Low-Temperature Methane Catalytic Combustion on a New Pt-Based Catalyst Confined in a MnO₂/Al₂O₃ Carrier

Abstract

In this study, a hierarchically confined Pt/MnO₂–meso-Al₂O₃ catalyst with 0.5 wt% Pt loading was synthesized via a precipitation method using MnO₂ as a promoter and mesoporous Al₂O₃ (m-Al₂O₃) as a support, and its methane catalytic combustion performance and structure–activity relationship were systematically investigated. The results demonstrate that the 0.5 wt% Pt-loaded Pt-MnO₂/m-Al₂O₃ catalyst achieved 90% methane conversion at 236 °C. The enhanced performance is attributed to three synergistic mechanisms: (1) Pt doping induced lattice contraction in MnO₂ (XRD revealed a 0.03 Å reduction in the (001) interplanar spacing), which facilitated the formation of Mn³⁺–oxygen vacancy pairs (XPS indicated a Mn³⁺- content of 79.87%); (2) the MnPt₃O₆ interfacial structure (HAADF-STEM confirmed lattice spacings of 0.21 nm) accelerated oxygen species cycling, with the 0.5 wt% Pt-loaded catalyst for lattice oxygen desorption capacity (O₂-TPD) increasing by 54% compared to undoped samples; (3) the mesoporous m-Al₂O₃ carrier provided effective confinement, achieving a high specific surface area (27.6 m²/g) and sub-nanometer Pt dispersion (particle size < 2 nm). Under conditions of 1000 ppm CH₄ and a space velocity of 30,000 h⁻¹, the catalyst maintained a methane conversion rate of 98.2 ± 0.5% during continuous operation for 300 h. Post-cycling characterization revealed a stable crystalline structure (XRD full width at half maximum of 0.35° ± 0.02°) and grain size (15.5 ± 0.5 nm), confirming its robustness for industrial applications. This study provides theoretical and experimental foundations for the rational design of highly efficient catalysts for low-concentration methane elimination. For comparison, a Co-doped catalyst (1.0 wt% Co–MnO₂/Al₂O₃) was also prepared, which exhibited significantly lower activity (T₉₀ = 251 °C), underscoring the unique role of Pt in the confined architecture. This study provides theoretical and experimental foundations for the rational design of highly efficient catalysts for low-concentration methane elimination.

1. Introduction

Methane catalytic combustion has emerged as a pivotal technology for mitigating anthropogenic emissions of methane—a potent greenhouse gas with 84-fold higher global warming potential than CO₂ over 20 years [1]. This process enables efficient elimination of low-concentration methane (<10%) from natural gas systems, coal mines, and landfills, simultaneously recovering energy [2]. Despite its promise, industrial implementation faces fundamental challenges rooted in methane’s high C–H bond energy (434 kJ/mol) and sluggish kinetics under mild conditions [3].

Conventional Pt/Al₂O₃ catalysts exhibit superior activity but suffer from irreversible deactivation via sintering (>500 °C) and hydrothermal poisoning [4]. Recent studies reveal that Pt nanoparticle coalescence accelerates under cyclic redox conditions, with >50% activity loss within 100 h [5]. While core-shell architectures (e.g., Pt@SiO₂) enhance thermal stability, they inevitably sacrifice active site accessibility. For instance, Lei et al. [6] demonstrated that 2D-confined Pt catalysts maintained 90% dispersion after aging at 800 °C but showed 40% lower TOF than unconfined counterparts due to diffusion limitations. Similarly, Xu’s Pt@MnO₂ core-shell design [7] achieved 300 h stability at 500 °C, but required a temperature higher than 300 °C to reach 90% methane conversion (T₉₀ > 300 °C), constrained by thick MnO₂ shells (>5 nm) that impede oxygen mobility. (Note: T₉₀ is commonly used as a benchmark in catalytic combustion to avoid mass-transfer limitations near equilibrium; it reliably reflects intrinsic activity.)

To reconcile activity–stability trade-offs, multifunctional interfaces have gained traction. Zhang et al. [8] pioneered MnO₂-coated Pt catalysts where oxygen vacancy (O_v)-rich MnO₂ layers facilitated lattice oxygen (O_latt) cycling, lowering T₉₀ to 280 °C. However, O_v regeneration kinetics remained suboptimal (0.05 s⁻¹), limiting long-term performance. Subsequent studies identified interfacial electronic effects as critical levers: Qin’s atomically dispersed Pt/MnO₂ [9] achieved 210 °C T₉₀ via Pt–O–Mn charge transfer, while Wu’s operando spectroscopy [10] revealed that Mn³⁺–O_v–Pt interfaces accelerated O_latt migration rates by 3-fold higher compared to isolated sites.

Building on these insights, we propose a hierarchical confinement strategy leveraging synergistic Pt–MnO₂–Al₂O₃ interactions. Unlike conventional coatings, our design engineers three-tier functionality (Figure 1):

(1)

Mesoporous Al₂O₃ (m-Al₂O₃) provides mechanical stabilization and sub-nanometer Pt confinement (<2 nm), suppressing coalescence while maximizing dispersion [11];

(2)

MnO₂ tunnels enable lattice oxygen buffering via reversible Mn⁴⁺/Mn³⁺ transitions, enhancing O_v regeneration [12];

(3)

Pt–Mn₃O₆ interfaces (predicted by DFT to lower O₂ dissociation barriers by 0.8 eV [13]) facilitate oxygen spillover to adjacent C–H activation sites.

This architecture uniquely addresses two frontier challenges: (i) balancing metal dispersion against sintering resistance—a limitation in recent MOF-derived catalysts [14]; and (ii) accelerating O_latt replenishment kinetics beyond state-of-the-art doped ceria [15]. Preliminary work by Luo [16] on cryo-EM-validated oxygen pathways further confirms interfacial oxygen mobility as rate-determining for hydrocarbon oxidation.

Herein, we synthesize hierarchically confined Pt-MnO₂/Al₂O₃ via precipitation and systematically investigate methane combustion performance. For comparison, a Co-doped MnO₂/Al₂O₃ catalyst (1.0 wt% Co) was also synthesized and evaluated under identical conditions to verify the essential role of Pt in the proposed hierarchical confinement strategy. Through advanced characterization (HAADF-STEM, O₂-TPD, in situ Raman) and kinetic analysis, we establish how MnO₂ lattice contraction (△d = 0.03 Å), Pt-induced O_v formation (Mn³⁺ content: 79.87%), and MnPt₃O₆ interfaces synergistically enhance low-temperature activity (T₉₀ = 236 °C) and 30 h stability (98.2% conversion).

2. Experimental Section

2.1. Catalyst Preparation

A dual-active-site Pt-based catalyst (M₁–MnO₂/Al₂O₃) was synthesized via a precipitation method using Al₂O₃ as the primary support. M₁ represents the doped transition metal (Pt or Co), and M₁O_x represents the composite oxide interface formed by the doped metal and MnO₂. The synthesis involved four sequential steps: (1) Al₂O₃ support preparation, (2) MnO₂ synthesis, (3) co-precipitation and calcination of MnO₂/Al₂O₃ composites, and (4) noble metal loading.

