Preparation of Mn-Doped Co₃O₄ Catalysts by an Eco-Friendly Solid-State Method for Catalytic Combustion of Low-Concentration Methane

Abstract

Coalbed methane is a significant source of methane in the atmosphere, which is a potent greenhouse gas with a considerable contribution to global warming, thus it is of great importance to remove methane in coalbed gas before the emission. Exploring the economical non-noble metal catalysts for catalytic methane combustion (CMC) has been a wide concern to mitigate the greenhouse effect caused by the emitted low-concentration methane. Herein, a series of Mn-doped Co₃O₄ catalysts have been synthesized by the environmentally friendly solid-state method. As a result, the Mn_0.05Co₁ catalyst performed the best CMC activity (T₉₀ = 370 °C) and good moisture tolerance (3 vol% steam). The introduction of an appropriate amount of manganese conduced Co₃O₄ lattice distortion and transformed Co³⁺ to Co²⁺, thus producing more active oxygen vacancies. Mn_0.05Co₁ exhibited better reducibility and oxygen mobility. In situ studies revealed that methane was adsorbed and oxidized much easier on Mn_0.05Co₁, which is the crucial reason for its superior catalytic performance.

1. Introduction

Methane (CH₄) is one of the most significant greenhouse gases (GHG), whose global warming potential is about 25 times over a 100-year time horizon and about 72 times over a 20-year time horizon than that of CO₂, thus the direct emission of methane can cause a series of severe environmental issues [1,2]. There are many sources of CH₄ in the atmosphere including biogenic, geogenic, and anthropogenic sources. As the main ingredient of natural gas, one of the largest anthropogenic emissions of CH₄ comes from the coal mining process [3,4]. This part of CH₄ is further diluted to 0.1–1.0% after mixing with air during underground mining, resulting in low-concentration coalbed methane. According to the requirements of relevant environmental protection policies, CH₄ emissions need to be strictly controlled. Catalytic methane combustion (CMC) helps to reduce the emission of CH₄ to meet the increasingly strict regulations on emissions reduction [5,6]. Therefore, it is of great significance to develop efficient CMC catalysts. Furthermore, the activation of the methane C–H bond in the gas phase usually requires a very high temperature due to the high stability of the molecular structure [7]. As a result, it is necessary to further optimize the low-temperature activity and thermal stability of CMC catalysts, which is in favor of industrial development and environmental protection.

Noble metal catalysts of Pd, Pt, and Rh are enriched with high activity for the catalytic combustion of low-concentration methane, but their industrial applications are restricted due to their low thermal stability, the shortage of resources, and high expense of the raw materials [8,9,10]. Considering cost control and the easy availability of raw materials, transition metal (Mn, Fe, Co, Ni, Cu, etc.) oxides are thought to be a feasible substitution for noble metal catalysts as they are prone to realize redox cycles among different oxidation states [9]. The commonly used transition metal oxide catalysts include MnO_x [11,12,13], Co₃O₄ [5,6,14], CeO₂ [15], NiO [16], and their mixed components [17,18,19,20,21,22]. A lot of research has proven that Co₃O₄ is one of the most promising transition metal oxides among different CMC catalysts [23,24]. Adding dopants of Ce, Zr, and Mn with higher redox advantages can further enhance catalytic activity by introducing lattice defects and increasing active oxygen species. Specifically, the enhancing effect of Mn is the most remarkable [25,26,27]. As for Mn–Co composite oxides, different procedures of hydrothermal, co-precipitation, and sol–gel methods are commonly applied to introduce dopants into the support structure. Chang et al. [28] elucidated that the introduction of Mn can promote the formation and mobility of active oxygen as well as the redox cycle between Mn⁴⁺/Mn³⁺ and Co²⁺/Co³⁺, accelerating the deeper transformation from intermediates to CO₂ and H₂O.

The synthesis method plays a significant impact on the catalytic performance of transition metal oxide catalysts. Compared with the methods above-mentioned, the solid-state method with simple procedures and easily accessible precursors is quite promising in industrial applications. Moreover, no organic solvent is needed in the preparation process with no harmful wastewater emission [29,30,31]. Akbari et al. [32] ground the nitrates of Mn and Ba into a paste, resulting in a BaO-doped solid-state mixing catalyst. The decent elemental doping under the solid-state method promoted a favorable regulation of the redox property and bi-component synergistic effect of the catalyst, providing more active oxygen for the reaction [33]. However, the emission of nitrogen oxides (NO_x) is still a problem when nitrates are used as precursors. Synthesizing composite oxides by the solid-state method using carbonates as precursors is a green route without wastewater and exhaust gas. However, the application of this method in CMC catalyst preparation has not been reported.

