Enhanced Heck reaction on flower-like Co(Mg or Ni)Al layered double hydroxides supported ultrafine PdCo alloy nanoclusters catalysts: the promotional effect of Co
A series of PdCo alloy nanoclusters (NCs) catalysts x-PdCor/Co(Mg or Ni)Al-LDH (x: Pd loading, r: Co/Pd molar ratio) were synthesized by immobilizing ultrafine PdCor-PVP NCs on flower-like layered double hydroxides (LDH) supports. The sizes of PdCo alloy NCs of the catalysts can be elaborately tuned in ~1.6 – 3.2 nm by both Co/Pd ratios and Pd loadings, and the PdCo NCs are mainly dispersed on the edge sites of LDH nanosheets upon flower-like morphology. The PdCo bimetallic catalysts 0.81-PdCo0.10/MgAl-LDH (2.6 ± 0.6 nm), 0.86-PdCo0.28/MgAl-LDH (2.3 ± 0.7 nm) and 0.79-PdCo0.54/MgAl-LDH (3.2 ± 0.9 nm) exhibit enhanced activity compared with monometallic Pd catalyst for Heck coupling of iodobenzene with styrene, especially the 0.86-PdCo0.28/MgAl-LDH shows the highest activity, attributed to its smallest PdCo0.28 alloy NCs, the maximum electron density of Pd0 center resulted from the electrons transferring from Co and the strongest PdCo0.28 NCs – LDH synergistic effect. The 0.67-PdCo0.28/CoAl-LDH shows much better activity than 0.64-PdCo0.28/NiAl-LDH and 0.86-PdCo0.28/MgAl-LDH. The lowest Pd loading sample 0.01-PdCo0.28/CoAl-LDH (1.6 ± 0.4 nm) shows ultrahigh turnover frequency of 163 000 h-1 (Pd: 1.9×10-5 mol%), which is the highest value so far. Meanwhile, the catalyst shows excellent adaptability for the substrates and can be reused 12 runs without significant loss of activity. The present work may provide a new idea for the simple and green synthesis of ultrafine Pd-based non-noble bimetallic catalysts for varied catalysis processes.