Work Function Study of the Adsorption, Lateral Repulsion, and Fragmentation of CH₃Br on Ru(001)

Abstract

The chemistry of methyl bromide on Ru(001) has been studied utilizing work function change (Δφ) measurements and temperature-programmed desorption (TPD) in the crystal temperature range of 82−1350 K. Employing a Δφ-TPD mode, chemical changes in the adsorbed state could be detected at temperatures below the onset for desorption. A decrease in work function of 2.15 ± 0.02 V has been measured at the completion of a monolayer coverage, which has been determined to consist of (3.6 ± 0.3) × 1014 molecules/cm2, equivalent to CH3Br/Ru = 0.22 ± 0.02. The onset for C−Br bond cleavage near 125 K was observed. 50% of the initial 1 monolayer methyl bromide molecules decompose to adsorbed methyl and bromine. A low-temperature increase in work function was found to precede dissociation or desorption as coverage increases. This change in work function is discussed in terms of several possible mechanisms, including multiple sites population, molecular rearrangement, and tilt angle, that change with coverage and surface temperature. A thermally activated flipping mechanism in which a fraction of the adsorbate molecules rearrange to adsorb with the methyl group facing the surface is found to be most consistent with the observed results. Sequential dehydrogenation of the methyl fragments, competing with minor methane production at higher coverages, was directly observed by employing the differential work function measurements. The corresponding surface temperature window for each of these decomposition steps has been determined, and the detailed reaction mechanism is discussed. Bromine atoms on Ru(001) were found to decrease the work function by 320 mV at a coverage Br/Ru = 0.3, indicating a complex charge redistribution upon adsorption. Deuterium preadsorption, which significantly passivates the surface, has been employed to better understand the various reactivity steps of the hydrocarbon fragments. Finally, work function measurements indicate the presence of strong interactions of the methyl bromide molecules with the metal surface up to the third layer. Alternating contributions to the work function of the first three layers are observed. This is understood in terms of an opposite adsorption geometry in which bromine faces the surface in the first layer, methyl in the second, and bromine again in the third. Upon heating, the third and fourth layers rearrange in a bilayer-like structure before the completion of the fourth layer, leading to higher stability of the combined two layers compared with the third alone. This structure is rather similar to that of methyl bromide in its molecular crystal.

Adsorption and reactions of CH2I2 on Ru(001) surface

“…Adsorbed iodine occupies one third of surface Ru atoms at saturation of a multilayer. The value of T p is nearly the same as determined for Rh(111) (T p =205 K ) and [34]. …”

“…5). In addithe surface normal [34]. The more molecules are on the surface, the larger is the tilting angle, with tion, a new H 2 peak developed with T p =300 K at higher surface concentration of CH 2 I 2 , very likely the results that (i) bonding I atoms are not equivalent and the CH 2 I 2 molecule is in a stretched due to the influence of adsorbed iodine.…”

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

“…Sample cleaning in UHV followed standard procedures as described elsewhere. 3,4,12 The surface was sputter-cleaned prior to each experiment in order to remove carbon impurities. Surface cleanliness was checked by measuring the CO and O 2 thermal desorption profiles following the oxygen cleaning procedure.…”

“…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).…”

Dipole–dipole interactions among CH3Cl molecules on Ru(001): Correlation between work function change and thermal desorption studies

“…Under UHV conditions the reactivity toward C-X bond cleavage follows the trend IϾBrϾCl on several catalytic metal surfaces. On Ru͑001͒ CH 3 I was found to completely dissociate upon adsorption at 100 K, 3 and CH 3 Br to partly ͑55%͒ dissociate above 125 K. 4 In contrast, CH 3 Cl does not dissociate on Ru͑001͒, Pt͑111͒, 5 or Pd͑100͒ 6,7 surfaces.…”