In this work, a solid-state method with carbonates as precursors was applied to investigate the CMC activities of a series of Mn-doped Co₃O₄ catalysts (Mn-Co₃O₄). This method is environmentally friendly with no wastewater or harmful gas emissions. Various characterization measures were applied to investigate the effect of Mn doping on the chemical state of active components, redox properties, and catalytic performance. Adjusting the Mn doping amount to 5.0% not only resulted in the formation of a Co–Mn solid solution and lattice defects, but also modified the surface properties and increased the active oxygen species of Co₃O₄, which was found to be beneficial for improving the catalytic performance. The cost of a prepared Mn_0.05Co₁ catalyst was lower than that of noble metals, and it has the advantages of high catalytic activity and good water resistance, so it has a good prospect of industrial application.

2. Results and Discussion

2.1. Structural and Textural Properties

The morphologies and microstructures of Co₃O₄, MnO_x, and the synthesized Mn_xCo₁ (x = 0.025, 0.05, and 0.1) catalysts were characterized by SEM and HRTEM. Pure Co₃O₄ (Figure 1a–c) mainly exhibited spherical and polygonal nanostructures with a diameter of 30–50 nm. On doping a low amount of Mn into Co₃O₄, the morphologies and crystal sizes of the obtained Mn_0.025Co₁ (Figure 1d–f), Mn_0.05Co₁ (Figure 1g–i), and Mn_0.1Co₁ (Figure 1j–l) catalysts became irregular with a wider size distribution (10–50 nm). Moreover, the catalyst edges became rougher and more blurred after Mn doping, implicating that the growth of the crystallite had been restrained and the amount of crystal defects increased. Generally, the formation of crystal defects facilitates the formation of more reactive sites and oxygen vacancies for CMC, thus enhancing the combustion activity of the Mn–Co catalyst [34]. Pure MnO_x (Figure 1m–o) showed a random accumulation of worm-like nanograins (20–50 nm). Clear lattice stripes of all catalysts can be seen in the HRTEM photos, indicating a high crystallinity.

The crystalline phase structures of Co₃O₄ and Mn_xCo₁ were characterized by XRD (Figure 2a). The Mn_xCo₁ catalysts mainly exhibited the characteristic diffraction peaks of Co₃O₄ (JCPDS No. 42-1467), and almost no obvious characteristic diffraction peaks of MnO_x were observed in the XRD patterns, except for the Mn_0.1Co₁ catalyst, which may be the result of the incorporation of Mn into the Co₃O₄ lattice to form a solid solution or the high dispersion of Mn on the surface of Co₃O₄ [28]. The main diffraction peaks of the Mn-Co₃O₄ catalysts were exposed at 2θ = 19.0°, 31.3°, 36.8°, 38.7°, 44.8°, 55.8°, 59.4°, and 65.2°, which can be attributed to the (111), (220), (311), (222), (400), (422), (511), and (440) facets of the Co₃O₄ phase [35,36,37]. With the increase in the Mn doping amount to 10.0 mol%, weak diffraction peaks at around 2θ = 33.0° and 55.0° were observed on the Mn_0.1Co₁ catalyst, which were well-matched with the characteristic diffraction peaks of the Mn₂O₃ (JCPDS No. 41-1442) and Mn₃O₄ (JCPDS No. 04-0732) phases, respectively. This indicates that increasing the Mn doping amount to a certain extent will lead to the formation of some Mn₂O₃ and Mn₃O₄ oxides instead of producing a Mn–Co solid solution. When the doping amount of Mn increased, the intensity of the diffraction peak slightly decreased, while the half-peak width slightly rose, resulting in a gradual reduction in the crystal size. The crystal sizes of the prepared catalysts were calculated by the Scherrer equation (

Table 1. Textual properties of the as-obtained catalysts.

