Uranium oxide catalysts

Certain types of catalyst uranium oxide and chromium oxide may be used as a promoter. This is reported to give a higher resistance to catalyst poisoning by sulfur components and a lower tendency to form carbon deposits. 

The Sohio technology is based on a catalyst of bismuth an4 molybdenum oxides. Subsequent catalyst improvements came from the use of bismuth phosphomolybdate on a silica gel, and more recently, antimony-uranium oxides. Each change in catalyst was motivated Jby a higher conversion rate per pass to acrylonitrile. 

Taylor, H. and O Leary, R. A study of uranium oxide based catalysts for the oxidative destruction of short chain alkanes, Appl. Catal, B Environmental, 2000, Volume 25, Issues 2-3, 137-149. 

Strangely enough, a combination similar to the ammonia catalyst, iron oxide plus alumina, yielded particularly good results (32). Together with Ch. Beck, the author found that other combinations such as iron oxide with chromium oxide, zinc oxide with chromium oxide, lead oxide with uranium oxide, copper oxide with zirconium oxide, manganese oxide with chromium oxide, and similar multicomponent systems were quite effective catalysts for the same reaction (33). 

Lead hydroxide is used in making porous glass in electrical-insulating paper in electrolytes in sealed nickel-cadmium batteries in recovery of uranium from seawater and as a catalyst for oxidation of cyclododecanol. 

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). 

It is claimed that a catalyst mixture of 93% uranium oxide and 7% molybdenum oxide gives relatively high yields. The oxidation is sometimes carried out in the liquid phase by using manganese dioxide/sulfuric acid at 40°C. 

Historical Uses of Uranium Oxides as Catalysts 545 Table 13.2 Catalytic data for toluene oxidation using uranium oxide based catalysts [6]. 

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. 

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. 

The catalysts were evaluated by exposure to a simulated automobile exhaust gas stream composed of 0.2% isopentane, 2% carbon monoxide, 4% oxygen and a balance of nitrogen. The temperature required to oxidize the isopentane and carbon monoxide was used to compare catalyst performance. The chromium-promoted catalyst oxidized isopentane at the lowest temperature, and a mixed chromium/copper-promoted catalyst proved the most efficient for oxidizing carbon monoxide and isopentane. It is interesting to note that the test rig used a stationary engine with 21 pounds of catalyst. Although the catalyst was very effective it is difficult to envisage uranium oxide catalysts employed for emission control of mobile sources. 

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... 

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. 

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... 

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. 

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. 

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

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. 

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

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