In Situ Growth of Mn-Co₃O₄ on Mesoporous ZSM-5 Zeolite for Boosting Lean Methane Catalytic Oxidation

Abstract

The low-temperature oxidation of methane gas in coal mine exhaust gas is important for reducing the greenhouse effect and protecting the environment. Unfortunately, the carbon–hydrogen bonds in methane molecules are highly stable, requiring higher reaction temperatures to achieve effective catalytic oxidation. However, metal oxide-based catalysts face the problem of easy sintering and the deactivation of active components at high temperatures, which is an important challenge that catalysts need to overcome in practical applications. In this work, a series of Mn-Co₃O₄ active components were grown in situ on ZSM-5 zeolite with mesoporous pore structures treated with an alkaline solution via a hydrothermal synthesis method. Due to the presence of polyethylene glycol as a structure-directing agent, manganese can be uniformly doped into the Co₃O₄ lattice. The large specific surface area of ZSM-5 zeolite allows the active component Mn-Co₃O₄ to be uniformly dispersed, effectively preventing the sintering and growth of active component particles during the catalytic reaction process. It is worth mentioning that the Mn-Co₃O₄/meso-ZSM-5-6.67 catalyst has a methane conversion rate of up to 90% at a space velocity of 36,000 mL·g⁻¹·h⁻¹ and a reaction temperature of 363 °C. This is mainly due to the mesoporous ZSM-5 carrier with a high specific surface area, which is conducive to the adsorption and mass transfer of reaction molecules. The active component has an abundance of oxygen vacancies, which is conducive to the activation of reaction molecules and enhances its catalytic activity, which is even higher than that of noble metal-based catalysts. The new ideas for the preparation of metal oxide-based low-temperature methane oxidation catalysts proposed in this work are expected to provide new solutions for low-temperature methane oxidation reactions and promote technological progress in related fields.

1. Introduction

The gas that leaks from gas mines contains approximately 0.1% to 1% low-concentration methane. It has been estimated that these leaked methane gases account for approximately 70% of the total methane gas present in the atmosphere [1]. Methane contributes 21 times more to the greenhouse effect than the same volume of carbon dioxide. Its atmospheric lifespan is 12 ± 3 years [2], which poses a significant threat to the natural environment. The challenge of methane combustion lies in the activation of strong C-H bonds in methane and the subsequent combination of this step with activated oxygen. Methane has a stable tetrahedral structure with an intramolecular C-H bond energy of up to 415.48 kJ/mol [3], requiring higher temperatures during combustion. In order to reduce energy wastage during methane combustion and to improve the safety of the treatment process, researchers have started to study the factors and mechanisms influencing the combustion of methane at low concentrations in an attempt to achieve complete catalytic combustion of methane at lower temperatures (<600 °C) [4]. Among them, noble metal-based catalysts (such as Pd and Pt [5]) exhibit good catalytic activity in low-temperature methane catalytic combustion, but their large-scale preparation and application are limited due to their high cost and easy sintering deactivation during the reaction process.

It is worth noting that transition metal oxides have shown tremendous potential in this area. Transition metal oxides usually have rich valence state changes, which help to provide multiple active sites in catalytic reactions [6], thereby improving catalytic efficiency. The spinel structure of Co₃O₄ is the best alternative to precious metal catalysts. The metal-oxygen bond (M-O) of Co₃O₄ is weaker than that of other transition metal oxides, and the surface oxygen species activity is higher, allowing the Co₃O₄ catalyst to catalyze methane efficiently at low temperatures. For Co₃O₄, researchers have found a more significant grain size effect compared to other transition metal oxide catalysts, meaning that as the catalyst grain size decreases, the methane conversion rate increases up to a certain limit and then decreases. Pandey et al. found that the catalytic activity of the catalyst is highest when the size of the Co₃O₄ particles is 2.3 nm, while the catalytic activity decreases when the size is below or above 2.3 nm. Therefore, by precisely adjusting the particle size, cobalt trioxide catalysts can achieve optimal catalytic performance and are more convenient to use than other transition metal oxide catalysts. Co₃O₄, as a low-cost and highly catalytic transition metal oxide, has received much attention from researchers in recent years [7].

The heterogeneous composite of Co₃O₄ with other oxides enhances the catalytic oxidation performance due to the highly dispersed active components caused by the interaction between oxide particles. Li et al. [8] proposed a combination of Co₃O₄ and SiAl@Al₂O₃, which was applied for the first time in the catalytic combustion of methane, improving the thermal environment of the catalyst bed and achieving high efficiency in the utilization of lean methane. G. Di et al. [9] prepared Co₃O₄/CeO₂ composite oxides with different cobalt loadings. The addition of 70% CeO₂ to the catalyst significantly improved its activity and long-term stability compared with pure Co₃O₄. The microstructure of the catalyst is also an important factor affecting its catalytic performance. Co₃O₄ nanorods containing a large number of exposed (110) crystal planes exhibit excellent catalytic activity against spherical particles mainly surrounded by (111) crystal planes and can catalyze methane at lower temperatures. This confirms that controlling the morphology of nanostructured cobalt oxides is beneficial for exposing more catalytic active sites [10]. A number of researchers have demonstrated that flower-shaped Co₃O₄ can fully expose specific crystal planes [11], thereby increasing the concentration of surface active oxygen species and promoting the catalytic effect on CH₄.

