Pd Nanoparticles Versus Single Atoms on CuTi-LDH: Reversing Photoinduced Charge Transfer to Switch Radical Generation Pathway for Selective Methane Photooxidation

Photocatalytic methane conversion to liquid products offers a promising route for the efficient utilization of methane while enabling the generation of value-added chemicals. However, the inherent complexity and uncontrollable nature of the reaction pathways make it extremely challenging to simultaneously achieve high C₁ product yield with high selectivity. In this study, we report a Pd nanoparticle-supported CuTi layered double hydroxide (LDH) catalyst (Pd_NPs-CuTi-LDH), which achieves a high C₁ liquid-oxygenates yield of 7220.7 µmol g^-1 h^-1, exceeding that of Pd single atoms-supported CuTi-LDH (Pd₁-CuTi-LDH) by threefold, while maintaining an outstanding selectivity of 99.7%. Mechanistic investigations reveal the amounts of ·OH and ·OOH radicals as the key determinant of catalytic performance. Interestingly, in situ XAFS analyses indicate that Pd size modulation induces a spatial separation of photogenerated electrons and holes under light irradiation, directing them toward different metal species. Further radical kinetic evaluations demonstrate the distinct charge-transfer behavior results in ·OH radicals mainly originating from H₂O on Pd_NPs-CuTi-LDH, whereas they predominantly arise from O₂ on Pd₁-CuTi-LDH. Consistently, theoretical calculations reveal the Pd_NPs-CuTi-LDH possesses lower formation energies for both ·OH (ΔE = -0.86 eV) and ·OOH (ΔE = -0.27 eV), thereby promoting more efficient methane conversion.