One-dimensional CO island formation in the coadsorption of H 2 and CO on the steps of Pt(112)

Abstract

The interaction of coadsorbed CO and H 2 on the steps of Pt(112) [Pt(S)[3(111)×(001)]] was examined by digital electron-stimulated desorption ion angular distribution (ESDIAD), low-energy electron diffraction (LEED) and temperature-programmed desorption (TPD). On the clean surface, CO and H 2 both adsorb preferentially on step sites at low coverages; CO at stop sites on the step edge and H 2, as atoms, in the step 4-fold sites. For pure CO adsorption, an ordered (2× n) structure of step CO is observed at 0.19 ML (atop step sites half filled), and at least two CO structural transformations occur as the step sites fill further. These structural transformations, due to COCO repulsions, involve the production of tilted CO species whose tilt directions are related to the local CO coverage in one-dimensional chains of CO on the step edges. The structural transformations described above for pure CO layers on the step edges can also be produced by coadsorption of hydrogen on the (2× n) step CO structure. The CO structural transformations in the coadsorbed system are driven by H adsorption, due to the formation of immiscible one-dimensional CO and H islands at the steps. Similar results are observed for low coverages of CO adsorbed on the H atom saturated surface.