Although Co₃O₄-based catalysts exhibit good catalytic activity in methane catalytic combustion, there are still some challenges that need to be overcome. For instance, there is a need to enhance its catalytic activity, stability, and anti-sintering performance. Additionally, there is a requirement to optimize the preparation process in order to facilitate large-scale production. Researchers are conducting in-depth research on these issues, including improvements to the formulation of catalysts, optimization of preparation conditions, and exploration of new synthesis methods. Guo et al. [12] adopted low-temperature phosphating technology for the first time to prepare a phosphorus-atom-doped Co₃O₄ nanowire array (P-Co₃O₄/NF) supported by foam nickel. In comparison to Co₃O₄, the electrocatalytic performance of P-Co₃O₄/NF has been significantly enhanced. Pu et al. [13] prepared Co₃O₄ catalysts under different pH environments using a simple precipitation method. At a pH of 9.0, the catalyst exhibited a high O_ads/O_latt ratio, which enabled it to exhibit high catalytic efficiency at low temperatures. Furthermore, the catalyst prepared using Co(C₂H₃O₂)₂ as a precursor exhibited superior catalytic activity.

It is important to highlight that catalyst carriers play a pivotal role in methane catalytic combustion. (1) The carrier can enhance the catalytic activity of the catalyst by modifying its dispersion state and the degree of exposure of active components. For instance, the surface of the carrier is replete with active sites that can form robust interactions with the active components of the catalyst, thereby augmenting catalytic activity. (2) The carrier can enhance the mechanical strength and thermal stability of the catalyst, preventing it from sintering or agglomeration at high temperatures. By fixing the active components of the catalyst on the surface of the carrier, the migration and aggregation of catalyst particles can be prevented, thereby improving the heat resistance and stability of the catalyst. (3) The interaction between the carrier and the active components of the catalyst can influence the reaction pathway and product distribution, thereby facilitating the efficient conversion of methane to carbon dioxide and water. (4) The high dispersion of active component particles on the surface of the carrier results in a larger contact area between the catalytic active component particles and the reaction gas, thereby improving the utilization rate of the unit catalyst.

ZSM-5 zeolite exhibits a considerable specific surface area and a distinctive pore structure [14], in addition to remarkable thermal stability and acidic properties [15]. It also exhibits catalytic activity. Following the loading of the active catalyst, the catalytic activity of the composite catalyst will also be significantly enhanced. In a study by Fei et al. [16], core–shell structured Co₃O₄-ZSM-5 composite catalysts were synthesized using different hydrothermal methods, and their catalytic performance on dichloromethane was subsequently evaluated. The results demonstrated that the catalysts prepared via the microwave hydrothermal method exhibited a higher proportion of Co³⁺/C_O²⁺, superior oxygen mobility, and superior catalytic performance. Furthermore, the synthesis of the Co₃O₄/ZSM-5 core–shell catalyst was enhanced by the introduction of additional elements, including Cr, Ce, Nb, and Mn [17]. The incorporation of Cr₂O₃, generated through the addition of Cr, was found to interact with Co₃O₄, resulting in a high Co³⁺/C_O²⁺ ratio and enhanced catalytic activity. In a separate study, Li et al. [18] prepared a series of ZSM-5/SBA-15 composite-supported Co catalysts and evaluated their suitability for Fischer–Tropsch synthesis. At a ZSM-5 loading of 20%, Co₃O₄ exhibited high dispersion and the most effective catalytic activity on CH₄.

To sum up, the Co₃O₄-catalysed methane catalytic combustion reaction is significantly promoted by the catalyst’s high specific surface area, high Co³⁺/Co²⁺ ratio, and high surface active oxygen concentration. Nevertheless, the current Co₃O₄ particles are susceptible to deactivation due to sintering in catalytic reactions. In this work, a series of Mn-doped Co₃O₄ particles were synthesized and grown in situ on ZSM-5 zeolite with a mesoporous pore structure, which had been treated with an alkaline solution in order to address the issue of the facile sintering of active components. The large specific surface area of ZSM-5 zeolite facilitates the uniform dispersion of the active component Mn-Co₃O₄. The Mn-Co₃O₄/ZSM-5-6.67 catalyst exhibited a methane conversion rate of 90% at a space velocity of 36,000 mL·g⁻¹·h⁻¹ and a reaction temperature of 363 °C. The principal reason for this is that the mesoporous ZSM-5 carrier with a high specific surface area is conducive to an enhanced methane conversion rate. The adsorption and mass transfer of reaction molecules results in the formation of abundant oxygen vacancies in the active components, which is beneficial for the activation of reaction molecules and enhances their catalytic activity.

2. Results and Discussion

The preparation process of the Mn-Co₃O₄/meso-ZSM-5-6.67 is illustrated in Scheme 1. Firstly, dissolve Co(NO₃)₂ and Mn(CH₃COOH)₂ in deionized water to obtain a solution, and then add ZSM-5, which has been treated with an alkaline solution. Secondly, dissolve NH₄HCO₃ in deionized water to obtain a solution, and then mix the above two solutions evenly. In the following hydrothermal process, the Co element combines with oxygen species to enable CoCO₃ to grow in situ on the surface of meso-ZSM-5 zeolite. Subsequently, the final catalyst Mn-Co₃O₄/meso-ZSM-5-6.67 was obtained through calcination. Figure 1 illustrates the phase composition between the hydrothermal reaction and calcination stages during the preparation of the meso-ZSM-5 supported Mn-Co₃O₄. The products obtained from the hydrothermal reaction at 170 °C exhibited distinct characteristic diffraction peaks at 24.83°, 32.35°, 38.37°, 42.41°, 46.30°, and 53.28°, corresponding to the (012), (104), (110), (113), (202) and (116) crystal planes, respectively. This is consistent with the diffraction peak of standard CoCO₃ (PDF # 11-0692), but there is a certain shift to the left, which is caused by manganese doping. The ionic radius of Mn³⁺ (0.065 nm) is greater than that of Co³⁺ (0.061 nm), which results in some Mn³⁺ occupying the position of Co³⁺ in CoCO₃. This leads to an increase in the lattice spacing. According to the Bragg formula 2dsinθ = nλ, it can be seen that as the interplanar spacing increases, the diffraction angle 2θ decreases. Consequently, the diffraction peak shifts to the left.

