Synergistic Effect of Iron Doping and Oxide Hybridization Enables Enhanced Low-Temperature NH₃-SCR Performance of Manganese Oxide Catalyst

Synergistic Effect of Iron Doping and Oxide Hybridization Enables Enhanced Low-Temperature NH₃-SCR Performance of Manganese Oxide Catalyst | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Synergistic Effect of Iron Doping and Oxide Hybridization Enables Enhanced Low-Temperature NH₃-SCR Performance of Manganese Oxide Catalyst Xuewen Li, Yaru Li, Menglin Wang, Can Wang, Wenxian Jing, Lisheng Fang, and 3 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6504997/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 9 You are reading this latest preprint version Abstract Manganese oxides (MnO x ) catalysts are promising for low-temperature ammonia-selective catalytic reduction (NH₃-SCR), however, the limited N₂ selectivity and the narrow operational temperature range remain challenges. To address these issues, we developed a method involving impregnation doping and high-temperature calcination to hybridize Mn₂O₃ with iron lattice and oxide. This hybrid catalyst maintains a NO x conversion rate above 90% within the broad temperature window of 175–300°C, while achieving N₂ selectivity above 99%. The as prepared Fe-Mn (0.15) exhibits spherical morphology with Fe and Mn uniformly distributed. It was investigated that the doping of lattice Fe contributes to a slight reduction in the catalyst's reducibility and a moderate increase the amounts of Lewis acid sites. Fe₂O₃, which produced through calcination, plays a crucial role in enhancing surface-adsorbed oxygen and Bronsted acid sites. These synergistic effects regulate both the acidic and redox properties of the catalyst, facilitating NH₃ adsorption and activation while controlling NH₃ overoxidation, thus broadening the operational temperature range and improving N₂ selectivity. Furthermore, in situ diffuse reflectance infrared spectroscopy (DRIFTS) characterization demonstrated that the NH₃-SCR reaction on the catalyst primarily follows an Eley-Rideal (E-R) mechanism. This work reveals the synergistic effects of Fe lattice doping and Fe₂O₃ composite on MnO x , offering new insights for developing advanced low-temperature catalysts. Low-temperature NH3-SCR Manganese oxides Iron lattice doping Fe₂O₃ E-R mechanism Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 1 Introduction With the advancement of industrialization, activities such as fuel combustion and chemical production have led to significant nitrogen oxide (NO x ) emissions, which are major contributors to severe air pollution [ 1 ]. Consequently, controlling NO x emissions has become a critical challenge for achieving sustainable development. Among the various NO x treatment methods, ammonia-selective catalytic reduction (NH₃-SCR) stands out for its high efficiency in reducing nitrogen oxide emissions [ 2 ]. This process relies on a catalyst, with NH 3 acting as a reductant, to catalyze the reduction of NO x to nitrogen (N₂) and water (H₂O). V₂O₅-WO₃/MoO₃-TiO₂ based catalysts are widely employed in commercial SCR processes [ 3 , 4 ]. However, their optimal NH₃-SCR activity is limited to a temperature range of 300–400°C, which restricts their application in flue gas treatment at lower temperatures [ 5 , 6 ]. Therefore, there is a critical need for developing highly active catalysts effective at lower temperatures (< 300°C), enabling more widespread and efficient NO x control across a broader range of industrial applications. Manganese oxide (MnO x ), due to its variable valence states and exceptional redox properties, demonstrates excellent low-temperature (LT) denitrification (de-NO x ) activity, making it a key focus in catalyst development. However, the robust redox capabilities of MnO x result in a narrow operating temperature window and low N₂ selectivity, limiting its further application in LT NH₃-SCR process [ 7 – 9 ]. Studies have demonstrated that doping transition metal elements into MnO x can significantly improve the catalytic activity of MnO x [ 10 – 13 ]. Especially, iron (Fe) is considered an excellent doping element due to its ease of availability and flexible valence states. For example, Zhang et al. demonstrated that doping lattice iron into Fe-Mn/TiO 2 catalysts increased the ratio of high-valent manganese to surface-chemically adsorbed oxygen, which lowered the catalyst's redox temperature and enhanced its LT de-NO x performance [ 14 ]. Meanwhile, the presence of Fe 2 O 3 in Fe-modified MnO x /TiO₂ catalysts has been shown to suppress N₂O formation and enhance N₂ selectivity by regulating the aggregation and dispersion of Mn species on the TiO₂ support [ 15 ]. Fe 2 O 3 , prepared by calcination at 300°C, exhibits over 95% NO x conversion and N 2 selectivity in the temperature range of 140°C to 280°C [ 16 ]. Moreover, researches highlight the impact of lattice doping and the coexistence of different metal oxides on catalyst performance. For instance, Cu₁Co₂Fe₁O x exhibits improved toluene oxidation performance compared to Co₃Fe₁O x and Cu₃Fe₁O x , primarily due to its rich multiphase interface (CuO/Fe₃O₄/Co₃O₄) and metal doping effects, which facilitate the creation of oxygen vacancies [ 17 ]. Likewise, Li et al. showed that the doping of indium as InMnO₃, In₂MnO₄, and In₂O₃ into MnO x catalysts promotes interactions with manganese, leading to the formation of Mn⁴⁺ and surface oxygen. Moderate indium modification significantly improves the LT catalytic activity of manganese oxide catalysts, resulting in nearly 100% de-NO x efficiency over the temperature range of 75–200°C [ 18 ]. Inspiringly, designing a strategy that incorporates Fe lattice doping and iron oxide hybridization might optimally enhance the NH₃-SCR performance of MnO x . Furthermore, exploring the synergistic effects of these modifications will offer valuable insights into expanding the design strategies for NH₃-SCR catalysts. In this study, we developed a simple method of impregnation doping followed by high-temperature calcination to prepare Fe-Mn metal oxide catalyst with both Fe lattice doping and Fe 2 O 3 composition. Fe-Mn (X) significantly enhanced LT de-NO x activity and improved N₂ selectivity. The synergistic effects of Fe incorporation were systematically evaluated. Furthermore, the reaction mechanism was explored through in situ DRIFTS analysis. 2 Experiment section 2.1 Preparation of catalysts MnO₂ was synthesized using a chemical precipitation method. Initially, 21 mmol of MnSO₄·H₂O (AR, Aladdin) and 42 mmol of NaHCO₃ (AR, Aladdin) were separately dissolved in 200 mL of deionized (DI) water, forming Solution Ⅰ and Solution Ⅱ. Then, Solution Ⅱ was slowly added to Solution Ⅰ, and the mixture was stirred for 10 minutes. After aging for 6 hours, the resulting white precipitate was centrifuged and washed with deionized water and anhydrous ethanol three times. The precipitate was dried in an oven at 80°C for 12 hours and then was calcined in air at 400°C for 5 hours to produce MnO₂. Fe-Mn oxide catalysts were synthesized via impregnation, followed by high-temperature calcination. Initially, 0.8 g of the as-prepared MnO₂ was sonicated in 30 mL of DI water and stirred for 30 minutes. Subsequently, varying amounts of Fe(NO₃)₃·9H₂O (AR, Aladdin) were added, and the mixture was stirred for 2 hours. The mixture was then dried at 80°C overnight and calcined in air at 500°C for 5 hours. The catalysts obtained were designated as Fe-Mn (X), where X denotes the Fe:Mn atomic ratio based on the precursor amounts of Fe(NO₃)₃·9H₂O and MnO₂, with values of 0.05, 0.1, 0.15, and 0.2. For comparison, Fe(NO₃)₃·9H₂O and MnO₂ were individually calcined at 500°C for 5 hours, yielding Fe-500 and Mn-500, respectively. These two were then physically mixed at an Fe:Mn atomic ratio of 0.15:1 to obtain the Fe–Mn-MG sample. 2.2 Characterization The morphology of catalysts was obtained using a Zeiss Sigma 300 field-emission scanning electron microscope. A Rigaku Ultima III X-ray diffractometer was used to obtain crystal structure of catalysts, within a scan range of 5–80°. Nitrogen adsorption-desorption isotherms were acquired via a JW-BK132F volume adsorption analyzer. The surface chemical information was detected through X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific ESCALAB QXi), and all spectra were calibrated using the C1s peak (284.8 eV) as a reference. X-ray absorption fine structure (XAFS) spectroscopy was carried out using the Deep-Inspectra-X1(Beijing SciStar Technology Co., Ltd.) by transmission mode at 20 kV and 20 mA, and the Si (553) spherically bent crystal analyzer with a radius of curvature of 500 mm was used for Cu. The H 2 -TPR and NH 3 -TPD were measured on AMI-300 instrument, connected to a thermal conductivity detector. Before measurement, the samples were pretreated with a helium flow at 300℃ for 1 hour, followed by cooling to 50℃. For the H₂-TPR tests, the samples were subjected to a 10% H₂/Ar flow (30 mL·min⁻¹), heated from 50°C to 700°C (10°C·min⁻¹), and reduced by hydrogen. For the NH₃-TPD experiment, the sample was saturated with a 10% NH₃/He flow (30 mL·min⁻¹) and heated at a rate of 10°C·min⁻¹ to 650°C in a helium flow, followed by NH₃ desorption. In situ DRIFTS were acquired on a Nicolet NEXUS 870 spectroscopy equipped with a liquid-nitrogen cooled mercury cadmium telluride detector. All samples were initially preheated in an Ar atmosphere at 200°C for 30 minutes, followed by cooling to 150°C to collect the background spectrum. Next, a gas mixture containing 500 ppm NH₃/Ar or 500 ppm NO + 3% O₂ was passed over the samples until the sample surface reached saturation. The total gas flow was 100 mL·min⁻¹. Subsequently, the samples were purged with an Ar flow for 30 minutes, followed by treatment with either 500 ppm NO + 3% O₂ or 500 ppm NH₃/Ar. Data were accumulated through 32 scans to obtain a spectral resolution of 4 cm⁻¹. 2.3 NH 3 -SCR activity test A quartz tube fixed-bed reactor containing 0.3 g of catalyst (40–60 mesh) was employed to simulate flue gas conditions and evaluate the catalytic activity. The reaction gas included 500 ppm NO, 500 ppm NH 3 and 3 vol% O 2 , and Ar serves as carrier gas. The Gas Hourly Space Velocity (GHSV) for the reaction was 60,000 h⁻¹. The catalytic performance for NH₃-SCR denitrification was analyzed based on the following equations: NO x conversion (%) = \(\:\frac{\left({\left[{\text{N}\text{O}}_{x}\right]}_{\text{i}\text{n}}-{\left[{\text{N}\text{O}}_{x}\right]}_{\text{o}\text{u}\text{t}}\right)}{{\left[{\text{N}\text{O}}_{x}\right]}_{\text{i}\text{n}}}\times\:100\%\) N 2 selectivity (%) = \(\:\:\left(1-\frac{2{\left[{\text{N}}_{2}\text{O}\right]}_{\text{o}\text{u}\text{t}}+{\left[\text{N}{\text{O}}_{2}\right]}_{\text{o}\text{u}\text{t}}}{{\left[\text{N}\text{O}\right]}_{\text{i}\text{n}}+{\left[\text{N}{\text{H}}_{3}\right]}_{\text{i}\text{n}}-{\left[\text{N}\text{O}\right]}_{\text{o}\text{u}\text{t}}-{\left[\text{N}{\text{H}}_{3}\right]}_{\text{o}\text{u}\text{t}}}\right)\times\:100\%\) where [NO] in and [NH 3 ] in correspond to the inlet concentrations of NO and NH 3 in the fixed-bed reactor, while [NO] out , [NH 3 ] out , [NO 2 ] out , and [N 2 O] out denote their respective outlet concentrations. 