Stability of adsorbed states and site-conversion kinetics: CO on Ni(100).

Abstract

The site conversion of adsorbed CO between the terminal site and the bridged site on Ni(100) was studied by means of infrared reflection absorption spectroscopy (IRAS). The temperature dependence of the relative occupation for two sites was measured from 80 to 266 K in detail, where the binding-energy difference was determined to be 11 meV. The driving force for the predominant occupation of the terminal site at higher temperature is ascribed to the vibrational entropy of the low-energy degenerate-hindered translational mode of the terminal CO. The kinetics of approaching the equilibrium was studied by time-resolved IRAS combined with a pulsed gas dose. Following a rapid dose, CO molecules are initially adsorbed at the terminal site and the bridged site with the a priori ratio of 1:2, indicating that gas-phase CO molecules are directly trapped by the potential minima initially, are thermalized, and migrate on the surface to approach the equilibrium occupation ratio. The microscopic hopping rate from the terminal site to the bridged site was estimated to be 0.02 ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ and that from the bridged site to the terminal site was estimated to be 0.005 ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ at 83 K. A random-walk model assuming the microscopic hopping rates gives a self-diffusion coefficient of 3.1\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}19}$ ${\mathrm{cm}}^{2}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ at 83 K, which is in good agreement with the previously reported macroscopic results. Thus, the elementary step of surface diffusion is ascribed to the hopping between the terminal site and the bridged site. The difference between the estimated barrier by assuming a harmonic potential and the activation energy for diffusion suggests the presence of anharmonicity in the potential between the terminal site and the bridged site.