The calcined products exhibit distinct characteristic diffraction peaks at 19.32°, 31.12°, 36.76°, 44.96°, 55.43°, 59.17°, and 64.98°, corresponding to (111), (220), (331), (400), (422), (511), and (440) crystal planes, respectively. These peaks are consistent with the diffraction peaks of standard Co₃O₄ (PDF # 42-1467).

The XRD patterns of Mn-Co₃O₄/meso-ZSM-5 catalysts with varying amounts of ZSM-5 addition are shown in Figure 2. Pure ZSM-5 exhibits distinct characteristic peaks at 7.95°, 8.80°, 23.10°, 23.37°, and 24.00°, corresponding to (101), (200), (332), (051), and (303) crystal planes, respectively (Figure 2a). It can be observed that the diffraction peak of the ZSM-5 sample can be considered to be in good agreement with the diffraction peak of the standard ZSM-5 (PDF # 42-0305), indicating that the purity of the ZSM-5 sample used in the experiment is relatively high.

The quantity of carrier employed will influence the concentration of active components within the catalyst, which in turn affects the diffraction peak intensity of the active components. Upon the addition of a small quantity of ZSM-5 to the cobalt manganese solution system, the diffraction peaks of Co₃O₄ become more pronounced (Figure 2b). Distinct diffraction peaks can be observed at 36.76°, 59.17°, 44.92°, and 64.98°, corresponding to the (331), (511), (400) and (440) crystal planes of Co₃O₄, respectively. Figure 2e illustrates the diffraction angle 2θ. The diffraction peak in the range of 36°~38° corresponds to a diffraction angle of approximately 36.63° for the (331) crystal face of Co₃O₄. It can be observed that as the quantity of ZSM-5 incorporated into the catalyst increases (Figure 2b–d), the diffraction peak intensity of the (331) crystal plane gradually diminishes. This is attributed to a reduction in the relative content of the active component Mn-Co₃O₄ in the catalyst.

In order to gain insight into the internal structure and chemical bonds of the catalyst, we conducted infrared spectroscopy experiments on the sample. Figure 3a illustrates the FTIR spectra of Mn-Co₃O₄/meso-ZSM-5 catalysts with varying amounts of ZSM-5 incorporation. The peak at 1638 cm⁻¹ in each group of samples is indicative of a bending vibration peak of water, thereby suggesting that each group of samples contains adsorbed water [19]. A group of peaks at 1233 cm⁻¹ and 1079 cm⁻¹ can be observed, which are related to the asymmetric stretching vibration of Si-O-Si bonds. These peaks can be attributed to the internal chemical bonds in SiO₄ or AlO₄ in ZSM-5 [20]. The external asymmetric stretching vibration near 1233 cm⁻¹ is considered to be the characteristic vibration of the double pentagonal ring in ZSM-5 [21]. The absorption peak near 1079 cm⁻¹ is attributed to the asymmetric tensile vibration within the Si-O-T bond rod. This change can be attributed to the slightly lower relative atomic mass of aluminum compared to silicon [22]. The absorption peaks at approximately 544 cm⁻¹ and 447 cm⁻¹ are characteristic absorption peaks of the ZSM-5 crystal structure [21]. The absorption peak at 447 cm⁻¹ is attributed to the T-O bending vibration of the tetrahedra within the silicate and AlO₄. Furthermore, Co₃O₄ exhibits a significant absorption peak at 665 cm⁻¹, which is attributed to the O-Co-O bond bridging vibration [23]. Figure 3b,c illustrate the position and Raman spectra of the spectral samples collected from the Mn-Co₃O₄/meso-ZSM-5-6.67 catalyst. As shown in Figure 3c, there are five distinct peaks at 150 cm⁻¹ to 800 cm⁻¹, whose peak positions are consistent with those of pure Co₃O₄ [24]. The 185.7 cm⁻¹ peak corresponds to the ${\mathrm{F}}_{2\mathrm{g}}^1$ mode of tetrahedral CoO₄ sites, the 742.0 cm⁻¹ peak corresponds to the E_g mode, the 541.1 cm⁻¹ peak corresponds to the ${\mathrm{F}}_{2\mathrm{g}}^2$ mode, the 600.2 cm⁻¹ peak corresponds to the ${\mathrm{F}}_{2\mathrm{g}}^3$ mode, and the 676.3 cm⁻¹ peak corresponds to the A_1g mode of the octahedral CoO₆ site in the Co₃O₄ phase of the crystal [25].

