Efficient methane oxidation to formaldehyde via photon-phonon cascade catalysis

Methane, the major component of natural gas and methane hydrates, is an abundant feedstock whose low-carbon conversion to liquid chemicals could reduce greenhouse gas emissions and enable sustainable petrochemistry. Formaldehyde is a high-volume commodity used in resins, plastics, textiles and pharmaceuticals. Industrially, HCHO is produced indirectly via steam reforming of methane to syngas, methanol synthesis, then methanol oxidation over Ag- or Fe–Mo-based catalysts at ~350 °C, a multi-step and energy-intensive process suited only to large-scale centralized plants. Direct oxidation of methane to HCHO could simplify the process but is challenged by methane’s inert C–H bond (439 kJ mol−1), requiring >500 °C in thermal catalysis and causing overoxidation of HCHO to CO/CO2, limiting selectivity (<65%). Photocatalysis can activate methane under mild conditions and has achieved high selectivity (~90%) to HCHO on several metal-oxide semiconductors, but production rates remain low (~20 µmol h−1) and prolonged reactions at room temperature lead to HCHO overoxidation due to slow desorption. The authors hypothesize that combining photons (to overcome the first C–H activation barrier) with phonon energy (to accelerate kinetics and product desorption, suppressing overoxidation) in a cascade could deliver both high efficiency and selectivity. They therefore design Ru single-atom modified ZnO to couple photocatalytic CH3OOH formation from CH4 and H2O with a phonon-driven decomposition step to HCHO, targeting efficient, selective, and milder methane valorization.

Prior approaches for direct methane-to-formaldehyde conversion include thermal oxidation over Mo-oxide-, V-oxide-, and other metal-oxide or phosphate catalysts, which require >500 °C and suffer from overoxidation, yielding <65% HCHO selectivity. Photocatalytic systems based on TiO2, ZnO, WO3, In2O3 and BiVO4 have enabled CH4 photo-oxidation to oxygenates at mild conditions, with reports of ~90% HCHO selectivity on biphasic TiO2, Au-modified In2O3, single-atom-modified oxides, and Cu-modified defective WO3. However, these systems typically exhibit low HCHO production rates (~20 µmol h−1) and risk overoxidation during long reactions due to sluggish product desorption at room temperature. Benchmark comparisons cited show that previous high-selectivity photocatalysts achieved an order of magnitude lower methane conversion than the present work, while thermal catalysts achieving comparable conversion require >500 °C and still lower HCHO selectivity.

Catalyst synthesis: ZnO nanoparticles were prepared by precipitation of zinc oxalate (0.025 mol Zn(NO3)2 and 0.025 mol oxalic acid in 500 ml water), drying at 60 °C and calcining at 350 °C for 6 h. Ru was deposited on ZnO by chemical reduction: ZnO (200 mg) dispersed in 100 ml water, addition of RuCl3·xH2O solution (0.5 mg ml−1 as Ru), stirring 60 min, followed by slow addition of 2 ml NaBH4 (0.1 M), stirring 60 min, washing and drying at 60 °C. Pd, Au, Pt, and Ag were deposited similarly using respective precursors. Ru loading was optimized, with 0.5 wt% as optimal for HCHO. Characterization: UV-Vis DRS (Shimadzu UV-2550), XRD (Stoe STADI-P, Cu Kα), XPS (Thermo, Al Kα), TEM (JEOL 2010), HAADF-STEM (JEM ARM200CF), Ru K-edge XANES/EXAFS (SSRF BL14W, fluorescence mode), PL (Renishaw InVia, 325 nm excitation). In situ UV-Vis-NIR DRS/PIA used an Agilent Cary 5000 with a Praying Mantis accessory and Harrick cell; 365 nm LED as pump, reflectance measured 500–2,700 nm under Ar, air, or CH4. Electrochemistry: Photoelectrodes prepared by spin-coating catalyst ink on FTO. Three-electrode setup in 0.5 M Na2SO4; Pt counter, Ag/AgCl reference (vs RHE). ORR measured from 1 to −0.2 V vs RHE (10 mV s−1). Transient photocurrent at 0.6 V vs RHE under a 150 W Xe lamp (2 s chopping). Radical detection and quantification: EPR (Bruker E580, DMPO spin trap) for •OOH (in methanol, air) and •OH (in water). •OOH quantified via NBT reduction kinetics (0.04 mM NBT, 365 nm illumination), •OH quantified via coumarin to 7-hydroxycoumarin PL at 454 nm (excitation 335 nm). In situ DRIFTS: Nicolet iS50 with Harrick cell; CH4/O2/H2O-vapour feed (CH4/O2 = 20:0.75), 365 nm LED irradiation, spectra collected at 1–40 min. Reaction testing: Batch reactor (230 ml) with quartz window; typical conditions: 10 mg catalyst, 180 ml water, purge with Ar, pressurize with CH4 20 bar and O2 0.75 bar, heat to 150 °C (thermocouple in liquid), 365 nm LED (100 W; 75 mW cm−2 through window; 12.56 cm2 illumination area), stirring 800 rpm. Post-reaction cooling <25 °C; gas analyzed by GC (TCD/methanizer/FID). Liquid CH3OOH and CH3OH quantified by 1H NMR (D2O with DMSO internal standard); HCHO quantified colorimetrically with ammonium acetate–acetylacetone reagent (absorbance at 412 nm). Selectivity based on observable products (CH3OOH, CH3OH, HCHO, CO, CO2). Stability: 7 cycles, 2 h each. Isotopic labeling: Oxygen source probed using 16/18O2 and H2 18O under standard conditions at 150 °C with 365 nm LED for 2 h; HCHO isotopologues analyzed by GC-MS. DFT: VASP with PBE-GGA, PAW, ZnO(001) 4×4 slab (5 layers, 15 Å vacuum), 3×3×1 k-points, 400 eV cutoff; convergence 1e−5 eV and 0.05 eV Å−1; computed energetics for intermediates and Ru valence states to compare pathways on Ruδ− single-atom sites vs Ru nanoparticles.

