Alloys metal atom cluster

In multi-component liquids, stabilization of the liquid is revealed by the formation of eutectics where the freezing temperature is suppressed. In such liquids, the atomic species (say A and B) are not distributed at random. There are more associated AB pairs (or other clusters) than expected for a random distribution. As a result in binary metal-metalloid alloys, such as Fe-B, the low melting-point eutectics occur at preferential compositions. The most common of these is at about 17 at. % B, or an atom ratio of one B for five Fe atoms (Gilman, 1978). This suggests that clusters of metal atoms surrounding metalloid atoms form (trigonal bipyramids). These probably share corners, edges, and faces. 

Hetero-atomic clusters, moreover, may be derived from the binary structures mainly through the introduction of late transition or earlier post-transition elements. Examples of ternary alloys containing such structures are the alkali metal salts of centred clusters In10Me10 (Me = Ni, Pd, Pt), Tl12 Me12- (Me = Mg, Zn, Cd, Hg), etc. The crystal structure of the phase Na T Cdi x)27 (0.24 < x < 0.33)... 

Synthesis of nano-structured alloys by the inert gas evaporation technique A precursor material, either a single metal or a compound, is evaporated at low temperature, producing atom clusters through homogeneous condensation via collisions with gas atoms in the proximity of a cold collection surface. To avoid cluster coalescence, the clusters are removed from the deposition region by natural gas convection or forced gas flow. A similar technique is sputtering (ejection of atoms or clusters by an accelerated focused beam of an inert gas, see 6.9.3). 

One of the important applications of mono- and multimetallic clusters is to be used as catalysts [186]. Their catalytic properties depend on the nature of metal atoms accessible to the reactants at the surface. The possible control through the radiolytic synthesis of the alloying of various metals, all present at the surface, is therefore particularly important for the catalysis of multistep reactions. The role of the size is twofold. It governs the kinetics by the number of active sites, which increase with the specific area. However, the most crucial role is played by the cluster potential, which depends on the nuclearity and controls the thermodynamics, possibly with a threshold. For example, in the catalysis of electron transfer (Fig. 14), the cluster is able to efficiently relay electrons from a donor to an acceptor, provided the potential value is intermediate between those of the reactants [49]. Below or above these two thresholds, the transfer to or from the cluster, respectively, is thermodynamically inhibited and the cluster is unable to act as a relay. The optimum range is adjustable by the size [63]. 

Indeed, eobalt and a promoter metal may form an integral metal particle deposited on the support oxide, altering the electronic properties of the surface cobalt metal atoms (Figure 4C). Depending on the promoter element added to the Co cluster, alloying might lead to an increased catalyst activity, selectivity, as well as stability. 

Another non-conventional preparative route to bimetallic catalysts has been developed where metal atoms (vapors) have been trapped at low temperature in solvating media. (A review has recently appeared).(17) By solvating two metals at the same time (eg. Co in toluene and Mn in toluene), followed by warming, bimetallic clusters/particles form. In the presence of a catalyst support, surface -OH groups can have a dramitic affect on the structure of the small bimetallic cluster produced. For example, with Co and Mn, a layered structure of MnOx covered by Co° in a particle of about 25 A was formed.(iS) With Fe and Co combinations, a layer of FeOx followed by Fe°Co° alloy and a surface rich in Co° was formed. (19)... 

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. 

Ablation is a powerful technique that uses high-energy lasers to vaporize or ablate materials from the surface. The wavelength of the laser is tuned for the specific material to achieve maximum absorption of the energy, most often ultraviolet. The target is vaporized, creating a plume of neutral metal atoms. The plume is then cooled with a carrier gas to form clusters. It is possible to couple laser evaporation with laser pyrolysis to form alloys. 

Bigger clusters have been formed, for instance, by the expansion of laser evaporated material in a gas still under vacuum. For metal-carbon cluster systems (including M C + of Ti, Zr and V), their formation and the origin of delayed atomic ions were studied in a laser vaporization source coupled to a time-of-flight mass spectrometer. The mass spectrum of metal-carbon cluster ions (TiC2 and Zr C j+ cluster ions) obtained by using a titanium-zirconium (50 50) mixed alloy rod produced in a laser vaporization source (Nd YAG, X = 532 nm) and subsequently ionized by a XeCl excimer laser (308 nm) is shown in Figure 9.61. For cluster formation, methane ( 15% seeded in helium) is pulsed over the rod and the produced clusters are supersonically expanded in the vacuum. The mass spectrum shows the production of many zirconium-carbon clusters. Under these conditions only the titanium monomer, titanium dioxide and titanium dicarbide ions are formed. 

As we will see more in Chapter 7, the resolution of electron microscopes is now suitable for the easy visualization of small atomic cluster arrays. Figure 3.22 illustrates a well-ordered array of Fe-Pd alloy nanoparticles. Interestingly, even though Fe (bcc) and Pd (fee) do not share the same crystal structure, each nanoparticle crystallite comprises only one lattice, indicating that the Fe and Pd metals form a solid solution. It is acmally common for the bcc lattice of iron to change to fee when alloyed with metals such as Pt, Pd, Cu, or Ni (Figure 3.23). 

Within the tight-binding (TB) approach. Slater and Roster [64] described the linear combination of atomic orbitals (LCAO) method as an eflRcient scheme for calculation of the electronic structure of periodic solids. As this method is computationally much less demanding than other methods such as the plane-wave methods, it has been extensively employed to calculate electronic structures of various metals, semiconductors, clusters and a number of complex systems such as alloys and doped systems. The calculation of the electronic structure requires solving the Schrodinger equation with the TB Hamiltonian given by... 

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