The activity of catalysts is related to their specific surface area and pore structure. The Brunauer–Emmett–Teller (BET) method can be employed to determine the specific surface area and pore structure of the catalyst, thereby facilitating a more comprehensive understanding of its performance and mechanism. Figure 4a presents a comparison of adsorption and desorption in meso-ZSM-5 and Mn-Co₃O₄/meso-ZSM-5-6.67 catalyst. It can be observed that both the pure meso-ZSM-5 zeolite and the Mn-Co₃O₄/meso-ZSM-5 catalyst exhibit typical Type IV isotherm characteristics and H₃ hysteresis loops [26], indicating that both the pure meso-ZSM-5 zeolite and the Mn-Co₃O₄/meso-ZSM-5-6.67 catalyst have mesoporous structures. Figure 4b presents a comparison of the pore size distribution in meso-ZSM-5 and Mn-Co₃O₄/meso-ZSM-5-6.67 catalyst. The specific surface area of pure ZSM-5 is 343.63 cm²/g, while that of the Mn-Co₃O₄/meso-ZSM-5-6.67 catalyst is 295.77 cm²/g. The objective of this study is to determine the pore size distribution of mesoporous materials using the BJH model. From Figure 4b, it can be observed that for pure meso-ZSM-5, mesopores emerge as a result of alkaline etching [27], with the majority of mesoporous pore sizes concentrated around 3.5 nm. In the case of the Mn-Co₃O₄/meso-ZSM-5-6.67 catalyst, the small-sized mesoporous pores are concentrated around 3.7 nm, while the large-sized mesopores are concentrated around 8.6 nm. This is due to the mesopores formed by the accumulation of Co₃O₄ particles. The objective of this study is to determine the pore size distribution of microporous materials using the Horvath Kawazoe (HK) model. From Figure 4c, it can be seen that the micropores of pure meso-ZSM-5 are concentrated at 0.57 nm, while those of the Mn-Co₃O₄/meso-ZSM-5-6.67 catalyst are concentrated at 0.42 nm. The aforementioned micropores are intrinsic to the ZSM-5 material. It can be observed that the mesopore size of the Mn-Co₃O₄/meso-ZSM-5-6.67 catalyst is larger than that of pure meso-ZSM-5. The presence of mesopores is beneficial for the mass transfer of gas molecules.

In order to ascertain the microstructure of the catalyst, we proceeded to conduct scanning electron microscopy (SEM) testing on the samples. Figure 5a,c present the SEM images of the ZSM-5 and Mn-Co₃O₄/meso-ZSM-5-6.67 catalysts, respectively. From the figures, it can be observed that the microstructure of ZSM-5 exhibits a tetragonal shape. Upon loading ZSM-5 with Mn-Co₃O₄, the particle morphology tends to become spherical, and the average particle size decreases. From Figure 5b, d, we can see that the average particle size of ZSM-5 particles is 2.19 $\mathsf{\mu }\mathrm{m}$, while the average particle size of Mn-Co₃O₄/meso-ZSM-5-6.67 particles is 1.55 $\mathsf{\mu }\mathrm{m}$. The reduction in particle size can be attributed to the acidity of the cobalt nitrate aqueous solution, which has an etching effect on ZSM-5 under high-temperature hydrothermal conditions.

To gain further insight into the distribution of active components within ZSM-5 zeolite and Mn-Co₃O₄/meso-ZSM-5-6.67 catalyst, transmission electron microscopy (TEM) analysis was conducted on ZSM-5 and Mn-Co₃O₄/meso-ZSM-5-6.67 catalyst. Figure 5e shows the transmission electron microscopy (TEM) image of the Mn-Co₃O₄/meso-ZSM-5-6.67 catalyst. It can be observed that the Mn-Co₃O₄/meso-ZSM-5-6.67 catalyst comprises elliptical grains with rough grain boundaries, indicative of the loading of Mn-Co₃O₄ particles on the surface of ZSM-5. It was observed that a cavity structure appeared in the TEM image at this time, which can be attributed to the etching effect of the alkaline solution during the alkaline treatment. This process resulted in the removal of the Al element from the ZSM-5 zeolite, thereby forming a cavity. Figure 5f presents an enlarged TEM image of the Mn-Co₃O₄/meso-ZSM-5-6.67 catalyst. In Figure 5g, portions of the crystal planes of Co₃O₄ are indicated, including the (311) crystal plane with a crystal plane spacing of d = 0.244 nm and the (400) crystal plane with a crystal plane spacing of d = 0.202 nm. Figure 5h represents the HAADF-STEM mapping image of the Mn-Co₃O₄/meso-ZSM-5-6.67 catalyst. Additionally, an elemental surface scanning analysis was conducted on the particles in the Mn-Co₃O₄/meso-ZSM-5-6.67 catalyst (Figure 5i–n), which clearly demonstrated the presence of Co and Mn elements. This further substantiates the successful loading of Mn-Co₃O₄ on the surface of ZSM-5 zeolite.

The results of the methane catalytic performance testing of the Mn-Co₃O₄/meso-ZSM-5 catalyst under different concentrations of meso-ZSM-5 zeolite addition are presented in Figure 6. The optimal catalytic activity of the catalyst is observed at a concentration of 6.67 g/L of ZSM-5 zeolite. At this point in time, the T₅₀ value of the catalyst is 305 °C, while the T₉₀ value of the catalyst is 365 °C. It is possible to achieve a conversion rate of 100% under 400 °C (GHSV = 36,000 mL·g⁻¹·h⁻¹). At a concentration of 20.00 g/L of ZSM-5 zeolite, the T₅₀ value and the T₉₀ value of the catalyst are 362 °C and 448 °C, respectively. At a concentration of 53.33 g/L, the temperature at which 50% conversion occurs is 428 °C, while the temperature at which 90% conversion occurs is 583 °C. It can be observed that the catalytic performance of pure ZSM-5 is the lowest due to the absence of the loaded active component Mn-Co₃O₄. As the proportion of the active component Mn-Co₃O₄ increases, the catalytic performance of the catalyst also improves.