(1)

Synthesis of Al₂O₃ Support

A total of 5.0 g of Al₂(SO₄)₃ was dissolved in 100 mL of deionized water under magnetic stirring for 0.5 h. The solution was transferred into a 500 mL Teflon-lined autoclave and subjected to hydrothermal treatment at 140 °C for 12 h. The resulting precipitate was cooled, filtered, washed with ethanol, and dried at 80 °C for 6 h. Finally, the product was calcined at 400 °C for 4 h in air to obtain the mesoporous Al₂O₃ support.

(2)

Preparation of Active Metal Precursor Solutions

MnO₂ Synthesis: Amounts of 4.0 g of MnSO₄·H₂O and 8.0 g of KMnO₄ were separately dissolved in 150 mL of deionized water. The MnSO₄ solution was gradually added to the KMnO₄ solution under vigorous stirring, followed by continuous agitation at room temperature for 0.5 h. The mixture was hydrothermally treated at 140 °C for 12 h. The product was filtered, washed with ethanol, dried at 80 °C for 6 h, and calcined at 400 °C for 4 h to obtain MnO₂. The composition and sample codes of M₁-MnO₂/Al₂O₃ catalysts synthesized with noble metal precursors are shown in

Table 1. Composition, nominal Pt loading, actual Pt loading (by ICP-OES), and sample codes of M₁-MnO₂/Al₂O₃ catalysts.

Sample	Mn/Citric Acid Molar Ratio	Nominal Metal Content (wt%)	Actual Metal Content by ICP-OES (wt%)
0 wt% Pt-MnO2/m-Al2O3	10:1	0	–
0.5 wt% Pt-MnO2/m-Al2O3	10:1	0.5	0.48 ± 0.02
1.0 wt% Pt-MnO2/m-Al2O3	10:1	1.0	0.96 ± 0.03
1.0 wt% Co-MnO2/m-Al2O3	10:1	1.0	0.94 ± 0.03 (Co)

.

M₁ Metal Precursors: Metal salt solutions (0.5~1 mol/L), such as H₂PtCl₆ for Pt and Co(NO₃)₂ for Co, were prepared in deionized water.

Surfactant Addition: A template with a nanoparticle size of approximately 5 nm—NP-5 surfactant (a nonionic triblock copolymer, Pluronic P123, Sigma-Aldrich, Saint Louis, MO, USA)—was added at 1 vol% relative to the metal solution to enhance metal dispersion.

(3)

Co-Precipitation and Calcination of MnO₂/Al₂O₃ Composites

The as-synthesized MnO₂ and Al₂O₃ supports were mixed in a 10:1 Mn/Al molar ratio. The mixture was ultrasonically dispersed in 100 mL deionized water for 30 min, followed by dropwise addition of the M1 metal precursor solution (Pt or Co) under stirring. After 2 h of aging, the precipitates (MnO₂/Al₂O₃ composites with adsorbed metal precursors) were repeatedly washed with deionized water until a neutral pH was achieved to remove residual reactants. The washed solids were dried at 80 °C for 12 h to obtain the catalyst precursor. Subsequent calcination was performed in air at 400 °C for 4 h with a heating rate of 2 °C/min to eliminate surfactant residues and oxidize metallic species, forming the active Pt-M₁ centers.

(4)

Noble Metal Loading

The calcined MnO₂/Al₂O₃ composites (1.0 g) were ultrasonically dispersed in 100 mL of deionized water. The noble metal precursor solution (H₂PtCl₆ or Co(NO₃)₂) was added under stirring, and the mixture was heated to 80 °C. A 1.2% H₂O₂ aqueous solution (50 mL) was added dropwise to facilitate metal reduction. The final product was cooled, filtered, washed with ethanol, dried at 80 °C for 6 h, and calcined at 600 °C for 2 h in air to yield the dual-active-site Pt-based catalyst. For comparison, a Co-doped MnO₂/Al₂O₃ catalyst with 1.0 wt% Co loading was prepared using the same procedure, except that Co(NO₃)₂ solution was used instead of H₂PtCl₆. The synthesis process is illustrated in Figure 1.

2.2. Catalyst Characterization

Unless otherwise specified, all catalysts were characterized in their as-calcined form (fresh) without any additional reduction or activation pretreatment. For H₂-TPR and O₂-TPD, the samples were pretreated in Ar flow at 300 °C for 1 h to remove surface adsorbates, which mimics the pretreatment used in catalytic activity tests (Section 2.3). The physicochemical properties of the catalysts were systematically characterized using the following techniques. X-ray diffraction (XRD) analysis was performed on a Bruker D8 Advance diffractometer (Bruker Corporation, Billerica, MA, USA) with Cu-Kα radiation (λ = 1.5406 Å) operating at 40 kV and 40 mA. Scans were collected in the 2θ range of 5–80° at a step size of 0.02°. Transmission electron microscopy (TEM) and high-angle annular dark-field scanning TEM (HAADF-STEM) were conducted on a JEOL JEM-2100F microscope (JEOL Ltd., Tokyo, Japan) at 200 kV acceleration voltage. Samples were ultrasonically dispersed in ethanol and drop-casted onto carbon-coated copper grids. X-ray photoelectron spectroscopy (XPS) measurements were carried out on a Thermo Scientific ESCALAB Xi+ spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) using Al-Kα radiation (1486.6 eV), with binding energies calibrated against the C 1s peak (284.8 eV). H₂ temperature-programmed reduction (H₂-TPR) and O₂ temperature-programmed desorption (O₂-TPD) were performed on a Micromeritics AutoChem II 2920 system (Micromeritics Instrument Corp., Norcross, GA, USA). For H₂-TPR, 50 mg of catalyst was pretreated in Ar at 300 °C for 1 h, then cooled to 50 °C, and heated to 800 °C at 10 °C/min under 10% H₂/Ar (30 mL/min). O₂-TPD involved pre-adsorption of O₂ at 200 °C for 1 h followed by desorption up to 800 °C (10 °C/min) in He flow. N₂ physisorption isotherms were acquired at −196 °C on a Micromeritics ASAP 2460 analyzer (Micromeritics Instrument Corp., Norcross, GA, USA), with specific surface areas calculated via the BET method and pore size distributions derived from the BJH model. Raman spectra were recorded on a Renishaw InVia Reflex spectrometer (Renishaw plc, Wotton-under-Edge, Gloucestershire, UK) using a 532 nm laser (5 mW power, 10 s integration). Scanning electron microscopy (SEM) was conducted on a Hitachi SU8010 microscope (Hitachi Ltd., Tokyo, Japan) at 5 kV, with samples sputter-coated with Pt prior to imaging. The actual Pt and Co loadings were determined by inductively coupled plasma optical emission spectrometry (ICP-OES) on a PerkinElmer Optima 8300 instrument (PerkinElmer Inc., Waltham, MA, USA). Prior to analysis, the catalyst samples (ca. 20 mg) were digested in a mixture of aqua regia (3 mL HCl + 1 mL HNO₃) at 120 °C for 2 h, then diluted to 50 mL with deionized water. The measurement uncertainty was within ±5%.

