Alloys surface composition

In the field of alloy surface composition, both theory and experimental determination achieved much progress in recent years. The present state of the art does not, unfortunately, allow one to predict quantitatively the surface composition from the bulk concentrations, but calculations on models allow one to estimate various effects and to make interesting conclusions and sometimes even semiquantitative predictions. 

Figure 9 shows a plot of the open-circuit potential as a function of the alloy-surface composition in an 02-saturated 0.1 M H2SO4 solution. The highest voltage, 0.86 V, was obtained for the alloy of composition that corresponded to PtsCo. Such value was 0.68 V higher than for pure Co and 0.22 V better than for pure Pt. However, the value is still considerably lower than the ideal potential of 1.229 V. In a control experiment, in which the sulfuric acid solution was thoroughly deaerated with ultrapure N2, an OCP value (ca. 0.5 V) was obtained that was independent of the alloy surface composition. 

Much of the earlier work on alloy ftlms was confused by the faa that binary systems are very susceptible to surface enrichment by one of the components. "" Techniques for measuring alloy surface compositions have led to a much greater understanding. " - Current views are that... 

F.L. Williams and D. Nason. Binary Alloy Surface Compositions from Bulk Alloy Thermodynamic Data. Surf. Sci. 45 377 (1974). 

Williams FL, Nason D (1974) Binary alloy surface compositions from bulk alloy thermodynamic data. Surf Sci 45(2) 377-408... 

J.K.G. Panitz, D.J. Sharp, The effect of different alloy surface compositions on barrier anodic film formation, J. Electrochem. Soc. 131 (10) (1984) 2227. 

The broken bond approach has been extended by Nason and co-workers (see Ref. 85) to calculate as a function of surface composition for alloys. The surface free energy follows on adding an entropy of mixing term, and the free energy is then minimized. 

Metals A and B form an alloy or solid solution. To take a hypothetical case, suppose that the structure is simple cubic, so that each interior atom has six nearest neighbors and each surface atom has five. A particular alloy has a bulk mole fraction XA = 0.50, the side of the unit cell is 4.0 A, and the energies of vaporization Ea and Eb are 30 and 35 kcal/mol for the respective pure metals. The A—A bond energy is aa and the B—B bond energy is bb assume that ab = j( aa + bb)- Calculate the surface energy as a function of surface composition. What should the surface composition be at 0 K In what direction should it change on heaf)pg, and why ... 

It is in this area that most work has been carried out, particularly in relation to corrosion resistance in sulphuric acid solutionsBourelier etal. and Raicheff etal. investigated the inhibitive effect of chloride ions on corrosion in sulphuric acid. The inhibition efficiency was found to depend on the alloy composition, alloy surface and chloride concentration. The more aggressive the environment, the greater the inhibition efficiency. Yagupol skaya etal studied the effect of iodine additions to sulphuric acid on the corrosion resistance of Ni and Ni-Fe alloys. Again there was an inhibitive effect caused by the halide ion. 

One of the features of liquid as well as solid alloys is that their bulk and surface compositions are as a rule substantially different because one of the components is more surface active than the other. In the... 

Adsorption of various organic compounds (e.g., cyclohexanol, adamantanol-1, and camphor) has been studied at a renewed Sn + Pb alloy/electrolyte interface.820-824 The time variation of the surface composition depends on the solution composition, the nature and concentration of the surface-active substance, and on E. The " of cyclohexanol for just-renewed Sn + Pb alloys shifts toward more negative E with time, i.e., as the amount of Pb at the Sn + Pb alloy surface increases. 

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... 

Various pc electrode models have been tested.827 Using the independent diffuse layer electrode model74,262 the value of E n = -0.88 V (SCE) can be simulated for Cd + Pb alloys with 63% Pb if bulk and surface compositions coincide. However, large deviations of calculated and experimental C,E curves are observed at a 0. Better correspondence between experimental and calculated C,E curves was obtained with the common diffuse-layer electrode model,262 if the Pb percentage in the solid phase is taken as 20%. However, the calculated C, at a Ois noticeably lower than the experimental one. It has been concluded that Pb is the surface-active component in Cd + Pb alloys, but there are noticeable deviations from electrical double-layer models for composite electrodes.827... 

Liquid alloys of Hg with a variety of metals (amalgams) constitute particularly complex systems in view of the potential dependence of surface composition. A detailed study of In and T1 amalgams, with... 

In alloys, the component with the lower surface free energy segregates to the surface, making the surface composition different from that of the bulk (Scheme 5.1). 

Discuss the role of the surface free-energy in phenomena such as alloy segregation, surface reconstruction, faceting and sintering of small particles. How does a gas environment (air, vacuum) affect the surface composition of alloys ... 

Data accunnilated in the last years on the Ft/Cu alloys, in particular on the 1) surface composition, 2) electronic structure, 3) adsorption properties, 4) catalytic behaviour and 5) various side effects, make a detailed discussion possible of the catalytic selectivity and mechanism of hydrocarbon reactions. 

Figure 1. Variation of the surface composition of Ni Si alloys as a function of bulk Ni content (relative standard derivation of + 10%).

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