Controllable Synthesis of Cobalt-Containing Nanosheet Array-Like Ternary CuCoAl-LDH/rGO Hybrids To Boost the Catalytic Efficiency for 4-Nitrophenol Reduction

A series of Co-doped ternary CuxCo3-xAl-layered double hydroxide (LDH)/rGO nanosheet array hybrids (x = 0.5, 1.0, 1.5, and 2.0) were successfully prepared using the preconditioned pH value aqueous-phase coprecipitation strategy. The CuxCo3-xAl-LDH/rGO hybrids are featured as hexagonal CuCoAl-LDH nanosheets in situ anchoring onto both sides of the rGO surface in an ab-plane vertically interlaced growth pattern. The CuxCo3-xAl-LDH/rGO hybrids show excellent activity for the complete conversion of 4-nitrophenol to 4-aminophenol, especially Cu1.5Co1.5Al-LDH/rGO with the highest kapp value of 49.2 × 10-3 s-1 and TOF of 232.8 h-1, clearly higher than most copper-containing samples in the literature and even some precious ones. Thermodynamic analysis was carried out, and the values of Ea, ΔH#, ΔS#, and ΔG# were estimated. The best activity of Cu1.5Co1.5Al-LDH/rGO can be mainly ascribed to the in situ-formed ultrafine Cu2O NPs (∼4.3 nm) along with a small amount of Cu0 species, the electron transfer effect induced by atomically dispersed Co2+ species leading to the formation of electron-rich Cu species along with the Co2+/Co3+ redox couple, the strong Cu2O-CuCoAl-LDH-rGO synergy upon the nanosheet array morphology with a high surface area and pore volume, and enhanced adsorption of reactants upon π-π stacking via an rGO layer. Meanwhile, the Cu1.5Co1.5Al-LDH/rGO exhibits an excellent universality and good cycling stability for 10 continuous runs. The Cu1.5Co1.5Al-LDH/rGO also shows superior efficiency in the catalytic reduction of 4-NP solution with a high concentration (20 mM) and displays excellent reduction performance in the fixed-bed test, implying the potential applications of the current Co-doped hierarchical ternary Cu-based LDH/rGO hybrids in the continuous treatment of practical wastewater.