Oxidation of uranium

Uranium pentabromide [13775-16-1], UBr, is unstable toward reduction and the pentaiodide is unknown. Two synthetic methods utilized for the production of UBr involve the oxidation of uranium tetrabromide [13470-20-7], UBr, by Br2 or by bromination of uranium turnings with Br2 in acetonitrile. The metastable pentabromide is isostmctural with the pentachloride, being dimeric with edge-sharing octahedra U2Br2Q. 

Today, the air oxidation of toluene is the source of most of the world s synthetic benzaldehyde. Both vapor- and Hquid-phase air oxidation processes have been used. In the vapor-phase process, a mixture of air and toluene vapor is passed over a catalyst consisting of the oxides of uranium, molybdenum, or related metals. High temperatures and short contact times are essential to maximize yields. Small amounts of copper oxide maybe added to the catalyst mixture to reduce formation of by-product maleic anhydride. 

The only anhydrous trioxide is UO3, a common form of which (y-U03) is obtained by heating U02(N03).6H20 in air at 400°C six other forms are also known.Heating any of these, or indeed any other oxide of uranium, in air at 800-900°C yields U3O8 which contains pentagonal bipyramidal UO7 units and can be used in gravimetric determinations of uranium. Reduction with H2 or H2S leads to a series of intermediate... 

Extensive work into the corrosion and oxidation of uranium and its alloys has been undertaken over the past decade but much of this is in the form of Ministry and industrial reports which are not necessarily readily available. The present review concentrates on the work published in the normal scientific and technical press. 

Uranium(IV) chalcogenolate compounds have been synthesizing with several stoichiometries as the homoleptic [U(SR)4] (R = Et, Pr, Ph) obtained by reaction of [U(BH4)4] or [U(NEt2)4] with thiols or by oxidation of uranium with the disulfide,571 or those bearing cyclopentadienyl rings as [UCp3(ER)]... 

Uranium is the fourth metal in the actinide series. It looks much like other actinide metallic elements with a silvery luster. It is comparatively heavy, yet malleable and ductile. It reacts with air to form an oxide of uranium. It is one of the few naturally radioactive elements that is fissionable, meaning that as it absorbs more neutrons, it splits into a series of other lighter elements (lower atomic weights) through a process of alpha decay and beta emission that is known as the uranium decay series, as follows U-238—> Th-234—>Pa-234—>U-234—> Th-230 Ra-226 Rn-222 Po-218 Pb-2l4 At-218 Bi-2l4 Rn-218 Po-2l4 Ti-210—>Pb-210—>Bi-210 Ti-206—>Pb-206 (stable isotope of lead,... 

The first and thus far only silsesquioxane complex of an actinide element is [Cy7Si70i2]2U (100). This colorless, nicely crystalline uranium(VI) compound is formed upon reaction of 3 with any uranium precursor, e.g., UCI4 in the presence of NEt3. In all cases oxidation of uranium to the hexavalent oxidation state is observed. The best synthetic route leading to 100 in ca. 80% yield is the reaction of 3 with uranocene as outlined in Scheme 33. 

Figure 23 shows the different oxides of uranium . As expected in an ionic picture, the 5f emission decreases with decreasing occupation number of the 5f shell, to disappear completely in P-UO3 (5f configuration of the ion). 

In 1823 J. A. Arfwedson reduced the green oxide of uranium (then believed to be the lowest oxide) with hydrogen, and obtained a brown powder which he took to be the metal, but which is now known to be uranous oxide, U02 (25, 30). In 1841 Peligot, on analyzing anhydrous uranous chloride, UC14, found that 100 parts of this chloride apparently yielded about 110 parts of its elements uranium and chlorine. His explanation of this seemingly impossible result was that the uranous chloride reacts with water in the following manner ... 

One last point. In the reaction of uranium(IV) where it is convenient to do a tracer experiment because there is only one metal ion product, we have actually determined the number of oxygens transferred to the uranyl ion product from the chlorite, and this number corresponds to 1.3 oxygen per chlorite transferred to the uranium. This is consistent with the results we reported some years ago (5) on the oxidation of uranium (IV) with Pb02 and Mn02, where indeed more than one oxygen is transferred. In conclusion, we feel that we have some direct evidence for two-electron transfer in these reactions and the formation of a chlorine(I) intermediate followed by the formation of chlorate. 

U02.25 corresponds to U4O9, which is a well-characterized oxide of uranium known at low temperature. UO2 has the fluorite structure. The unit cell is depicted in Figure 5.27(a) and contains four formula units of UO2. (There are four uranium ions contained within the cell boundaries the eight oxide ions come from (8x /8)=l at the corners ... 