Catalyst	Crystallite Size 1 (nm)	Specific Surface Area 2 (m2·g−1)	Pore Volume 3 (cm3·g−1)	Average Pore Diameter 4 (nm)
Co3O4	34	14	0.09	39.7
Mn0.025Co1	30	26	0.20	28.8
Mn0.05Co1	27	33	0.21	15.6
Mn0.1Co1	25	39	0.21	11.0
MnOx	28	28	0.22	20.5

¹ Calculated from the line broadening of diffraction peaks by the Scherrer equation from XRD patterns. ² Determined by the BET method. ³ Total pore volume adsorbed at P/P₀ = 0.99. ⁴ Determined by the BJH method.

) in which the mean crystal sizes decreased from 34 nm to 25 nm with an increasing Mn doping amount, which is consistent with the microscope images. Smaller crystals may also boost the formation of low-coordinated defect sites that can promote catalyst activities including surface oxygen vacancies [34]. Based on Mars−van-Krevelen (MvK) mechanisms, the surface-active oxygen species play a decisive role in CH₄ oxidation, providing more assistance for acceleration of the chemical reactions [38]. In addition, the diffraction peak of the Co₃O₄ (311) facet at 36.8° (Figure 2b) gradually shifted toward lower angles, and the corresponding lattice constant (Figure 2c) increased from 8.0608 Å (Co₃O₄) to 8.0809 Å (Mn_0.05Co₁), with a maximum increase of 0.25%. Considering the crystal radius of Co³⁺ (0.685 Å), which is lower than that of Mn³⁺ (high spin, 0.785 Å; low spin, 0.72 Å), substituting Mn³⁺ for Co³⁺ can result in the expansion of interplanar spacing [39]. Therefore, the shift in the diffraction position and the increase in the lattice constant can be ascribed to the incorporation of Mn ions. Furthermore, incorporating Mn ions also led to the accompanying transformation from Co³⁺ into Co²⁺ [25,40,41]. All of the mentioned changes directly prove that the doped Mn had been incorporated into the Co₃O₄ lattice and a Mn–Co solid solution formed. However, further increasing the doping amount to 10.0% (Mn_0.1Co₁) resulted in a slight decrease in the lattice constant, indicating no further incorporation of Mn into the Co₃O₄ lattice and no further formation of the Mn–Co solid solution.

The textual properties of the prepared Co₃O₄, Mn_xCo₁, and MnO_x samples were characterized by N₂ adsorption–desorption isotherms. Figures S1 and S2 illustrate the isotherms and the pore size distributions of the catalysts, respectively. Detailed data about the specific surface areas, pore volumes, and mean pore sizes of the catalysts are listed in Table 1. Compared with pure Co₃O₄, the Mn-Co₃O₄ catalysts possess a larger specific surface area and total pore volume as well as a smaller mean pore size. It has been proven that larger specific surface area increases will be beneficial for the transfer of CH₄ molecules inside the catalyst pores and can promote contact between CH₄ molecules and active sites, enhancing the efficiency of CH₄ oxidation [42]. When the doping amount of Mn increased, the specific surface area increased from 14 m²·g⁻¹ (Co₃O₄) to 26 m²·g⁻¹ (Mn_0.025Co₁), 33 m²·g⁻¹ (Mn_0.05Co₁), and 39 m²·g⁻¹ (Mn_0.1Co₁).

2.2. Surface Chemical States

The impacts of properties including the surface elemental valance states and oxygen species on the catalytic activities were characterized by XPS. The XPS spectra of Co 2p_3/2 (Figure 3a) can be divided into two peaks, with which the peak with a binding energy (BE) of 779.4–780.1 eV can be attributed to octahedral Co³⁺, while the other with a higher BE of 780.4–781.1 eV can be assigned to tetrahedral Co²⁺ [43,44]. When the doping amount of Mn increased, the Co²⁺ ratio (Co²⁺/(Co²⁺ + Co³⁺)) value increased at first and then decreased: Mn_0.05Co₁ (34.89%) > Mn_0.025Co₁ (32.16%) > Mn_0.1Co₁ (28.50%) > Co₃O₄ (27.41%), indicating a highest surface Co²⁺ content of Mn_0.05Co₁. The introduction of a decent amount of Mn into the Co₃O₄ lattice will facilitate the formation of crystal defects such as oxygen vacancy, leading to the reduction of neighbored Co³⁺ ions into Co²⁺. Excessive Co²⁺ can bring about a surface charge imbalance, causing other metal components to shift from a lower valence state to a higher valence state [45]. This further stimulates the redox balance between Co ions and Mn ions, in which the doping of a decent amount of Mn can effectively facilitate the breakage of Co–O bonds and create more oxygen vacancies [46].

