DFT study on the mechanism of manganese(II)-catalyzed destruction of oxalic acid in nitric acid systems

In the plutonium purification cycle of the PUREX process, the decomposition process in nitric acid systems is critical for the treatment of oxalic acid in the post-nuclear fuel reprocessing. One possible solution for the removal of oxalic acid is the catalyzed oxidation by introducing transition metal cations such as manganese(II) ions (Mn(II)), while its underlying mechanism remains ambiguous. This study employs density functional theory (DFT) to elucidate the catalytic mechanism involving Mn(II), focusing on the oxidation of oxalic acid at 298.15 K. The overall reaction process can be categorized as a series of multi-step elementary reactions. Firstly, nitric acid oxidizes the hexa-aqua-manganese(II) ion to the high-spin hexa-aqua-manganese(III) ion. Subsequently, two oxalate ligands (C $$_{2}$$ O $$_{4}{}^{2-}$$ ) displace water then one coordinated oxalate ligand decomposes quickly to generate the $$\cdot $$ C $$_{2}$$ O $$_{4}{}^{-}$$ radical via intramolecular charge transfer mechanism. Based on this, the $$\cdot $$ C $$_{2}$$ O $$_{4}{}^{-}$$ further decomposes to produce the $$\cdot $$ CO $$_{2}{}^{-}$$ radical and CO $$_{2}$$ molecule. Finally, the $$\cdot $$ CO $$_{2}{}^{-}$$ radical reacts with another Mn(III) to generate another CO $$_{2}$$ molecule, while the Mn(III) ion is reduced to manganese(II), allowing the catalytic cycle to continue. The key step that determines the rate of the whole reaction is identified as the generation of $$\cdot $$ C $$_{2}$$ O $$_{4}{}^{-}$$ radical from high-spin Mn(III) oxalic acid complex. The activation energy of the rate-controlling step is calculated as 22.37 kcal/mol (93.60 kJ/mol) aligning well with experimental values of approximately 78 – 101 kJ/mol (18.64 – 24.10 kcal/mol). By providing the full thermodynamic profile and identifying the rate-determining step, this work establishes a rigorous theoretical foundation for optimizing the industrial process of oxalic acid destruction.