Outermost surface layer

Metals Dispersion from LEISS. Since He scattering Is very selective to Che outermost surface layer, one should anticipate that LEISS would be a valuable Cool for studies of metals dispersion for supported catalysts. For low oietal concentrations on high area supports, the (oietal/support) LEISS Intensity ratio should be directly proportional to metals dispersion. Recent stiidles In our laboratory have confirmed that expectation. 

In the end, what matters obviously is whether the features of interest distinctly show up in the analytical data, e.g., in a spectrum or a map or image. Thus, even though IR is not ultimately a surface specific technique, when an outermost surface layer reveals characteristic absorption bands that are related to say a property such as adhesion, the technique can be a very valuable one and is extensively applied. 

Dwyer et al. (43) have also reported that dealumination of Y zeolites by a steam/acid leaching process produces a more uniform composition than dealumination by EDTA. The later method caused a depletion of Al in the outermost surface layer, producing a compositional gradient in the zeolite crystals. The conclusions reached by J. Dwyer in his studies of aluminum-deficient zeolites using the FABMS method are summarized in Table IV. 

The nautre of the He-surface interaction potential determines the major characteristics of the He beam as surface analytical tool. At larger distances the He atom is weakly attracted due to dispersion forces. At a closer approach, the electronic densities of the He atom and of the surface atoms overlap, giving rise to a steep repulsion. The classical turning point for thermal He is a few angstroms in front of the outermost surface layer. This makes the He atom sensitive exclusively to the outermost layer. The low energy of the He atoms and their inert nature ensures that He scattering is a completely nondestructive surface probe. This is particularly important when delicate phases, like physisorbed layers, are investigated. 

Change in Composition of Surface Film with Treatment. Since trivalent chromium hydroxide can be generally expressed as Cr203 nH20, the composition of the outermost surface layer is assumed to be expressed by "Equation 2" and the hydration degree n and covering rate a of chromium... 

The condensation of two neighboring hydroxyl groups leads to the formation of a water molecule that is then expelled from the surface. It is now agreed that this process leaves an oxide ion in the outermost surface layer and an exposed incompletely coordinated aluminum ion in the next-lower layer this exposed cation is located in a hole that is electron-deficient and acts thus as a Lewis acid site. 

Heckman and Harling57 examined the gas-phase oxidation of carbon black micro-structures and showed that oxidative attack of carbon crystallites was concentrated on the small crystallites, at the edges of layer planes and at lattice defects. Partial graphitization of a carbon black, so that only the outermost surface layers are well-ordered, causes oxidative corrosion within the core of the carbon particle, leaving an outer shell . Consequently, similar behavior can be expected for ungraphitized and partially graphitized carbons in electrochemical environments. 

The feasibility of determining approximately the surface composition of silica-supported Ru-Pt catalysts by using infrared spectroscopy and two adsorbing gases has been explored, as a possible alternative to more complex instrumental methods and with the advantage of greater sensitivity to the outermost surface layer. The infrared absorption bands of CO selectively adsorbed on Pt sites and NO selectively adsorbed on Ru sites were stable in vacuo at room temperature. If both extinction coefficients and stoicheiometric coefficients are taken to be independent of surface composition, a simple correlation between the absorbance ratio and the bulk metal composition was obtained, suggesting that the surface and bulk compositions were similar. 

Rapid, initially reversible hydrolysis and exchange of charge-balancing cations in the outermost surface layer (approximately one or two unit cells thick) for hydronium ions (Nash and Marshall, 1954 Garrels and Howard, 1959). 

The CTR shape is sensitive to the termination of the crystal surface. Calculations show (Fig. 5B) that CTR data are sensitive not only to the presence of the crystal termination but also to the detailed termination of the lattice. Here, we compare the scattering intensity for a semi-infinite lattice in which the outermost surface layer is ideally terminated with a bulk-like termination, or has been modified, either by its position, ds, or its scattering strength, fs. The total structure factor of the crystal with a modified surface is just the sum of individual structure factors for all atoms in the crystal (a table of commonly used structure factors is given in Appendix 3). Conceptually, this quantity can be broken into two parts, consisting of contributions from the modified surface layer, Fsurf, and from the semi-infinite substrate, Fsub -... 

