Oxide-supported metal clusters: models for heterogeneous catalysts

Abstract

Understanding the size-dependent electronic, structural and chemical properties ofmetal clusters on oxide supports is an important aspect of heterogeneouscatalysis. Recently model oxide-supported metal catalysts have been prepared byvapour deposition of catalytically relevant metals onto ultra-thin oxide filmsgrown on a refractory metal substrate. Reactivity and spectroscopic/microscopicstudies have shown that these ultra-thin oxide films are excellent models for thecorresponding bulk oxides, yet are sufficiently electrically conductive foruse with various modern surface probes including scanning tunnellingmicroscopy (STM). Measurements on metal clusters have revealed a metalto nonmetal transition as well as changes in the crystal and electronicstructures (including lattice parameters, band width, band splitting andcore-level binding energy shifts) as a function of cluster size. Size-dependentcatalytic reactivity studies have been carried out for several importantreactions, and time-dependent catalytic deactivation has been shown to arisefrom sintering of metal particles under elevated gas pressures and/orreactor temperatures. In situ STM methodologies have been developed tofollow the growth and sintering kinetics on a cluster-by-cluster basis.Although several critical issues have been addressed by several groupsworldwide, much more remains to be done. This article highlights some ofthese accomplishments and summarizes the challenges that lie ahead.