Density functional and infrared spectroscopy studies of bonding and vibrations of NH species adsorbed on the Ru(001) surface: A reassignment of the bending mode band

The electron energy-loss feature at about 1350 cm^-1 often observed on the Ru(001) surface (and on other transition-metal surfaces) with N-and H-containing adsorbates is commonly assigned to the bending mode of the adsorbed NH species. Since it is widely used in the literature as a fingerprint for tilted NH species adsorbed on metals, we have inspected this assignment with the help of density functional model cluster calculations and infrared spectroscopy experiments. Adsorption in three-fold hollow sites with the N-H axis oriented perpendicular to the surface is computed to be energetically favored. Consequently, the Ru-N-H bending mode is dipole-forbidden in this adsorption complex, but this mode should be dipole-allowed for the tilted NH moiety calculated to be weaker bound in the on-top position of Ru(001). The vibrational frequencies of the Ru-N-H bending mode for both conceivable structures of the adsorption complexes are calculated to be lower by more than 500 cm^-1 than the experimental value assigned to this mode. Newly recorded infrared spectra of the reactive N+O+H(D)/Ru(001) system could easily have detected N-H (N-D) stretching modes of adsorbed NH (ND) species. However, no maxima were found in the region from 700 to 2000 cm^-1 which can be related to adsorbed NH, in full agreement with the density functional cluster-model results. Therefore, the electron energy-loss feature at about 1350 cm^-1 has to be due to a different NH-related surface species.