Ultrafine PdCu Nanoclusters by Ultrasonic-Assisted Reduction on the LDHs/rGO Hybrid with Significantly Enhanced Heck Reactivity

A series of hierarchical nanosheet array-like Co–Al layered double hydroxides (LDHs)/reduced graphene oxide (rGO) hybrid supported ultrafine PdCu nanocluster (NC) catalysts m-PdCux/LDHs/rGO (x: Cu/Pd molar ratio of 1.5, 3.0, and 5.5; m: Pd loadings of ∼0.80, 0.40, 0.11, and 0.01 wt %) were assembled via an ultrasonic-assisted NaBH4 reduction–sol immobilization strategy. The as-obtained catalysts display ultrafine PdCu alloy NCs with sizes of ∼0.9–1.8 nm finely tuned by both Cu/Pd ratios and Pd loadings and mainly distributed on the edge sites of LDH nanosheets and part of LDHs–rGO junctions upon the unique hierarchical nanosheet array-like structure. Three catalysts 0.85-PdCu1.5/LDHs/rGO, 0.83-PdCu3.0/LDHs/rGO, and 0.80-PdCu5.5/LDHs/rGO exhibit excellent Heck reactivity for iodobenzene with styrene, of which the 0.83-PdCu3.0/LDHs/rGO shows the highest activity, much higher than Pd/LDHs/rGO and single LDHs or GO supported PdCu3.0 catalysts, attributed to the ultrafine PdCu3.0 NCs, the largest electron density of the Pd0 center, and the strongest PdCu3.0 NCs–LDHs–rGO three-phase synergistic effect. The lowest Pd-loading sample 0.01-PdCu3.0/LDHs/rGO shows an unprecedented turnover frequency of 210 000 h–1 (Pd dosage: 2 × 10–5 mol %) with the highest value so far, excellent adaptability for substrates, and reusability. The present work provides a versatile method for designing hierarchically structured ultrafine Pd–M alloy NC catalysts for varied catalysis processes.