266753 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).