THE ROLE OF DEFECTS IN THE SPECIFIC ADSORPTION OF ANIONS ON PT(111)

The specific adsorption of anions from hydrochloric and sulfuric acids was studied on Pt(111) surfaces containing two different types of deliberately induced defects. One type of defect surface used was the step-terrace structure produced by cutting a [111] oriented crystal a few degrees off the [111] zone axis. A second type was a randomly up-and-down stepped surface produced by ion bombardment and annealing. The atomic structure of the defect surfaces was determined by spot-profile analysis of LEED patterns in a UHV chamber directly coupled to the electrochemical cell. The vacuum work function of the surfaces was determined by UV photoemission. All of the step defects of the type studied lowered the work function of the surface in direct proportion to the step density, indicative of a local dipole at the step with the positive end of the dipole outward from the surface. The local work function at the step, and thus the local potential of zero charge (pzc), is lower at the step than at the atomically flat terrace. This difference in local pzc controls the coupling of the processes of hydrogen adsorption and anion desorption. On the atomically flat (111) terraces, which have the most positive pzc of all Pt surfaces, the processes are completely decoupled in dilute (mM) acids, with complete desorption of anions from the terraces at potentials more anodic than for the onset of hydrogen adsorption. The lower pzc for steps places the potential region for anion desorption/adsorption in the potential region for hydrogen adsorption/desorption, e.g. anion desorption from steps occurs simultaneously with hydrogen adsorption on the terraces.