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

Abstract

Work function change (∆Φ) measurements during adsorption and surface heating in a ∆Φ-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 (η 2 (C,C)CHCH 2 ) during ethylene dehydrogenation to ethylidine. Employing a derivative mode with respect to temperature -d(∆Φ)/dT, we find an early onset for ethylidine decomposition near 265 K. ∆Φ-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 T s ) 560 K for the first time using work function change measurements. Good agreement was found between the contribution to ∆Φ by the graphite layer formed on the Ru(001) surface and ab initio calculations performed previously on this system.

Hydrogenation of 1,3-butadiene on Pd(111) and PdSn/Pd(111) surface alloys under UHV conditions

“…We assumed a linear relationship between the work function change and surface coverage, which at the least can be considered an approximation to trends in reaction behavior. This method has already been successfully applied by Livneh and Asscher [28], who studied the decomposition of ethene on Ru(001) using work function measurements and found that the work function change during decomposition was linear at lower coverage but reached saturation at high coverage.…”

Experimental Characterization of CCH(ads) and CCH2(ads) during the Thermal Decomposition of Methane and Ethylene on Ru(0 0 0 1)

“…As noted above, in prior EELS‐based characterization of Ru(0 0 0 1) samples dosed in a high‐pressure cell with T Ru ≈400 K and carbon coverages ≥0.6 ML of carbon, Goodman and co‐workers reported the presence of CCH 2 (ads) (vinyledine) by observation of its C=C stretching mode at ≈1160 cm −1 and the CH 2 scissor mode of CCH 2 (ads) at 1400 cm −1 . Prior studies have concluded that CCH 2 assumes a configuration on Ru(0 0 0 1) in which the C=C bond is orthogonal to the plane of the surface . In this configuration the two CH stretches would be expected to couple, giving symmetric and asymmetric stretches, with the transition dipole of the symmetric stretch orthogonal to the surface and that of the asymmetric stretch parallel.…”

“… Dissociation pathway of C 2 H 4 dosed on Ru(0 0 0 1), predicted by EELS, RAIRS, and temperature‐programmed oxidation (TPO) measurements . The dissociation steps from CCH 3 to CCH remain unclear; CCH 2 is expected to be an intermediate, but has not been previously reported.…”

“…Similar gaps in understanding also exist for ethylene dissociation on Ru(0 0 0 1). The results of prior work–function measurements, electron‐energy‐loss spectroscopy (EELS) and reflection absorption infrared spectroscopy (RAIRS) studies are summarized in Figure . Because of the relatively large sticking coefficient of ethylene, in each case the sample was created by dosing at <150 K and was subsequently thermally dissociated.…”

Electron-induced chemistry of methyl chloride caged within amorphous solid water

“…The experiments described here were conducted in a previously described 32 Ultra High Vacuum (UHV) system at a base pressure of ∼2 × 10 −10 Torr. A 0.8 × 0.8 cm 2 Ru(0001) single crystal is daily cleaned via a 10 min sputter using high energy ions (Ar + at 700 eV) with subsequent flash annealing to 1600 K and then stay at 1400 K for two more minutes.…”