Surface Chemistry of CH₃Br and Methyl Modified by Copper Deposition on Ru(001)

Abstract

The chemistry of methyl bromide on Cu/Ru(001) has been studied utilizing work function change (∆ ) and temperature-programmed desorption (TPD) measurements. The remarkable modification in the methyl fragments dehydrogenation at the completion of a single copper layer and the significant difference in reactivity of the Cu(2 ML)/Ru(001) or Cu(111) surfaces are the focus of this study. A decrease in work function at the completion of 1 ML CH 3 Br of 2.15 ( 0.02 eV and 1.33 ( 0.05 eV was measured, respectively, for Ru (001) and Cu(2 ML)/Ru(001) held at 82 K. Methyl bromide does not dissociate upon adsorption on clean or the copper-covered surfaces, and it is bound with the bromine down. Copper modifies the reactivity of the Ru substrate, gradually decreasing the dissociated fraction of CH 3 Br from 0.55 of the initial one monolayer on clean Ru(001) to 0.06 on Cu(2 ML)/Ru(001), probably because of defects in the copper layer. The methyl fragment dehydrogenation rate slows as the copper coverage increases. At a narrow copper coverage range between 0.8 and 0.95 ML, adsorbed hydrogen and methyl fragments coexist on the surface in the temperature range 230-280 K. Sequential decomposition channels of the parent molecules and the methyl fragment lead to a unique enhancement of methane production rate, this on the account of further hydrocarbon dehydrogenation, as reflected in both ∆p and ∆ TPD measurements. Methane is formed on top of copper terraces as a result of "spill-over" of both methyl and hydrogen atoms, similar to the chemistry over Cu(111) and Cu(110) single-crystal surfaces. The dipole moment of adsorbed methyl is reported here for the first time on metal surfaces, being 0.48 D on top of Cu(2 ML)/Ru(001).

Photoinduced Fragmentation of Multilayer CH3Br on Cu/Ru(001)

“…Briefly, the saturation coverage of the CH 3 Br relative to the number of ruthenium atoms was determined to be 0.22 ( 0.02 for both surfaces 18 with the monolayer coverage (1 ML) consisting of 3.6 ( 0.3 × 10 14 and 4.0 ( 0.3 × 10 14 molecules/cm 2 , respectively. The density of the crystalline CH 3 Br in the (001) plane is 6.96 × 10 14 molecules/cm 2 , as obtained from X-ray studies.…”

“…We note that our interpretation of a monolayer is different from other studies [2][3][4]7 for which the definition of 1ML is correlated with a definition of a bilayer (2ML) in the present study. 18 4 desorption is observed, associated with parent molecules dissociation at defects on the copper layer that is subsequently followed by bimolecular reaction with adsorbed hydrogen to form methane. Cleavage of the C-Br bond is the result of the UV irradiation, producing "hot" CH 3 radicals.…”

“…Destabilization of the third layer and then a more stable fourth and thicker layers on the clean Ru(001) were discussed in terms of bulklike molecular crystalline structure formation. 18 It turns out that on the copper covered surface destabilization of the third layer is not observed. Based on work function change measurements taken during adsorption, we concluded that, unlike the clean Ru(001) surface, on the Cu(2ML)/Ru(001) surface, CH 3 Br molecules do not form the molecular crystalline-like structure based on antiparallel arrangement.…”

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

“…The overall change in ∆Φ at the end of the ∆Φ-TPD run (750 K) relative to that at the beginning (82 K) is +0.1 V. This indicates that the carbon atoms, probably in the form of carbides, 10,13 contribute positively to the surface work function. Increasing the ethylene exposure to 0.17 L (b) a different ∆Φ-TPD profile above 400 K is observed: an increase of ∆Φ up to 560 K followed by a monotonic decrease.…”

“…Structural rearrangements were observed prior to and during desorption in the case of nonreactive systems (∆Φ-TPD) 11 and decomposition of intermediates in reactive systems (∆Φ-TPR). 12,13 It is known that ethylene rehybridized to a di-σ bonded structure upon adsorption on the Ru (001) surface at 80 K. 4 At a coverage of θ ) C 2 H 4 /Ru (001) ) 0.30 (defined as 1 ML), 80% of the di-σ-bonded ethylene dissociates within the range 150-280 K. The other (20%) desorbs molecularly up to 250 K. 4 Although it is agreed that at higher temperatures ethylene (C 2 H 4 ) irreversibly converts to ethylidine (CCH 3 ), the actual conversion pathway remained controversial. According to Hills et al 4 ethylidine is directly produced from di-σ-bonded ethylene but not as a sole product; simultaneously the production of acetylide (CCH) takes place as a competing channel with a probability of 0.3 (compared with 0.5 for the ethylidine route).…”