Steric Hindrance of NH3 Diffusion on Pt(111) by Co-Adsorbed O-Atoms

A detailed velocity-resolved kinetics study of NH₃ thermal desorption rates from p(2 × 2) O/Pt(111) is presented. We find a large reduction in the NH₃ desorption rate due to adsorption of O-atoms on Pt(111). A physical model describing the interactions between adsorbed NH₃ and O-atoms explains these observations. By fitting the model to the derived desorption rate constants, we find an NH₃ stabilization on p(2 × 2) O/Pt(111) of 0.147_-0.014^+0.023 eV compared to Pt(111) and a rotational barrier of 0.084_-0.022^+0.049 eV, which is not present on Pt(111). The model also quantitatively predicts the steric hindrance of NH₃ diffusion on Pt(111) due to co-adsorbed O-atoms. The derived diffusion barrier of NH₃ on p(2 × 2) O/Pt(111) is 1.10_-0.13^+0.22 eV, which is 0.39_-0.14^+0.22 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 NH₃-O adsorbate-adsorbate interactions and NH₃ binding energies at Pt(111) and p(2 × 2) O/Pt(111), which indicates the importance of dispersion interactions for both systems.