O₇ catalyst, a unique NH₄⁺ band linked to Brønsted acidic sites appears at 1,442 cm^-1, which is beneficial to the reaction^[67].

Display Full Size

Figure 11. In-situ DRIFTS study on Fe/Ce₂Sn₂O_8+x and Fe/Ce₂Sn₂O₇ catalysts at 150 °C; (A1) and (B1) in a 500 ppm of NH₃/Ar flow, (A2) and (B2) in a 500 ppm of NO+5% O₂/Ar flow after pre-adsorbing NH₃. DRIFTS: diffuse reflectance infrared Fourier transform spectroscopy.

After 40 min exposure in NH₃, the catalyst surfaces were saturated, as indicated by the stabilized peak intensity. The catalysts were then flushed by a 50 mL/min Ar flow for 1 h to remove physically adsorbed NH₃. Afterward, a 500 ppm NO+5 vol % O₂+Ar balance flow was switched into the in-situ cell. Figure 11A2 shows that on the Fe/Ce₂Sn₂O_8+x catalyst, three bands at 1,559, 1,542 and 1,305 cm^-1 assigned to bidentate nitrate, and one peak at 1,359 cm^-1 assigned to cis-N₂O₂^2- appeared rapidly in the first 1 minute^[15,68]. With the increase of the exposing time, the peaks for NH₃ molecules at 3,394, 3,258 and 3,150 cm^-1, and NH₃ attached on Lewis acidic sites at 1,599 and 1,160 cm^-1 weakened until they vanished in the end^[69].

Compared with the Fe/Ce₂Sn₂O_8+x catalyst, Figure 11B2 shows that on Fe/Ce₂Sn₂O₇, the decrease of NH₄⁺ groups adsorbed on the Brønsted acidic sites at 1,442 cm^-1 is observed as well by extending the exposing time in NO+O₂. This indicates that both NH₄⁺ and coordinated NH₃ species can be involved in the reaction over Fe/Ce₂Sn₂O₇ pyrochlore catalyst.

Summarizing these results, it is rational to conclude that NH₃-SCR reaction over both Fe/Ce₂Sn₂O₇ and Fe/Ce₂Sn₂O_8+x catalysts follows a Langmuir-Hinshelwood mechanism, which involves the adsorption of both NO_x and NH₃ molecules on the surface of the catalysts during the reaction. However, on Fe/Ce₂Sn₂O₇, additional active intermediates can be formed for both NO_x and NH₃ adsorption compared with Fe/Ce₂Sn₂O_8+x, which accounts for its better NH₃-SCR performance.

3.8.3 Reaction between O₂ and pre-adsorbed CO species

Figure 12 displays the in-situ DRIFTS results of the transient reaction between O₂ and pre-adsorbed CO on the two catalysts. As displayed in Figure 12A1, for Fe/Ce₂Sn₂O_8+x, after 60 minutes of CO treatment, monodentate carbonate (1,548 and 1,359 cm^-1), bidentate carbonate (1,650 and 1,268 cm^-1), and polydentate carbonate (1,192 cm^-1) were detected^[70,71]. In contrast, the Fe/Ce₂Sn₂O₇ catalyst predominantly exhibited bidentate carbonate (1,650, 1,582, and 1,236 cm^-1) along with bridged carbonate (1,759 cm^-1)^[72,73], as shown in Figure 12B1. Meanwhile, characteristic peaks attributable to gaseous CO₂ were observed for both catalysts in the range of 2,300-2,400 cm^-1, indicating that lattice oxygen participated in the CO oxidation process, following the Mars-van Krevelen (MvK) mechanism.

Display Full Size

Figure 12. In-situ DRIFTS study on Fe/Ce₂Sn₂O_8+x and Fe/Ce₂Sn₂O₇ catalysts at 150 °C. (A1) and (B1) exposed in a 2,000 ppm CO/Ar flow, (A2) and (B2) exposed in a 5% O₂/Ar flow after pre-adsorbing CO. DRIFTS: diffuse reflectance infrared Fourier transform spectroscopy.

