KINETICS OF THE ETHYLENE TO ETHYLIDYNE CONVERSION REACTION ON PT(111) STUDIED BY LASER-INDUCED THERMAL-DESORPTION FOURIER-TRANSFORM MASS-SPECTROMETRY

The rate coefficients for the conversion of ethylene to ethylidyne on the Pt(111) surface at several surface temperatures (236-274 K) have been measured by using laser-induced thermal desorption/Fourier transform mass spectrometry (LITD/FTMS) to monitor the change in the amount of ethylene on the surface as a function of time. The results yield an activation energy of 14.9 ± 1.4 kcal/mol and a preexponential factor of 4 × 1010±1.2 s-1 for a 2-langmuir near-saturation exposure of ethylene. Approximately 20-25% of the ethylene thermally desorbs under the conditions of these experiments, and Auger electron spectra (AES) show that this thermal desorption process is much faster than the conversion reaction. These data indicate that both reversibly and irreversibly adsorbed ethylene are important in the description of the surface chemistry of ethylene on Pt(111) following a 2-langmuir exposure. The thermal desorption and conversion to ethylidyne are sequential processes, and it is the irreversibly adsorbed ethylene that converts to ethylidyne. In addition, the AES studies show that the total amount of ethylene thermal desorption does not change substantially under isothermal conditions at different temperatures in the range of ∼236-274 K. © 1990 American Chemical Society.