Abstract Submitted to the    CCTN09 Symposium:

NH₃ adsorption and dissociation on a nanosized iron cluster

Giorgio Lanzani, Kari Laasonen

Department of Chemistry - University of Oulu

giorgio.lanzani@oulu.fi

I. CVD Synthesis of Carbon Nanotubes

In doped nanotubes either or both boron and nitrogen atoms replace carbon atoms within the structure and are covalently bound. The main target is to control the electrical properties of the nanotubes, with special attention to control the number of layers. The important industrial potential is demonstrated by developing transparent, conductive, flexible nanotube mats. Despite widespread interest, the chemistry on the surface of BNC catalyst transition-metal nanoparticles is largely unknown though it is believed that the precursor molecules chemisorb dissociatively on their surface and there are major uncertainties in the carbon precursor surface decomposition reaction kinetics and the role of certain species which are believed to etch amorphous the nitrogen/carbon deposits from catalyst particles and thus permit surface decomposition reactions to persist for continuous BNC tube's growth [1,2]. We employed spin-polarized periodic density functional theory (DFT) [3] to study the bonding and chemistry of NH₃ and their fragment on a Fe55 icosahedral cluster. We investigate the site preference for NH₃ with J. P. Perdew, K. Burke, and M. Ernzerhof (PBE) functional [3] and our results, which agree with recent experimental studies, suggest that for NH₃, only the interaction perpendicular at the cluster is favorauble (-0.37 eV < B.E.(NH₃) < +0.05 eV). When the geometry optimization is started with the molecule placed flat on the surfece, it's flipping up during the process. Anyway, also the perpendicolar adsorption are not favourable when the hydrogen is toward the surface. Stable geometries of N and H on the high simmetry adsorption site of Fe₅₅ have been calculated as well. Both of the atom present similar behaviour: the hollow or top are the only stable sites and from the bridge sites, the adsorbed migrates on the nearest hollow site. For the atomic nitrogen adsorption, FeN and Fe₃N conformation are observed. The most stable adsorption site is the hollow one (Fe₃N): -1.35 eV. The nitrogen on top sites bind to one iron and the interation is less favourable: B.E.(N) = -0.05 eV.
[1] A.G. Nasibulin, D. P. Brown, P. Queipo, D. Gonzalez, H. Jiang, A. S. Anisimov, E. I. Kauppinen 243 13 (2006) 3087.
[2] A.G. Nasibulin, P. Queipo, S.D. Shandakov, D.P. Brown, H. Jiang, P.V. Pikhitsa, O.V. Tolochko, and E.I. Kauppinen, Journal of Nanoscience and Nanotechnology, 6 5 (2006) 1233.G. Kresse, J. Hafner, J. Phys. Condens. Matter 6 (1994) 8245.
[3] J. P. Perdew, K. Burke, and M. Ernzerhof, Phys. Rev. Lett.77, 3865 (1996).

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