Moreover, the deconvolution of O 1s XPS spectra with two subpeaks is represented in Figure 3b, in which the peaks at 530.8 eV and 529.6 eV can be assigned to the surface-active oxygen species (O_surf, e.g., O₂⁻, O⁻, and O₂²⁻) associated with the oxygen vacancies and the lattice oxygen (O_latt, e.g., O²⁻), respectively [47]. O_surf is the active oxygen species in catalytic oxidation [48]. The formation of active O_surf is related to the generation of oxygen vacancies, which is conducive to the adsorption and activation of oxygen molecules [49,50,51]. As a result, when the doping amount of Mn increases, the O_surf ratio (O_surf/(O_surf + O_latt)) value increases at first—from 54.17% (Co₃O₄) to 65.33% (Mn_0.05Co₁)—and then decreases, suggesting a superior catalytic activity of the Mn_0.05Co₁ catalyst when compared to other Mn-Co₃O₄ catalysts. After being consumed by the oxidation of methane and intermediates, the dissipative surface-active oxygen species can be replenished by the migration of the bulk lattice oxygen species and the supplement of the gaseous O₂, following the MvK mechanism [21,38]. The active oxygen species released from Co₃O₄ due to the reduction of Co³⁺ will transfer to the Mn species, benefiting the redox reaction cycle (Co³⁺ + Mn³⁺ ↔ Co²⁺ + Mn⁴⁺), which is responsible for the improved catalytic performance [28,46].

2.3. Redox Capabilities

The redox capacities of different catalysts are characterized by H₂-TPR (Figure 4). The spectrum of pure Co₃O₄ mainly consists of two reducing peaks, in which the peak at 250–400 °C is attributed to the reduction of Co₃O₄ to CoO, while the peak at 370–500 °C to the reduction of CoO to Co⁰ [52,53,54]. The spectrum of pure MnO_x mainly consists of two reducing peaks (300–350 °C and 400–450 °C) as well, which are attributed to the reduction of highly-dispersed MnO₂ and Mn³⁺ to Mn²⁺, respectively [40,54,55]. The internal reduction temperatures of Co and Mn oxides are rather close, and thus it is difficult to thoroughly split their respective reducing peaks apart [40,54]. The first peak of Mn_0.05Co₁ at 328 °C can be assigned to the combined reduction of Co³⁺ and Mn⁴⁺ to Co²⁺ and Mn³⁺, the second peak at 401 °C is attributed to the continuous combined reduction of Co²⁺ and Mn³⁺, while the third peak at 503 °C is attributed to the reduction of Co²⁺ to Co⁰. When Mn is doped into Co₃O₄, both the first and second reducing peaks of Mn_xCo₁ (x = 0.025, 0.05, and 0.1) are shifted to lower temperatures, indicating the improvement in the redox capacity, which is beneficial for the CMC process. Combined with the previous results, we suggest that Mn doping can increase the number of oxygen vacancies and active oxygen species, and the oxygen mobility can be correspondingly enhanced, thus causing the reduction profiles to shift to lower temperatures. However, when the Mn doping amount is further increased from 5.0% to 10.0%, the areas of the reduction peaks decrease significantly, indicating the decreased content of oxygen vacancies and surface active oxygen species, which is consistent with the XPS results. The above results show that the redox capacity of Mn_0.05Co₁ can be improved by Mn doping, which can promote the redox cycles among the reducible Mn–Co species.

