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Modern surface chemistry has made significant contributions to the molecular level understanding of macroscopic surface phenomena including adsorption, bonding, catalysis and tribology. According to the classical view, surface atoms are rigid and occupy equilibrium sites dictated by the bulk unit cell. Modern surface science techniques that permit atomic scale monitoring of the structures of both the surface atoms and adsorbates during surface processes have shattered the myth of the rigid surface. A new model of the surface has been adopted, the so-called flexible surface. Upon adsorption the surface restructures and the metal atoms move into new sites, dictated by the chemisorption bond so as to optimize the strength of that bond, thereby creating the active sites for surface chemical processes. In this paper we show experimental evidence for the dynamic character of surfaces during chemical processes.
First a historical perspective is presented and basic concepts such as surface concentration and dispersion are introduced. Molecular sieves serve as examples for materials with large internal surfaces (microporous solids). Then the necessity of an ultra high vacuum environment to keep surfaces clean is illustrated and an introduction to gas adsorption is given. The most frequently used surface science techniques to determine surface properties such as structure, composition, oxidation states, chemical, electronic, and mechanical properties are briefly explained.
The article then focuses on new phenomena which were discovered by molecular surface chemistry. It is demonstrated that clean surfaces undergo relaxation and reconstruction. Adsorbate-induced restructuring of metal surfaces and coadsorption-induced ordering are revealed by high pressure scanning tunneling microscopy experiments. The structures and chemical rearrangements of ethylene upon adsorption are described. The importance of atomically rough (stepped and kinked) surfaces for carrying out surface reactions at high rates is illustrated by results of molecular beam surface scattering experiments. The bonding of hydrocarbons (aliphatic and aromatic) on metal surfaces is identified to being similar to that in organo-metallic clusters. Finally the present and future technological impact of molecular surface chemistry on catalysis and semiconductor devices is discussed.
For more information: http://www.cchem.berkeley.edu/~gasgrp
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