The Adsorption and Decomposition of C2H4 on Ru(001): A Combined TPR and Work Function Change Study

Abstract

Work function change (ΔΦ) measurements during adsorption and surface heating in a ΔΦ−temperature programmed reaction (TPR) mode combined with TPD are demonstrated to provide new information on the interaction and chemistry of ethylene on Ru(001). Rearrangement of second layer ethylene molecules has been observed between 82 and 120 K. This is a competing process with molecular desorption, interpreted as a result of migration of second layer molecules toward the surface. Our results are consistent with and support previous studies that suggested the formation of a surface intermediate (η2(C,C)CHCH2) during ethylene dehydrogenation to ethylidine. Employing a derivative mode with respect to temperature −d(ΔΦ)/dT, we find an early onset for ethylidine decomposition near 265 K. ΔΦ−TPR measurements in the range 560−720 K reveal three distinct CH decomposition peaks, reflecting different activation energies for the decomposition reaction sites. The dipole moment of an adsorbed CH has been determined to be μ = 0.43 D, suggesting a rather polarized Ru−CH complex; its structure is independent of the adsorption site. Finally, carbide polymerization to form graphite has been detected above Ts = 560 K for the first time using work function change measurements. Good agreement was found between the contribution to ΔΦ by the graphite layer formed on the Ru(001) surface and ab initio calculations performed previously on this system.