Water enables mild oxidation of methane to methanol on gold single-atom catalysts

The study addresses the long-standing challenge of directly converting methane to methanol with high selectivity under mild conditions. Methane is abundant but difficult to activate due to its strong C–H bond (439.3 kJ mol⁻¹) and nonpolar tetrahedral structure. Moreover, once activated, methanol is thermodynamically prone to over-oxidation to CO or CO₂. Aerobic oxidation of methane to methanol is ideal for atom economy but faces spin-state constraints between singlet CH₄ and triplet O₂ and the selectivity issue. Existing industrial routes (reforming to syngas followed by downstream synthesis) require high temperatures (>700 °C) and suffer from coking. The purpose here is to realize selective, low-temperature aerobic methane oxidation by leveraging photocatalysis and the role of water on gold single-atom sites supported on black phosphorus nanosheets, aiming for high methanol selectivity and activity under light irradiation.

Direct methane functionalization has been achieved under mild conditions but often relies on corrosive or costly oxidants/media (e.g., H₂O₂ with Au–Pd colloids at 50 °C under 30 bar CH₄; SO₃-based routes to methanesulfonic acid; processes in oleum, trifluoroacetic or hydrobromic acid), limiting scalability. Photocatalytic approaches have shown promise for selective methane oxidation (e.g., TiO₂-supported Fe species; modified zeolites; polymeric carbon nitride systems). However, achieving aerobic oxidation to methanol selectively, without sacrificial or corrosive oxidants, remains challenging. The present work builds on these advances by using water and O₂ under light to drive selective oxidation over Au single atoms on black phosphorus, exploiting BP’s broadband absorption and single-atom catalysis.

Catalyst synthesis: Bulk black phosphorus (BP) was synthesized via a low-pressure transport method using red phosphorus, Sn, and SnI₄, followed by washing and drying. BP nanosheets were obtained by liquid exfoliation in NMP with prolonged sonication in an ice bath, sequential centrifugation, solvent washing, and vacuum drying. Au₁/BP nanosheets were prepared by injecting HAuCl₄ solution into an ethanol dispersion of BP under N₂ using a syringe pump; product collected and washed, with Au loading determined as 0.2 wt%. Analogous methods yielded Pt₁/BP, Rh₁/BP, Pd₁/BP (0.2 wt% metal). Au, Pt, Rh, and Pd nanoparticles (1.0 wt% loadings) were synthesized by NaBH₄ reduction. Characterization: HAADF-STEM confirmed isolated Au atoms without nanoparticles at 0.2 wt% loading; Au NPs (~6 nm) formed at higher precursor concentration. XRD matched orthorhombic BP with no Au phase. Raman showed characteristic BP modes (A_g^1, B_2g, A_g^2). Au L₃-edge XANES/EXAFS indicated Au in an intermediate oxidation state with Au–P coordination (CN ~2.0 at 2.33 Å) and no Au–Au or Au–O contributions; XPS showed no residual Cl. DFT modeling placed Au single atoms at bridge P sites forming two Au–P bonds. UV–vis–NIR, Tauc plots, Mott–Schottky, and valence XPS established similar band structures for BP and Au₁/BP with bandgap ~1.45 eV and flat band potential −0.35 V vs RHE. DRIFTS with CO probe distinguished single atoms (linear CO adsorption only) from nanoparticles (linear and bridge CO adsorption). Catalytic testing: Reactions were performed in a 180-mL stainless-steel slurry reactor with a sapphire window for Xe-lamp irradiation (full spectrum). Standard condition: 200 mg catalyst in 20 mL H₂O; pressurized at room temperature with CH₄ 30 bar and O₂ 3 bar (CH₄:O₂ = 10:1, total 33 bar), heated to 90 °C for 2 h under illumination (1.2 W; irradiation area 3.14 cm²). Gas products analyzed by GC-TCD/FID; liquid by ¹H NMR using DMF internal standard. Light intensity varied (0.4–1.2 W) for rate/Arrhenius studies. Solvent and oxidant controls used acetonitrile or N₂ in place of water or O₂. Stirring rate and water volume were varied to probe mass transfer. In situ cycles: 10 successive 2-h runs without catalyst removal, re-pressurizing between cycles. Apparent quantum yields (AQY) measured under monochromatic light (350–765 nm) with band-pass filters and irradiance quantification; AQY calculated assuming one electron per CH₃OH formed. Spectroscopic/mechanistic studies: In situ DRIFTS under 1 bar flows (N₂, O₂, N₂/H₂O, O₂/H₂O) at 90 °C in dark or under light (1.2 W); CH₄-DRIFTS after pre-treatments to track P=O, P–O–P, P–OH, CH₄, and CH₃ features. Quasi-situ XPS O 1s and Au 4f post-treatments to assign hydroxyl on P (P–OH) rather than Au. ¹H MAS NMR referenced to Au(OH)₃ to exclude Au–OH formation. In situ ESR with TEMP (for ¹O₂) and DMPO (for O₂⁻ and •OH) under light in water saturated with CH₄ and O₂. TPSR–MS with H₂¹⁸O labeling and ³²O₂ pre-treatments under dark/light; CH₄ introduced and m/z signals monitored while ramping 50–300 °C. Computations: Spin-polarized DFT (VASP) with PAW, optB86b-vdW functional, 400 eV cutoff, BP monolayer (3×3) supercell with 12 Å vacuum; k-point mesh 4×3×1; CI-NEB for transition states. Explored O₂ activation pathways (dark and light-induced via charge carriers), adsorption configurations of O and OH species, and CH₄ activation/oxidation sequences on Au₁/BP with and without light. TOF and conversion calculated via defined equations using metal dispersion assumptions; AQY computed from photon flux and methanol counts.