To further study the oxygen properties and verify the mobility of oxygen, O₂-TPD experiments of Co₃O₄ and Mn_xCo₁ catalysts were performed, and the results are shown in Figure 5. For all the samples, oxygen desorption peaks appeared at about 600 °C, which can be attributed to lattice oxygen species released from the Co₃O₄ bulk [56,57]. Pure Co₃O₄ has a small oxygen desorption peak with a center temperature of 667 °C. After doping a decent amount of manganese into Co₃O₄, the desorption temperatures of the composite oxides significantly decreased, indicating the improved oxygen mobility of Mn_xCo₁. Moreover, Mn_0.05Co₁ exhibited the lowest O₂ desorption temperature at 570 °C, then the desorption temperature rose to 580 °C when the doping amount of manganese was further increased to 0.1, demonstrating that active oxygen vacancies most easily formed on Mn_0.05Co₁, which agrees with the XPS results. Based on the above investigations, it can be estimated that the doping of Mn into the Co₃O₄ lattice facilitates the release of lattice oxygen at high temperatures, and significantly enhances the redox capacities and oxygen mobility, which is beneficial to promoting CMC activities.

2.4. Catalytic Performance for Low-Concentration CH₄ Combustion

The CMC catalytic activities of different catalysts were evaluated (Figure 6) including Mn-doped Co₃O₄ with different doping amounts, pure Co₃O₄, and MnO_x. The Mn doping amounts in the Mn-Co₃O₄ samples were 2.5%, 5.0%, 10.0%, and 20.0%, respectively. As shown in Figure 6a, among all of the tested catalysts, pure MnO_x showed a low activity for CMC, in which the temperature needed for 50% conversion of CH₄ (T₅₀) was over 408 °C. On the other hand, pure Co₃O₄ had good activity, in which T₅₀ was about 333 °C, indicating that in the whole system, Co₃O₄ plays a major role in CH₄ catalytic oxidation. When small amounts of Mn were doped into Co₃O₄, the low-concentration CMC catalytic performances of the obtained Mn-Co₃O₄ samples were improved to a certain extent. When the doping amount of Mn was increased from 2.5% to 20.0%, the catalytic activity first increased and then decreased. When the doping amount of Mn increased to 20.0%, the activity of Mn_0.2Co₁ was much lower than that of Co₃O₄, indicating that manganese is no longer easily incorporated into the Co₃O₄ lattice to form a Mn–Co solid solution at this time, but mainly exists in the form of aggregated Mn₂O₃ oxide, as shown in the XRD pattern of Figure S3. From the experimental results, the Mn_0.05Co₁ catalyst with the Mn doping amount of 5.0% had the best activity, in which T₅₀ and T₉₀ (the temperature needed for 90% CH₄ conversion) were 310 °C and 370 °C, respectively. It can be concluded that doping the proper amount of Mn is beneficial to improving the catalytic performance of Co₃O₄.

To compare the catalytic activity differences of the samples more clearly, the temperatures needed for 10%, 50%, and 90% CH₄ conversion (T₁₀, T₅₀, and T₉₀) of all the catalysts as well as their reaction rates per unit area per second at 250 °C are summarized in

Table 2. Catalytic activities over the Co₃O₄, Mn_xCo₁ (x = 0.025, 0.05, and 0.1), and MnO_x catalysts.

Catalysts	T10 (°C)	T50 (°C)	T90 (°C)	r 1 × 109 (mol·m−2·s−1)
Co3O4	269	333	395	1.57
Mn0.025Co1	262	321	379	1.63
Mn0.05Co1	250	310	370	2.14
Mn0.1Co1	269	328	385	0.66
MnOx	323	408	468	0.06

¹ The feed gas was 0.5 vol% CH₄ and 20 vol% O₂/N₂ at T = 250 °C.

. The reaction rates per unit area per second of the relevant catalysts were calculated on the activity data of a CH₄ conversion lower than 20%. The Arrhenius equations for CMC reactions of Co₃O₄, Mn_0.025Co₁, Mn_0.05Co₁, and Mn_0.1Co₁ catalysts were calculated according to their reaction rates (Figure 6b). The apparent activation energy of CH₄ oxidation was determined by the slope of the Arrhenius equation, and for the above catalysts, the activation energies were Mn_0.1Co₁ (109.2 ± 0.4 kJ·mol⁻¹) > Co₃O₄ (102.1 ± 0.2 kJ·mol⁻¹) > Mn_0.025Co₁ (97.2 ± 0.5 kJ·mol⁻¹) > Mn_0.05Co₁ (88.4 ± 0.2 kJ·mol⁻¹), which was consistent with the CMC reaction results. This further proves that the Mn_0.05Co₁ catalyst with a small Mn doping amount had the highest catalytic activity. It can be concluded that the formation of the Mn–Co solid solution and the increase in the redox pairs of the Co/Mn multi-valence ions can effectively improve the oxygen mobility and increase oxygen vacancies. Although the specific surface area of the Mn_0.05Co₁ catalyst increases due to manganese incorporation, the more essential reasons for its performance enhancements were the higher oxygen mobility and more oxygen vacancies.

