Synthesis of amino talc-like clay-supported Au nanoparticles with outstanding catalytic activity for 4-nitrophenol reduction

Abstract

Studies on synthesizing gold nanoparticle-based catalysts to reduce the amount of gold and improve the catalytic activity at the same time are still crucial. The reduction reaction of 4-nitrophenol (a toxic compound) to 4-aminophenol (a non-toxic compound and essential material for the pharmaceutical industries) is a standard reaction for analyzing the catalytic activity of AuNPs. A capping agent is indispensable for producing stable and size-controlled gold nanoparticles (AuNPs) and in maintaining the catalytic activity of the AuNPs. However, some previous studies reported that capping agents reduced the catalytic activity of AuNPs through a surface-blocking phenomenon. Moreover, introducing a framework for capped AuNPs could be another consideration to increase the dispersion of AuNPs in an aqueous medium, eventually improving the catalytic activity of the AuNPs. Amino talc-like clay, a relatively non-toxic and well-dispersed layered material in aqueous solution, has been used as a framework for AuNPs. The present study reports the AuNP synthesis using delaminated amino talc-like clay (DAC) as a framework that brings about gold nanoparticles with superior catalytic activity to other supports. Moreover, the effect of the reducing agent concentration, Mn addition and pH conditions are discussed in determining this excellent property. DAC shows a promising role as a AuNP framework which increases the catalytic activity by improving the dispersion and stability of the material in the colloidal system. The pH-dependent stacking structure arrangement of the DAC support was a determining factor of Kapp for the 4-nitrophenol reduction reaction by DAC stabilized AuNPs. Under acidic conditions, the stacking structure of DAC was destroyed, providing more open space for the AuNPs attached to the clay surface, which increased the Kapp value. On the contrary, under basic conditions, the stacking structure of DAC was re-arranged, closing the surface space of the AuNPs attached to the interlayer clay, which decreased the Kapp value. Moreover, the DAC stabilizer provided a partially capped AuNP structure in neutral and acidic aqueous solutions, which prevented the catalytically active surface blocking phenomenon occurring in the ordinary capping type. Mn addition during the synthesis process controlled the production of small-size gold nanoparticles and increased the catalytic activity of the material.