Ab-initio simulations of atomic hydrogen interaction with Nb and V at clean and oxygen covered surfaces

Superpermeation allows for hydrogen fluxes through metal foil membranes at rates orders of magnitude higher than pressure driven permeation. This process occurs only for hydrogen isotopes, meaning it is hydrogenselective, and it can work against a pressure gradient, implying pumping capabilities. These characteristics allow for using superpermeation as the base process for a very efficient, selective separation of hydrogen from other gases. However, the efficacy of superpermeation needs further research both experimentally and theoretically. Its efficiency relies on a surface energetic barrier that hinders both adsorption of molecular hydrogen on the downstream side and desorption on the upstream side, while leaving atomic hydrogen absorption unaffected. Such a barrier can be created by a monolayer of non-metallic impurities (usually oxygen) that naturally develops at group 5 metal surfaces. The physics explaining why such a monolayer drastically affects atomic hydrogen reactions are being explored in this work via density functional theory (DFT) calculations for the implementation of which we use the Vienna ab-initio Simulations Package (VASP). By performing structural relaxations and saddle point-searching calculations deploying a dimer method using VASP, energy diagrams for atomic hydrogen absorption are obtained for two representative materials, namely niobium and vanadium. The differences in these diagrams are analyzed and compared in order to determine which material is optimal for superpermeation. To that end, slabs with (100) surface orientation are compared for the case with and without an O monolayer coverage. The characteristic energies involved according to the diagrams and the types of absorption sites will be key parameters to understand and, eventually, optimize for the emerging phenomena. It was found that the presence of an oxygen monolayer is necessary of superpermeation to occur, and that for the 100 orientation, the vanadium system provides better characteristics.