- Activity/selectivity: Au₁/BP produced 22.7 µmol methanol in 2 h under standard conditions with >99% selectivity and no by-products; mass activity 113.5 µmol g_catal⁻¹ and TOF 5.6 h⁻¹ at 90 °C, 33 bar (CH₄:O₂=10:1), 1.2 W light. BP alone was inactive; in the absence of light or O₂, or when using acetonitrile solvent, no detectable products formed. Methanol was not further oxidized under reaction conditions. - Kinetics: Activity increased with light intensity; apparent activation energy E_a ≈ 43.4 kJ mol⁻¹ (nearly independent of light power between 0.4–1.2 W). Reaction was heterogeneous (supernatant and homogeneous Au precursors inactive). AQY reached 17.4% at 350 nm; lower at longer wavelengths. - Reactant effects: Higher CH₄ partial pressure increased conversion while maintaining >99% selectivity; higher O₂ partial pressure increased conversion but decreased selectivity due to CO₂ formation. Reaction was not diffusion-limited (insensitive to stirring rate and water volume). - Stability: Over 10 in situ cycles (20 h total), cumulative methanol 205.2 µmol with TOF fluctuation <7%; Au remained atomically dispersed with Au–P bonds and no Au–Au or Au–O signals post-reaction. - Mechanism—O₂ activation and role of water: In situ DRIFTS showed formation of P=O (1246 cm⁻¹) and P–O–P (911 cm⁻¹) upon O₂ exposure in dark; under O₂/H₂O and light, a 3350 cm⁻¹ band indicated P–OH formation. Quasi-situ XPS (O 1s) and ¹H MAS NMR confirmed hydroxyls on P (P–OH), not on Au. ESR under light detected O₂⁻, ¹O₂, and •OH on both BP and Au₁/BP, supporting a light-driven sequence: ³O₂ + e⁻ → O₂⁻; O₂⁻ + h⁺ → ¹O₂; ¹O₂ + H₂O → HOO• + •OH; HOO• → •OH + •O. - CH₄ activation/oxidation: DRIFTS with CH₄ showed consumption of P=O and CH₄ with formation of P–OH and CH₃ in dark (CH₄ + P=O → CH₃ + P–OH), but no methanol formed without light. Under light, P–OH and P=O decreased with CH₃ formation and methanol detected in catalysis. TPSR–MS after light pre-treatment with H₂¹⁸O/O₂ showed CH₃¹⁸OH and CH₃¹⁶OH formation upon heating, indicating incorporation of oxygen from both water and O₂. - DFT energetics: In dark (both O₂ activation and CH₄ oxidation), conversion of CH₃ to CH₃OH has a high apparent barrier (2.62 eV), favoring CH₃ + H recombination to CH₄. After light-activated O₂ but CH₄ oxidation in dark, CH₄ C–H activation by OH to CH₃ + H₂O has barrier 0.58 eV; subsequent steps lead to methanol with a rate-limiting barrier 1.15 eV and apparent barrier 1.03 eV. Under continuous light, CH₄ activation by OH to CH₃ + H₂O proceeds with 0.58 eV apparent barrier; coupling of OH with CH₃ on Au to CH₃OH has barrier 1.10 eV (rate-limiting), while an alternative co-adsorbed OH/CH₃ reaction step has as low as 0.25 eV barrier. Residual oxygen species can re-activate CH₄ (barrier 1.19 eV), closing the cycle. - Selectivity origin: Deeper dehydrogenation steps (CHₓ→CHₓ₋₁+H, x=1–3) are endothermic; Au single atoms stabilize CH₃, suppressing over-oxidation. Methanol oxidation barriers (e.g., 1.31 eV via P–OH; 1.82 eV via P=O) exceed methanol desorption (0.90–0.83 eV) and methane activation (0.58 eV), favoring methanol desorption into water. Overall conversion after 2 h under standard conditions was ~0.01%, selected to enable kinetic analysis.

