Kinetic Analysis of Catalytic Reduction of 4-Nitrophenol by Metallic Nanoparticles Immobilized in Spherical Polyelectrolyte Brushes

TLDR

The study investigates the catalytic reduction of 4‑nitrophenol by sodium borohydride using metal nanoparticles immobilized in spherical polyelectrolyte brushes. The 2‑nm nanoparticles are grafted onto a polystyrene core with dense cationic polyelectrolyte brushes, and the reaction follows a Langmuir–Hinshelwood mechanism in which borohydride transfers surface‑hydrogen to adsorbed 4‑nitrophenol, with the rate determined by surface area, kinetic constants, and adsorption equilibria. Kinetic measurements show an induction time of minutes that scales inversely with nitrophenol concentration, indicating that a slow surface reconstruction of the nanoparticles governs the rate, consistent with the Langmuir–Hinshelwood model.

Abstract

We present a study on the catalytic reduction of 4-nitrophenol by sodium borohydride in the presence of metal nanoparticles. The nanoparticles are embedded in spherical polyelectrolyte brushes, which consist of a polystyrene core onto which a dense layer of cationic polyelectrolyte brushes are grafted. The average size of the nanoparticles is approximately 2 nm. The kinetic data obtained by monitoring the reduction of 4-nitrophenol by UV/vis-spectroscopy could be explained in terms of the Langmuir−Hinshelwood model: The borohydride ions transfer a surface-hydrogen species in a reversible manner to the surface. Concomitantly 4-nitrophenol is adsorbed and the rate-determining step consists of the reduction of nitrophenol by the surface-hydrogen species. The apparent reaction rate can therefore be related to the total surface S of the nanoparticles, to the kinetic constant k related to the rate-determining step, and to the adsorption constants KNip and KBH4 of nitrophenol and of borohydride, respectively. In all cases, an induction time t0 was observed of the order of minutes. The reciprocal induction time can be treated as a reaction rate that is directly related to the kinetics of the surface reaction because there is a linear relation between 1/(kt0) and the concentration of nitrophenol in the solution. All data obtained for t0 so far and a comparison with data from literature indicate that the induction time is related to a slow surface reconstruction of the nanoparticles, the rate of which is directly related to the surface reaction.