Adsorbate surface restructuring

Since the most active catalytic sites are usually steps, kinks, and surface defects, atomically resolved structural information including atomic distribution and surface structure at low pressure, possible surface restructuring, and the mobility of adsorbate molecules and of the atoms of the catalyst surface at high temperature and high pressure is crucial to understanding catalytic mechanisms on transition metal surfaces. The importance of studying the structural evolution ofboth adsorbates... 

Another example of the flexibility of the Pt catalyst is the reconstruction of a stepped Pt(l 11) crystal with adsorbed sulfur upon exposure to CO [25]. Single-crystal Pt(l 1 1) cut at an angle of approximately 5° from the (1 1 1) direction consists of numerous terraces with a width of 20-60 A separated by steps with single-atom height. The adsorption of sulfur atoms restructures the clean stepped Pt(l 1 1) surface with single-atom steps into a sulfur-adsorbed surface with double-atom... 

This is different at semiconductor surfaces where the covalent bonds between the substrate atoms are often strongly perturbed by the presence of adsorbates. This can result in a significant surface restructuring. Hence the dynamics of the substrate atoms has to be explicitly taken into account which of course increases the complexity of the modelling of the adsorption/desorption dynamics, as will be shown below for the H2/Si system. 

Oscillations connected with adsorbate-induced surface restructuring were studied also in [29]. The model used was aimed at mimicking oscillations in NO reduction by H2 on a mesoscopic Pt particle containing two catalytically active (100) areas connected by an inactive (111) area that only adsorbed NO reversibly. NO diffusion on and between facets was much faster than other steps. The results obtained show that the coupling of the catalytically active sublattices may synchronize nearly harmonic oscillations observed on these sublattices and also may result in the appearance of aperiodic partially synchronized oscillations. The spatiotemporal patterns corresponding to these regimes are nontrivial. In particular, the model predicts that, due to phase separation, the reaction may be accompanied by the formation of narrow NO-covered zones on the (100) sublattices near the (lOO)-(lll) boundaries. These zones partly prevent NO supply from the (111) sublattice to the (100) sublattices. 

Adsorbate Structure and Adsorbate-Induced Restructuring at Metal Surfaces... 

Tabulations of some surface structures may be found in a review by Van Hove and coworkers (Van Hove et al., 1989) and the reviews by Watson that compare the results of surface structure determinations utilizing different crystallographic techniques (Watson, 1990, 1992). Van Hove has also published a recent review of crystal surface structure, without the tabular presentation of the structural data (Van Hove, 1992). Van Hove and Somorjai have reviewed surface structure from the point-of-view of adsorbate induced restructuring of surfaces (Van Hove and Somorjai, 1989). Ohtani and coworkers have listed all observed overlayer structures and surface symmetries, albeit without any reference to the detailed surface structure (Ohtani et al., 1987). 

Silver deposition is important for technical applications and is achieved by electroless deposition (from a silver-cyanide bath) or bulk electrodeposition from the same bath. It has been studied by a large number of techniques [59,62,63,87], and most of them on gold and platinum in sulfate or perchlorate electrolytes [88-95]. However, there is lack of information on the problem of co-adsorption of metal and anions and the surface restructuring caused by the presence of strong adsorbable anions such as halides. 

A hard or A type system is one that is not restructured in the presence of an adsorbed film or on contact with the bulk liquid. For it, equations such as Equations 1 and 2 should apply approximately. A soft or B type system shows significant surface restructuring. Additivity and geometric mean rules are likely invalid the entropy of Interface formation will be important. Schematic illustrations of the two cases are shown in Figure 4. 

Ordered Monolayers and the Reasons for Ordering Adsorbate-Induced Restructuring Atomic Adsorption and Penetration into Substrates Metals on Metals Epitaxial Growth Growth Modes at Metal Surfaces Molecular Adsorption... 

The chemical bonds formed during adsorption between the adsorbate and the substrate lead to epitaxy or may induce surface restructuring. 

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