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

Abstract

We present a study on the catalytic\nreduction of 4-nitrophenol (Nip) to 4-aminophenol (Amp) by sodium\nborohydride (BH₄^–) in the presence of\nmetal nanoparticles in aqueous solution. This reaction which proceeds\nvia the intermediate 4-hydroxylaminophenol has been used abundantly\nas a model reaction to check the catalytic activity of metallic nanoparticles.\nHere we present a full kinetic scheme that includes the intermediate\n4-hydroxylaminophenol. All steps of the reaction are assumed to proceed\nsolely on the surface of metal nanoparticles (Langmuir–Hinshelwood\nmodel). The discussion of the resulting kinetic equations shows that\nthere is a stationary state in which the concentration of the intermediate\n4-hydroxylaminophenol stays approximately constant. The resulting\nkinetic expression had been used previously to evaluate the kinetic\nconstants for this reaction. In this stationary state there are isosbestic\npoints in the UV/vis-spectra which are in full agreement with most\npublished data. We compare the full kinetic equations to experimental\ndata given by the temporal decay of the concentration of Nip. Good\nagreement is found underlining the general validity of the scheme.\nThe kinetic constants derived from this analysis demonstrate that\nthe second step, namely the reduction of the 4-hydroxylaminophenol\nis the rate-determining step.