In methane catalytic oxidation reactions, the higher the catalytic activity of the catalyst, the lower the corresponding activation energy. Figure 6b–d illustrate the Arrhenius fitting curves of the three catalysts. It can be observed that the Mn-Co₃O₄/meso-ZSM-5-6.67 catalyst exhibits the highest catalytic activity and the lowest activation energy, with a value of only 8.71 KJ/mol. As the quantity of ZSM-5 zeolite employed increases, the proportion of active components diminishes, the catalytic activity of the catalyst also declines, and the corresponding activation energy also rises. The activation energies of Mn-Co₃O₄/meso-ZSM-5-6.67 and Mn-Co₃O₄/meso-ZSM-5-6.67 were found to be 14.18 KJ/mol and 35.54 KJ/mol, respectively.

To further investigate the effect of ZSM-5 zeolite with different Si/Al ratios on the methane conversion rate, two ZSM-5-supported Mn-Co₃O₄ catalysts with Si/Al ratios of 70 and 170 were prepared, respectively, using the same preparation method as in this experiment. From Figure 6e,f, it can be seen that as the Si/Al ratio in ZSM-5 zeolite increases, the methane conversion rate gradually decreases. When the Si/Al ratio of ZSM-5 zeolite changed from the original 30 to 70 and 170, the T₅₀ value of the catalyst also changed from the original 305 °C to 357 °C and 380 °C, and the T₉₀ value changed from the original 363 °C to 434 °C and 493 °C. Correspondingly, the activation energy of the catalytic reaction gradually increased to 12.31 KJ/mol and 23.57 KJ/mol, both higher than the original value of 8.71 KJ/mol. As the Si/Al ratio in ZSM-5 zeolite continues to increase, the acidity of the zeolite support decreases. The highly acidic zeolite support can effectively suppress the removal of aluminum elements during catalytic reactions, making the catalyst highly stable and active. Conversely, zeolite supports with lower acidity lead to a decrease in catalyst performance due to their inability to effectively inhibit aluminum removal.

Table 1. Overview of activity of catalysts in the catalytic combustion of methane.

Catalysts	Reaction Conditions	T50/°C	T90/°C	Reference
Mn-Co3O4/meso-ZSM-5-6.67	1%CH4, 99%air, 36,000 mL·g−1·h−1	305	363	this work
Pd@S-1	1%CH4, 20%O2, 24,000 mL·g−1·h−1	308	360	[28]
CM-Mg0.3	1%CH4, 20%O2, balance N2, 36,000 mL·g−1·h−1	318	445	[29]
Pd (PdO)/Co3O4@SiO2	1%CH4, 21%O2, 30,000 mL·g−1·h−1	357	445	[30]
Pd-Pt/SnO2	1%CH4, 19.5%O2, 21,000 mL·g−1·h−1	413	450	[31]
Al2O3/C-Pd/SiO2	1%CH4, 10% O2/Ar, 36,000 mL·g−1·h−1	325	390	[32]
Co3O4/CeO2	0.3%CH4, 0.6%O2, 36,000 mL·g−1·h−1	343	450	[33]
Co3O4 (Flower-like)	1%CH4, 99%air, 36,000 mL·g−1·h−1	380	430	[11]
Co3O4 (Rod-like)	1%CH4, 30,000 mL·g−1·h−1	320	385	[34]
Co3O4-NW	1%CH4, 20%O2, 30,000 mL·g−1·h−1	308	366	[35]

demonstrates that the prepared catalyst exhibits superior catalytic performance to most existing noble metal-based catalysts and metal oxide catalysts [11,28,29,30,31,32,33,34,35]. It is capable of catalyzing methane at lower temperatures. It can be seen in Table 1 that the T₉₀ values of most precious metal-based catalysts exceed 390 °C, whereas the T₉₀ values of the same metal oxide catalyst are predominantly above 400 °C. In order to more effectively illustrate the remarkable catalytic efficacy of the catalyst synthesized in this study, the specific catalytic reaction conditions are presented in Table 1.

The catalytic oxidation mechanism of methane on Mn-Co₃O₄/meso-ZSM-5 catalyst was analyzed using in situ infrared spectroscopy. Figure 7a depicts the in situ infrared spectrum of methane with a volume fraction of 1% adsorbed on the Mn-Co₃O₄/meso-ZSM-5-6.67 catalyst at 250 °C. From the graph, it can be observed that there are distinct absorption bands at 3014 cm⁻¹, 2349 cm⁻¹, and 1304 cm⁻¹. The vibration peaks observed at 3014 cm⁻¹ and 1304 cm⁻¹ are attributed to the antisymmetric stretching vibration of the C-H bond of gaseous methane molecules. This is a fundamental requirement for the occurrence of catalytic combustion reactions [36]. The C=O band is located at 1600–1800 cm⁻¹ and belongs to the formic acid and carbonate groups [37], which are the main intermediate substances in the catalytic combustion process of methane. The asymmetric stretching vibration peak at 2349 cm⁻¹ is attributed to gaseous carbon dioxide [38], indicating that carbon dioxide is formed in the absence of oxygen molecules. Furthermore, the intensity of the vibration peak is observed to gradually increase with the increase of adsorption time.

