Metallic rhenium

Metallic rhenium, 21 687 Metallic silver growth of, 19 352 in images, 19 366-369 morphology of, 19 359 optical density of, 19 369 rate of formation of, 19 207 Metallic silver clusters, filamentary, 19 367-368... 

Rhenium ranges in color from silvery-white to gray to a black powder. It is a rather dense element. As a refined metal, rhenium is ductile, but because it is rather rare, its properties have not found many uses. Rhenium does have the widest range of valences. In addition to its common valences of 4, 6, and 7, it also has the uncommon valences of 2, -1, and -7. 

The first genuine transuranic element was discovered at Berkeley, where Edwin McMillan used Lawrence s cyclotron in 1939 to bombard uranium with slow neutrons. He saw beta decay from what he predicted was element 93, and set about trying to isolate it. McMillan saw that the element sits beneath the transition metal rhenium in the Periodic Table, and so he assumed it should share some of rhenium s chemical properties. But when he and Fermi s one-time collaborator Emilio Segre performed a chemical analysis, they found that eka-rhenium (in Mendeleyev s terminology) behaved instead like a lanthanide, the series of fourteen elements that loops out of the table after lanthanum (see page 152). Disappointed, they figured that all they had found was one of these known elements. 

Two methods are in common use for the production of metallic rhenium. The first of these involves a direct reduction of commercial potassium perrhenate (procedure A) and yields a product that usually contains a small amount of alkali but is pure enough for most preparative purposes. The second method (procedure B) is slightly more complicated in that a preliminary precipitation of rhenium heptasulfide is followed by conversion to ammonium perrhenate which is subsequently reduced to the metal.1,2 Metallic rhenium so produced is usually purer than that prepared by direct reduction of the potassium salt. 

Metallic rhenium prepared by this method is generally between 99.0 and 99.8 per cent pure. The chief impurity is potassium, probably as potassium hydroxide. The product is a dense gray-black powder. The yield is between 85 per cent and 95 per cent based upon the potassium per-rhenate used. 

Rhenium pentachloride is prepared by the direct combustion of metallic rhenium in dry chlorine.1... 

Metallic rhenium, prepared by the reduction of either potassium perrhenate (see synthesis 60A) or ammonium perrhenate (see synthesis 60B), is placed in a previously ignited porcelain boat and inserted in a pyrex combustion tube of the type shown in Fig. 31. All air in the train is displaced with nitrogen that has been passed through alkaline pyrogallol A and sulfuric acid B. 

Kirlin, P. S., Knozinger, H., and Gates, B. C., Mononuclear, trinuclear, and metallic rhenium catalysts supported on MgO Effects of structure on catalyst performance. J. Phys. Chem. 94, 8451 (1990). 

Space technology often uses alloys that are too expensive for everyday use. An example is the propulsion systems used for keeping satellites in place. Some of these systems use alloys made of iridium and another platinum metal, rhenium. These alloys remain strong at high temperatures and are not attacked by fuels used in the systems. 

Kirlin PS, van Zon FBM, Koningsberger DC, Gates BC (1990) Surface catalytic sites prepared from [HRe(CO)j] and [H3Re3(CO)jJ Mononuclear, trinuclear, and metallic rhenium catalysts supported on magnesia. J Phys Chem 94 8439... 

In contrast to the X-ray diffraction pattern of alumina-supported rhenium oxide, the pattern for the silica-supported samples gives diffraction lines characteristic of metallic rhenium. The metal particle size is about 75 A. Initial kinetic studies with propylene indicated that the silica-supported samples were inactive for the disproportionation reactions up to 180°C. X.p.s. studies of rhenium-supported catalysts show that the state of the initial and reduced rhenium on silica surface is quite different from that on 7-alumina and is dependent on the rhenium compound used to prepare the catalysts. Because of a stronger interaction of Re with the alumina surface, the reducibility of rhenium on alumina is much less than on silica. 

Interestingly, the electron reservoir properties of redox active ligands are also found to be useful to impose one-electron transformation on late transition metals. Rhenium complexes are known to be powerful oxo-transfer reagents [27]. However, closed-shell... 

The lower rate of deactivation of platinum-rhenium catalysts relative to platinum catalysts cannot be attributed to a lower rate of accumulation of carbonaceous residues on the surface. For a given time on stream, the amount of such residues on the surface is not decreased by the presence of the rhenium. This point is interesting because metallic rhenium, like metallic iridium, has much higher hydrogenolysis activity than platinum (26). It is possible that the difference between platinum-rhenium and platinum-iridium catalysts is due to the strong retention of sulfur by the former. The inhibiting effect of sulfur on hydrogenolysis activity is well known. The improved activity maintenance of a platinum-rhenium catalyst relative to a platinum catalyst is due to better tolerance of the carbonaceous residues. 

The most active Tc metal catalyst was obtained at the lowest temperature used for the reduction of TCO2 to Tc metal. Die apparent activation energy for the dehydrogenation process was 13.7 kcal/mole. At 220 C 30 % of isopropyl alcohol was converted to acetone. ITie activity of the technetium metal catalyst for the dehydrogenation of isopropyl alcohol is superior to that of manganese and close to that of metallic rhenium when, however, the content of rhenium in the catalyst was 30 wt% instead of around 0.2 wt% of technetium [11]. 

Rhenium, properties of. Transition metals Rhenium trioxide, 440-444, 450, 466f, 471... 

In previous studies, in addition to metallic rhenium, ionic rhenium species were also present in the catalysts active in the hydrogenation of carbonyl bond of different substrates. Therefore, atomic closeness of metallic and ionic species might be required to obtain bimetallic Re-Pt based catalysts for the hydrogenation of esters such as butyl acetate. The formation of Re -Pt ensembles may be achieved by a calcination/reduction treatment of the catalyst carried out at moderate temperatures. Such a treatment may provide high catalytic activity. 

The alkali metal rhenium- and technetium chalcogenide clusters described in Section 5.6 are also octahedral.These clusters have M6Es units with eight face-capping anions on the octahedral faces in contrast to the niobium halide or oxide clusters. The A4M6S14 (A = alkali metal, M — Re, Tc), Cs6Rc6Si2, and... 

The crystal structures of every known ternary alkali-metal rhenium sulfide and se-lenide all contain [ReeXg] structural units in which X = S or Se. These structural... 

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