THE SURFACE-CHEMISTRY OF 2-BUTENES ADSORBED ON MO(100), OXYGEN-COVERED MO(100) AND MOO2

2-Butenes adsorbed on Mo(100) and oxygen-covered Mo(100) can either th ermally decompose to yield hydrogen and adsorbed carbon, desorb molecu larly, self-hydrogenate to produce butane, or dissociate to form a C-2 species which can further decompose to desorb methane. 2-Butenes are proposed to completely thermally decompose on the four-fold sires on M o(100) since the hydrogen yield decreases linearly with oxygen coverag e and oxygen blocks the four-fold sites. The butene desorption and sel f-hydrogenation activation energy increases with increasing oxygen cov erage suggesting that 2-butenes bond to Mo(100) predominantly by donat ion of pi electrons to the molybdenum. Finally, methane is also formed . This is proposed to occur via the intervening formation of methylene carbenes formed by direct carbon-carbon double-bond cleavage. The met hane yield is much larger than that found following both ethylene and propylene adsorption on oxygen-modified molybdenum. This effect is rat ionalized using the bonding model outlined above from the variation in the position of the Ti orbital through the homologous series ethylene , propylene, 2-butenes. (C) 1998 Elsevier Science B.V.