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Selective uranium oxides

The fifth component is the stmcture, a material selected for weak absorption for neutrons, and having adequate strength and resistance to corrosion. In thermal reactors, uranium oxide pellets are held and supported by metal tubes, called the cladding. The cladding is composed of zirconium, in the form of an alloy called Zircaloy. Some early reactors used aluminum fast reactors use stainless steel. Additional hardware is required to hold the bundles of fuel rods within a fuel assembly and to support the assembhes that are inserted and removed from the reactor core. Stainless steel is commonly used for such hardware. If the reactor is operated at high temperature and pressure, a thick-walled steel reactor vessel is needed. [Pg.210]

Here, the main features of the valence band results for Th02 and UO2 will be illustrated. Since a large number of publications exists in this field (especially for uranium oxides), reference will be made only to a few selected investigations, chosen for the purpose of highlighting those aspects of the oxide bond discussed previously. A very comprehensive review of these results can be found (and references therein electronic and spectroscopic properties in Refs. 109-111). Figure 21 shows the photoemission spectrum of Th02 and UO2 up to Et = 45 eV The valence band region extends to about 10 eV. The marked difference is the appearance in UO2 of a sharp and intense peak at Et =... [Pg.240]

Thus, historically, uranium oxides have been used as catalysts, and more often they have been used as catalyst components in combination with other metal oxides. Often it is difficult to identify the catalysts unambiguously there is little characterization data in the studies, and it is most likely that the specific stoichiometries of uranium oxides quoted as catalysts are not correct. There are many other examples of the use of uranium oxides for heterogeneous catalysis and the few examples presented in this section are typical of some of the earliest uses. It is interesting to note that, although some of the work highlighted was carried out over 80 years ago, some of the aims, such as selective hydrocarbon oxidation, are still major research aims for heterogeneous catalysis today. [Pg.546]

Uranium oxides have been investigated as catalysts and catalyst components for selective oxidation. They are more commonly used as catalyst components, but there are also reports of uranium oxide alone as a selective oxidation catalyst The oxidation of ethylene over UO3 has been studied by Idriss and Madhavaram [40] using the technique of temperature programmed desorption (TPD). Table 13.3 shows the desorption products formed during TPD after ethylene adsorption at room temperature on UO3. The production of acetaldehyde from ethylene indicates... [Pg.548]

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. [Pg.553]

A study has been undertaken to compare the effectiveness of molybdenum and uranium oxide and iron sodalite catalysts with the homogeneous gas-phase oxida-hon of methane [54]. Catalyst performance was evaluated in a high-pressure annular reactor and data were compared to the reactivity of the empty reactor. It was concluded that none of the catalysts gave any advantage over the homogeneous reaction. Indeed, using a catalyst only reduced the selectivity to the desired partial oxidation products. Similar conclusions have been reached for many catalysts used for the partial oxidation of methane, and therefore it is perhaps not surprising that uranium oxide catalysts are no different... [Pg.553]

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. [Pg.555]

Uranium oxide catalysts have largely been employed for the reduction of organic species but, in a series of interesting studies, a uranium oxide catalyst has also been used for the reduction of NO, and simultaneous oxidahon of CO [66]. Studies showed that NO was converted to N2 with 100% selectivity under favorable reachon conditions. Using a mixture of 4%NO, 4%CO with a balance of He, different uranium oxides were tested in a fixed bed micro-reactor. The results obtained are shown in Table 13.5, and compared with a conventional supported Pt catalyst. [Pg.555]

Table 13.5 NO conversion and selectivity to N2 over uranium oxide catalysts [66]. Table 13.5 NO conversion and selectivity to N2 over uranium oxide catalysts [66].
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. [Pg.248]

Over the course of the 18 month program, the NRPCT evaluated all reasonable reactor and energy conversion technologies and selected a reference coolant and energy conversion approach that offered the best prospect of meeting NASA mission requirements within the schedule and cost constraints. The NRPCT also recommended the use of uranium oxide fuel. NRPCT was working to select reference core and plant structural materials, a... [Pg.36]

Weiss and Downs described the use of vanadium pentoxide for the catalytic oxidation of toluene and naphthalene. Subsequently, Graver suggested a mixed oxide catalyst derived from uranium oxide and molybdenum trioxide in molar ratios ranging from 3-13 1. Copper oxide was also suggested as a possible promoter. When using the vanadium pentoxide catalyst, benzoic acid was the main product at temperatures below 400°C, with some benzaldehyde formed at higher temperatures. Selectivity was only about 50% at 5% conversion. [Pg.291]

Any canning metal had to be compatible not only with the uranium oxide fuel but with carbon dioxide. A form of stainless steel (containing 20% chromium and 25% nickel together with niobium) was found to be satisfactory at temperatures up to about 850°C. It had a melting point of almost 1,500°C. The maximum can surface temperature selected was 650°C (this meant the system could produce steam at the same temperature as conventional power stations), which allowed for local hot spots. One drawback to stainless steel was that it had a relatively high cross-section area for thermal neutrons, which meant using enriched fuel, of the order of 2.5% enrichment. [Pg.266]

Although acrylonitrile manufacture from propylene and ammonia was first patented in 1949 (30), it was not until 1959, when Sohio developed a catalyst capable of producing acrylonitrile with high selectivity, that commercial manufacture from propylene became economically viable (1). Production improvements over the past 30 years have stemmed largely from development of several generations of increasingly more efficient catalysts. These catalysts are multicomponent mixed metal oxides mostly based on bismuth—molybdenum oxide. Other types of catalysts that have been used commercially are based on iron—antimony oxide, uranium—antimony oxide, and tellurium-molybdenum oxide. [Pg.182]

If the metallic compound in the ore can be selectively leached by acid or base without dissolving much of the remaining ore, then the energy requirement is only about 10 J/kg. Examples are leaching of oxide ores of copper, zinc, or uranium with sulfuric acid. [Pg.771]


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See also in sourсe #XX -- [ Pg.548 ]




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