The dissolution time for the unreprocessed fuel would be at least 1 million years due to the limited water supply, even if a rapid oxidation of uranium to the hexavalent state and a subse-guent formation of water soluble carbonate complexes are assumed (15). Since the conditions are reducing in the groundwater (see beTow) the dissolution time would probably be several orders of magnitude larger. The unsignificant dissolution of uranium and fission products observed in the Oklo-deposit (16) is an example of a similar extremely slow leaching process in the natural environment. 

Gold Topas Color.—Six hundredweight of batch with three pounds of oxide of uranium. 

Oxides.—Amongst the simple oxides may be classed —oxide of chromium, oxide of iron, oxide of uranium, oxide of manganese, oxide of zinc, oxide of cobalt, oxide of antimony, oxide of copper, oxide of tin. 

Uranium Arsenide, U3As4, may be obtained i by passing hydrogen over a fused mixture of sodium uranous chloride and sodium arsenide. It is a greyish powder which readily burns in the air. Sometimes it is obtained in a pyrophoric condition. An aluminium-containing product results when the aluminothermic process, using an oxide of uranium and arsenious oxide, is employed. The purest arsenide is obtained, in the crystalline form, when a mixture of hydrogen and arsenic vapour is passed over sodium uranium chloride. It is rapidly decomposed by nitric acid. 

The oxidation of uranium (IV) complexes has already been mentioned on discussion of the synthesis of U (V) derivatives. They can even be transformed directly into the U(VI) compounds on action of strong dielectron oxidants in the basic media [432] ... 

Distribution ratios and transport were carried out on real HAW arising from dissolution of a mixed oxide of uranium and plutonium (MOX) fuel (burnup 34,650 MW d/tU), where uranium and plutonium have been previously extracted by TBP.86 The experiments were performed in the CARMEN hot cell of CEA Fontenay aux Roses with two dialkoxy-calix[4]arene-crown-6 derivatives (diisopropoxy and dini-trophenyl-octyloxy). High cesium distribution ratios were obtained (higher than 50) by contacting the HAW solution with diisopropoxy calix[4]arene-crown-6 (0.1 M in NPHE). Moreover, the high selectivity observed with the simulated waste was confirmed for most of the elements and radionuclides (actinides or fission products Eu, Sb, Ce, Mo, Zr, and Nd). The residual concentration or activity of elements, other than cesium, was less than 1% in the stripping solution, except for iron (2%) and ruthenium (8%) the extraction of these two cations, probably under a complexed... 

Can failures occur from time to time. The release of fission products from them depends on the temperature and type of fuel. If the fuel is uranium metal, as in the Windscale and Magnox reactors, and the can fails, the uranium will oxidise in air or C02. In laboratory experiments, the mass median aerodynamic equivalent diameter (MMAD) of the particles produced by oxidation of uranium increased from about 40 ptm when the temperature of oxidation was 600°C to 500 jum at 1000°C (Megaw et al., 1961). At high temperature, a coherent sintered oxide layer formed on the uranium and this hindered the formation of particles. 

The particle size of Pu aerosols is very variable, depending on the mode of formation. In Fig. 5.2, curves A, B and C show size spectra obtained by Carter Stewart (1971) in laboratory experiments on the oxidation of Pu metal in air. In controlled oxidation at temperatures below the ignition point (about 500°C), scaly, friable, oxide particles were produced, with median diameter increasing with temperature. Few particles less than 1 jum in diameter were found. When the delta alloy of Pu was used, the oxide was more adherent, and the particle size larger. Increase of particle size with increase of temperature was also found in laboratory oxidation of uranium metal (Megaw et al., 1961), and was ascribed to sintering of the oxide layer. 

It is necessary to mention that uranium p-diketonates were also obtained by a direct electrochemical procedure. Thus, the electrochemical oxidation of uranium leads to chelates of the type UL4 and U02L2 (LH = diketone) [14,343-347]. In addition to these complexes, the compound having composition UO2L2(HL)05 was also isolated [344] the structure showed in 974 (compare with 973) was proposed on IR spectroscopy data and, in our opinion, requires a more detailed analysis. 

The reduction of U03 to U02 has drawn much attention in connection with the preparation of fuels for nuclear reactors. Above 400° C, in which temperature range most of the reductions have been studied, the oxides of uranium take the forms, U02+, U409, U308-x(U02 6) and U03, as in Fig. 5 [65], In common with other gas—solid reactions, the details of the kinetics vary depending on the origin of the sample [69], the surface area [68], etc. Some authors claim that the reduction by hydrogen proceeds stepwise U03 -> U3Og - U02 [66—69], but there is a report [70] that no evidence for the existence of the intermediate oxides was found by X-ray diffraction at the reaction boundary of U03 and U02. 

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