High Coverage CO Activation Mechanisms on Fe(100) from Computations

CO activation on Fe(100) at different coverage was systematically computed on the basis of spin-polarized density functional theory. At the saturated coverage (11CO) on a p(3 x 4) surface size (24 exposed Fe atoms), top (1CO), bridge (3CO) and 4-fold hollow (7CO) adsorption configurations coexist. The stepwise adsorption energies and dissociation barriers at different coverage reveal equilibriums between desorption and dissociation of adsorbed CO molecules. It is found that only molecular adsorption is likely for n(CO) = 8-11, and mixed molecular and dissociative adsorption becomes possible for n(CO) = 5-7, while only dissociative adsorption is favorable for n(CO) = 1-4. The computed CO adsorption configurations and stretching frequencies as well as desorption temperatures from ab initio thermodynamic analysis agree well with the available experimental data.