Subsurface alloy

Figure 10. Pt/Pd (shell/core) stability and impact of Pt thickness on surface segregation. Under 0.25 ML of oxygen adsorbed on the fee site, Pt remains stable on the siuface (no Pd segregation) in all cases. Pd forms subsurface alloys in the 2 and 3-layer Pt shells. Grey spheres are Pt atoms, blue spheres are Pd atoms.

In the following, surface and sub-surface alloy formation of ordered systems in ultra high vacuum will be discussed as an option to generate different surface configurations with dissimilar properties from the same set of material composition. Certainly, alloys develop also at the liquid-solid interface [10-13], yet the topic will not be covered in this chapter being of special devotion to ordering effects. Surfaces of ordered bulk alloys shall be reviewed in a first part. A second subdivision includes the formation of surface and subsurface alloys, whereas in a third section applications are discussed to grow ordered superstructures on top of alloy surfaces. 

Stephens lEL, Bondarenko AS, Perez-Alonso FJ, Calle-Vallejo F, Bech L, Johansson TP, Jepsen AK, Frydendal R, Knudsen BP, Rossmeisl J, Chorkendorff I (2011) Tuning the activity of Pt(lll) for oxygen electroreduction by subsurface alloying. J Am Chem Soc... 

Analytical treatments for predicting the changes in the subsurface alloy composition during cyclic oxidation have been developed by Whittle (1972) and Wahl (1983) for complete scale spallation after each cycle, and by Nesbitt (1989) for partial spallation (i.e., a thin layer of scale remains on the alloy). The main purpose of these treatments was to arrive at a criterion to enable prediction of the minimum bulk solute concentration required for repeated reformation of a protective solute-oxide scale during cyclic oxidation. The most advanced treatment is that by Nesbitt (1989), who eombined a dif-... 

Figure 12.14 Ordered Type II near-surface alloy Cu/(Cu-F lr)/Cu(100). (a) STM image of Cu(lOO) after deposition of 0.5 ML Ir at 520 K. (b) Top view and (c) side view of a structure model of the Cu(100)-(2 x l)-lr subsurface alloy. (From Ref [40].)...

In the context of (electro)chemical model studies, Pt(lll) surfaces modified by subsurface 3d transition metals are a well-studied class of Type II (sub)surface alloys. These are of particular interest because of their tuning effect on the chemical and (electro)catalytic properties of Pt(lll) [42]. Among those, Ni/Pt(lll) has become a prototype system for subsurface alloying. The reader is referred to... 

In the cited work, curves as in Figure 12.18 are hsted for aU combinations of transition and noble metal classes [4]. Specifically, the first and second derivatives at foreign = tabulated. For aU systems listed in this chapter as Type I surface alloys, the tabulated DFT data correctly predicted this behavior. Moreover, Ru Ptj /Pt(lll) is predicted to belong to class (c), which fits to the observed tendency toward the formation of a subsurface alloy overgrown by the host metal Pt (Figure 12.13). 

A collaborative test programme covering low-alloy and high-alloy steels was carried out by the Central Electricity Generating Board and various steelmakers. Samples were exposed in specially constructed chambers held at 566°C, 593°C and 621 °C fed with power-station steam at a pressure of 3-45 MN/m for times of up to 16 286 h. In the assessment of the results both metal lost from the surface and subsurface penetration were measured. The results have been reported by King, Robinson, Howarth and Perry in a C.E.G.B. report. Selected data are shown in Fig. 7.32, in which the broken lines have been obtained by extrapolation of the experimental results. 

Alloy Mass loss (%) Depth of subsurface attack (mm)... 

Greeley J, Mavrikakis M. 2005. Surface and subsurface hydrogen adsorption properties on transition metals and near-surface alloys. J Phys Chem B 109 3460-3471. 

This allows a direct influence of the alloying component on the electronic properties of these unique Pt near-surface formations from subsurface layers, which is the crucial difference in these materials. In addition, the electronic and geometric structures of skin and skeleton were found to be different for example, the skin surface is smoother and the band center position with respect to the metallic Fermi level is downshifted for skin surfaces (Fig. 8.12) [Stamenkovic et al., 2006a] owing to the higher content of non-Pt atoms in the second layer. On both types of surface, the relationship between the specific activity for the oxygen reduction reaction (ORR) and the tf-band center position exhibits a volcano-shape, with the maximum... 

The proposal that Cd always forms the first atomic layer is based on several factors. First, it is clear from our previous work that Cd can react with multiple layers of chalcogenide, and that chalcogenides can react with multiple layers of Cd, that is, that Cd can go subsurface into Te. In addition, it is clear from Cd cyclic voltammetry that an atomic layer of Te suppresses the deposition of Cd on Au, as the Cd UPD peak is shifted negatively in the presence of an atomic layer of Te. This suggests that Cd is more stable on Au than on Te. As mentioned above, Cd forms an alloy with Au, which is a clear sign of the high affinity of Cd for Au, and supports the idea that Cd maybe inserting between the Te and the Au surface. Similar behavior has been... 

As alluded to before, the adsorption of atoms and molecules may also induce segregation in alloys. Upon revisiting the thermodynamic behavior of the improved Cu-Ag alloy catalysts for ethylene epoxidation synthesized by Linic et al, (section 2.1) Piccinin et al. calculated that, while in the absence of oxygen Cu prefers to stay in the subsurface layers, oxygen adsorption causes it to segregate to the surface which then phase-separates into clean Ag(lll) and various Cu surface oxides under typical industrial conditions (Fig. 7). This casts doubt on the active state of the previous Cu-Ag catalysts being a well-mixed surface Ag-Cu alloy. 

A very important characteristic of the subsurface region is the surface concentration of the atoms. For alloys, it is customary to speak of the surface segregation of the component whose surface concentration exceeds the bulk one. With an increase in the distance from the surface, the local concentrations of the particles tend to their bulk values. This also relates to the other characteristics of particle distribution determining short- and long-range orders, which in the subsurface region can have a great anisotropy. 

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