Catalytic Reduction of 4-Nitrophenol by Gold Catalysts: The Influence of Borohydride Concentration on the Induction Time

A comprehensive mechanistic framework is key to the effective utilization of model catalytic reactions. Although the reduction of 4-nitrophenol by borohydride has emerged as one of the most widely used model reactions for accessing the catalytic activity of nanostructures, there still exist knowledge gaps. The cause of the induction time, which is a period at the beginning of the reaction where no reaction seemingly occurs, has long been the subject of debate. Recent work provides compelling experimental evidence that links the induction time to the consumption of dissolved oxygen within the aqueous reactants and provides a mechanistic understanding based on a previously unknown side reaction. A concern has, however, been raised that the proposed mechanism is unable to account for prior work showing the induction time to be independent of the borohydride concentration. Here, a systematic study is presented that re-examines this dependency where reactions are simultaneously monitored using spectroscopy and an in situ dissolved oxygen probe. The dependency is shown to be far more involved than prior studies suggest because it varies with the amount of catalyst added. The understanding obtained is consistent with the previously proposed mechanism for the induction time and resolves perceived inconsistencies with earlier work.