THE SURFACE-CHEMISTRY OF ETHYLENE ADSORBED ON MO(100), OXYGEN-COVEREDMO(100) AND MOO2

Ethylene adsorbed on Mo(100) and oxygen-covered Mo(100) can thermally decompose to yield hydrogen and adsorbed carbon, desorb molecularly, s elf-hydrogenate to produce ethane or dissociate to form adsorbed C-1 s pecies which can hydrogenate to form methane. Complete thermal decompo sition of the ethylene is proposed to take place on the four-fold site s on Mo(100) since the hydrogen yield decreases linearly with oxygen c overage. The ethylene desorption activation energy increases with incr easing oxygen coverage suggesting that ethylene bonds to Mo(100) predo minantly by donation of pi electrons to the molybdenum surface. The et hylene hydrogenation activation increases as a function of oxygen cove rage in accord with this effect. The yield of methane also varies with oxygen coverage so that no methane desorption is detected for clean M o(100) but the yield increases with oxygen coverage reaching a maximum at a coverage of similar to 0.6 ML and decreasing at higher coverages . Photoelectron spectroscopy results suggest that adsorbed oxygen incr eases the dissociative probability of ethylene. In addition, experimen ts in which carbenes are grafted onto the surface by decomposing methy lene iodide show that carbenes are stabilized by the addition of oxyge n to the surface. These effects both explain the increase in methane y ield as a function of increasing oxygen coverage. The decrease at high er coverage is likely due, at least in part, to the lack of hydrogen. The ethane yield also decreases at higher coverages due to a similar e ffect. (C) 1997 Elsevier Science B,V.