On the other hand, the CMC stability test for the Mn_0.05Co₁ catalyst with the best performance was carried out at 370 °C (Figure 6c). After continuous reaction for 100 h, the CH₄ conversion was maintained at above 85%, expressing good durability, which provides the potential possibility for its application in the CH₄ treatment industry. To further investigate the water resistance, water vapor (3 vol%) was further introduced after 50 h. The CH₄ conversion of Mn_0.05Co₁ first decreased from 86.6% to 79% and then fluctuated in the range of 76–78% for 25 h. When the vapor was evacuated, the activity of the Mn_0.05Co₁ catalyst could be thoroughly recovered to the initial level, giving a CH₄ conversion of 87.4% again, which could be maintained above 85% in the next 25 h of operation. It was found that the methane conversion could quickly increase to the previous level after stopping the water addition and still showed good stability. Based on the above results, we can conclude that the introduction of steam had a certain effect on the CMC reactivity of the Mn_0.05Co₁ sample, but this effect is reversible because its activity can be recovered after the water is removed, illustrating that the Mn_0.05Co₁ catalyst has a certain degree of water resistance.

2.5. In Situ DRIFTS Study

The CH₄ adsorption processes at room temperature on the Mn_0.05Co₁ and Co₃O₄ catalysts over time were observed by in situ DRIFTS spectra (Figure 7a,b). The peaks at 3016 cm⁻¹ and 1304 cm⁻¹ were attributed to the C–H vibration of CH₄, while the peak near 1613 cm⁻¹ was attributed to the antisymmetric stretching vibrations of formates (HCO or HCOO) [58,59,60]. For Mn_0.05Co₁, the formate species can be instantly well-observed after introducing the CH₄ stream for 1 min, while the intensities of the same peaks on Co₃O₄ are very weak, indicating that the CH₄ molecules are more easily oxidized by the surface-active lattice oxygen on the Mn_0.05Co₁ surface under O₂-free circumstances, which obey the fundamental properties of the MvK mechanism. Furthermore, the surface-active lattice oxygen on Mn_0.05Co₁ is more reactive than that on Co₃O₄. For a more visual comparison of the CH₄ relative adsorption capacity, the peak signals of Mn_0.05Co₁ and Co₃O₄ after 15-min of CH₄ adsorption were compared under the same testing conditions after weighing the same amount of samples accurately (Figure S4). Mn_0.05Co₁ had a higher maximum peak intensity than Co₃O₄, suggesting that more CH₄ was absorbed on Mn_0.05Co₁, which had a higher surface CH₄ adsorption capacity.

Moreover, the in situ DRIFTS spectra of the Mn_0.05Co₁ and Co₃O₄ catalysts with a temperature rise in an atmosphere of 1% CH₄ + O₂ were characterized (Figure 7c,d) to identify the intermediate species and their contents changed during CH₄ oxidation. For Mn_0.05Co₁, the peak at 1530 cm⁻¹ was attributed to formates, while the peaks at 1373 cm⁻¹, 1341 cm⁻¹, and 1260 cm⁻¹ were attributed to carbonates. For Co₃O₄, the peak at 1530 cm⁻¹ was attributed to formates, while the peaks at 1371 cm⁻¹, 1341 cm⁻¹, and 1250 cm⁻¹ were attributed to carbonates [59,60,61]. At low temperatures (100–350 °C), a certain amount of formate and carbonate intermediates were detected for Mn_0.05Co_1, while almost no intermediates were detected at the same temperature for Co₃O₄. Merely a small number of formates and carbonates were detected on the Co₃O₄ surface with an increase in temperature to 250 °C. On the other hand, at high temperatures (350–450 °C), the intensity of the formate peak signals on the Mn_0.05Co₁ surface weakened as the temperature increased, and due to the fast reaction depletion at 400 °C, the formate peaks almost disappeared. At the same time, the peaks at 2300–2400 cm⁻¹ could be observed at 450 °C, which were attributed to the C=O stretch of the vibration of gaseous CO₂. This means that the intermediates undergo rapid conversion and are more likely to further oxidize into carbonates and the product CO₂ on the Mn_0.05Co₁ surface [62]. The above results indicate that Mn_0.05Co₁ has a stronger capacity for CH₄ and O₂ activation, assisting in the production of intermediates and CO₂. This is one of the significant reasons for the CMC performance difference between the two catalysts.

