Surface metal atom, adsorption-induced reconstruction

Adsorption-induced reconstruction, surface metal atom, 245 Adsorption isotherms, development, 4/,5 Alcohols, role in photooxidation reaction of water on n-Ti02 electrode, 297-308 Alloy single-crystal surface, thin anodic oxide overlayers, 236-244 Alloy surfaces, chemical reconstruction, 246-247... 

So far, it has been accepted that molecules or atoms are adsorbed on sites being in registry with the substrate lattice, and the catalysis is explained by their reaction over the surface. By the adsorption of molecules or atoms, surface metal atom undergoes so often short distance shift from their equilibrium position, which is the adsorption induced reconstruction. It should be pointed out that such short distance shift of the... 

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 appears that a significant amount of energy is required for CH activation, the primary elementary step of the hydrocarbon conversion reaction. If one chemisorbs methane at low temperature on a transition metal surface, it desorbs before reaction can occur. Aliphatic hydrocarbons can dissociate from a preadsorbed state, if they contain enough carbon atoms in their chain to induce a high heat of adsorption whereby CH dissociation can take place at a rate large compared to the desorption rate. So far this has only been found for surfaces containing highly active metal atoms, such as the reconstructed Ir (110) surface or stepped surfaces l. As we will see, both electronic and steric effects may play a role. 

The final section in Chapter 2 deals with the molecular asp>ects of transition-metal catalysis. It serves as an introduction to Chapter 3. A characteristic feature of the transition-metal surfaces under catalytic conditions is their potential to restructure. Adsorbate overlayer adsorption can induce the surface to reconstruct with rapid diffusion of the metal as well as the overlayer atoms. The state of the surface may start to resemble that of a solid state compound. The state of the surface is not only strongly influenced by the composition of the reactant gas, but can also be strongly affected by the addition of promoters or other modifiers, that can result in alloy formation or new complex surface phases. 

By adsorption of metal atoms, one can induce various reconstructed surfaces that show metal-insulator phase transitions. Some transitions apparently are of the Mott type [73], whereas the driving mechanism of others is still subject to discussion, particularly considering the role of defects. Glasslike, disordered states have been found, which are very similar to theoretical predictions for phase separation in correlated electron systems [105]. 

A quite different class of adsorbate-induced surface reconstruction is formed by those systems involving pseudo-(lOO) reconstruction of the outermost atomic layer this behaviour has been found to occur on fcc(lll) and (110) surfaces in several metal/adsorbate combinations. The essential driving force for such reconstructions appears to be that adsorption on a (100) surface (typically in a c(2 x 2) arrangement) is so energetically favourable that, even on a surface with a different lateral periodicity (and point-group symmetry), reconstruction of the outermost layer or layers to form this (100)-like geometry is favoured. This must occur despite the introduction of strain energy at the interface between the substrate and the... 

The surface structure of single-crystal faces of noble metals such as KgQtkl) and AvL hkJ) were found to be unreconstructed under defined electrochemical conditions. For example, a unreconstructed Ag(lll) surface domain on a terrace in contact with perchloric acid solution is shown in Fig. 1. Surface reconstruction changing the interatomic distance and symmetry of surface atoms can be induced thermally or by potential, as observed by other authors [12]. This phenomenon can be lifted by potential or by adsorption processes. Surface defects such as monatomic steps and... 

The adsorbed H atoms vibrate around their equilibrium position (e.g. on top of a surface atom, or a bridge over two surface atoms) diffuse along the surface and form disordered or ordered surface structures according to temperature, coverage and substrate. The adsorption often induces a rearrangement of the surface atoms of the substrate, a surface reconstruction, or even a surface segregation if the substrate is a binary compound. Thus in a description of the topic H on metals" the following aspects should be considered ... 

Kirsch and Harris [176] calculated Ni(OOl) surface reconstructions induced by C, N, and O adsorption and suggested that C and N atoms prefer the nearly coplanar sites with the top Ni surface, which induce the clock reconstruction of the surface. However, O atoms prefer sites slightly above the Ni(lOO) surface plane and have little effect on the overall surface structure. The local environments of the C, N, and O atoms on these surfaces are similar to their environments in a series of late transition metal carbonyl clusters, suggesting that some of the same electronic factors may play a role in favouring different structures. Results of the calculations suggest that adsorbates occupy coplanar sites on Ni(lOO) surface disrupt the Ni-Ni bonding within the surface layer and bonds between the surface and second layers. 

However, adsorbate could induce various kinds of stresses accompanied with versatile patterns of relaxation and reconstruction [59, 81, 82]. The spacing between the first and the second atomic layers expands if an adsorbate such as C, N, and O buckles into space between the atomic layers even if there is contraction of bonds between the adsorbates and the host atoms [81]. For example, H, C, N, O, S, and CO adsorbates on a metal surface could change the surface stress and cause surface reconstmction because of bond making and breaking. Surface adsorption of sodium ions also increases the stiffness of a microcantilever [83]. 

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