Highly Active and Water-Resistant Mn-Loaded MgAlO_x Catalysts for NH₃-SCR at Low Temperature

Abstract

Advancing catalysts for low-temperature NH₃-SCR enhances their viability as a terminal flue gas denitration solution across diverse operating regimes. A high-performance, hydrothermally stable catalyst for low-temperature SCR was synthesized by depositing MnO_x onto MgAlO_x composite oxide supports. These supports, featuring varied Mg/Al ratios, originated from layered double hydroxide (LDH) precursors. The obtained catalyst with the Mg/Al ratio of 2 (Mn/Mg₂AlO_x) possesses relatively high concentrations of active oxygen species (O_α) and Mn⁴⁺ and exhibits remarkable catalytic performance. The Mn/Mg₂AlO_x catalyst exhibits a wide operating temperature range (100–300 °C) for denitration, achieving over 80% NO_x conversion, along with robust water resistance. The temperature-programed surface reactions and NO oxidation reactions are performed to elucidate the promoting effect of water on N₂ selectivity, which is not only due to inhibition of catalyst oxidation capacity at high temperature but also is related to the competing adsorption of water and NH₃. In situ DRIFTS analysis confirmed that the NH₃-SCR mechanism over Mn/Mg₂AlO_x adheres to the Eley–Rideal (E–R) pathway. These findings highlight the significant promise of Mn/MgAlO_x catalysts for deployment as downstream denitration units within exhaust treatment systems.

1. Introduction

Generated during industrial fossil fuel combustion, nitrogen oxides (NO_x) represent significant atmospheric contaminants. These compounds facilitate the formation of secondary pollutants, notably fine particulate matter (PM_2.5) and ozone (O₃) [1,2]. The selective catalytic reduction with ammonia (NH₃-SCR) technique is a high-efficiency denitration method. As the fundamental component governing NH₃-SCR system viability, the catalyst dictates operational practicality. Since the 1970s, V₂O₅-WO₃(MoO₃)/TiO₂ has served as a highly effective medium-temperature (300–400 °C) catalyst in this technology [3,4]. Zeolite-based catalysts with unique pore structure and excellent catalytic performance have also received more and more attention and application in petrochemical, energy and environmental fields [5,6]. However, it has numerous inherent disadvantages. For example, V₂O₅ with biologic toxicity is prone to sublimation and shedding during actual operation as well as diffusion into the external environment with the fumes, causing serious harm to humans and the environment. Furthermore, the high quantities of SO₂ and dust in the exhaust might nevertheless damage the V-based catalyst over a prolonged period of operation, leading to blockage or covering of the active sites and, subsequently, deactivation. The installation of NH₃-SCR reactors at the end of a purification unit can reduce the impact of toxic flue gases on the catalyst, where the gas temperature is lower [7,8]. The hydrothermal stability of zeolite catalysts still needs to be improved, and the denitrification efficiency at low temperature is still not ideal [9]. Designing catalysts that are operational at low temperatures enables broader deployment of NH₃-SCR as a terminal flue gas denitrification solution, ensuring adaptability across diverse operating conditions.

The multivalence and robust redox capacity of Mn-based catalysts enable “outstanding low-temperature performance”, making them well-suited for implementation in final-stage flue gas cleaning processes [10,11,12]. Stringent SO₂ emission regulations (e.g., a limit of 35 mg/m³ in flue gas) present new opportunities for Mn-based catalysts in NO_x removal applications. Even after desulfurization and the Cottrell process, the flue gas still contains some residual H₂O, for example, the exhaust of waste-to-power (generating electricity through waste incineration), gas turbines and biomass combustion, which is the biggest obstruction to the application in fact of pure MnO_x in terminal denitration systems. As a solution, one of the cutting-edge strategies for catalyst design is to effectively improve the water tolerance [13]. Gu et al. [14] synthesized Mn–Fe mixed oxides loaded with titanium silicalite-1 (TS-1) for NH₃-SCR, which exhibited high water resistance and maintained about 94% NO_x conversion, while 10% H₂O was present for 12 h at 180 °C. The addition of Ti provided the active ingredient with additional surface acidic sites and stronger redox capacity. Zhao et al. [15] reported that a low-level MnCoO_x doping on coal fly ash-derived 13X zeolite enables high-efficiency, low-temperature NH₃-SCR, with over 80% NO conversion, 90% N₂ selectivity and significant cost reduction. Pu et al. [16] developed a CNTs-functionalized CeO₂/CNTs-GAC catalyst that achieves high NO conversion and superior SO₂ tolerance in low-temperature NH₃-SCR of NO. Xu et al. [17] reported that the inclusion of CNTs improved the H₂O tolerance of the MnCe/GAC-CNTs catalyst by promoting the creation of mesopores and electron transfer. Previous reports [18,19] confirmed that the suitable support materials not only offer a lot of surface area for dispersing the active component, preventing the growth of large crystal particles, but also provide enough room for the catalytic reaction.

