The potential of photocatalytic conversion of methane using water as an oxidant has opened doors to on-site and on-demand green chemical technology. However, the lack of understanding of the oxidation kinetics, active sites, and photocatalytic performance has hindered the rational design of next-generation photocatalysts. To address this issue, a research group led by Toshiki Sugimoto at the Institute for Molecular Science has shed light on the critical role of metal cocatalysts in modulating oxidation kinetics and selectivity.
Surface Oxidation Kinetics and Selectivity
By conducting real-time mass spectrometric analysis under controlled methane pressures, the research group discovered that the Pt-loaded Ga2O3 photocatalyst significantly promoted the total oxidation of methane to CO2 on its surface. On the other hand, the Pd-loaded photocatalyst exhibited a higher selectivity for the formation of C2H6 through the gas-phase coupling of free •CH3. This difference in oxidation kinetics was further confirmed by operando infrared absorption spectroscopy, which observed surface intermediates during the photocatalytic process. Additionally, the researchers found that the Pt cocatalyst itself was oxidized by photogenerated holes. These findings highlight the crucial roles of metal cocatalysts as reservoirs of photogenerated holes and effective reaction sites for methane oxidation.
A New Understanding of Metal Cocatalysts
Traditionally, metal cocatalysts have been viewed as reduction cocatalysts that accumulate photogenerated electrons and promote reduction reactions like H2 evolution. This conventional assumption led to the belief that hole-accumulated metal cocatalysts hinder 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 new understanding challenges the long-held assumptions about metal cocatalysts and paves the way for a cocatalyst-based surface engineering strategy for controlling non-thermal oxidation reactions.
Through their 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 opens up new possibilities for the design and optimization of next-generation photocatalysts for environmentally friendly and sustainable chemical processes.
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