Oxidation as an Early Stage in the Multistep Thermal Decomposition of Uranium(IV) Oxalate into UO.

The thermal decomposition of actinide oxalates is greatly dependent 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(CO)·6HO can be viewed as a peculiar case, as its sensibility toward oxidation leads to a specific series of reactions when heating under an oxygen atmosphere. In order to clarify the disagreements existing in the literature, particularly concerning potential carbonate intermediates and the possible transitory existence of UO, 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 still bearing adsorbed carbonates. The first pure oxide formed during the thermal conversion was further identified to substoichiometric UO after heating at 550 °C. Finally, UO 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 high-temperature environmental scanning electron microscopy observations: decomposition led to a 20% reduction in the size of the aggregates, while uranium oxidation clearly promoted growth within the agglomerates.