Oxidation of ethylene on Pd(100); bonding of ethylene and scavenging of dehydrogenation fragments by surface oxygen

The adsorption and reaction of C₂H₄ on oxygen covered Pd(100) was studied with high resolution electron energy loss spectroscopy (EELS) and temperature programmed reaction spectroscopy (TPRS). The clean Pd(100) surface at 300 K was exposed to O₂ to produce atomic oxygen in the p(2×2) structure for coverages between 0.05 and 0.25. The EELS and TPRS measurements were conducted following saturation coverage of the oxygen covered surface by C₂H₄ at 80 K. Both the di-σ- and π-bonded forms of C₂H₄ were stable on the surface for θ_O less than 0.25. The π-bonded form desorbed without reaction between 100 and 300 K, but the di-σ-bonded form underwent dehydrogenation above 250 K. The C₂H₄ dehydrogenation products were reactive towards atomic oxygen and produced H₂, H₂O, CO, CO₂, and adsorbed C. Oxygen preadsorption inhibited C₂H₄ Oxidation by limiting the formation of di-σ-bonded C₂H₄, and the fully developed p(2×2)O overlayer, corresponding to θ_O = 0.25, was sufficient to block completely the reaction of ethylene. The extent of reaction decreased in a 2:1 ratio to the increase in oxygen coverage, and indicated that oxygen islands blocked C₂H₄ dissociation. Only the π-bonded form of C₂H₄ was stable on the surface for θ_O greater than 0.25; the saturation coverage of π-bonded C₂H₄ of 0.25 was the same as for clean Pd(100).