Adsorbate-enhanced chemisorption in the CO/Re(001) system: Experiment and theory

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, β-CO state and the less tightly bound α-CO state; basically, the α-state starts to populate when the β-sites approach saturation. Furthermore, the increase in β-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 α-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 β-sites are treated as traps for mobile α-CO admolecules. Occupied β-sites then serve as nucleation centers for enhanced, extrinsic precursor-mediated, chemisorption and island growth. The nonmonotonic variation of α-CO adsorption energy, and the appearance of a shoulder in the α-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.