Measurement of the Binding Energy for Di-sigma C_2H_4/Pt{lll}: Does a Radical Intermediate Form During Thermal Desorption?

摘要

The thermal desorption of an intact molecule from a well-defined surface represents one of the simplest surface chemical reactions available for investigation. For adsorbate systems where several reaction channels are accessed, determining the mechanism of desorption is virtually impossible from the analysis of final state products. In this paper we address the mechanism of thermal desorption of di-cr C2H4 from Pt{111}, as shown in Scheme 1, using an impulsive (femtosecond time scale) scattering probe of the initial state of the reactant. Many aspects of the surface chemistry of the C_2H_4/Pt{111} system have been studied intensively over the last 20 years, and the understanding of this multichannel reactivity has evolved considerably with the application of new, time-resolved surface analysis probes. The specific reaction shown in Scheme 1 is of interest because the mechanism of desorption, involving rehybridization of the carbon atoms, remains almost totally unknown. The reaction is of technological interest because the rate of ethylene desorption competes effectively with the rate of hydrogenation. Furthermore, the mechanism of olefin hydrogenation in general remains uncertain, and our measurements suggest that a radical intermediate may be available for hydrogen addition during the thermal desorption reaction.