Metal alloys surface free energy

Ruthenium and copper are not miscible hence, homogeneous alloy particles will not be formed in supported Ru-Cu catalysts. As copper has a smaller surface free energy than ruthenium, we expect that if the two metals are present in one particle, copper will be at the surface and ruthenium in the interior (see also Appendix 1). This is indeed what chemisorption experiments and catalytic tests suggest [40], EXAFS, being a probe for local structure, is of particular interest here because it investigates the environment of both Ru and Cu in the catalysts. 

Ordered metal alloy systems might expose profound different surface characteristics even though consisting of the same elemental composition in the bulk. Intermixing or phase separation is correlated with the surface composition and structure. Differences appear associated by the influence of the free surface energy with segregation and surface ordering. Some prospects have been illustrated at specific metal alloy surfaces. 

Simple criteria for surface segregation in alloys (relative melting points, enthalpies of sublimation, metal atom radii, surface free energies of the pure metals) all indicate that surface segregation of titanium should occur on Pt/Ti alloys in vacuo. However, this is inadequate because of the large departures from ideality in Pt/Ti alloys. Analysis (11) of a broken bond model of the system, especially with the use of data directly determined with Pt/Ti alloys, gives a more reliable result. 

Figure 3.17 Computational high throughput screening for 736 pure metals and surface alloys. The rows indicate the identity of the pure metal substrates, and the columns indicate the identity of the solute embedded in the surface layer of the substrate. The solute coverage is (a) ilVIL, (b) ML, and (c) 1 ML, and the adsorbed hydrogen coverage is also jML. The diagonals of the plots correspond to the hydrogen adsorption free energy on the pure metals. Adapted from [Greeley et al., 2006] see this reference for more details.

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],... 

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