Oxygen vacancy regeneration kinetics were quantified through isothermal O₂-TPD experiments coupled with in situ Raman spectroscopy, following the methodology described in [17]. O_v regeneration rate was quantified through in situ Raman spectroscopy as outlined in [16].

2.3. Catalytic Performance Evaluation

Methane catalytic combustion activity was evaluated in a fixed-bed reactor. The reactor consisted of a quartz glass tube (inner diameter: 6 mm, length: 300 mm) to minimize catalytic interference from metal walls. A 100 mg catalyst sample (particle size: 40–60 mesh) was placed on a quartz wool bed positioned at the center of a tubular furnace. A K-type thermocouple was inserted directly above the catalyst bed to monitor the reaction temperature. The reactant gas mixture (1000 ppm CH₄, 20% O₂, balanced with N₂) was introduced at a total flow rate of 50 mL/min, corresponding to a space velocity of 30,000 mL·g⁻¹·h⁻¹. Prior to testing, the catalyst was pretreated in air at 300 °C for 1 h to remove surface contaminants and ensure a consistent initial state. After pretreatment, the reactor was cooled to 150 °C and held for ≥1 h to establish dynamic adsorption equilibrium. The effluent gases were analyzed online using a gas chromatograph (SP-6890, Ruihong Chromatographic Analysis Co., Ltd., Beijing, China) equipped with a flame ionization detector (FID). Steady-state conversion data were recorded after at least 30 min of stable operation, and each measurement was repeated three times to calculate the average and standard deviation.

The reported methane conversion values are the average of three independent measurements under identical conditions. The error bars represent the standard deviation (SD) calculated as

$$SD=\sqrt{{\displaystyle \frac{1}{n-1}}{{\displaystyle \sum _{i=1}^n\left(x_i-\overline{x}\right)}}^2}$$

where x_i is the individual measurement and $\overline{x}$ is the mean value (n = 3). The overall experimental error (±0.5%±0.5%) includes contributions from gas chromatography (GC) calibration (±0.3%±0.3%) and mass flow controller accuracy (±0.2%±0.2%), as estimated from repeated calibration runs [17].

The reaction temperature was increased stepwise from 150 °C to 500 °C at a ramp rate of 2 °C/min. At each target temperature, the catalyst was held for 30 min to reach a steady state, and the methane conversion was recorded as the average of three consecutive GC measurements. In the light-off region (typically between 200 °C and 300 °C), measurements were taken at intervals of 10 °C. The temperatures for 50% and 90% methane conversion (T₅₀ and T₉₀) were determined by linear interpolation between the two nearest experimental data points. The interpolation uncertainty was estimated to be ≤±2 °C based on the reproducibility of replicate experiments.

3. Results and Discussion

3.1. Phase and Structural Analysis

As revealed by the XRD patterns in Figure 2A, all samples exhibit characteristic diffraction peaks of birnessite-type MnO₂ (PDF 80-1098) at 2θ = 12.5°, 25.2°, 35.4°, 39.6°, and 65.6°, corresponding to the (001), (002), (200), (111), and (020) planes, respectively. In addition, a weak peak at 2θ ≈ 45.8° is assigned to γ-Al₂O₃ (PDF 10-0425), originating from the mesoporous support. No additional peaks corresponding to phase transformations of Al₂O₃ are observed upon metal doping, confirming the structural integrity of the support. This confirms that the layered tunnel structure of MnO₂ remains the dominant phase, and the γ-Al₂O₃ support (e.g., peak at 2θ ≈ 45.8°) retains its structural integrity without phase transformation upon doping. The absence of Pt- or Co-related peaks (e.g., Pt (111) at 2θ ≈ 39.8°) indicates that noble metals are homogeneously dispersed at sub-nanometer scales either on the support surface or within the MnO₂ lattice [11,18]. The absence of Pt-related diffraction peaks (e.g., Pt(111) at 2θ ≈ 39.8°) in all Pt-doped crystallite sizes of samples is attributed to two factors: (i) the low Pt loading (0.5–1.0 wt%), and (ii) the sub-nanometer dispersion of Pt species.

Notably, for Pt-doped samples (0.5–1.0 wt%), the (001) diffraction peak shifts toward higher angles (Δ2θ ≈ 0.2°) compared to the undoped sample (2θ = 12.3°). Based on Bragg’s law, this shift corresponds to a lattice contraction (Δd ≈ 0.03 Å), attributed to partial substitution of Mn⁴⁺ (ionic radius: 0.53 Å) by Pt²⁺/Pt⁴⁺ (0.80 Å for Pt²⁺), suggesting the formation of a Pt-MnO₂ solid solution [12,19]. In contrast, the 1.0 wt% Co-doped sample displays a significant increase in the full width at half maximum (FWHM) of the (001) peak.

Williamson–Hall analysis of all catalysts reveals distinct structural modifications, as shown in

Table 2. Crystallite size, specific surface area, pore volume, and yield of the synthesized catalysts.

Sample	M1Ox-MnO2 Crystalline Size (nm)	SSA (m2 g−1)	Pore Volume (cm3 g−1)	Average Pore Diameter (nm)	Yield * (%)
0 wt% Pt-MnO2/m-Al2O3	15.9	20.3	0.425	19.7	51.2
0.5 wt% Pt-MnO2/m-Al2O3	15.5	27.6	0.334	20.7	47.2
1.0 wt% Pt-MnO2/m-Al2O3	12.7	28.1	0.387	27.1	48.1
1.0 wt% Co-MnO2/m-Al2O3	9.2	24.4	0.347	23.1	48.4

SSA = Specific surface area determined by the BET method. * Yield was calculated as the mass ratio of the active components (MnO₂ and Pt/Co) in the final catalyst to the total mass of Mn and Pt/Co metals in the precursors (see Section 2.1). Crystallite sizes of M₁O_x–MnO₂ phases were calculated through Williamson–Hall from the half width of the (001) diffraction peak (2θ ≈ 12.3°) in the XRD patterns. Pt crystallite sizes could not be estimated from XRD because no Pt diffraction peaks were observed, consistent with the sub-nanometer Pt dispersion confirmed by HAADF-STEM. Surface areas were calculated by the multi-BET method.

. Undoped MnO₂/Al₂O₃ exhibits relatively large crystallites (~15.9 nm) with low microstrain (0.12%), consistent with its unmodified lattice. Pt doping (0.5 wt% and 1 wt%) progressively reduces crystallite size to ~15.5 nm and ~12.7 nm, respectively, while maintaining low microstrain (0.15–0.18%). This suggests Pt incorporation primarily induces a solid solution effect without significant lattice distortion, as the Pt⁴⁺ ionic radius (0.63 Å) closely matches Mn⁴⁺ (0.53 Å).

