The reaction of propene with oxygen-covered Pd(1 0 0)

Abstract

The effect of oxygen coverage and surface structure on the total oxidation of propene over Pd(1 0 0) was studied using temperature-programmed reaction (TPR), isothermal kinetic measurements, low energy electron diffraction (LEED), and scanning tunneling microscopy (STM). TPR revealed CO 2 production peaks at 430 and 550 K. When the surface was covered by chemisorbed oxygen in (2 × 2) structures the 550 K peak dominated, while only the 430 K peak was seen at low propene doses and high oxygen coverages (∼0.8 ML) where a ( 5 × 5 ) R27° reconstruction covered the surface. Regardless of the oxygen coverage, water desorbed at 430 K indicating that the 550 K CO 2 peak was due to oxidation of C deposited by propene dissociation. The initial propene sticking coefficient at 330 K was a factor of five greater for (2 × 2)-O surfaces than for the ( 5 × 5 ) R27°-O surface, thus the lower activation energy pathway favored at high oxygen coverages did not necessarily translate into higher reaction rates. Above 450 K, the isothermal kinetic measurements showed that the reaction rate increases with decreasing oxygen coverage until the reaction becomes starved for oxygen. LEED measurements showed that the rate increases as the ( 5 × 5 ) R27° structure is replaced by the (2 × 2) structures. At lower temperatures, however, the oxidation rate of C deposited on the surface by propene dissociation at low oxygen coverages is slow and so higher rates were seen at high oxygen coverages. At room temperature, STM images showed that propene initially slowly randomly adsorbs atop the ( 5 × 5 ) R27° surface. As the propene coverage increased, however, adsorbates tended to cluster together forming disordered regions; as the surface disordered the adsorption rate increased. At 550 K, STM movies recorded during propene exposure to oxygen-covered surfaces showed the slow removal of ( 5 × 5 ) R27° domains followed by rapid dissolution of islands formed during oxygen adsorption. The results indicate that oxidizing the Pd surface affects hydrocarbon oxidation in two opposing ways: it decreases the hydrocarbon adsorption probability but also favors an easier oxidation pathway once the molecules adsorb.