Low temperature adsorption and site-conversion process of CO on the Ni(111) surface

Abstract

Low-temperature (25K) adsorption states and the site conversion of adsorbed CO between the ontop and the hollow sites on Ni(111) were studied by means of temperature programmed desorption and infrared reflection absorption spectroscopy. The activation energy and pre-exponential factor of desorption were estimated to be 1.2eV and 2.6×1013s−1, respectively, in the limit of zero coverage. At low coverage, CO molecules preferentially adsorbed at the hollow sites below 100K. With increasing temperature, the ontop sites were also occupied. Using a van't Hoff plot, the enthalpy and the entropy differences between the hollow and ontop CO were estimated to be 36meV and 0.043meVK−1, respectively, and the vibrational entropy difference was estimated to be 0.085meVK−1. The positive entropy difference was the result of the low-energy frustrated translational mode of the ontop CO, which was estimated to be 4.6±0.3meV. With the harmonic approximation, the upper limit of the activation energy of site hopping from ontop sites to hollow sites was estimated to be 61meV. In addition, it was suggested that the activation energy of hollow-to-hollow site hopping via a bridge site was less than 37meV.