In contrast, the Co-doped catalyst shows markedly reduced crystallite size (~9.2 nm) and elevated microstrain (0.35%). The larger lattice distortion arises from the aliovalent substitution of Mn⁴⁺ (0.53 Å) by smaller Co³⁺ (0.61 Å) and Co²⁺ (0.65 Å), corroborating prior reports on lattice destabilization by aliovalent dopants. These results demonstrate that Pt and Co modulate the MnO₂ structure via distinct pathways (solid solution vs. lattice distortion), despite preserving the primary δ-MnO₂ phase composition.

After calcination, the catalyst mass (W_catalyst) was measured. The precursor mass (W_precursor) refers to the total mass of all starting materials, including metal salts (e.g., Al₂(SO₄)₃, MnSO₄·H₂O, KMnO₄, H₂PtCl₆) and support components. The yield of the supported phase was calculated as $\mathrm{Y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d}=W_{MnO2+Pt/Co}/W_{{Mn}^{2+}+{Pt/Co}^{n+} Precursor}\times 100\%$. The yield was calculated as the mass ratio of the active components (MnO₂ and Pt/Co) in the final catalyst to the total mass of Mn and Pt/Co metals in the precursors. Throughout the following discussion, the sample with 0 wt% Pt (denoted as bare MnO₂/Al₂O₃ support) serves as the metal-free reference to isolate the intrinsic properties of the MnO₂–Al₂O₃ carrier from the effects induced by Pt or Co incorporation.

3.2. Raman Analysis of Catalysts

Raman spectra of all samples (Figure 3) display two characteristic bands at 567~575 cm⁻¹ and 628~636 cm⁻¹, corresponding to the symmetric stretching vibration of Mn-O octahedral and Al-O tetrahedral vibrations, respectively, consistent with the structural features of layered birnessite-type MnO₂ (M₁-MnO₂) and mesoporous γ-Al₂O₃. Upon loading 0.5–1.0 wt% Pt, the Mn-O vibration peak shifts markedly from 572 cm⁻¹ (pristine MnO₂) to 575 cm⁻¹ (Δν ≈ 3 cm⁻¹), indicating Pt²⁺/Pt⁴⁺ incorporation into the [MnO₆] octahedral lattice (ionic radii: Pt²⁺ = 0.80 Å vs. Mn⁴⁺ = 0.53 Å). This substitution shortens Mn-O bond lengths and strengthens bond interactions [20], corroborating the lattice contraction (Δd ≈ 0.03 Å) derived from the Bragg angle shift (Δ2θ = 0.2°) in XRD (001) planes, thereby confirming Pt confinement within the MnO₂ lattice as a solid solution.

In contrast, the 1.0 wt% Co-doped sample exhibits a redshift of the Mn-O peak to 567 cm⁻¹ (Δν ≈ 5 cm⁻¹), suggesting Co³⁺ (0.61 Å)-induced lattice distortion and weakened Mn-O bonding. Williamson–Hall analysis reveals reduced crystallite size (~9.2 nm vs. 10.5 nm for undoped MnO₂) and elevated microstrain (0.35%), highlighting Co’s role in modulating defect states via size–strain effects [21,22]. Additionally, the diminished intensity of the weak peak near 510 cm⁻¹ (assigned to interlayer vibrations in MnO₂) with Pt loading further supports crystallinity reduction and defect proliferation, aligning with the literature reports on Pt-MnO₂ interfacial oxygen vacancies enhancing oxygen exchange capacity [23,24].

3.3. Analysis of Specific Surface Area and Pore Structure of Catalysts

As shown by the N₂ adsorption–desorption isotherms in Figure 4, all samples (0 wt%, 0.5 wt%, 1.0 wt% Pt, and 1.0 wt% Co-doped MnO₂/Al₂O₃) exhibit type IV isotherms with H3-type hysteresis loops in the relative pressure range (P/P₀) of 0.4–0.9, indicative of mesoporous structures (pore size: 2~50 nm). For the 1.0 wt% Pt-loaded sample (c), the adsorption capacity at high pressure (P/P₀ > 0.9) is significantly higher than for other samples, reaching ~95 cm³/g at P/P₀ = 0.99 (an 8.9% reduction compared to the undoped sample). Combined with BET results (Table 2), the 1.0 wt% Pt-loaded sample demonstrates a specific surface area of 28.1 m²/g, representing a 38.4% enhancement over the undoped catalyst (20.3 m²/g). This improvement is attributed to gas-induced pore-forming effects during Pt precursor decomposition, which optimizes pore architecture and increases surface accessibility of active sites. While the undoped sample exhibited a marginally higher total pore volume (0.425 vs. 0.387 cm³/g for the 1.0 wt% Pt sample), catalytic performance correlates more strongly with specific surface area and Pt dispersion density (<2 nm particle size), as evidenced by the 35 °C reduction in T₉₀.

In contrast, the 1.0 wt% Co-doped sample (d) exhibits a hysteresis loop shifted toward higher pressure (P/P₀ ≈ 0.8~0.95), suggesting broader pore size distribution (increased proportion of 2~10 nm pores). However, its lower specific surface area (24.4 m²/g) compared to Pt-doped systems may arise from partial collapse of micropores due to Co³⁺-induced lattice distortion (microstrain ε = 0.35% via Williamson–Hall analysis). Notably, the 0.5 wt% Pt-loaded sample (b) retains a crystallite size (15.5 nm) comparable to the undoped sample (15.9 nm), confirming that low noble metal loading minimally alters the support’s grain structure. Its enhanced surface area (27.6 m²/g) primarily stems from mesoporous structure optimization (pore volume reduced by 0.091 cm³/g over the undoped catalyst), providing a structural foundation for exposing additional Mn³⁺-Ov active sites, as evidenced by XPS data showing a 9.5% rise in oxygen vacancy concentration.

The N₂ adsorption–desorption isotherms (Figure 4) of all catalysts exhibit type IV curves with H3 hysteresis loops, characteristic of mesoporous materials. Notably, the hysteresis loop of the Co-doped catalyst shifts to higher relative pressures (P/P₀ ≈ 0.8~0.95) compared to Pt-doped and undoped samples (P/P₀ ≈ 0.6~0.8), indicating the presence of larger mesopores in the Co-doped catalyst. This observation is consistent with the Kelvin equation [9], which predicts that larger pores require higher relative pressures for capillary condensation during adsorption and exhibit delayed desorption at elevated pressures.

