Mixed metal oxides uranium compounds

Another propylene ammoxidation catalyst that was used commercially was U-Sb-0. This catalyst system was discovered and patented by SOHIO in the mid-1960s (26,27). Optimum yield of acrylonitrile from propylene required sufficient antimony in the formulation in order to ensure the presence of the USbaOio phase rather than the alternative uranium antimonate compound USbOs (28-30). The need for high antimony content was understood to stem from the necessity to isolate the uranium cations on the surface, which were presumed to be the sites for partial oxidation of propylene. Isolation by the relatively inactive antimony cation prevented complete oxidation of propylene to CO2. Later publications and patents showed that the activity of the U-Sb-0 catalyst is increased by more than an order of magnitude by the substitution of a tetravalent cation, tin, titanium, and zirconium (31). Titanium was found to be especially effective. The promoting effect results in the formation of a solid solution by isomorphous substitution of the tetravalent cation for Sb + within the catalytically active USbaOio- phase. This substitution produces o gen vacancies in the lattice and thus increases the facility for diffusion of lattice o gen in the solid structure. As is discussed below, the enhanced diffusion of o gen is directly linked to increased activity of selective (amm)oxidation catalysts based on mixed metal oxides. 

The most important fuel for currently operated nuclear power stations (mainly light-water reactors) is - U-enriched uranium(IV) oxide. Also of importance are metallic uranium for the Magnox reactors and a few research reactors and uranium-plutonium mixed oxides for light-water reactors. Fuel production comprises extraction and dressing of uranium ores to uranium concentrates, conversion into UF, the uranium compound used for enrichment of the BSy.jjjotope, enrichment of and production of fuel from enriched UF5 (reconversion). 

Protactinium is a relatively heavy, silvery-white metal that, when freshly cut, slowly oxidizes in air. AH the isotopes of protactinium and its compounds are extremely radioactive and poisonous. Proctatinium-231, the isotope with the longest half-life, is one of the scarcest and most expensive elements known. It is found in very small quantities as a decay product of uranium mixed with pitchblende, the ore of uranium. Protactiniums odd atomic number (gjPa) supports the observation that elements having odd atomic numbers are scarcer than those with even atomic numbers. 

The two most important sources of uranium are the minerals carnotite, where uranium occurs in the hexavalent oxide or hydrated oxide, and pitchblende, where uranium occurs mostly in the tetravalent state as a compound salt with other metals. It also occurs as a mixed oxide with titanium, thorium, and niobium in the tetravalent form. The tetravalent uranium minerals appear to have been geologically formed in the presence of reducing agents such as hydrocarbon minerals, graphite, native metals, and sulfide minerals, while such association is rarely observed with the hexavalent uranium minerals. 

Plutonium Dioxide in Molten Equimolar Sodium-Potassium Nitrate. The behavior of plutonium dioxide in molten alkali metal nitrates is an area of major concern. Claims that alkali metal plutonates are formed (1, 2, 3, 5, 6) are not substantiated by definitive analytical results. In some cases (5, 6), sodium peroxide was added as an oxidant to either an alkali metal nitrate melt (6) or to an alkali metal hydroxide melt (5). If the temperature is great enough, for example above 700°C, thermal decomposition of the nitrate melt produces peroxide species. Other studies (4, , 12, 17) do not claim formation of a plutonate species, but only state that an insoluble plutonium-containing compound exists. However, in all the references cited, the results were given for mixed uranium-plutonium dioxide definitive analytical results were not given. 

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