Ce/Mn Co-Doping Induces Synergistic Effects for Low-Temperature NH₃-SCR over Ba₂Ti₅O₁₂ Catalysts

Abstract

To develop eco-friendly low-temperature NH₃-SCR catalysts for the non-electric industry, a series of CeMn-modified Ba₂Ti₅O₁₂ catalysts were synthesized using the sol-gel method to achieve denitrification. Activity tests revealed that Ce-Mn-modified Ba₂Ti₅O₁₂ catalysts exhibit excellent low-temperature denitrification performance with a broad operational temperature window. Characterization through XRD, XPS, BET, NH₃-TPD, and EPR indicated that Ce-Mn modification enhances surface oxygen chemisorption and increases acidity, significantly improving NO_x reduction. Notably, the optimal catalyst achieved NO_x conversion rates exceeding 90% within the temperature range of 90 to 240 °C under a gas hourly space velocity (GHSV) of 28,000 h⁻¹. In particular, the coexistence of Ce and Mn species promotes the oxidation of NO to NO₂, facilitating the “fast SCR” reaction. The abundance of valence states further enhances the catalyst’s ultra-low-temperature NH₃-SCR denitration performance.

1. Introduction

Nitrogen oxides (NOx) are major contributors to critical environmental issues, including acid rain, ozone layer depletion, and photochemical smog. In recent years, controlling NO_x emissions in the non-electric industry has become a significant focus, with sectors such as the cement and steel industries accounting for a large proportion of NO_x emissions [1,2,3]. Selective catalytic reduction with NH₃ (NH₃-SCR) has emerged as an effective strategy to mitigate NO_x pollution, using catalysts to convert NO_x into harmless nitrogen and water vapor [4,5]. Industrially, V₂O₅/TiO₂ catalysts are widely used for flue gas denitrification. However, the active vanadium (V) component poses environmental and health risks due to its toxicity. Moreover, these catalysts have a limited operational temperature range and poor redox performance at low temperatures. Therefore, it is essential to develop non-toxic, vanadium-free catalysts suitable for low-temperature denitrification.

Perovskite oxides have emerged as promising materials for environmental protection and other applications due to their structural stability, rich surface acidity, and tunable redox properties [6]. Among these, Ba₂Ti₅O₁₂, a typical perovskite, has been identified to possess oxygen vacancy defects on its surface, which contribute to the SCR process [7]. The crystalline structure of perovskite oxides can be finely tuned by doping metals, introducing lattice distortions and surface vacancy defects without significantly altering the overall structure. This approach has been shown to improve the material’s redox properties and surface acidity. For example, Ao Ran, et al. [8] demonstrated enhanced denitrification activity and structural stability in LaCoO₃ perovskite catalysts doped with Sr via the sol-gel method. Transition metal manganese (Mn), renowned for its redox capability due to unfilled d-orbitals, has become a focal point in low-temperature SCR research. Mn doping has been shown to improve catalyst acidity and redox properties, significantly boosting low-temperature denitrification activity [9,10]. Similarly, cerium (Ce), a rare earth element with unique properties, has gained attention for its ability to enhance low-temperature SCR activity when incorporated into catalysts [11]. For instance, Shi et al. [12] synthesized a series of Ce-modified La-Mn perovskite catalysts, achieving enhanced surface area, acidity, and chemisorptive oxygen capacity, with a NO_x conversion rate of 90% at 135 °C. Liu et al. [13] investigated the effects of Mn-V or Mn-Ti co-doping on CoCr₂O₄ catalysts and observed synergistic improvements in lattice defects, electron migration, and SCR reaction kinetics, as well as enhanced sulfur and water resistance.

Despite extensive research on perovskite oxides for industrial catalysis and environmental protection, the use of Ba₂Ti₅O₁₂ in NH₃-SCR denitrification has been largely unexplored in the scientific literature [14]. This study aims to address that gap by preparing Ba₂Ti₅O₁₂ perovskite oxide via the sol-gel method and pioneering its application in NH₃-SCR denitrification. To develop a non-vanadium catalyst with a wide operational temperature range and superior low-temperature SCR performance, Ba₂Ti₅O₁₂ was doped with the transition metals manganese (Mn) and cerium (Ce). A comprehensive physicochemical analysis of the catalyst’s structure, redox properties, and surface acidity was conducted using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Brunauer–Emmett–Teller (BET) measurements, ammonia temperature-programmed desorption (NH₃-TPD), and electron paramagnetic resonance (EPR). Notably, the optimal catalyst achieved NO_x conversion rates exceeding 90% within the temperature range of 90 to 240 °C under a gas hourly space velocity (GHSV) of 28,000 h⁻¹. These characterization techniques provided some insights into the synergistic effects of Mn, Ce, and BaTiO₃, which contribute to the enhanced denitrification performance.