The in situ infrared spectra of methane oxidation on Mn-Co₃O₄/meso-ZSM-5-6.67 catalysts at different temperatures are shown in Figure 7b. The stretching vibration peaks of the C-H bond at 3014 cm⁻¹ and 1304 cm⁻¹ are attributed to gaseous CH₄ [39]. As the temperature increases, the methane signal gradually decreases, indicating that the catalytic oxidation rate of methane is continuously increasing. The absorption band centered at 1504 cm⁻¹ represents the asymmetric stretching of ${\mathrm{C}\mathrm{H}}_3^{-}$ [37], while the vibration peak of -CH₂- appears at 2849 cm⁻¹. Concomitantly, the intensity of the ${\mathrm{C}\mathrm{H}}_3^{-}$ and -CH₂- bands increases with rising temperature [40]. The absorption band at 2384–2292 cm⁻¹ is attributed to the C=O vibration mode of methane combustion product CO₂ [40]. This signal is observed to increase with temperature. Furthermore, the absorption peak observed at 3743 cm⁻¹ is attributed to the -OH group generated and adsorbed by the methane oxidation reaction. [38], yet it is absent in the absence of gaseous O₂ (Figure 7a), indicating that hydroxyl groups are not readily formed in the absence of gaseous O₂.

The in situ infrared results allow us to propose a reaction mechanism for methane oxidation on the Mn−Co₃O₄/meso−ZSM-5-6.67 catalyst. Scheme 2 depicts the reaction mechanism on it. As illustrated in Scheme 2, the mechanism of the reaction between reactants and catalyst lattice oxygen ions can be observed. The initial stage of the reaction involves the reactant methane (CH₄) combining with the lattice oxygen on the catalyst surface. This process results in the formation of an intermediate product, which is subsequently oxidized to yield the final products H₂O and CO₂. Concurrently, the catalyst generates oxygen vacancies that are subsequently reduced. The second step is to replenish the oxygen vacancies on the surface of the catalyst and re-oxidize them in order to achieve regeneration.

In order to further investigate the changes in elemental valence states of pure ZSM-5 and Mn−Co₃O₄/meso−ZSM-5 catalysts and to verify the active site role of Co³⁺ in the catalytic process, XPS testing was conducted on the samples. The characteristic peaks of Co 2p and Mn 2p are marked in Figure 8 [41,42,43,44]. It can be observed that the ZSM-5 zeolite does not contain any Co or Mn elements prior to loading, yet these elements become apparent in Mn−Co₃O₄/meso−ZSM-5-6.67 catalyst, thereby indicating that the Co and Mn elements have been successfully loaded onto the ZSM-5 zeolite.

Furthermore, we conducted a detailed analysis and comparison of the surface states of the catalyst before and after the reaction. Figure 9a presents a comparative analysis of the Al 2p spectra of pure ZSM-5 zeolite and Mn-Co₃O₄/meso-ZSM-5-6.67 catalyst. A clear Al peak can be observed in pure ZSM-5 zeolite, with a peak position of 74.6 eV. However, the Al peak in Mn-Co₃O₄/meso-ZSM-5-6.67 catalyst did not show a clear peak position, indicating a significant decrease in the surface Al element content of the catalyst. This is due to the fact that cobalt trioxide particles are loaded on the surface of ZSM-5 zeolite, which makes it challenging to detect the Al element. Figure 9b presents a comparison of the Si 2p spectra of the sample before and after loading the catalyst. The peak position of the Si 2p spectrum in pure ZSM-5 zeolite is approximately 103.2 eV, which is comparable to the peak position of 102.8 eV in Mn-Co₃O₄/meso-ZSM-5-6.67 catalyst. This indicates that the valence state of the Si element has not been altered. The observed decrease in the peak position of the Si element is attributed to the shielding effect of the cobalt trioxide loaded on the surface of the zeolite, which effectively reduces the sensitivity of the Si element to the electron beam.

Figure 9c presents a detailed analysis of the Co 2p spectrum of the Mn-Co₃O₄/meso-ZSM-5-6.67 catalyst. Two distinct peaks are observed in the spectrum, with binding energies of 796.1 eV and 780.4 eV [45]. These peaks are attributed to the spin orbitals of Co 2p_1/2 and Co 2p_3/2, respectively. The peak positions of Co²⁺ vibrational satellites on the Co 2p_1/2 and Co 2p_3/2 spin orbits are 803.8 eV and 787.1 eV, respectively. The binding energy of Co³⁺ is located at 795.9 eV (Co 2p_1/2) and 779.9 eV (Co 2p_3/2), while the binding energy of Co²⁺ is located at 797.9 eV (Co 2p_1/2) and 781.6 eV (Co 2p_3/2). The ratio of Co³⁺/CO²⁺ is 0.368. Figure 9d presents the analysis of the Co 2p orbitals of the Mn-Co₃O₄/meso-ZSM-5-6.67 catalyst following the stability assessment testing. The two distinct peaks observed in the spectrum, situated at approximately 796.5 eV and 780.7 eV [46], respectively, can be attributed to the spin orbitals of Co 2p_1/2 and Co 2p_3/2. The peak positions of the Co²⁺ vibration satellites on these two spin orbits are 804.0 eV and 787.3 eV, respectively. The binding energies of Co³⁺ are 795.4 eV (Co 2p_1/2) and 780.2 eV (Co 2p_3/2), while those of Co²⁺ are 796.9 eV (Co 2p_1/2) and 781.4 eV (Co 2p_3/2), respectively. The ratio of Co³⁺/CO²⁺ is 0.948. The binding energy peaks at 796.9 eV~797.9 eV and 781.4 eV~781.6 eV are indicative of tetrahedral coordination of Co²⁺, while the binding energy peaks at 795.4 eV~795.9 eV and 779.4 eV~780.2 eV are indicative of octahedral coordination of Co³⁺. The ratio of Co³⁺/CO²⁺ increased from 0.368 to 0.948, accompanied by a proportional increase in the relative amount of Co³⁺. This is due to the fact that under an oxidizing atmosphere, Co²⁺ is gradually oxidized to Co³⁺, and the catalytic reaction is accompanied by a transformation of the valence state of this Co element.

