THE CONVERSION OF ETHYLENE TO ETHYLIDYNE ON PT(111) - NON-FIRST ORDER KINETICS AND ENSEMBLE EFFECTS

The conversion of ethylene to ethylidyne on Pt(111) has been studied in detail by laser induced thermal desorption (LITD) in conjunction with Fourier transform mass spectrometry (FTMS) and Fourier transform infrared reflection absorption spectroscopy (FT-IRAS). The formation of ethylidyne shows strictly first order kinetics over the entire coverage range. In contrast, this holds only for low initial coverages in case of the decomposition of ethylene, whereas at high coverages a pronounced non-first order behavior is observed. The latter observation can be explained by an ensemble effect which assumes an increased decomposition rate for ethylene molecules in the immediate vicinity of an ensemble of free Pt sites. Using a simple Monte Carlo calculation, not only the measured decomposition rates can be reproduced but, in addition it shows that the reaction proceeds in patches rather than uniformly. This result is in agreement with recent scanning tunnelling microscope (STM) observations.