(149b) Theoretical and Experimental Approach to Nitrogen Selective Membranes

The main objective of the proposed research project
is to develop N₂-selective catalytic membrane technology with
potential sustainability benefits of indirect CO₂ capture and
ammonia synthesis. The N₂-selective membrane technology benefits
from the driving force of N₂ flue gas (73 wt.%)
stream for indirect CO₂ capture as it provides atomic nitrogen on
the permeate side of the membrane during separation. With these properties if
the H₂ is provided as a sweep gas alongside this novel technology,
additional benefits, such as formation
of ammonia as a byproduct of the N₂ separation process would
be observed.

Within
this study, N₂ in particular, has been investigated to determine the
mechanism of surface adsorption, dissociation and subsequent atomic diffusion
into the bulk crystal structure. Theoretical calculations
have been carried out using Vienna ab initio
Simulation Package (VASP)[i]^,[ii].
The electron exchange correlation effects are described by a
generalized-gradient approximation (GGA) using the Perdew-Burke-Ernzerhof functional[iii]
with a plane-wave expansion with a cutoff of 600 eV.
The surface Brillouin zone integration has been
calculated using a 7x7x7 Monkhorst-Pack
mesh[iv].
For absorption energy estimates, binding energy calculations of atomic N in the
BCC structure of bulk V and V-Ru alloys have been
simulated using a unit cell of 16 atoms with an optimized lattice constant of
2.98Å (3.03Å experiment). Atomic N was found to be stable in the octahedral,
face-octahedral and tetrahedral interstitial crystal sites (Figure 1.). It was
determined that the octahedral site is the most favorable binding site for N
within bulk V, with a binding energy of -2.132 eV. N
binding in V is nearly two orders of magnitude stronger compared to the
well-known H binding[v]
case (-0.076 eV). These strong N binding energies
indicate that N may have difficulty diffusing through the material. We found
through alloying V with Ru, that this binding energy
could be tuned so that it approaches that of H. Alloying V with 6.25 at.% Ru yields a weaker N binding
energy of -0.889. It is anticipated that increasing the content of Ru in the alloy will lead to a further reduction in the
stability of N within the crystal lattice. Optimal N permeability includes a
careful balance between N solubility and diffusivity within a given material.

Binding energy calculations are
carried at different interstitial sites of Vanadium bulk structure. Nitrogen was
found to be stable primarily at O-sites within the bulk V lattice. Bader charge
analysis has been carried out to investigate the mechanism associated with bulk
absorption phenomena. Additionally, alloys of ruthenium (Ru)-V
have been studied indicating that Ru can be used as a dopant to tune the
electronic structure of the bulk to enhance atomic diffusion. Surface
calculations on the 3
low-index surfaces, V(110), V(100) and V(111)
based on DFT indicated
that the V(111) surface binds N₂ the strongest at the fcc site, with an adsorption energy of 1.4 eV.

Experimental efforts
involve pure and mixed gas permeation to determine the N₂ and CO₂permeabilities of membrane foils comprised of Group V
metals and their different composition alloys with Ru
over a range of temperatures from 500 to 1000 °C. The schematic of the
permeation test set-up is given in Figure 2. In this study, linear regression
analyses of the flux isotherms were performed to obtain Sieverts'
Constant (n) and it was shown to be 0.5 for N₂ permeation through V
and Nb membranes. Unlike the lighter gases such as H₂
and N₂ permeating through the metallic membranes by
solution-diffusion mechanism, CO₂ was shown to diffuse only through
the grain boundaries or defects on the dense metal surfaces. As the CO₂
fluxes were compared with N₂ fluxes for the pure foil membranes, N₂
fluxes were found to be 2 to 3 orders of magnitudes higher than the CO₂
fluxes.  Characterization of the foils before
and after the permeation tests was performed by XRD, XPS and SEM techniques to
investigate the surface and subsurface structural changes in the membranes. In
total, characterization and flux measurements were compared alongside
electronic structure predictions of DFT to determine the performance of Group V
metals and their alloys for N₂-selective membrane application.

Figure 2.Schematic of Experimental Set-Up

References: