1 research outputs found
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/Al<sub>2</sub>O<sub>3</sub> and Au/TiO<sub>2</sub>, were found
to have Au:S surface stoichiometries of ∼2:1, whereas a Au/SiO<sub>2</sub> catalyst had a Au:S surface stoichiometry of ∼1:1.
The room temperature equilibrium binding constants for PEM adsorption
on the Au/Al<sub>2</sub>O<sub>3</sub> and Au/TiO<sub>2</sub> catalysts
were similar (∼3 × 10<sup>5</sup> M<sup>–1</sup>; Δ<i>G</i> ≈ −31 kJ/mol); the PEM–Au/SiO<sub>2</sub> binding constant was somewhat larger (∼2 × 10<sup>6</sup> M<sup>–1</sup>; 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