The adsorption thermodynamics of H2O and CO2 on PuO2(1 1 1) surface: A comparative study based on DFT+U-D3

Abstract

The adsorption thermodynamics of active molecules on PuO2 surface are very important for the long-term storage and surface corrosion of Pu. Here, a comparative study is performed within DFT+U-D3 scheme to investigate the adsorption mechanisms, morphology, and phase diagrams of H2O and CO2 on PuO2(111) surface. The adsorption morphology of H2O will transition from the molecular state to a mixed adsorption state when the coverage is increasing, but CO2 always adsorbs in the molecular state. The differences in adsorption behaviors are mainly due to the hydrogen bonds and Coulomb interactions between the adsorbed molecules. The hydrogen bonds promote the polymolecular adsorption of H2O and the further partial dissociation at high coverage. The Coulomb attractive force among H2O is larger than CO2, and the attractive force among CO2 rapidly decreases as the coverage increases. Unlike the multilayer BET model of H2O adsorption, CO2 adsorption accords with monolayer Freundlich model as revealed by the calculated isotherms. The desorption temperature (Tdes) of H2O and CO2 are agreeing with the experimental results, and the Tdes of H2O is lower than CO2 at the same pressure. This work clarifies the adsorption behaviors of H2O and CO2, and it could provide some important insights into the prediction of the surface corrosion reactions of PuO2 in storage containers.