2. Results and Discussion

2.1. Catalytic Performance

The BaTi catalyst, synthesized with a stoichiometric Ba:Ti molar ratio of 1:1 and calcined at 650 °C, exhibited moderate NH₃-SCR activity, achieving ~70% NO_x conversion at 240 °C. However, its active temperature window remained relatively narrow compared to other tested catalysts (Figure 1a), indicating limited practical applicability. To optimize the catalytic performance, a series of Mn-doped BaTi catalysts with varying Mn/Ba/Ti ratios (e.g., 1:1:1, 2:1:1, 3:1:1, and 4:1:1) were systematically evaluated for NO_x removal. As shown in Figure 1b, the Mn_3.0BaTi catalyst (Mn/Ba/Ti = 3:1:1) exhibited superior low-temperature NO_x conversion efficiency compared to other ratios, suggesting an optimal Mn loading for redox performance. Based on this optimal composition, we subsequently focused on examining the influence of Ce doping levels while maintaining the fixed Mn/Ba/Ti = 3:1:1 ratio. While the effects of Ce doping on catalysts with other Mn/Ba/Ti ratios were not explored in this study, such investigations could provide further insights into synergistic interactions between Mn-Ce and host composition, which merits future work. Remarkably, Ce incorporation significantly broadened the active temperature window while improving low-temperature activity (Figure 1a). Among all evaluated catalysts, the Ce_1.5Mn_3.0BaTi composite demonstrated exceptional performance, achieving nearly 99% NO_x conversion across a wide operating range (90–210 °C). This outstanding activity can be attributed to the synergistic interactions between Ce and Mn species, which enhances redox properties, surface acidity, and reactive oxygen mobility that are key factors governing NH₃-SCR efficiency. Notably, the NO_x conversion efficiency of Ce_1.5Mn_3.0BaTi surpassed that of all control samples with varying Mn/Ba/Ti ratios, underscoring the critical role of optimal dopant concentrations. The results reveal a clear composition-performance relationship, where NO_x conversion is highly dependent on the Ce/Mn ratio, with an identified optimum Ce content for maximizing denitrification efficiency in simulated flue gas conditions. These findings highlight the potential of Ce/Mn co-doped Ba₂Ti₅O₁₂ as a highly effective, low-temperature NH₃-SCR catalyst for industrial applications.

Catalyst stability is a decisive factor in determining practical applicability [15], particularly for industrial NH₃-SCR systems requiring prolonged operation under harsh flue gas conditions. While the primary focus of this work was to optimize the NO_x removal activity through composition tuning, we specifically evaluated the operational stability of the most active catalyst, Ce_1.5Mn_3.0BaTi. To assess the durability of the optimized Ce_1.5Mn_3.0BaTi catalyst, continuous stability tests were conducted at both 180 °C (within its peak activity window, 90–210 °C) and 60 °C (a lower-conversion regime). As illustrated in Figure 2, the catalyst exhibited exceptional stability, maintaining near-constant NO_x conversion (~100% at 180 °C and ~73% at 60 °C) over a 25 h reaction period without noticeable deactivation. The consistency of time-on-stream performance across these conditions confirms that the observed stability is not an artifact of excessive active sites at 180 °C, but reflects intrinsic catalyst robustness. Systematic investigation of long-term durability across all compositions will be addressed in future work, as stability may be influenced by both Mn/Ba/Ti ratios and Ce loading levels.

