Surfaces and Chemisorption

Heterogeneous catalysis deals with reactions between species that are adsorbed on the surface of a catalyst. The role of the catalytic surface is to provide an energetically favorable pathway for the reaction. In order to find an explanation for the catalytic activity of substances, it is essential that we look at properties of the surface, rather than at a collective property of the bulk of the catalyst. 

The distinctive feature of a surface atom is that it has fewer neighbors than an atom in the interior. This unsaturated coordination forms the reason why the electronic and vibrational properties and sometimes the crystallographic positions of surface atoms differ from those of the bulk atoms. 

This appendix begins with a brief introduction to the physics of metal surfaces. We limit ourselves to those properties of surfaces that play a role in catalysis or in catalyst characterization. The second part is an introduction to the theory of 

The most important metals for catalysis are those of Groups VIII and I-B of the periodic system. Three crystal structures are important, face-centered cubic (fee Ni, Cu, Rh, Pd, Ag, Ir, Pt, Au), hexagonal close-packed (hep Co, Ru, Os) and body-centered cubic (bcc Fe) [9, 10]. Before discussing the surfaces that these lattices expose, we mention a few general properties. 

If the lattices are viewed as close-packed spheres, the fee and the hep lattices have the highest density they possess about 26% empty space. Each atom in the interior has 12 nearest neighbors, or, in other words, an atom in the interior has a coordination number of 12. The bcc lattice is slightly more open and contains about 

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Oct. 14, 1922, Kromeriz, then Czechoslovakia - Aug. 10, 2005, Berlin, Germany) Koutecky was a theoretical electrochemist, quantum chemist, solid state physicist (surfaces and chemisorption), and expert in the theory of clusters. He received his PhD in theoretical physics, was later a co-worker of -> Brdicka in Prague, and since 1967 professor of physical chemistry at Charles University, Prague. Since 1973 he was professor of physical chemistry at Freie Universitat, Berlin, Germany. Koutecky solved differential equations relevant to spherical -> diffusion, slow electrode reaction, - kinetic currents, -> catalytic currents, to currents controlled by nonlinear chemical reactions, and to combinations of these [i-v]. For a comprehensive review of his work on spherical diffusion and kinetic currents see [vi]. See also Koutecky-Levich plot. 

Adsorption of atoms and molecules on surfaces plays a fundamental role in catalysis a distinction can be made between physisorption, in which weak Van der Waals forces bind the atom/molecule to the surface, and chemisorption in which chemical bonds dominate. Much experimental and theoretical work is devoted to studying energy changes as a molecule approaches the surface and dissociates (or doesn t) into separate atoms on the surface. Here we concentrate on the relation between structure and bonding for chemisorbed atoms in their equilibrium sites on the surface (ch. 1, sect. 2.4). 

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