Upon the subsequent introduction of O₂, the bidentate carbonate (1,650 cm^-1) and monodentate carbonate (1,359 cm^-1) species adsorbed on the Fe/Ce₂Sn₂O_8+x surface, and the bidentate carbonate (1,236 cm^-1) on the Fe/Ce₂Sn₂O₇ surface gradually diminished, accompanied by a progressive increase in the intensity of the gaseous CO₂ peak (Figure 12A2,B2). This suggests that the carbonate intermediates formed with lattice oxygen sites actively participate in the CO oxidation reaction, a mark of the Mars–van Krevelen (MvK) mechanism. Remarkably, both catalysts exhibited a time-dependent intensification of the bidentate carbonate band at 1,523 cm^-1[73,74]. This phenomenon can be attributed to the adsorption of O₂ at oxygen vacancies upon O₂ introduction, leading to the formation of peroxide species (1,065 cm^-1)^[75]. These O₂^- species subsequently react with surface carbonates, facilitating their transformation into more stable carbonates, which can ultimately promote the evolution of CO₂^[76]. It is noteworthy that the active bidentate carbonate species on Fe/Ce₂Sn₂O₇ decreases more rapidly than that on Fe/Ce₂Sn₂O_8+x, testifying that Fe/Ce₂Sn₂O₇ is more active in CO oxidation, in line with the activity tests.

Based on the in-situ DRIFTS results (Figure 10, Figure 11 and Figure 12), we have drawn the possible reaction pathways on the Fe/Ce₂Sn₂O₇ catalyst (Scheme 1). First, gas-phase NO_x adsorbs on the catalyst surface to form active surface bidentate nitrate and bridge nitrite. Simultaneously, gas-phase NH₃ adsorbs on the catalyst surface to form coordinated NH₃ (B-NH₄⁺ and L-NH₃). Notably, on Fe/Ce₂Sn₂O₇, the additional B-NH₄⁺ ions generated on the extra Brønsted acid sites are also highly reactive, especially at low temperatures. The formed nitrate intermediates and NH₃ intermediates subsequently react with each other to produce the final products, following a Langmuir-Hinshelwood mechanism.

Display Full Size

Scheme 1. Illustration of NH₃-SCR and CO oxidation reaction on Fe/Ce₂Sn₂O₇ catalysts.

The CO oxidation pathway on the surface oxygen vacancies of Fe/Ce₂Sn₂O₇ proceeds as follows: CO reacts with adjacent surface-active oxygen to form carbonate intermediates (e.g., bridged carbonate or bidentate carbonate). These carbonate intermediates are unstable and decompose to generate CO₂. In turn, gas-phase O₂ molecules will adsorb onto the oxygen vacancies, and restore the consumed active oxygen species, following the MvK mechanism.

Fe/Ce₂Sn₂O₇ uniquely generates Brønsted-bound NH₄⁺ and bridge nitrite intermediates during NH₃-SCR, which are absent on Fe/Ce₂Sn₂O_8+x. These species accelerate the Langmuir-Hinshelwood surface reaction, leading to faster NO conversion. For CO oxidation, the higher oxygen vacancy density on Fe/Ce₂Sn₂O₇ promotes the Mars-van Krevelen cycle, as evidenced by the faster consumption of bidentate carbonate intermediates and the stronger peroxide signal upon O₂ introduction, thus improving the CO oxidation activity.