A phenomenon that sometimes occurs in bulk alloys is the preferential segregation of one of the components at the surface, that is, the enrichment of the outermost surface layers(s) in one of the components. The slight outward shift of the Na atoms relative to the Li atoms can be interpreted as a manifestation of this tendency. The difference between the Wigner-Seitz radii of Na and Li is JR -s(Na-Li) = 0.72 a.u. This difference increases for K-Na and Cs-Na, namely dRH s(K-Na) = 0.87 a.u. and dRirs(Cs-Na) = 1.81 a.u. Since the properties of alkali metals and alkali atoms vary smoothly with atomic number, we expect that the effects observed in Li-Na clusters will be enhanced for K-Na and Cs-Na clusters. 

The dynamic mode uses a higher ion current density and the sputtering rate is typically 1 pmol h . So, in the static mode the outermost surface layer may exist for hours before being completely removed in contrast to the dynamic mode where the surface layers are stepwise removed and depth profile analysis is possible. 

In the majority of mechanical applications of materials, their surfaces experience contact with another material and take the external load before the bulk of the material is influenced. In some cases, surface interactions influence the bulk (e.g., propagation of cracks dislocations or point defects from the surface in depth). In many cases, only the outermost surface layer is affected by the surface contact with no detectable changes in the bulk of the material. This is like a storm that is frightening and destructive on the ocean surface, but does not have any influence on deep-water life. We are primarily concerned in this review with that kind of interaction. The surface layer thickness affected by external mechanical forces ranges from nanometers to microns. Thus, in our case, the definition of surface is different from the one used by surface scientists, that is, physicists and chemists. We introduce here an engineering definition of surface the outermost layer of the material that can be influenced by physical and/or chemical interaction with other surfaces and/or the environment. In this chapter, we consider only mechanical effects, but both mechanical and chemical interactions are possible and their synergy can lead to mechanochemical alteration of a material surface. 

Penning ioni2ation electron spectroscopy is also called (i) metastable atom electron spectroscopy (MAES), (ii) metastable deexdtation spectroscopy (MDS), (iii) metastable quenching spectroscopy (MQS), and (iv) metastable impact electron spectroscopy (MIES), where atom at metastable state is used instead of photon to ionize the target material When a slow, long-lived, electronically excited metastable atom hits a solid surface, most of its excitation energy is used to eject electrons from the surface. Unlike photons used for UPS, metastable atoms do not penetrate into the bulk of the solid. PIES, therefore, excites the outermost surface layer selectively (39). 

The number of available surface sites for isopropanol adsorption of bulk tungsten trioxide and monolayer supported tungsten oxide catalysts, even mesoporous nanoparticles, is orders of magnitude (0.9-6 pmol/m ) lower than the heteropolyacids (8-55 pmol/m ). This observation cannot be attributed to the surface structure because all these materials possess polymerized WOg species with octahedral coordination (see Table 5.1). Moreover, no correlation is observed with the specific surface area and the pore diameter because high surface area, mesoporous monolayer supported catalysts possess lower Ns than HPAs. In fact. Table 5.1 shows that those materials do not possess microporosity the specific surface area is only external surface area. This observation shows that the adsorption of alcohol occurs exclusively at the outermost surface layer of WO3 and monolayer supported tungsten oxide catalysts. 

Fig. 8 Schematic real-space model and normalized electron density profiles < p(z)>/pix> (where p is the bulk electron density of mercury) obtained from the fits of reflectivity data to a density-dependent model for n-octadecanethiol (bold line) and n-dodecanethiol (thin line). The upper and lower figures are aligned with each other. Vertical lines in the model mark the position of the three outermost surface layers of mercury, with the origin of z coinciding with the first mercury layer [89]. 

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