A first principles study of water adsorption on α-Pu (0 2 0) surface

Adsorptions of water in molecular (H2O) and dissociative (OH + H, H + O + H) configurations on the α-Pu (0 2 0) surface have been studied using ab initio methods. The full-potential FP/LAPW + lo method has been used to calculate the adsorption energies at the scalar relativistic with no spin–orbit coupling (NSOC) and fully relativistic with spin–orbit coupling (SOC) theoretical levels. It is found that the SOC effect increases the adsorption energies by ∼0.30 eV for the two dissociative adsorptions. Weak physisorptions have been observed for the molecule H2O on the α-Pu (0 2 0) surface with primarily a covalent bonding, while the two dissociative adsorptions are chemisorptive with ionic bonding. The one-fold top site with an almost flat-lying orientation is found to be the most stable site for the adsorbed H2O molecule. At the SOC level, the most stable adsorption energy is 0.58 eV, the corresponding values being 5.44 eV and 5.73 eV for the partial dissociation and complete dissociation cases, respectively. The analysis of the local projected density of states shows that the surface Pu-5f electrons remain primarily chemically inert in the molecular water adsorption process. Completely dissociative adsorption at a long bridge site for the dissociated O atom and two short bridge sites for the two dissociated H atoms is the most stable adsorption site. Hybridizations of O(2p)–H(1s)–Pu(5f)–Pu(6d) are observed for the two dissociative adsorptions, implying that some of the Pu-5f electrons become further delocalized and participate in chemical bonding. Work functions decrease for the molecular and the partial adsorption processes while it increases for complete dissociation.