Interaction of Hydrogen with Actinide Dioxide (111) Surfaces

The interaction of atomic and molecular hydrogen with the actinide dioxide (AnO₂, An = U, Np, Pu) (111) surfaces has been investigated by DFT+U, where noncollinear 3k antiferromagnetic (AFM) behaviour and spin-orbit interactions (SOI) are considered. The adsorption of atomic hydrogen forms a hydroxide group, and is coupled to the reduction of an actinide ion. The energy of atomic hydrogen adsorption on the UO₂ (0.82 eV), NpO₂ (-0.10 eV), and PuO₂ (-1.25 eV) surfaces has been calculated. The dissociation of molecular hydrogen is not observed, and shown to be due to kinetic rather than thermodynamic factors. As a barrier in the formation of a second hydroxyl group, an unusual charge distribution has been shown. This is possibly a limitation of a (1·1) unit cell method. The recombination of hydrogen ions on the AnO₂ (111) surfaces is favoured over hydroxide formation.