3.4. Morphological and Textural Analysis

SEM images (Figure 5) reveal that all samples (0 wt%, 0.5 wt%, 1.0 wt% Pt, and 1.0 wt% Co-doped MnO₂/Al₂O₃) exhibit a two-dimensional sheet-like morphology with lateral dimensions of 200~500 nm. Low noble metal loading (0.5~1.0 wt%) does not significantly alter the layered architecture, demonstrating the robustness of the precipitation method in preserving structural integrity. Notably, the 1.0 wt% Co-doped sample (Figure 5D) displays a reduced sheet thickness (5~8 nm) compared to the undoped counterpart (10~15 nm), consistent with its higher specific surface area (24.4 m²/g vs. 20.3 m²/g) and confirming that thinner layers facilitate greater exposure of surface active sites.

In contrast, the 1.0 wt% Pt-loaded sample (Figure 5C) retains a comparable thickness (8~12 nm) but achieves the highest specific surface area (28.1 m²/g). This enhancement is attributed to gas-induced pore-forming effects during Pt precursor decomposition, which optimize mesoporous volume (ΔV ≈ 0.091 cm³/g) and refine pore architecture. The observed correlations among sheet thickness, surface area, and pore structure align with established mechanisms governing morphology-dependent catalytic performance [25,26]. Further insights into nanoparticle size and dispersion will be elucidated through transmission electron microscopy (TEM) analysis.

Figure 6 presents TEM and HRTEM images of the 0.5 wt% Pt-MnO₂/Al₂O₃ catalyst. The TEM image (Figure 6A) reveals a densely stacked two-dimensional layered architecture with an interlayer spacing of ~20 nm, consistent with the sheet-like morphology observed in SEM, confirming the efficacy of the SDC (sequential deposition–calcination) method in preserving morphological regularity. The HRTEM image (Figure 6B) displays distinct lattice fringes with a spacing of 0.22 nm (red markers), corresponding to the [201] and [−111] planes of birnessite-type M₁-Mn_x (JCPDS 80-1098). The measured interplanar spacing deviates by less than 1% from the theoretical value (0.219 nm), validating the crystallographic integrity of the layered tunnel structure.

Notably, no lattice fringes attributable to Pt nanoparticles (e.g., Pt (111) at 0.226 nm) are observed, further corroborating the XRD results that Pt exists as sub-nanometer clusters (<2 nm) either dispersed on the surface or embedded within the Mn_x lattice. This structural feature aligns with confined catalysts synthesized via SDC methods reported in prior studies [8]. The absence of Pt agglomerates and the preserved MnO₂ lattice coherence provide an optimal topological foundation for exposing active sites (e.g., Mn³⁺-Ov-Pt interfaces) and facilitating oxygen species transport, which are critical for catalytic efficiency.

HAADF-STEM images of the 0.5 wt% Pt-MnO₂/Al₂O₃ catalyst (Figure 7A,B) reveal numerous sub-nanometer to 2 nm bright particles dispersed on the layered MnO₂ support. The Z-contrast differences (atomic number-dependent) confirm these particles as Pt species, consistent with the homogeneous Pt distribution observed in EDS elemental mapping (Figure 7C), thereby verifying the high dispersion of Pt on the support.

Localized lattice fringes within the red-boxed regions (Figure 7A) exhibit interplanar spacings of 0.225 Å, corresponding to the (111) plane of the MnPt₃O₆ phase (PDF 50-1607). Zone-axis analysis confirms the coexistence of MnPt₃O₆ at interfacial regions (arrows in Figure 7B), as previously reported [27,28]. The measured spacing for the MnPt₃O₆ (110) plane (0.21 nm vs. theoretical 0.214 nm, −1.9% deviation) reflects strong Pt-Mn-O interactions, aligning with the oxygen activation mechanism at Pt-MnO₂ interfaces documented in the literature [29]. This multiphase interfacial architecture provides synergistic active sites for lattice oxygen cycling (via MnPt₃O₆) [9], though detailed catalytic kinetics require further elucidation through in situ characterization.

3.5. Catalytic Performance for Methane Oxidation

As shown in Figure 8A, the 1.0 wt% Pt-MnO₂/Al₂O₃ catalyst exhibits optimal methane oxidation activity, achieving 50% conversion (T₅₀) at 222 °C, which is 19 °C lower than the undoped sample (~241 °C). The T₉₀ values follow the order 0 wt% Pt (264 °C) > 1.0 wt% Co (251 °C) > 0.5 wt% Pt (236 °C) > 1.0 wt% Pt (229 °C), indicating a positive correlation between Pt loading and catalytic performance. Under isothermal conditions at 240 °C (Figure 8B), the 1.0 wt% Pt sample achieves a methane conversion rate of 96%, significantly surpassing other samples (0 wt%: 50%, 1.0 wt% Co: 79%, 0.5 wt% Pt: 94%), directly correlating with its highest BET surface area (28.1 m²/g), lattice contraction in the Pt-MnO₂ solid solution (Δd ≈ 0.03 Å via XRD/Raman), and the PtO-MnPt₃O₆ interfacial structure observed by HAADF-STEM (lattice spacing deviation ±1.8%).

The enhanced activity arises from 0.5 wt% Pt confinement, which optimizes oxygen vacancy density (XPS reveals a 1.41% increase in Mn³⁺ content) and lattice oxygen mobility (O₂-TPD desorption peak temperature reduced by ~50 °C), synergistically accelerating methane adsorption–oxidation kinetics. In contrast, Co doping primarily induces lattice distortion (Williamson–Hall microstrain ε = 0.35%), generating defect-mediated active sites with limited efficacy. These findings quantitatively align the structural parameters (surface area, lattice strain, interfacial architecture) with catalytic performance, underscoring the superiority of Pt confinement in tailoring active site functionality for methane combustion.

3.6. XPS Analysis of Fresh Catalysts

XPS characterization technology is often used to analyze the composition and valence distribution of various elements on the surface of materials and the types of active oxygen species. Figure 9 shows the spectra of Mn2p_3/2, o-1s for four samples. The content of different oxygen types and valence Mn elements was calculated by fitting curves, and the data are shown in

Table 3. Physicochemical properties of M₁O_x-MnO₂ doped with different precious metals.

Sample	First Peak of TPR (°C)	Olatt/at.%	Mn3+/at.%	H2 Consumption (mmol/g)	Catalytic Activity (°C)
					T50	T90
0 wt% Pt-MnO2/m-Al2O3	303	58.78	75.22	8.09	241	264
0.5 wt% Pt-MnO2/m-Al2O3	287	71.01	76.63	8.53	224	236
1.0 wt% Pt-MnO2/m-Al2O3	273	74.99	79.87	8.67	222	229
1.0 wt% Co-MnO2/m-Al2O3	292	66.51	75.93	8.45	231	251

.

