Migration of surface atoms

Lennard-Jones suggested that the migration of surface atoms may be the first stage of melting of a solid this may be one of the preliminaries of large scale molting. It is very probable that migration of adsorbed... 

There aie a number of major indusuial problems in the operation of the steam reforming of metlrane. These include the formation of carbon on the surface of the catalyst, the sulphidation of the catalyst by the H2S impurity in commercial natural gas, and die decline of catalytic activity due to Ostwald ripening of the supported catalyst particles by migration of catalyst atoms from the smaller to tire larger particles, as the temperamre is increased. A consideration of tire thermodynamics of the principal reaction alone would suggest that the reaction shifts more favourably to the completion of the reaction as the temperature is increased. 

Fig.4.12. The sample construction 1 - polished quartz plate 2 — semiconductor sensor (ZnO) i - a strip of marblyte glass 4 - a layer of titanium (palladium) X = 0.027 cm ( the length of surface migration of H atoms h = 0.0025 cm ( the air gap between the quartz plate and the glass strip).

The explanation of these phenomena is that at room temperature the mobility of the potassium atoms of the surface of the metal is high enough to cause a migration of these atoms onto and over every area of surface hydride which is formed by the take up of hydrogen atoms. Consequently the photoelectric current rises in direct proportion to the number of atoms adsorbed and thus with time (Fig. 22). The photoelectric cathode which is formed by these phenomena may be represented by the symbol 206) ... 

The first is the choice of fairly large, finite-sized van der Waals clusters for the environment of the chemical reaction (10 -10 atoms or molecules in our case). At such sizes, the number of surface atoms is relatively important compared with the number of atoms staying inside the cluster, and in our case the reactants deposited on the clusters stay at the surface, but are free to migrate, to collide with each other and eventually to react. 

The dissolution of hydrogen in palladium to form a- and (i-phase palladium hydrides has been assumed to present problems in determining the surface area of palladium catalysts by hydrogen chemisorption, but under normal chemisorption conditions this is probably not a factor. The a-phase Pd-H forms initially by the migration of hydrogen atoms from the surface into the interstitial volume of the palladium crystals. Hydrogen pressures near atmospheric are... 

Physical characteristics of a support, namely porosity and specific surface area, have long been understood to play a key role in stabilizing active components of the catalysts in dispersed state. Explicitly or implicitly, they reflect topological properties of the carbon surface, namely the nature and quantity of (1) traps (potential wells for atoms and metal particles), which behave as sites for nucleation and growth of metal crystallites and (2) hindrances (potential barriers) for migration of these atoms and particles [4,5]. An increase in the specific surface area and the micropore volume results, as a rule, in a decrease in the size of supported metal particles. Formal kinetic equations of sintering of supported catalysts always take into consideration these characteristics of a support [6]. 

In general, the sites are not simple pair sites but involve additional surface species. In the absence of hydrocarbons, migration of deuterium atoms between sites or adjacency of sites must be assumed for sites A and D (which we tentatively assume are the same) because H2 + Dz = 2 HD is very fast. However, reaction (—I) is slow in the presence of hydrocarbons on sites A (and D). Adsorbed hydrocarbon residues largely or partially block sites which permit Hz + Dz = 2 HD. Alkane is an exception. 

NiO(250°) contains more metallic nickel than NiO(200°). Magnetic susceptibility measurements have shown that carbon monoxide is adsorbed in part on the metal (33) and infrared absorption spectra have confirmed this result since the intensity of the bands at 2060 cm-i and 1960-1970 cm-1 is greater when carbon monoxide is adsorbed at room temperature on samples of nickel oxide prepared at temperatures higher than 200° and containing therefore more metallic nickel (60). Differences in the adsorption of carbon monoxide on both oxides are not explained entirely, however, by a different metal content in NiO(200°) and NiO(250°). Differences in the surface structures of the oxides are most probably responsible also for the modification of their reactivity toward carbon monoxide. In the surface of NiO(250°), anionic vacancies are formed by the removal of oxygen at 250° and cationic vacancies are created by the migration of nickel atoms to form metal crystallites. Carbon monoxide may be adsorbed in principle on both types of surface vacancies. Adsorption experiments on doped nickel oxides, which are reported in Section VI, B, have shown, however, that anionic vacancies present a very small affinity for carbon monoxide whereas cationic vacancies are very active sites. It appears, therefore, that a modification of the surface defect structure of nickel oxide influences the affinity of the surface for the adsorption of carbon monoxide. The same conclusion has already been proposed in the case of the adsorption of oxygen. 

The dynamics of adsorbed species over MgO(OOl) surface was studied by MD method. The migration of the adsorbed species was found to depend on the morphology of MgO and the thermal vibration of surface atoms in MgO lattice. Further, the situation where the supercritical fluid and adsorbed species exist together was simulated. The collision of supercritical fluid with the adsorbed species was identified as the primary cause of extraction. Additionally, the supercritical fluid form clusters around the desorbed species avoiding the readsorption. Thus, clustering is the secondary cause for the increased efficiency of supercritical extraction even above the critical conditions. The details of these simulation studies are given in the following section. 

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