- Demonstration of a photon-phonon cascade for methane oxidation to formaldehyde using Ru single-atom-decorated ZnO (0.5Ru-ZnO), achieving an HCHO production rate of 401.5 µmol h−1 (40,150 µmol g−1 h−1) with 90.4% selectivity at 150 °C and 0.75 bar O2 under 365 nm irradiation. - Methane conversion reached 1.1% after 1 h at 20 bar CH4 (optimized 0.5Ru-ZnO), exceeding prior high-selectivity photocatalysts by about an order of magnitude, and comparable to thermal catalysts operating >500 °C, while maintaining >90% HCHO selectivity. - Temperature dependence (0.5Ru-ZnO): At 15 °C, CH3OOH dominates (89% selectivity) and HCHO is ~9%. At 50 °C, HCHO production rises (40.8 µmol h−1, 21% selectivity). At 100 °C, HCHO reaches 198.0 µmol h−1 (66.0% selectivity) while CH3OOH falls (39.0 µmol h−1, 14.4%). At 150 °C, HCHO reaches 448.5 µmol h−1 (85.0% selectivity). At 200 °C, HCHO increases but CO2 rises (~18%), lowering HCHO selectivity to 71.7%. - O2 partial pressure optimization: Best HCHO selectivity (90.4%) and high rate (401.5 µmol h−1) at 0.75 bar O2 with 20 bar CH4; lower O2 yields minimal CO2. - Time profile: Initial 0–60 min shows high HCHO rate (~400 µmol h−1). From 60–120 min, average HCHO rate drops to ~100 µmol h−1 due to surface accumulation; beyond 120 min HCHO plateaus while CO2 increases, indicating HCHO overoxidation at high concentration. - Stability: 0.5Ru-ZnO maintains constant activity over seven 2 h cycles (14 h total); XRD/XPS indicate unchanged ZnO phase and Ru valence after long reaction. - Mechanism and charge dynamics: Electrochemistry shows enhanced oxygen reduction current with 0.5Ru-ZnO; lower photocurrent in CH4 suggests Ru acts as electron acceptor. PL quenching and PIA indicate improved charge separation and conduction-band electron quenching by Ru. EPR/NBT and coumarin assays show 0.5Ru-ZnO produces more •OOH and •OH than ZnO; 2Ru-ZnO (with Ru nanoparticles) shows decreased radical generation due to light scattering. - Isotopic labeling: Oxygen in HCHO predominantly originates from H2O (not O2) over 0.5Ru-ZnO (and similarly 0.5Au-ZnO, 2Ru-ZnO), whereas oxygen in CH3OH derives from O2, indicating HCHO is not produced via CH3OH oxidation but through a distinct pathway involving water-derived oxygen. - In situ DRIFTS: 0.5Ru-ZnO shows bands for adsorbed CH3 (2854 cm−1) and CH2OOH (1335/1350 cm−1) growing with irradiation, consistent with efficient methane activation and CH3OOH intermediate formation; 2Ru-ZnO and 0.5Au-ZnO show methoxy/CH3OH-related bands (1052, 2939, 2971 cm−1), aligning with higher CH3OH selectivity on nanoparticle-loaded catalysts. - Cascade validation: Photocatalysis at 30 °C forms CH3OOH; subsequent dark heating at 150 °C converts CH3OOH largely to HCHO (~80%), with CH3OH and CO2 as minors, and similar conversion in absence of catalyst, confirming a phonon-driven decomposition step. - DFT and pathway proposal: On Ru single-atom ZnO (Ruδ−-ZnO), photogenerated electrons reduce Ru4+ to Ru3+ and facilitate O2 reduction. Water oxidation by holes yields •OH that abstracts H from CH4 to form adsorbed −CH3, which couples with OH to −CH3OH and then to −CH3OOH, the main intermediate on Ruδ−-ZnO. Desorption/dehydration yields HCHO. On Ru nanoparticle ZnO, a different pathway favors CH3OH formation via •CH3 and •OH recombination on nanoparticles.

