Theoretical study of ammonia oxidation on platinum clusters – Adsorption of ammonia and water fragments

Abstract

This study examined the adsorption of reactants (NH3 and OH) and intermediates (NH2, NH, N and H2O) formed during the oxidation of ammonia by hydroxyl on platinum. Specifically, four clusters were used to model the catalytic surface in the Pt(111) orientation. Structural, electronic and vibrational properties were calculated using Density Functional Theory (DFT). The molecules resided in the favored positions predicted by prior experimental observations and DFT calculations, while the adsorption energies followed the trend: H2O<NH3<OH<NH2<NH<N, with the weakest bonds formed by charge transfer and the strongest bonds formed by orbital overlap of unpaired electrons of the radicals and the d orbital of adjacent Pt atoms. Calculated frequency vibrations in this work showed sufficient agreement with experimental observation, challenged previously assigned frequency modes for NH2, NH and H2O and predicted the correct shift in frequency vibrations upon adsorption on platinum when compared with prior DFT calculations.