Adsorption on nanosurfaces: A detailed look at metal clusters using infrared spectroscopy

Abstract

A technique known as infrared photodissociation spectroscopy is being used at Argonne to probe the intimate details of how molecules and atoms adsorb onto metal clusters. Clusters of transition metal atoms, produced by laser vaporization of a metal target, are allowed to react with small molecules, producing cluster complexes whose properties mimic those of the small metal-containing particles that make up many industrial catalysts. A powerful infrared laser is used to excite the characteristic vibrations of the atoms or molecules adsorbed on the surfaces of the clusters, causing the complexes to fragment. The resulting photodissociation spectrum is capable of revealing whether the adsorbed molecules have undergone a chemical reaction after sticking to the cluster surface. This techniques has been used to show that methyl alcohol molecules readily adsorb to the surfaces of small iron clusters, but do not undergo further reaction once they are there. This behavior is fundamentally different from that observed on macroscopic iron surfaces, where methyl alcohol readily reacts to form smaller fragment species. It is anticipated that these experiments will contribute to the understanding of particle size effects and their influence on reaction mechanisms and pathways in heterogeneous catalysis systems.