Island formation

Bonig L, Liu S and Metiu FI 1996 An effective medium theory study of Au islands on the Au(IOO) surface reconstruction, adatom diffusion, and island formation Surf. Sot 365 87... 

Yan C, Jensen J A and Kummel A 1994 Large island formation versus single site adsorption for CI2 chemisorption onto Si(111 )-7 X 7 surfaces Phys. Rev. Lett. 72 4017-20... 

There are several reasons for deviations from the LHHW kinetics Surface heterogeneity, surface reconstruction, adsorbate island formation and, most important, lateral coadsorbate interactions.18,19 All these factors lead to significant deviations from the fundamental assumption of the Langmuir isotherm, i.e. constancy of AHa (and AHB) with varying coverage. 

The oxidation of CO on Pt is one of the best studied catalytic systems. It proceeds via the reaction of chemisorbed CO and O. Despite its complexities, which include island formation, surface reconstruction and self-sustained oscillations, the reaction is a textbook example of a Langmuir-Hinshelwood mechanism the kinetics of which can be described qualitatively by a LHHW rate expression. This is shown in Figure 2.39 for the unpromoted Pt( 111) surface.112 For low Pco/po2 ratios the rate is first order in CO and negative order in 02, for high pco/po2 ratios the rate becomes negative order in CO and positive order in 02. Thus for low Pcc/po2 ratios the Pt(l 11) surface is covered predominantly by O, at high pco/po2 ratios the Pt surface is predominantly covered by CO. 

Riedmiiller B, Papageorgopoulos DC, Berenbak B, van Santen RA, Kleyn AW. 2002. Magic island formation of CO coadsorbed with H on Ru(OOOl). Surf Sci 515 323. 

The rate of y -alumina island formation essentially depends on the nature of the electrolyte used. If outwards migrating (in the terms of Xu et al.102) anions, such as tungstates and molybdates, are used in the anodization process, y- alumina seed crystals are surrounded by pure alumina and crystallization occurs easily. In the case of inwards migrating anions (e.g., citrates, phosphates, tartrates), the oxide material surrounding the y-nuclei is enriched... 

Fig. 66. STM image showing island formation as the Cd-Au alloy begins to form. Adapted from ref. [273],...

The interpretation of these coverage-dependent effects involves such ideas as island formation, mixed domains of CO and 0, the mobility of CO and 0 and the adsorption of CO on oxygen-covered regions (1,18,26, 27). A deeper understanding of the roles of these processes comes from isothermal experiments involving pressure transients. 

P. Fanson, W. Delgass, and J. Lauterbach, Island formation during kinetic rate oscillations in the oxidation of CO over R/Si02 A transient Fourier transform infrared spectrometry study, J. Catal. 204, 35-52 (2001). 

In the Goldenville Group, the Moshers Island Formation also has two compositional members an upper one explained by mixtures of chlorite, kaolinite illite, and smectite, and a lower one explained by only illite and smectite. 

Shalaby F, Rossant J, YamaguchiTP, etal. Failure of blood-island formation and vasculogenesis in Flk-... 

Island Formation and Segregation. Island formation affects the kinetics because often only the adsorbates at the edges of islands can react with other molecules. (Exceptions are adsorbates that form islands and that can react with each other, and if there is an Eley-Rideal mechanism.) To form islands it seems necessary to have an attractive interaction that keeps adsorbates together. Indeed, if there is just one type of adsorbate an attractive interaction leads to island formation at low temperatures, whereas a repulsive one does not. If there is more than one type of adsorbate, then one can even have island formation if all lateral interactions are repulsive. The term segregation may be more appropriate for such a situation. 

As an example of the analysis of the ordering of an overlayer of adsorbates, we may take the question of detecting island formation. LEED provides a means for identifying when island formation takes place " although it does not always give a definitive answer. To monitor island formation the presence of adsorbate-induced extra spots in the diffraction is necessary. Thus the adsorbate must produce a superlattice and we assume this case in the following discussion. 

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