Insights into the structure and thermal stability of uranyl aluminate nanoparticles

Ultrasonically assisted hydrolytic precipitation of U(VI) in the presence of mesoporous alumina followed by thermal treatment of a solid precursor enabled the formation of crystallized uranyl aluminate (URAL) nanoparticles (NPs) dispersed in the alumina matrix. The effect of U(VI) concentration and calcination temperature on the yield of URAL NPs was studied using XRD, XAFS and HRTEM techniques. At 800 degrees C, URAL NPs (d approximate to 5 nm) are formed only for a low uranium loading of about 5 wt% whereas for a higher content of uranium, larger U3O8 NPs (d approximate to 20 nm) were identified as the principal uranium species. At 500 degrees C, URAL NPs are formed even for 25 wt% of uranium. The U L-III edge EXAFS spectra pointed out that uranyl cations in URAL are coordinated by bidentate aluminate groups. Presumably URAL is formed during the heating of the 2UO(3).NH3.2H(2)O/AlO(OH) precursor. However, high temperature and higher content of uranium promote transformation of URAL to more thermodynamically stable U3O8. This process is accompanied by the growth of uranium NPs via the Ostwald ripening mechanism.