DFT and TPD study of the role of steps in the adsorption of CO on copper: Cu(4 1 0) versus Cu(1 0 0).

Adsorption of carbon monoxide (CO) was studied on stepped Cu(4 1 0) by temperature programmed desorption (TPD) and density-functional-theory (DFT) calculations. For comparison, the adsorption of CO was characterized also on Cu(1 0 0) by DFT calculations. On Cu(4 1 0) TPD reveals two desorption peaks: a high temperature peak (∼210 K) is attributed to the desorption of CO from step-edge sites and low temperature peak (∼170 K) to desorption from terrace sites. According to DFT calculations, CO prefers to adsorb at step-edges of Cu(4 1 0), although the step-edge versus terrace site preference is rather small at low coverage of 1/16 ML, about 0.05 eV; the respective DFT predicted CO binding energies are  -0.89 and  -0.84 eV at the step-edge and terrace top sites, whereas the value calculated at top sites of Cu(1 0 0) is  -0.86 eV. Although this small step-edge over terrace site preference of 0.05 eV cannot explain the temperature difference of 40 K between the two TPD peaks, when the lateral intermolecular interactions are neglected, it is sufficient that the CO adsorbs almost exclusively at step-edges at low coverage (at 200 K the 0.05 eV corresponds to 3 kT). The emergence of the two TPD peaks on Cu(4 1 0) is therefore attributed to a combination of step-edge preference and lateral repulsion between CO molecules, which increases with increasing coverages and diminishes the net desorption energy of CO. DFT calculations further reveal that the reason for the significant increase of saturation coverage on Cu(4 1 0) compared to Cu(1 0 0) is related to the geometry of the step-edge that allows the CO molecules adsorbed thereon to tilt away from the nearest neighboring CO molecules adsorbed at the terrace and therefore to effectively reduce the lateral repulsion.