3 Results and discussion 3.1 Morphological and structural properties The crystal structure of various samples was analyzed by XRD patterns. As observed in Fig. 1 a, three distinct diffraction peaks at 37.12°, 42.40° and 56.03° in MnO 2 , which match well with the (100), (101) and (102) planes of ε-MnO 2 (PDF#30–0802). After calcined at 500°C, MnO₂ and Fe(NO₃)₃·9H₂O are transforms to Mn₂O₃ (PDF#41-1442) and Fe₂O₃ (PDF#33–0664), respectively. For Fe-Mn (X), diffraction peaks at 23.12°, 32.93°, 38.20°, 45.14°, 49.31°, 55.14°, and 65.74° correspond to the (211), (222), (400), (332), (431), (440), and (622) crystal planes of Mn₂O₃. Moreover, with increasing Fe addition, the diffraction peaks shift slightly to higher angles, and their intensity gradually decreases (Fig. 1 b). This is attributed to the partial substitution of Mn³⁺ by Fe³⁺, with Fe³⁺ incorporating into the Mn₂O₃ lattice [ 19 ]. When the Fe:Mn atomic ratio exceeds 0.05, characteristic peaks of Fe₂O₃ gradually appear in Fe-Mn (X), indicating that excess Fe exists in the form of Fe₂O₃ (Fig. 1 c). This suggests that Fe modifies Mn₂O₃ through lattice doping as well as by forming a composite with iron oxides, where lattice Fe and iron oxides coexist in Fe-Mn (X) (X = 0.1, 0.15, 0.2). The shift and intensity variation of the Fe₂O₃ diffraction peaks may be related to its interaction with manganese oxides [ 20 ]. SEM was conducted to study the morphology of various catalysts. As depicted in Fig. 1 d and e, MnO 2 exhibits a spherical morphology of approximately 2 µm, while Mn-500 particles are slightly smaller, measuring around 1.2 µm. After the introduction of Fe (Fig. 1 f ~ i), the size of Fe-Mn (X) is approximately 1.5 µm, slightly larger than that of Mn-500. This increase in size is related to the reaction between Fe and MnO₂ during high-temperature calcination, where excess Fe is transformed into Fe 2 O 3 , contributing to particle growth. The surface of Fe-Mn (0.05) exhibits no significant change, maintaining a particulate structure. As the level of iron incorporation increases, the surface of Fe-Mn (X, X = 0.1, 0.15) displays a needle-like structure formed by the stacking of nanosheets. With a further increase in Fe content, the nanosheet structure on the surface of Fe-Mn (0.2) gradually disappears, and the surface becomes rough, possibly due to the excessive deposition of Fe₂O₃ aggregation. EDS elemental mapping reveals a homogeneous distribution of O, Mn and Fe elements (Fig. 1 j ~ l) in Fe-Mn (0.15), further demonstrating that Fe is involved in the formation process of the catalysts. This uniform distribution facilitates synergistic interactions between Mn and Fe, promoting efficient electron transfer and enhancing the NH₃-SCR catalytic activity [ 21 ]. The pore texture of the catalysts was determined by nitrogen adsorption-desorption analysis. Fig. S1 a illustrates that the isothermal adsorption-desorption curves of MnO₂, Mn-500, and Fe-Mn (X) are classified as type IV isotherms with H1-type hysteresis loops, indicating mesoporous structures [ 22 ]. The pore size distribution in Fig. S1 b, reveals that after Fe loading, the catalyst's pore size increases, primarily within the range of 7–27 nm, which facilitates the adsorption and transfer of reactive gases [ 9 ]. According to Table S1 , MnO₂ has a specific surface area of 104.0 m²·g − 1 and an average pore diameter of 5.61 nm, while Mn-500 exhibits a lower specific surface area (26.2 m²·g − 1 ) and a larger average pore diameter (23.31 nm). This change may result from the formation of a new phase, where smaller pores become blocked, and the proportion of larger pores increases. Fe-Mn (X) catalysts show higher specific surface areas and smaller average pore diameters compared to Mn-500. Nevertheless, excess Fe₂O₃ agglomeration blocks pores, reducing the specific surface area of Fe-Mn (0.2). 3.2 Surface chemistry properties The surface chemistry of various Fe-Mn (X) catalysts was investigated using an XPS analysis. Figure 2 a shows the Mn 2p reveal two distinct peaks in the range of 638–658 eV, corresponding to Mn 2p 3/2 and Mn 2p 1/2 . The Mn 2p 3/2 peak can be further fitted into three peaks at approximately 640.2 eV, 641.6 eV, and 643.2 eV, suggesting that Mn primarily exists as a mixture of Mn 2+ , Mn 3+ , and Mn 4+ [ 23 ]. These coexisting Mn species enhance redox activity and promote the creation of additional oxygen vacancies [ 24 ]. As shown in Table S2, the Mn⁴⁺/(Mn⁴⁺ + Mn³⁺ + Mn²⁺) ratio increases with Fe introducing but slightly decreases at higher Fe levels. This indicates that the introduction of Fe enhances the proportion of higher oxidation states of Mn in Fe-Mn oxides. It was reported that high valence state Mn 4+ had the potential to enhance the low-temperature de-NO x activity by facilitating the conversion of NO to NO 2 and finally reduced to N 2 through NH₃ [ 25 ]. Figure 2 b shows that Fe 2p consists of Fe 2p₃/₂ (~ 711 eV) and Fe 2p₁/₂ (~ 725 eV). The Fe 2p₃/₂ peak can be further fitted into Fe²⁺ (~ 710.6 eV) and Fe³⁺ (~ 713 eV) [ 26 ]. Previous study demonstrated that Fe³⁺ exhibits higher catalytic activity than Fe²⁺ by suppressing NH₃ oxidation and enhancing the “fast SCR” mechanism, improving NO x conversion efficiency [ 27 ]. As indicated in Table S2, the introduction of Fe gradually increases the Fe³⁺/(Fe³⁺ + Fe²⁺) ratio, which could promote redox reactions with Mnⁿ⁺, such as Mn³⁺ + Fe³⁺ ↔ Mn⁴⁺ + Fe²⁺ and Mn³⁺ + Fe²⁺ ↔ Mn²⁺ + Fe³⁺, leading to the coexistence of Fe and Mn in various valence states on the catalyst surface [ 14 ]. In addition, Fig. 2 c illustrates that the O1s peaks of all catalysts can be divided into lattice oxygen (O α ) at ~ 530 eV, surface-chemically adsorbed oxygen (O β ) at ~ 531 eV, and -OH (O γ ) at ~ 533 eV [ 28 ]. Fe-Mn (0.6) achieves the highest proportion of O β in comparison with other catalysts (Table S2). Compared to O α , O β is proposed to have higher chemical reactivity and electron mobility, influencing oxidation reactions. It may support NO oxidation to NO₂, which could play a role in facilitating the “fast SCR” reaction. Therefore, a higher O β /(O α + O β + O γ ) ratio is suggested to correlate with enhanced NH₃-SCR catalytic performance [ 29 ]. To further investigate the chemical bonding information in Fe-Mn (0.15), XAFS spectroscopy was analyzed. As shown in Fig. 3 a, the Fe K-edge XANES signal of Fe-Mn (0.15) is located between that of Fe foil (7122.8 eV) and the Fe₂O₃ standard sample (7125.3 eV). The spectral peaks closely resemble those of the Fe₂O₃ standard sample, indicating that Fe in Fe-Mn (0.15) predominantly exists in the form of Fe₂O₃ [ 30 ]. Comparison with the standard Mn₂O₃ spectrum indicates that Fe-Mn (0.15) primarily exhibits an Mn₂O₃ structure, which is consistent with the XRD results. Furthermore, Fig. 3 b shows that the XANES spectrum of Fe-Mn (0.15) shifts toward the higher energy region, indicating that Fe doping increases the average oxidation state of Mn [ 31 ]. This result is consistent with the XPS data. In the Fe K-edge extended X-ray absorption fine structure (EXAFS) spectra (Fig. 3 c), Fe foil exhibits a characteristic signal of the Fe–Fe bond at 2.2 Å, while the Fe₂O₃ standard sample presents distinct oscillation peaks at approximately 1.5 Å and 2.6 Å, corresponding to Fe–O and Fe–Fe bonds, respectively. The Fe-Mn (0.15) sample exhibits oscillation peaks at 1.8 Å and 2.6 Å, similar to those of the Fe₂O₃ standard sample, corresponding to Fe–O and Fe–Fe (Mn) bonds [ 32 ]. In the Mn K-edge EXAFS spectra (Fig. 3 d), the Mn-500 and Mn₂O₃ standard sample exhibit peaks at 1.4 Å and 2.7 Å, which are assigned to Mn–O and Mn–Mn bonds, respectively. The Fe-Mn (0.15) sample displays similar oscillation peaks, with peaks at 1.4 Å and 2.8 Å corresponding to Mn–O and Mn–Mn (Fe) bonds, respectively [ 33 ]. These results confirm that Fe is successfully incorporated into the Mn₂O₃ lattice in Fe-Mn (0.15), while excess Fe primarily exists in the form of iron oxides. In addition, wavelet transform analyses of the Fe and Mn K-edge further verify these findings (Fig. 3 e ~ j). Compared with Fe₂O₃ and Mn₂O₃, Fe-Mn (0.15) spectra exhibit higher k-values of Fe–Fe (Mn) and Mn–Mn (Fe) signals, which could be attributed to Fe doping and Mn–Fe signal overlap [ 34 , 35 ]. Figure 4 a shows H 2 -TPR profiles of the catalysts. Mn-500 exhibits two sequential peaks, reflecting its stepwise reduction to Mn₃O₄ at lower temperatures and subsequently to MnO at higher temperatures [ 36 ]. In the Fe- Mn (X) catalyst, peaks around 330°C and 450°C correspond to the reduction processes of Mn₂O₃ → Mn₃O₄ and Fe₂O₃ → Fe₃O₄, and the composite reduction of Mn₃O₄ → MnO and Fe₃O₄ → FeO, respectively [ 37 ]. Compared to Mn-500, the reduction peaks of Fe-Mn (0.05) shift to higher temperatures, suggesting that Fe doping slightly weakens the catalyst's reduction capacity. This modulation is believed to suppress the excessive oxidation of NH₃, which may contribute to expanding the operating temperature window and improving the catalytic performance of the NH₃-SCR reaction [ 38 ]. When Fe is present in the form of Fe₂O₃, the Fe-Mn (0.15) catalyst exhibits a new reduction peak at 284°C, corresponding to the reduction of surface-adsorbed oxygen (generation of oxygen vacancies) [ 39 ]. This result indicates that the synergy between Mn₂O₃ and Fe₂O₃ enhances oxygen adsorption on the catalyst surface, which is consistent with the XPS data and benefits the NH₃-SCR performance. The acidity properties of the catalysts were analyzed using NH₃-TPD, with the results shown in Fig. 4 b. The NH₃-TPD profiles of all catalysts exhibit three NH₃ desorption peak regions located at I (50–200°C), II (200–400°C), and III (400–500°C). Region I corresponds to NH₃ desorption from weak acidic sites (Lewis acid sites), primarily including physically adsorbed NH₃. Region II represents moderate Lewis (L) acid sites linked to Fe species. Region III corresponds to strong acidic sites, mainly involving NH₄⁺ (Bronsted acid sites) desorption and newly generated L acid sites from Fe species [ 40 – 42 ]. As shown in Table S3, compared to Mn-500, the Fe-Mn (0.05) catalyst exhibits a larger integrated area of Region I, indicating that lattice Fe doping enhances the strength of weak acid sites. As the Fe₂O₃ content increases, the number of weak acid sites on the Fe-Mn (X, X = 0.1, 0.15, 0.2) catalysts gradually increases, while the number of strong acid sites initially decreases and then increases. This indicates that an appropriate amount of Fe₂O₃ promotes the formation of strong acid sites on the catalyst surface. Among these catalysts, Fe-Mn (0.15) exhibits the highest number of NH₃ desorption peaks, implying the highest acidity and acid strength, thereby enhancing their ability to adsorb and activate NH₃ [ 43 ]. A higher concentration of weak acid sites is more favorable for low-temperature NH₃-SCR reactions [ 44 ]. 