A facile approach to fabricate halloysite/metal nanocomposites with preformed and in situ synthesized metal nanoparticles: a comparative study of their enhanced catalytic activity

Abstract

A widespread study has been carried out by the researcher during the last few decades to improve the properties and performance of halloysite-based inorganic–organic hybrid nanocomposites (NCs) because of their excellent structural features and characteristic properties. Here, we report the fabrication of halloysite/metal nanocomposites on a large-scale through the immobilization of metal precursors followed by reduction or direct loading of preformed metal nanoparticles (NPs) over the surface of aminosilane modified halloysite nanotubes (HNTs), which in turn develops environmentally benign and low-cost heterogeneous catalysts. Characterization by different physical methods authenticates the successful fabrication of four different HNTs/metal NCs by these two different synthetic approaches, having monodispersed spherical morphology of the metal NPs. The catalytic activity and recyclability of all the NCs have been evaluated considering the reduction of 4-nitrophenol using sodium borohydride as a model reaction, attributed to their almost comparable catalytic efficiency. However, a detailed kinetic study demonstrates the enhanced catalytic activity of in situ synthesized HNTs/Ag among the four NCs, owing to the absence of any capping materials over the surface of NPs. Activation energy, pre-exponential factor, and entropy of activation have been estimated for this reduction reaction. A comparison study of their catalytic activity has been carried out with the reported heterogeneous catalysts, indicating the higher activity of these NCs for the reduction of nitroaromatics. Nevertheless, such an outstanding catalytic efficiency was only observed for HNTs/Au and HNTs/Ag NCs, with no activity of HNTs or aminosilane modified HNTs.