The flexible surface: new techniques for molecular level studies of time dependent changes in metal surface structure and adsorbate structure during catalytic reactions

Abstract

The new model that emerges from the results of recent surface science studies is one of a dynamic, flexible surface that undergoes rapid adsorbate-induced restructuring on the time scale of chemisorption, or at times, slower restructuring on the time scale of catalytic surface reactions, atom diffusion-controlled faceting, or solid state reactions. Many of the unique chemical properties of the surface can be associated with its ability to restructure rapidly as adsorbate bonds form or break. Surface specific vibrational spectroscopy by sum frequency generation (SFG) and high pressure/high temperature scanning tunneling microscopy (STM) are two independent but complimentary techniques which permit molecular level studies of surfaces under dynamic conditions at high pressures. The application of these techniques allows in situ analysis of reactive surfaces during catalytic conditions. These techniques have been used to study ethylene and propylene hydrogenation on platinum surfaces. The investigations provide definitive evidence for physisorbed intermediates, stagnant chemisorbed species, and an unreconstructed surface during catalysis. The role of carbon-containing overlayers covering the surface during reactions has been identified. A detailed molecular mechanism for these two reactions can be proposed.