Ab initio thermodynamic and kinetic modeling of molecular adsorption and reaction properties on PuO2(111) surface under exposure to environmental gases

Abstract

Adsorption and reaction properties of environmental gases including O₂, H₂, and H₂O on the PuO₂(111) surface were studied via density functional theory simulations along with thermodynamic and kinetic analysis. Simulation results show that the stoichiometric PuO₂(111) remains intact under O₂ atmosphere and extremely low or high O₂ pressure is required to form oxygen vacancy or adsorbed-O on the surface. The H₂O prefers to stay as molecular state when adsorbing on PuO₂(111) and a relatively high humidity is required for H₂O to be stably binding on the surface. For H₂ interaction with PuO₂, the dissociative adsorption of H₂ molecule induces reduction of Pu(IV) ions to Pu(III), and remains thermodynamically stable at H₂ pressure as low as ∼10⁻³⁵ bar under room temperature. Kinetic modeling shows that at temperature below 350 K, the PuO₂(111) surface is mainly covered by OH species when exposing to H₂ environment while bare metal sites appear with increased temperature and reaction time.