Computational study of HCl adsorption on stoichiometric and oxygen vacancy PuO2 {111}, {110} and {100} surfaces

The interactions between HCl and both the pristine and defect {111}, {110} and {100} surfaces of PuO2 are modelled using hybrid density functional theory, within the periodic electrostatic embedded cluster method. In the case of the pristine surfaces, adsorptions onto the {110} surface are the most stable, likely due in part to the presence of surface hydrogen bonds. In addition, results suggest that even when dissociatively adsorbed onto the surface, the proximity of the hydrogen and chlorine atoms has a significant impact on the stability of the system. The electronic structure of both the pristine and reduced surfaces of PuO2 and UO2 is also probed, with unpaired electrons left behind in a neutral oxygen vacancy defect site having a greater tendency to delocalize on the surface of UO2 than PuO2. HCl adsorptions on the reduced surfaces reveal that configurations in which the chlorine atom attempts to “heal” the gap left by the oxygen vacancy in the {111} surface are by far the most stable of all considered. Finally, molecular thermodynamics is employed to translate adsorption energies to HCl thermal desorption temperatures, for each geometry considered. Particularly when a chlorine atom is embedded in the surface of PuO2, the temperatures required for thermal desorption to occur are high, implying that although thermal treatment is likely to remove some chlorine contamination from PuO2 samples, some will likely remain bound to defect sites within the material.