Core/shell composite microparticles for catalytic reduction of p-nitrophenol: kinetic and thermodynamic study

Abstract

A facile and single-step in situ chemical reduction method has been adopted to effectively assemble silver nanoparticles in already prepared polystyrene-poly(N-isopropyl methacrylamide-acrylic acid) core/shell microgels. The successful endowment of silver nanoparticles in microparticles was confirmed by UV–visible spectroscopy, Fourier transform infrared spectroscopy, dynamic light scattering and transmission electron microscopic analysis. Microscopic analysis showed core–shell-type morphology of pure microparticles and composite microparticles. Reported composite system exhibited high catalytic activity toward reduction of p-nitrophenol due to the presence of silver nanoparticles in a thin-layered shell of core/shell polymer system. Composite microparticles showed a change in the value of apparent rate constant with a change in reaction conditions such as concentration of NaBH4, p-nitrophenol, catalyst and temperature of the medium. The temperature dependence of rate constant of catalytic reduction of p-nitrophenol reveals that the catalytic activity of composite microparticles is thermally tunable and p-nitrophenol is converted into p-aminophenol through an activated complex. The value of apparent rate constant was found in the range of 0.0141–0.0356 s−1 depending upon medium temperature. The positive values of enthalpy of activation (ΔH* = 17,334.69 J/mol) and entropy of activation (ΔS* = 37,105.4 J/mol K) suggest that the process of formation of activated complex is endothermic and entropy driven. The composite microparticles were proved to be promising and easily recoverable catalysts with maintained catalytic activity up to four cycles.