2.2. Phase Analysis of Catalysts

The XRD pattern of Ce_1.5Mn_3.0BaTi catalysts (Mn/Ce-doped Ba₂Ti₅O₁₂) shows distinct diffraction peaks corresponding to the Ba₂Ti₅O₁₂ phase (JCPDS No. 17-0661) and secondary BaCO₃ phase (JCPDS No. 44-1487). The absence of detectable Ce/Mn-related phases suggests successful incorporation of dopants into the Ba₂Ti₅O₁₂ lattice, due to ionic radius mismatch (Ce⁴⁺: 0.97 Å, Mn⁴⁺: 0.53 Å vs. Ti⁴⁺: 0.605 Å). The BaCO₃ impurity likely originates from incomplete decomposition of barium precursors during synthesis, as its complete conversion requires higher temperatures (>800 °C). The XRD analysis indicates effective dopant integration into the Ba₂Ti₅O₁₂ lattice, with no evidence of segregated MnO_x phases (e.g., no peaks at 32.9° for Mn₂O₃ or 55.2° for Mn₃O₄) (Figure 3). While the Ba₂Ti₅O₁₂ (311) peak at 28.5° overlaps with the expected CeO₂ (111) reflection, the presence of a discernible peak at 47.5° (attributed to CeO₂ (220)) suggests trace amounts of CeO₂ may coexist. However, the absence of other characteristic CeO₂ peaks (e.g., 33.1° (200)) and the dominant Ba₂Ti₅O₁₂ phase signal confirm that most Ce species are atomically dispersed, with minimal phase segregation. This structural modification, maintaining the host framework integrity, creates oxygen vacancies (EPR-confirmed) and enhances Lewis acidity (NH₃-TPD) while preserving the matrix’s stability, collectively contributing to the improved catalytic performance observed in NH₃-SCR tests. The results highlight remarkable capacity of Ba₂Ti₅O₁₂ to accommodate dual dopants while retaining its fundamental structure-function relationships.

2.3. XPS Analysis

Comprehensive X-ray photoelectron spectroscopy (XPS) characterization of the Mn_3.0BaTi and Ce_1.5Mn_3.0BaTi catalysts was performed to elucidate the chemical state evolution and electronic interactions [16,17]. The high-resolution Ti 2p spectra (Figure 4a) exhibit characteristic Ti⁴⁺ signatures at 458.22 eV (2p_3/2), 463.95 eV (2p_1/2), and 470.91 eV (satellite peak), confirming the preservation of titanium’s tetravalent state in both catalysts. Notably, Ce incorporation induces a systematic 0.3 eV positive binding energy shift in the Ti 2p_3/2 peak, indicating enhanced electron withdrawal from Ti sites due to the following: (1) the higher electronegativity of Ce³⁺/Ce⁴⁺ relative to Ba²⁺; (2) possible electron transfer through newly formed Ti-O-Ce linkages; and (3) local lattice distortions from Ce doping [18]. This electronic perturbation, while maintaining the fundamental TiO₆ coordination (evidenced by unchanged satellite peaks), significantly modifies the catalyst’s redox properties. Moreover, the strengthened Ti⁴⁺ electron-binding capacity promotes oxygen vacancy formation and improves surface oxygen mobility, both critical for the SCR mechanism.

Figure 4b displays the deconvoluted O 1s XPS spectra of Mn_3.0BaTi and Ce_1.5Mn_3.0BaTi catalysts, obtained through Lorentz–Gaussian fitting analysis. The spectra reveal three characteristic oxygen species: (i) lattice oxygen (O_L) at 529.23 eV, (ii) surface chemisorbed oxygen (O_V, present as reactive superoxide radicals O₂⁻ at 531.28 eV, and (iii) hydroxyl oxygen (OH) at 532.51 eV [19]. Quantitative analysis (

Table 1. The proportion of oxygen species, Mn species and Ce species in Mn_3.0BaTi or Ce_1.5Mn_3.0BaTi catalysts.

Samples	Lattice Oxygen (OL)/%	Surface Adsorption Oxygen (OV)/%	Hydroxyl Oxygen (OH)/%	Mn2+/%	Mn3+/%	Mn4+/%	Mn4++Mn3+/Mnn+	Ce3+/%	Ce4+/%
Mn3.0BaTi	69.21	21.13	9.66	33.28	66.72	0	66.72	0	0
Ce1.5Mn3.0BaTi	46.66	32.71	20.63	13.72	55.17	31.11	86.28	34.06	65.94