3.3 NH 3 -SCR performance The catalytic performance of different catalysts is illustrated in Fig. 4 c and d. As observed in Fig. 4 c, Fe-500 demonstrates relatively low NO x conversion efficiency due to its inherent medium- and high-temperature de-NO x characteristics [ 45 ]. The de-NO x activity of MnO 2 is higher than that of Mn-500, attributed to the higher oxidation state of Mn 4+ in MnO 2 , which facilitates NO x conversion [ 46 ]. Although Fe-500 and Mn-500 mechanical mixtures (Fe-Mn-MG) can achieve up to 80% NO x conversion, they are constrained by a narrow operating temperature range. By contrast, the introduction of Fe effectively promotes the catalytic activity of Fe-Mn (X), with Fe-Mn (0.15) demonstrating the best overall performance. Notably, within the 175–300°C range, it achieves a NO x conversion rate of 96.8% with N 2 selectivity exceeding 99% (Fig. 4 d). This exceptional catalytic activity results from the synergistic effects of an optimal amount of Fe lattice doping and Fe₂O₃ hybridization, which increase acidic sites, enhance NH₃ adsorption, and broaden the operational temperature window. Additionally, Fe doping contributes to a moderate reduction in reducibility, which helps suppress excessive NH₃ oxidation and improves N₂ selectivity [ 47 , 48 ]. Excessive Fe content, as in Fe-Mn (0.2), slightly reduces performance. This is due to excess Fe₂O₃ blocking the pore structure, decreasing the specific surface area, and limiting reactant gas adsorption [ 49 ]. Compared with catalysts in the literature (Table S4), Fe-Mn (0.15) demonstrates competitive NO x conversion efficiency across a wide temperature range, even under a high GHSV of 60,000 h⁻¹. 4 NH-SCR mechanism The adsorption behavior and reaction mechanism of NH₃ and NO on Mn-500 and Fe-Mn (0.15) catalysts were studied using in situ DRIFTS. 4.1 NH 3 adsorption In situ DRIFTS spectra of the adsorption of NH 3 at different times over Mn-500 and Fe-Mn (0.15) are presented in Fig. 5 a and b. According to Fig. 5 a, Mn-500 exhibits NH₃ adsorption peaks at 1170 cm⁻¹, 1190 cm⁻¹, 1228 cm⁻¹, 1606 cm⁻¹, and 1625 cm⁻¹, corresponding to L-NH 3 adsorbed on L acid sites [ 50 – 52 ] The peak at 1411 cm − 1 was assigned to B-NH 4 + adsorbed on Bronsted (B) acid sites [ 53 ]. As observed in Fig. 5 b, for Fe-Mn (0.15), NH₃ adsorption peaks at 1163 cm⁻¹, 1174 cm⁻¹, 1187 cm⁻¹, 1200 cm⁻¹, 1224 cm⁻¹, and 1593 cm⁻¹ are ascribed to L-NH 3 [ 38 ]. Additional peaks at 1309 cm⁻¹, 1419 cm⁻¹ and 1435 cm⁻¹ are associated with correspond to the B-NH 4 + species adsorbed on the catalyst surface[ 54 ]. These observations reveal that both Mn-500 and Fe-Mn (0.15) adsorb more L-NH₃ species than B-NH₄⁺ species. Notably, after Fe lattice doping and Fe₂O₃ combination, the number of spectral bands corresponding to B-NH₄⁺ increases, suggesting that Fe incorporation enhanced the catalyst's acidity, in agreement with the NH 3 -TPD analysis. After 30 min of Ar purge, the in situ DRIFTS spectra of Mn-500 and Fe-Mn (0.15) show no significant changes, confirming that L-NH 3 and B- NH 4 + species stably adsorbed on the catalyst surface[ 55 ]. 4.2 NO + O 2 adsorption Figure 5 c and d display the DRIFT spectra of surface-adsorbed species formed by NO + O₂ co-adsorption on Mn-500 and Fe-Mn (0.15) at various time intervals. In terms of Mn-500, peaks at 1215 cm⁻¹ and 1261 cm⁻¹ are assigned to bridged nitrite and monodentate nitrate, while peaks at 1311 cm⁻¹, 1548 cm⁻¹, and 1558 cm⁻¹ are attributed to bidentate nitrate, and the peak at 1435 cm⁻¹ represents ionic nitrate[ 56 ]. On the Fe-Mn (0.15) surface, peaks at 1201 cm⁻¹ and 1610 cm⁻¹ are assigned to bridged nitrite, peaks at 1307 cm⁻¹ and 1315 cm⁻¹ originate from bidentate nitrate, and the absorbance peak at 1448 cm⁻¹ correspond to ionic nitrate [ 36 ]. Both Mn-500 and Fe-Mn (0.15) primarily convert adsorbed NO into various nitrate species, with bidentate nitrates enhancing low-temperature de-NO x performance [ 21 ]. Compared to Mn-500, Fe-Mn (0.15) exhibits stronger NO-related peaks, indicating a superior ability to adsorb and activate NO, thus enhancing the de-NO x activity [ 41 , 57 ]. Similarly, the 30-minute Ar purge had a slight effect on the NO-related peaks, indicating the stable retention of NO-related active species on the catalyst surfaces. The infrared absorption peaks of the catalyst are relatively weak, and several negative peaks are observed. This may be attributed to the black color of catalyst, which can strongly absorb or scatter infrared light, thereby reducing signal intensity, affecting spectral detection and causing inconsistent scaling in infrared spectra. This is similar to previous reports in the literature[ 56 ]. 4.3 Reaction between NO + O 2 and pre-adsorbed NH 3 The DRIFT spectra of NH 3 pre-adsorbed Mn-500 and Fe-Mn (0.15) after treatment with NO + O₂ at 150°C are depicted in Fig. 5 e and f. After introducing NO + O₂, the Mn-500 catalyst shows new peaks at 1307 cm⁻¹, 1319 cm⁻¹, and 1546 cm⁻¹, corresponding to bidentate nitrates. Simultaneously, the peak at 1626 cm⁻¹, associated with bridged nitrates, gradually increases. The L-NH₃ peaks at 1228 cm⁻¹ and 1606 cm⁻¹ weaken and eventually disappear over time, indicating their involvement in the reaction with NO [ 53 ]. For Fe-Mn (0.15), the L-NH₃ peaks at 1172 cm⁻¹, 1186 cm⁻¹, and 1209 cm⁻¹ also fade, while new peaks at 1311 cm⁻¹ and 1593 cm⁻¹ for bidentate nitrates appear, along with additional peaks at 1438 cm⁻¹ and 1580 cm⁻¹ for ionic and bridged nitrates. These peaks increase over time, indicating the simultaneous formation of various nitrates, with L-NH₃ predominantly involved in the reaction. Therefore, both Mn-500 and Fe-Mn (0.6) follow the Eley-Rideal (E-R) mechanism [ 58 ]. 4.4 Reaction between NH 3 and pre-adsorbed NO + O 2 Figure 5 g and h show the DRIFT spectra of NO + O₂ pre-adsorbed Mn-500 and Fe-Mn (0.15) catalysts after different NH₃ treatment times. As shown in Fig. 5 g, upon NH 3 purging, peaks corresponding to bridged nitrites (1215 cm⁻¹), monodentate nitrates (1261 cm⁻¹), and bidentate nitrates (1556 cm⁻¹) on the Mn-500 slightly decrease [ 36 , 59 ]. Simultaneously, a new peak corresponding to L-NH₃ emerges at 1213 cm⁻¹, and its intensity gradually increases with purging time. This indicates a reaction occurs between NO-related species and NH₃, though the effect is not pronounced. The peak at 1435 cm⁻¹ gradually broadens during NH 3 purging, likely due to the overlap of B-NH 4 + and ionic nitrate peaks. After pre-adsorption of NO + O 2 (Fig. 5 f), purging Fe-Mn (0.15) with NH 3 leads to the appearance of new peaks at 1230 cm⁻¹, corresponding to L-NH₃. Meanwhile, the peaks at 1201 cm⁻¹ (bridged nitrite) [ 60 ], 1307 cm⁻¹ (bidentate nitrate), and 1315 cm⁻¹ (ionic nitrate) gradually disappear, while the peaks at 1448 cm⁻¹ and 1610 cm⁻¹ slightly decrease, revealing a reaction with NH₃. These findings confirm that nitrate adsorption on Fe-Mn (0.15) is more stable, and the NH 3 -SCR reaction follows the Langmuir-Hinshelwood (L-H) mechanism. Compared to the reaction between NO + O₂ and pre-adsorbed NH₃, the reaction between NH₃ and pre-adsorbed NO + O₂ is less likely to occur [ 61 ]. Therefore, based on the above, lattice doping of Fe and the combination with Fe₂O₃ could enhance the adsorption of NH₃ on L and B acid sites of Fe-Mn (0.15), promoting the activation of NO. As depicted in Fig. 5 i, the NH₃-SCR reaction over Fe-Mn (0.15) proceeds through both the E-R and L-H mechanisms, where the E-R mechanism plays the primary role. In the E-R mechanism, NH₃ is initially adsorbed onto both L and B acid sites of the catalyst surface, forming L-NH₃ (ad) and B-NH₄ + (ad) , respectively. These adsorbed species are then converted into -NH₂ (ad) through reaction with active oxygen species (O (ad) ), which are generated by the adsorption of O₂(g) (O₂(g) → O (ad) ). The resulting -NH₂ (ad) subsequently reacts with NO, forming the intermediate NH₂NO (ad) , which decomposes into the final products, N₂ and H₂O. In the L-H pathway, both NH₃ and NO adsorb onto the catalyst. The adsorbed NO species react with L-NH₃(ad) and B-NH₄⁺(ad), ultimately forming N₂ and H₂O. 5 Conclusion In summary, we successfully synthesized Fe-Mn oxide catalysts with simultaneous Fe lattice doping and Fe₂O₃ composite through a simple impregnation doping technique followed by high-temperature calcination. The homogeneously of Mn and Fe in the Fe-Mn (0.15) spherical catalyst is beneficial to the synergistic interactions between Mn and Fe. The incorporation of lattice Fe moderately reduces the reducibility and enhances the weak acid site acidity of the catalyst. The incorporation of Fe₂O₃ significantly increases the amount of chemical adsorbed oxygen on the catalyst surface, while an appropriate amount of Fe₂O₃ also enhances the formation of strong acid sites. By regulating the amount of Fe introduced, the catalyst’s acidity and redox properties can be balanced, thereby improving NO x conversion and N 2 selectivity at lower temperatures. The Fe-Mn (0.15), with abundant surface Mn⁴⁺, chemisorbed oxygen, high reducibility and acidic sites, favors the “fast SCR” reaction, achieving a NO x conversion rate of 96.8% at 200°C. In situ DRIFTS analysis indicated that L-NH₃ adsorption and activation play a critical role in the NH₃-SCR reaction. At 150°C, the Fe-Mn (0.15) follows both the E-R and L-H mechanisms, with the E-R mechanism being predominant. This study underscores the significance of tuning Fe lattice doping and Fe₂O₃ composite in MnO x catalysts for optimizing L-T NH₃-SCR performance. Abbreviations MnO x Manganese oxides NH₃-SCR Ammonia-selective catalytic reduction DRIFTS Diffuse reflectance infrared spectroscopy E-R Eley-Rideal NO x Nitrogen oxide LT Low-temperature de-NO x Denitrification DI Deionized XRD X-ray difraction SEM Scanning electron microscope XPS X-ray photoelectron spectroscopy NH 3 -TPD Temperature-programmed desorption of ammonia H 2 -TPR Hydrogen temperature-programmed reduction XAFS X-ray absorption fine structure spectroscopy BET Brunauer–Emmett–Teller GHSV Gas Hourly Space Velocity L acid site Lewis acid site B acid site Bronsted acid site L-H Langmuir-Hinshelwood XANES X-ray absorption near edge structure EXAFS Extended X-ray absorption fine structure ad adsorbed Declarations Ethics and Consent to Participate Not applicable. Consent for Publication Not applicable. Competing Interest The authors declare no competing interests. Author Contributions Xuewen Li: Conceptualization, Methodology, Formal analysis, Writing–original draft. Yaru Li: Investigation, Formal analysis, Validation, Writing–review & editing. Menglin Wang: Investigation, Methodology. Can Wang: Formal analysis, Writing–review & editing. Wenxian Jing: Methodology, Validation. Lisheng Fang: Validation, Resources. Yonghua Hu: Project administration, Resources, Supervision. Yundong Liang: Project administration, Supervision. Xianbiao Wang: Funding acquisition, Resources, Writing–review & editing. Funding This work was supported by the Natural Science Foundation of Anhui Province (2408085MB026), the Anhui Provincial Key Research and Development Plan (2022h11020025), the National Natural Science Foundation of China (21976003), the Anhui Provincial Key Natural Science Research Project of Education Department (2023AH050195) and the Excellent Scientific Research and Innovation Team in Colleges and Universities of Anhui Province (2022AH010017). We thank the Beijing SciStar Technology Co., Ltd. for XAFS measurements and analysis. Availability of data and materials The authors declare that the data supporting the findings of this study are available within the paper and its Supplementary Information files. Should any raw data files be needed in another format they are available from the corresponding author upon reasonable request. Source data are provided with this paper. Acknowledgments This work was supported by the Natural Science Foundation of Anhui Province (2408085MB026), the Anhui Provincial Key Research and Development Plan (2022h11020025), the National Natural Science Foundation of China (21976003), the Anhui Provincial Key Natural Science Research Project of Education Department (2023AH050195) and the Excellent Scientific Research and Innovation Team in Colleges and Universities of Anhui Province (2022AH010017). We thank the Beijing SciStar Technology Co., Ltd. for XAFS measurements and analysis. