Advances in hydraulic fracturing have led to the discovery of substantial natural gas reserves, primarily composed of methane. Currently, much of this methane is combusted for energy, despite it being a potent greenhouse gas (25 times the global warming potential of CO₂). Developing efficient methane upgrading techniques is crucial. The industrial standard involves steam or dry reforming to produce syngas (CO and H₂), followed by Fischer-Tropsch or methanol synthesis. This high-temperature process (>700 °C) demands significant energy and causes catalyst deactivation. Direct conversion of methane under milder conditions is highly desirable. While methods exist using oxidants like H₂O₂, oleum, or other corrosive/expensive reagents, these are not commercially viable. Aerobic oxidation of methane to methanol offers 100% atom economy, but activation of methane and preventing methanol over-oxidation pose significant challenges. Methane's non-polar tetrahedral structure and high C-H bond dissociation energy, coupled with the spin-forbidden reaction between singlet methane and triplet O₂, hinder direct oxidation. This study aimed to overcome these challenges using photocatalysis, a method enabling thermodynamically unfavorable reactions under mild conditions, employing black phosphorus (BP) nanosheets as a broadband solar absorber support and Au single atoms as active sites.

Existing literature highlights the challenges in direct methane oxidation to methanol. Studies have reported success using aqueous Au-Pd colloids with H₂O₂ as an oxidant under pressure and temperature, and methane oxidation to methanesulfonic acid using SO₃ over an electrophilic initiator. However, these methods rely on corrosive or expensive reagents, limiting their scalability. Other research focuses on using various catalysts and supports, such as using hydrophobic zeolite modification for in situ peroxide formation, titanium dioxide-supported iron species, and Cu-Oxo clusters stabilized in metal-organic frameworks. However, these often have limitations in terms of selectivity, efficiency or operating conditions. The development of a robust, selective, and environmentally benign approach for direct methane oxidation remains a significant research goal.

The researchers synthesized Au₁/BP nanosheets through a process involving liquid exfoliation of bulk black phosphorus to create nanosheets and subsequent injection of HAuCl₄ solution into a BP nanosheet and ethanol mixture. This procedure resulted in the formation of Au single atoms dispersed on the BP nanosheets, confirmed by techniques such as HAADF-STEM, XRD, Raman spectroscopy, XANES, and EXAFS. The characterization methods included HAADF-STEM to visualize the structure, XRD and Raman to identify the crystallographic phases and vibrational modes, and XANES and EXAFS to analyze the local atomic environment around the Au atoms. The electronic structure was explored using UV-vis-NIR absorption and valence XPS. Control catalysts, including Pt, Pd, and Rh single atoms and nanoparticles on BP, were also prepared and characterized for comparison. Catalytic tests were performed in a slurry reactor equipped with a sapphire window to allow light irradiation from a Xe lamp. The reaction parameters, including temperature, pressure, light intensity, and the partial pressures of CH₄ and O₂, were varied to investigate their impact on the reaction performance. The product yields were determined using GC and ¹H NMR spectroscopy. To study the reaction mechanism, in situ DRIFTS, quasi in situ XPS, in situ ESR using trapping agents (TEMP and DMPO), ¹H solid-state NMR, and temperature-programmed surface reaction coupled with mass spectrometry (TPSR-MS) were conducted. DFT calculations were performed to understand the electronic structure and reaction pathways.

The Au₁/BP nanosheets demonstrated significantly enhanced catalytic activity towards partial methane oxidation compared to control catalysts. Under optimal conditions (33 bar mixed gas (CH₄:O₂ = 10:1), 90 °C, 1.2 W light irradiation), the mass activity reached 113.5 µmol g_catal⁻¹, with >99% selectivity for methanol. The light irradiation was found to be essential for the reaction, with activity increasing with light intensity, while the activation energy remained relatively constant. The reaction was determined to be heterogeneous. Water played a crucial role in the process, with no methanol formation observed without it or when acetonitrile was used as a solvent. O₂ was also essential, as no methanol was produced in its absence. Mechanistic studies using DRIFTS revealed that the presence of water led to the formation of reactive hydroxyl groups and •OH radicals under light irradiation. The hydroxyl groups reacted with methane at the Au single atom sites to form CH₃• species, which were subsequently oxidized by the •OH radicals into methanol. This was further supported by in-situ ESR, identifying O2⁻, ¹O2, and •OH radicals. DFT calculations showed that the reaction pathway involved the activation of O₂ by photogenerated electrons in the presence of water, forming hydroxyl groups which subsequently react with methane to generate CH₃ species, followed by oxidation to methanol. The apparent activation energy was 43.4 kJ mol⁻¹. The catalyst showed good stability over ten consecutive reaction cycles, maintaining high activity and selectivity. Isotope labeling experiments using H₂¹⁸O confirmed the participation of water-derived hydroxyl groups in methanol formation. DFT calculations provided insights into the energy barriers for the various reaction steps, supporting the proposed reaction mechanism.

This study demonstrates a highly selective and efficient method for methane oxidation to methanol under mild conditions. The use of Au single atoms on black phosphorus nanosheets, combined with the crucial role of water under light irradiation, overcomes many of the limitations of previous approaches. The low activation energy and high selectivity observed highlight the potential of this catalyst for practical applications in methane valorization. The detailed mechanistic studies provide valuable insights into the reaction pathways involved, emphasizing the importance of water-assisted O₂ activation in the formation of reactive hydroxyl groups and •OH radicals. The results are significant in the context of greenhouse gas mitigation and the sustainable utilization of natural gas resources. This approach addresses the need for environmentally friendly and energy-efficient technologies for methane conversion.

This research successfully demonstrated a highly selective and efficient method for converting methane to methanol under mild conditions using Au₁/BP nanosheets and light irradiation, with water acting as a crucial catalyst. The detailed mechanistic investigation clarifies the reaction pathways and highlights the role of water in activating oxygen. This work contributes significantly to the field of methane oxidation and opens potential avenues for sustainable and efficient natural gas utilization. Future work could focus on exploring other single-atom catalysts on various supports and optimizing reaction conditions for further improvements in activity and stability.

While the study demonstrates high selectivity and activity, the relatively low overall conversion of methane may limit its immediate industrial applicability. The study's reliance on light irradiation may necessitate further research to develop more energy-efficient and scalable light sources. Long-term stability tests exceeding ten cycles would provide a more comprehensive evaluation of the catalyst's durability. The specific synthesis procedure may require further optimization for broader applicability and scalability.