) demonstrates that the Mn_3.0BaTi catalyst comprises 69.21% O_L, 21.13% O_V, and 9.66% OH. Notably, Ce modification induces substantial compositional changes in Ce_1.5Mn_3.0BaTi, with the oxygen species distribution shifting to 46.66% O_L, 32.71% O_V, and 20.63% OH. The pronounced increase in O_V and OH content following Ce doping signifies enhanced oxygen vacancy formation, which critically influences the catalytic mechanism in two key aspects: First, the abundant O_V species facilitate NO oxidation to NO₂ via their highly active superoxide radicals (O₂⁻), while the acidic OH groups promote NH₃ adsorption and activation; both processes synergistically enhance the NH₃-SCR reaction kinetics [20]. Second, the oxygen vacancies substantially improve the redox properties of the catalyst by enabling efficient electron transfer between active sites and modulating local electron density distributions. Moreover, these vacancies actively participate in the catalytic cycle by interacting with gaseous oxygen to generate additional reactive oxygen species (e.g., O₂⁻). This dynamic regeneration mechanism not only accelerates the SCR reaction cycle but also significantly enhances the overall denitration performance through improved redox properties and optimized reaction kinetics.

Figure 4c presents the Mn 2p XPS spectra of the Mn_3.0BaTi and Ce_1.5Mn_3.0BaTi catalysts. In the Ce_1.5Mn_3.0BaTi catalyst, the Mn 2p spectrum exhibits two primary spin–orbit doublets corresponding to Mn 2p_1/2 and Mn 2p_3/2, accompanied by a characteristic satellite peak. Deconvolution analysis reveals the coexistence of three manganese oxidation states: Mn²⁺, Mn³⁺, and Mn⁴⁺ [21]. Previous studies have established that the Mn 3s core-level XPS spectrum exhibits a characteristic doublet structure, which serves as a more reliable indicator for determining manganese oxidation states compared to other spectral features [22]. The energy separation (ΔE) between these Mn 3s peaks typically ranges from 4.7 to 5.8 eV for various manganese oxides [23]. As shown in Figure 5, the Ce_1.5Mn_3.0BaTi catalyst displays a ΔE value of 5.3 eV, falling within this characteristic range. This observation provides clear evidence for the coexistence of multiple manganese oxidation states (Mn²⁺, Mn³⁺, and Mn⁴⁺) in the catalyst, in good agreement with numerous reported studies on mixed-valence manganese systems [24,25]. Notably, the emergence of Mn⁴⁺ in the Ce-doped catalyst can be ascribed to the redox interplay between Ce and Mn, wherein Ce⁴⁺ promotes the oxidation of Mn²⁺ and Mn³⁺ to Mn⁴⁺. This observation is in agreement with the findings of Kapteijn et al. [26], who demonstrated that Mn⁴⁺ possesses superior SCR activity compared to other manganese oxides. Quantitative analysis (Table 1) further confirms a decrease in Mn²⁺ and Mn³⁺ content alongside a marked increase in Mn⁴⁺ concentration in the Ce_1.5Mn_3.0BaTi catalyst. This shift in oxidation state distribution correlates well with the enhanced redox capability observed in the Ce-modified catalyst, reinforcing the critical role of Mn⁴⁺ in improving catalytic performance.

Figure 4d displays the deconvoluted Ce 3d XPS spectrum of the Ce_1.5Mn_3.0BaTi catalyst, revealing ten characteristic peaks corresponding to the spin–orbit split 3d_3/2 and 3d_5/2 states of both Ce³⁺ and Ce⁴⁺ species [23]. The spectral features arise from well-documented final-state effects in core-level photoemission, where Ce³⁺ exhibits two distinct states per spin–orbit component (main 3d_5/2 peak at 881.5 eV and shake-up satellite at 885.8 eV), characteristic of electron correlation effects during photoemission. For Ce⁴⁺, we observe three states per component (main 3d_5/2 peak at 882.9 eV, shake-up satellite at 888.1 eV, and final-state multiplet at 898.2 eV), resulting from multiplet splitting effects. Similar spectral features were consistently observed for the corresponding 3d_3/2 components, with obvious binding energies shift due to spin–orbit coupling, further confirming the coexistence of both oxidation states in the catalyst. Quantitative analysis (Table 1) reveals that the catalyst surface contains 34.06% Ce³⁺ and 65.94% Ce⁴⁺, confirming the coexistence of these two oxidation states. This mixed-valence configuration is particularly significant as it facilitates efficient redox cycling between Ce³⁺ and Ce⁴⁺, a process that has been demonstrated to be critical for enhancing catalytic activity [14]. The presence of this redox pair contributes to improved oxygen storage capacity and facilitates electron transfer processes, both of which are essential for optimal catalytic performance.

