Catalytic Activity of Faceted Gold Nanoparticles Studied by a Model Reaction: Evidence for Substrate-Induced Surface Restructuring

We present the analysis of the catalytic activity of gold nanoparticles in aqueous solution as a function of temperature. As a model reaction, the reduction of p-nitrophenol (Nip) by sodium borohydride (BH4–) is used. The gold nanoparticles are immobilized on cationic spherical polyelectrolyte brushes that ensure their stability against aggregation. High-resolution transmission electron microscopy shows that the Au nanoparticles are faceted nanocrystals. The average size of the nanoparticles is 2.2 nm, and the total surface area of all nanoparticles could be determined precisely and was used in the subsequent kinetic analysis. Kinetic data have been obtained between 10 and 30 °C by monitoring the concentrations of Nip and BH4– by UV–vis spectroscopy. The reaction starts after an induction time t0, and the subsequent stationary phase yields the apparent reaction rate, kapp. All kinetic data could be modeled in terms of the Langmuir–Hinshelwood model; that is, both reactants must be adsorbed onto the surface to react. The analysis of the temperature dependence of kapp leads to the heat of adsorption of both Nip and BH4– and the surface of the Au nanoparticles. Moreover, the true activation energy of the surface reaction is obtained. The analysis of t0 reveals clearly that the induction period is not related to the limitations due to diffusion but to the surface restructuring of the Au nanoparticles induced by the adsorbed Nip. The rate 1/t0 of this substrate-induced surface restructuring is found to be proportional to the square of the surface coverage, θNip, by Nip and therefore points to a cooperative process.