Kinetic Analysis of the Catalytic Reduction of 4-Nitrophenol by Metallic Nanoparticles

We present a study on the catalytic reduction of 4-nitrophenol (Nip) to 4-aminophenol (Amp) by sodium borohydride (BH4-) in the presence of metal nanoparticles in aqueous solution. This reaction which proceeds via the intermediate 4-hydroxylaminophenol has been used abundantly as a model reaction to check the catalytic activity of metallic nanoparticles. Here we present a full kinetic scheme that includes the intermediate 4-hydroxylaminophenol. All steps of the reaction are assumed to proceed solely on the surface of metal nanoparticles (Langmuir-Hinshelwood model). The discussion of the resulting kinetic equations shows that there is a stationary state in which the concentration of the intermediate 4-hydroxylaminophenol stays approximately constant. The resulting kinetic expression had been used previously to evaluate the kinetic constants for this reaction. In this stationary state there are isosbestic points in the UV/vis-spectra which are in full agreement with most published data. We compare the full kinetic equations to experimental data given by the temporal decay of the concentration of Nip. Good agreement is found underlining the general validity of the scheme. The kinetic constants derived from this analysis demonstrate that the second step, namely the reduction of the 4-hydroxylaminophenol is the rate-determining step.