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

Abstract

Catalytic nitrogen (N 2 )-selective membrane technology with potential applications of indirect CO 2 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 (V x Ru 100−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.

Unprecedented thermal stability of inherently metastable titanium aluminum nitride by point defect engineering

“…Hence, assuming that the reaction of nitrogen with the growing film is not kinetically limited since nitrogen adsorption and desorption are single-step processes with activation energy in the order of one electronvolt [26,27], the nitrogen concentration will be determined by the driving force for the following chemical reaction:…”

“…Hence, to enable diffusion metal atoms need to overcome an activation energy in the order of one electronvolt many tens or even hundreds of times [24,25]. Nitrogen adsorption and desorption on the other hand are single-step processes with activation energy in the same order of magnitude [26,27]. The nature of point defects that accommodates off-stoichiometry during growth remains unknown.…”

Adsorption and dissociation of dinitrogen on transition metal (Ta, W and Re) doped MgO surface

“…Clearly, the adsorption of parallel configuration is favored for Ta and W doped cases whereas for the Re doped case, the adsorption in perpendicular configuration is favored. It is known that the adsorption of N 2 in perpendicular configuration is the least favored on V(1 1 0) surface [20] whereas it is the most favored on Fe (1 1 1) surface [21]. The N-N bond lengths for Ta, W and Re doped cases are 1.19, 1.18 and 1.17 Å respectively.…”

Investigation of New Ammonia Synthesis Process Utilizing Vanadium-Based Hydrogen Permeable Alloy Membrane

“…13) When nitrogen gas is flowed on the outlet side surface of the V-based hydrogen permeable alloy membrane, strong NÔN bond will be dissociated by the catalytic activity of vanadium, and atomic nitrogen is supplied to the outlet side surface of the membrane. Thus, the hydrogen and the nitrogen atoms coexist on the outlet side surface of the membrane.…”

“…8(c), the color of the solution changes into light yellow, indicating that ammonia is synthesized with sample C. In this case, both V-Fe alloy and Pd-Ag alloy coexist on the outlet side surface of sample C in a grid pattern. Therefore, hydrogen atoms permeating through the part of Pd-Ag alloy and nitrogen atoms dissociated from nitrogen molecule on the part of V-Fe alloy 13) react with each other to form ammonia. Thus, there is a possibility that ammonia can be synthesized under moderate temperature and pressure conditions by utilizing V-based hydrogen permeable alloy membrane.…”