Layered double hydroxide (LDH)-derived MgAl composite oxides (MgAlO_x) exhibit “substantial specific surface area and exceptional thermal stability”. These characteristics establish them as high-performance catalyst supports. Due to the interlayer confinement effect of LDHs, MgAlO_x as support can prevent the aggregation of metal nanoparticles in favor of uniform dispersion [20,21]. The hydroxyl sites, basic sites and cation of MgAlO_x combined with active components play a synergistic catalytic role [22,23]. For a long time, MgAlO_x have been extensively studied for NO_x adsorption. In NO_x storage and reduction (NSR) testing, the newly designed Pt(Cu)/MgAlO_x catalyst demonstrated exceptional low-temperature performance and robust sulfur tolerance [24]. Our research group [25] constructed Mn-loaded catalysts with MgAlO_x derived from LDHs as the support for low-temperature NH₃-SCR. Calcination at elevated temperatures induced strong MgAlO_x–manganese interactions, stabilizing manganese as surface MnO_x species while simultaneously doping Mn³⁺ into spinel-phase MgMn₂O₄, with both mechanisms collectively increasing surface chemisorbed oxygen. Our previous studies have proved that not only the different Mn salt precursors but also the properties of the MgAlO_x support have an effect in significant ways on the catalytic performance. Gao et al. [26] systematically explored the role of different Mg/Al on the high-temperature CO₂ adsorption capacity of MgAl-LDO catalysts and found that further modulation of the acid–base properties of the support could be achieved by appropriately adjusting the Mg/Al ratio in the interlayer of hydrotalcite precursors. Wu et al. [27] synthesized a series of CuMgFe-LDHs with varying Cu/Mg/Fe molar ratios and derived mixed oxide catalysts (LDO) for NH₃-SCR denitration. The Cu_0.5Mg_2.5Fe₁-LDO catalyst exhibited superior performance, achieving over 80% NO_x conversion between 180 and 250 °C and demonstrating strong resistance to SO₂. Wu et al. [28] also synthesized a series of NiTi-LDHs with varying Ni/Ti molar ratios for NH₃-SCR denitration. The Ni₄Ti₁-LDO catalyst calcined at 400 °C demonstrated optimal performance, achieving over 90% NO_x conversion between 240 and 360 °C with nearly 95% N₂ selectivity. Hou et al. [29] reported the tunable synthesis of highly dispersed Ni–Mn layered double oxide catalysts from Ni–Mn LDH precursors, achieving optimal Ni/Mn ratios (Ni₅Mn-LDO) that deliver excellent low-temperature NH₃-SCR performance. Consequently, our focus shifts to enhancing MgAlO_x support acidity through controlled Mg/Al ratio variation in LDH precursors. This approach enables the development of hydrothermally stable, low-temperature SCR catalysts with high activity.

This study systematically modulates Mg/Al ratios in MgAlO_x composite oxides to investigate their influence on redox properties, surface oxygen vacancies, acidity and corresponding NH₃-SCR performance. Ultimately, the optimal Mn/Mg₂AlO_x has a wide temperature window for denitration, removing 80% of NO_x at 100–300 °C, as well as satisfactory water resistance activity. A multi-technique investigation (kinetics, XPS, H₂-TPR, NH₃-TPD, in situ DRIFTS) revealed the water-resistance behavior and denitration pathways of Mn/Mg_tAlO_x catalysts.

2. Materials and Methods

2.1. Synthesis of the Mn/Mg_tAlO_x Catalysts

Al(NO₃)₃·9H₂O (A.R.) and Mg(NO₃)₂·6H₂O (A.R.) are provided by Tianjin Damao Chemical Reagent Co., Ltd. (Tianjin, China). MnC₄H₆O₆·4H₂O (A.R.) and Na₂CO₃ (A.R.) are provided by Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China). NaOH (A.R.) is provided by Sinopharm Chemical Reagent Co., Ltd. (Beijing, China).

Mn/Mg_tAlO_x (t = 1, 2, 3, 4) catalysts with varied Mg/Al molar ratios were prepared using a combination of the co-precipitation approach and the simple wet impregnation method. Taking Mn/Mg₂AlO_x as an example, 9.4 g of Al(NO₃)₃·9H₂O and 12.8 g of Mg(NO₃)₂·6H₂O were dissolved in 50 mL deionized water to form a mixed nitrate solution. At room temperature, a mixed base solution was prepared in a volumetric flask by dissolving 2 M NaOH and 1 M Na₂CO₃ solids with a molar ratio of (CO₃²⁻):(OH⁻) = 1:2. A beaker filled with 50 mL of deionized water was vigorously stirred as the two solutions were introduced drop by drop, and the pH was regulated at roughly 10 throughout the precipitation by adjusting the drop speed. The resulting mixture underwent aging at 60 °C for 18 h under continuous agitation. Subsequent filtration isolated the precipitate, which was then subjected to repeated deionized water washings until a neutral pH (~7) was attained. The filtered precipitate underwent drying at 120 °C for 12 h to yield the MgAl-LDH precursor (Mg/Al = 2). The resulting material was then calcined at 800 °C for 5 h with a heating rate of 2 °C/min to generate the Mg₂AlO_x support. Manganese acetate (MnC₄H₆O₆·4H₂O) was then impregnated onto this support following established protocols from prior research [25]. A series of 50 wt%Mn/Mg_tAlO_x catalysts were prepared via the conventional impregnation method. Taking the Mn/Mg₂AlO_x as an example, a stoichiometric amount of MnC₄H₆O₆·4H₂O was added to an aqueous suspension of Mg₂AlO_x support and stirred to achieve homogeneous dispersion of the precursor. The resulting mixture was then evaporated in a water bath and dried overnight. Finally, the obtained material was calcined at 500 °C for 5 h with a heating rate of 2 °C/min to obtain the Mn/Mg₂AlO_x catalyst.

The actual elemental composition and Mg/Al molar ratios of the catalysts are provided in Table S2.

