Surface properties of NpO2 and water reacting with stoichiometric and reduced NpO2 (111), (110), and (100) surfaces from ab initio atomistic thermodynamics

Abstract

We have investigated the low-index (111), (110), and (100) surfaces of NpO2 as well as the adsorption and dissociation behaviors of water on these surfaces by using DFT+U method. The calculated surface energies for the (111), (110), and (100) surfaces are 0.81, 1.14, and 1.67Jm−2, respectively. Based on our calculations, the presence of surface oxygen vacancy enhances the dissociation adsorption of water molecule. There is no significant coverage dependence for molecular adsorption of water on the NpO2 (111), (110), and (100) surfaces. The average adsorption energy of water in dissociative states increases slightly from 1/4 to 2/4ML and rises significantly with the coverage increase to 1ML on these surfaces. In addition, we have analyzed the water adsorption on the NpO2 surfaces for different temperatures and H2O partial pressures and have plotted the pressure–temperature phase diagrams by using the “ab initio atomistic thermodynamics” approach.