The potential of using water as an oxidant for photocatalytic conversion of methane has brought forth new opportunities in green chemical technology. However, the lack of understanding regarding oxidation kinetics, active sites, and photocatalytic performance has impeded the development of advanced photocatalysts. To address this issue, a research group at the Institute for Molecular Science, led by Toshiki Sugimoto, has delved into the critical role of metal cocatalysts in modulating oxidation kinetics and selectivity.
Through real-time mass spectrometric analysis conducted under controlled methane pressures, the research group made a significant discovery. They found that the Pt-loaded Ga2O3 photocatalyst effectively facilitated the complete oxidation of methane on its surface, leading to the production of CO2. In contrast, the Pd-loaded photocatalyst exhibited higher selectivity for the formation of C2H6 through the gas-phase coupling of free •CH3. Operando infrared absorption spectroscopy further confirmed the differences in oxidation kinetics, revealing surface intermediates during the photocatalytic process. Additionally, the researchers observed that the Pt cocatalyst itself underwent oxidation due to photogenerated holes. These findings emphasize the vital role of metal cocatalysts as reservoirs of photogenerated holes and as effective reaction sites for methane oxidation.
Traditionally, metal cocatalysts were seen as reduction cocatalysts that accumulate photogenerated electrons to promote reduction reactions such as H2 evolution. This conventional assumption led to the belief that hole-accumulated metal cocatalysts obstruct photocatalysis by acting as charge recombination centers. However, the research group’s experiments contradicted this belief. They demonstrated that metal cocatalyst loading actually accelerated both H2 evolution and methane oxidation. This suggests that photogenerated electrons and holes are trapped separately at different metal cocatalyst particles, preventing charge recombination and promoting redox reactions. This breakthrough challenges long-held assumptions about metal cocatalysts and opens avenues for a cocatalyst-based surface engineering strategy to control non-thermal oxidation reactions.
Through a systematic investigation of the photocatalytic oxidation of methane and water, the research group has provided valuable insights into the role of metal cocatalysts in photocatalysis. By elucidating the oxidation kinetics and selectivity, as well as the oxidation of metal cocatalysts themselves, they have expanded our understanding of how metal cocatalysts can influence photocatalytic reactions. This knowledge unlocks new possibilities for the design and optimization of next-generation photocatalysts, facilitating environmentally friendly and sustainable chemical processes.
The research group’s findings shed light on the critical role of metal cocatalysts in modulating oxidation kinetics and selectivity in photocatalytic oxidation reactions. By challenging traditional assumptions, they have paved the way for a cocatalyst-based surface engineering strategy to control non-thermal oxidation reactions. This breakthrough opens up new opportunities for the design and optimization of next-generation photocatalysts, propelling the development of environmentally friendly and sustainable chemical processes.
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