U3O8 Uranium oxide

The effect of water addihon on the complete oxidahon of benzene and propane VOCs by uranium oxide catalysts has been inveshgated [37]. Benzene oxidahon was studied using a silica-supported U3O8 catalyst Complete oxidahon was promoted by the addition of 2.6% water compared with the reachvity when no water was added to the reactant feed. Increasing the water concentrahon to 12.1% resulted in a suppression of oxidahon achvity. Inveshgahon of propane oxidahon using U3O8 showed a dramatic promohon of achvity. Propane conversion was ca 50% at 600 °C without added water, whilst it increased to 100% at 400 °C with the... 

In a much earlier patent, the removal of organics from exhaust gases by oxidation over a supported uranium oxide catalyst was reported by Hofer and Anderson [39]. The catalyst was 4% U3O8 supported on alumina spheres. The authors used the incipient wetness technique to impregnate alumina with uranyl nitrate solution. In this case the catalyst precursors were calcined at 700°C for 3 h to decompose the uranium salt. The use of other uranium compounds as starting materials was mentioned and these included uranyl acetate, uranium ammonium carbonate and uranyl chloride. The alumina-supported catalyst had a surface area of ca 400m g and further added components, such as copper, chromium and iron, were highlighted as efficient additives to increase activity. 

Under the reaction conditions used, a U3O8 catalyst demonstrated appreciable selective oxidation activity. The best results, in terms of both activity and selectivity to benzaldehyde, were obtained with the mixed oxides with U Mo atomic ratios in the range 8 2 to 9 1. The maximum yield of benzaldehyde was 40 mol%. On the other hand, antimony-based uranium oxides were not found to be effective as catalyst for this reaction. U—Mo and Bi—Mo mixtures also exhibited promising activity and selectivity to benzaldehyde. Bi—Mo and Bi—Mo—P—Si catalysts were also tested. Qualitahvely there was little difference between the product distributions from the two catalysts. The major products formed were benzaldehyde, benzene and carbon oxides, as well as traces of anthraquinone and benzoic acid. 

Nicklin, with others, filed several early patents describing the use of uranium oxides as steam reforming catalysts [69], U3O8 was used along with nickel oxide as the basis of a steam reforming catalyst, and it was modified with potassium species (potassium hydroxide, potassium oxide and/or potassium carbonate), all supported on either alumina or a mix of alumina and magnesium oxide. The uranium and nickel catalysts proved to be extremely efficient for steam reforming. 

Catalysts based on uranium oxide are also particularly active for the destruction of the chlorinated VOCs chlorobenzene and chlorobutane [77]. Both were destroyed by U3O8 at 350°C and 70,000 h space velocity, showing 99.7% and >99.5% conversions respectively. Time-on-line studies for the destruction of 0.12% chlorobenzene at 450°C showed that the catalyst was not deactivated as 99.9% conversion was maintained during 400 hours continuous operation. These catalysts were also active for the oxidative abatement of other VOCs and it has been demonstrated that toluene, butylacetate and cyclohexanone can also be destroyed at relatively low temperatures. Considering the high space velocities employed in these studies, uranium based catalysts are amongst some of the most active oxide catalysts investigated for VOC destruction. 

Carbothermic Plasma-Chemical Reduction of Uranium Oxide (U3O8). Analyze the stoichiometry of the carbothermic reduction of U3O8 (7-24). Explain why the ratio of molar fractions of CO2 and CO in products of the process is not fixed. Find out the relation between the molar fractions of CO2 and CO in the products as a function of initial composition of the solid mixture U3O8-C. Explain why the carbothermic reduction process of UO2 (7-23) assumes only CO in products, while that of U3O8 (7-24) expects formation of CO and CO2. 

Uranium Oxides. The important oxides of uranium are UO2, U3O8 and UO3. The dioxide (m.p. 2880 C theoretical density 10.%) is used as n nuclear fuel element. Uranium oxide has been used to produce red and yellow glazes and ceramic colours. 

Uranium oxide catalysts have been reported to have high activity for oxidation of volatile organics [60]. Activity for oxidation of short chain linear alkanes improved when Cr was added as a modifier. It is reported that the addition of Cr increases the defects density of U3O8 phase. Deep oxidation activity of UsOg was enhanced when 2.6% of water was cofed with VOC [61]. It is proposed that this improvement is due to contributions from the other reaction pathways, such as steam reforming. However, the addition of more water caused the catalytic activity to decrease and similar treatment decreased the VOC oxidation activity of Mn203. 

Chemical stability of uranium oxides. Both the early Manhattan Project work [9] and the ORNL work [10] indicated that uranium trioxide would be the probable stable form of uranium oxide under the radiolytic gas formed by the radiation-induced decomposition of water in a reactor. Uranium dioxide in an aqueous slurry at 250°C was oxidized to uranium trioxide in the presence of oxygen overpressure and even in the presence of excess hydrogen gas. The extent of this oxidation depended on the oxygen pressure, and seemed to be independent of the partial pressure of hydrogen (Table 4-1). The extent of oxidation of U3O8 to uranium trioxide depended on both temperature and oxygen pressure. The presence... 

Uranium, too, is widely distributed and, since it probably crystallized late in the formation of igneous rocks, tends to be scattered in the faults of older rocks. Some concentration by leaching and subsequent re-precipitation has produced a large number of oxide minerals of which the most important are pitchblende or uraninite, U3O8, and camotite, K2(U02)2(V04)2.3H20. However, even these are usually dispersed so that typical ores contain only about 0.1% U, and many of the more readily exploited deposits are nearing exhaustion. The principal sources are Canada, Africa and countries of the former USSR. 

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... 

Uranium forms several oxides. The main oxides are brown-black UO2, orange yellow UO3, and nonstoichiometric greenish black U3O8. The most stable oxide is dioxide, UO2. Heating the metal in air or oxygen at 150 to 350°C forms UO2 and UsOs. A trihydride, UH3, is obtained when metal is heated in hydrogen at 250°C. 

The temperature must not exceed 400°C, to prevent the formation of U3O8. The nitrous gases produced are processed to nitrie aeid, whieh is recycled. The subsequent reduction of uranium(VI) oxide to uranium(IV) oxide with hydrogen at 500°C also proceeds in the fluidized bed furnace. 

Pulverization takes place by oxidation ofQthe uranium dioxide (UO2) with air at elevated temperatures ( 400UC) which expands the fuel volume if the oxidation is continued until U3O8 is obtained, a 30% volume expansion is achieved. The volume increase ruptures the cladding and pulverizes the fuel. Complete oxidation of UO2 to UoOg is not required to obtain sufficient volume expansion for pulverization. 

The nuclear fuel consists of uranium, usually in the form of its oxide, U3O8 (Figure 23.12). Naturally occurring uranium contains about 0.7 percent of the uranium-235 isotope, which is too low a concentration to sustain a small-scale chain reaction. For effective operation of a light water reactor, uranium-235 must be enriched to a concentration of 3 or 4 percent. In principle, the main difference between an atomic bomb and a nuclear reactor is that the chain reaction that takes place in a nuclear reactor is kept under control at all times. The factor limiting the rate of the reaction is the number of neutrons present. This can be controlled by lowering cadmium or boron rods between the fuel elements. These rods capture neutrons according to the equations... 

---