A near ambient pressure XPS study of subnanometer silver clusters on Al2O3 and TiO2 ultrathin film supports.

We have investigated model systems of silver clusters with different sizes (3 and 15 atoms) deposited on alumina and titania supports using ambient pressure X-ray photoelectron spectroscopy. The electronic structures of silver clusters and support materials are studied upon exposure to various atmospheres (ultrahigh vacuum, O2 and CO) at different temperatures. Compared to bulk silver, the binding energies of silver clusters are about 0.55 eV higher on TiO2 and 0.95 eV higher on Al2O3 due to the final state effect and the interaction with supports. No clear size effect of the silver XPS peak is observed on different silver clusters among these samples. Silver clusters on titania show better stability against sintering. Al 2p and Ti 2p core level peak positions of the alumina and titania support surfaces change upon exposure to oxygen while the Ag 3d core level position remains unchanged. We discuss the origin of these core level shifts and their implications for catalytic properties of Ag clusters.

Oxidation and reduction of size-selected subnanometer Pd clusters on Al2O3 surface.

In this paper, we investigate uniformly dispersed size-selected Pd(n) clusters (n = 4, 10, and 17) on alumina supports. We study the changes of clustered Pd atoms under oxidizing and reducing (O2 and CO, respectively) conditions in situ using ambient pressure XPS. The behavior of Pd in the clusters is quite different from that of Pd foil under the same conditions. For all Pd clusters, we observe only one Pd peak. The binding energy of this Pd 3d peak is ~1-1.4 eV higher than that of metallic Pd species and changes slightly in CO and O2 environments. On the Pd foil however many different Pd species co-exist on the surface and change their oxidation states under different conditions. We find that the Pd atoms in direct contact with Al2O3 differ in oxidation state from the surface Pd atoms in a foil under reaction conditions. Compared to previous literature, we find that Pd 3d peak positions are greatly influenced by the different types of Al2O3 supports due to the combination of both initial and final state effects.