The study addresses the central challenge in direct methane oxidation: overcoming the initial C–H activation barrier under mild conditions while preventing overoxidation of desired oxygenates. By integrating photocatalysis (photon-driven) with a subsequent phonon-driven thermal step, the cascade exploits photons to initiate selective CH3OOH formation from CH4 and H2O at low temperature and uses moderate heat (150 °C) to accelerate CH3OOH decomposition and product desorption, thereby enhancing rate and suppressing overoxidation. Single-atom Ru on ZnO improves charge separation and oxygen reduction, boosting radical generation (•OH, •OOH) that underpins selective CH3OOH formation with water-derived oxygen, as confirmed by isotopic labeling. The cascade delivers both high HCHO productivity and selectivity at relatively mild temperature and high CH4 pressure, achieving methane conversion and selectivity surpassing prior photocatalysts and rivaling high-temperature thermal systems. Mechanistic spectroscopy (PIA, EPR, DRIFTS) and DFT corroborate the role of Ru single atoms as electron acceptors and the unique CH3OOH-mediated route to HCHO, distinct from CH3OH oxidation pathways prominent on nanoparticle cocatalysts. These findings demonstrate a generalizable strategy for coupling photon and phonon energies to steer kinetics and selectivity in light alkane oxyfunctionalization.

The work establishes a photon-phonon-driven cascade catalysis platform using Ru single-atom-decorated ZnO to convert methane to formaldehyde with benchmark performance: 401.5 µmol h−1 HCHO rate, >90% selectivity at 150 °C and 0.75 bar O2. Mechanistic studies show Ru single atoms act as electron acceptors to enhance charge separation and oxygen reduction, enabling photocatalytic formation of CH3OOH from CH4 and H2O, followed by a phonon-driven decomposition to HCHO. Compared to photocatalysis alone, the cascade increases HCHO yield nearly 30-fold and improves selectivity about eightfold, while operating under milder conditions than thermal catalysis. This approach provides a promising, potentially lower-carbon route for methane valorization to HCHO. Future work could focus on continuous-flow reactor designs, optimizing light utilization (e.g., visible-light-responsive systems), reducing pressure requirements, managing product accumulation to suppress overoxidation, extending the strategy to other light alkanes, and scaling catalyst synthesis while preserving single-atom dispersion.

- Reaction relies on UV irradiation (365 nm) and elevated CH4 pressure (20 bar); performance under lower pressures or visible light is not demonstrated here. - At higher temperature (200 °C) or prolonged reaction (>120 min), CO2 formation increases and HCHO selectivity declines due to overoxidation and product accumulation on the catalyst surface. - The process shows diminishing HCHO production rate over time due to surface-accumulated HCHO, necessitating cycle operation or product removal strategies. - High Ru loading (e.g., 2 wt%) forms nanoparticles that scatter light, reduce radical generation, and shift selectivity toward methanol, indicating a narrow optimal loading window to maintain single-atom dispersion. - Thermocatalysis alone at 150 °C yields only trace HCHO, underscoring dependence on the photocatalytic step; cascade effectiveness without UV or with solar spectrum was not evaluated.