ENSEMBLE EFFECTS IN THE OXYGEN CHLORINE/PD(100) SYSTEM/

Chlorohydrocarbons chemisorb dissociatively on Pd surfaces at greater than or equal to 200 K and greater than or equal to 10(-8) Torr, as sh own by high-resolution electron energy loss spectroscopy (HREELS) and C Is, Cl 2p, and Pd 3d surface core level shifts (SCLS). From CH2Cl2-g enerated overlayers on Pd(100), hydrogen is removed thermally and carb on is removed by oxidation as CO and CO2, leaving voids between the ch lorine (Cl) ensembles that are accessible to other adsorbates. The res ulting Cl overlayers are partially ordered depending on initial condit ions. The concomitant low-energy electron diffraction (LEED) patterns show that the Cl ensembles are stable to high-temperature reaction cyc les. The order-disorder phenomena observed in the temperature range 30 0-900 K include the generation of domains consisting of Cl only that s urround reactive sites of the metal. With CH2Cl2/Pd(100). these domain s are formed by lateral packing of 16 Pd/CCl2 units that restrict the supply of O-(a) for oxidation of C. Selectivity is switched from CO2 t o CO with increasing Cl concentration. Lateral interactions are of two types: mobile O-immobile Cl and mobile O-mobile O. This is reflected in a lowering of the O-2 temperature programmed desorption (TPD) maxim a with increasing Cl concentration. A statistical-mechanical model is presented for the effects of Cl-(a) with phase-equilibration between a dense and a rare phase of O-(a).