Cubic metal particles

Fig. 13. Square-shaped layers around the cubic metal particles (a) at the surface (top view) and (b) in the bulk (view perpendicular to the siu-face).

This study could be extended to the synthesis of iron nanoparticles. Using Fe[N(SiMe3)2]2 as precursor and a mixture of HDA and oleic acid, spherical nanoparticles are initially formed as in the case of cobalt. However, a thermal treatment at 150 °C in the presence of H2 leads to coalescence of the particles into cubic particles of 7 nm side length. Furthermore, these particles self-organize into cubic super-structures (cubes of cubes Fig. ) [79]. The nanoparticles are very air-sensitive but consist of zerovalent iron as evidenced by Mossbauer spectroscopy. The fact that the spherical particles present at the early stage of the reaction coalesce into rods in the case of cobalt and cubes in the case of iron is attributed to the crystal structure of the metal particles hep for cobalt, bcc for iron. 

Similarly, monometallic Rh, Pd, and Au and bimetallic Pt-Rh and Pt-Pd nanowires were prepared in FSM-16 or HMM-1 by the photoreduction method [30,33,34]. The bimetallic wires gave lattice fringes in the HRTEM images, and the EDX analysis indicated the homogeneous composition of the two metals. These results show that the wires are alloys of Pt-Rh and Pt-Pd. Mesoporous silica films were also used as a template for the synthesis of uniform metal particles and wires in the channels [35,36]. Recently, highly ordered Pt nanodot arrays were synthesized in a mesoporous silica thin film with cubic symmetry by the photoreduction method [37]. The... 

Recently, Chaudhari compared the activity of dispersed nanosized metal particles prepared by chemical or radiolytic reduction and stabilized by various polymers (PVP, PVA or poly(methylvinyl ether)) with the one of conventional supported metal catalysts in the partial hydrogenation of 2-butyne-l,4-diol. Several transition metals (e.g., Pd, Pt, Rh, Ru, Ni) were prepared according to conventional methods and subsequently investigated [89]. In general, the catalysts prepared by chemical reduction methods were more active than those prepared by radiolysis, and in all cases aqueous colloids showed a higher catalytic activity (up to 40-fold) in comparison with corresponding conventional catalysts. The best results were obtained with cubic Pd nanosized particles obtained by chemical reduction (Table 9.13). 

Particles of face-centered cubic metals of diameter 5 nm of more have been studied extensively by high resolution electron microscopy, diffraction and other methods. It has been shown that such particles are usually multiply twinned, often conforming approximately the idealized models of decahedral and icosahedral particles consisting of clusters of five or twenty tetrahedrally... 

Metals generally have face-centred cubic (fee), body-centred cubic (bee) or hexagonal structures. The simplest is fee. In the bee structure, if the central atom is different, the lattice is known as a CsCl (cesium chloride) structure. A bee structure can be considered as two interpenetrating cubic lattices. These are shown schematically in figure 1.3. In catalysis, nanoscopic metallic particles supported on ceramic supports or carbon are employed in many catalytic applications as we show in chapter 5. Increasingly, a combination of two metals (bimetallic) or alloys of two or more metals with special properties are used for specific catalytic applications. 

Fig. 4. EDS spectroscopy results of E-particles from a high bum-up LWR fuel superimposed on the isothermal section of ternary phase diagram from Kleykamp (1985) at 1700 CC. These analyses show that there is distinct heterogeneity in the composition of metallic particles in the fuels. Hence, spot analysis of an individual e-parlicle may not provide direct evidence of corrosion. The metallic system is dominated by the hexagonal close packing (e) that occupies most of the phase space. The tr-space and the body centered cubic

Phase analysis and texture of the metal particles. Over the whole composition range, whatever the particle diameter, a face-centered cubic (fee) phase is always observed (Fig. 9.2. J 3) by x-ray diffraction (XRD) either as a single phase (Ni and CovNi) v with x < 0.35) or beside a hexagonal close-packed (hep) phase with broad lines (Co and Co,Ni (with x 2 0.35). The lattice parameter of the fee phase shows... 