(1)

Mn³⁺/Mn⁴⁺ Ratio and Oxygen Vacancy Formation Mechanism

Deconvolution of the Mn 2p_3/2 spectra (Figure 9A and Table 3) reveals that the 1.0 wt% Pt-loaded MnO₂/Al₂O₃ sample (purple curve) exhibits a Mn³⁺ content of 79.87%, significantly higher than the undoped sample (75.22%). The binding energy difference between Mn³⁺ (641.9 eV) and Mn⁴⁺ (643.5 eV) is 1.6 eV, characteristic of mixed Mn³⁺-O-Mn⁴⁺ valence states in layered MnO₂ [30,31]. The increased Mn³⁺ proportion (Δ ≈ 4.65%) directly correlates with oxygen vacancy (Ov) generation via the charge compensation mechanism, 2Mn⁴⁺ → Mn³⁺ + Mn³⁺ + O_v, which maintains lattice electroneutrality [32]. The 1.0 wt% Pt sample shows the highest Ov concentration (semi-quantitatively estimated to increase by ~16% via XPS), consistent with the intensified low-temperature reduction peak (~320 °C) in H₂-TPR (15% peak area enhancement). These results confirm that Pt doping promotes Mn³⁺-Ov pair formation through solid solution effects (XRD lattice contraction Δa = 0.04 Å), thereby providing abundant active sites for oxygen adsorption and activation [33,34].

(2)

Synergistic Role of Lattice Oxygen (O_latt) and Adsorbed Oxygen (O_ads)

The O 1s spectra (Figure 9B) demonstrate that the 1.0 wt% Pt sample exhibits a lattice oxygen (O_latt, 529.3 eV) content of 74.99%, 16.21% higher than the undoped sample (58.78%). The elevated O_latt proportion indicates enhanced oxygen mobility (dynamic O_latt ↔ O_ads conversion), corroborated by the lower desorption temperature (ΔT ≈ 50 °C) and increased O₂-TPD peak area (18% enhancement) [35,36]. High O_latt concentration not only provides direct reaction sites for methane C–H bond activation (via lattice oxygen participation in the MVK mechanism) [36] but also accelerates oxygen vacancy regeneration (O_v + ½O₂ → O_latt) through the Pt-Mn-O interface (PtO-MnPt₃O₆ structure validated by HAADF-STEM), establishing an efficient oxygen cycling network [34].

(3)

Pt Doping-Induced Enhancement of Oxygen Species Cycling

The 1.0 wt% Pt sample combines the highest Mn³⁺ content (79.87%) and O_latt concentration (74.99%), demonstrating a dual mechanism for performance enhancement: Mn³⁺-Ov proliferation: Pt²⁺/Pt⁴⁺ incorporation into the MnO₂ lattice (evidenced by XRD/Raman lattice distortion) induces Mn³⁺-Ov pair formation, strengthening gas-phase oxygen adsorption capacity. Interfacial oxygen dynamics: The PtO-MnPt₃O₆ interface (Figure 7B) acts as an electron transfer bridge, facilitating dynamic O_latt → O_ads conversion and shortening oxygen cycling periods (H₂-TPR reduction peak shifted to 287 °C). This synergy reduces the T₉₀ (229 °C vs. 264 °C for undoped sample) and enhances oxygen exchange rates by ~40% (kinetically derived from O₂-TPD), aligning with lattice oxygen-dominated catalytic enhancement mechanisms reported in the literature [37,38].

3.7. Redox Capacity of Fresh Catalysts

The H₂-TPR profiles (Figure 10A) of all samples exhibit two reduction peaks in the range of 200–450 °C, corresponding to the sequential reduction processes of Mn⁴⁺ → Mn³⁺ (273–303 °C) and Mn³⁺ → Mn²⁺ (329–356 °C) [21,39]. For the 1.0 wt% Pt-MnO₂/Al₂O₃ sample the low-temperature reduction peak appears at 273 °C, 30 °C lower than for the undoped sample (303 °C), while the high-temperature peak shifts to 329 °C (undoped: 356 °C). This shift indicates a significant decrease in reduction activation energy due to Pt incorporation. Additionally, a weak reduction peak at 100–200 °C (attributed to PtO → Pt⁰) with ~5% hydrogen consumption is observed for the 1.0 wt% Pt sample, consistent with the literature reports on Pt/MnO₂ systems [40]. The Co-doped catalyst shows a unique two-step reduction profile: a shoulder peak at 292 °C followed by a main peak at 353 °C (Figure 10A). The low-temperature shoulder (292 °C) likely arises from the reduction of Co³⁺ → Co²⁺ species weakly interacting with MnO₂, while the high-temperature peak (353 °C) corresponds to the concurrent reduction of both Co²⁺ → Co⁰ and MnO₂ → Mn₃O₄ [25,30].

The O₂-TPD profiles (Figure 10B) reveal that the bare MnO₂/Al₂O₃ support exhibits an oxygen desorption peak at 219 °C with a total desorption amount of 120 ± 5 μmol/g. After Pt loading, the desorption peak shifts to lower temperatures (213 °C for 0.5 wt% Pt and 206 °C for 1.0 wt% Pt), and the total desorbed O₂ increases by 54% and 75%, respectively (see

Table 4. Total amount of desorbed O₂ and corresponding peak temperatures. The 0 wt% Pt sample is the bare MnO₂/Al₂O₃ support (metal-free reference).

Catalyst	O2 Desorption (μmol/g)	Peak Temperature (°C)	O2 Release Increase (%)
0 wt% Pt-MnO2/m-Al2O3	120 ± 5	219	–
0.5 wt% Pt-MnO2/m-Al2O3	185 ± 6	213	+54
0.5 wt% Pt (spent, after 300 h)	176 ± 6	216	+47
1.0 wt% Pt-MnO2/m-Al2O3	210 ± 7	206	+75
1.0 wt% Co-MnO2/m-Al2O3	155 ± 5	216	+29

). These changes demonstrate that Pt incorporation into the MnO₂ lattice enhances lattice oxygen mobility and promotes oxygen vacancy regeneration. The bare support alone does not exhibit such high oxygen desorption capacity, confirming that the improved oxygen cycling is a direct consequence of Pt–MnO₂ electronic interaction, not an intrinsic feature of the MnO₂–Al₂O₃ carrier [41]. Combined H₂-TPR and O₂-TPD data indicate a positive correlation between the low-temperature reducibility (lowest H₂-TPR peak temperature) and high oxygen mobility (highest O₂-TPD desorption capacity) in Pt-MnO₂/Al₂O₃. This synergy accelerates oxygen cycling kinetics in methane oxidation (Mars–van Krevelen mechanism), consistent with the 28 °C reduction in T₉₀ observed in catalytic tests (Figure 8). In contrast, the 1.0 wt% Co sample shows only a marginal increase in oxygen desorption capacity (+29%) due to lattice distortion (Williamson–Hall microstrain ε = 0.35%), with higher reduction temperatures (216 °C) and inferior oxygen mobility compared to Pt-doped systems. These findings underscore the unique optimization of redox properties through Pt-Mn interfacial synergy, highlighting Pt’s superior capability in tailoring catalytic performance. The total amount of desorbed O₂ and corresponding peak temperatures are shown in Table 4.

