Surface enrichment, segregation

N1 surface concentrations determined from ESCA are plotted as a function of bulk N1 content in Figures 1 and 2. In the case of homogeneous alloys the points should fall on the 45 diagonal line. It can be seen that In both (N1 SI ) and (N1 Th ) series the surfaces of the alloys are nickel-poor, Ss compared to tHe bulk. Similar observations have been made In the case of N1 A1 (11,12) and Co Th (13) alloys. Surface enrichment In Si or tS i2 to be expected be cause of the higher heats of formation of S10 and ThO, compared to NiO (-210, -292, and -58.4 kcal/mol, respectively). This would lead to a higher chemical affinity of SI and Th toward the ambient gas and consequently an Increased driving force of SI and Th for segregation. 

Recently characterization of bimetallic nanoparticles by EXAFS were extensively reported [122-124,176], Structural transformation of bimetallic Pd/Pt nanoparticles, which were prepared by a sequential loading of H2PtClg onto the Pd loaded catalyst, was investigated with EXAFS at high temperatures [176], The results of EXAFS at Pd K and Pt L-III edges showed that Pt was surface-enriched or anchored on the Pd metal core with an increase of the Pt content. The structure of the obtained bimetallic Pd/Pt nanoparticles seemed to be retained upon heating up to 1273 K under ambient condition [176], Pt/ Au bimetallic nanoparticles can be prepared by polyol method and stabilized by PVP [122], XANES and EXAFS studies were also performed on the samples and their results supported the idea of a Pt-core/Au-shell structure with the elements segregated from each other [122],... 

Pt-Rh/AROs catalysts are widely used in automotive-exhaust emission control. In these systems, Pt is generally used for the oxidation of CO and hydrocarbons and Rh is active for the reduction of nitric oxide to N2. HRTEM and AEM show two discrete particle morphologies and Pt-Rh alloy particles (Lakis et al 1995). EM studies aimed at understanding the factors leading to deactivation, surface segregation of one metal over the other and SMSI are limited. There are great opportunities for EM studies, in particular, of surface enrichment, and defects and dislocations in the complex alloy catalysts as sites for SMSI. 

For such free surfaces with no energetic preference of the surface for one of the components (because of the arbitrary choice of a purely symmetric choice of interactions, UAA=UBB ) the surface enrichment effects are typically rather weak (Fig. 16b), even if one considers asymmetry of chain lengths. Even for %=0 the shorter chains are then attracted to the surface, but this purely entropic effect is particularly weak. For the enthalpy is less effective near the free surface in comparison with the entropy of mixing, and this effect is for small volume fraction < >A b of the minority component nearly independent of the asymmetry in chain length (Fig. 16b) i.e., even if the minority chains are much longer (NA= 2Nb) they segregate to the surface, contrary to the non-interacting case. 

FIG. 12 Blend surface segregation in PLA/PSA blends. Surface enrichment is evident from the higher surface molar percentage of PLA compared to the bulk surface percentage of PLA. (From Ref. 41.)... 

In the case of mixed Ce-Zr oxides, both reducibility and segregation effects have been monitored. Tlius, high reducibility of Ce has been reported in powder (Ce,Zr)02 samples displaying no surface enrichment in any of both cations [141] surface enrichment in Ce was however observed in other high surface area samples with Ce Zr ratio=l l, accompanied by significant Ce reduction (ca. 50% decrease in %(u )) [142] a trend towards surface Ce enrichment upon calcination was found also for low-Ce (Ce,Zr)02 aerogels [143]. An important spontaneous reduction... 

As shown by Knipping et al. (2000) and Knipping and Dabdub (2002) for chlorine, surface segregation has the potential of increasing the release of halogens from sea salt aerosol. This should be pursued further, especially for Br and I , where the surface enrichment effects are predicted to be highest. These effects also likely play a role in polar ODEs. 

Chemical intuition would also lead to the expectation of segregation of surface species, corrosive chemisorption, and adsorption-induced surface enrichments. There is in addition a range of more subtle and unexpected changes which might be collected under the heading of surface rearrangements. 

The divergent predictions of theories on entropic surface segregation advocating the surface enrichment in more flexible [184, 186, 205, 206], more stiff [200,204,207,209,214], or both types of molecules [183,208] may be compared with experimental results obtained for blends composed of the statistical olefines E EEx. The more branched (i.e., more flexible) component was found preferred at the free surface (with an exception of enrichment-depletion duality described in Sect. 3.1.2.4) but no segregation was observed at silicon or gold interfaces. This leads to the conclusion [120] that the entropic driving force alone cannot be in charge of the enrichment because in such a case the enrichment... 

A conventional understanding of the surface segregation from polymer blends is that the surface should be enriched in the component with lower bare surface free energy fs, regardless of the value of bulk composition This is however true only when (-dfs/d(j))s does not change its sign when surface concentration is varied (see Fig. 14b). For such blends, surface enrichment in the same... 

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