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-6504997","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":450787669,"identity":"fd88cbd5-f9c6-429c-8228-7f1acb077f62","order_by":0,"name":"Xuewen Li","email":"","orcid":"","institution":"Anhui Jianzhu University","correspondingAuthor":false,"prefix":"","firstName":"Xuewen","middleName":"","lastName":"Li","suffix":""},{"id":450787672,"identity":"26ad5192-7dd0-4ca6-b629-c3a04ceae709","order_by":1,"name":"Yaru Li","email":"","orcid":"","institution":"Anhui Jianzhu University","correspondingAuthor":false,"prefix":"","firstName":"Yaru","middleName":"","lastName":"Li","suffix":""},{"id":450787675,"identity":"399acc60-a09b-48cf-a73e-cd258428ed81","order_by":2,"name":"Menglin Wang","email":"","orcid":"","institution":"Anhui Jianzhu University","correspondingAuthor":false,"prefix":"","firstName":"Menglin","middleName":"","lastName":"Wang","suffix":""},{"id":450787677,"identity":"fd9695f1-a6d6-44d5-8429-92a98b71bd43","order_by":3,"name":"Can Wang","email":"","orcid":"","institution":"Anhui Jianzhu University","correspondingAuthor":false,"prefix":"","firstName":"Can","middleName":"","lastName":"Wang","suffix":""},{"id":450787678,"identity":"2439ada9-1ed3-4f11-9d86-f9ceae1e20c1","order_by":4,"name":"Wenxian Jing","email":"","orcid":"","institution":"Anhui Shiqing Environmental Protection Technology Co., Ltd","correspondingAuthor":false,"prefix":"","firstName":"Wenxian","middleName":"","lastName":"Jing","suffix":""},{"id":450787680,"identity":"555a568a-7582-4cfb-a398-6e906e337e86","order_by":5,"name":"Lisheng Fang","email":"","orcid":"","institution":"Anhui Shiqing Environmental Protection Technology Co., Ltd","correspondingAuthor":false,"prefix":"","firstName":"Lisheng","middleName":"","lastName":"Fang","suffix":""},{"id":450787681,"identity":"d8542ae2-b9eb-452b-b5a0-61a30bbe4e51","order_by":6,"name":"Yonghua Hu","email":"","orcid":"","institution":"China Tobacco Anhui Industrial Co, Ltd","correspondingAuthor":false,"prefix":"","firstName":"Yonghua","middleName":"","lastName":"Hu","suffix":""},{"id":450787682,"identity":"7c86116d-fc7b-4590-997e-f9c7d35f33dc","order_by":7,"name":"Yundong Liang","email":"","orcid":"","institution":"Anhui Shiqing Environmental Protection Technology Co., Ltd","correspondingAuthor":false,"prefix":"","firstName":"Yundong","middleName":"","lastName":"Liang","suffix":""},{"id":450787683,"identity":"2b9a4ba3-c3bc-4e18-a8c5-acc2f6fd67ae","order_by":8,"name":"Xianbiao Wang","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA50lEQVRIiWNgGAWjYDACCTBpw2DAcADEYCZaSxrpWg4DtTAQqcXgdo/h54Jf5+XNGY8/k2CosE5sYD97AK8WyTlnjKVn9t023NlwIE2C4Ux6YgNPXgJeLfwSuRukeXtuM244cOCYBGPb4cQGCR4DvFrYJHI3/+btOWe/4cDBNgnGf0RoAdqyTZrnx4HEDQcOs0kwNhChRXLO+W/WvA3JyRsOHGO2SDiWbtzGk4Nfi8HttuTbPH/sbDfcOP7wxocaa9l+9jP4tYABYxuQkDjAwJAA8h1h9SDwB+SrBuLUjoJRMApGwcgDAJ0oSid591aUAAAAAElFTkSuQmCC","orcid":"","institution":"Anhui Jianzhu University","correspondingAuthor":true,"prefix":"","firstName":"Xianbiao","middleName":"","lastName":"Wang","suffix":""}],"badges":[],"createdAt":"2025-04-22 13:53:26","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6504997/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6504997/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":81959288,"identity":"6a0e9667-b752-4278-8c80-12d61fffa303","added_by":"auto","created_at":"2025-05-05 10:25:31","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":397190,"visible":true,"origin":"","legend":"\u003cp\u003e(a) XRD patterns of catalysts, (b) enlarged XRD patterns from 22° to 34°, (c) enlarged XRD patterns from 35° to 38°; SEM images of (d) MnO\u003csub\u003e2\u003c/sub\u003e, (e) Mn-500, (f) Fe-Mn (0.05), (g) Fe-Mn (0.1), (h) Fe-Mn (0.15), and (i) Fe-Mn (0.2); (j) O element, (k) Mn element and (l) Fe element mapping of Fe-Mn (0.15).\u003c/p\u003e","description":"","filename":"image1.png","url":"https://assets-eu.researchsquare.com/files/rs-6504997/v1/fc69cae9d4b38153a516968b.png"},{"id":81959675,"identity":"bde2a012-49ca-49fa-a165-583b30164f83","added_by":"auto","created_at":"2025-05-05 10:33:31","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":8042546,"visible":true,"origin":"","legend":"\u003cp\u003eXPS spectra of (a) Mn 2p (b) Fe 2p and (c) O1s of Fe-Mn (X).\u003c/p\u003e","description":"","filename":"image2.png","url":"https://assets-eu.researchsquare.com/files/rs-6504997/v1/46b8c30d56853b8c8ae00e4a.png"},{"id":81959673,"identity":"ac2c0c56-3f07-4e9b-a23c-fa2b0c039a21","added_by":"auto","created_at":"2025-05-05 10:33:31","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":317374,"visible":true,"origin":"","legend":"\u003cp\u003eNormalized (a) Fe and (b) Mn K-edge XANES spectra; EXAFS R space fitting curves of (c) Fe and (d) Mn; Wavelet transform analysis of Fe for (e) Fe\u003csub\u003e \u003c/sub\u003efoil, (f) Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e standard sample and (g) Fe-Mn (0.15); Wavelet transform analysis of Mn for (h) Mn\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e standard sample, (i) Mn-500 and (j) Fe-Mn (0.15).\u003c/p\u003e","description":"","filename":"image3.png","url":"https://assets-eu.researchsquare.com/files/rs-6504997/v1/ff6d5ecb844a644945d075c5.png"},{"id":81959285,"identity":"18f1d882-a1b7-4b4a-b351-bd3e3006cb6e","added_by":"auto","created_at":"2025-05-05 10:25:31","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":240263,"visible":true,"origin":"","legend":"\u003cp\u003e(a) H\u003csub\u003e2\u003c/sub\u003e-TPR profiles, (b) NH\u003csub\u003e3\u003c/sub\u003e-TPD profiles, (c) NO\u003csub\u003ex\u003c/sub\u003e conversion and (d) N\u003csub\u003e2\u003c/sub\u003e selectivity of various catalysts.\u003c/p\u003e","description":"","filename":"image4.png","url":"https://assets-eu.researchsquare.com/files/rs-6504997/v1/295798be9d14e13bddae6280.png"},{"id":81959287,"identity":"eade0bce-7716-42e5-a7db-baf08d47786c","added_by":"auto","created_at":"2025-05-05 10:25:31","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":487773,"visible":true,"origin":"","legend":"\u003cp\u003eIn situ DRIFTS spectra at 150°C: (a) Mn-500 and (b) Fe-Mn (0.15) in NH\u003csub\u003e3\u003c/sub\u003e flow; (c) Mn-500 and (d) Fe-Mn (0.15) in NO + O\u003csub\u003e2\u003c/sub\u003e flow at different times, followed by 30 minutes of Ar purge; NH\u003csub\u003e3\u003c/sub\u003e pre-adsorbed (e) Mn-500 and (f) Fe-Mn (0.15) in NO + O\u003csub\u003e2\u003c/sub\u003e flow; NO + O\u003csub\u003e2\u003c/sub\u003e pre-adsorbed (g) Mn-500 and (h) Fe-Mn (0.15) in NH\u003csub\u003e3\u003c/sub\u003e flow at different times; (i) The NH\u003csub\u003e3\u003c/sub\u003e-SCR reaction mechanism on Fe-Mn (0.6).\u003c/p\u003e","description":"","filename":"image5.png","url":"https://assets-eu.researchsquare.com/files/rs-6504997/v1/f13f1ceefede69b7212df62f.png"},{"id":81961518,"identity":"6d2ccf32-b530-4748-a4a0-7acfe6c7cb9a","added_by":"auto","created_at":"2025-05-05 10:57:37","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":8264227,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6504997/v1/a0baa873-ca09-4dfc-a26f-2607fb7179dc.pdf"},{"id":81959674,"identity":"51a76750-aa75-4620-99a1-557a0d842720","added_by":"auto","created_at":"2025-05-05 10:33:31","extension":"docx","order_by":0,"title":"","display":"","copyAsset":false,"role":"supplement","size":142030,"visible":true,"origin":"","legend":"","description":"","filename":"SupplementaryInformation.docx","url":"https://assets-eu.researchsquare.com/files/rs-6504997/v1/d8cb9a451b806d6ae7d69861.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Synergistic Effect of Iron Doping and Oxide Hybridization Enables Enhanced Low-Temperature NH₃-SCR Performance of Manganese Oxide Catalyst","fulltext":[{"header":"1 Introduction","content":"\u003cp\u003eWith the advancement of industrialization, activities such as fuel combustion and chemical production have led to significant nitrogen oxide (NO\u003csub\u003ex\u003c/sub\u003e) emissions, which are major contributors to severe air pollution [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. Consequently, controlling NO\u003csub\u003ex\u003c/sub\u003e emissions has become a critical challenge for achieving sustainable development. Among the various NO\u003csub\u003ex\u003c/sub\u003e treatment methods, ammonia-selective catalytic reduction (NH₃-SCR) stands out for its high efficiency in reducing nitrogen oxide emissions [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. This process relies on a catalyst, with NH\u003csub\u003e3\u003c/sub\u003e acting as a reductant, to catalyze the reduction of NO\u003csub\u003ex\u003c/sub\u003e to nitrogen (N₂) and water (H₂O). V₂O₅-WO₃/MoO₃-TiO₂ based catalysts are widely employed in commercial SCR processes [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. However, their optimal NH₃-SCR activity is limited to a temperature range of 300\u0026ndash;400\u0026deg;C, which restricts their application in flue gas treatment at lower temperatures [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. Therefore, there is a critical need for developing highly active catalysts effective at lower temperatures (\u0026lt;\u0026thinsp;300\u0026deg;C), enabling more widespread and efficient NO\u003csub\u003ex\u003c/sub\u003e control across a broader range of industrial applications.\u003c/p\u003e \u003cp\u003eManganese oxide (MnO\u003csub\u003ex\u003c/sub\u003e), due to its variable valence states and exceptional redox properties, demonstrates excellent low-temperature (LT) denitrification (de-NO\u003csub\u003ex\u003c/sub\u003e) activity, making it a key focus in catalyst development. However, the robust redox capabilities of MnO\u003csub\u003ex\u003c/sub\u003e result in a narrow operating temperature window and low N₂ selectivity, limiting its further application in LT NH₃-SCR process [\u003cspan additionalcitationids=\"CR8\" citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eStudies have demonstrated that doping transition metal elements into MnO\u003csub\u003ex\u003c/sub\u003e can significantly improve the catalytic activity of MnO\u003csub\u003ex\u003c/sub\u003e [\u003cspan additionalcitationids=\"CR11 CR12\" citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. Especially, iron (Fe) is considered an excellent doping element due to its ease of availability and flexible valence states. For example, Zhang et al. demonstrated that doping lattice iron into Fe-Mn/TiO\u003csub\u003e2\u003c/sub\u003e catalysts increased the ratio of high-valent manganese to surface-chemically adsorbed oxygen, which lowered the catalyst's redox temperature and enhanced its LT de-NO\u003csub\u003ex\u003c/sub\u003e performance [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. Meanwhile, the presence of Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e in Fe-modified MnO\u003csub\u003ex\u003c/sub\u003e/TiO₂ catalysts has been shown to suppress N₂O formation and enhance N₂ selectivity by regulating the aggregation and dispersion of Mn species on the TiO₂ support [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, prepared by calcination at 300\u0026deg;C, exhibits over 95% NO\u003csub\u003ex\u003c/sub\u003e conversion and N\u003csub\u003e2\u003c/sub\u003e selectivity in the temperature range of 140\u0026deg;C to 280\u0026deg;C [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. Moreover, researches highlight the impact of lattice doping and the coexistence of different metal oxides on catalyst performance. For instance, Cu₁Co₂Fe₁O\u003csub\u003ex\u003c/sub\u003e exhibits improved toluene oxidation performance compared to Co₃Fe₁O\u003csub\u003ex\u003c/sub\u003e and Cu₃Fe₁O\u003csub\u003ex\u003c/sub\u003e, primarily due to its rich multiphase interface (CuO/Fe₃O₄/Co₃O₄) and metal doping effects, which facilitate the creation of oxygen vacancies [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. Likewise, Li et al. showed that the doping of indium as InMnO₃, In₂MnO₄, and In₂O₃ into MnO\u003csub\u003ex\u003c/sub\u003e catalysts promotes interactions with manganese, leading to the formation of Mn⁴⁺ and surface oxygen. Moderate indium modification significantly improves the LT catalytic activity of manganese oxide catalysts, resulting in nearly 100% de-NO\u003csub\u003ex\u003c/sub\u003e efficiency over the temperature range of 75\u0026ndash;200\u0026deg;C [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. Inspiringly, designing a strategy that incorporates Fe lattice doping and iron oxide hybridization might optimally enhance the NH₃-SCR performance of MnO\u003csub\u003ex\u003c/sub\u003e. Furthermore, exploring the synergistic effects of these modifications will offer valuable insights into expanding the design strategies for NH₃-SCR catalysts.