Converting methane (CH4) into valuable chemicals under mild conditions is crucial for maximizing CH4 utilization and mitigating the greenhouse effect. Partial oxidation of CH4 at low temperatures (<200 °C) is an attractive approach to produce oxygenates like methanol, formaldehyde, formic acid, and acetic acid (CH3COOH), minimizing energy input and carbon emissions compared to traditional gas-phase CH4 conversion. CH3COOH is a vital feedstock in chemical industries. Traditional CH3COOH synthesis from CH4 involves a three-step process (syngas and methanol production), which is resource-intensive and poses safety concerns. Therefore, developing a green, direct conversion method from CH4 to CH3COOH is imperative. While oxidative carbonylation of CH4 to CH3COOH has been demonstrated in thermocatalytic processes, the need for additional oxidants (e.g., O2 and H2SO4) and/or CO limits their applicability. Numerous side reactions generate undesirable products, reducing CH3COOH selectivity. Photocatalysis offers a potential green solution, where OH radicals from water oxidation replace additional oxidants. Metal-decorated semiconductor photocatalysts, exhibiting synergistic effects on electronic structure, charge separation, and intermediate adsorption, are effective for CH4 activation. Pd-based photocatalysts have shown promise in converting CH4 into C1 oxygenates. However, photocatalytic CH3COOH production remains challenging due to difficulties in forming carbonyl intermediates and controlling methyl-carbonyl coupling. The formation of carbonyl intermediates is critical for CH3COOH production from CH4 as the sole carbon source, demanding precise engineering of catalytically active sites. If carbonyl intermediates can be formed in situ from CH4 oxidation, the carbonylation process would eliminate the need for added CO. Thermocatalysis studies have shown that in situ generated carbonyl groups from CH4 oxidation can couple with adsorbed methyl groups, leading to CH3COOH formation. This research aims to develop a method for direct conversion of CH4 to CH3COOH through rational design of catalytic active sites.

Previous research has explored various methods for methane conversion, including thermocatalytic and photocatalytic approaches. Thermocatalytic methods often require high temperatures and pressures and may produce a mixture of products, making separation challenging. Photocatalytic methods offer a more sustainable alternative by utilizing light energy to drive the reaction at ambient temperatures and pressures. Several studies have demonstrated the successful photocatalytic conversion of methane to C1 oxygenates such as methanol. However, the selective production of C2 oxygenates, specifically acetic acid, has remained a significant challenge due to the complex reaction pathways and the difficulty in controlling intermediate formation and coupling. Existing literature highlights the potential of Pd-based catalysts for CH4 activation, but the specific role of PdO and the creation of a suitable heterojunction for efficient carbonyl formation and methyl-carbonyl coupling have not been fully investigated. This study builds upon existing knowledge by strategically designing a catalyst material and reaction system to overcome these challenges.

The study focused on synthesizing a PdO/Pd-WO3 heterointerface nanocomposite. WO3 nanosheets were prepared via a two-step hydrothermal method, followed by loading Pd nanoparticles (NPs) onto the nanosheets using NaBH4 reduction of PdCl2. Subsequent thermal annealing at varying temperatures (120–450 °C) introduced PdO species, creating the PdO/Pd-WO3 nanocomposite. The samples were characterized using various techniques such as powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), electron energy loss spectroscopy (EELS), and electron paramagnetic resonance (EPR). The photocatalytic CH4 conversion was evaluated in a custom-made quartz tube reactor under a CH4 atmosphere (0.1 MPa) at room temperature using a 300 W xenon lamp. Gas products were analyzed using gas chromatography (GC) equipped with a thermal conductivity detector (TCD) and a flame ionization detector (FID). In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and near ambient pressure XPS (NAP-XPS) were employed to identify reaction intermediates. Isotope labeling experiments using 18O-labeled PdO were conducted to determine the oxygen source in carbonyl intermediate formation. H2-temperature-programmed reduction (H2-TPR) was used to quantify PdO content. Finally, a photochemical flow reactor with arc-shaped flow channels was designed to enhance CH3COOH production by promoting continuous methyl-carbonyl coupling. The performance of the flow reactor was assessed in terms of production rate and selectivity for CH3COOH. The reproducibility and durability were evaluated using reaction-regeneration cycles.

The PdO/Pd-WO3 heterointerface nanocomposite demonstrated high selectivity and production rate in the photocatalytic conversion of CH4 to CH3COOH. In situ characterization using DRIFTS and NAP-XPS revealed the presence of key intermediates including methyl, carbonyl, and acetyl species. The DRIFTS spectra showed the characteristic vibrational modes of CH3COOH, indicating its formation during the reaction. The NAP-XPS results confirmed the presence of surface carbon and oxygen species, corroborating the DRIFTS findings. Isotope labeling experiments using 18O-labeled PdO definitively confirmed that the oxygen atom in the carbonyl intermediate originates from the lattice oxygen of PdO. A strong correlation was observed between the amount of PdO consumed and the CH3COOH yield, indicating that PdO plays a critical role in the oxygenate production. The study also found that the Pd-PdO interface is crucial for CH3COOH generation, with control experiments showing that simply mixing Pd/WO3 and PdO did not result in CH3COOH production. A photochemical flow reactor with arc-shaped flow channels significantly improved the efficiency and selectivity of CH3COOH production, achieving a remarkable production rate of 1.5 mmol gpd⁻¹ h⁻¹ and selectivity of 91.6%. The flow reactor design facilitated continuous methyl-carbonyl coupling, leading to enhanced performance. The system also displayed good durability with negligible Pd leaching during cyclic testing.

The findings demonstrate a highly efficient and selective photocatalytic method for converting methane directly to acetic acid, addressing the long-standing challenge of selective C2 oxygenate production from methane. The rational design of the PdO/Pd-WO3 heterointerface nanocomposite effectively controls intermediate formation and coupling, maximizing CH3COOH yield. The results highlight the importance of material design and interface engineering in photocatalysis. The successful implementation of a flow reactor further demonstrates the scalability and practical potential of the method. This approach provides a sustainable and environmentally friendly alternative to traditional methods of acetic acid synthesis, reducing reliance on resource-intensive and potentially hazardous processes. The work opens up new avenues for research into selective C2 oxygenate synthesis from methane using other heterojunctions and advanced reactor designs.

This research successfully demonstrated a direct light-driven synthesis of CH3COOH from CH4 using a rationally designed PdO/Pd-WO3 heterointerface nanocomposite. The control of carbonyl intermediate formation and methyl-carbonyl coupling, coupled with a novel flow reactor design, resulted in high selectivity (91.6%) and production rate (1.5 mmol gpd⁻¹ h⁻¹). The study provides significant insights into methane activation and selective C2 oxygenate synthesis under mild conditions, offering a potential pathway for sustainable chemical production. Future research could explore other heterojunction catalysts and optimization of flow reactor designs to further enhance efficiency and broaden the range of applicable methane conversion processes.

While the flow reactor design significantly improved performance, further optimization may be possible to achieve even higher production rates and selectivities. The study focused on specific reaction conditions; investigating the effects of varying parameters (e.g., light intensity, CH4 concentration, temperature) could provide more comprehensive insights into the reaction kinetics and mechanism. Long-term stability testing over extended periods could provide a more robust assessment of the catalyst’s durability under continuous operation. The current work primarily focuses on laboratory-scale experiments. Scaling up the process for industrial applications requires further development and optimization.