2.4. NH₃-TPD

Surface acidity is a pivotal attribute of NH₃-SCR catalysts, significantly influencing their catalytic performance. To evaluate this, NH₃-Temperature Programmed Desorption (NH₃-TPD) was employed, leveraging the principle that desorption temperatures for NH₃, an alkaline gas molecule, are higher over strong acid sites compared to weak ones. The Mn_3.0BaTi and Ce_1.5Mn_3.0BaTi catalysts underwent a controlled heating desorption process ranging from 300 to 900 °C, and the outcomes are depicted in Figure 6. Distinct from the Mn_3.0BaTi catalyst, which displayed a single desorption peak between 500 and 600 °C, the Ce_1.5Mn_3.0BaT catalyst exhibited an expanded desorption profile spanning 500 to 900 °C. This broadened peak is attributed to the interaction between NH₃ and the metal cations at the strong acid sites [27], indicative of a modified acid site landscape post-Ce doping.

The Ce_1.5Mn_3.0BaT catalyst not only demonstrated a higher desorption temperature but also an increased peak area compared to the Mn_3.0BaTi catalyst. These observations suggest that Ce doping significantly amplifies the acidic intensity of the catalyst. The substantial rise in the number of strong acid sites, particularly those of the Lewis type, is a direct consequence of Ce incorporation [28]. This enhancement in acidity is pivotal as it augments the catalyst’s ability to adsorb and activate NH₃ more effectively. The improved NH₃ management at the catalyst surface is a key determinant in boosting the catalyst’s denitration performance, aligning with the enhanced redox properties conferred by Ce doping as previously discussed [29].

2.5. EPR Surface Features

Electron Paramagnetic Resonance (EPR) technology, capable of detecting unpaired electrons and thereby identifying vacancy defects and free radicals [30] was utilized to examine the Mn_3.0BaTi and Ce_1.5Mn_3.0BaT catalysts. The results of this analysis are presented in Figure 7. In the EPR spectrum of the Mn_3.0BaTi catalyst, a distinct symmetrical peak is observed, with the magnetic field strength at the peak center measured to be 3506.7 G. Utilizing the relationship between the Planck constant (h), the magnetic field strength (B), the Bohr magneton (β), the electromagnetic wave frequency (ν), and the g-factor, the g-value for the Mn_3.0BaTi catalyst’s EPR spectrum is calculated to be 2.0081. This g-value, when compared to the standard value for unpaired electrons, corresponds to the solitary electron generated at oxygen vacancies, thereby confirming the presence of such vacancies on the surface of the Mn_3.0BaTi catalyst. These oxygen vacancies, as a type of anionic defect, not only serve as active sites themselves but also modulate the electron density of adjacent atoms, enhancing electron mobility and consequently improving the catalyst’s denitration activity [20].

Furthermore, the EPR spectrum of the Mn_3.0BaTi catalyst reveals six peaks labeled from I to VI. Peaks I, II, IV, and VI exhibit nearly identical intensities, while the peaks at III and V are relatively weaker. These characteristics are indicative of the fingerprint features of the superoxide radical (O₂⁻). As an intermediate active species, the superoxide radical facilitates rapid SCR reactions by promoting the conversion of NO to NO₂. Additionally, it enhances the generation and reactivity of intermediate species by abstracting hydrogen atoms from NH₃, thereby improving the catalyst’s denitration activity. This enhancement is a significant reason for the increased low-temperature activity of Mn-doped catalysts [31].

Upon Ce doping, the Ce_1.5Mn_3.0BaT catalyst’s EPR spectrum retains the six peak signals, with a calculated g-factor of 2.0081, aligning with the standard superoxide radical signal [32]. This indicates that Ce doping does not alter the type of radical present on the catalyst surface. However, the intensity of the superoxide radical peaks in the Ce_1.5Mn_3.0BaT catalyst is significantly higher than that in the Mn_3.0BaT catalyst, suggesting that Ce doping increases the quantity of superoxide radicals. This increase is consistent with the elevated surface oxygen species content observed by XPS analysis following Ce doping. The superoxide radical, as an active species, is capable of promoting the generation of NO from intermediates such as NO₃⁻ and NO₂⁻ in the SCR reaction, thereby accelerating the chemical reaction rate and enhancing the catalyst’s denitration activity.

