N2-selective metal membranes could be used to couple extraction of N2 from flue gas and NH3 synthesis within a single membrane reactor. The larger interstitial spaces of BCC metals makes them attractive for fast permeation of the large N atom. The caveat is that N permeabilities reported thus far for pure BCC metals (e.g. V, Cr, Fe, Mo, W) are insufficient for practical application. However, practical permeabilities could be achieved by alloying these metals. Here, we use density functional theory (DFT) to study how alloying V—a metal with high N solubility but low N diffusivity—with Fe and Cr impacts the dissociation, dissolution and diffusion of nitrogen. Although N binding at vacancies was studied, binding at these defects was not particularly strong compared to regular interstitial sites, and, thus, may not significantly affect solubility. The alloys present two types of interstitial octahedral (o) sites: V-rich (o1) and V-depleted (o2). Nitrogen binding strength correlates with number of V nearest-neighbors, hence binding in o1 sites is in average 1 eV stronger than in o2 sites. Ab initio<br/>thermodynamics suggests that this combination of strong and weak binding sites mitigates the reduction in solubility expected from the alloying (at least at relevant operating conditions). However, the heterogeneity of the interstitial sites 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 V0.25Fe0.75 alloy (1.01 eV). Based on our calculations, at 673 K and 5 bar (N2 pressure), N solubility and diffusivity in V0.25Fe0.75would be ~3 times smaller and ~53 times larger, respectively, than pure V. According to the solution-diffusion model, this finding indicates that a ~18-fold higher permeability would be expected for V0.25Fe0.75 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.

Dissociation, dissolution and diffusion of nitrogen on VxFey and VxCry alloy membranes studied by first principles

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.