\u003c/p\u003e \u003cp\u003eIn this study, we developed a simple method of impregnation doping followed by high-temperature calcination to prepare Fe-Mn metal oxide catalyst with both Fe lattice doping and Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e composition. Fe-Mn (X) significantly enhanced LT de-NO\u003csub\u003ex\u003c/sub\u003e activity and improved N₂ selectivity. The synergistic effects of Fe incorporation were systematically evaluated. Furthermore, the reaction mechanism was explored through in situ DRIFTS analysis.\u003c/p\u003e"},{"header":"2 Experiment section","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Preparation of catalysts\u003c/h2\u003e \u003cp\u003eMnO₂ was synthesized using a chemical precipitation method. Initially, 21 mmol of MnSO₄\u0026middot;H₂O (AR, Aladdin) and 42 mmol of NaHCO₃ (AR, Aladdin) were separately dissolved in 200 mL of deionized (DI) water, forming Solution Ⅰ and Solution Ⅱ. Then, Solution Ⅱ was slowly added to Solution Ⅰ, and the mixture was stirred for 10 minutes. After aging for 6 hours, the resulting white precipitate was centrifuged and washed with deionized water and anhydrous ethanol three times. The precipitate was dried in an oven at 80\u0026deg;C for 12 hours and then was calcined in air at 400\u0026deg;C for 5 hours to produce MnO₂.\u003c/p\u003e \u003cp\u003eFe-Mn oxide catalysts were synthesized via impregnation, followed by high-temperature calcination. Initially, 0.8 g of the as-prepared MnO₂ was sonicated in 30 mL of DI water and stirred for 30 minutes. Subsequently, varying amounts of Fe(NO₃)₃\u0026middot;9H₂O (AR, Aladdin) were added, and the mixture was stirred for 2 hours. The mixture was then dried at 80\u0026deg;C overnight and calcined in air at 500\u0026deg;C for 5 hours. The catalysts obtained were designated as Fe-Mn (X), where X denotes the Fe:Mn atomic ratio based on the precursor amounts of Fe(NO₃)₃\u0026middot;9H₂O and MnO₂, with values of 0.05, 0.1, 0.15, and 0.2. For comparison, Fe(NO₃)₃\u0026middot;9H₂O and MnO₂ were individually calcined at 500\u0026deg;C for 5 hours, yielding Fe-500 and Mn-500, respectively. These two were then physically mixed at an Fe:Mn atomic ratio of 0.15:1 to obtain the Fe\u0026ndash;Mn-MG sample.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Characterization\u003c/h2\u003e \u003cp\u003eThe morphology of catalysts was obtained using a Zeiss Sigma 300 field-emission scanning electron microscope. A Rigaku Ultima III X-ray diffractometer was used to obtain crystal structure of catalysts, within a scan range of 5\u0026ndash;80\u0026deg;. Nitrogen adsorption-desorption isotherms were acquired via a JW-BK132F volume adsorption analyzer. The surface chemical information was detected through X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific ESCALAB QXi), and all spectra were calibrated using the C1s peak (284.8 eV) as a reference. X-ray absorption fine structure (XAFS) spectroscopy was carried out using the Deep-Inspectra-X1(Beijing SciStar Technology Co., Ltd.) by transmission mode at 20 kV and 20 mA, and the Si (553) spherically bent crystal analyzer with a radius of curvature of 500 mm was used for Cu. The H\u003csub\u003e2\u003c/sub\u003e-TPR and NH\u003csub\u003e3\u003c/sub\u003e-TPD were measured on AMI-300 instrument, connected to a thermal conductivity detector. Before measurement, the samples were pretreated with a helium flow at 300℃ for 1 hour, followed by cooling to 50℃. For the H₂-TPR tests, the samples were subjected to a 10% H₂/Ar flow (30 mL\u0026middot;min⁻\u0026sup1;), heated from 50\u0026deg;C to 700\u0026deg;C (10\u0026deg;C\u0026middot;min⁻\u0026sup1;), and reduced by hydrogen. For the NH₃-TPD experiment, the sample was saturated with a 10% NH₃/He flow (30 mL\u0026middot;min⁻\u0026sup1;) and heated at a rate of 10\u0026deg;C\u0026middot;min⁻\u0026sup1; to 650\u0026deg;C in a helium flow, followed by NH₃ desorption. In situ DRIFTS were acquired on a Nicolet NEXUS 870 spectroscopy equipped with a liquid-nitrogen cooled mercury cadmium telluride detector. All samples were initially preheated in an Ar atmosphere at 200\u0026deg;C for 30 minutes, followed by cooling to 150\u0026deg;C to collect the background spectrum. Next, a gas mixture containing 500 ppm NH₃/Ar or 500 ppm NO\u0026thinsp;+\u0026thinsp;3% O₂ was passed over the samples until the sample surface reached saturation. The total gas flow was 100 mL\u0026middot;min⁻\u0026sup1;. Subsequently, the samples were purged with an Ar flow for 30 minutes, followed by treatment with either 500 ppm NO\u0026thinsp;+\u0026thinsp;3% O₂ or 500 ppm NH₃/Ar. Data were accumulated through 32 scans to obtain a spectral resolution of 4 cm⁻\u0026sup1;.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3 NH\u003csub\u003e3\u003c/sub\u003e-SCR activity test\u003c/h2\u003e \u003cp\u003eA quartz tube fixed-bed reactor containing 0.3 g of catalyst (40\u0026ndash;60 mesh) was employed to simulate flue gas conditions and evaluate the catalytic activity. The reaction gas included 500 ppm NO, 500 ppm NH\u003csub\u003e3\u003c/sub\u003e and 3 vol% O\u003csub\u003e2\u003c/sub\u003e, and Ar serves as carrier gas. The Gas Hourly Space Velocity (GHSV) for the reaction was 60,000 h⁻\u0026sup1;. The catalytic performance for NH₃-SCR denitrification was analyzed based on the following equations:\u003c/p\u003e \u003cp\u003eNO\u003csub\u003ex\u003c/sub\u003e conversion (%) = \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\frac{\\left({\\left[{\\text{N}\\text{O}}_{x}\\right]}_{\\text{i}\\text{n}}-{\\left[{\\text{N}\\text{O}}_{x}\\right]}_{\\text{o}\\text{u}\\text{t}}\\right)}{{\\left[{\\text{N}\\text{O}}_{x}\\right]}_{\\text{i}\\text{n}}}\\times\\:100\\%\\)\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e \u003cp\u003eN\u003csub\u003e2\u003c/sub\u003e selectivity (%) =\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\:\\left(1-\\frac{2{\\left[{\\text{N}}_{2}\\text{O}\\right]}_{\\text{o}\\text{u}\\text{t}}+{\\left[\\text{N}{\\text{O}}_{2}\\right]}_{\\text{o}\\text{u}\\text{t}}}{{\\left[\\text{N}\\text{O}\\right]}_{\\text{i}\\text{n}}+{\\left[\\text{N}{\\text{H}}_{3}\\right]}_{\\text{i}\\text{n}}-{\\left[\\text{N}\\text{O}\\right]}_{\\text{o}\\text{u}\\text{t}}-{\\left[\\text{N}{\\text{H}}_{3}\\right]}_{\\text{o}\\text{u}\\text{t}}}\\right)\\times\\:100\\%\\)\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e \u003cp\u003ewhere [NO]\u003csub\u003ein\u003c/sub\u003e and [NH\u003csub\u003e3\u003c/sub\u003e]\u003csub\u003ein\u003c/sub\u003e correspond to the inlet concentrations of NO and NH\u003csub\u003e3\u003c/sub\u003e in the fixed-bed reactor, while [NO]\u003csub\u003eout\u003c/sub\u003e, [NH\u003csub\u003e3\u003c/sub\u003e]\u003csub\u003eout\u003c/sub\u003e, [NO\u003csub\u003e2\u003c/sub\u003e]\u003csub\u003eout\u003c/sub\u003e, and [N\u003csub\u003e2\u003c/sub\u003eO]\u003csub\u003eout\u003c/sub\u003e denote their respective outlet concentrations.\u003c/p\u003e \u003c/div\u003e"},{"header":"3 Results and discussion","content":"\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e3.1 Morphological and structural properties\u003c/h2\u003e \u003cp\u003eThe crystal structure of various samples was analyzed by XRD patterns. As observed in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea, three distinct diffraction peaks at 37.12\u0026deg;, 42.40\u0026deg; and 56.03\u0026deg; in MnO\u003csub\u003e2\u003c/sub\u003e, which match well with the (100), (101) and (102) planes of ε-MnO\u003csub\u003e2\u003c/sub\u003e (PDF#30\u0026ndash;0802). After calcined at 500\u0026deg;C, MnO₂ and Fe(NO₃)₃\u0026middot;9H₂O are transforms to Mn₂O₃ (PDF#41-1442) and Fe₂O₃ (PDF#33\u0026ndash;0664), respectively. For Fe-Mn (X), diffraction peaks at 23.12\u0026deg;, 32.93\u0026deg;, 38.20\u0026deg;, 45.14\u0026deg;, 49.31\u0026deg;, 55.14\u0026deg;, and 65.74\u0026deg; correspond to the (211), (222), (400), (332), (431), (440), and (622) crystal planes of Mn₂O₃. Moreover, with increasing Fe addition, the diffraction peaks shift slightly to higher angles, and their intensity gradually decreases (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb). This is attributed to the partial substitution of Mn\u0026sup3;⁺ by Fe\u0026sup3;⁺, with Fe\u0026sup3;⁺ incorporating into the Mn₂O₃ lattice [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]. When the Fe:Mn atomic ratio exceeds 0.05, characteristic peaks of Fe₂O₃ gradually appear in Fe-Mn (X), indicating that excess Fe exists in the form of Fe₂O₃ (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec). This suggests that Fe modifies Mn₂O₃ through lattice doping as well as by forming a composite with iron oxides, where lattice Fe and iron oxides coexist in Fe-Mn (X) (X\u0026thinsp;=\u0026thinsp;0.1, 0.15, 0.2). The shift and intensity variation of the Fe₂O₃ diffraction peaks may be related to its interaction with manganese oxides [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eSEM was conducted to study the morphology of various catalysts. As depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ed and e, MnO\u003csub\u003e2\u003c/sub\u003e exhibits a spherical morphology of approximately 2 \u0026micro;m, while Mn-500 particles are slightly smaller, measuring around 1.2 \u0026micro;m. After the introduction of Fe (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ef\u0026thinsp;~\u0026thinsp;i), the size of Fe-Mn (X) is approximately 1.5 \u0026micro;m, slightly larger than that of Mn-500. This increase in size is related to the reaction between Fe and MnO₂ during high-temperature calcination, where excess Fe is transformed into Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, contributing to particle growth. The surface of Fe-Mn (0.05) exhibits no significant change, maintaining a particulate structure. As the level of iron incorporation increases, the surface of Fe-Mn (X, X\u0026thinsp;=\u0026thinsp;0.1, 0.15) displays a needle-like structure formed by the stacking of nanosheets. With a further increase in Fe content, the nanosheet structure on the surface of Fe-Mn (0.2) gradually disappears, and the surface becomes rough, possibly due to the excessive deposition of Fe₂O₃ aggregation. EDS elemental mapping reveals a homogeneous distribution of O, Mn and Fe elements (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ej\u0026thinsp;~\u0026thinsp;l) in Fe-Mn (0.15), further demonstrating that Fe is involved in the formation