Figure 9e presents a detailed analysis of the Mn 2p spectrum of the Mn-Co₃O₄/meso-ZSM-5-6.67 catalyst. The two characteristic peaks observed at approximately 653.3 eV and 642.5 eV [47] in the spectrum are attributed to the spin orbitals of Mn 2p_1/2 and Mn 2p_3/2, respectively. The binding energy of Mn³⁺ is located at 654.6 eV (Mn 2p_1/2) and 643.9 eV (Mn 2p_3/2), while that of Mn²⁺ is located at 652.7 eV (Mn 2p_1/2) and 641.6 eV (Mn 2p_3/2). The ratio of Mn³⁺/Mn²⁺ is 0.891. Figure 9f presents the analysis of the Mn 2p orbitals of the Mn-Co₃O₄/mesoZSM-5-6.67 catalyst following the stability assessment testing. The two characteristic peaks observed at approximately 651.2 eV and 641.2 eV in the spectrum are attributed to the spin orbitals of Mn 2p_1/2 and Mn 2p_3/2, respectively. The binding energy of Mn³⁺ is located at 653.6 eV (Mn 2p_1/2) and 642.5 eV (Mn 2p_3/2), while the binding energies of Mn²⁺ are located at 651.9 eV (Mn 2p_1/2) and 640.9 eV (Mn 2p_3/2). The ratio of Mn³⁺/Mn²⁺ was found to be 1.041. The ratio of Mn³⁺/Mn²⁺ before and after the reaction increased from 0.891 to 1.041, indicating that Mn²⁺ was converted to Mn³⁺ during the reaction process. This is beneficial for the conversion of Co²⁺ to Co³⁺, as it can expose more Co³⁺ sites on the catalyst surface, thereby further verifying the active site role of Co³⁺ in the catalytic process.

Figure 9g illustrates that the peak position of O 1s in pure ZSM-5 is approximately 532.5 eV [48]. Figure 9h,i demonstrate that the peak position of O 1s in the Mn-Co₃O₄/meso-ZSM-5-6.67 catalyst and catalyst following the stability assessment testing are approximately 532.1 eV and 531.2 eV [49], respectively. The three peaks exhibit a similar profile, indicating that the valence state of oxygen has not undergone any significant changes. Two distinct oxygen species were identified in the O 1s spectra following Mn-Co₃O₄/meso-ZSM-5-6.67 catalyst and catalyst following the stability assessment testing: surface adsorbed oxygen (O_ads) and surface lattice oxygen (O_latt). The ratio of O_ads/O_latt in Mn-Co₃O₄/meso-ZSM-5-6.67 catalyst is 2.623, while that of the sample after stability testing is 1.083. A greater ratio of O_ads/O_latt indicates a greater quantity of oxygen adsorbed on the surface of the sample, which in turn leads to the formation of more oxygen vacancies on the surface. This results in a higher concentration of active oxygen species, which in turn enhances the catalyst activity.

3. Materials and Methods

3.1. Experimental Supplies

The drug information utilized in this experiment is as follows: Sodium hydroxide was procured from Tianjin Oubokai Chemical Co., Ltd. (Tianjin, China) with analytical grade purity. Tetrapropyl ammonium hydroxide was procured from Shanghai McLean Biochemical Technology Co., Ltd. (Shanghai, China) with a concentration of 25%. ZSM-5 zeolite was procured from the Catalyst Plant of Nankai University, with the ratio of Si/Al is 25–30. Cobalt nitrate hexahydrate was procured from Aladdin Reagent (Shanghai) Co., Ltd. (Shanghai, China) with analytical purity. Manganese acetate tetrahydrate was procured from Aladdin Reagent (Shanghai) Co., Ltd. with analytical purity. Ammonium bicarbonate was procured from Aladdin Reagent (Shanghai) Co., Ltd. with analytical purity.

3.2. ZSM-5 Zeolite Pretreatment

The pretreatment procedure is as follows: Weigh 0.80 g of sodium hydroxide and 4.07 g of tetrapropyl ammonium hydroxide using an electronic balance. Dissolve them in 100 mL of deionized water and stir the mixed alkaline solution at 65 °C for 30 min. The ZSM-5 zeolite was ground in a mortar and placed in a beaker. The aforementioned mixed alkaline solution was added, and the mixture was stirred evenly. It was then left to stand, centrifuged, and separated to obtain the solid product. The sample was dried at 110 °C for 12 h and finally calcinated at 550 °C in an air atmosphere for 5 h to remove tetrapropyl ammonium hydroxide from the sample. This process yielded modified ZSM-5 zeolite with a micro-mesoporous structure.

