ETHYLENE HYDROGENATION ON PT(111) MONITORED IN-SITU AT HIGH-PRESSURESUSING SUM-FREQUENCY GENERATION

Infrared-visible sum frequency generation (SFG) has been used to monit or the surface vibrational spectrum in situ during ethylene hydrogenat ion on Pt(111). Measurements were made near 1 atm of total pressure of ethylene and hydrogen and at 295 K. Kinetic information was obtained simultaneously with the surface vibrational spectroscopy by monitoring the reaction rate with gas chromatography. The macroscopic turnover r ate and surface adsorbate concentration could then be correlated. Duri ng the reaction ethylidyne, di-sigma-bonded ethylene, ethyl, and pi-bo nded ethylene were observed on the surface in various amounts dependin g on conditions. Ethylidyne, a spectator species during hydrogenation, competed directly for sites with di-sigma-bonded ethylene and its sur face concentration could be shown to be completely uncorrelated with t he rate of hydrogenation. In contrast, pi-bonded ethylene did not comp ete for sites with the ethylidyne overlayer and was observed on the su rface regardless of the surface concentration of ethylidyne. Evidence points to the pi-bonded species as being the primary intermediate in e thylene hydrogenation on Pt(111). The surface concentration of this sp ecies is about 0.04 ML (ML = monolayer) during reaction. Thus, the tur nover rate per reaction intermediate is 25 times faster than the turno ver rate if measured per surface platinum atom.