Abstract:

The selective conversion of methane to C₂ hydrocarbons offers a sustainable approach to utilize natural gas efficiently and reduce reliance on conventional fossil fuels. Unlike the conventional thermal catalytic conversion that requires high temperatures and pressures, the photocatalytic pathway enables methane activation and selective conversion under mild conditions, holding great promise as a sustainable method. However, achieving the efficient generation of C₂ compounds under flowing conditions using cost-effective photocatalysts remains great challenge. In this work, we synthesized an ultralow loading AuPd alloy nanoparticle-supported on TiO₂ (Au_0.05-Pd_0.05/TiO₂) photocatalyst via simple chemical reduction. Characterization using X-ray diffraction (XRD), aberration corrected high-angle annular dark field scanning transmission electron microscopy (AC-HAADF-STEM) and in situ CO-diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) confirmed its composition and structure. The performance of the Au_0.05-Pd_0.05/TiO₂ photocatalyst in methane conversion was evaluated under flow-reaction conditions. Remarkably, the photocatalyst efficiently converted methane containing water vapor into C₂ compounds, including ethane and ethylene, with a remarkable C₂ production rate of up to 10092 μmol∙g⁻¹∙h⁻¹ and a selectivity of 77%. While water vapor was not essential for methane conversion, its presence enhanced the production of ethane and ethylene while suppressing overoxidation to CO₂. The photocatalyst demonstrated excellent stability, maintaining its catalytic activity even after continuous reaction for 32 h, surpassing previously reported results. With the assistant of transient photocurrent response test, in situ X-ray photoelectron spectroscopy spectra and in situ DRIFTS, we uncovered that the exceptional catalytic activity of Au_0.05-Pd_0.05/TiO₂ originates from the synergistic effect of Au and Pd, which promotes the separation of photogenerated carriers and facilitates the C-C bond coupling of ·CH₃ to produce C₂ compounds. Furthermore, XPS characterization revealed that the introduction of water vapor replenished consumed lattice oxygen during the methane activation process, thus contributing to the catalyst's stability. This study not only offers a cost-effective and efficient photocatalyst for methane conversion but also provides insights into the fundamental mechanism of photocatalytic methane conversion. We believe that our work will inspire the exploration of inexpensive catalysts with simple preparation methods, driving advancements in efficient methane to C₂ compound conversion and contributing to sustainable photocatalytic pathways for the future.

Key words: Photocatalysis, Methane, Flow-reaction, Water vapor, AuPd alloy nanoparticle, C₂ compounds