Effect of Nanoparticle Ligands on 4‑Nitrophenol\nReduction: Reaction Rate, Induction Time, and Ligand Desorption

Abstract

Ligands\nare the quintessential synthesis tool in the preparation\nof colloidal metal catalysts, allowing for the rational design of\nnanostructured surfaces with high activity and selectivity. These\nsame agents can, however, strongly influence the catalytic performance\nof metal nanostructures in aqueous media. In this regard, the current\nliterature describing the influence of ligands on the model catalytic\nreaction that sees 4-nitrophenol reduced to 4-aminophenol by borohydride\nis highly fragmented in that the understanding of reaction rate, induction\ntime, and ligand desorption phenomena is disconnected and, at times,\ncontradictory. Herein, we present a study in which three chemically\ndistinct ligands are applied to bare gold catalysts followed by their\nexposure to aqueous solutions of relevance to 4-nitrophenol reduction\nwhile simultaneously tracking the ligand whereabouts. It is shown\nthat the exposure of ligands to borohydride leads to their near-complete\nremoval from the gold catalyst. This, in turn, gives rise to severe\ndisruptions to the rate of 4-nitrophenol reduction for the scenario\nwhere the aqueous reactants are purged of dissolved oxygen and ligand\ndesorption times are slow. In sharp contrast, the reaction rate is\nlittle affected when the same reactants contain dissolved oxygen because\nthe resulting induction period provides ample time for the ligands\nto desorb prior to the onset of the reaction. Moreover, strongly bound\nligands are shown to give rise to an induction-time-like feature that\nis only observable when the reactants are free of dissolved oxygen.\nCollectively, these findings advocate procedures for catalytic benchmarking\nthat differ from current best practices and underscore the point that\na fundamental understanding of 4-nitrophenol catalysis must adopt\na holistic approach that accounts for ligand–nanostructure\ninterdependencies.