A DFT study of the adsorption and dissociation of CO on Fe(100) : influence of surface coverage on the nature of accessible adsorption states

Adsorption energies, structures, and vibrational frequencies of CO on Fe(100) for several adsorption states and at three surface coverages are reported. A full anal. of the vibrational frequencies of CO was performed, thus detg. what structures are stable adsorption states and characterizing the transition-state structure for CO dissocn. The activation energy of dissocn. of CO at 0.25 ML (ML = monolayers) was calcd. as well as at 0.5 ML; the dissocn. at 0.5 ML was studied to quantify the destabilization effect on the CO(a3) mols. when a neighboring CO mol. dissocs. The no. and nature of likely adsorption states is coverage dependent. Evidence is presented that shows that the CO mol. adsorbs on Fe(100) at fourfold hollow sites with the mol. axis tilted away from the surface normal by 51.0 Deg. The adsorption energy of the CO mol. is -2.54 eV and the C-O stretching frequency is 1156 cm-1. This adsorption state corresponds to the a3 mol. desorption state reported in temp. programmed desorption (TPD) expts. However, the activation energy of dissocn. of CO(a3) mols. at 0.25 ML is only 1.11 eV (~25.60 kcal mol-1) and the gain in energy is 1.17 eV; thus, the dissocn. of CO is largely favored at low coverages. The activation energy of dissocn. of CO at 0.5 ML is 1.18 eV (~27.21 kcal mol-1), very similar to that calcd. at 0.25 ML. However, the dissocn. reaction at 0.5 ML is slightly endothermic, with a total change in energy of 0.10 eV. Consequently, mol. adsorption is stabilized with respect to CO dissocn. when the CO coverage is increased from 0.25 to 0.5 ML. [on SciFinder (R)]