Hierarchical Pd Nanoclusters/CuCoAl-LDH/Graphene Catalyst: Pd–Cu₂O Synergistic Enhancement Effect for Highly Improved Reduction of Nitro Compounds

A series of Pd nanocluster (PdNC) catalysts <i>x</i>-PdNCs/CuCoAl(O)/rGO-<i>T</i> (<i>x</i>: Pd loading (= 0.04-0.24 wt %), <i>T</i> = 260-340 °C) are synthesized by loading water-soluble captopril-protected PdNCs on CuCoAl-layered-double-hydroxide/reduced-graphene-oxide using electrostatic adsorption followed by proper calcinations. The catalysts possess nanosheet array-like structures with PdNCs (∼1.4-2.0 nm) uniformly dispersed on the CuCoAl(O) nanosheet edges decorated with Cu₂O nanoparticles (2.65-3.95 nm). All catalysts exhibit outstanding catalytic activities for 4-nitrophenol (4-NP) reduction, especially the 0.1-PdNCs/CuCoAl(O)/rGO-300 with the highest TOF value of 76,608 h^-1, which is 2.5-folds higher than the 0.17-PdNCs/CoAl(O)/rGO-300 without Cu doping and obviously superior to most reported noble-metal catalysts. Meantime, the 0.1-PdNCs/CuCoAl(O)/rGO-300 exhibits an excellent cycling stability with complete conversion of 4-NP in 60 s after 40 successive runs without removing the product. The present catalyst also exhibits good universality for a variety of nitroaromatics and organic dyes. Moreover, the 0.1-PdNCs/CuCoAl(O)/rGO-300 shows outstanding catalytic activity in continuous flow reactions via a simulated fixed-bed test. The superior catalytic performance can be attributed to the ultrafine electron-enriched PdNCs, the electron promotion of Cu₂O nanoparticles, and the PdNCs-CuCoAl(O)-rGO three-phase synergistic effect. The present work offers novel insight into the design and synthesis of supported noble-metal catalysts in a large variety of applications.