2.2. Catalytic Activity Test

Catalytic activity assessments employed a continuous-flow, fixed-bed quartz reactor (laboratory-built). The simulated flue gas contained 500 ppm NH₃, 500 ppm NO, 5% O₂ and N₂ balance, with 5% H₂O vapor introduced where applicable. Testing occurred at 40,000 h⁻¹ gas hourly space velocity (GHSV). Catalysts underwent initial thermal conditioning at 350 °C for 20 min under reaction gas flow. Subsequently, the temperature was reduced to 50 °C at a controlled 5 °C·min⁻¹ ramp rate, with 20 min stabilization periods at each intermediate measurement point. A chemiluminescence NO_x analyzer (Thermo 42i-HL) (Thermo Fisher Scientific, Waltham, MA, USA)recorded NO_x concentrations in the products in real time. N₂O concentrations in effluent gases were quantified via gas chromatography (Agilent 7890B, Agilent Technologies, Santa Clara, CA, USA) equipped with an electron capture detector (ECD). Catalyst performance metrics—NO_x conversion and N₂ selectivity—were derived from Equations (1) and (2):

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

(1)

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

(2)

where $\left[{\mathrm{NO}}_{\mathrm{x}}\right]=\left[\mathrm{NO}\right]+\left[{\mathrm{NO}}_2\right]$ and the subscript “in” and “out” denote, respectively, the concentration of the corresponding gas at the inlet and output.

2.3. Reaction Kinetics Testing

Identical reactor configurations assessed NH₃-SCR kinetics. To eliminate internal/external diffusion limitations, differential reactor conditions were maintained (5–20% NO_x conversion, GHSV = 200,000 h⁻¹). Kinetic parameters were normalized using Equations (3) and (4):

$$k=-{\displaystyle \frac{F_{\mathrm{NOx}}}{w}}\times \ln (1-\mathrm{X})$$

(3)

$$\ln k=-{\displaystyle \frac{E_a}{RT}}+\ln \mathrm{A}$$

(4)

where k denotes the reaction rate constant (mol·g⁻¹·s⁻¹), F_NOx is the NO_x molar flow rate (mol·s⁻¹), w represents catalyst mass (g), and X signifies NO_x conversion.

E_a corresponds to apparent activation energy (kJ·mol⁻¹), A is the pre-exponential factor, R is the molar gas constant (8.314 J·mol⁻¹·K⁻¹), and T is the reaction temperature (K).

2.4. Catalyst Characterization

Phase composition analysis was performed using a Bruker X-ray diffractometer with Cu Kα radiation (λ = 1.5418 Å). XRD patterns were acquired across 5–90° 2θ at 6 °·min⁻¹ scan rates. Average crystallite dimensions were derived via the Scherrer equation.

$$\mathrm{Crystallite} \mathrm{size}={\displaystyle \frac{k\lambda }{\beta cos\theta }}$$

(5)

Textural properties of synthesized catalysts were characterized through N₂ physisorption at −196 °C using a Micromeritics ASAP 2460 analyzer (Micromeritics Instrument Corporation, Norcross, GA, USA). Prior to the experiment, samples were degassed at 300 °C for 6 h to remove any adsorbed substances. Specific surface areas were determined via the Brunauer–Emmett–Teller (BET) method, while Barrett–Joyner–Halenda (BJH) analysis yielded pore size distributions and cumulative pore volumes.

XPS analysis was conducted using a Thermo Fisher Scientific K-Alpha spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) equipped with an Al Kα X-ray source. All binding energies were referenced to the adventitious carbon C 1s peak at 284.8 eV.

H₂-TPR analyses employed a quartz flow reactor with inline TCD (FX6080, Lunan Ruihong Chemical Instrument Co., Ltd., Zaozhuang, China). Samples (50 mg) underwent 500 °C pretreatment for 30 min under N₂ flow before cooling to ambient temperature. Reduction profiles were then acquired from 20 to 700 °C at 10 °C·min⁻¹ in 5% H₂/N₂.

NH₃-TPD analyses utilized a TCD-equipped quartz flow reactor (FX6080, Lunan Ruihong Chemical Instrument Co., Ltd.). Samples (50 mg) underwent N₂ pretreatment (500 °C, 30 min) and cooling to ambient temperature. The NH₃ adsorption was then conducted for 60 min under two distinct conditions: (a) under dry conditions using a 1250 ppm NH₃/N₂ gas mixture and (b) in the presence of water vapor using a 1250 ppm NH₃/N₂ and 5% H₂O gas mixture. After 30 min N₂ purging to remove physisorbed species, programmed desorption proceeded at 10 °C·min⁻¹ (20–550 °C).

In situ DRIFTS analysis utilized a Nicolet is 50 FTIR spectrometer with MCT detection (Thermo Fisher Scientific, Waltham, MA, USA). Samples underwent 30 min pretreatment at 200 °C under 50 mL·min⁻¹ N₂ flow. Background spectra were acquired at target temperatures in N₂ before introducing regulated gas streams (500 ppm NH₃/N₂ or 500 ppm NO/N₂ + 5% O₂), each supplied at a flow rate of 50 mL/min. Catalyst spectra were recorded after system stabilization.

