Theoretical Study of Nitrogen Absorption in Metals

Abstract

Nitrogen binding in open structure body-centered-cubic (bcc) metals was studied to understand atomic nitrogen absorption in these systems in order to assess their feasibility as membrane materials for nitrogen separation and subsequent reactivity for ammonia production. For a metallic membrane to be feasible for this application, it must exhibit adequate solubility that allows for sufficient permeability. Using first-principle calculations it was demonstrated that nitrogen is too soluble in pure vanadium due to its strong binding in this metal (i.e., binding energy of −2.80 eV/atom), but through alloying, the binding energy may be tuned to mimic the weaker hydrogen binding exhibited in Group V metals, leading to high permeability. In particular, alloys of Ru and Mo were investigated. The Bader charge and density of states analyses showed that nitrogen binding in pure V is enhanced by the electrostatic and covalent interaction between nitrogen and surrounding V atoms, whereas repulsive interaction with an alloying component, Ru or Mo, with nitrogen resulted in the less stable binding of nitrogen in the alloys. Reduced binding energies were observed in both alloys (e.g., −1.64 eV/atom for both V53Ru and V15Mo alloys). In particular, Mo13V3 alloy showed a nitrogen BE of −0.041 eV, which is very similar to the BE of H in Pd. The nitrogen solubility in Mo was estimated based upon a thermodynamic equilibrium assumption with systematic corrections to the calculated vibrational frequencies, showing agreement to the experimental solubility. Using the same correction scheme, nitrogen solubility in V–Mo was estimated to be 4 orders of magnitude higher at 1000 K for a 25 at. % V alloy composition compared to pure Mo. This theoretical study provides useful guidance for the discovery of promising materials for metallic membranes for ammonia synthesis.

Spectroscopic evaluation of charge-transfer doping and strain in graphene/ MoS2 heterostructures

“…i is the Grüneisen parameter corresponding to the frequency of peak i (ωi), and * + is the frequency corresponding to zero strain. The Grüneisen parameter has been experimentally determined for both graphene 21,[34][35][36][37] and MoS2. 38,39 Average room temperature values for uniaxial strain from the literature are 1.9, 2.6, 0.86, and 0.15 for the G, G', E' and A' peaks respectively.…”

“…The frequencies of the two major Raman peaks in graphene (G and G' peaks) and in MoS2 (E' and A' peaks) are influenced by charge transfer doping and strain, and correlation analysis between the two modes offers a way to decouple their effects. [15][16][17][18][19][20][21][22] That is, by plotting the frequencies of the G' (E') band against the G (A') band in graphene (MoS2), one can establish the extent of strain versus doping in each layer. In addition to Raman peaks, monolayer MoS2 exhibits strain-and dopingdependent photoluminescence (PL) emission, offering an additional means to study the interaction between graphene and MoS2.…”

“…CTE of the SiO2 substrate, as well as the differences between the lattice constants of graphene (2.40 Å) and MoS2(3.12 Å). Indeed, large tensile and compressive strains (up to 0.7%) have been observed in bare MoS2 and graphene on substrates upon heating and cooling 20,21,51. Note that compressive and tensile strains are also observed in astransferred graphene and MoS2 without any thermal annealing 15,18.…”

Dissociation, Dissolution, and Diffusion of Nitrogen on VxFey and VxCry Alloy Membranes Studied by First Principles

“…BCC metals are promising materials to make nitrogen-selective membranes. − While metal membranes have mostly gathered attention for permeating the smaller H atoms, − the larger interstitial spaces of BCC metals could facilitate the dissolution and diffusion of the larger N atoms. Indeed, Liguori et al .…”

“…Previous density functional theory (DFT) efforts on nitrogen in BCC metals include studies of N binding in the bulk of V, W, , Fe, , and N 2 adsorption and dissociation in Fe − and V . On the other hand, DFT studies on the dissociation, dissolution, and diffusion of nitrogen on V alloys have been largely absent.…”