Nearly atomic precise gold nanoclusters on nickel-based layered double hydroxides for extraordinarily efficient aerobic oxidation of alcohols†
A series of nickel-based layered double hydroxides supported nearly atomic precise Au25 nanoclusters catalysts, and especially Au25/NixAl-LDH systems (x = Ni/Al, 2, 3, 4) were fabricated via modified electrostatic adsorption of captopril-capped clusters Au25Capt18 onto the predispersed positively-charged NixAl-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 NixAl-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/NixMn-LDH and Au25/Ni3−xMnxFe-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 (Ea) of Au25/NixAl-LDH catalysts for 1-phenylethanol oxidation, and the order of k and Ea values act in accordance with their reactivities.