3. Results and Discussion

3.1. Catalytic Performance of Catalysts

Catalytic activities for NH₃-SCR over prepared Mn/Mg_tAlO_x systems are shown in Figure 1. Temperature-dependent NO_x removal efficiencies display characteristic volcano trends across all samples (Figure 1a). At Mg/Al = 1, Mn/MgAlO_x exhibits notably low activity, reaching only 70% peak NO_x conversion at 200 °C. Progressively higher Mg/Al ratios substantially enhance catalytic performance across all samples. Notably, Mn/Mg₂AlO_x exhibits substantially enhanced low-temperature activity, attaining 60% NO_x conversion at 50 °C. The catalyst also demonstrates an expanded operational range, sustaining >80% NO_x conversion between 100 and 300 °C. Regarding Mn/Mg₃AlO_x, the temperature interval is narrowest, even if the almost 100% NO_x conversion may be attained at 200 °C. Catalytic activity undergoes marked deterioration when temperatures exceed 250 °C, as evidenced by declining NO_x conversion rates. Mn/Mg₄AlO_x with elevated Mg/Al ratios exhibits constrained low-temperature activity but sustains stable high-temperature performance, maintaining >80% NO_x conversion between 150 and 300 °C. Figure 1b shows that altering of Mg/Al improves the N₂ selectivity of the various samples over the temperature range tested, as shown by the N₂ selectivity of Mn/Mg_tAlO_x being higher than 80% at low temperatures (150 °C), demonstrating the prepared catalysts are highly efficient in the reduction of NO to environmentally friendly N₂ via NH₃. Elevated temperatures progressively reduce N₂ selectivity over Mn/Mg_tAlO_x catalysts, attributable to excessive NH₃ oxidation at high temperatures. A comprehensive evaluation of low-temperature activity, operational temperature breadth and N₂ selectivity confirms Mn/Mg₂AlO_x as the optimal NH₃-SCR catalyst. We also added Table S3 comparing NO_x conversion, temperature window and H₂O resistance with other Mn-based catalysts.

We conducted comparative experiments on Mg₂AlO_x supports and Mn/γ-Al₂O₃ catalysts. As shown in Figure S2, the Mg₂AlO_x support exhibited a relatively low NO_x conversion (approximately 10%) in the low-temperature range (100–250 °C). The Mn/Mg₂AlO_x catalyst demonstrated significantly superior low-temperature activity compared to Mn/γ-Al₂O₃. This highlights the high SCR activity contributed by manganese oxides and the crucial role of the LDH-derived Mg₂AlO_x support.

Literature [30] attributes SCR catalyst deactivation in humid reaction environments to competitive adsorption between H₂O vapor and reactants (NH₃, NO) at catalytic sites. Figure 1c presents the variation in the NO_x conversion over time for the Mn/Mg₂AlO_x at 200 °C with a continuous flow of 5 vol% H₂O into the reaction atmosphere for 8 h. First of all, the NO_x conversion is stable at 92% for the first two hours under the standard reaction atmosphere. After introducing H₂O for 8 h, the NO_x conversion of Mn/Mg₂AlO_x decreases slightly by about 4%. With the suspension of H₂O supply, the activity of Mn/Mg₂AlO_x can be recovered to the initial state, implying that the inhibitory action of water over catalyst is reversible. According to the results above, the Mn/Mg₂AlO_x catalyst exhibits excellent H₂O resistance ability. Figure 1d illustrates the influence of 5 vol% H₂O on Mn/Mg₂AlO_x-catalyzed NO_x reduction across temperatures. Below 250 °C, water vapor suppresses catalytic activity relative to anhydrous conditions, evidenced by a 100 °C conversion decline from 83% to 56%. Above this threshold, hydrothermal effects become negligible. These findings demonstrate temperature-dependent water adsorption behavior. Strong H₂O adsorption at low temperatures competitively inhibits catalytic sites, significantly suppressing activity. Above ~250 °C, diminished water adsorption attenuates this inhibitory effect, with some studies reporting enhanced NO_x reduction efficiency under such conditions [31,32]. Moreover, the presence of water improved the N₂ selectivity of the catalyst to a certain degree, which increased from 65% to 91% at 200 °C. The previous researchers [33] have proposed that the inhibition of N₂O production by water vapor is also related to its inhibition of catalyst oxidation ability. Figure 1e exhibits the inhibition of NO oxidation over Mn/Mg₂AlO_x by H₂O molecules, with a decrease in NO conversion when the reaction temperature exceeds 200 °C. As the NO is oxidized to NO₂ by reactive oxygen species, it can trigger the “fast SCR” reaction, which has a lower activation energy and higher catalytic activity than typical NH₃-SCR. However, over-oxidation capacity can entail excessive oxidation of NH₃, causing the reductant to be rapidly consumed by O₂, whereby the SCR reaction does not proceed properly, as well as generating byproducts of N₂O, which reduces the NO_x removal efficiency and narrows the operation temperature range of the catalyst. Accordingly, the N₂ selectivity is somewhat improved when the oxidation capacity over Mn/Mg₂AlO_x is suppressed by H₂O at high temperature, which is consistent with the findings of the activity tests. A comparison of the catalytic performance with othe catalysts reported in the literature is provided in the Supplementary Material (Table S3), which demonstrates the competitive activity and superior selectivity of our catalyst.

NH₃ oxidation (NH₃-SCO) acts as a kinetic competitor to NH₃-SCR by depleting NH₃ and lattice/surface oxygen, thereby narrowing the effective activity window at elevated temperatures. In our catalysts, water vapor moderates the oxidation strength, which both restrains NH₃-SCO and aligns with the observed rise in N₂ selectivity under humid conditions and the reduced NO-oxidation ability in H₂O. Thus, while NH₃-SCO can become relevant above ~300 °C, its contribution under the humid low-temperature regime studied here is limited, helping maintain high N₂ selectivity.

