Preferential segregation

The overall behavior of the third type [mixed active phase-promoter ex (Nl Fej )Th] Is similar to the second. Preferential segregation of Fe, as compared to Nl, occurs on oxidation. A close correlation between CO conversion rate and surface Nl content has been observed. ... 

Compared to bulk polymer mixtures, the interfacial behaviour of polymer blends is essentially different [341]. The demixing process in thin films is strongly affected by the thin film confinement and the interfacial interactions of the blend components with the confining phases (e.g., substrate and air). Even in the one-phase region of the phase diagram, preferential segregation of the components at one of the interfaces leads to a certain composition profile as a function of the distance from the free surfaces and the substrate plane [342,343]. In the... 

Despite the utility of the physical methods described above, characterization of entities on the nanometer scale is still a problem. Well-mixed nanoparticles are not necessarily completely homogeneous, and one metal may preferentially segregate to the nanoparticle surface. Subtle differences in surface stoichiometries are presently extremely difficult to quantitatively evaluate with spectroscopy, even when the metal of interest has an intense surface plasmon. 

In catalysis it is important not only to be able to study the surface region of the catalyst itself, but also any adlayer that may be present. The latter may arise from adsorption or preferential segregation of one component from the bulk to the surface. The spatial distribution of the elements at the surface can be obtained from scanning AES. Experiments, in which the surface is progressively eroded, e.g., by ion-bombardment, with surface analysis by AES, XPS or ISS carried out after various times, may provide concentration-depth profiles of the chemical species. 

Luo L, Eisenberg A (2002) One-step preparation of block copolymer vesicles with preferentially segregated acidic and basic corona chains. Angew Chem Int Ed Engl 41 1001-1004... 

Figure 9 Preferential segregation of silver occurs at the surface of a Ag-Au alloy due to its lower surface tension...

Since the simple and attractive hypothesis of RBT in alloys has failed to stand the test of time, the geometric or ensemble-size model became very helpful to interpret the behavior of alloys. When applied to alloys between an active component A and an inactive one B, the ensemble-size model in its simplest form reflects the dilution in ensembles of smaller size of the active surface A by B these smaller ensembles of A are less prone to activate the reactant(s). The immediate consequence of this phenomenon is a sharp decrease of the TOP. Obviously, the fall of TOP should be all the more so if preferential segregation to the surface of the inactive component B occurs. In the classical treatment of diluting an inflnite surface A by B, the probability to find an ensemble of neighboring n A atoms is given by [5,30,31]... 

A more cogent explanation may lie in the possibility that at high temperatures preferential segregation of nickel occurs and this results in an enrichment of the particle surface in this component, conditions that do not favor dissociative chemisorption of CO on nickel is not a facile process (12,13). Furthermore, the reactions leading to carbon... 

The results of this investigation have highlighted the existence of an unusual deactivation phenomenon when certain bimetallic catalyst systems are reacted in either a hydrocarbon or carbon monoxide environment. The activation/deactivation behavior of the respective bimetallic catalysts appears to be completely reversible in nature and is found to be highly sensitive to both reaction temperature and of the composition of the reactant gas. Deactivation is attributed to the preferential segregation of the respective metal components to either the solid carbon or gas phase interfaces of the catalyst particles. When the temperature is subsequently increased or lowered to a previously active regime then catalytic activity is restored to a level comparable to that observed with a fresh sample. 

This study indicates preferential segregation of styrene at the surface of the block copolymers. 

Calcium addition removes the Ce -CO band, decreases the intensity of the Me -CO band, and shifts it to lower frequencies (cf. spectra 2 and 6, Fig. 2a). Since no appreciable variation in the density of bulk extended defects (Table 1) or Vis range absorption (Fig. 1) is observed, this result can be explained by preferential segregation of calcium cations in the vicinity of surface coordinatively unsaturated sites thus blocking them. 

Furthermore, we show that when Co is added to the catalyst as a promoter, it is initially homogeneously distributed throughout the hemihydrate platelet. As activation proceeds, however, the Co is seen to have limited solubility in the (VO)2P207 phase and preferentially segregates into, as well as structurally stabilising, the disordered matrix material. 

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