(428a) Dissociation, Dissolution, and Diffusion of Nitrogen on VxFey and VxCry Alloy Membranes Studied By First Principles

N₂-selective metal membranes could be used to couple the extraction of N₂ from flue gas and NH₃ synthesis within a single membrane reactor. The larger interstices of BCC metals make them attractive for fast permeation of the large N atom, but N permeabilities reported for pure BCC metals are still impractical. As practical permeabilities could be achieved by alloying these metals, we use density functional theory to study how alloying V, a metal with high N solubility but low N diffusivity, with Fe and Cr impacts N₂ dissociation, dissolution, and diffusion. Although N binding at vacancies was studied, binding energies indicate that these defects may not significantly affect solubility. The studied alloys present V-rich (o1) and V-depleted (o2) octahedral interstices. Nitrogen binding strength correlates with the number of V nearest-neighbors; hence, binding in o1 sites is on average 1 eV stronger than in o2 sites. Ab initio thermodynamics suggests that this combination of strong and weak binding sites mitigates the reduction in solubility expected from the alloying at relevant operating conditions. However, the heterogeneity of the interstices generally leads to higher energy barriers for N hopping than those encountered in pure V (1.24 eV). The exception to this observation was the V_0.25Fe_0.75 alloy (1.01 eV). Based on our calculations, at 673 K and 5 bar (N₂ pressure), N solubility and diffusivity in V_0.25Fe_0.75 would be ∼3 times smaller and ∼53 times larger than pure V, respectively. According to the solution–diffusion model, these findings indicate that an ∼18-fold higher permeability would be expected for V_0.25Fe_0.75 relative to V. Permeability is expected to be controlled by bulk diffusion rather than by surface processes, as in all alloys, we find the energy barrier for N₂ dissociation at the alloy surface to be lower than the barrier for bulk diffusion in the same alloy.