Kinetic studies employed high space velocities (GHSV = 200,000 h⁻¹) and low NO_x conversions (<20%) to eliminate diffusion limitations. Figure 1f derives apparent activation energies (E_a) from Arrhenius plot slopes for various catalysts. Mn/Mg₂AlO_x exhibits the lowest E_a (36 kJ·mol⁻¹), confirming Mg/Al ratio modulation effectively lowers reaction energy barriers and enhances catalytic activity.

3.2. Crystal Structure and Texture Property

XRD of the as-synthesized Mg-Al LDH (prior to calcination) confirmed a well-defined hydrotalcite structure for all Mg/Al ratios. The full detailed peaks/analysis of the LDH phase are available in the Supplementary Materials. The XRD patterns of Mg_tAlO_x supports obtained from MgAl-CO₃ LDH after calcination at 800 °C are shown in Figure 2a. The main component of Mg_tAlO_x is MgO species (PDF#74-1225), with a small amount of MgAl₂O₄ spinel phase (PDF#86-2258) incorporated. The strength of diffraction peaks ascribed to MgO increases when Mg content increases, while peaks related to MgAl₂O₄ decrease in intensity. When the Mg/Al ratio increases to 4, only a weak diffraction peak assigned to MgAl₂O₄ can be detected at 36.8°. As shown in Figure 2b, Mn/Mg_tAlO_x catalysts exhibit MnO₂ (PDF#44-0992) as the main crystal structure, with traces of Mn₃O₄ (PDF#80-0382) and MgMn₂O₄ (PDF#72-1336) crystalline phases. The diffraction peaks of MgMn₂O₄ become more intensified with increasing Mg/Al ratios, especially at 2θ = 44.7°, where the change is more pronounced. In addition, the Mn/MgAlO_x catalyst also involves Al₂O₃ (PDF#12-0539), showing weak diffraction peaks at 2θ = 16.3° and 25.7°. The absence of Al-associated reflections in other catalysts suggests either atomic-level dispersion of aluminum species or their existence in an XRD-amorphous phase.

N₂ adsorption–desorption isotherms and pore size distributions for the Mn/Mg_tAlO_x catalyst series appear in Figure 3. Per IUPAC classification, all Mn/Mg_tAlO_x catalysts display characteristic type II isotherms featuring H3 hysteresis loops across defined relative pressure ranges. This behavior confirms macroporous architectures within the catalysts [34]. Notably, Mn/Mg₂AlO_x exhibits hysteresis loop closure at reduced relative pressure (p/p₀ = 0.4), signaling enhanced microporosity. The broad pore size distributions in Figure 3b further reveal a hierarchical structure with pores spanning the micro-, meso-, and macroporous ranges. Although the average pore sizes are situated within the mesoporous regime (

Table 1. The textural parameters and the quantitative data from XPS spectra of Mn/Mg_tAlO_x catalysts.

Catalysts	SBET (m2·g−1) a	Vp (cm3·g−1) a	Dp (nm) a	Atomic Concentration (mol.%) b		Atomic Ratio (%) b
				Mn	O	Mn4+/Mn	Oα/(Oα + Oβ)
Mn/MgAlOx	45.4	0.19	14.7	14.3	56.6	31.8	53.7
Mn/Mg2AlOx	88.0	0.25	9.01	11.9	58.9	39.5	59.2
Mn/Mg3AlOx	56.4	0.29	17.2	11.9	63.4	36.6	56.6
Mn/Mg4AlOx	67.6	0.29	14.7	11.9	58.5	38.7	56.1

^a Estimated from N₂ physical adsorption. ^b Calculated from XPS analysis.

), the coexistence of larger and smaller pores is evident. As established in [35], coexisting micropores and mesopores increase accessible surface area, while macroporous domains facilitate mass transport. Consequently, optimal catalytic activity in Mn/Mg₂AlO_x requires a narrow pore size dispersion and a balanced microporous–mesoporous–macroporous hierarchy. Table 1 summarizes textural properties, revealing the minimal surface area of Mn/MgAlO_x (45.4 m²·g⁻¹), which is consistent with good crystallinity in XRD. On the contrary, Mn/Mg₂AlO_x exhibits the highest specific surface area of 88.0 m²·g⁻¹ and the smallest average pore size of 9.01 nm, which favors the dispersed distribution of the active components within the channels [36]. It can be seen that the pore structure properties of the prepared materials are important factors to influence their catalytic activity [37].

3.3. Acidic Property

The chemisorption and activation of ammonia constitute a fundamental NH₃-SCR mechanism stage, critically influenced by catalyst surface acidity during reactive intermediate formation [38]. Acid site concentration and strength across Mn/Mg_tAlO_x surfaces were evaluated through NH₃-TPD analysis (Figure 4). All catalysts display broad NH₃ desorption profiles originating from Lewis acid sites, with spectral deconvolution revealing two superimposed desorption features. Peaks below 200 °C correspond to NH₃ desorption from weak acid sites, indicating physisorbed or weakly chemisorbed ammonia. Desorption events between 200 and 350 °C reflect NH₃ release from medium-strength acid sites with stronger chemical binding, consistent with established NH₃-TPD interpretation frameworks [39]. It can be seen that there is no significant change in the position of the desorption peaks of catalysts with different Mg/Al ratios. Quantitative analysis reveals Mn/MgAlO_x possesses the lowest acid site density, equivalent to 57% of Mn/Mg₂AlO_x’s total acidity. Weak and medium-strong acid sites constitute 51% and 49% of acid sites, respectively. Acid site deficiency constrains SCR reactivity, explaining the pronounced activity loss. Conversely, Mn/Mg₂AlO_x exhibits greater total acidity, comprising 36% weak and 64% medium-strong acid sites. This promotes NH₃ adsorption, subsequently enhancing catalytic activity via the “NH₄NO₃ + NO” or “NH₂ + NO” pathway. Consequently, superior medium-to-high temperature performance is achieved [40,41].

