Effect of Nanoparticle Ligands on 4-Nitrophenol Reduction: Reaction Rate, Induction Time, and Ligand Desorption

Ligands are the quintessential synthesis tool in the preparation of colloidal metal catalysts, allowing for the rational design of nanostructured surfaces with high activity and selectivity. These same agents can, however, strongly influence the catalytic performance of metal nanostructures in aqueous media. In this regard, the current literature describing the influence of ligands on the model catalytic reaction that sees 4-nitrophenol reduced to 4-aminophenol by borohydride is highly fragmented in that the understanding of reaction rate, induction time, and ligand desorption phenomena is disconnected and, at times, contradictory. Herein, we present a study in which three chemically distinct ligands are applied to bare gold catalysts followed by their exposure to aqueous solutions of relevance to 4-nitrophenol reduction while simultaneously tracking the ligand whereabouts. It is shown that the exposure of ligands to borohydride leads to their near-complete removal from the gold catalyst. This, in turn, gives rise to severe disruptions to the rate of 4-nitrophenol reduction for the scenario where the aqueous reactants are purged of dissolved oxygen and ligand desorption times are slow. In sharp contrast, the reaction rate is little affected when the same reactants contain dissolved oxygen because the resulting induction period provides ample time for the ligands to desorb prior to the onset of the reaction. Moreover, strongly bound ligands are shown to give rise to an induction-time-like feature that is only observable when the reactants are free of dissolved oxygen. Collectively, these findings advocate procedures for catalytic benchmarking that differ from current best practices and underscore the point that a fundamental understanding of 4-nitrophenol catalysis must adopt a holistic approach that accounts for ligand-nanostructure interdependencies.