Metals, surface free energy data

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. 

Inequality (4), which conveys that the spreading of the oxide over the surface of the metal will occur if the interactions per unit interfacial area exceed twice the interfacial free energy of oxide-gas, is both simple and illuminating. Indeed, inequality (4) is satisfied if Ooxide-gas - -s sufficiently small and/or Ucs is sufficiently large. From the data available in the literature (20) on the surface free energies of oxides, those pertinent to the oxides used as catalytic substrates are summarized in Table I. 

This concept is illustrated in Fig. 8.11 for a poly(ethylene terephthalate) substrate and a mild steel (ferric oxide) substrate with, in both cases, water as the hostile environment. Values of y and yl of the various adhesives may be measured, as described in Chapter 2, or extracted from the literature (see Table 2.3) for example, considering a styrene-butadiene rubbery adhesive the values are 27.8 and 1.3 mJ/m, respectively, and for a typical epoxy adhesive they are 41.2 and 5.0 mJ/m, respectively. Hence, it is evident that these (and most other) adhesives will form an environmentally water-stable interface with the poly(ethylene terephthalate) substrate but an unstable interface with mild steel. Indeed, the data confirm that if only secondary molecular forces are acting across the interface then water will virtually always desorb organic adhesives, which typically have low surface free energies of less than about 60 mJ/m, from a metal oxide surface. Hence, for such interfaces, stronger intrinsic adhesion forces must be forged which are more resistant to rupture by water. 

The calculation of surface free energy and surface stress data depends critically on the model used for the interatomic interactions. Therefore, calculations for inert gas crystals, ionic crystals, covalent crystals and metals are treated separately as each crystal class is described best by different models of the interactions. 

Under reasonable assumptions about the structure of the film surface layer, the concentrations of different clusters in the pseudoliquid layer can be assessed based on the corresponding free energies. The computational results are shown in Figure 14.3. According to these data, under the typical experimental conditions of co-condensation (overall metal amount, lx 10-5 mol liquid phase volume, 0.1ml overall metal concentration [Mg]0, 0.1M), the concentration of Mg5 at 80 K should be six orders of magnitude lower than that of Mg4. At 120 K, this difference is four orders of magnitude. 

Porosity and pore size distributions, both before and after surface modification, are analyzed from N2 adsorption data. Both capacity as well as adsorption free energies are obtained from metal ion adsorption isotherms from aqueous solution as described in the experimental section. 

FIGURE 12.4 Top Experimentally measured exchange current, log(io), for the HER over different metal surfaces plotted as a function of the calculated hydrogen chemisorption energy per atom, AE (top axis). Single crystal data are indicated by open symbols. Bottom The result of the simple kinetic model plotted as a function of the free energy for hydrogen adsorption, AGg, = AE + 0.24 eV. (Erom Nprskov, J. K. et al., J. Electrochem. Soc., 152, J23, 2005. With permission.)... 

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