Adsorbate-enhanced chemisorption in the carbon monoxide/rhenium (001) system: experiment and theory

Abstract

The chemisorption kinetics of CO molecules on Re(001) crystal surface was studied by temperature-programmed desorption (TPD) experiments in the crystal temperature range of 80-360 K. Correlation was found between the population of the tightly bound, partially dissociated, {beta}-CO state and the less tightly bound {alpha}-CO state; basically, the {alpha}-state starts to populate when the {beta}-sites approach saturation. Furthermore, the increase in {beta}-CO coverage is accompanied by an increase in the overall sticking probability, as well as in the nonmonotonic coverage dependence of the peak desorption temperature of the {alpha}-state. The chemisorption kinetics was found to be crystal temperature independent. LEED analysis reveals that the CO overlayer is disordered, as reported previously. A theoretical model is proposed to account for the above observations. In this model, the {beta}-sites are treated as traps for mobile {alpha}-CO admolecules. Occupied {beta}-sites then serve as nucleation centers for enhanced, extrinsic precursor-mediated, chemisorption and island growth. The nonmonotonic variation of {alpha}-CO adsorption energy, and the appearance of a shoulder in the {alpha}-CO TPD peak at high coverages, are explained by a lattice gas model, incorporating repulsive nearest-neighbor and attractive next-nearest-neighbor lateral interactions between the chemisorbed molecules.