process of the catalysts. This uniform distribution facilitates synergistic interactions between Mn and Fe, promoting efficient electron transfer and enhancing the NH₃-SCR catalytic activity [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe pore texture of the catalysts was determined by nitrogen adsorption-desorption analysis. Fig. \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003ea illustrates that the isothermal adsorption-desorption curves of MnO₂, Mn-500, and Fe-Mn (X) are classified as type IV isotherms with H1-type hysteresis loops, indicating mesoporous structures [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. The pore size distribution in Fig. \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003eb, reveals that after Fe loading, the catalyst's pore size increases, primarily within the range of 7\u0026ndash;27 nm, which facilitates the adsorption and transfer of reactive gases [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eAccording to Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e, MnO₂ has a specific surface area of 104.0 m\u0026sup2;\u0026middot;g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and an average pore diameter of 5.61 nm, while Mn-500 exhibits a lower specific surface area (26.2 m\u0026sup2;\u0026middot;g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) and a larger average pore diameter (23.31 nm). This change may result from the formation of a new phase, where smaller pores become blocked, and the proportion of larger pores increases. Fe-Mn (X) catalysts show higher specific surface areas and smaller average pore diameters compared to Mn-500. Nevertheless, excess Fe₂O₃ agglomeration blocks pores, reducing the specific surface area of Fe-Mn (0.2).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e3.2 Surface chemistry properties\u003c/h2\u003e \u003cp\u003eThe surface chemistry of various Fe-Mn (X) catalysts was investigated using an XPS analysis. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea shows the Mn 2p reveal two distinct peaks in the range of 638\u0026ndash;658 eV, corresponding to Mn 2p\u003csub\u003e3/2\u003c/sub\u003e and Mn 2p\u003csub\u003e1/2\u003c/sub\u003e. The Mn 2p\u003csub\u003e3/2\u003c/sub\u003e peak can be further fitted into three peaks at approximately 640.2 eV, 641.6 eV, and 643.2 eV, suggesting that Mn primarily exists as a mixture of Mn\u003csup\u003e2+\u003c/sup\u003e, Mn\u003csup\u003e3+\u003c/sup\u003e, and Mn\u003csup\u003e4+\u003c/sup\u003e [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]. These coexisting Mn species enhance redox activity and promote the creation of additional oxygen vacancies [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]. As shown in Table S2, the Mn⁴⁺/(Mn⁴⁺ + Mn\u0026sup3;⁺ + Mn\u0026sup2;⁺) ratio increases with Fe introducing but slightly decreases at higher Fe levels. This indicates that the introduction of Fe enhances the proportion of higher oxidation states of Mn in Fe-Mn oxides. It was reported that high valence state Mn\u003csup\u003e4+\u003c/sup\u003e had the potential to enhance the low-temperature de-NO\u003csub\u003ex\u003c/sub\u003e activity by facilitating the conversion of NO to NO\u003csub\u003e2\u003c/sub\u003e and finally reduced to N\u003csub\u003e2\u003c/sub\u003e through NH₃ [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb shows that Fe 2p consists of Fe 2p₃/₂ (~\u0026thinsp;711 eV) and Fe 2p₁/₂ (~\u0026thinsp;725 eV). The Fe 2p₃/₂ peak can be further fitted into Fe\u0026sup2;⁺ (~\u0026thinsp;710.6 eV) and Fe\u0026sup3;⁺ (~\u0026thinsp;713 eV) [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]. Previous study demonstrated that Fe\u0026sup3;⁺ exhibits higher catalytic activity than Fe\u0026sup2;⁺ by suppressing NH₃ oxidation and enhancing the \u0026ldquo;fast SCR\u0026rdquo; mechanism, improving NO\u003csub\u003ex\u003c/sub\u003e conversion efficiency [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]. As indicated in Table S2, the introduction of Fe gradually increases the Fe\u0026sup3;⁺/(Fe\u0026sup3;⁺ + Fe\u0026sup2;⁺) ratio, which could promote redox reactions with Mnⁿ⁺, such as Mn\u0026sup3;⁺ + Fe\u0026sup3;⁺ \u0026harr; Mn⁴⁺ + Fe\u0026sup2;⁺ and Mn\u0026sup3;⁺ + Fe\u0026sup2;⁺ \u0026harr; Mn\u0026sup2;⁺ + Fe\u0026sup3;⁺, leading to the coexistence of Fe and Mn in various valence states on the catalyst surface [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eIn addition, Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec illustrates that the O1s peaks of all catalysts can be divided into lattice oxygen (O\u003csub\u003eα\u003c/sub\u003e) at ~\u0026thinsp;530 eV, surface-chemically adsorbed oxygen (O\u003csub\u003eβ\u003c/sub\u003e) at ~\u0026thinsp;531 eV, and -OH (O\u003csub\u003eγ\u003c/sub\u003e) at ~\u0026thinsp;533 eV [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]. Fe-Mn (0.6) achieves the highest proportion of O\u003csub\u003eβ\u003c/sub\u003e in comparison with other catalysts (Table S2). Compared to O\u003csub\u003eα\u003c/sub\u003e, O\u003csub\u003eβ\u003c/sub\u003e is proposed to have higher chemical reactivity and electron mobility, influencing oxidation reactions. It may support NO oxidation to NO₂, which could play a role in facilitating the \u0026ldquo;fast SCR\u0026rdquo; reaction. Therefore, a higher O\u003csub\u003eβ\u003c/sub\u003e/(O\u003csub\u003eα\u003c/sub\u003e + O\u003csub\u003eβ\u003c/sub\u003e+ O\u003csub\u003eγ\u003c/sub\u003e) ratio is suggested to correlate with enhanced NH₃-SCR catalytic performance [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo further investigate the chemical bonding information in Fe-Mn (0.15), XAFS spectroscopy was analyzed. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea, the Fe K-edge XANES signal of Fe-Mn (0.15) is located between that of Fe foil (7122.8 eV) and the Fe₂O₃ standard sample (7125.3 eV). The spectral peaks closely resemble those of the Fe₂O₃ standard sample, indicating that Fe in Fe-Mn (0.15) predominantly exists in the form of Fe₂O₃ [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]. Comparison with the standard Mn₂O₃ spectrum indicates that Fe-Mn (0.15) primarily exhibits an Mn₂O₃ structure, which is consistent with the XRD results. Furthermore, Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb shows that the XANES spectrum of Fe-Mn (0.15) shifts toward the higher energy region, indicating that Fe doping increases the average oxidation state of Mn [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e]. This result is consistent with the XPS data. In the Fe K-edge extended X-ray absorption fine structure (EXAFS) spectra (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec), Fe foil exhibits a characteristic signal of the Fe\u0026ndash;Fe bond at 2.2 \u0026Aring;, while the Fe₂O₃ standard sample presents distinct oscillation peaks at approximately 1.5 \u0026Aring; and 2.6 \u0026Aring;, corresponding to Fe\u0026ndash;O and Fe\u0026ndash;Fe bonds, respectively. The Fe-Mn (0.15) sample exhibits oscillation peaks at 1.8 \u0026Aring; and 2.6 \u0026Aring;, similar to those of the Fe₂O₃ standard sample, corresponding to Fe\u0026ndash;O and Fe\u0026ndash;Fe (Mn) bonds [\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e]. In the Mn K-edge EXAFS spectra (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed), the Mn-500 and Mn₂O₃ standard sample exhibit peaks at 1.4 \u0026Aring; and 2.7 \u0026Aring;, which are assigned to Mn\u0026ndash;O and Mn\u0026ndash;Mn bonds, respectively. The Fe-Mn (0.15) sample displays similar oscillation peaks, with peaks at 1.4 \u0026Aring; and 2.8 \u0026Aring; corresponding to Mn\u0026ndash;O and Mn\u0026ndash;Mn (Fe) bonds, respectively [\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]. These results confirm that Fe is successfully incorporated into the Mn₂O₃ lattice in Fe-Mn (0.15), while excess Fe primarily exists in the form of iron oxides. In addition, wavelet transform analyses of the Fe and Mn K-edge further verify these findings (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ee\u0026thinsp;~\u0026thinsp;j). Compared with Fe₂O₃ and Mn₂O₃, Fe-Mn (0.15) spectra exhibit higher k-values of Fe\u0026ndash;Fe (Mn) and Mn\u0026ndash;Mn (Fe) signals, which could be attributed to Fe doping and Mn\u0026ndash;Fe signal overlap [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e, \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea shows H\u003csub\u003e2\u003c/sub\u003e-TPR profiles of the catalysts. Mn-500 exhibits two sequential peaks, reflecting its stepwise reduction to Mn₃O₄ at lower temperatures and subsequently to MnO at higher temperatures [\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e]. In the Fe- Mn (X) catalyst, peaks around 330\u0026deg;C and 450\u0026deg;C correspond to the reduction processes of Mn₂O₃ \u0026rarr; Mn₃O₄ and Fe₂O₃ \u0026rarr; Fe₃O₄, and the composite reduction of Mn₃O₄ \u0026rarr; MnO and Fe₃O₄ \u0026rarr; FeO, respectively [\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e]. Compared to Mn-500, the reduction peaks of Fe-Mn (0.05) shift to higher temperatures, suggesting that Fe doping slightly weakens the catalyst's reduction capacity. This modulation is believed to suppress the excessive oxidation of NH₃, which may contribute to expanding the operating temperature window and improving the catalytic performance of the NH₃-SCR reaction [\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e]. When Fe is present in the form of Fe₂O₃, the Fe-Mn (0.15) catalyst exhibits a new reduction peak at 284\u0026deg;C, corresponding to the reduction of surface-adsorbed oxygen (generation of oxygen vacancies) [\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e]. This result indicates that the synergy between Mn₂O₃ and Fe₂O₃ enhances oxygen adsorption on the catalyst surface, which is consistent with the XPS data and benefits the NH₃-SCR performance.