Active uranium oxides

Uranium oxide [1344-57-6] from mills is converted into uranium hexafluoride [7783-81-5] FJF, for use in gaseous diffusion isotope separation plants (see Diffusion separation methods). The wastes from these operations are only slightly radioactive. Both uranium-235 and uranium-238 have long half-Hves, 7.08 x 10 and 4.46 x 10 yr, respectively. Uranium enriched to around 3 wt % is shipped to a reactor fuel fabrication plant (see Nuclear REACTORS, NUCLEAR FUEL reserves). There conversion to uranium dioxide is foUowed by peUet formation, sintering, and placement in tubes to form fuel rods. The rods are put in bundles to form fuel assembHes. Despite active recycling (qv), some low activity wastes are produced. 

In the 1960s, a number of binary oxides, including molybdenum, tellurium, and antimony, were found to be active for the reactions and some of them were actually used in commercial reactors. Typical commercial catalysts are Fe-Sb-O by Nitto Chemical Ind. Co. (62 -64) and U-Sb-O by SOHIO (65-67), and the former is still industrially used for the ammoxidation of propylene after repeated improvements. Several investigations were reported for the iron-antimony (68-72) and antimony-uranium oxide catalysts (73-75), but more investigations were directed at the bismuth molybdate catalysts. The accumulated investigations for these simple binary oxide catalysts are summarized in the preceding reviews (5-8). 

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. 

The dehydrogenation of ethylbenzene is an important process used for styrene manufacture, and uranium oxide catalysts have been inveshgated for this reaction. A catalyst of uranium dioxide supported on alumina showed high selectivity to styrene of 96% at high conversion [62, 63]. The catalyst was synthesized as a higher oxide of uranium and inihally it was not UO2. Consequently, over the initial onstream period only carbon dioxide and water were observed, as the catalyst produced total oxidahon products. However, as the reachon proceeded the uranium oxide was reduced in situ by the ethylbenzene and hydrogen to form the active UO2 phase. It was only when the uranium oxide was fully reduced to UO2 that styrene was produced with high selectivity. 

Table 13.6 Activity of nickel-uranium oxide catalysts for steam reforming of naphtha [72],...

The activity of a specimen is measured by comparing the number of impulses which it produces per minute in a Geiger counter with that produced by a substance of known specific activity, say uranium oxide, in the same position in the same counting equipment. 

Studies of the decomposition of methanol over various catalysts show that the same catalysts are active toward the decomposition reaction cat pressures of one atmosphere as are active toward the synthesis at the higher pressures. Indeed, to Patart is attributed the statement that the results from the work of Sabatier on the catalytic decomposition of methanol led directly to the use of certain of the methanol synthesis catalysts.188 144 Smith and Hawk 145 found that zinc oxide made by igniting die carbonate, mixtures of zinc and chromium oxides in the atomic proportions of 4 zinc to 1 chromium, mixtures of zinc and uranium oxides,... 

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