Ammonia activation on platinum {1 1 1}: A density functional theory study

By means of density functional theory calculations we have investigated the role of adsorbed atomic oxygen and adsorbed OH in the oxidation of ammonia on Pt{1 1 1}. We have investigated the dissociation of NH3,ads, NH2,ads and NHads on Pt{1 1 1} and the oxidation of these species by Oads and OHads. We have done normal mode frequency analysis and work function calculations to characterise reactant, product and transition states. We have determined reaction energies, activation entropies, kinetic parameters and corrected total energies with the zero point energy. We have shown that Oads only activates the dehydrogenation of NH3,ads and that OHads activates the dehydrogenation of all NHx,ads species and have reasoned this difference in activation by a bond order conservation principle. We have pointed out the importance of a zero point energy correction to the reaction energies and barriers. We have compared the calculated vibrational modes of the adsorbates with corresponding experimental EELS data. This has led to a revise of the frequency assignment of ν(Pt-OH2), a revise in the identification of a NH2 species on the Pt{1 1 1} surface after electron bombardment of pre-adsorbed NH3 and the confirmation of an ammonia dimer binding model at the expense of a hollow site occupation by ammonia on the Pt{1 1 1} surface.