The SCR of NO_x by NH₃ over Ce-Mn-modified Ba₂Ti₅O₁₂ catalysts involves a synergistic interplay between redox and acid–base sites, as demonstrated by comprehensive characterization. The redox cycle between Mn and Ce species plays a pivotal role, with XPS confirming the coexistence of multiple Mn oxidation states (Mn²⁺/Mn³⁺/Mn⁴⁺), where Mn⁴⁺ is particularly active in oxidizing NO to NO₂, enabling the “fast SCR”pathway (NO + NO₂ + 2NH₃ → 2N₂ + 3H₂O). The Ce³⁺/Ce⁴⁺ redox pair further enhances this process by facilitating oxygen vacancy formation and electron transfer, as evidenced by EPR and XPS, which also revealed a significant increase in O_V from 21.13% to 32.71% after Ce doping. These oxygen vacancies generate superoxide radicals (O₂⁻), promoting NO oxidation and improving electron mobility. Concurrently, NH₃-TPD results highlight the critical role of acidic sites, with Ce doping introducing strong Lewis acid sites and increasing Brønsted acidity (OH groups rising from 9.66% to 20.63%). These sites enhance NH₃ adsorption and activation, with Lewis acid sites (Mnⁿ⁺, Ce⁴⁺, Ti⁴⁺) coordinating NH₃ and Brønsted sites (-OH) protonating NH₃ to NH₄⁺. The combined redox and acid–base functionalities explain the superior SCR performance of Ce_1.5Mn_3.0BaTi catalyst.

3. Experimental Section

3.1. Samples Preparation

The Mn- and Ce-doped Ba₂Ti₅O₁₂ catalysts were synthesized using a one-step sol-gel method. First, 3.4 mL of n-butyl titanate, 5 mL of glacial acetic acid, and 10 mL of absolute ethanol were uniformly mixed to form solution A. Based on the molar ratio of barium to titanium, the required amount of barium acetate was dissolved in deionized water with sonication to produce solution B. Similarly, the required amounts of manganese acetate tetrahydrate and cerium nitrate hexahydrate were dissolved in deionized water via sonication, forming solution C. Subsequently, solutions A, B, and C were mixed and stirred magnetically for 2 h, then aged in a water bath at 80 °C for 8 h. The mixture was left to stand at room temperature to form a gel. This gel was then dried in an oven at 70 °C for 12 h. The dried solid was calcined in a muffle furnace at 650 °C for 2 h to produce the final catalyst, denoted as Ce_xMn_yBaTi (where x represents the Ce/Ti molar ratio and y represents the Mn/Ti molar ratio). Using the same procedure, pure Ba₂Ti₅O₁₂ (denoted as BaTi) and Mn-doped Ba₂Ti₅O₁₂ (denoted as Mn_yBaTi) were synthesized as comparison samples.

3.2. Activity Measurement

The catalytic activity for NH₃-SCR denitrification was evaluated using an integrated system consisting of a gas distribution unit, a fixed-bed reactor, and a flue gas analyzer (Model Testo 350, Testo SE & Co. KGaA, Lenzkirch, Germany). The gas distribution system utilized pre-calibrated cylinder gas to simulate exhaust gas containing 500 ppm NH₃, 500 ppm NO, 5 vol% O₂, and N₂ as the balance gas. The total flow rate of the simulated exhaust gas was maintained at 100 mL/min, corresponding to a gas hourly space velocity (GHSV) of 28,000 h⁻¹. The flue gas analyzer depicted in Figure 8 was employed to measure the NO_x concentration during the NH₃-SCR reaction. The flue gas analyzer used in this study can selectively detect NO and NO₂, but it does not measure N₂O or N₂. Therefore, while the NO_x conversion efficiency can be accurately assessed, the full nitrogen balance and final reduction products cannot be directly confirmed. However, based on well-established SCR mechanisms, including the standard SCR reaction (4NH₃ + 4NO + O₂ → 4N₂ + 6H₂O) and the fast SCR reaction (2NH₃ + NO + NO₂ → 2N₂ + 3H₂O), the main product under typical reaction conditions is expected to be N₂. This is especially valid under moderate temperatures and appropriate NO/NO₂ ratios. Although N₂O was not detected due to instrumental limitations, future work will involve advanced analytical techniques such as FTIR for N₂O quantification and gas chromatography for direct detection of N₂, enabling a complete evaluation of the nitrogen-containing products. Special consideration was given to oxygen consumption pathways during the reaction, which may occur through three primary mechanisms: (i) catalyst surface reoxidation of reduced active sites, (ii) selective catalytic reduction to N₂ (4NH₃ + 4NO + O₂ → 4N₂ + 6H₂O), and (iii) competitive oxidation reactions forming N₂O (2NH₃ + 2O₂ → N₂O + 3H₂O) or NO₂ (2NO + O₂ ↔ 2NO₂). The 5 vol% O₂ concentration was selected to align with typical flue gas conditions. Tests confirmed that higher O₂ levels did not further improve NO_x conversion, indicating the reaction is already oxygen-saturated at this concentration.

