The reduction of 4-nitrophenol (4-NP) by sodium borohydride serves as an important model catalytic reaction for assessing the catalytic activity of metal nanoparticles since it is readily monitored in real-time by spectroscopic means, occurs at room temperature, and does not proceed unless a catalyst is present. Critical to its standing as a model reaction is a comprehensive mechanistic framework able to explain the entire time evolution of the reaction. While the reaction kinetics are governed by Langmuir–Hinshelwood model, the cause of the induction time, a period where no reaction seemingly occurs, remains controversial. Numerous mechanisms have been proposed for the activation of a seemingly inactive catalyst. Early work demonstrated that the induction time could be eliminated for Ag nanoparticles if reactants were first purged with an N2 gas. Other work suggested that dissolved oxygen outcompetes 4-NP for borohydride and, as a result, the reaction could only start once all the dissolved oxygen was consumed. More recently, it was hypothesized that the reaction could only begin once the catalyst surface was restructured through the attachment of reactants. This surface reconstruction also accounted for the fact that no induction time is observed when the catalyst is reused. Another proposed mechanism attributes the induction time to the time required for borohydride ions to inject electrons into the catalyst. While the literature reveals that dissolved oxygen influences the catalytic reduction of 4-NP, a comprehensive understanding does not yet exist. Here, we present a study in which we spectroscopically monitored the reduction of 4-NP by borohydride using Au, Ag, and Pd nanoparticles while simultaneously carrying out an in situ measurement of the dissolved oxygen content. By purging the reactants with either O2 or Ar gas we were able to systematically vary the dissolved oxygen content and, hence, obtain a quantitative assessment of its influence on the reaction. It was determined that dissolved oxygen plays a crucial role in determining both the apparent reaction rate constant (kapp) and the duration of the induction period. Based on these results, we will present a mechanistic framework able to rationalize our results both self-consistently and within the context of the preceding literature. We will also demonstrate how we applied this understanding to generate catalytic reactions with turnover frequencies that exceed all previously reported values.

Quantitative assessment of the role of dissolved oxygen in catalytic reduction of 4-nitrophenol

253rd National Meeting (2017)

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