Single-atom catalysts (SACs) anchored on two-dimensional (2D) materials represent a cutting-edge approach to methane activation, offering atomic efficiency and tunable reactivity. The conversion of methane to higher-value products like methanol or ethylene is a critical challenge in catalysis due to the inertness of the C–H bond and the difficulty in controlling selectivity. SACs on graphene, transition metal dichalcogenides (TMDCs), and other 2D supports provide unique coordination environments and metal-support interactions that can be tailored for optimal performance.

The coordination environment of single atoms on 2D materials plays a pivotal role in catalytic activity. For instance, platinum (Pt) or iron (Fe) atoms anchored on molybdenum disulfide (MoS₂) exhibit distinct electronic properties depending on their binding sites. Sulfur vacancies in MoS₂ serve as anchoring points for metal atoms, creating stable single-atom sites with well-defined coordination. Density functional theory (DFT) calculations reveal that Pt atoms occupying sulfur vacancies in MoS₂ adopt a distorted tetrahedral geometry, leading to partial charge transfer from the support to the metal. This electronic perturbation enhances the ability of Pt to activate methane by weakening the C–H bond. Similarly, Fe atoms on MoS₂ exhibit high spin states, facilitating radical-mediated C–H cleavage.

Metal-support interactions are crucial for stabilizing SACs and modulating their reactivity. In the case of graphene-supported SACs, nitrogen doping introduces pyridinic or pyrrolic sites that anchor metal atoms such as Fe or Co. These sites prevent aggregation and provide electron-rich environments that promote methane adsorption. Experimental studies using X-ray absorption spectroscopy (XAS) confirm that Fe-N₄ centers in nitrogen-doped graphene exhibit a +2 oxidation state, which is optimal for methane activation. Meanwhile, on TMDCs like MoS₂, the chalcogenide lattice induces strong metal-support interactions that alter the d-band center of the anchored metal, influencing adsorption energies and reaction barriers.

The mechanistic pathways for methane activation on SACs involve both homolytic and heterolytic C–H bond cleavage. DFT simulations suggest that Pt/MoS₂ facilitates heterolytic cleavage, where the C–H bond breaks asymmetrically with proton transfer to a nearby sulfur atom and methyl binding to Pt. This pathway avoids high-energy intermediates and lowers the activation barrier compared to gas-phase reactions. For Fe/MoS₂, homolytic cleavage dominates, generating methyl radicals that can couple to form ethylene or react with hydroxyl groups to yield methanol. The selectivity between these products depends on the local environment, including the presence of oxygen or water.

Experimental studies corroborate these computational insights. Catalytic tests with Pt/MoS₂ show methane conversion to methanol at low temperatures, with in situ spectroscopy confirming the formation of Pt-CH₃ intermediates. Fe/MoS₂ systems, on the other hand, favor ethylene production due to radical recombination pathways. The difference in product distribution highlights the importance of metal identity and support effects in steering reaction selectivity.

Despite these advances, challenges remain in achieving high activity and selectivity simultaneously. The inertness of methane requires catalysts to balance C–H activation without over-oxidizing products. SACs on 2D materials mitigate this by offering isolated sites that prevent over-coordination of intermediates. However, competing reactions such as complete oxidation to CO₂ or unwanted C-C coupling can still occur. Strategies to improve selectivity include tuning the electronic structure of SACs via strain engineering or secondary coordination with heteroatoms. For example, introducing oxygen functionalities near Fe sites on graphene can stabilize methanol precursors and suppress further oxidation.

Another challenge is the stability of SACs under reaction conditions. Aggregation of metal atoms or poisoning by reaction intermediates can deactivate the catalyst. Encapsulating single atoms within defects or edges of 2D materials enhances durability. Studies show that Fe atoms embedded in graphene vacancies remain stable after multiple reaction cycles, whereas those on pristine surfaces tend to sinter. Similarly, sulfur-rich environments in TMDCs protect anchored metals from oxidation.

Future directions for SACs in methane activation include exploring non-traditional metals and hybrid supports. Nickel (Ni) and copper (Cu) SACs on boron nitride (hBN) or phosphorene may offer alternative pathways for C–H activation with lower energy barriers. Additionally, combining SACs with plasmonic effects or photoexcitation could enable room-temperature methane conversion. Advances in operando characterization techniques will further elucidate active sites and transient intermediates, guiding the design of next-generation catalysts.

In summary, single-atom catalysts on 2D materials provide a versatile platform for methane activation, with coordination environments and metal-support interactions dictating reactivity and selectivity. DFT and experimental studies have uncovered key mechanistic pathways, but challenges in stability and selectivity control persist. Addressing these limitations through precise engineering of SACs will unlock their full potential for sustainable methane utilization.