Using Thiol Adsorption on Supported Au Nanoparticle Catalysts To Evaluate Au Dispersion and the Number of Active Sites for Benzyl Alcohol Oxidation

Two techniques to study the surface chemistry of supported gold nanoparticles were developed. First, phenylethyl mercaptan (PEM) adsorption from hexane solution was followed with UV–vis spectroscopy to evaluate the total amount of surface Au available. Two catalysts, Au/Al₂O₃ and Au/TiO₂, were found to have Au:S surface stoichiometries of ∼2:1, whereas a Au/SiO₂ catalyst had a Au:S surface stoichiometry of ∼1:1. The room temperature equilibrium binding constants for PEM adsorption on the Au/Al₂O₃ and Au/TiO₂ catalysts were similar (∼3 × 10⁵ M^–1; Δ<i>G</i> ≈ −31 kJ/mol); the PEM–Au/SiO₂ binding constant was somewhat larger (∼2 × 10⁶ M^–1; Δ<i>G</i> ≈ −36 kJ/mol). XPS data for all of the catalysts showed no observable changes in the Au oxidation state upon adsorption of the thiol. Implications of these experiments regarding self-assembled monolayers and thiol-stabilized Au nanoparticles are discussed. Second, kinetic titrations (i.e., controlled thiol-poisoning experiments) were developed as a method for evaluating the number of active sites for selective 4-methoxybenzyl alcohol oxidation. These experiments suggested only a fraction of the surface Au (∼10–15% of the total Au) was active for the reaction. When thiol was added with the 4-methoxybenzyl alcohol substrate, more thiol was required to poison the catalyst, suggesting that the thiol and substrate compete for initial adsorption sites, possibly at the metal–support interface. These two methods were combined to evaluate the magnitude of the support effect on selective 4-methoxybenzyl alcohol oxidation. Correcting the catalytic activity of the catalysts to the number of sites determined by thiol titration provided clear evidence that the support has a strong influence on the catalytic activity of Au in benzyl alcohol oxidation