Theoretical and Experimental Investigations of N2-Selective Membranes

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 not great enough for traditional CO₂-selective polymer membrane processes. A co-benefit of a N₂-selective membrane is the potential for ammonia synthesis, provided a H₂ source is available. If H₂ is used as a sweep gas on the permeate side of the membrane, ammonia may be simultaneously produced. Preliminary material screening investigations indicate that Group V metals show strong-binding characteristics to diatomic molecules such as N₂, O₂, and H₂. These metals also exhibit remarkably high atomic nitrogen diffusivities. In this study, Group V-based metallic membranes and their alloys with ruthenium (Ru) have been investigated for catalytic selective N₂ separation. Investigations include both theoretical modeling with electronic structure theory calculations and experimental testing using a high-temperature bench-scale membrane reactor. The main aim of this work is to design a metallic membrane with optimal selectivity and permeability for the given application by combining the theoretical and the experimental findings. As the modeling component of this work enables screening the bulk and surface properties of all the possible Group V-based metallic alloys, the experimental efforts facilitates testing the activity of the most promising N₂-selective membrane candidates proposed from the theoretical calculations.

Fundamental investigations of molecular adsorption, dissociation of N₂ and subsequent atomic diffusion on V and V/Ru alloys have been performed by theoretical calculations and experiments. Electronic structure calculations based on density functional theory (DFT) have been carried out on the investigation of the molecular N₂adsorption at 1/4 coverage on the 3 low-index surfaces, V(110), V(100) and V(111). DFT-based calculations have also been carried out for N₂ binding at different interstitial sites within the bulk of the V crystal. Nitrogen was not found to be stable at T-sites within the bulk V lattice, but only stable at O-sites with a binding energy of -2.13 eV. The binding energy of N in bulk V is approximately an order of magnitude stronger compared to H binding in Pd, -0.11eV. The strong-binding nature of N in these metal systems indicates that N may have difficulty diffusing through the material. Strong attachment of nitrogen to bulk vanadium may be modified by alloying with ruthenium (Ru) at various concentrations for optimized N diffusion. For instance, alloying V with 6.25 at.% Ru yields a substantially weaker N binding energy of -0.889 eV, which is within an order of magnitude of H in Pd, our target system. Bader charge analysis has been carried out to investigate charge-transfer mechanisms associated with bulk nitrogen absorption. Further validation of the theoretical calculations have been performed by the experimental efforts.

Experimental efforts of this study mainly involve flux measurements of pure N₂ and CO₂ gas species and their mixtures through the foils comprised of V, Nb and their alloys with Ru. The concentration of the feed N₂-CO₂ gas mixtures is adjusted to represent the flue gas output of coal- and natural-gas-fired power plants. Flux measurements are used to determine the permeability and selectivity of the tested metallic membranes. Due to the high temperatures required for the dissociation of N₂ and diffusion of atomic N through the metals, the membranes have been tested over a range of temperatures from 500 to 1000 °C. The effect of the pressure gradient across the membrane and the feed flowrate are also measured during the experimental tests. Tested foils are characterized by different techniques such as XPS and XRD.