Surface segregation alloy

Alloy surface segregation and ordering phenomena recent progress... 

Kitchin JR, Reuter K, Scheffler M (2008) Alloy surface segregation in reactive environments first-principles atomistic thermodynamics study of AgjPd(l 11) in oxygen atmospheres. Phys... 

Studies of small particles by Sinfelt [29] and his co-workers have shown that when the particles sizes become very small and dispersions tend toward unity (that is, when virtually every atom is at the surface), alloy systems exhibit phase diagrams very different from those that characterize bulk systems. For example, microclusters containing Cu and Ru, Cu and Os, or Au and Ni can be produced in any ratio of the two elements, indicating complete miscibility or solid solution behavior. In the bulk phase these elements are completely immiscible. This very different behavior of the surface phases of bimetallic systems finds important applications in the design of catalysts to carry out selective chemical reactions. Moran-Ldpez and Falicov [30] developed a theory—using pairwise interactions—of alloy surface segregation that explains this effect. Bimetallic systems remain miscible at lower temperatures in two dimensions than in three dimensions. 

POSAP has also been applied to the phase chemistry of AlNiCo 2 magnet alloys the precipitation of copper in iron-based alloys surface segregation in ceramic oxide superconductors hetero-... 

Muller, S., Stohr, M., and Wieckhorst, O. (2006) Structure and stability of binary alloy surfaces segregation, relaxation, and ordering from first-principles calculations. Appl. Phys. A, 82, 415. 

Other recent investigations involving AES, often with depth profiling, deal with the surface segregation of Ag in Al-4.2 % Ag [2.163], of Sn in Cu and formation of superficial Sn-Cu alloy [2.164], of Mg in Al-Mg alloy [2.165], and of Sb in Ee-4% Sb alloy [2.166]. Note the need to differentiate between, particularly, segregation, i. e. original sample properties, from the artifact of preferential sputtering. 

Ab-initio studies of surface segregation in alloys are based on the Ising-type Hamiltonian, whose parameters are the effective cluster interactions (ECI). The ECIs for alloy surfaces can be determined by various methods, e.g., by the Connolly-Williams inversion scheme , or by the generalized perturbation method (GPM) . The GPM relies on the force theorem , according to which only the band term is mapped onto the Ising Hamiltonian in the bulk case. The case of macroscopically inhomogeneous systems, like disordered surfaces is more complex. The ECIs can be determined on two levels of sophistication ... 

This work has been carried out by Marcus and his co-workersand deals with the influence of sulphur on the passivation of Ni-Fe alloys. For sulphur-containing Ni-Fe alloys, sulphur segregates on the surface during anodic dissolution. Above a critical sulphur content a non-protective thin sulphide film is formed on the surface instead of the passive oxide film. 

Cd + Bi alloy electrodes (1 to 99.5% Bi) have been prepared by Shuganova etal. by remelting alloy surfaces in a vacuum chamber (10-6 torr) evacuated many times and thereafter filled with very pure H2. C dispersion in H20 + KF has been reported to be no more than 5 to 7%. C at Emin has been found to be independent of alloy composition and time. The Emin, independent of the Bi content, is close to that ofpc-Cd. Only at a Bi content 95% has a remarkable shift of toward less negative E (i.e., toward o ) been observed. This has been explained by the existence of very large crystallites (10-4 to 10-3 cm) at the alloy surface. Each component has been assumed to have its own electrical double layer (independent electrode model262,263). The behavior of Cd + Bi alloys has been explained by the eutectic nature of this system and by the surface segregation of Cd.826,827... 

Figure 4.28. STM image of a PtRh(lOO) surface. Although the bulk contains equal amounts of each element, the surface consists of 69% of platinum (dark) and 31 % of rhodium (bright), in agreement with the expected surface segregation of platinum on clean Pt-Rh alloys in ultrahigh vacuum. The black spots are due to carbon impurities. It is seen that platinum and rhodium have a tendency to cluster in small groups of the same elements.

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. 

Stamenkovic V, Schmidt TJ, Ross PN, Markovic NM. 2003. Surface segregation effects in electrocatalysis Kinetics of oxygen reduction reaction on polycrystalline PtsNi alloy surfaces. J Electroanal Chem 554 191 -199. 

Wang G, Van Hove MA, Ross PN, Baskes MI. 2005. Quantitative prediction of surface segregation in bimetaUic Pt-M alloy nanoparticles (M = Ni, Re, Mo). Prog Surf Sci 79 28-45. 

Bozzolo G, Noebe RD, Khalil J, Morse J. 2003. Atomistic analysis of surface segregation in Ni-Pd alloys. Appl Surf Sci 219 149-157. 

Ruban AV, Skriver HE, Nprskov JK. 1999. Surface segregation energies in transition-metal alloys. Phys Rev B 59 15990-16000. 

Bimetallic (98) and alloy catalysts (97), of interest for hydrogenation reactions, have been investigated in in situ characterizations of methanol synthesis from CO and H2 in the presence of novel Cu-Pd alloy catalysts supported on carbon the results show surface segregation of palladium on the catalyst particles in CO atmospheres, but surfaces with equal amounts of copper and palladium when the atmosphere is H2 (97). 

In this section, we extend the above formalism to that for an alloy surface within the CPA, which serves as the model for the pre-chemisorption substrate. The model discussed here is based on that of Ueba and Ichimura (1979a,b) and Parent et al (1980). For a comprehensive introduction to alloy surfaces see Turek et al (1996). A feature of surface-alloy models, which is different from bulk ones, is that the CP in layers near the surface is different from that in the bulk, due to the surface perturbation. Moreover, the alloy concentration in the surface layers may be quite different from that in the bulk, a feature known as surface segregation. (See Ducastelle et al 1990 and Modrak 1995 for recent reviews.) We assume that both of these surface effects are confined to the first surface layer only. 

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