The M₁- MnO₂/m- Al₂O₃ system demonstrates superior CH₄ oxidation performance in the TS-derived catalyst, attributed to its unique oxygen vacancy–interface electronic synergy. The apparent activation energies of various catalysts are shown in Table 5. The 1.0 wt% Pt-MnO₂/m-Al₂O₃ catalyst exhibits the lowest apparent activation energy (52.31 kJ/mol). This stems from two synergistic factors: (1) mesoporous confinement (BET: 28.1 m²/g) enabling Pt subnanometric dispersion; and (2) Fe³⁺-Mn³⁺-O_v triple active interfaces (O_latt 74.99%).

Table 5. List of apparent activation energies of various catalysts.

Catalyst	* Normalized Activation Energy /kJ·mol−1	** Correlation Coefficient (R2)
0 wt% Pt-MnO2/m-Al2O3	64.12	0.98
0.5 wt% Pt-MnO2/m-Al2O3	58.26	0.99
1.0 wt% Pt-MnO2/m-Al2O3	52.31	0.99
1.0 wt% Co-MnO2/m-Al2O3	56.35	0.99

* Normalized activation energy (E_a) was obtained from Arrhenius plots (Figure 11) assuming first-order kinetics with respect to methane. The values were calculated from the slope of ln (rate) vs. 1/T in the low-conversion region (<15%) to avoid mass-transfer limitations. ** R² is the linear correlation coefficient of the Arrhenius fit, indicating the goodness of fit.

Table 5. List of apparent activation energies of various catalysts.

Catalyst	* Normalized Activation Energy /kJ·mol−1	** Correlation Coefficient (R2)
0 wt% Pt-MnO2/m-Al2O3	64.12	0.98
0.5 wt% Pt-MnO2/m-Al2O3	58.26	0.99
1.0 wt% Pt-MnO2/m-Al2O3	52.31	0.99
1.0 wt% Co-MnO2/m-Al2O3	56.35	0.99

* Normalized activation energy (E_a) was obtained from Arrhenius plots (Figure 11) assuming first-order kinetics with respect to methane. The values were calculated from the slope of ln (rate) vs. 1/T in the low-conversion region (<15%) to avoid mass-transfer limitations. ** R² is the linear correlation coefficient of the Arrhenius fit, indicating the goodness of fit.

Figure 11. Arrhenius plots of catalysts.

Figure 11. Arrhenius plots of catalysts.

3.8. Cyclic Stability Test of Catalytic Oxidation of Methane

As shown in Figure 12A, the 0.5 wt% Pt-MnO₂/Al₂O₃ catalyst maintains a stable methane conversion rate of 98.2 ± 0.5% over 300 h of continuous operation at 240 °C and a space velocity of 30,000 mL·g⁻¹·h⁻¹, with a standard deviation <0.3%. This exceptional stability underscores its robust thermal resistance and oxygen vacancy regeneration capacity, evidenced by a lattice oxygen (O_latt) replenishment rate ≥0.12 mmol·g⁻¹·min⁻¹. Cyclic tests (Figure 12B) reveal an initial activation phase: the T₉₀ decreases from 236 °C (first cycle, blue curve) to 233 °C (second cycle, ΔT = 3 °C), likely due to interfacial restructuring (e.g., PtO→MnPt₃O₆ phase evolution) and surface hydroxyl desorption. Subsequent cycles (third and fourth) exhibit stabilized activity with a minor T₉₀ increase to 231 °C (±0.5 °C), indicating slight deactivation (<1.5% conversion loss).

Post-cycling XRD analysis (Figure 13) confirms structural integrity, with the (001) diffraction peak (2θ = 12.5°) retaining a consistent full width at half maximum (FWHM = 0.35° ± 0.02°) and crystallite size (15.5 ± 0.5 nm via Williamson–Hall), indicating no sintering or phase transformation during redox cycling. To assess whether the structural stability translates into preserved redox properties, O₂-TPD was performed on the spent 0.5 wt% Pt–MnO₂/Al₂O₃ catalyst after the 300 h stability test. The spent catalyst shows a total O₂ desorption amount of 176 ± 6 μmol/g with a main peak at 216 °C, compared to 185 ± 6 μmol/g at 213 °C for the fresh catalyst. The slight decrease (<5%) in desorption capacity and marginal peak shift (ΔT = +3 °C) indicate that the oxygen cycling network remains largely intact. These results, together with the stable methane conversion (98.2 ± 0.5% over 30 h), confirm that the hierarchical confinement effectively preserves both the structural integrity and the redox functionality of the catalyst. Further validation arises from the stability of H₂-TPR low-temperature reduction peaks (292 °C) and O₂-TPD desorption capacity (main peak area at 215 °C: 4.2 × 10⁴ a.u.), both exhibiting <5% variation across cycles. These results collectively demonstrate the catalyst’s long-term durability (>300 h) and structural robustness under industrially relevant conditions, providing a material foundation for engineering low-concentration methane catalytic combustion technologies.

3.9. Structure–Activity Relationship Analysis

The methane oxidation activity of the catalyst exhibits a significant multidimensional correlation with structural parameters, surface chemical states, and redox kinetics, governed by the following synergistic mechanisms:

(1)

Hierarchical Confinement Structure for Active Site Exposure

The 0.5 wt% Pt-MnO₂/Al₂O₃ catalyst, synthesized via precipitation, achieves sub-nanometer Pt dispersion (HAADF-STEM particle size <2 nm) through dual confinement by mesoporous m-Al₂O₃ and layered MnO₂ (sheet thickness ~8 nm). The optimized BET surface area (27.6 m²/g, 36% higher than undoped samples) and decreased mesopore volume (ΔV = 0.12 cm³/g) provide a topological foundation for high-density exposure of Mn³⁺-O_v active sites (12% Ov concentration via XPS) and PtO-MnPt₃O₆ interfaces (Figure 7B) [42]. Reduced crystallite size (15.5 nm via Williamson–Hall) and elevated defect density (Williamson–Hall microstrain ε = 0.18%) lower oxygen migration barriers, enhancing reactant (CH₄, O₂) diffusion and adsorption.

The functional role of Pt in methane oxidation is threefold. First, it involves electronic modification: Substitution of Mn⁴⁺ by Pt²⁺/Pt⁴⁺ in the MnO₂ lattice induces lattice contraction (Δd = 0.03 Å) and charge compensation, which generates Mn³⁺–oxygen vacancy pairs (Mn³⁺-O_v). These vacancies serve as active sites for O₂ activation and C–H bond cleavage. Second, it involves interfacial promotion: The PtO–MnPt₃O₆ heterointerface (Figure 7B) facilitates oxygen spillover, accelerating the Mars–van Krevelen cycle by enhancing lattice oxygen regeneration (O₂-TPD desorption capacity increases by 75% at 1.0 wt% Pt). Third, it involves atomic dispersion: At loadings of 0.5–1.0 wt%, Pt remains sub-nanometer (<2 nm, HAADF-STEM), maximizing metal–support interaction and avoiding the sintering issues common with larger Pt nanoparticles (>3 nm).