Beyond suppressing catalytic oxidation capacity, water also competes with NH₃ for adsorption sites during NH₃-SCR [4]. Consequently, Figure 4c presents Mn/Mg₂AlO_x TPD profiles after 1 h NH₃ exposure in humid atmospheres. Unlike dry conditions, the broad desorption feature resolves into distinct peaks at about 81 °C and 270 °C. Relative to anhydrous conditions, Mn/Mg₂AlO_x exhibits markedly reduced NH₃ adsorption capacity in humid environments, especially at medium-strength acid sites. This indicates that competitive displacement by H₂O restricts ammonia adsorption, consequently diminishing NH₃ oxidation and suppressing N₂O byproduct formation. The excellent water resistance of Mn/Mg₂AlO_x stems from its high surface area and macroporous architecture. At elevated temperatures, limited pore condensation of H₂O minimally impacts NH₃-SCR activity. Conversely, capillary-driven water accumulation in micropores at low temperatures blocks active sites, reducing catalytic efficiency [42]. In this way, the large pore volume and specific surface area of Mn/Mg₂AlO_x not only prevents the condensation of H₂O molecules but also allows more active sites to be fully exposed.

3.4. Surface Element Analysis and Redox Property

Elemental composition and manganese oxidation states in Mn/Mg_tAlO_x catalysts were characterized by XPS (Figure 5, Table 1). The Mn 2p spectrum (Figure 6a) displays characteristic spin-orbit splitting, with Mn 2p_3/2 and Mn 2p_1/2 peaks at ~641.5 eV and ~653.1 eV, respectively. Deconvolution of the Mn 2p_3/2 region reveals three oxidation states: Mn²⁺ (~640.3 eV), Mn³⁺ (~641.4 eV) and Mn⁴⁺ (~642.5 eV). The latter species enhances reactive oxygen activation and mobility, as documented in catalysis literature [43,44]; thus, the relative ratio of Mn⁴⁺ by calculating the peak area ratio of Mn⁴⁺/(Mn²⁺+Mn³⁺+Mn⁴⁺) is shown. The relative ratio of Mn⁴⁺ in the over Mn/Mg_tAlO_x serial catalysts follows an increased sequence of Mn/MgAlO_x < Mn/Mg₃AlO_x < Mn/Mg₄AlO_x < Mn/Mg₂AlO_x, in harmony with their catalytic activity. Consequently, abundant surface Mn⁴⁺ in Mn/Mg₂AlO_x accelerates the SCR redox cycle, constituting the primary mechanism for its superior catalytic performance. Complementary analysis of O 1s spectra (Figure 5b) resolves two components: adsorbed oxygen species O_α (531.2–531.6 eV) and lattice oxygen O_β (529.6 eV). Since O_α has a higher mobility than O_β, it tends to be more reactive in the oxidation of NO to NO₂. Table 1 reveals parallel trends in O_α and Mn⁴⁺ surface concentrations across catalysts. This correlation likely originates from enhanced oxygen vacancy formation during MnO_x to MnO₂ transformation [45,46], facilitating oxygen molecule activation. Subsequently, NO reacts with activated oxygen at Mn⁴⁺ sites, generating adsorbed NO₂ or nitrate intermediates that undergo reaction with ammonia.

Figure 6 displays the results of the H₂-TPR investigation into the redox capabilities of the Mn/Mg_tAlO_x catalysts. All samples show three reduction peaks at 100–700 °C, which are in relation to the gradual reduction of MnO_x. Reduction profiles exhibit three characteristic peaks: the low-temperature peak (Peak I, 302–349 °C) is attributed to MnO₂ to Mn₂O₃ transformation, the medium-temperature peak (Peak II, 440–460 °C) corresponds to Mn₂O₃ to Mn₃O₄ reduction, and the high-temperature peak (Peak III, 479–505 °C) originates from Mn₃O₄ to MnO phase transformation. Reduction peak temperatures directly reflect catalytic reducibility, with lower values indicating stronger reduction capability. Figure 6a demonstrates that reduction peaks broadly shift to higher temperatures with increasing Mg/Al ratios. Notably, the peaks associated with MnO₂ species exhibit the most pronounced displacement. This pattern generally indicates strengthened interaction between highly dispersed manganese oxides and the Mg_tAlO_x support. Elevated Mg²⁺ content within the catalyst enhances this effect, consequently increasing the reduction resistance of these MnO_x species [47]. Furthermore, H₂ consumption of samples is quantified by reference to the H₂-TPR curves spectrum of the quantitative standard CuO. As presented in Figure 6b, the order of H₂ consumption for the four samples is Mn/MgAlO_x (3.42 mmol·g⁻¹) < Mn/Mg₂AlO_x (3.78 mmol·g⁻¹) < Mn/Mg₄AlO_x (4.09 mmol·g⁻¹) < Mn/Mg₃AlO_x (4.23 mmol·g⁻¹). Elevated Mg²⁺ content correlates with increased reduction peak intensity, suggesting greater formation of redox-active sites on the catalyst surface. Moderate formation of the MgMn₂O₄ phase can enhance the catalyst’s redox capacity [48]. However, excessively strong redox capability promotes undesirable side reactions, including ammonia over-oxidation, which negatively impacts denitration performance.