Phase analysis and texture of the metal particles. Iron powders are constituted of the a-Fe phase with a body-centered cubic (bcc) lattice, whereas Fe-Co powders appear as a mixture of three phases that are quite similar to those of pure metals (bcc for a-Fe and a mixture of hep and fee for cobalt) (6). In the Fe.Nil(m system, a single fee phase is observed over the whole available composition range U s 25) with a linear dependence of the lattice parameter versus z, which shows the existence of a fee solid solution as already evidenced for the Co.rNiu)o-. system (33). The XRD patterns of the Fe [CovNi(1()o -,v)] i - powders depend on the composition An fee phase is always observed either as a single phase or as the main phase a second hep phase with weak and broad lines appears for a cobalt content x > 35 a third body-centered cubic (hcc) phase can be evidenced when x > 80. 

The observed pattern of segregation might well be a consequence of the accommodation of metallic particles in the free volume available in an assembly of large approximately spherical particles of PVC. For example, close packing of spheres in a face centered cubic system would provide a continuous network of open channels which in the (100) planes would provide spaces of the kind shown in Figure 7. Some features corresponding to the filling of such spaces by small metallic particles... 

There is a consensus from both theoretical and experimental studies that small particles may have unusual physical, chemical, and catalytic properties. Both in terms of numbers of sites of different co-ordination and with regard to electronic effects small means particles having diameters less than about 2 nm. For very small particles, sites having a particular co-ordination may be important, but the calculation of the number and distribution of such sites is subject to serious errors and requires assumptions about particle shapes, etc., which are difficult to confirm, and which may vary from one system to another. Although particles having unusual five-fold symmetry have been detected in certain circumstances, the large majority of small metal particles have conventional cubic symmetry. However, the difference in energy between two alternative structures is small - much smaller than typical heats of... 

For the beautiful tetracapped octahedral Os cluster, [OsioC(CO)24]2- with an interstitial C atom in the octahedral core, shown in Figure 3.10, the predicted eve count is 14(6) + 2 + 4(12) = 134, which agrees with that of the observed stoichiometry. It s a little bit harder to count the sep but give it a try. Each tetrahedral cap consists of an Os(CO)3 fragment and the other six fragments are Os(CO)2 so we have (4x2 + 6x0 + 4 + 2)/2 = 7 appropriate for an octahedron. If you look ahead in Chapter 6 (Exercise 6.1), you will find that this trigonal bipyramidal ten-atom core can be excised from a cubic close-packed metal lattice (ABC layers). [OsioC(CO)24]2- can be considered a nano-sized metal particle stabilized by the ligands in the same manner as Ni atoms are stabilized when removed from Ni metal by CO as Ni(CO)4 in the Mond process. 

Colors develop because of selective absorption by anisotropic silver crystals less than 500 A long. The anisotropic crystals are produced by precipitating a cubic alkali halide crystal, NaF, on silver metal particles (the normal photochromic process) and growing a second halide crystal (e.g., NaBr) on the NaF crystal. Growth occurs preferentially from the (100) face of the NaF crystal in the form of a pyramid consisting of very small dendrites. 

Researchers in the area of heterogeneous catalysis have recently focussed considerable attention to the relationships among catalytic activity, product selectivity and the size and shape of metal particles for reactions catalyzed by metals (15). Reactions that are influenced by the size and shape of metal particles or electronic interactions of the metal particles with the support are known as structure sensitive reactions. Theoretical calculations of various crystallographic structures (16) have shown that the number of specific type of surface atoms (face, corner, edge) change as a function of particle size. For example, for a face centered cubic system, the number of face atoms decreases as particle size decreases. If, therefore, a reaction is catalyzed on a face and there are a substantial number of face atoms necessary for catalysis to occur, then as particle size decreases catalytic activity will decrease. This idea often runs counter to principles discussed in general science texts (17). 

As an attempt to simulate real operating conditions of automotive converters, a laboratory bench has been designed and ageing procedures determined to reproduce simultaneous chemical and thermal modifications encountered by catalysts in the exhaust line. Characterization of commercial samples after ageing according to different temperature cycles evidences formation of both platinum/rhodium alloys and cubic perovskite-type compound, CeA103. Simultaneously with the formation of cerium aluminate, a thermal stabilization of catalysts is observed, in terms of mean noble metal particles size and concentration of rhodium in alloyed phases. An interpretation based on the crystallographic adaptation of alumina, cerium aluminate and ceria is proposed. 

The HREM study showed some additional features of the packing of the Pd atoms. " "" Although some cluster metal particles have a five-fold axis typical of an icosahedron (Fig. 2a), the more abundant metal particles seemed from the micrographs to be arranged according to face-centered cubic (f.c.c.) packing (Fig. 2b), and this was confirmed by the electron diffractograms for the same cluster samples. 

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