Effect of Ligands and Their Removal on the Au Nanoparticle-Catalyzed Reduction of 4-Nitrophenol

The catalytic activity of gold nanoparticles (Au NPs) is strongly affected by the organic ligands coating them, which afford colloidal stability and are most commonly attached during the synthesis of the Au NPs. However, different ligands also produce Au NPs of different sizes and morphologies, complicating the study of the impact of the ligands themselves. Alternatively, postsynthetic ligand exchange risks incomplete exchanges and a mixed ligand shell, as well as colloidal instability. Here, Au NPs are immobilized on glass supports to afford truly ligand-free, solvent-stable Au nanoislands (NIs). The catalytic activity of Au NIs, uncoated and coated with a variety of ligands, is evaluated for the catalytic reduction of 4-nitrophenol to 4-aminophenol, a common model reaction. The modification of stable, immobilized NIs ensures identical size and shape distributions across all cases and isolates the effect of ligands to reveal their absolute impact versus uncoated Au. All molecules are found to inhibit catalysis to varying degrees, with longer-chain molecules having a stronger influence. Ligands are partially removed by borohydride during the reaction, as confirmed by X-ray photoelectron spectroscopy measurements, leading to significantly less inhibition when the catalysts are reused. Kinetic analysis of the reaction for various ligand-coated catalysts shows that the reaction rate increases over time due to ligand removal, demonstrating the value of longer conversion studies versus the method of initial rates. These direct cross-class ligand comparisons, using the same NP core, provide valuable insights into complex NP–ligand–reactant interactions.