Molecular beam study of oxygen and C2 hydrocarbon chemisorption and reactions on Pt(111)

Abstract

The chemisorption and reactions of oxygen and the C₂ hydrocarbons (acetylene, ethylene, and ethane) on epitaxially grown Pt(111) crystals have been studied using modulated molecular beam techniques. The sticking coefficient of ethylene is observed to be about half that for acetylene, which is near unity on oxygen covered Pt(111) in the temperature range 550–1000 K. For ethane, S2 hydrocarbons on platinum at elevated temperatures results in a surface acetylenic species. The first step in the oxidation process appears to be the removal of hydrogen atoms from this surface acetylene, either by molecular desorption or by reaction with surface oxygen atoms. Surface carbon then reacts with surface oxygen to form CO which either desorbs or reacts further to form CO₂. At high oxygen pressures, the reaction rate is apparently limited by the mobility of surface oxygen atoms which have an activation barrier to diffusion on Pt(111) of about 16 kcal/mole. Some implications of this high surface mobility with regard to the gas–surface oxygen equilibrium are discussed. The observance here of two distinct components in the angular desorption distributions of CO and CO₂ is discussed in terms of finer details of the reaction mechanism.