| Abstract
| - The decomposition of ethene on the Pd(111) surface was studied at effective pressures in the 10-8 to 10-7mbar range and at sample temperatures between 300 and 700 K, using an effusive capillary array beam doserfor directional adsorption, LEED, AES, temperature programmed reaction, and TDS. In the temperature rangeof 350−440 K increasingly stronger dehydrogenation of the ethene molecule is observed. Whereas at 350 Kan 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 asteady-state surface C coverage is established, which decreases with temperature and is determined by detailedbalancing 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 PdxCy surface”. Above 540 K thesurface−bulk diffusion of adsorbed carbon becomes fast, and in the UHV experiment the ethene adsorptionrate becomes limited by the ethene gas-phase supply. The carbon bulk migration rate and the steady-statecarbon surface coverage were determined as a function of the sample temperature and the ethene flux. Anactivation energy of 107 kJ mol-1 for the process of C diffusion from surface adsorption sites into the subsurfaceregion was derived in the temperature range of 400−650 K by modeling the C surface coverage as a functionof 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 C2H4 adsorption.
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