\u003c/p\u003e \u003cp\u003eThe acidity properties of the catalysts were analyzed using NH₃-TPD, with the results shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb. The NH₃-TPD profiles of all catalysts exhibit three NH₃ desorption peak regions located at I (50\u0026ndash;200\u0026deg;C), II (200\u0026ndash;400\u0026deg;C), and III (400\u0026ndash;500\u0026deg;C). Region I corresponds to NH₃ desorption from weak acidic sites (Lewis acid sites), primarily including physically adsorbed NH₃. Region II represents moderate Lewis (L) acid sites linked to Fe species. Region III corresponds to strong acidic sites, mainly involving NH₄⁺ (Bronsted acid sites) desorption and newly generated L acid sites from Fe species [\u003cspan additionalcitationids=\"CR41\" citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e]. As shown in Table S3, compared to Mn-500, the Fe-Mn (0.05) catalyst exhibits a larger integrated area of Region I, indicating that lattice Fe doping enhances the strength of weak acid sites. As the Fe₂O₃ content increases, the number of weak acid sites on the Fe-Mn (X, X\u0026thinsp;=\u0026thinsp;0.1, 0.15, 0.2) catalysts gradually increases, while the number of strong acid sites initially decreases and then increases. This indicates that an appropriate amount of Fe₂O₃ promotes the formation of strong acid sites on the catalyst surface. Among these catalysts, Fe-Mn (0.15) exhibits the highest number of NH₃ desorption peaks, implying the highest acidity and acid strength, thereby enhancing their ability to adsorb and activate NH₃ [\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e]. A higher concentration of weak acid sites is more favorable for low-temperature NH₃-SCR reactions [\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e3.3 NH\u003csub\u003e3\u003c/sub\u003e-SCR performance\u003c/h2\u003e \u003cp\u003eThe catalytic performance of different catalysts is illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec and d. As observed in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec, Fe-500 demonstrates relatively low NO\u003csub\u003ex\u003c/sub\u003e conversion efficiency due to its inherent medium- and high-temperature de-NO\u003csub\u003ex\u003c/sub\u003e characteristics [\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e]. The de-NO\u003csub\u003ex\u003c/sub\u003e activity of MnO\u003csub\u003e2\u003c/sub\u003e is higher than that of Mn-500, attributed to the higher oxidation state of Mn\u003csup\u003e4+\u003c/sup\u003e in MnO\u003csub\u003e2\u003c/sub\u003e, which facilitates NO\u003csub\u003ex\u003c/sub\u003e conversion [\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e]. Although Fe-500 and Mn-500 mechanical mixtures (Fe-Mn-MG) can achieve up to 80% NO\u003csub\u003ex\u003c/sub\u003e conversion, they are constrained by a narrow operating temperature range. By contrast, the introduction of Fe effectively promotes the catalytic activity of Fe-Mn (X), with Fe-Mn (0.15) demonstrating the best overall performance. Notably, within the 175\u0026ndash;300\u0026deg;C range, it achieves a NO\u003csub\u003ex\u003c/sub\u003e conversion rate of 96.8% with N\u003csub\u003e2\u003c/sub\u003e selectivity exceeding 99% (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed). This exceptional catalytic activity results from the synergistic effects of an optimal amount of Fe lattice doping and Fe₂O₃ hybridization, which increase acidic sites, enhance NH₃ adsorption, and broaden the operational temperature window. Additionally, Fe doping contributes to a moderate reduction in reducibility, which helps suppress excessive NH₃ oxidation and improves N₂ selectivity [\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e, \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e]. Excessive Fe content, as in Fe-Mn (0.2), slightly reduces performance. This is due to excess Fe₂O₃ blocking the pore structure, decreasing the specific surface area, and limiting reactant gas adsorption [\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e]. Compared with catalysts in the literature (Table S4), Fe-Mn (0.15) demonstrates competitive NO\u003csub\u003ex\u003c/sub\u003e conversion efficiency across a wide temperature range, even under a high GHSV of 60,000 h⁻\u0026sup1;.\u003c/p\u003e \u003c/div\u003e"},{"header":"4 NH-SCR mechanism","content":"\u003cp\u003eThe adsorption behavior and reaction mechanism of NH₃ and NO on Mn-500 and Fe-Mn (0.15) catalysts were studied using in situ DRIFTS.\u003c/p\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003e4.1 NH\u003csub\u003e3\u003c/sub\u003e adsorption\u003c/h2\u003e \u003cp\u003eIn situ DRIFTS spectra of the adsorption of NH\u003csub\u003e3\u003c/sub\u003e at different times over Mn-500 and Fe-Mn (0.15) are presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea and b. According to Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea, Mn-500 exhibits NH₃ adsorption peaks at 1170 cm⁻\u0026sup1;, 1190 cm⁻\u0026sup1;, 1228 cm⁻\u0026sup1;, 1606 cm⁻\u0026sup1;, and 1625 cm⁻\u0026sup1;, corresponding to L-NH\u003csub\u003e3\u003c/sub\u003e adsorbed on L acid sites [\u003cspan additionalcitationids=\"CR51\" citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e] The peak at 1411 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e was assigned to B-NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e adsorbed on Bronsted (B) acid sites [\u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e53\u003c/span\u003e]. As observed in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb, for Fe-Mn (0.15), NH₃ adsorption peaks at 1163 cm⁻\u0026sup1;, 1174 cm⁻\u0026sup1;, 1187 cm⁻\u0026sup1;, 1200 cm⁻\u0026sup1;, 1224 cm⁻\u0026sup1;, and 1593 cm⁻\u0026sup1; are ascribed to L-NH\u003csub\u003e3\u003c/sub\u003e [\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e]. Additional peaks at 1309 cm⁻\u0026sup1;, 1419 cm⁻\u0026sup1; and 1435 cm⁻\u0026sup1; are associated with correspond to the B-NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e species adsorbed on the catalyst surface[\u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e54\u003c/span\u003e]. These observations reveal that both Mn-500 and Fe-Mn (0.15) adsorb more L-NH₃ species than B-NH₄⁺ species. Notably, after Fe lattice doping and Fe₂O₃ combination, the number of spectral bands corresponding to B-NH₄⁺ increases, suggesting that Fe incorporation enhanced the catalyst's acidity, in agreement with the NH\u003csub\u003e3\u003c/sub\u003e-TPD analysis. After 30 min of Ar purge, the in situ DRIFTS spectra of Mn-500 and Fe-Mn (0.15) show no significant changes, confirming that L-NH\u003csub\u003e3\u003c/sub\u003e and B- NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e species stably adsorbed on the catalyst surface[\u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e55\u003c/span\u003e].\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003e4.2 NO\u0026thinsp;+\u0026thinsp;O\u003csub\u003e2\u003c/sub\u003e adsorption\u003c/h2\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec and d display the DRIFT spectra of surface-adsorbed species formed by NO\u0026thinsp;+\u0026thinsp;O₂ co-adsorption on Mn-500 and Fe-Mn (0.15) at various time intervals. In terms of Mn-500, peaks at 1215 cm⁻\u0026sup1; and 1261 cm⁻\u0026sup1; are assigned to bridged nitrite and monodentate nitrate, while peaks at 1311 cm⁻\u0026sup1;, 1548 cm⁻\u0026sup1;, and 1558 cm⁻\u0026sup1; are attributed to bidentate nitrate, and the peak at 1435 cm⁻\u0026sup1; represents ionic nitrate[\u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e56\u003c/span\u003e]. On the Fe-Mn (0.15) surface, peaks at 1201 cm⁻\u0026sup1; and 1610 cm⁻\u0026sup1; are assigned to bridged nitrite, peaks at 1307 cm⁻\u0026sup1; and 1315 cm⁻\u0026sup1; originate from bidentate nitrate, and the absorbance peak at 1448 cm⁻\u0026sup1; correspond to ionic nitrate [\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e]. Both Mn-500 and Fe-Mn (0.15) primarily convert adsorbed NO into various nitrate species, with bidentate nitrates enhancing low-temperature de-NO\u003csub\u003ex\u003c/sub\u003e performance [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. Compared to Mn-500, Fe-Mn (0.15) exhibits stronger NO-related peaks, indicating a superior ability to adsorb and activate NO, thus enhancing the de-NO\u003csub\u003ex\u003c/sub\u003e activity [\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e, \u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e57\u003c/span\u003e]. Similarly, the 30-minute Ar purge had a slight effect on the NO-related peaks, indicating the stable retention of NO-related active species on the catalyst surfaces. The infrared absorption peaks of the catalyst are relatively weak, and several negative peaks are observed. This may be attributed to the black color of catalyst, which can strongly absorb or scatter infrared light, thereby reducing signal intensity, affecting spectral detection and causing inconsistent scaling in infrared spectra. This is similar to previous reports in the literature[\u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e56\u003c/span\u003e].\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section2\"\u003e \u003ch2\u003e4.3 Reaction between NO\u0026thinsp;+\u0026thinsp;O\u003csub\u003e2\u003c/sub\u003e and pre-adsorbed NH\u003csub\u003e3\u003c/sub\u003e\u003c/h2\u003e \u003cp\u003eThe DRIFT spectra of NH\u003csub\u003e3\u003c/sub\u003e pre-adsorbed Mn-500 and Fe-Mn (0.15) after treatment with NO\u0026thinsp;+\u0026thinsp;O₂ at 150\u0026deg;C are depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ee and f. After introducing NO\u0026thinsp;+\u0026thinsp;O₂, the Mn-500 catalyst shows new peaks at 1307 cm⁻\u0026sup1;, 1319 cm⁻\u0026sup1;, and 1546 cm⁻\u0026sup1;, corresponding to bidentate nitrates. Simultaneously, the peak at 1626 cm⁻\u0026sup1;, associated with bridged nitrates, gradually increases. The L-NH₃ peaks at 1228 cm⁻\u0026sup1; and 1606 cm⁻\u0026sup1; weaken and eventually disappear over time, indicating their involvement in the reaction with NO [\u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e53\u003c/span\u003e]. For Fe-Mn (0.15), the L-NH₃ peaks at 1172 cm⁻\u0026sup1;, 1186 cm⁻\u0026sup1;, and 1209 cm⁻\u0026sup1; also fade, while new peaks at 1311 cm⁻\u0026sup1; and 1593 cm⁻\u0026sup1; for bidentate nitrates appear, along with additional peaks at 1438 cm⁻\u0026sup1; and 1580 cm⁻\u0026sup1; for ionic and bridged nitrates. These peaks increase over time, indicating the simultaneous formation of various nitrates, with L-NH₃ predominantly involved in the reaction. Therefore, both Mn-500 and Fe-Mn (0.6) follow the Eley-Rideal (E-R) mechanism [\u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e58\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec14\" class=\"Section2\"\u003e \u003ch2\u003e4.4 Reaction between NH\u003csub\u003e3\u003c/sub\u003e and pre-adsorbed NO\u0026thinsp;+\u0026thinsp;O\u003csub\u003e2\u003c/sub\u003e\u003c/h2\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eg and h show the DRIFT spectra of NO\u0026thinsp;+\u0026thinsp;O₂ pre-adsorbed Mn-500 and Fe-Mn (0.15) catalysts after different NH₃ treatment times. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eg, upon NH\u003csub\u003e3\u003c/sub\u003e purging, peaks corresponding to bridged nitrites (1215 cm⁻\u0026sup1;), monodentate nitrates (1261 cm⁻\u0026sup1;), and bidentate nitrates (1556 cm⁻\u0026sup1;) on the Mn-500 slightly decrease [\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e, \u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e59\u003c/span\u003e]. Simultaneously, a new peak corresponding to L-NH₃ emerges at 1213 cm⁻\u0026sup1;, and its intensity gradually increases with purging time. This indicates a reaction occurs between NO-related species and NH₃, though the effect is not pronounced. The peak at 1435 cm⁻\u0026sup1; gradually broadens during NH\u003csub\u003e3\u003c/sub\u003e purging, likely due to the overlap of B-NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e and ionic nitrate peaks. After pre-adsorption of NO\u0026thinsp;+\u0026thinsp;O\u003csub\u003e2\u003c/sub\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ef), purging Fe-Mn (0.15) with NH\u003csub\u003e3\u003c/sub\u003e leads to the appearance of new peaks at 1230 cm⁻\u0026sup1;, corresponding to L-NH₃. Meanwhile, the peaks at 1201 cm⁻\u0026sup1; (bridged nitrite) [\u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e60\u003c/span\u003e], 1307 cm⁻\u0026sup1; (bidentate nitrate), and 1315 cm⁻\u0026sup1; (ionic nitrate) gradually disappear, while the peaks at 1448 cm⁻\u0026sup1; and 1610 cm⁻\u0026sup1; slightly decrease, revealing a reaction with NH₃. These findings confirm that nitrate adsorption on Fe-Mn (0.15) is more stable, and the NH\u003csub\u003e3\u003c/sub\u003e-SCR reaction follows the Langmuir-Hinshelwood (L-H) mechanism. Compared to the reaction between NO\u0026thinsp;+\u0026thinsp;O₂ and pre-adsorbed NH₃, the reaction between NH₃ and pre-adsorbed NO\u0026thinsp;+\u0026thinsp;O₂ is less likely to occur [\u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e61\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eTherefore, based on the above, lattice doping of Fe and the combination with Fe₂O₃ could enhance the adsorption of NH₃ on L and B acid sites of Fe-Mn (0.15), promoting the activation of NO. As depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ei, the NH₃-SCR reaction over Fe-Mn (0.15) proceeds through both the E-R and L-H mechanisms, where the E-R mechanism plays the primary role. In the E-R mechanism, NH₃ is initially adsorbed onto both L and B acid sites of the catalyst surface, forming L-NH₃\u003csub\u003e(ad)\u003c/sub\u003e and B-NH₄\u003csup\u003e+\u003c/sup\u003e\u003csub\u003e(ad)\u003c/sub\u003e, respectively. These adsorbed species are then converted into -NH₂\u003csub\u003e(ad)\u003c/sub\u003e through reaction with active oxygen species (O\u003csub\u003e(ad)\u003c/sub\u003e), which are generated by the adsorption of O₂(g) (O₂(g) \u0026rarr; O\u003csub\u003e(ad)\u003c/sub\u003e). The resulting -NH₂\u003csub\u003e(ad)\u003c/sub\u003e subsequently reacts with NO, forming the intermediate NH₂NO\u003csub\u003e(ad)\u003c/sub\u003e, which decomposes into the final products, N₂ and H₂O. In the L-H pathway, both NH₃ and NO adsorb onto the catalyst. The adsorbed NO species react with L-NH₃(ad) and B-NH₄⁺(ad), ultimately forming N₂ and H₂O.