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

Abstract

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.