Ruthenium surfaces

Zero-order desorption occurs if the rate of desorption does not depend on the adsorption coverage, as seen with relatively large silver islands on a ruthenium surface (Fig. 7.7), where the Ag atoms desorb from the edges of the island. As the 0" term in Eq. (12) vanishes, the curves exhibit a clearly recognizable exponential shape on the leading side. Such situations are rare. 

Figure 7.8. The compensation effect in the desorption ofAg from a ruthenium surface activation energy and pre-exponential factor depend in the same way on coverage. The...

In the following we consider nitrogen atoms adsorbed on a ruthenium surface that is not completely flat but has an atomic step for each one hundred terrace atoms in a specific direction. The nitrogen atoms bond stronger to the steps than to the terrace sites by 20 kj mok. The vibrational contributions of the adsorbed atoms can be assumed to be equal for the two types of sites. (Is that a good assumption ) Determine how the coverage of the step sites varies with terrace coverage. 

Waszczuk P, Lu GU, Wieckowski A, Lu C, Rice C, Masel MI. 2002. UHV and electrochemical studies of CO and methanol adsorbed at platinum/ruthenium surfaces, and reference to fuel cell catalysis. Electrochim Acta 47 3637-3652. 

Nitric oxide is dissociatively chemisorbed at Ru(0001) at 295 K, with Zambelli et al.n establishing the role of a surface step in the dynamics of the dissociation process. Figure 8.3 shows an STM image taken 30min after exposure of the ruthenium surface to nitric oxide at 315 K. There is clearly a preponderance of dark features concentrated around the atomic step (black strip), which are disordered nitrogen adatoms, while the islands of black dots further away... 

The variations in syngas conversion and C +C2 selectively could be due to the difference in ruthenium surface areas as a result of different preparations. 

The top spectrum of Fig. 3.22 corresponds to a complete monolayer of Xe on the sample. It can be used for a quantitative titration of sites. The area under the two peaks indicates that about 27% of the Xe in the monolayer is adsorbed on silver, indicating that 27% of the ruthenium surface is covered by silver islands. As a peak at 7.25 eV, which would be characteristic of Ag-Ru boundary sites, is not observed, the results suggest that Ag is present in fairly large islands. 

Fig. 13.15 Computer-generated view of the heme group and four ruthenium surface histidines in sperm-whale myoglobin. Closest heme-oj-Ru(His) edge-to-edge distances are 14.6 (His48), 19.1 (His8l). 20.1 (Hi ll6). and 22.1 A (Hisl2). (From Mayo.

Mossbauer studies of the impregnation of silica with ruthenium chloride solution and subsequently dried at 383 K have reported (59) the presence of a ruthenium surface complex resembling RuC13 xH20. Recent work (128) has shown that Mossbauer spectra of "Ru supported on alumina, silica, activated charcoal, and X- and Y-zeolite are sensitive to the nature of the preparation and treatment of the samples. 

FIGURE 7 A comparison of the energies of CO dissociation on a stepped and a nonstepped ruthenium surface. 

A result similar to that found for ruthenium was found for the Co(OOOl) terrace. Inderwildi et al. (62) reported an activation energy for formyl formation quite similar to that foimd for the ruthenium surface. Moreover, the results reported by Inderwildi et al. indicate a significantly lower barrier for the cleavage of the C—O bond in formaldehyde than for that in the CO molecule. 

On nickel (a metal with a low reactivity for CO activation), hydrogen-assisted CO dissociation is always the most favorable pafhway, independent of the nature of fhe surface site. On ruthenium, two scenarios must be differentiated. On stepped ruthenium surfaces, direcf CO dissociation is the reaction path with the lowest energy barrier, whereas on ruthenium terraces, hydrogen-assisted C—O cleavage is again favored over direct dissociation. 

Figure 12 shows computed activation energies and relative energies of reaction intermediates for "Ci hydrogenation. Two different ruthenium surfaces are considered. It is important to realize that the relative stabilities of the various CH species on the two surfaces differ considerably. These differences in stability are crucial because they determine the relative concentrations of the CH species, which in turn control to a significant extent the relative rates of the CH —CHj, recombination reactions. 

On ruthenium surfaces, a growth of hydrocarbon chains up to a length of 30 carbon atoms could be observed [115]. The results obtained by electron... 

SFG spectra of CO adsorbed on nickel have been reported 116,118,416,417), as have spectra characterizing NH3 adsorption/dissociation on Fe(l 1 1) 418). UHV SFG investigations of formic acid decomposition on NiO(l 1 1) were also reported 419,420). Investigations of ruthenium surfaces 147,148,157,421-425) and of CO adsorbed on Ir(l 1 1) are also available 426). 

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