The reaction temperature of the catalyst was varied from 90 to 300 °C, and the NO_x conversion efficiency was calculated using the Formula (1):

$${\mathrm{NO}}_{\mathrm{x}}\mathrm{conversion} (\%)={\displaystyle \frac{{\left({\mathrm{N}\mathrm{O}}_{\mathrm{x}}\right)}_{\mathrm{i}\mathrm{n}}-{\left({\mathrm{N}\mathrm{O}}_{\mathrm{x}}\right)}_{\mathrm{o}\mathrm{u}\mathrm{t}}}{{\left({\mathrm{N}\mathrm{O}}_{\mathrm{x}}\right)}_{\mathrm{i}\mathrm{n}}}}\times 100\mathrm{\%}$$

(1)

3.3. Characterization

The crystalline phase structure of the catalysts was analyzed using X-ray diffraction (XRD) on a Bruker D8 Advance X-ray diffractometer. The analysis employed a copper target X-ray source with Cu Kα radiation (λ = 0.15406 nm), covering a diffraction angle (2θ) range of 5° to 80°. The surface elemental composition and chemical valence states of the catalysts were analyzed by X-ray photoelectron spectroscopy (XPS) using a ULVAC-PHI 5000 Versa Probe spectrometer (ULVAC-PHI, Inc., Chigasaki, Japan) equipped with a monochromatic Al-Kα X-ray source (hν = 1486.6 eV, operated at 15 kV and 25 W). The spectra were acquired using a hemispherical energy analyzer with a step size of 0.1 eV. All binding energies were calibrated against the C 1s peak at 284.8 eV as an internal reference. The specific surface area and pore structure of the catalysts were assessed through nitrogen adsorption-desorption isotherms using an ASAP 2020 analyzer (Micromeritics Instrument Corporation, Norcross, GA, USA). Before analysis, samples were degassed under vacuum at 100 °C for 200 min to eliminate contaminants. The Brunauer–Emmett–Teller (BET) method was used to calculate the specific surface area, while the Barrett-Joyner-Halenda (BJH) method was applied to determine pore size distribution and volume. Ammonia temperature-programmed desorption (NH₃-TPD) analysis was performed using an AutoChem II 2920 analyzer (Micromeritics Instrument Corporation, Norcross, GA, USA). Samples were pretreated in nitrogen flow at 350 °C for 1 h and saturated with a 2% NH₃-He mixture at 40 mL/min for 60 min. After saturation, the catalyst was purged with He, and the temperature was ramped at 10 °C per minute to the final set temperature. The NH₃-TPD spectrum was recorded once the baseline stabilized, providing data on the strength and quantity of acid sites. Electron paramagnetic resonance (EPR) spectroscopy was employed to detect unpaired electrons indicative of free radicals or vacancy defects on the catalyst surface. The measurements were performed using a Bruker EMX-10/12x spectrometer (Bruker BioSpin GmbH, Rheinstetten, Germany) operating at a frequency of 9.7 GHz. This technique provided valuable insights into the surface active sites of the catalysts.

4. Conclusions

In this study, Ce,Mn-codoped Ba₂Ti₅O₁₂ (CeMnBaTi) catalysts were successfully synthesized via the sol-gel method, demonstrating excellent NO_x reduction performance through a synergistic mechanism. Mn doping introduced lattice defects and surface superoxide radicals (O₂⁻), providing strong redox activity, while Ce incorporation further enhanced reactive oxygen species (ROS) concentration, stabilized Mn⁴⁺/Ce⁴⁺ redox pairs, and increased medium-strong Lewis acid sites, improving NH₃ adsorption and activation. EPR analysis confirmed the presence of oxygen vacancies and O₂⁻ radicals, which promoted NO oxidation to NO₂ and hydrogen abstraction from NH₃, thereby accelerating SCR reaction kinetics. The combined effects of Ce-Mn doping resulted in superior low-temperature denitration activity, making the CeMnBaTi catalyst a promising candidate for practical NO_x control applications.