Highly Dispersed Pd Nanoclusters on Layered Double Hydroxides with Proper Calcination Improving Solvent-Free Oxidation of Benzyl Alcohol

Abstract

Highly dispersed Pd nanocluster (NC) catalysts x-PdNCs/LDH-T (where x is the Pd loading, T is the calcination temperature, and LDH refers to Ni3Al-layered double hydroxides) were synthesized by the electrostatic adsorption of atomically precise captopril (Capt)-protected Pd17Capt8 clusters (∼1.4 nm) onto the LDH support followed by proper calcination. The 0.15-PdNCs/LDH-T (where T = 250, 270, and 280 °C) catalysts show similar Pd NCs with sizes of ∼1.6 nm highly distributed on the verge of small-sized LDH nanoplates (30–50 × ∼15 nm), while the 0.16-PdNCs/LDH-300 catalyst shows slightly increased Pd NCs size of ∼1.8 nm, attributable to the sintering between neighboring NCs. The as-obtained catalysts x-PdNCs/LDH-T all show excellent solvent-free aerobic oxidation activity of benzyl alcohol, and 0.15-PdNCs/LDH-280 exhibits the highest turnover frequency of 108,670 h–1. This unprecedented activity can be attributed to the ultrafine Pd NCs with the complete exposure of interfacial sites Pd0/Pd2+–Ni2+–OH after removing the surface ligands contributing to the adsorption and activation of the reactants, the electron-rich Pd0 species stemmed from the electron transfer from Ni2+–OH groups to Pd clusters promoting the β-hydride elimination of metal-alkoxide intermediates and strong PdNCs–LDH synergistic interactions, while other catalysts with residual ligands or enlarged PdNCs along with less surface hydroxyls as well as a newly generated NiO phase exhibit reduced activity owing to the blocked or reduced active sites. Additionally, the present catalyst shows no significant activity loss after six runs, indicating its excellent recyclability.

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