The influence of chlorine on the dispersion of Cu particles on Cu/ZnO(0001) model catalysts

One of the ways in which chlorine is thought to poison metal catalysts on o xide supports is by altering their dispersion. The effect of chlorine on Cu /ZnO(0001) model catalysts was studied by vapor-depositing Cu onto Zn-termi nated ZnO(0001), both with and without preadsorbed Cl-2, using XPS, ion sca ttering spectroscopy (ISS), temperature-programmed desorption (TPD), work f unction, and band bending measurements. A disordered, but nearly close-pack ed overlayer of Cl adatoms forms at saturation with similar to 0.30 Cl adat oms per Zn site. Without Cl, vapor-deposited Cu grows in two-dimensional is lands that cover similar to 33% of the ZnO, after which these islands thick en (i.e., as 3D Cu particles) while the clean ZnO between these Cu islands gets covered with Cu only very slowly. Preadsorbed Cl decreases the fractio n of the surface that is covered by Cu islands by a factor of three, so Cl( a) either decreases the number of 2D Cu islands or their critical area befo re thickening. Both are consistent with weaker binding of Cu to the Cl-cove red surface than to the clean ZnO. The TPD features for formate decompositi on after HCOOH adsorption onto Cu/ZnO(0001) were suppressed with preadsorbe d Cl, but the CO2 : CO selectivity increased. When Cu was deposited onto Cl -presaturated ZnO, neither the Zn- nor Cu-formate peaks were observed, show ing that Cl covers both the Zn sites and the growing Cu islands, as suggest ed by ISS also.