Pd composition and dispersion control selectivity in photocatalytic methane oxidation

Abstract

Selective methane oxidation to methanol under mild conditions presented a significant challenge due to the high bond dissociation energy of methane and the difficulty in achieving high selectivity. This study investigated how Pd composition and dispersion influenced photocatalytic oxidation of methane over brookite TiO₂ (BTO) nanorods, using molecular O₂ and water as oxidants. In situ time-resolved transient absorption (TA) spectroscopy in the visible to mid-infrared (IR) range, along with X-ray photoelectron spectroscopy (XPS) analysis before and after methane oxidation, revealed that BTO nanorods with high Pd loading and lower Pd dispersion exhibited significant electron trapping in the Pd/PdO nanoparticles under light irradiation. This electron trapping promoted the self-reduction of PdO to metallic Pd (Pd⁰), which in turn drove the selective production of CH₃OH with 98% selectivity. In contrast, a moderate Pd loading and dispersion on the BTO nanorods enhanced electron transfer to O₂, reducing electron trapping, and resulted in a higher concentrations of mixed Pd⁰/PdO nanoparticles, favoring the formation of primary oxygenates – CH₃OOH and CH₃OH. DFT calculations and experimental findings showed that Pd⁰ facilitates the direct three-electron reduction of O₂ to *OH or ˙OH, which subsequently coupled with *CH₃ or ˙CH₃ to form CH₃OH. Meanwhile PdO promotes the one-electron reduction of O₂ to *OOH or ˙OOH, leading to CH₃OOH formation. This work provides valuable insights into the design of efficient photocatalysts for selective methane oxidation and underscores the critical role of Pd composition and dispersion in modulating charge dynamics and steering product selectivity in methane.