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Oxidation as an early stage in the multistep thermal decomposition of uranium(IV) oxalate into U3O8

Abstract : The thermal decomposition of actinides oxalates greatly depends on the oxidation state of the cation, the gas involved and the physical characteristics of the precursor. In the actinides series, uranium(IV) oxalate U(C 2 O 4) 2 .6H 2 O can be viewed as a peculiar case, as its sensibility towards oxidation leads to a specific series of reactions when heating under oxygen atmosphere. In order to clarify the disagreements existing in the literature, particularly concerning potential carbonate intermediates and the possible transitory existence of UO 3 , we show here an extended characterization of the different intermediates through a combination of X-Ray diffraction, vibrational spectroscopies and X-Ray absorption near edge spectroscopy. In this frame, uranium oxidation was found to occur at low temperature (200°C) concomitantly to the onset of oxalate groups decomposition, leading to an amorphous oxo-oxalato compound. Pursuing the thermal conversion up to 350°C led to complete oxidation of U(IV) into U(VI), then to the formation of amorphous UO 3 still bearing adsorbed carbonates. The first pure oxide formed during the thermal conversion was further identified to sub-stoichiometric UO 3- after heating at 550°C. Finally, U 3 O 8 was obtained as the final stable phase after heating above 660°C. The mechanism of thermal conversion of uranium(IV) oxalate into oxide under oxygen is then driven by a complex interplay between redox reactions and decomposition of the organic fractions. Such chemical reactions were also found to significantly modify the morphology of the powder through HT-ESEM observations : decomposition led the size of the aggregates to reduce by 20% while uranium oxidation clearly promoted growth within the agglomerates. 2
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Lénaïc Desfougeres, Eleonore Welcomme, Maelig Ollivier, Philippe Martin, Julie Hennuyer, et al.. Oxidation as an early stage in the multistep thermal decomposition of uranium(IV) oxalate into U3O8. Inorganic Chemistry, American Chemical Society, In press, ⟨10.1021/acs.inorgchem.0c01047⟩. ⟨hal-02736864⟩

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