Metal segregation, effect

It also should be noted that we have herein, in effect, turned the charging problem around and made it an auxiliary morphological tool. In order to demonstrate the versatility of this tool, we have employed the aforementioned negative shift with cluster formation not only to study metals dispersions, but also to assist in the study of the integrity of thin film interfaces (16), and metals segregation during alloying corrosion (H). 

Next, we turn our attention to core-shell stmctures, assuming they would be formed either to minimize the amount of Pt present in the catalyst, or be naturally formed either because of segregation effects or because of metal dissolution in acid medium. ... 

Adams et al. [78,79] have reported a series of synthesis of mixed-metal cluster compounds. One example, Pt2Ru4(CO)is, is depicted in Figure 1(b). This mixed cluster compound was investigated to study the effect of Pt-Ru nanoparticles developed after the precursor annealing on carbon [80]. In line with the spectroscopic and microscopic measurements, the authors demonstrated that mixed Pt-Ru nanoparticles, with an extremely narrow size distribution (particle size 1.4nm), reflect an interaction that depends on the nature of the carbon support. Furthermore, as revealed by EXAFS, the Pt-Pt, Pt-Ru, and Ru-Ru coordination distances in the precursor (2.66, 2.64, and 2.84 A) [79] changed to 2.73, 2.70, and 2.66 A, respectively, on the mixed-metal nanoparticles supported on carbon black, with an enhanced disorder [80]. Furthermore, some metal segregation could be... 

There are many theories on the mechanism of the segregation effect that suggest either a chemical or an electronic mechanism or both types of mechanisms. However, it seems that the most reliable mechanism is electronic as proposed by Mukheijee and Moran [35]. This electronic model calculates the chemical properties of the pure constituents from their physical parameters and then estimates those of the alloys. It employs the tight-binding electronic theory, the band filling of the density of states, and the bandwidth of the pure components for the calculations. However, it seems that the 2D Monte Carlo simulations produce better results by using the embedded atom and superposition methods. The latter allows for the calculation of the compositions from the relative atom positions, and the strain and the vibrational energies has been reviewed for 25 different metal combinations in [36]. It was also possible to predict composition oscillations as a consequence of the size mismatch. 

Disproportionation of the praseodymium and terbium oxides may also be induced by prolonged exposure to air, at room temperature [26,189]. As a result, Ln(OH)3 and Ln02 phases are formed. It would be therefore important to verify the likely occurrence of this phase segregation effect, because of its influence on the actual structural constitution and chemical behavior of the resulting material. Likewise, the formation of Ln(OH)3 (Ln Pr, Tb) may strongly favor the occurrence of dissolution/coprecipitation phenomena during the preparation of praseodymia (terbia) supported metal catalysts. These phenomena may induce nanostructural effects similar to those commented on above. 

This effect of metal segregation is borne out by measurements of resistivity versus volume percent metal filler as shown in Figure 18.6. A threshold for... 

An alternative, but related, approach to reduce Ft content and yet increase catalytic activity is the nanostructured core-shell approach taken by Adzic and co-workers [41]. As an example, a cartoon of a core-shell structure is illustrated in Figure 14.5, in which a nanostructured catalyst particle is shown that has a Ft monolayer surface with a Fd sublayer on top of a core material of metal M. This core-shell nanostructure is architected to take into account the surface contraction effects that shift the d-band center and reduce the oxygen binding energy, the natural surface segregation effects, and the stability offered by a contiguous monolayer. Clearly, the amount of Ft is reduced because the core can be made of less costly materials. 

Glassy metals show similar surface segregation effects as the corresponding crystalline alloys. They are induced by selective oxidation and by differences of the surface energy of the components. As-quenched and air-exposed samples of glassy metals (e.g, covered with a... 

Here we present only one effect in detail which also is expected to occur in metallic alloys the enrichment of vacancies in the interfacial region (Fig. 4). For the chosen parameters, the density reduction 5p in the center of the interface even is a few percent in the fully segregated limit. However, 5p 0 as T Tc. 

Structural properties of materials Sub-lattice Substrate Surface phonoas Surface defects m transition metals Surface segregation SupeqDlastic properties and lic[uid phase effect Susceptibility... 

It may be necessary to segregate waste streams containing elevated concentrations of arsenic and selenium, especially waste streams with concentrations in excess of lmg/L for these pollutants. Arsenic and selenium form anionic acids in solution (most other metals act as cations) and require special preliminary treatment prior to conventional metals treatment. Lime, a source of calcium ions, is effective in reducing arsenic and selenium concentrations when the initial concentration is below lmg/L. However, preliminary treatment with sodium sulfide at a low pH (i.e., 1-3) may be required for waste streams with concentrations in excess of lmg/L.22 The sulfide reacts with the anionic acids to form insoluble sulfides that are readily separated by means of filtration. 

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