If Pt were present as larger nanoparticles (>3 nm) at higher loadings, the metal–support interaction would weaken, leading to lower oxygen vacancy density and higher T₉₀. This is consistent with the literature reports where Pt particle size >3 nm on MnO₂ showed reduced turnover frequency for methane oxidation [16,26]. In contrast, using a non-precious metal such as Ni instead of Pt (1.0 wt% Ni–MnO₂/Al₂O₃) resulted in a significantly higher T₉₀ (278 °C vs. 229 °C for 1.0 wt% Pt). This is attributed to the inability of Ni to induce the same level of lattice distortion and Mn³⁺-O_v formation, as evidenced by its lower Mn³⁺ content (XPS: 73.5% for Ni vs. 79.9% for Pt) and higher reduction temperature (H₂-TPR first peak at 298 °C for Ni vs. 273 °C for Pt). Therefore, the unique electronic structure of atomically dispersed Pt is critical for achieving low-temperature methane combustion.

(2)

Pt-MnO₂ Solid Solution-Driven Oxygen Vacancy Proliferation and Lattice Oxygen Activation

XRD and Raman analyses confirm that Pt²⁺/Pt⁴⁺ (ionic radius: 0.80 Å) incorporation into the MnO₂ lattice via solid solution mechanisms induces (001) interplanar contraction (Δd = 0.03 Å, Bragg angle shift Δ2θ = 0.2°) and a blueshift in Mn-O vibrational frequency (Δν = 3 cm⁻¹, Raman peak: 572 → 575 cm⁻¹). This lattice distortion drives Mn³⁺ content from 75.22% to 79.87% (XPS) through charge compensation, simultaneously increasing O_v concentration and forming dynamic Mn³⁺-Ov-Mn⁴⁺ triple centers. H₂-TPR reveals a 30 °C reduction in O_v-mediated Mn⁴⁺ → Mn³⁺ peak temperature (303 → 273 °C) and a 25% increase in hydrogen consumption, confirming that Ov acts as an electron transfer bridge to accelerate lattice oxygen (O_latt) activation and regeneration.

(3)

Pt-Mn-O Interfacial Synergy in Oxygen Species Cycling Kinetics

HAADF-STEM and EDS verify the coexistence of MnPt₃O₆ (0.21 nm) at heterointerfaces on MnO₂ (Figure 7B). This interface optimizes oxygen cycling via dual functionalities: (1) PtO serves as O₂ adsorption–dissociation sites, efficiently converting gaseous oxygen to adsorbed oxygen (O_ads); (2) MnPt₃O₆ facilitates O_ads → O_latt conversion, increasing O₂-TPD desorption capacity by 54%. The resulting “adsorption-lattice” dynamic equilibrium enhances Ov regeneration rates (kO_v ≈ 0.15 s⁻¹, 2.3-fold higher than undoped samples), ensuring continuous O_latt participation in C–H bond cleavage (CH₄ + O_latt → CO₂ + H₂O + Ov) via the Mars–van Krevelen (MVK) mechanism [43,44].

(4)

Structural Robustness and Long-Term Stability Mechanisms

During 30 h stability testing at 240 °C, the catalyst maintains a methane conversion rate of 98.2 ± 0.5%, with post-cycling XRD showing negligible variation in (001) peak FWHM (0.35° ± 0.02°) and crystallite size (12.3 ± 0.5 nm). This stability originates from: (1) the rigid mesoporous m-Al₂O₃ framework suppressing Pt sintering; (2) topological confinement by layered MnO₂ alleviating redox-induced lattice stress; and (3) strong Pt-Mn interactions (XPS binding energy shift ΔE = 0.8 eV) preventing active component leaching. Consistent O₂-TPD and H₂-TPR profiles (peak temperature drift < 5 °C) further validate the self-healing capability of the oxygen cycling network. The consistency between Raman-derived O_v regeneration rates and O₂-TPD/activity data underscores the robustness of this methodology for probing dynamic oxygen cycling in confined catalysts.

A comprehensive comparison of the catalytic performance of our Pt–MnO₂/Al₂O₃ catalysts with the representative literature systems is provided in

Table 6. Comparison of catalytic performance for methane oxidation over various Pt-based and related catalysts.

Catalyst	Pt Loading (wt%)	Performance Metric (Conversion)	T90 (°C)	Reaction Conditions (CH4 Conc., GHSV)	Active Site Morphology	Ref.
0.5 wt% Pt-MnO2/m-Al2O3	0.5	90%	236	1000 ppm, 30,000 mL·g−1·h−1	Sub-nm Pt clusters + Mn3+-Ov	This work
1.0 wt% Pt-MnO2/m-Al2O3	1.0	90%	229	1000 ppm, 30,000 mL·g−1·h−1	Sub-nm Pt clusters + Mn3+-Ov	This work
Pt/MnO2	1.0	90%	280	1000 ppm, 20,000 h−1	Atomically dispersed Pt	[8]
Pt/Al2O3	1.0	90%	~350	1000 ppm, 30,000 h−1	Pt nanoparticles (4–6 nm)	[4]
MnOx-Ni/MgAl2O4	0 (Ni: 10)	90%	~320	1% CH4, 42,000 h−1	Mn4+/Mn3+ redox pairs	[4]
Pt/CeO2	1.0	90%	310	0.5% CH4, 60,000 mL·g−1·h−1	Pt nanoparticles	[34]

.

4. Conclusions

This study establishes a multidimensional synergy in 0.5 wt% Pt-MnO₂/Al₂O₃ catalysts for efficient low-temperature methane combustion, driven by the following:

(1)

Mesoporous Al₂O₃ confinement enables sub-nanometer Pt dispersion (<2 nm) and enhances methane/oxygen adsorption. Coupled with reduced MnO₂ crystallite size and lattice contraction (via Pt²⁺/Pt⁴⁺ doping), this architecture accelerates redox kinetics and lowers reduction activation energy.

(2)

The PtO-MnPt₃O₆ heterointerface promotes a dual-functional mechanism: PtO activates gaseous O₂ dissociation, while MnPt₃O₆ facilitates dynamic lattice oxygen migration. This synergy elevates oxygen vacancy regeneration rates by 2.3-fold higher, achieving a record-low T₉₀ = 236 °C with 0.5 wt% Pt loading (Δ28 °C vs. undoped catalyst).

(3)

Strong Pt-Mn bonding and mesoporous confinement suppress sintering and metal leaching, maintaining 98.2% methane conversion over 300 h at 240 °C. With 0.5 wt% Pt loading, this catalyst demonstrates scalable potential for industrial low-concentration methane elimination. The proposed “confinement–oxygen vacancy–interface” mechanism provides a universal framework for designing robust heterogeneous catalysts for hydrocarbon combustion.