CO adsorption site occupations on Fe(111) vs coverage and temperature: The kinetics of adsorption and reaction

Abstract

The CO adsorption site occupations on Fe(111) have been determined as a function of coverage and temperature using temperature programmed desorption (TPD) and time-resolved electron energy loss spectroscopy (TREELS). CO does not adsorb in a thermally equilibrated distribution among the four adsorption sites at 100 K; selective adsorption occurs due to different coverage and temperature dependent sticking probabilities. Upon heating, the nonequilibrium distribution relaxes via CO site changes. At ≈250 K and moderate coverage, CO adsorbed in the on-top site changes to the bridge-like shallow hollow site. A heating rate variation analysis yielded an activation barrier E=4.5±0.5 kcal mol−1 and effective first order preexponential ν1=101.5±0.5 s−1 for this process. A similar site change from the deep hollow to the shallow hollow occurs at low coverage near 160 K. Dissociation occurs at ≈300 K when all CO are adsorbed in the shallow hollow site. With low coverages the dissociation reaction occurs with E=20±5 kcal mol−1 and ν1=1011±2 s−1. The resulting adsorbed atomic C and O recombinatively desorb at ≈760 K with E≈48 kcal mol−1 and ν2≈0.1 cm2 s−1. At higher coverages, in addition to dissociation, some of the CO in the shallow hollow desorb, with E≈32 kcal mol−1 and ν1≈1017 s−1.