DFT Study of NHx (x=1-3) Adsorption on RuO2(110) Surfaces

We have used density functional theory (DFT) to investigate the adsorption of NHx species (x = 1-3) onto RuO2(110) surfaces at various monolayer coverages and onto oxygen-rich RuO2(110) surfaces. We characterized the structures, vibrational frequencies, and detailed bonding interactions between the adsorbates and the surfaces. The binding energy increased as the number of hydrogen atoms bonded to the N atom decreased, i.e., NH3-cus binds most weakly to the surface, whereas NHcus has the highest binding energy. We investigated the nature of the metal-NHx bonding through analyses of the density of states (DOS) and electron density difference contour maps. The DOS analysis allowed us to characterize the state interactions between the NHx species and the surface; the electron density difference maps provided evidence that was consist with the DOS analysis. A greater number of state interactions led to the higher binding energy of an NHx species: for NH3-cus bound through a single sigma bond, the binding energy was 1.56 eV; it increases to 4.23 eV when two pi bonds were present in the NHcus bonding mode. In addition, hydrogen bonding played an important role in the binding of NH3-cus and NHcus, especially for the weakly coordinated NH3-cus. The formation of hydrogen bonds was evident from changes in the bond length and stretching frequency of the N-H bonds.