3.5. In Situ DRIFTs Analysis

To obtain insight into the reaction process of catalysts, the adsorption behavior of reactants is studied. Figure 7a,b show in situ DRIFTS spectra of NH₃ adsorption over Mn/Mg_tAlO_x at 100 °C. After the addition of NH₃, it is clear that both catalysts have comparable characteristics. Infrared bands at approximately 1188 and 1616 cm⁻¹ correspond to NH₃ adsorbed at Lewis acid sites (predominantly Mn⁴⁺ cations, with a contribution from Al³⁺ sites of the support) [49,50]. Conversely, the weaker band near 1427 cm⁻¹ arises from NH₄⁺ species bound to Brønsted acid sites. Additionally, N-H stretching vibrations of Lewis-adsorbed NH₃ generate peaks at 3172, 3253 and 3361 cm⁻¹ [51,52]. O-H stretching vibrations at 3693 and 3747 cm⁻¹ produce negative bands, arising from interactions between adsorbed NH₃ and surface hydroxyl groups. This process consumes hydroxyl groups while generating ionic NH₄⁺ species [53]. Furthermore, spectral band intensities progressively increase during extended NH₃ exposure to catalysts, attaining saturation within 30 min.

Figure 7b reveals that Lewis acid sites (predominantly Mn⁴⁺ cations, with a contribution from Al³⁺ sites of the support) dominate over Brønsted sites in both catalysts, suggesting that adsorbed NH₃ constitutes the predominant ammonia species. Distinct from Mn/Mg₃AlO_x, infrared bands at 1336 and 1565 cm⁻¹ confirm surface NH₂ species on Mn/Mg₂AlO_x [54,55]. Reactive oxygen species oxidize adsorbed NH₃ to form NH₂⁻, establishing this intermediate as critical for the reaction mechanism. The detection of NH₂⁻ on Mn/Mg₂AlO_x signifies concurrent NH₃ adsorption and activation. This surface process mediates subsequent reactions with NO, yielding N₂ and H₂O, thereby enhancing NO_x removal efficiency. Relative to Mn/Mg₃AlO_x, Mn/Mg₂AlO_x exhibits markedly more intense NH₃ absorption bands, signifying enhanced surface acidity and ammonia adsorption capacity following Mg/Al ratio modification. These observations align with NH₃-TPD results. Figure 7c,d present in situ DRIFTS spectra for NO + O₂ co-adsorption on Mn/Mg_tAlO_x at 100 °C. With the introduction of NO_x, absorption peaks belonging to a variety of nitrate species appeared on the catalyst’s surface. The peaks locating at 1218, 1307, 1475 and 1554 cm⁻¹ are ascribed to bridging nitrate, monodentate nitrate, linear nitrite and bidentate nitrate species, respectively, while the newly appearing peak at 1650 cm⁻¹ is ascribed to the gaseous NO₂ [56,57,58]. Mn/Mg₂AlO_x exhibits an additional absorption band at 1394 cm⁻¹, assigned to monodentate nitrate [59], which is not observed on Mn/Mg₃AlO_x. Nitrate-related band intensities increase progressively over time. Figure 7d demonstrates that all nitrate species signals are substantially stronger on Mn/Mg₂AlO_x versus Mn/Mg₃AlO_x, revealing that Mg/Al ratio modulation enhances NO_x adsorption and activation. This augmented NO + O₂ co-adsorption generates greater surface nitrate concentrations, thereby accelerating the “fast SCR” pathway [60].

To elucidate NH₃-NO_x reaction pathways on Mn/Mg₂AlO_x, in situ DRIFTS spectra were acquired at 100 °C using sequential steps: 1 h NH₃ pre-adsorption, 30 min N₂ purge, and NO + O₂ exposure, with results shown in Figure 8a,b. Post-adsorption spectral features confirm Lewis-bound NH₃ (1188, 1616 cm⁻¹), Brønsted-bound NH₄⁺ (1427 cm⁻¹) and NH₂⁻ intermediates (1336, 1565 cm⁻¹) on the catalyst. All ammonia-associated bands diminish within 3 min of NO + O₂ introduction, indicating efficient acid site-mediated adsorption and activation of ammonia species for subsequent NO_x reactions [61]. Subsequent spectral features reveal nitrate species deposition, including bridged nitrate (1218 cm⁻¹), monodentate nitrate (1307 and 1394 cm⁻¹) and gaseous NO₂ (1650 cm⁻¹) [62,63]. Nitrate-associated band intensities steadily intensify during prolonged exposure, reflecting progressive accumulation on the catalyst surface. These observations collectively demonstrate that all surface ammonia species participate in NH₃-SCR reactions over Mn/Mg₂AlO_x via the Eley–Rideal (E–R) mechanism.

