Theoretical Study of Nitrogen Absorption in Metals

5 years ago

Theoretical Study of Nitrogen Absorption in Metals

Simona Liguori, Kyoungjin Lee, Jennifer Wilcox, Peter Psarras

Nitrogen binding in open structure body-centered-cubic (bcc) metals was studied to understand atomic nitrogen absorption in these systems in order to assess their feasibility as membrane materials for nitrogen separation and subsequent reactivity for ammonia production. For a metallic membrane to be feasible for this application, it must exhibit adequate solubility that allows for sufficient permeability. Using first-principle calculations it was demonstrated that nitrogen is too soluble in pure vanadium due to its strong binding in this metal (i.e., binding energy of −2.80 eV/atom), but through alloying, the binding energy may be tuned to mimic the weaker hydrogen binding exhibited in Group V metals, leading to high permeability. In particular, alloys of Ru and Mo were investigated. The Bader charge and density of states analyses showed that nitrogen binding in pure V is enhanced by the electrostatic and covalent interaction between nitrogen and surrounding V atoms, whereas repulsive interaction with an alloying component, Ru or Mo, with nitrogen resulted in the less stable binding of nitrogen in the alloys. Reduced binding energies were observed in both alloys (e.g., −1.64 eV/atom for both V₅₃Ru and V₁₅Mo alloys). In particular, Mo₁₃V₃ alloy showed a nitrogen BE of −0.041 eV, which is very similar to the BE of H in Pd. The nitrogen solubility in Mo was estimated based upon a thermodynamic equilibrium assumption with systematic corrections to the calculated vibrational frequencies, showing agreement to the experimental solubility. Using the same correction scheme, nitrogen solubility in V–Mo was estimated to be 4 orders of magnitude higher at 1000 K for a 25 at. % V alloy composition compared to pure Mo. This theoretical study provides useful guidance for the discovery of promising materials for metallic membranes for ammonia synthesis.

Publisher URL: http://dx.doi.org/10.1021/acs.jpcc.7b05315

DOI: 10.1021/acs.jpcc.7b05315