One-pot synthesis of bimetallic Ni/Ag nanosphere inside colloidal silica cavities for in situ SERS monitoring of the elementary steps of chemoselective nitroarene reduction evidenced by DFTB calculation

Abstract

Spherically shaped bimetallic Ni/Ag nanocomposite is synthesized in one-pot within colloidal silica cavities in aqueous media in an efficient way and is characterized using UV-Vis, IR, EPR, Raman, powder XRD, SEM, TEM, and SERS. This could trigger the chemoselective NaBH4 reduction of nitoarenes under solvent free conditions at ambient temperature in excellent yields. The process could be scaled up to multi-gram scale without any need for temperature control and organic extractor. This spherical nanocomposite showed good operational stability, robustness and recyclability as is evidenced by a model reaction of 2-nitophenol (2-NP) to 2-aminophenol (2-AP). Reduction of nitroarenes is widely used for the studies of catalytic efficacy of nanoparticles as its mechanism is still unproven in heterogenous condition. Although complicated, a real time monitoring of surface-enhanced Raman scattering (SERS) with discrete steps, by placing the surface-immobilized reactant molecules at the well-defined spherical nanocomposite, we identified dihydroxyazobenzene (DHAB) as the intermediate. The catalysing process was further studied computationally using DFTB level of theory through Nudged elastic band (NEB) method to locate the involved transition states and intermediates. Overall, the results demonstrate that spherical Ni/Ag@silica bimetallic nanocomposite could catalyse the selective activation of NaBH4 reduction of nitoarenes in a fast, clean process and SERS as a powerful tool for studying heterogeneous catalysis and hence, the present catalyst system constitutes one straightforward and environmentally safe approach to a class of much sought organic transformation.