(392a) Theoretical and Experimental Investigations of N2-Separation and Ammonia Formation Via Group V Metal 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 (NH₃) synthesis by sweeping 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.

Previous investigations have shown that Group V metals strongly bind diatomic molecules such as N₂, O₂, and H₂, and that these metals also exhibit remarkably high atomic N diffusivities. In this study, Group V-based metallic membranes and their alloys with ruthenium (Ru) have been investigated for catalytic selective N₂ separation and subsequent NH₃ formation. Investigations include both theoretical modeling based upon electronic structure calculations and experimental testing using a high-temperature bench-scale membrane reactor. 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 facilitate testing the activity of the most promising N₂-selective membrane candidates proposed from the theoretical calculations. This work will provide fundamental knowledge toward the design of a metallic membrane with optimal selectivity and permeability of N₂ for post-combustion CO₂ capture with simultaneous NH₃ production.

Density functional theory (DFT) calculations have been carried out on the investigation of the molecular N₂ adsorption at different low-index surfaces of vanadium (V) and niobium (Nb). DFT-based calculations have also been carried out for N₂ binding at different interstitial sites within the bulk of the V and Nb crystal. The binding energy of N at O-sites in bulk V is approximately an order of magnitude stronger compared to H binding in Pd. The strong-binding nature of N in these metal systems indicates that N may have difficulties in diffusing through the bulk material. The binding energy can be modified by alloying Ru of various concentrations. 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. This binding energy is approximately within an order of magnitude of H binding in Pd, which is a commercial example of a metallic membrane application. Bader charge analysis was carried out to investigate charge-transfer mechanisms associated with bulk nitrogen absorption.

In permeation experiments, we measured the flux of pure N₂ and CO₂, and their mixtures through foils comprised of V, Nb and their alloys with Ru. The concentration of the feed N₂-CO₂ gas mixtures has been adjusted to represent the flue gas out of coal- and natural gas-fired power plants. 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. Permeability and selectivity of each metallic membrane were determined and the tested membranes have been characterized by different techniques such as x-ray photoelectron spectroscopy (XPS), x-ray diffraction (XRD), and scanning electron microscopy (SEM).