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

Aggregation Behavior of Ligand-Protected Au₉ Clusters on Sputtered Atomic Layer Deposition TiO₂

[Au₉(PPh₃)₈)]­(NO₃)₃ (Au₉) clusters were deposited onto sputtered ALD titania surfaces. Atomic force microscopy (AFM) was used to determine the height and distributions of the Au₉ clusters over the titania surface fabricated using atomic layer deposition (ALD). Synchrotron X-ray photoelectron spectroscopy (XPS) was used to derive information about the degree of agglomeration of the Au₉ clusters due to the annealing process. Both AFM and XPS show that the Au₉ clusters deposited on ALD titania are partially agglomerated after annealing. Deposition of the [Au₉(PPh₃)₈)]­(NO₃)₃ clusters on sputtered ALD titania is compared with deposition of the same cluster on titania nanosheets of previous work

Investigation of Ligand-Stabilized Gold Clusters on Defect-Rich Titania

Chemically synthesized atomically precise gold clusters stabilized by triphenylphosphine ligands [Au₉(PPh₃)₈]­(NO₃)₃] were deposited onto the surface of titania fabricated via atomic layer deposition. The titania surface was pretreated by heating and sputtering. After deposition of the clusters onto pretreated titania, the samples were heated at 200 °C for 20 min under ultrahigh vacuum and subsequently investigated using metastable-induced electron spectroscopy to study the electronic structure of the outermost layer of the sample and X-ray photoelectron spectroscopy to determine the chemical composition of the surface of the sample. The former study revealed that two reference spectra are needed to explain the electronic structure of the sample. One reference spectrum is related to the titania substrate, while the second spectrum is related to the presence of the Au cluster cores and the ligands removed from the cluster cores. The latter study found that the Au 4f peak is shifted to lower binding energy and the P 2p peak to higher binding energy after heating. These are interpreted in the light of ligand removal and size evolution of Au particles upon heating of the clusters on titania. The important outcome of the present work is that defects introduced at the ALD titania surface via sputtering and heating strongly reduce the agglomeration of the Au clusters adsorbed to the surface

Toward Control of Gold Cluster Aggregation on TiO₂ via Surface Treatments

Well-defined Au–TiO₂ materials were synthesized by deposition of triphenylphosphine-protected Au₉ clusters on TiO₂ (Aeroxide P-25), pre-treated in eight different ways and subsequently exposed to two post-treatments. X-ray photoelectron spectroscopy and UV–vis diffuse reflectance spectroscopy studies showed that in most cases the PPh₃ ligand shell was removed upon deposition even before post-treatment, coinciding with some cluster aggregation. However, clusters deposited on TiO₂ treated using H₂SO₄ and H₂O₂ showed remarkable resistance to aggregation, even after high-temperature calcination, while clusters on H₂-treated TiO₂ showed the greatest resistance to aggregation under ozonolysis