(681c) Thermodynamics of Surface Reactions: Relationship to Reactivity

Campbell, C., University of Washington

The energetics of adsorbed species provide key insight into surface reactions of importance in catalysis. Novel calorimetric methods for measuring adsorption energies will be described, and some systematic measurements of energies will be discussed. The adsorption energy of metal atoms on a variety of surfaces provides fundamental insight into morphology of the resulting metal film (or particles) that grow, and helps explain the surface chemical reactivity of bimetallics and oxide-supported metal nanoparticles. Calorimetric adsorption energies of organic molecules on transition metals provide direct measurements of adsorbate ? metal surface bond energies, which can be extrapolated to provide approximate bond energies for related organic species.

The oxides of many transition metals wet their own metal surface. The adhesion energy at this interface (Eadh) provides extra stabilization which lowers the O2 pressure required for oxide stability as a thin film below that required for bulk oxide stability by the factor exp[(2g - Eadh) / (tNRT)], where g is the surface energy of the oxide, t is the oxide film thickness, and N is the oxygen concentration in the bulk oxide (moles O2 per volume). For oxide films ~ 5 atomic layers thick, this correction can be many orders of magnitude, and can decide whether catalysis by transition metals is really happening on a metallic surface or instead on the surface of its oxide. Experimental evidence for such a thin-film oxide which is stable well outside the stability limits of its bulk oxide will be presented.

? Supported by NSF and DOE-BES Chemical Sciences.