In solid or liquid states, ammonia salts and solutions are the active components of most synthetic fertilizers used in agriculture, which consume 83% of the world’s ammonia. Today, ammonia for fertilizers is industrially produced via the Haber-Bosch process at 400-500 °C and at pressures up to 30 MPa (300 bar). These harsh operating conditions are necessary due to the high affinity of dissociated nitrogen atoms towards the catalyst surface in addition to the high barrier associated with N2 dissociation. For these reasons, the need for advanced catalytic methods for the reduction of N2 to ammonia remains a requirement for sustainability in the food and energy cycle.
The aim of this work is to explore the potential of metallic membranes for N2 separation with the final intent to produce NH3. Based on a preliminary theoretical investigation using density functional theory, the Group V transition metals (e.g., vanadium (V), niobium (Nb) and tantalum (Ta)) show strong affinity toward N2.
Moreover, from solubility and diffusivity values taken from the literature, iron (Fe) is a suitable fit for this application. The first experimental study showed that V, Ta, Nb, and Fe have a N2-permeating fluxes on the order of 10-5 and 10-4 molN2/m2·sec, depending on the operating condition used and that V-based metallic membranes have excellent properties for the preferential transport of N2 over CO2 and CH4 with near-infinite selectivity.
The V metal is, hence, used as a potential membrane to produce NH3. Specifically, it is housed in the membrane reactor where a H2 stream is used as the sweep gas to promote the NH3 reaction. Lower pressures than the conventional Haber-Bosch process are used. Specifically, a range of operating pressure from 30 to 60 bar, instead of 200 bar (Haber-Bosch process), and a range of temperature from 350 °C to 500 °C are used as conditions to produce NH3. The performance of the membrane reactor in terms of NH3 conversion is, hence, investigated.