4. A Brief Discussion

In this study, to design efficient and applicable catalysts for elimination of NO and CO together, a paramorphism effect strategy has been adopted to fabricate catalysts by loading FeO_x onto Ce₂Sn₂O₇ pyrochlore and Ce₂Sn₂O_8+x solid solutions supports. The XRD, Raman and TEM results have proven that a disordered pyrochlore phase has formed in Ce₂Sn₂O₇ support. In contrast, mixed solid solutions with mainly Sn⁴⁺ doped into the CeO₂ matrix phase and minorly Ce⁴⁺ doped into the SnO₂ lattice phase are present in Ce₂Sn₂O_8+x support. Although the metal element composition of both supports is the same, after supporting FeO_x, Fe/Ce₂Sn₂O₇ exhibits obviously better catalytic performance than Fe/Ce₂Sn₂O_8+x. A paramorphs effect is distinctly observed, which could be a useful strategy to design catalysts not only for NH₃-SCR, but also for many other catalytic reactions.

Noteworthily, in NH₃-SCR reaction, the surface acidity and redox property of a catalyst are critical factors to determine its catalytic performance. As testified by EPR, more oxygen vacancies are present on Fe/Ce₂Sn₂O₇ than on Fe/Ce₂Sn₂O_8+x, thus it can adsorb and activate gaseous O₂ molecules more easily than Fe/Ce₂Sn₂O_8+x. DFT calculation has also confirmed that the oxygen vacancy formation energy of Fe/Ce₂Sn₂O₇ is lower than that of Fe/Ce₂Sn₂O_8+x. A₂B₂O₇ pyrochlores are stoichiometric chemicals with a definite structure, which generally have superior thermal resistance, and possess structure 8a oxygen defects that can extend to the surface^[34]. Naturally, Fe/Ce₂Sn₂O₇ could possess more surface oxygen vacancies and a better capability than Fe/Ce₂Sn₂O_8+x to activate gas phase O₂. In addition, on Fe/Ce₂Sn₂O₇, more Ce³⁺ cations are formed than on Fe/Ce₂Sn₂O_8+x, which is an additional factor to promote the O₂ activation. As substantiated by H₂-TPR, O₂-TPD and XPS O1s signals, Fe/Ce₂Sn₂O₇ owns a richer amount of active oxygen anions (O₂^2- and O₂^-) on its surface than Fe/Ce₂Sn₂O_8+x. Figure 13A shows that the R_w and R_s on the two catalysts are positively correlated to the number of active surface oxygen sites, testifying that it is one of the crucial elements to determine the reaction performance.

Display Full Size

Figure 13. R_w and R_s of the catalysts at 115 °C versus (O₂^-+O₂^2-)/O_total amount (A), R_w and R_s of catalysts at 115 °C versus NH₃ desorption amount.

Moreover, in NH₃-SCR reaction, surface acidic sites are also of great importance, which can adsorb and activate NH₃. NH₃-TPD experiments have testified that a larger number of acidic sites are formed on Fe/Ce₂Sn₂O₇ than on Fe/Ce₂Sn₂O_8+x, which could also be closely related to the abundance of surface defects. What is more interesting, NH₃ adsorption in-situ DRIFTS tests have proved that adsorbed NH₄⁺ groups attached to Brønsted acidic sites have been uniquely formed on Fe/Ce₂Sn₂O₇, which is in additional to the adsorbed NH₃ species on Lewis acid sites found on both catalysts. It was reported that the NH₄⁺ species can contribute to the generation of NH₄NO₂ or NH₄NO₃ intermediates to improve NH₃-SCR performance at low temperatures^[77]. Figure 13B shows that the R_w and R_s are also positively correlated to the number of acid sites, demonstrating that it is another crucial reason to control the reaction performance.

In terms of CO oxidation, the better performance of Fe/Ce₂Sn₂O₇ than Fe/Ce₂Sn₂O_8+x can be attributed to its higher concentration of surface oxygen vacancies, as evidenced by EPR and DFT calculations. These vacancies facilitate the activation of gaseous O₂ into reactive oxygen species (O₂^2- and O₂^-), which play a pivotal role in the Mars–van Krevelen mechanism. In-situ DRIFTS results further reveal that carbonate intermediates formed on Fe/Ce₂Sn₂O₇ are more reactive and readily transformed into CO₂ upon O₂ exposure. This highlights the critical role of oxygen vacancies/reactive oxygen species in determining the activity for CO oxidation.

