Ethylene hydrogenation on Pt(111) monitored in situ at high pressures using sum frequency generation

Infrared-visible sum frequency generation (SFG) has been used to monitor the surface vibrational spectrum in situ during ethylene hydrogenation 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 rate and surface adsorbate concentration could then be correlated. During the reaction ethylidyne, di-sigma-bonded ethylene, ethyl, and pi-bonded ethylene were observed on the surface in various amounts depending on conditions. Ethylidyne, a spectator species during hydrogenation, competed directly for sites with di-sigma-bonded ethylene and its surface concentration could be shown to be completely uncorrelated with the rate of hydrogenation. In contrast, pi-bonded ethylene did not compete for sites with the ethylidyne overlayer and was observed on the surface regardless of the surface concentration of ethylidyne. Evidence points to the pi-bonded species as being the primary intermediate in ethylene hydrogenation on Pt(111). The surface concentration of this species is about 0.04 ML (ML = monolayer) during reaction. Thus, the turnover rate per reaction intermediate is 25 times faster than the turnover rate if measured per surface platinum atom.