(241f) N2/O2 Adsorption, Dissociation and Diffusion In Group V Metals

It is well known that Group V metals, vanadium (V), niobium (Nb) and tantalum (Ta) have strong-binding characteristics to diatomic molecules such as N₂, O₂, H₂ and CO. Because of this phenomenon, these metals are interesting to investigate for small molecule reactivity to the extent of potential atomic N, O, H, and C diffusion into their bulk crystal structures. The applications of these investigations are toward the design of N₂- or O₂-selective catalytic membrane for gas separation. The feasibility of the membrane is dependent upon N₂ or O₂ dissociation across the surface with subsequent diffusion through the dense metal as atomic N or O, respectively. Successful implementation of this membrane technology has the capability of separating N₂ or O₂ from air with potentially lower energy requirements for oxy-combustion applications compared to traditional noncatalytic techniques such as sorbents or cryogenic separation. Another application may be for N₂ separation followed by reaction of hydrogen, which could lead to lower energy requirements associated with the ammonia synthesis process.

In the current study, fundamental investigations of molecular adsorption, dissociation of small molecules (N₂ and O₂) and potential subsequent atomic diffusion on V and V/Ru alloys are performed by theoretical calculations and experiments. Electronic structure calculations based on density functional theory (DFT) have been carried out on the investigation of the molecular N₂ adsorption at 1/4 coverage on the 3 low-index surfaces, V(110), V(100) and V(111). Preliminary investigations indicate that the V(111) surface binds N₂ the strongest at the bridge site, with an adsorption energy of 1.26 eV. N₂ is known to be the most difficult diatomic molecule to dissociate due to its triple bond; therefore, the energy barrier associated with N₂ dissociation is a crucial parameter in determining its surface reactivity. The calculations for the minimum energy path along with the associated activation barrier and transition structures are ongoing using the nudge elastic band (NEB) method. In addition to the theoretical calculations, experiments performed using synchrotron-based x-rays in a UHV environment have been investigated to provide information on the N₂ and O₂ dissociation pathway and compared to the theoretical predictions.

For the bulk diffusion simulations, atomic N was found to be stable in the octahedral (O-site), face-octahedral (face O-site) and tetrahedral (T-site) interstitial crystal sites. It was determined that the octahedral site is the most favorable binding site for N within bulk V, with a binding energy of -2.132 eV. Nitrogen binding in V is nearly two orders of magnitude stronger compared to the well-known H-binding case (-0.076 eV). These strong N binding energies indicate that N may have difficulty diffusing through the material. Ruthenium doping of V was investigated since Ru is a well-known catalyst for the Haber-Bosch ammonia synthesis process. Through alloying V with Ru, it has been found that the binding energy may be tuned so that it approaches that of the H binding energy in the bulk V system. We found that the addition of the Ru as a doping material acts to reduce the interaction between V and N thereby weakening the binding energy to -0.889 eV.

N₂ and O₂ permeation experiments have also been carried out to determine the permeabilities of membrane foils comprised of V and their alloys. Investigations range from single-component gas streams to mixtures including air.