The surface chemistry of 2-butenes adsorbed on Mo(100), oxygen-covered Mo(100) and MoO2

2-Butenes adsorbed on M o (100) and oxygen-covered Mo(100) can either thermally decompose to yaeld hydrogen and adsorbed carbon, desorb molecularly, 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 sites on Mo (100) smce the hydrogen yield decreases hnearly with oxygen coverage and oxygen blocks the four-fold sites. The butene desorptxon and self-hydrogenation ac twanon energy increases with increasing oxygen coverage suggesting that 2-butenes bond to M o (100) predominantly by donation of ~z electrons to the molybdenum Finally, methane is also formed This is proposed to occur vla the intervening formation of methylene carbenes formed by direct ca rbon-carbon double-bond cleavage The methane yield is much larger than that found followang both ethylene and propylene adsorption on oxygen-modified molybdenum This effect is rationalized using the bonding model outhned above from the variation in the position of the ~ orbital through the homologous seraes ethylene, propylene, 2-butenes © 1998 Elsevier Science B V