Metals modification

Iodine cataly2es the conversion of amorphous selenium to the black, semiconducting metallic modification, and is used for this purpose in the manufacture of photoelectric cells and electric rectifiers (see Seleniumand selenium compounds). 

Crystalline selenium exhibits two monochnic forms an alpha form constituting dark red transparent crystals, density 4.50 g/cm. The alpha form converts to a metastable beta form of hexagonal crystal structure when heated to about 170°C. Both the crystalline forms are insoluble in water soluble in sulfuric and nitric acids very slightly soluble in carbon disulfide. Also, both the crystalline forms convert into gray metallic modification on heating. 

The grey form may be obtained from the yellow in carbon disulphide solution by treating with alcohol, or by cooling with carbon dioxide and ether or with liquid air. It is stable towards atmospheric oxygen, and is oxidised by nitric acid more slowly than the brown and metallic modifications. 

Similarly to the lanthanides, actinides in the elemental state are reactive electropositive metals and pyrophoric in finely dispersed form. Strong reducing agents are necessary to prepare the metals from their compounds, for instance reduction of the halides by Ca or Ba at 1200°C (e.g. Pup4 + 2Ca Pu -I- 2CaF2). Some properties of the actinides in the metallic state are fisted in Table 14.5. The number of metallic modifications and the densities are remarkably high for U, Np and Pu. Some modifications of these elements are of low symmetry this is an exception for metals that is explained by the influence of the f electrons. The properties of Am and the following elements correspond to those of the lanthanides. 

It was regarded as a suboxide and also as metal modification. Today it is described as a metastable metal phase, which is stabilized by small amounts of oxygen. A further stabilization of P-W occurs by the presence of different foreign elements, such as K, Be, P, B, As, Ce, Th, and Al. All these elements have in common that they form rather stable ternary and quaternary tungsten oxides. 

Within the homologous series C—Si-Ge—Sn all of the elements are stable (or metastable) in the diamond stmcture (coordination number = 4). Pressure converts these non-conductors or semi-conductors into metallic modifications with coordination number of 6 (which is the stable modification of tin at room temperature and atmospheric pressure (6,17, 18) (Table 4). 

ZSM-5 zeolite catalysts are well known for their shape selective and acidic properties, and low deactivation rates in efficient transformation of a number of hydrocarbon molecules[3-5]. Xylene isomerization. Toluene disproportionation. Methanol to gasoline and olefins, M-2 forming are some of the important ZSM-5 based processes[6-l 1]. These catalysts are also known to increase LPG range products when they are used as FCC additives. These considerations lead us to the development of ZSM-5 based catalysts such, that optimization of LPG or gasoline can be made by suitable choice of modifying procedure such as acid modification or metal modification[12-17j. 

Product distribution of the light naphtha conversion (LNl) over the metal incorporated zeolite catalyst is given in Table 8. As can be seen from data presented, the metal modified catalyst is highly active for aromatization, evidenced by increased conversion to 91.5 wt % and aromatic yield 35.2 wt %. This is more obvious when we compare with the results on the parent catalyst (before metal modification) HZSM-5, which showed only 84% conversion and 22.4% aromatic yield. Increase in aromatic yields obtained over the metal incorporated catalyst can be explained by the active participation of metal in the olefin production by dehydrogenation and aromatization steps of the reaction [39-41]. Since aromatization of paraffins is an endothermic reaction, higher reaction temperatures (500 C) were employed for the maximum production of aromatics. 

Methods of metal modification used in the catalyst preparation have been reported to influence the state of metal, dispersion, concentration of metal, size and surface area. Further more the metal-support interactions, physico-chemical and catalytic properties of the catalysts have also been observed to be influenced by the methods of catalyst preparation. Use of computational and combinatorial methods for catalyst preparation, characterisation and evaluation of catalysts can enhance the possibility of finding new catalytic materials and potential application areas [1-4]. Zeolites and mesoporous molecular sieves are often modified with metals by ion-exchange method, however other methods... 

To further emphasize these findings and to better define the metal modifications which occur under different activating conditions we have carried out an XPS investigation within the same experimental conditions reported above... 

Several approaches have been made in the chlorophyll a molecule 15, by introducing a large number of metals," modification of the isocyclic ring, modification of the vinyl group at position 3, as well as by replacing the phytyl group at the propionyl residue throu either trans-esterification or conversion of the ester functionality into the corresponding amide derivatives (see Scheme 4, also see Chapter 1 by Smith). 

Three allotrope modifications are known. The stable form is the grey metallic modification with a sublimation temperature of 613°C. Melting is possible in closed tubes at 817°C and a pressure of 28 bar. The metallic allotrope exists in a crystalline (specific weight 5.73 g/cm ) and an amorphous form (specific weight 4.7-5.1 g/cm. The yellow crystalline and the black amorphous modifications are metastable and transform into the grey arsenic under the influence of light or heat. Arsenic is nontoxic in its elementary form [1-5]. 

These results agree well with electrochemical experiments. If dichloromethane solutions of different Msj dusters are contacted to Pt electrodes to which 20 V dc is applied, the duster molecules are degraded as a result of the contact with the electrodes. [109] Polarization effects may be the reason for the decomposition. Electrophoresis is observed without any indication of duster decomposition if the platinum electrodes dip into water layers covering the organic phase in a U-tube. The black, thermodynamically unstable microcrystalline products formed on the Pt surfaces have been identified by X-ray powder diffraction to be novel [(Mi3)J metal modifications. The results from the diffraction experiments indicate a structure consisting of cubic dose packed M13 dusters which are linked via their triangular faces to form a kind of pseudo dose packed structure with M13 dusters as building blocks. 

Thin composite films are formed by simultaneous polymerization of compounds during the vacuum evaporation of metals. Modifications have been reported using different modes of preparation and introducing nanoparticles. One version includes simultaneous evaporation of a monomer and metal from different sources, while another relies on the combination of plasma polymerization and metal evaporation." Plasma-induced graft polymerization of traditional monomers (e.g., vinylim-idazole on a capron film) " was carried out during metal evaporation. The plasma polymerization of organometallic compoimds is of special interest." For example, plasma-induced polymerization of diethylberyllimn is remarkable for the simplicity of the equipment required. "... 

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