The work demonstrates that water, under light irradiation, enables selective aerobic oxidation of methane to methanol on Au single atoms supported on black phosphorus. The photocatalyst harnesses photoexcited charge carriers in BP to activate O₂ into reactive species (O₂⁻, ¹O₂, •OH) and generates surface P–OH groups. These hydroxyl species perform mild C–H activation of CH₄ at the Au single-atom sites to yield CH₃ and water, while •OH facilitates rapid oxidation of CH₃ to methanol. DFT and in situ spectroscopy coherently support this mechanism and rationalize why light is essential: continuous generation of reactive oxygen species lowers apparent barriers and provides an energetically feasible pathway at 90 °C. The Au single atoms stabilize CH₃ intermediates, and methanol is kinetically and thermodynamically favored to desorb rather than over-oxidize, explaining the >99% selectivity. The findings address the activation–over-oxidation dilemma by coupling photocatalytic O₂ activation with water-mediated C–H activation on isolated Au sites, offering a low-temperature, selective route for methane upgrading.

This study establishes Au single atoms on black phosphorus as an efficient photocatalyst for selective methane-to-methanol conversion using O₂ and water under mild conditions. Key contributions include: identifying water’s catalytic role in O₂ activation to generate P–OH and •OH; elucidating a light-driven mechanism for CH₄ activation at Au sites and selective CH₃ oxidation; demonstrating high methanol selectivity (>99%), meaningful activity (113.5 µmol g_catal⁻¹ in 2 h), favorable kinetics (E_a ≈ 43.4 kJ mol⁻¹), high AQY at 350 nm (17.4%), and stability over multiple cycles with preserved single-atom dispersion. Future work could aim at increasing overall conversion and productivity at lower pressures and under broader solar spectra, optimizing supports and metal loadings to enhance photon utilization and active-site density, and integrating reactor designs for continuous operation and product separation.

- The overall methane conversion under standard conditions was low (≈0.01% after 2 h at 30 bar CH₄ and 3 bar O₂), chosen for kinetic measurements but limiting immediate practical throughput. - The process requires light irradiation and elevated total pressure (33 bar) to achieve activity; no activity was observed without light or when O₂ was absent. - Selectivity decreases at higher O₂ partial pressures due to CO₂ formation, indicating a narrow optimal operating window. - Stability was demonstrated over 10 in situ cycles (20 h); longer-term durability and scale-up performance were not reported. - TPSR indicated that without continuous light, higher temperatures are needed to form methanol, underscoring reliance on photoactivation.