Nearly atomic precise gold nanoclusters on nickel-based layered double hydroxides for extraordinarily efficient aerobic oxidation of alcohols

Abstract

A series of nickel-based layered double hydroxides supported nearly atomic precise Au₂₅ nanoclusters catalysts, and especially Au₂₅/Ni_xAl-LDH systems (x = Ni/Al, 2, 3, 4) were fabricated via modified electrostatic adsorption of captopril-capped clusters Au₂₅Capt₁₈ onto the predispersed positively-charged Ni_xAl-LDH supports followed by proper calcination. Detailed characterizations show that the ultrafine gold clusters of ∼0.9 nm were well dispersed on the edge sites of hexagonal plate-like particles of Ni_xAl-LDH (x = 2, 3) originated from their ordered LDH layers with more Ni–OH sites and strong Au–LDH synergy, while slightly aggregated to ∼1.1 nm on the irregular Ni₄Al-LDH due to its poor layer structure along with doped nickel oxide. The catalysts exhibit excellent activity for selective oxidation of 1-phenylethanol to acetophenone with molecular oxygen under base-free, and the activity follows an increased order of Au₂₅/Ni₄Al-LDH < Au₂₅/Ni₂Al-LDH < Au₂₅/Ni₃Al-LDH. The Au₂₅/Ni₃Al-LDH shows the highest activity with TOF of 6780 h⁻¹ in toluene and 118 500 h⁻¹ in solvent-free and can be applied for a wide range of alcohols, mainly ascribed to ultrafine gold clusters and strongest gold–LDH interaction associated with the highly ordered Ni₃Al-LDH layers. Similar regularity is found in the Au₂₅/Ni_xMn-LDH and Au₂₅/Ni_3−xMn_xFe-LDH systems. The Au₂₅/Ni₃Al-LDH can be reused 5 times without loss of activity. The least-squares fit analysis yields the rate constant (k) and apparent activation energy (E_a) of Au₂₅/Ni_xAl-LDH catalysts for 1-phenylethanol oxidation, and the order of k and E_a values act in accordance with their reactivities.