Highly efficient reduction of 4-nitrophenol by heterostructured gold-magnetite nanocatalysts

Abstract

In this study, the catalytic reduction of 4-nitrophenol by heterostructured Au–Fe₃O₄ nanocatalysts using NaBH₄ as the reducing agent was investigated under various environmental conditions. The electron behaviors at the interface of Au and Fe₃O₄ nanoparticles were examined to elucidate the reaction mechanisms for 4-nitrophenol reduction. The transmission electron microscopic images show that the average particle size of Au–Fe₃O₄ heterostructures increases slightly from 14 to 18 nm after phase transfer from oil phase to aqueous solution. The X-ray photoelectron and X-ray absorption near edge spectroscopic results show the electron flow from Au seeds to Fe₃O₄, resulting in the formation of positively charged Au surface to accelerate the catalytic reduction efficiency and rate of 4-nitrophenol. In addition, the reduction of 4-nitrophenol is a surface-mediated reaction and the catalytic efficiency and rate of 4-nitrophenol is highly dependent on the initial 4-nitrophenol concentration, pH, and reaction temperature. The increase in pH lowers the reduction efficiency and rate of 4-nitrophenol and a 2.4-fold decrease in the pseudo-first-order rate constant is observed when pH increases from 5 to 9. In addition, the Au–Fe₃O₄ nanocatalysts show a good separation ability and reusability which can be repeatedly applied for complete reduction of 4-nitrophenol for at least six successive cycles without the loss of morphology and saturation magnetization. Results obtained in this study clearly demonstrate that the Au–Fe₃O₄ heterostructures are excellent nanocatalysts which can be applied in heterogeneous catalysis, water treatment, and green chemistry.