Fabrication of bimetallic Co/Ni composite oxides on Ni-foam as a highly active monolithic membrane catalyst for the reduction of 4-nitrophenol

The development of highly active non-noble metal catalysts is the prerequisite for the practical application of the reduction of 4-nitrophenol (4-NP) into 4-aminophenol (4-AP) by NaBH4. In this work, inexpensive Co/Ni bimetallic composite oxide integral membrane catalysts with different metal-ion molar ratios towards 4-NP reduction were grown on the nickel foam (NF) substrate by a facile oxalate-pyrolysis method. Benefiting from the synergistic effect between Co and Ni elements, special crystalline phase of NiCo2O4 with high electron transfer capacity and uniform nanoflake array structure, the NF-based monolithic membrane catalyst with a Co/Ni molar ratio of 2:1 (Co2Ni1) exhibited the optimal activity for the catalytic reduction of 4-NP. A reduction ratio of 96.71% for 250 mL of 4-NP with an initial concentration of 15 mg/L could be reached within 25 min under the reaction conditions as follows: two pieces of Co2Ni1 catalytic membranes (20 × 40 mm2), 0.015 mol/L NaBH4 and a temperature of 25 °C. The corresponding kinetic reaction rate constant was 0.1410 min−1, which was 2.0 and 15.5 times higher than that of pure Co3O4 and NiO. Moreover, the activation energy, enthalpy and entropy for the reaction were calculated as 17.44 kJ/mol, 14.97 kJ/mol and −210.61 J•mol−1•K−1, respectively. Notably, the reduction rates of 4-NP remained at around 97% during thirteen consecutive catalytic runs, indicating the good reusability of the Co2Ni1 integral membrane catalyst. These results show that NF-based Co/Ni composite oxide membrane is a promising integral catalyst for converting of 4-NP into 4-AP owing to its excellent properties and easy recovery feature.