The adsorption and decomposition of C2H4 on Ru(001): A combined TPR and work function change study

Work function change (Delta Phi) measurements during adsorption and surface heating in a Delta Phi-temperature programmed reaction (TPR) mode combined with TPD are demonstrated to provide new information on the interaction an d chemistry of ethylene on Ru(001). Rearrangement of second layer ethylene molecules has been observed between 82 and 120 K. This is a competing proce ss with molecular desorption, interpreted as a result of migration of secon d layer molecules toward the surface. Our results are consistent with and s upport previous studies that suggested the formation of a surface intermedi ate (eta(2)(C,C)CHCH2) during ethylene dehydrogenation to ethylidine. Emplo ying a derivative mode with respect to temperature -d(Delta Phi)/dT, we fin d an early onset for ethylidine decomposition near 265 K. Delta Phi-TPR mea surements in the range 560-720 K reveal three distinct CH decomposition pea ks, reflecting different activation energies for the decomposition reaction sites. The dipole moment of an adsorbed CH has been determined to be mu = 0.43 D,suggesting a rather polarized Ru-CH complex; its structure is indepe ndent of the adsorption site. Finally, carbide polymerization to form graph ite has been detected above T-s = 560 K for the first time using work funct ion change measurements. Good agreement was found between the contribution to Delta Phi by the graphite layer formed on the Ru(001) surface and ab ini tio calculations performed previously on this system.