3. Materials and Methods

3.1. Catalyst Preparation

Basic cobalt carbonate ((CoCO₃)₂·[Co(OH)₂]₃·xH₂O, 99%) was purchased from Sinopharm (Shanghai, China). Manganese carbonate (MnCO₃, 99%) was purchased from Aladdin (Shanghai, China). A series of improved catalysts of Mn-doped Co₃O₄ with different doping amounts was synthesized by the solid-state reactions of basic cobalt carbonate and manganese carbonate. In the synthesis procedure, the scaled carbonate precursors were placed in the grinding jar, in which the grinding media of zirconium oxide balls occupied a 1/3 of the whole volume. The precursors were then ground and mixed at room temperature for 8 h using a planetary ball mill at a speed of 200 rpm. The doping amount of Mn was adjusted at the range of 2.5–20.0 mol% than that of Co. The obtained powder was dried overnight at 110 °C and then calcined at 550 °C for 4 h. The obtained mixture after calcination is denoted as Mn_xCo₁, in which x stands for the molar ratio of Mn/Co. The reference catalysts of pure MnO_x and Co₃O₄ samples were synthesized according to the same procedure using basic cobalt carbonate and manganese carbonate, respectively.

3.2. Catalyst Characterizations

X-ray powder diffraction (XRD) was carried out on a Bruker AXS D8 diffractometer (AXS D8, Bruker, Madison, WI, USA). The XRD spectra were obtained in the 2θ of 10–80° to identify the crystal structures of the calcinated samples. The micromorphology of the catalysts was observed by field-emission scanning electron microscopy (FESEM) and high-resolution transmission electron microscopy (HRTEM). FESEM images were obtained on a Gemini 500 microscope (Gemini 500, Zeiss, Oberkochen, Ostalbkreis, Baden-Württemberg, Germany) at a working voltage of 3 kV. HRTEM images were obtained on a Tecnai G2 F20 S-Twin microscope (Tecnai G2 F20 S-Twin, FEI, Waltham, MA, USA) at a working voltage of 200 kV. The specific surface areas (SSA), pore volumes, and pore distributions were characterized by nitrogen adsorption–desorption isotherms on a Quadrasorb evo analyzer (Quadrasorb evo, Quantachrome, Boynton Beach, FL, USA). The pore structure data were then calculated according to the Brunauer–Emmett–Teller (BET) equation and the Barrett–Joyner–Halenda (BJH) model (Quadrasorb evo, Quantachrome, Boynton Beach, FL, USA). X-ray photoelectronic spectroscopy (XPS) was carried out on a PHI 5000C spectrometer (PHI 5000C, ULVCA-PHI, Chanhassen, MN, USA) to identify the surface element composition, valence states, and content of the catalysts. The reductivity and oxygen mobility of the catalysts were characterized by hydrogen temperature-programmed reduction (H₂-TPR) and oxygen temperature-programmed desorption (O₂-TPD) on a ChemiSorb 2720 automatic multi-purpose adsorption instrument (Auto Chem II 2720, Micromeritics, Norcross, GA, USA). Before the H₂-TPR experiments, the 40–60-mesh sample of 100 mg was filled in a quartz tube and pretreated under helium (flow rate 30 mL·min⁻¹) at 200 °C for 2 h (heating rate 10 °C·min⁻¹). The sample was then cooled to room temperature under a helium atmosphere. After that, the sample was heated under 10% H₂/Ar to 800 °C (heating rate 10 °C·min⁻¹), during which the curve was recorded. In the O₂-TPD experiments, the 40–60-mesh sample of 100 mg was pretreated under 5% O₂/He (flow rate 30 mL·min⁻¹) at 300 °C for 1 h (heating rate 10 °C·min⁻¹). After that, the sample was purged using helium for 0.5 h at 300 °C and then heated to 800 °C with a heating rate of 10 °C·min⁻¹, during which the curve was recorded. In situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) was carried out on a Nicolet 6700 spectrometer (Nicolet 6700, Thermo Fisher Scientific, Waltham, MA, USA). Before the experiments, the catalyst was pretreated at 300 °C for 1 h and cooled to 30 °C under helium to obtain the background spectrum. The pre-obtained background spectrum at the respective temperature was deducted in the following adsorption/desorption and transient reaction experiments.

