Surface binary alloys

A general description of the multilayer surface binary alloys formed by Na, and the multilayer surface ternary alloys formed by coadsorption of Na with K,... 

Dilute binary alloys of nickel with elements such as aluminium, beryllium and manganese which form more stable sulphides than does nickel, are more resistant to attack by sulphur than nickel itself. Pfeiffer measured the rate of attack in sulphur vapour (13 Pa) at 620°C. Values around 0- 15gm s were reported for Ni and Ni-0-5Fe, compared with about 0-07-0-1 gm s for dilute alloys with 0-05% Be, 0-5% Al or 1-5% Mn. In such alloys a parabolic rate law is obeyed the rate-determining factor is most probably the diffusion of nickel ions, which is impeded by the formation of very thin surface layers of the more stable sulphides of the solute elements. Iron additions have little effect on the resistance to attack of nickel as both metals have similar affinities for sulphur. Alloying with other elements, of which silver is an example, produced decreased resistance to sulphur attack. In the case of dilute chromium additions Mrowec reported that at low levels (<2%) rates of attack were increased, whereas at a level of 4% a reduction in the parabolic rate constant was observed. The increased rates were attributed to Wagner doping effects, while the reduction was believed to result from the... 

Vasihev MA. 1997. Surface effects of ordering in binary alloys. J Phys D Appl Phys 30 3037-3070. 

According to the data in Table III, the value of the ratio P)Mm is approximately the same for the metals Au, Fe, Co, Ni, and Pd. Binary alloys formed from any pair of these metals can therefore be expected to evaporate without substantial fractionation. On the other hand, films evaporated from Ag-Pd and Cu-Ni alloys can be expected to be enriched in Ag and Cu, respectively. These predictions are largely confirmed by experiment. For example, the composition of Pd-Au films was found to be the same as the wires which were evaporated (46), but in the case of Pd-Ag, evaporation of a 30% Ag-Pd alloy ware yielded a 50% Ag-Pd alloy film (47)- Alexander and Russell evaporated a number of alloys from pellets in the reaction vessel as shown in Fig. 5 (48) The alloy pellet was placed in a small quartz cup with its surface equidistant from the hemispherical top of the reaction vessel. The pellet was evaporated by... 

The electrodeposition of alloys at potentials positive of the reversible potential of the less noble species has been observed in several binary alloy systems. This shift in the deposition potential of the less noble species has been attributed to the decrease in free energy accompanying the formation of solid solutions and/or intermetallic compounds [61, 62], Co-deposition of this type is often called underpotential alloy deposition to distinguish it from the classical phenomenon of underpotential deposition (UPD) of monolayers onto metal surfaces [63],... 

For our purpose here, the surface concentration of B, xs, in a binary alloy can be assumed to be given by... 

Figure 6.30. The surface activities of various Pt-Ru-M ternary alloys. The additional alloy atoms, M, on the surface are listed along the horizontal axis. Pure Pt denotes monometal Pt Pt denotes the binary alloy Pt/Ru (adapted from [69]).

Dowden and Reynolds (49,50) in further experimental work on the hydrogenation of benzene and styrene with nickel-copper alloys as catalysts, found a similar dependence. The specific activities of the nickel-copper alloy catalysts decreased with increasing copper content to a negligible value at 60% copper and 30-40% copper for benzene and styrene, respectively. Low-temperature specific heat data indicated a sharp fall (1) in the energy density of electron levels N(E) at the Fermi surface, where the d-band of nickel becomes filled at 60 % copper, and (2) from nickel to the binary alloy 80 nickel -)- 20 iron. Further work by these authors (50) on styrene hydrogenation with nickel-iron alloy... 

Small particles of binary alloys have been investigated in detail in static EXAFS experiments, but if information about the dynamic behavior of the alloy composition and the segregation phenomena is desired, time-resolved combined EXAFS/XRD studies are necessary. Figure 18 shows the atomic structure of a small binary particle of a Ni-Au alloy as predicted from Monte Carlo simulations (60). Ni and Au do not form a miscible alloy in the bulk but can form a stable alloy at the surface. The structural and chemical changes that occur when such particles are exposed to different... 

A system which can easily be treated in this way is a single phase binary alloy. For preparation, however, let us consider an A crystal with a vacancy flux driven across it. In view of the fact that jA +y v = 0 in the steady state lattice system, the vacancy flux induces a counterflux of A, which shifts the whole crystal in the direction of the surface where the vacancy source is located. The shift velocity vb is yV... 

X-ray diffraction work showed the existence of two phases. The work function data suggest that the copper-rich alloy in the two-phase system is located at the surface and the nickel-rich phase below the surface. To check this, CO was admitted at a pressure of 10 8 Torr. The gas is strongly adsorbed on nickel, but not on copper at such low pressures. The work function of copper was not altered. The binary alloys showed a constant increase in work function between 0.04 and 0.11 eV. Therefore, the adsorbing surface belonged to the copper-rich phase. Chemisorption of H2 on Ni-Cu films (40) leads to essentially the same conclusions. At temperatures below the miscibility gap, several classes of alloy systems characterized by their concentration ranges can be distinguished (4c), as illustrated in Fig. 3 ... 

As a whole, considerable experimental data have been accumulated in the literature concerning the influence of the gas phase on the surface state of adsorbents and catalysts. The surface state is used specially to preliminarily produce the required surface composition of a catalyst. At the same time, quantitative characteristics of these changes are mainly available only for binary alloys contacting with molecules of H2, CO, and 02 [41,42]. Investigations conducted in recent years reveal that the influence of adsorbed particles on the state of a solid surface is apparently more significant than is customarily considered at present. The structural transformations in the surface layers (an example is the rearrangement of the surface layer of platinum in the adsorption of CO and 02 molecules [43]) and the processes of formation of new phases in them, which are similar to three-dimensional topochemical processes [44], may be of a major significance. 

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