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

Propylene adsorbed on Mo(100) and oxygen-covered Mo(100) thermally dec omposes to desorb hydrogen and deposit carbon, desorbs molecularly, se lf-hydrogenates forming propane, or decomposes to ultimately form adso rbed C-1 species, which hydrogenate to form methane. Since the hydroge n desorption yield decreases linearly with increasing oxygen coverage, it is proposed that propylene completely thermally decomposes on the four-fold hollow sites on Mo(100). Adsorbed propylene can self-hydroge nate to form propane and predosing the surface increases the propane y ield indicating that it is formed by reaction of propylene with surfac e hydrogen. The activation energy for both of these processes increase s with increasing oxygen coverage, a trend that is rationalized by pro posing that propylene adsorbs on molybdenum by pi donation to the surf ace. Finally, methane is formed in yields greater than found for ethyl ene on oxygen-covered Mo(100) and the methane yield increases with inc reasing oxygen coverage up to 0.67 monolayers and decreases at higher coverages, No other hydrocarbons are detected following propylene adso rption on any oxygen-covered surface and it is suggested that an initi al carbon-carbon bond cleavage leads to the ultimate formation of meth ane by reaction of C-1 species with adsorbed hydrogen. (C) 1997 Elsevi er Science B.V.