THE FLEXIBLE SURFACE - CORRELATION BETWEEN REACTIVITY AND RESTRUCTURING ABILITY

Studies of the surface structure of metals on the atomic scale indicate the restructuring the clean surfaces with respect to their bulk atomic structure. Inward relaxation and reconstruction are more pronounced at steps, at open surfaces, and at low coordination sites. Chemisorption causes adsorbate-induced restructuring which drastically changes the surface structure. Such restructuring events occur on the time scale of chemisorption, on the time scale of catalytic reactions, and on a longer time scale that leads to redispersion and sintering. Thermodynamic considerations indicate more facile adsorption-induced restructuring for more open surfaces where atoms have fewer nearest neighbors (flexible surfaces) while close packed surfaces are less likely to restructure (rigid surfaces). The dynamical restructuring of surfaces on the atomic scale during chemisorption and catalytic reactions explains several puzzles of surface chemistry and heterogeneous catalysis. In surface chemistry, these include (1) the transition from physisorption to chemisorption, (2) why rough surfaces are more active in chemical bond breaking and in catalysis, (3) epitaxy, and (4) unusual coverage-dependent changes of bonding. In heterogeneous catalysis the dynamic model of surfaces may also shed light on why certain reactions are structure insensitive and on the unique activity of bimetallic and oxide-metal interface systems.