OXYGEN-ACTIVATED COMBUSTION OF ALKENES ON THE PD(100) SURFACE

Reactions of ethylene, propene, 1-butene, and 1,3-butadiene have been investigated on atomic oxygen-covered Pd(100)-p(2 x 2)-O surface using temperature-programmed reaction spectroscopy. All four alkenes undergo combustion yielding H2O, CO, and CO2. No partial oxidation products were observed. H-2 is evolved after the oxygen is consumed. Compared to dehydrogenation reactions on the clean Pd(100) surface, O(a) inhibits both adsorption and reaction of ethylene. For propene, 1-butene, and 1,3-butadiene, in contrast, O(a) does not inhibit adsorption and also activates reactions-all C-H bonds react facilely with O(a) forming H2O below the dehydrogenation temperature on the clean surface, Isotope experiments show that initial reactions occur predominantly with the vinylic C-H bonds of 1-alkenes, Low-temperature CO2 evolution and desorption-limited CO evolution provide evidence for oxygen addition to the carbon atoms of dehydrogenated alkene intermediates. Indirect reactions resulting from dehydrogenation at clean sites and oxidation of H(a) and C(a) lead to high-temperature H2O evolution above 300 K and CO and CO2 evolution above 400 K.