Atomic displacements, induced adsorption

Asymmetric diarylmethanes, hydrogenolytic behaviors, 29 229-270, 247-252 catalytic hydrogenolysis, 29 243-258 kinetics and scheme, 29 252-258 M0O3-AI2O3 catalyst, 29 259-269 relative reactivity, 29 255-257 schematic model, 29 254 Asymmetric hydrogenations, 42 490-491 Asymmetric synthesis, 25 82, 83 examples of, 25 82 Asymmetry factor, 42 123-124 Atom-by-species matrix, 32 302-303, 318-319 Atomic absorption, 27 317 Atomic catalytic activities of sites, 34 183 Atomic displacements, induced by adsorption, 21 212, 213 Atomic rate or reaction definition, 36 72-73 structure sensitivity and, 36 86-87 Atomic species, see also specific elements adsorbed... 

The ready alleviation of surface distortion by adsorption of foreign atoms is of course only a special case of induced atomic displacement. Small motions of substrate atoms away from regular lattice sites can frequently be expected also. These are probably rather common after adsorption on any type of substrate, whether a metal, semiconductor or... 

Upon increasing the coverage, adsorbed H atoms form disordered or, at lower temperatures, ordered surface phases. Adsorption itself can induce relaxation or reconstruction of the substrate surface or may even lift the relaxation or reconstruction of the clean substrate surface, for example Ref [34]. The van der Waals type bonding of physisorbed H2 however is too weak to cause noticeable displacement of substrate atoms. There is experimental and theoretical evidence [34, 36, 57-60]... 

By relaxation of a substrate surface we mean small atomic displacements from ideal or clean-substrate positions. The displacements shall be small compared to near-neighbor distances, such that no rebonding (bond breaking or new bond formation) takes place within the substrate. Such relaxation may induce the formation of a two-dimensional superlattice if the displacements modify the initial substrate surface symmetry. This is especially common in adsorption, since the adsorbate itself often creates a superlattice. 

Reconstructed surfaces of metals are also perturbed by the Ceo molecules, as has been shown for bare Au(l 10)-p(l x 2) where a Au(110)-p(6 x 5) superstructure is induced (Pedio et al., 2000). Again, the adsorption of Ceo is accompanied by important displacements of underlying gold atoms. 

It has been established for Pt(100) and Pt(l 11) that when the specific adsorption of hydrochloric acid is strong enough, anions are partially displaced from the surface-adsorbed oxygen with negligible transient currents. For Pt(100), this strong specific adsorption also induces a surface atomic rearrangement toward a more stable surface with a more densely packed structure similar to the (5 x 20) reconstruction observed in the gas phase. 

The chemisorption of an atom or a molecule often induces rearrangement of the substrate atoms around the adsorption site. For example, the chemisorption of carbon atoms on the nickel (100) surface occurs at the fourfold site. The nearest-neighbor nickel atoms are displaced away from the carbon, permitting it to move more into the metal surface and bond to the metal atom in the second layer [5, 6]. A small in-plane rotation of the surface nickel atoms around the carbon, shown in Figure... 

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