Steric Hindrance of NH 3 Diffusion on Pt(111) by Co-Adsorbed O-Atoms.

A detailed velocity-resolved kinetics study of NH thermal desorption rates from (2 × 2) O/Pt(111) is presented. We find a large reduction in the NH desorption rate due to adsorption of O-atoms on Pt(111). A physical model describing the interactions between adsorbed NH and O-atoms explains these observations. By fitting the model to the derived desorption rate constants, we find an NH stabilization on (2 × 2) O/Pt(111) of 0.147 eV compared to Pt(111) and a rotational barrier of 0.084 eV, which is not present on Pt(111). The model also quantitatively predicts the steric hindrance of NH diffusion on Pt(111) due to co-adsorbed O-atoms. The derived diffusion barrier of NH on (2 × 2) O/Pt(111) is 1.10 eV, which is 0.39 eV higher than that on pristine Pt(111). We find that Perdew Burke Ernzerhof (PBE) and revised Perdew Burke Ernzerhof (RPBE) exchange-correlation functionals are unable to reproduce the experimentally observed NH-O adsorbate-adsorbate interactions and NH binding energies at Pt(111) and (2 × 2) O/Pt(111), which indicates the importance of dispersion interactions for both systems.