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Abstract

The decomposition of ethene on Pd(111) surface was studies at effective pressure in the 10^-8 to 10^-7 mbar range and at sample temperature between 300 and 700 K, using an effusive capillary array beam doser for directional adorption, LEED, AES, temperature programmed reaction, and TDS. In the temperature range of 350-440 K increasingly stronger dehydrogenation of the ethene molecule is observed. Whereas at 350 K an ethylidyne adlayer is still present after adsorption, already at temperatures around 440 K complete coverageof the surface by carbon is attained, while the bulk still retains the properties of pure Pd. Beyond 440 K a steady-state surface C coverage is established, which decreases with temperature and is determind bz detailed balancing between the ethene gas-phase adsorption rate and the migration rate of carbon into the Pd bulk. This process gives rise to the formation of a "partially carbon-covered Pd_xC_y surface". Above 540 K the surface-bulk diffusion of adsorbed carbon becomes fast, and in the UHV experiment the ethene adsorption rate becomes limited by the ethene gas-phase supply. The carbon bulk migration rate and the steady-state carbon surface coverage were determind as a function of the sample temperature and the ethene flux. An activation energy of 107kJmol^-1 for the process of C diffusion from the surface adsorption site into the subsurface region was derieved in the temperature range of 400-650 K by modeling the C surface coverage as a function of temperature on the basis of steady-state reaction kinetics, assuming a first-order process for C surface-subsurface diffusion and a second -order process for C(ads) formation by dissociative C₂H₄adsorption.