Existence of uranyl aluminate at the nanoscale

The aluminate complexes of uranyl ion cannot be obtained in aqueous solutions due to the hydrolysis of U(VI). However, U(VI) is known to be readily adsorbed with alumina from aqueous near-neutral solutions owing to the formation of inner-sphere, bidentate complexes at the surface [1]. Herein, for the first time we present the data indicating possibility of uranyl aluminate (UrAl) formation as nanoparticles dispersed in alumina matrix.
The precursor for UrAl was prepared by U(VI) precipitation with ammonia (pH=11) in the presence of mesoporous alumina MSU-X under the effect of power ultrasound (f = 20 kHz, Pac = 0.6 Wcm-2, Ar, T = 37°C). Finally, solids were centrifuged, washed until neutral pH with pure water, dried at 70°C and annealed at 800°C prior to low resolution TEM, U LIII XAFS and 27Al MAS NMR analysis.
The sample with 5 weight % of uranium yields an orange air-stable product after annealing. TEM images show uranium nanoparticles with an average size of about 5 nm dispersed in alumina matrix. XAFS spectrum of this sample reveals the presence of uranyl group with a U=O trans-dioxo distance equal to 1.80 (0.01) Å. Both XANES and EXAFS spectra are quite similar to the spectra obtained with uranyl ions adsorbed onto alumina at pH 6.
27Al MAS NMR spectra of initial MSU-X alumina exhibits three peaks assigned to octahedral, tetrahedral, and pentagonal local environment of aluminum. According to literature, pentagonal coordination of aluminum is typical for protonated boehmite-like forms of alumina, AlO(OH) [2]. Heating of MSU-X without uranium causes sharp decrease in pentagonal aluminum occupancy. By contrast, with 5% of uranium NMR spectra after annealing at 800°C clearly indicate the presence of aluminum ions coordinated by five oxygen atoms, which could be assigned to uranyl coordination with AlO2- anions.
The increase of uranium concentration to 30 weight % causes formation of green-black solids after annealing at 800°C. TEM measurements demonstrate formation of 50 nm uranium crystallites embedded into alumina matrix. EXAFS spectrum of this sample exhibits the absence of UO22+ groups and a strong U-U interaction with a characteristic distance of 4.39 (0.01) Å comparable with U3O8 reference sample.
Presumably UrAl is formed during the calcination of precursor only at low content of uranium hydrolyzed species finely dispersed or adsorbed at alumina surface. Otherwise, annealing leads to predominated formation of uranium oxides, probably, due to the favorable kinetics of solid state reaction. Thus, interaction of uranyl ions with alumina matrix represents a relatively rare example of real nanochemistry, when the chemical composition of species is a strong function of the particle size. In conclusion, it should be emphasized that a further structural study is necessary to refine the UrAl structure.
Acknowledgements This work was supported by European ACTINET and French PARIS Research Programs.
References
1. Sylwester E.R., Hudson E.A., Allen P.G. Geochim. Cosmochim. Acta, 64 (2000) 2431-2438.
2. Wang J.A., Bokhimi X., Morales A., Novaro O., Lopez T., Gomez R. J. Phys. Chem. B, 103 (1999) 299-303.