Theoretical study of ammonia oxidation on platinum clusters--adsorption of intermediate nitrogen dimer molecules.

Density Functional Theory calculations with the hybrid B3LYP functional and the LANL2DZ and 6-311++g(**) basis sets were used to calculate the adsorption energies, geometries and vibration modes of N2Hz (z=0-4) molecules on a cluster of 20 platinum atoms. Based on calculated binding energies, the trans conformations of N2H4 and N2H2 were predicted to adsorb with one nitrogen in contact with the cluster; N2H3 and N2H radicals adsorb with both nitrogen atoms in contact with the catalyst; while N2 was not found to adsorb to any appreciable degree. Calculated frequencies showed N-N bond stretching frequency occurs at 913 cm(-1) and 953 cm(-1) for N2H4 and N2H3, respectively and is blueshifted to 1603 cm(-1) and 1631 cm(-1) for N2H and N2H2, respectively. This trend suggests that the formation of this bond could indicate the presence of these species during ammonia oxidation as a shift from 900 to 1600 cm(-1) is expected when the single bond becomes a double bond. Finally, this study combined with earlier studies predicts the following trend for the adsorption energies of species formed: N2<H2O<NH3<N2H2<N2H4<N2H<N2H3<OH<NH2<NH<N.