Polynitrogen/Nanoaluminum Surface Interactions

First-principles density functional theory (DFT) calculations using the generalized gradient approximation (GGA) have been used to expand the analysis of the adsorption properties of polynitrogen and high nitrogen compounds on Al(111) and AlN(0001) surfaces. The electron-ion interaction has been described using ultrasoft (PAW) pseudopotentials in the case of Al (AlN). All calculations have been done using the PW91 generalized gradient approximation. In the case of Al, the calculations employ periodic slab models with 4 layers, ranging in size from (4×4) to (6×6) surface unit cells, and containing up to 144 Al atoms. For AlN, slab models with (2×2) and (3×3) unit cells and with 8 and 4 dual AlN layers, respectively, have been considered. Complementary quantum chemical calculations, utilizing DFT and second-order perturbation theory methods, of the ground state potential energy surfaces of the corresponding polynitrogen/high nitrogen species in the absence of the aluminum surface also have been performed. Previous results obtained in the first two years of this Challenge Project, related to adsorption properties of all nitrogen Nx(x=1-6,8-12) compounds or of non-substituted and mono-substituted triazenes have been extended to the case of high nitrogen heterocycle systems. Specifically, we have considered representative six-membered P-N-C heterocycles containing azide ligands on the C and P atoms such as C2N15P, CN18P2 and N21P3 systems. Additionally, we extended our analysis by including compounds containing the nitrogen rich CN7 - anion (i.e., the energetic salts NH4 +CN7 -, N2H5 +CN7 -, and CN4H7 +CN7 -) and heterocyclic-based ionic systems (C2N3H4 +- NO3 -) for which direct optimizations and ab initio molecular dynamics simulations have been performed. Finally, the interaction properties of atomic and molecular oxygen and of other energetic species like nitromethane (CH3NO2) with AlN(0001) and AlN(000 1 ) surfaces have been analyzed. For these systems the individual bonding mechanism involving either nondissociative (molecular) or dissociative processes has been determined, depending on both the molecular orientation and the surface sites involved. The binding energies were found to be highly sensitive to the specific adsorbed species and its local configuration. Dissociation processes were also found to be highly exothermic.