Establishing a Au Nanoparticle Size Effect in the Oxidation of Cyclohexene Using Gradually Changing Au Catalysts

The effect of the size of gold nanoparticles on their catalytic activity in aerobic oxidation of cyclohexene was established using supported gold nanoparticles that gradually undergo a change in size during the catalytic reaction. Two triphenylphosphine-stabilized clusters, Au9(PPh3)8(NO3)3 and Au101(PPh3)21Cl5, were synthesized and deposited on SiO2. The clusters did not retain their structure during the catalytic reaction; larger particles with mean diameters of ∼5–10 nm gradually formed. By combining kinetic experiments with the monitoring of catalyst transformations using transmission electron microscopy, diffuse-reflectance ultraviolet–visible spectroscopy, and X-ray photoelectron spectroscopy, we showed that catalytic activity appeared only after >2 nm Au0 particles had formed, while intact clusters and phosphine-free <2 nm particles were inactive in cyclohexene oxidation under the studied conditions.