Bulk alloy types

During annealing between 400 and 450 °C, the particles assume the f ordered structure of the bulk alloy (type CsCl). They are polyhedral and limited at the edges... 

Clusters and alloys are molecular species that may show different catalytic activity, selectivity and stability than bulk metals and alloys. Small metal clusters and alloy clusters have been studied reeendy for potential use as catalysts, ceramic precursors, and as thin films. Several fundamental questions regarding such clusters are apparent. How many atoms are needed before metallic properties are observed How are steric and electronic properties related to the number, type and structure of such clusters Do mixed metal clusters behave like bulk alloy phases ... 

Alloys are employed as catalysts because there is the opportunity for continuous variation of their electronic properties with composition. A small number of metal atoms having various compositions have been employed to simulate alloys using the MO-type calculations. Whether such a model can be an adequate representation for bulk alloy particles is questionable in light of the differences between properties of Ag or Pd clusters and those of bulk metals noted earlier in this paper. Therefore, these data are best taken to represent small mixed-metal cluster particles. 

In many systems, 3D Me-S bulk alloy formation is connected with considerable changes of the lattice type, unit cell, and/or interatomic distances producing significant internal mechanical stress. This is illustrated, for example, for the electrochemical formation of the p phase of the Li-Al alloy in the system A1 (polycrystalline)/molten Li, IC, cr at AEfs 260 mV [3.345]. As a result, acoustic emission is generated during the 3D Me-S bulk alloy formation as illustrated in Fig. 3.62 [3.345]. 

The BFS method has been applied to a variety of problems, ranging from the determination of bulk properties of solid solution fee and bee alloys and the defeet strueture in ordered bee alloys [28] to more speeifie applieations ineluding detailed studies of the strueture and eomposition of alloy surfaees [29], ternary [30] and quaternary alloy surfaees and bulk alloys [31,32], and even the determination of the phase strueture of a 5-element alloy [33]. Previous appheations have foeused on fundamental features in monatomie [26] and alloy surfaces [29] surface energies, reconstructions, surface structure and surface segregation in binary and higher order alloys [34,35] and multilayer relaxations [36,37]. While most of the work deals with metallic systems, the lack of restrictions on the type of system that can be studied translated into the extension of BFS to the study of semiconductors [38]. 

DFT calculations were particularly useful for this system in that they enabled further studies of systems which are not easy to prepare experimentally. Thus calculations for five-layer surface alloys showed that the lower layers of the surface alloy revert to a Cu3Au-type structure, but that the stacking fault at the surface which converts a CusAu-type structure into an AlsTi-type structure (see Fig. 19) is still present. Further calculations of the surface structure of bulk alloys then led to the prediction that the surface structure of the bulk AlsLi alloy has a stacking fault, such that it consists of a three-layer Al3Ti-type structure on a CusAu-type bulk. At the present time these novel theoretical predictions await experimental confirmation. Attempts to prepare thicker surface alloys by deposition of Li on Al(lOO) indicate that diffusion of Li into the bulk occurs... 

The multilayer surface alloy formed by Li adsorption on Al(lOO) is exceptional in that substitution of 1/2 ML A1 by Li occurs in both the first and third A1 layers. An unexpected feature of this structure is that the registry of Li atoms in the first and third layers is such that they are staggered along the surface normal direction, as in the Al3Ti-type bulk alloy structure, rather than collinear, as in the expected CusAu-type bulk alloy structure known to be adopted by the metastable, AlsLi bulk alloy. DPT calculations for this system lead to the novel prediction that the AlaLi bulk alloy has a stacking fault at the surface, such that it can be described as an AlaTi-type surface on a CuaAu-type bulk. 

Theoretical and experimental studies of model bimetallic catalysts in recent years have distinguished between thermodynamically stable bulk alloys and so-called near surface alloys. Near surface alloys are materials where the top few surface layers are created in a chemically heterogeneous way, for example, by depositing a monolayer of one metal on top of another metal. These structures are often not the thermodynamic equilibrium states of the material. To give one example, Ni and Pt form an fee bulk solid solution under most (but not all) conditions,73 so if a monolayer of Ni is deposited on Pt and the system comes to equilibrium, all of the deposited Ni will dissolve into the bulk. There is, however, a considerable kinetic barrier to this process, so the near surface alloy of a monolayer on Ni on Pt(lll) is quite stable provided a moderate temperature is used.191 If the deposited monolayer in systems of this type has a tendency to segregate away from the surface, a common near surface alloy structure is the formation of a subsurface layer of the deposited metal.85 The deposition of V on Pd(lll) is one example of this behavior.192... 

Compilation of structural parameters for the surfaces of unreconstructed disordered metallic alloys. Cl—C4 are the percentage of atom type A in the corresponding layer of the bulk alloy AB. Atom type A is the first element listed in the alloy column of the table, ddn is the change in the first intcrplanar spacing expressed as a percentage of the (mean) bulk interlayer spacing of the disordered alloy. 9 3 and 3 4 are the equivalent quantities for deeper layers. 

It should be emphasized that after TPRe run up to 350 °C, all alloy-type Sn-Pt/Si02 catalysts without re-reduction had very low activity. Thus, on platinum nanoclusters covered by bulk type tin-oxide layer the number of required metal ion - metal nanocluster ensemble sites is very low. The experimental data given in Table 11 strongly indicated that the activity of the alloy type Sn-Pt/Si02 catalysts was controlled by the surface composition of the bimetallic nanoclusters and the reduced form of the Sn-Pt nanoclusters is more active than a fully oxidized form. Additional experiments have proven that the activity of catalysts used in TPRe experiments can be completely restored after reduction in hydrogen at 340 °C. 

It is possible to make some remarks by comparison with known bulk alloys bulk GdAu is CsCl-type (bcc) with a = 3.60 A, while the indexing of the film leads to a fee structure. Bulk GdAu2 is tetragonal (MoSia-type) with a = 3.73 A and c = 9.02 A, whereas the indexing leads to a fee structure with a = 3.75 A and a hexagonal one with a - 3.70 A. The nature of these systems is complex and they can possibly be interpreted as ternary alloys or oxidized compounds. 

One approach for preparing carbon supported, non-noble metal core, Pt shell type particles is depicted in Fig. 9.11. First, a bulk alloy of Co(Ni)Au(Pd) is formed on the carbon support by reduction of the metal salt precursors. Surface segregation of the noble metal is achieved by hydrogen treatment at temperatures between 600 and 850°C. After this, a Cu monolayer is deposited at underpotential (Cu UPD) and displaced by Pt atoms. °... 

Figure 12.23 shows atomically resolved STM images with chemical contrasts of Type III surface alloys of the system Cu Pdj /Ru(0001), whose fabrication is demonstrated in Figure 12.19. The images show no indication of LRO in the atom distribution. One might expect such an LRO for surface compositions close to those forming ordered bulk alloys [78], that is, Cu/Pd= 1 1 (Figure 12.23c) or Cu/Pd = 3 1 (Figure 12.23e).

The second type of magnetic investigation was made by Curie temperature determinations in a thermomagnetlc balance (ref.11). The Curie temperatures of the catalysts were also very near to those of bulk alloys (figure 6). 

It is important to emphasize again that this is not normally an equilibrium structure in the bulk of the solid. Calculations show that a random bulk alloy is typically favored over the CuPt or CuAu organization. Ordering of this type in alloys is a result of the growth process and is directly tied to the surface structure as growth occurs. This is why sometimes one order is found, sometimes another, and sometimes no order is present. 

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