Substrate-Dependent Role of a Pd Dopant in PdAu₁₂ Catalysts in the Oxidation of <i>p</i>-Substituted Benzyl Alcohols: Promotion of Hydride Abstraction and Reductive Activation of O₂

Single-atom doping, which is the primary step toward multimetallization, on atomically size-controlled metal nanoclusters facilitates the elucidation of doping effects on catalysis. Herein, we synthesized MAu12 (M = Au, Ir, Rh, Pt, or Pd) nanoclusters on double metal hydroxide composed of Co and Ce by thermal treatment of diphosphine-protected MAu12(dppe)5Cl2 (dppe: 1,2-bis(diphenylphosphino)ethane). The successful formation of the MAu12 nanocluster under an optimized thermal treatment condition (175 °C, 8 h) was confirmed by X-ray absorption fine structure spectroscopy and aberration-corrected high-angle annular dark-field scanning transmission electron microscopy. Among the five catalysts, PdAu12 exhibited 4.4 times higher activity than Au13 in the aqueous phase benzyl alcohol (BnOH) oxidation, while the other MAu12 (M = Ir, Rh, or Pt) catalysts showed comparable activity to Au13. On the basis of the kinetic experiments under different partial pressures of O2 with p-substituted BnOH (R-BnOH; R = MeO, Me, H, Cl, or CF3), we concluded that the single Pd atom dopant plays a dual role in the oxidation depending on the nature of the R group: abstraction of H– from the adsorbed alkoxide for R = Cl or CF3 and reductive activation of O2 for R = MeO, Me, or H. Theoretical studies on model structures have shown that O2 is more efficiently activated by PdAu12 than by Au13, thereby the different mechanism mediated by activated O2 appears with the single Pd doping.