Effect of Ligands\nand Their Removal on the Au Nanoparticle-Catalyzed\nReduction of 4‑Nitrophenol

Abstract

The catalytic activity of gold nanoparticles (Au NPs)\nis strongly\naffected by the organic ligands coating them, which afford colloidal\nstability and are most commonly attached during the synthesis of the\nAu NPs. However, different ligands also produce Au NPs of different\nsizes and morphologies, complicating the study of the impact of the\nligands themselves. Alternatively, postsynthetic ligand exchange risks\nincomplete exchanges and a mixed ligand shell, as well as colloidal\ninstability. Here, Au NPs are immobilized on glass supports to afford\ntruly ligand-free, solvent-stable Au nanoislands (NIs). The catalytic\nactivity of Au NIs, uncoated and coated with a variety of ligands,\nis evaluated for the catalytic reduction of 4-nitrophenol to 4-aminophenol,\na common model reaction. The modification of stable, immobilized NIs\nensures identical size and shape distributions across all cases and\nisolates the effect of ligands to reveal their absolute impact versus\nuncoated Au. All molecules are found to inhibit catalysis to varying\ndegrees, with longer-chain molecules having a stronger influence.\nLigands are partially removed by borohydride during the reaction,\nas confirmed by X-ray photoelectron spectroscopy measurements, leading\nto significantly less inhibition when the catalysts are reused. Kinetic\nanalysis of the reaction for various ligand-coated catalysts shows\nthat the reaction rate increases over time due to ligand removal,\ndemonstrating the value of longer conversion studies versus the method\nof initial rates. These direct cross-class ligand comparisons, using\nthe same NP core, provide valuable insights into complex NP–ligand–reactant\ninteractions.