Photocatalytic oxidation of methane over SrCO₃ decorated SrTiO₃ nanocatalysts via a synergistic effect

Because of the high C-H bond energy as well as the non-polar feature of CH₄ molecules, oxidation of methane under mild conditions remains a challenging task for both C1 utilization and atmospheric environmental cleansing. Here we report that by using a sol-gel method SrCO₃ decorated SrTiO₃ nanocatalysts (SrTiO₃-S) with an average particle size of ∼25 nm can be readily prepared, which surprisingly show efficient performance for photocatalytic oxidation of methane with the activity close to fourfold of P25, a benchmark photocatalyst. Further investigation revealed a synergistic effect between SrCO₃ and SrTiO₃ when combined together into a composite material as both of which are totally inactive for methane oxidation if used alone. Gas adsorption characterization disclosed that the SrCO₃ can adsorb methane and cannot adsorb carbon dioxide, whereas the SrTiO₃ will preferentially adsorb CO₂ instead of CH₄. Photocurrent and photoluminescence measurements indicate that SrCO₃ exhibits a negligible photocurrent response relative to the SrTiO₃ semiconductor under simulated solar light illumination but the formation of the SrCO₃/SrTiO₃ junction structure (SrTiO₃-S) helps reduce surface recombination of the photogenerated electrons and holes. All these results refer to the synergistic mechanism in which the SrCO₃ acts as a trapping agent to adsorb methane and weaken its C-H bond while the SrTiO₃ acts as a photocatalyst to activate and oxidize methane under light illumination. The underlying photooxidation mechanism is further investigated with the aid of in situ electron paramagnetic resonance and infrared spectroscopy.