Spinel-type oxides, particularly cobalt oxide (Co3O4) and manganese oxide (Mn3O4), have emerged as highly effective nanostructured catalysts for low-temperature methane combustion. Their unique crystal structure, redox properties, and tunable surface chemistry make them ideal for this application. Methane combustion is critical for reducing emissions from natural gas vehicles and power plants, where low-temperature activity is essential for energy efficiency and operational flexibility. This article examines the synthesis, facet engineering, catalytic mechanisms, and stability of spinel oxides in methane combustion, focusing on performance metrics such as light-off temperatures, water tolerance, and sintering resistance.

Flame spray pyrolysis (FSP) is a versatile method for synthesizing spinel-type oxide nanoparticles with high purity and controlled morphology. In this process, precursor solutions containing metal salts (e.g., cobalt or manganese acetylacetonates) are atomized into a flame, where rapid combustion leads to the formation of oxide nanoparticles. The high-temperature environment ensures crystallinity while allowing precise control over particle size (typically 10–50 nm) and composition. FSP offers scalability and the ability to dope spinel oxides with promoters (e.g., Pd, Pt, or Ce) to enhance catalytic activity. The resulting materials exhibit high surface areas (50–150 m²/g), which are crucial for maximizing active sites in methane oxidation.

The catalytic performance of spinel oxides is strongly influenced by exposed crystal facets. Co3O4 nanoparticles with dominant {111} facets demonstrate superior methane oxidation activity compared to those with {100} facets. The {111} surfaces are rich in coordinatively unsaturated Co³⁺ sites, which facilitate oxygen activation and methane C–H bond cleavage. In contrast, {100} facets are more stable but less reactive due to their lower density of active sites. Facet engineering via synthetic control (e.g., adjusting FSP parameters or post-treatment conditions) can optimize the surface structure for catalytic activity. Mn3O4, while less studied, shows similar facet-dependent behavior, with Mn²⁺/Mn³⁺ redox pairs playing a key role in the reaction mechanism.

The redox mechanism of methane combustion over spinel oxides involves Mars-van Krevelen kinetics, where lattice oxygen participates in the reaction. Methane adsorbs on the catalyst surface, and C–H bonds are cleaved by surface oxygen species, forming intermediates such as methoxy and formate. These intermediates further oxidize to CO2 and H2O, while the reduced catalyst surface is reoxidized by gas-phase oxygen. Co3O4 excels in this process due to its high mobility of lattice oxygen and the coexistence of Co²⁺/Co³⁺ redox couples. Mn3O4 operates similarly, though its lower oxygen mobility often results in higher light-off temperatures. Doping with noble metals (e.g., Pd) can enhance the redox properties by providing additional activation sites for oxygen dissociation.

Light-off temperature (T50, the temperature at which 50% methane conversion is achieved) is a critical metric for evaluating catalytic performance. Undoped Co3O4 typically achieves T50 between 300–350°C, while Mn3O4 requires higher temperatures (350–400°C). Incorporating Pd or Pt nanoparticles (1–5 wt%) can lower T50 by 50–100°C by promoting oxygen activation. The presence of water vapor in exhaust streams can inhibit catalytic activity due to competitive adsorption on active sites. Co3O4 exhibits better water tolerance than Mn3O4, as its surface hydroxyl groups are more easily displaced by methane. Strategies to improve water resistance include hydrophobic coatings (e.g., silica overlayers) or doping with CeO2, which stabilizes active sites under wet conditions.

Sintering is a major deactivation mechanism for spinel oxide catalysts at elevated temperatures (>600°C). Nanoparticle agglomeration reduces surface area and active site density, leading to irreversible activity loss. To suppress sintering, structural promoters such as Al2O3 or ZrO2 can be incorporated to create physical barriers between spinel particles. Alternatively, embedding Co3O4 or Mn3O4 nanoparticles in a thermally stable matrix (e.g., mesoporous silica or carbon) confines their growth. Pre-treatment in oxidative atmospheres can also stabilize the spinel structure by preventing phase segregation. For Mn3O4, maintaining the spinel phase under reaction conditions is challenging, as it tends to transform into Mn2O3 or MnO2 at high temperatures, further emphasizing the need for stabilization strategies.

Long-term stability under realistic conditions is essential for industrial adoption. Accelerated aging tests (e.g., thermal cycling in humid air) reveal that Co3O4-based catalysts retain >80% of their initial activity after 100 hours at 500°C when properly stabilized. Mn3O4 catalysts, while less durable, can achieve comparable stability with optimized dopants and supports. The balance between activity and stability remains a key research focus, with advanced characterization techniques (e.g., in situ XRD and XAS) providing insights into degradation pathways.

Future developments in spinel oxide catalysts for methane combustion will likely focus on atomic-scale engineering of active sites, such as single-atom catalysts or defect-rich surfaces. Combining experimental and computational approaches can accelerate the discovery of optimal compositions and structures for low-temperature activity and long-term stability. The role of interfacial effects in supported catalysts and the impact of complex exhaust gas compositions (e.g., SO2 or CO) also warrant further investigation. By addressing these challenges, spinel-type oxides can play a pivotal role in enabling cleaner and more efficient methane utilization technologies.