Monodisperse Pd and Au nanoparticles of 1-2 nm in size were created using poly(amidoamine) dendrimers as templates. Once synthesized, these dendrimer-encapsulated nanoparticles (DENs) were used to catalyze the reduction of p-nitrophenol (p-NP) to p-aminophenol via sodium borohydride. In the presence of a large excess of NaBH4, this reaction appears pseudo first-order with respect to p-NP, and so the apparent rate constant for the reaction was measured according to classical first-order kinetics. The apparent rate constant was measured for several different starting concentrations of p-NP.  The apparent rate constant decreased with increasing concentration of p-NP, suggesting that this may be a surface area mediated reaction that can be modeled using a Langmuir-Hinshelwood type isotherm.  Indeed, we were able to fit the data according to the Langmuir-Hinshelwood model, and hence we were able to derive values of the binding of p-NP and sodium borohydride to the surface of the nanoparticles. At a constant sodium borohydride concentration of 40mM, the binding constant for p-NP to Au nanoparticles was an order of magnitude higher than the binding constant to Pd nanoparticles. The lower binding constant for Pd DENs provides a mechanistic explanation of kinetic data showing Pd DENs to be a better catalyst for the reduction of p-NP:  the lower affinity of p-NP for the palladium surface may allow better surface coverage by NaBH4 on the DENs, thereby increasing the rate of reaction.  We conclude that the reduction of p-NP by NaBH4 catalyzed by metal nanoparticles is a surface-mediated reaction that can be modeled by a Langmuir-Hinshelwood isotherm.

Surface restructuring of palladium and gold dendrimer-encapsulated nanoparticles

247th National Meeting (2014)

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