3.3. Evaluation of the Catalytic Performance

To evaluate the catalytic activity for the CMC reaction at atmospheric pressure, CMC catalytic experiments toward low-concentration CH₄ were carried out on a fixed-bed reactor. The as-made powder was tablet pressed and sieved (40–60 mesh), and 200 mg of the obtained particles were packed in a quartz tube (inner diameter 4.5 mm). The total flow rate of the feeding gas (0.5 vol% CH₄, and 20 vol% O₂ with N₂ balanced) was 40 mL·min⁻¹, and the gas hourly space velocity (GHSV) was 12,000 mL g⁻¹ h⁻¹. The reaction products were detected and analyzed online by the FTD detector of a Thermo trace GC Ultra gas chromatography. The CH₄ conversion ($X_{{\mathrm{CH}}_4}$) was calculated by the following formula:

$$X_{{\mathrm{CH}}_4}\left(\%\right)=\frac{{\left[{\mathrm{CH}}_4\right]}_{\mathrm{in}}-{\left[{\mathrm{CH}}_4\right]}_{\mathrm{out}}}{{\left[{\mathrm{CH}}_4\right]}_{\mathrm{in}}}\times 100\%$$

(1)

where [CH₄]_in and [CH₄]_out are the inlet and outlet CH₄ concentrations at the steady state, respectively.

The CMC reaction rates per unit area per second of the catalysts can be calculated by the following formulas:

$$r\left(\mathrm{mol}\cdot {\mathrm{m}}^{-2}\cdot {\mathrm{s}}^{-1}\right)=\frac{X_{{\mathrm{CH}}_4}\cdot q\cdot {\left[{\mathrm{CH}}_4\right]}_{\mathrm{in}}}{V_{\mathrm{m}}\cdot W\cdot S}$$

(2)

$$\ln r=\frac{-E_{\mathrm{a}}}{RT}+C$$

(3)

where q, V_m, W, S, E_a, R, and T are the volume flow rate (40 mL·min⁻¹), the standard molar volume of gases (22.4 mL·mmol⁻¹), catalyst mass (0.2 g), SSA of the catalyst (m²·g⁻¹), activation energy (kJ·mol⁻¹), universal gas constant (8.314 J·mol⁻¹·K⁻¹), and thermodynamic temperature (K), respectively.

4. Conclusions

In summary, a series of Mn-Co₃O₄ catalysts with different Mn doping amounts synthesized by a solid-state method was applied for catalytic methane combustion. The XRD and XPS results revealed that the formation of a Co–Mn solid solution could introduce more crystal defects and oxygen vacancies, inducing higher surface concentrations of Co²⁺ and active oxygen species. The study of H₂-TPR and O₂-TPD analysis indicated the enhancement in redox capacity and oxygen mobility due to more active oxygen vacancies. Compared with the pure Co₃O₄, the Mn_0.05Co₁ sample exhibited superior catalytic performance in low-concentration methane combustion. The T₅₀ and T₉₀ of Mn_0.05Co₁ were 310 and 370 °C, respectively, which is outstanding as a transition metal catalyst compared with other components. Furthermore, the Mn_0.05Co₁ catalyst also possessed good stability and water resistance (3 vol% steam). In situ DRIFTS results confirmed that Mn_0.05Co₁ held stronger capabilities for CH₄ adsorption and provided more abundant active oxygen species for the reaction, which helped to improve the reaction rates and catalytic performance. It can be concluded that the Mn_0.05Co₁ catalyst not only had a lower cost than noble metal materials, but also had a good performance in the catalytic combustion reaction of low-concentration methane, making it of great research value and industrial application potential. Based on these results, we propose an effective strategy to develop promising high-performance non-noble metal catalysts for low-concentration methane combustion.