The thermal decomposition of uranium(IV) and of uranyl(VI) formates

Abstract

Initial experiments involving thermogravimetry and differential thermal analysis have established the stages in the decomposition of uranyl(VI) formate monohydrate and anhydrous uranium(IV) formate in either inert or oxidising atmospheres.

Analysis of the gases evolved when anhydrous uranyl(VI) formate is heated in a vacuum at 280° indicates that the reaction is mainly represented by the equation: 3UO₂(HCO₂)₂→ 2HCO₂H + 4CO + H₂O + 3α-UO₃. A secondary reaction occurs to a small extent where a basic formate, [UO(OH)(HCO₂)], is formed which subsequently decomposes to give a form of uranium trioxide.

Studies on uranium(IV) formate at 280° have indicated that in vacuum the reaction is: 2U(HCO₂)₄→ 3HCO₂H + 4CO + H₂+ CO₂+ 2UO₂. In air the salt is subject to oxidation and the first stage is that where uranyl(VI) formate is produced.

The dehydration of the uranyl(VI) formate monohydrate followed contracting-sphere kinetics with energy of activation 36·2 ± 1·0 kcal./mole. The decarboxylation of the uranyl(VI) formate follows a Prout–Tompkins equation with energy of activation 40·4 ± 1·0 kcal./mole. In the case of uranium(IV) formate, the reaction in inert conditions followed a linear law with an energy of activation 28·5 ± 1·0 kcal./mole.

Mechanisms which may account for the behaviour are discussed.