Figure 8c,d show in situ DRIFTS spectra of the adsorbed state species on the surface of Mn/Mg₂AlO_x produced by first pre-adsorbing NO_x at 100 °C for 1 h, then purging with N₂ for 30 min and finally feeding NH₃. Following NO_x saturation, spectral features emerge for bridging nitrate (1215 cm⁻¹), monodentate nitrate (1307 and 1394 cm⁻¹), linear nitrite (1475 cm⁻¹) and gaseous NO₂ (1650 cm⁻¹). Upon NH₃ introduction, only linear nitrite and gaseous NO₂ bands diminish over time, indicating partial nitrate reactivity with ammonia. Conversely, monodentate nitrate peaks remain stable, demonstrating surface accumulation. Bridging nitrate signals intensify progressively due to spectral overlap between accumulating Lewis-adsorbed NH₃ (1210 cm⁻¹) and pre-existing bridging nitrate species [64]. Infrared bands at 3263 and 3373 cm⁻¹ correspond to N-H stretching vibrations of Lewis-bound NH₃ species. Stability analyses reveal that nitrate species exhibit greater surface persistence than ammonia on Mn/Mg₂AlO_x, with NH₃ reactions involving pre-adsorbed NO_x proving kinetically challenging. Consequently, NH₃-SCR over Mn/Mg₂AlO_x predominantly follows the Eley–Rideal (E–R) mechanism, involving adsorbed NH₃ reacting with gaseous NO_x species [65,66].

3.6. The Reaction Mechanism of NO_x over the Mn/Mg₂AlO_x

Based on the preceding in situ DRIFTS analysis, primary Eley–Rideal (E–R) mechanisms on the catalyst surface involve gaseous NH₃ first adsorbing at Lewis acid sites (predominantly the Mn⁴⁺ cations, with a secondary contribution from Al³⁺ sites on the support), followed by Mn⁴⁺-mediated activation to NH₂⁻ intermediates. These species subsequently reduce gaseous NO to environmentally benign N₂ and H₂O (Equations (6)–(8)). However, excessive NH₃ dehydrogenation yielding NH radicals initiates detrimental side reactions. NH reacts with NO to form N₂O (reducing N₂ selectivity), while NH over-oxidation generates NO, thereby compromising overall NO_x removal efficiency (Equations (9)–(11)). The reactive oxygen species then further oxidize the metal ions in the low-valence states (Mn³⁺, Mn²⁺) to metal ions in the high-valence state (Mn⁴⁺), which are then used to initiate additional reduction processes (Equations (12) and (13)). Elevated Mn⁴⁺ concentrations and ROS abundance within Mn/Mg₂AlO_x enhance the kinetics of NH₃ dehydrogenation and NO oxidation processes, thus elevating SCR performance [67,68].

$${\mathrm{NH}}_3\left(\mathrm{g}\right)\to {\mathrm{NH}}_3\left(\mathrm{ad}\right) \mathrm{on} \mathrm{Lewis} \mathrm{acid} \mathrm{sites} ({Mn}^{4+}, {Al}^{3+})$$

(6)

$${\mathrm{Mn}}^{4+}+{\mathrm{NH}}_3\left(\mathrm{ad}\right)\to {\mathrm{Mn}}^{3+}+-{\mathrm{NH}}_2+{\mathrm{H}}^{+}$$

(7)

$$-{\mathrm{NH}}_2+\mathrm{NO}\left(\mathrm{g}\right)\to {\mathrm{N}}_2+{\mathrm{H}}_2\mathrm{O}$$

(8)

$$-{\mathrm{NH}}_2+{\mathrm{Mn}}^{4+}\to \mathrm{NH}+{\mathrm{Mn}}^{3+}+{\mathrm{H}}^{+}$$

(9)

$$\mathrm{NH}+\mathrm{NO}\left(\mathrm{g}\right)+{\mathrm{Mn}}^{4+}\to {\mathrm{N}}_2\mathrm{O}+{\mathrm{Mn}}^{3+}+{\mathrm{H}}^{+}$$

(10)

$$\mathrm{NH}+1/2{\mathrm{O}}_2\left(\mathrm{g}\right)+{\mathrm{Mn}}^{4+}\to \mathrm{NO}+{\mathrm{Mn}}^{3+}+{\mathrm{H}}^{+}$$

(11)

$${\mathrm{O}}_2\left(\mathrm{g}\right)\to {2\mathrm{O}}_{(\alpha )}$$

(12)

$${\mathrm{Mn}}^{3+}+{\mathrm{O}}_{(\alpha )}\to {\mathrm{Mn}}^{4+}-\mathrm{O}$$

(13)

4. Conclusions

In summary, a novel series of Mn/Mg_tAlO_x catalysts derived from LDH precursors were developed for low-temperature NH₃-SCR. The optimized Mn/Mg₂AlO_x catalyst demonstrated superior NO_x conversion within a broad temperature window (100–300 °C) and outstanding water resistance. More importantly, this study unveils that the exceptional performance is not attributable to a single factor but to a synergistic effect arising from its hierarchical porosity (facilitating mass transfer), balanced acidic properties (favoring NH₃ adsorption) and abundant surface Mn⁴⁺ and active oxygen species (promoting the redox cycle). Furthermore, in situ DRIFTS studies confirmed that the reaction follows the Eley–Rideal mechanism and provided molecular-level insight into the unique promoting effect of H₂O on N₂ selectivity by suppressing the over-oxidation of NH₃.

Despite the promising results, several challenges remain to be addressed before practical application. Firstly, the resistance to SO₂ poisoning, a ubiquitous component in flue gas, must be thoroughly investigated in the future. Secondly, the long-term stability under real flue gas conditions containing both H₂O and SO₂ is essential to evaluate. Finally, scaling up the catalyst and formulating it into a monolithic structure will be a critical step towards industrial implementation. This work provides a fundamental understanding and a robust catalyst platform, paving the way for the development of high-performance, water-resistant SCR catalysts for terminal denitrification applications.