\u003c/p\u003e \u003c/div\u003e"},{"header":"5 Conclusion","content":"\u003cp\u003eIn summary, we successfully synthesized Fe-Mn oxide catalysts with simultaneous Fe lattice doping and Fe₂O₃ composite through a simple impregnation doping technique followed by high-temperature calcination. The homogeneously of Mn and Fe in the Fe-Mn (0.15) spherical catalyst is beneficial to the synergistic interactions between Mn and Fe. The incorporation of lattice Fe moderately reduces the reducibility and enhances the weak acid site acidity of the catalyst. The incorporation of Fe₂O₃ significantly increases the amount of chemical adsorbed oxygen on the catalyst surface, while an appropriate amount of Fe₂O₃ also enhances the formation of strong acid sites. By regulating the amount of Fe introduced, the catalyst\u0026rsquo;s acidity and redox properties can be balanced, thereby improving NO\u003csub\u003ex\u003c/sub\u003e conversion and N\u003csub\u003e2\u003c/sub\u003e selectivity at lower temperatures. The Fe-Mn (0.15), with abundant surface Mn⁴⁺, chemisorbed oxygen, high reducibility and acidic sites, favors the \u0026ldquo;fast SCR\u0026rdquo; reaction, achieving a NO\u003csub\u003ex\u003c/sub\u003e conversion rate of 96.8% at 200\u0026deg;C. In situ DRIFTS analysis indicated that L-NH₃ adsorption and activation play a critical role in the NH₃-SCR reaction. At 150\u0026deg;C, the Fe-Mn (0.15) follows both the E-R and L-H mechanisms, with the E-R mechanism being predominant. This study underscores the significance of tuning Fe lattice doping and Fe₂O₃ composite in MnO\u003csub\u003ex\u003c/sub\u003e catalysts for optimizing L-T NH₃-SCR performance.\u003c/p\u003e"},{"header":"Abbreviations","content":"\u003cp\u003eMnO\u003csub\u003ex\u003c/sub\u003e Manganese oxides\u003c/p\u003e \u003cp\u003eNH₃-SCR Ammonia-selective catalytic reduction\u003c/p\u003e \u003cp\u003eDRIFTS Diffuse reflectance infrared spectroscopy\u003c/p\u003e \u003cp\u003eE-R Eley-Rideal\u003c/p\u003e \u003cp\u003eNO\u003csub\u003ex\u003c/sub\u003e Nitrogen oxide\u003c/p\u003e \u003cp\u003eLT Low-temperature\u003c/p\u003e \u003cp\u003ede-NO\u003csub\u003ex\u003c/sub\u003e Denitrification\u003c/p\u003e \u003cp\u003eDI Deionized\u003c/p\u003e \u003cp\u003eXRD X-ray difraction\u003c/p\u003e \u003cp\u003eSEM Scanning electron microscope\u003c/p\u003e \u003cp\u003eXPS X-ray photoelectron spectroscopy\u003c/p\u003e \u003cp\u003eNH\u003csub\u003e3\u003c/sub\u003e-TPD Temperature-programmed desorption of ammonia\u003c/p\u003e \u003cp\u003eH\u003csub\u003e2\u003c/sub\u003e-TPR Hydrogen temperature-programmed reduction\u003c/p\u003e \u003cp\u003eXAFS X-ray absorption fine structure spectroscopy\u003c/p\u003e \u003cp\u003eBET Brunauer\u0026ndash;Emmett\u0026ndash;Teller\u003c/p\u003e \u003cp\u003eGHSV Gas Hourly Space Velocity\u003c/p\u003e \u003cp\u003eL acid site Lewis acid site\u003c/p\u003e \u003cp\u003eB acid site Bronsted acid site\u003c/p\u003e \u003cp\u003eL-H Langmuir-Hinshelwood\u003c/p\u003e \u003cp\u003eXANES X-ray absorption near edge structure\u003c/p\u003e \u003cp\u003eEXAFS Extended X-ray absorption fine structure\u003c/p\u003e \u003cp\u003ead adsorbed\u003c/p\u003e "},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eEthics and Consent to Participate\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent for Publication\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting Interest\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing interests.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor Contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eXuewen Li: Conceptualization, Methodology, Formal analysis, Writing\u0026ndash;original draft. Yaru Li: Investigation, Formal analysis, Validation, Writing\u0026ndash;review \u0026amp; editing. Menglin Wang: Investigation, Methodology. Can Wang: Formal analysis, Writing\u0026ndash;review \u0026amp; editing. Wenxian Jing: Methodology, Validation. Lisheng Fang: Validation, Resources. Yonghua Hu: Project administration, Resources, Supervision. Yundong Liang: Project administration, Supervision. Xianbiao Wang: Funding acquisition, Resources, Writing\u0026ndash;review \u0026amp; editing.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis work was supported by the Natural Science Foundation of Anhui Province (2408085MB026), the Anhui Provincial Key Research and Development Plan (2022h11020025), the National Natural Science Foundation of China (21976003), the Anhui Provincial Key Natural Science Research Project of Education Department (2023AH050195) and the Excellent Scientific Research and Innovation Team in Colleges and Universities of Anhui Province (2022AH010017). We thank the Beijing SciStar Technology Co., Ltd. for XAFS measurements and analysis.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAvailability of data and materials\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that the data supporting the findings of this study are available within the paper and its Supplementary Information files. Should any raw data files be needed in another format they are available from the corresponding author upon reasonable request. Source data are provided with this paper.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgments\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis work was supported by the Natural Science Foundation of Anhui Province (2408085MB026), the Anhui Provincial Key Research and Development Plan (2022h11020025), the National Natural Science Foundation of China (21976003), the Anhui Provincial Key Natural Science Research Project of Education Department (2023AH050195) and the Excellent Scientific Research and Innovation Team in Colleges and Universities of Anhui Province (2022AH010017). We thank the Beijing SciStar Technology Co., Ltd. for XAFS measurements and analysis.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eLv Z, He G, Zhang W, Liu J, Lian Z, Yang Y, Yan Z, Xu G, Shan W, Yu Y, He H (2024) Interface sites on vanadia-based catalysts are highly active for NO\u003csub\u003ex\u003c/sub\u003e removal under realistic conditions. 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Fuel 376:132756\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"catalysis-letters","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"Learn more about [Catalysis Letters](https://link.springer.com/journal/10562)","snPcode":"10562","submissionUrl":"https://submission.springernature.com/new-submission/10562/3","title":"Catalysis Letters","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Low-temperature NH3-SCR, Manganese oxides, Iron lattice doping, Fe₂O₃, E-R mechanism","lastPublishedDoi":"10.21203/rs.3.rs-6504997/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6504997/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eManganese oxides (MnO\u003csub\u003ex\u003c/sub\u003e) catalysts are promising for low-temperature ammonia-selective catalytic reduction (NH₃-SCR), however, the limited N₂ selectivity and the narrow operational temperature range remain challenges. To address these issues, we developed a method involving impregnation doping and high-temperature calcination to hybridize Mn₂O₃ with iron lattice and oxide. This hybrid catalyst maintains a NO\u003csub\u003ex\u003c/sub\u003e conversion rate above 90% within the broad temperature window of 175\u0026ndash;300\u0026deg;C, while achieving N₂ selectivity above 99%. The as prepared Fe-Mn (0.15) exhibits spherical morphology with Fe and Mn uniformly distributed. It was investigated that the doping of lattice Fe contributes to a slight reduction in the catalyst's reducibility and a moderate increase the amounts of Lewis acid sites. Fe₂O₃, which produced through calcination, plays a crucial role in enhancing surface-adsorbed oxygen and Bronsted acid sites. These synergistic effects regulate both the acidic and redox properties of the catalyst, facilitating NH₃ adsorption and activation while controlling NH₃ overoxidation, thus broadening the operational temperature range and improving N₂ selectivity. Furthermore, in situ diffuse reflectance infrared spectroscopy (DRIFTS) characterization demonstrated that the NH₃-SCR reaction on the catalyst primarily follows an Eley-Rideal (E-R) mechanism. This work reveals the synergistic effects of Fe lattice doping and Fe₂O₃ composite on MnO\u003csub\u003ex\u003c/sub\u003e, offering new insights for developing advanced low-temperature catalysts.\u003c/p\u003e","manuscriptTitle":"Synergistic Effect of Iron Doping and Oxide Hybridization Enables Enhanced Low-Temperature NH₃-SCR Performance of Manganese Oxide Catalyst","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-05-05 10:25:26","doi":"10.21203/rs.3.rs-6504997/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2025-05-31T22:32:48+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-05-29T07:47:25+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"55987686811026014804253075923813282995","date":"2025-05-21T01:00:21+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-05-06T02:10:48+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"178023697403501195504039370420042138381","date":"2025-04-29T22:44:25+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-04-29T19:06:50+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-04-24T07:45:28+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-04-24T07:44:23+00:00","index":"","fulltext":""},{"type":"submitted","content":"Catalysis Letters","date":"2025-04-22T13:47:06+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"catalysis-letters","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"Learn more about [Catalysis Letters](https://link.springer.com/journal/10562)","snPcode":"10562","submissionUrl":"https://submission.springernature.com/new-submission/10562/3","title":"Catalysis Letters","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"efd3ae0f-29f4-4be7-8f7e-61e6e97d51d8","owner":[],"postedDate":"May 5th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2025-08-03T14:23:04+00:00","versionOfRecord":[],"versionCreatedAt":"2025-05-05 10:25:26","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-6504997","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6504997","identity":"rs-6504997","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}