Group V Metal and Metal Alloy Membranes for Nitrogen Separation: Theoretical and Experimental Approaches

Nitrogen(N₂)-selective membrane technology is promising for post-combustion capture of CO₂ from coal- or natural gas-fired power plants, where CO₂ concentrations are potentially too dilute for traditional CO₂-selective polymer membrane processes. A co-benefit of a N₂-selective membrane is the potential for ammonia (NH₃) synthesis by sweeping hydrogen (H₂) on the permeate side of the membrane, provided a H₂ source is available. The produced NH₃ can be used as a viable fertilizer or a clean energy source and may partially offset the costs of capture. In this study, pure vanadium (V) metal and their alloys with ruthenium (Ru) have been investigated as membrane materials for selective N₂ separation. Investigations include both theoretical modeling based upon electronic structure calculations and experimental testing using a high-temperature bench-scale membrane reactor. This work will provide fundamental knowledge toward the design of a metallic membrane with optimal selectivity and permeability of N₂ for post-combustion CO₂capture.

Atomic-scale simulations based on density functional theory (DFT) have been carried out to investigate absorption of nitrogen atom in the bulk metal interstitial sites. Octahedral interstitial site in V provides a strong binding site for nitrogen, indicating that nitrogen will have a high solubility but a low diffusivity in V. This could limit the nitrogen permeability across the V metallic membrane. Alloying various concentrations of Ru with V lowered the nitrogen binding energy to be within an order of magnitude of H binding energy in Pd, which is a commercial example of a metallic membrane application. For instance, alloying V with 6.25 at.% Ru yields a substantially weaker N binding energy of -0.889 eV than that of -2.13 eV before alloying.

In the permeation experiments, we measured the flux of pure N₂, Ar, and He through discs comprised of V and its alloys with Ru. Due to high temperatures required for the dissociation of N₂ and diffusion of atomic nitrogen through the metals, the membranes have been tested over a range of temperatures from 600 to 900 °C. Feed pressure of 100 – 200 psig was applied while permeate side was in ambient pressure. Direct flow was measured by using bubble flow meter and flux composition was analyzed by using an electron ionization quadrupole mass spectrometer (EI-QMS) with sensitivity in the low ppm range. The tested membranes were characterized by x-ray photoelectron spectroscopy (XPS), x-ray diffraction (XRD), and scanning electron microscopy (SEM) with electron dispersive spectroscopy (EDS). A thin oxide layer as well as bulk vanadium nitride phases were observed from the analyses, indicating that atomic nitrogen penetrates into the bulk and permeates through the membrane.