5. Conclusions

In this work, a paramorphism strategy has been adopted to design efficient catalysts for elimination of NO and CO together. Specifically, a Ce₂Sn₂O₇ pyrochlore and a Ce₂Sn₂O_8+x solid solution paramorph compounds have been successfully synthesized with a hydrothermal method to support the FeO_x active component. It is discovered that Fe/Ce₂Sn₂O₇ displays better catalytic performance than Fe/Ce₂Sn₂O_8+x, showing an evident paramorphism effect.

As proved by XRD, Raman, TEM/HRTEM and EDS mapping tests, a disordered cubic pyrochlore phase is formed in Ce₂Sn₂O₇ samples. In contrast, mixed solid solutions are present in Ce₂Sn₂O_8+x samples. The loaded FeO_x, which is finely dispersed in an amorphous state or small clusters, has different interactions with the two types of supports.

EPR and DFT calculations have testified that it is easier for Fe/Ce₂Sn₂O₇ to generate more surface defects than for Fe/Ce₂Sn₂O_8+x, thus forming a richer amount of active O₂^2- and O₂^- sites. Furthermore, the number of acid sites on Fe/Ce₂Sn₂O₇ is larger than on Fe/Ce₂Sn₂O_8+x. As a plus, NH₄⁺ linked to Brønsted acidic sites and bridged nitrite are uniquely formed as reactive intermediates on Fe/Ce₂Sn₂O₇, as testified by the in-situ DRIFTS experiments. Noteworthy, Fe/Ce₂Sn₂O₇ also exhibits better sulfur and water vapor tolerance than Fe/Ce₂Sn₂O_8+x.

In-situ DRIFTS tests have testified that NH₃-SCR reaction on both catalysts follows a Langmuir-Hinshelwood mechanism, involving the adsorption of both NO and NH₃ to form reactive surface intermediates critical to the reaction. It is also demonstrated that both catalysts follow the Mars-van Krevelen mechanism for CO oxidation. The higher density of oxygen vacancies on the Fe/Ce₂Sn₂O₇ catalyst promotes O₂ activation to form O₂^2- species, thus resulting in its better CO oxidation activity than Fe/Ce₂Sn₂O_8+x.

Supplementary materials

The supplementary material for this article is available at: Supplementary materials.

Authors contribution

Yang Y: Data curation, formal analysis, investigation, methodology, writing-original draft, funding acquisition.

Song T: Data curation, formal analysis, investigation, methodology, visualization, writing-original draft.

Ma J, Zhang S: Investigation, formal analysis, methodology, validation.

Huang J: Investigation, formal analysis.

Shen J: Investigation, formal analysis, funding acquisition.

Xu X: Project administration, resources, software.

Xu J: Formal analysis, project administration, methodology, software, resources.

Fang X: Project administration, resources.

Wang X: Conceptualization, data curation, formal analysis, funding acquisition, validation, project administration, supervision, writing-review & editing

Conflicts of interest

The authors declare no conflicts of interest.

Ethical approval

Not applicable.

Consent to participate

Not applicable.

Consent for publication

Not applicable.

Availability of data and materials

All data generated or analyzed during this study are included in this article and its supplementary information files.

Funding

The authors acknowledge deeply the financial supporting by the National Natural Science Foundation of China (Grant No. 22376090, 22362026, 22172071), the Natural Science Foundation of Jiangxi Province (Grant No. 20242BAB26029), the Key Laboratory Foundation of Jiangxi Province for Environment and Energy Catalysis (Grant No. 20242BCC32041), and the Graduate Students Innovation Special Foundation of Jiangxi Province (Grant No. YC2024-B011).

Copyright

© The Author(s) 2026.