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

The conversion of ethylene to ethylidyne on Pt(111) has been studied i n detail by laser induced thermal desorption (LITD) in conjunction wit h Fourier transform mass spectrometry (FTMS) and Fourier transform inf rared reflection absorption spectroscopy (FT-IRAS). The formation of e thylidyne 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 pronoun ced non-first order behavior is observed. The latter observation can b e explained by an ensemble effect which assumes an increased decomposi tion rate for ethylene molecules in the immediate vicinity of an ensem ble of free Pt sites. Using a simple Monte Carlo calculation, not only the measured decomposition rates can be reproduced but, in addition i t shows that the reaction proceeds in patches rather than uniformly. T his result is in agreement with recent scanning tunnelling microscope (STM) observations.