Nitrogen Adsorption, Dissociation, and Subsurface Diffusion on the Vanadium (110) Surface: A DFT Study for the Nitrogen-Selective Catalytic Membrane Application

Catalytic nitrogen (N-2)-selective membrane technology with potential applications of indirect CO2 capture and ammonia synthesis is introduced. Metallic membranes made from Earth-abundant group V metals, i.e., vanadium (V), and alloys with ruthenium (Ru) are considered. Similar to a traditional palladium (Pd)-based hydrogen (H-2)-selective membrane for hydrogen purification, N-2 molecules preferentially adsorb on the catalytic membrane and dissociate into two nitrogen atoms. Atomic nitrogen subsequently diffuses through the crystal lattice by hopping through the interstitial crystal sites of the bulk metal, ultimately leading to atomic nitrogen on the permeate side of the membrane. This study is focused on the nitrogen interactions only at the membrane surface and the first subsurface layer. The adsorption energies of molecular as well as atomic nitrogen on the vanadium surface (V(110)) and Ru-alloyed V surface (VxRu100-x/V(110), where x is the atomic composition of vanadium in the alloy) are calculated using first-principles and compared against traditional catalysts for ammonia synthesis, i.e., iron (Fe). The N-2 dissociation pathway and its corresponding activation barrier are also determined. Additionally, the diffusion of atomic nitrogen from the V(110) surface to its subsurface layers is investigated to determine the rate-limiting step of nitrogen transportation across membrane surface. It has been found that N-2 and atomic nitrogen bind on the V(110) surface very strongly compared to adsorption on corresponding Fe surfaces. Although the activation energy (ca. 0.4 eV) for nitrogen dissociation on the V(110) surface is greater than that of the Fe surfaces, it is comparable to that of the Ru surfaces. Atomic nitrogen slightly prefers to stay on the V(110) surface rather than in the subsurface layers. Coupling this with the relatively high activation barrier for subsurface diffusion (ca. 1.4 eV), it is likely that the subsurface diffusion of nitrogen is the rate-limiting step of nitrogen transport across a membrane. Alloying Ru with V reduces the adsorption energy of atomic nitrogen on the Ru-alloyed V(110) surface in addition to the subsurface layer. Therefore, it is expected to facilitate nitrogen transport across the membrane surface.