3.3. Catalysts Preparation

Firstly, weigh 4.35 g of cobalt nitrate and 0.73 g of manganese acetate, then dissolve them in 60 mL of deionized water. Once dissolved, the solution should be stirred well and divided into three equal parts. Subsequently, weigh 0.2 g, 0.6 g, and 1.6 g of ZSM-5 zeolite treated with an alkaline solution, add them to the aforementioned cobalt manganese mixed solution, and stir thoroughly to obtain the precursor mixture. Subsequently, 1.42 g of ammonium bicarbonate should be weighed and dissolved in 30 mL of deionized water. The solution of ammonium bicarbonate should be stirred thoroughly and divided into three equal parts. Subsequently, the precursor mixture is to be added to the ammonium bicarbonate solution in a continuous and slow manner, with stirring throughout the process. Once all the components have been added, the solution should be left to stand at room temperature and stirred continuously for a period of two hours in order to ensure thorough mixing. Subsequently, the suspension is placed in a high-pressure reactor and subjected to a 170 °C drying oven for 12 h, during which time it undergoes a crystallization process. Subsequently, the product is subjected to centrifugation, washing, and filtration with anhydrous ethanol and deionized water. Subsequently, the solid product is subjected to a vacuum drying process at a temperature of 70 °C for a period of six hours. Subsequently, the dried sample was subjected to calcination in a muffle furnace at 350 °C for a period of four hours, resulting in the formation of a black product. The preparation process is illustrated in Scheme 1. The catalyst should be named according to the mass concentration of ZSM-5 zeolite. For example, the addition of 0.2 g of ZSM-5 zeolite to 30 mL of deionized water results in the formation of a catalyst designated as Mn-Co₃O₄/meso-ZSM-5-6.67. Similarly, the catalyst with 0.6 g of ZSM-5 zeolite added is designated as Mn-Co₃O₄/meso-ZSM-5-20.00, while the catalyst with 1.6 g of ZSM-5 zeolite added is designated as Mn-Co₃O₄/meso-ZSM-5-53.33.

3.4. Catalysts Characterization

X-ray diffraction (XRD) was conducted on a DX-2500 X-ray diffractometer (Dandong Fangyuan Instrument Co., Ltd., Dandong, China) with a working voltage of 40 kV, a working current of 40 mA, and the scanning angle range was 5° to 90°. The Fourier transform infrared spectrometer (FTIR-8400s, Shimadzu Co., Ltd., Kyoto, Japan) was employed for FTIR testing, with a scanning range of 3200 cm⁻¹ to 400 cm⁻¹. Electron microscopy measurements were conducted using scanning electron microscopy (SEM) from Phenom Prox (Phenom World Company, Eindhoven, The Netherlands), with an accelerated voltage of 10 kV and maintained in a vacuum state. A gap-specific surface area analyzer (SSA-4000) was employed for specific surface area testing (Beijing Builder Electronic Technology Co., Ltd., Beijing, China). X-ray photoelectron spectroscopy (XPS) was utilized for XPS testing, with excitation rays of Al K_α. The working voltage was 15 kV, the working current was 15 mA, and the power was 225 W. The calibration standard for the electron binding energy of other elements is a polluted carbon C 1s (284.8eV). Transmission electron microscopy (TEM) testing was conducted using a Tecnai G2 F20 transmission electron microscope (Hillsboro, OR, USA). Raman spectroscopy analysis was performed using the LabRAM HR-800 Raman spectrometer (RS) from Horiba Jobin Yvon, Palaiseau, France. In situ infrared testing was conducted using an FTIR-8400s Fourier transform infrared spectrometer (Shimadzu Co., Ltd.), with a scanning range of 4000 cm⁻¹ to 1000 cm⁻¹ during the experiment. Brunauer-Emmett-Teller test (BET) of catalyst particles was conducted using a Kubo X1000 Surface Area and Pore Size Analyzer (Beijing Builder Electronic Technology Co., Ltd., Beijing, China).

3.5. Catalysts Performance Evaluation

The Mn-Co₃O₄/meso-ZSM-5 composite catalysts synthesized in this experiment were evaluated for the catalytic oxidation performance of lean methane in a fixed bed reactor. The reactor is equipped with a quartz reaction tube with a fixed sand core in the middle. During the experiment, A gas mixture composed of 1 vol% CH₄ and 99% equilibrium air was passed through a mass flow meter with a controlled flow rate of 60 mL/min, an air velocity (GHSV) of 36,000 mL·g⁻¹·h⁻¹, and a reaction temperature of 200 °C to 700 °C. The gas composition of the inlet and outlet of the reaction tube was analyzed online by a gas chromatograph equipped with a TCD detector. (Best Instrument Technology Co., Ltd., Beijing, China)

In this experiment, T₅₀ and T₉₀, namely the temperature at which CH₄ is converted to 50% and 90%, were selected as the criteria for evaluating the catalytic performance of the catalyst. The formula for calculating the conversion rate of low-concentration CH₄ is X = (C_in − C_out)/C_in × 100%.

In the formula, X is the conversion rate of low-concentration methane gas, C_in is the initial concentration of methane before catalytic combustion, and C_out is the residual concentration of methane after catalytic combustion.

4. Conclusions

This study first synthesized a series of Mn-CoCO₃/meso-ZSM-5 catalyst precursors via the hydrothermal method. Subsequently, the Mn-Co₃O₄/meso-ZSM-5 catalysts were prepared through the calcination method. The introduction of manganese ions into the lattice of Co₃O₄ results in lattice distortion, which in turn causes the formation of oxygen vacancies and the continuous conversion of Co²⁺ to Co³⁺ during the catalytic reaction. This enhances the catalytic activity of the catalyst for methane combustion. Among the catalysts, the Mn-Co₃O₄/meso-ZSM-5-6.67 catalyst exhibits the most promising catalytic activity for methane combustion (T₅₀ = 305 °C, T₉₀ = 363 °C). Its catalytic activity is superior to that of similar cobalt manganese catalyst systems and even better than that of precious metal-based catalysts. The primary reason for this is attributed to the mesoporous ZSM-5 carrier with a high specific surface area, which is conducive to the adsorption and mass transfer of reaction molecules. The active component is characterized by a high density of oxygen vacancies, which facilitate the activation of reaction molecules and, as a consequence, enhance their catalytic activity. The novel approaches to the synthesis of metal oxide-based methane low-temperature oxidation catalysts proposed in this study are anticipated to offer novel solutions to methane low-temperature oxidation reactions and facilitate technological advancement in related fields.