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a slightly decreasing trend with coverage up to one molecule per two metal surface atoms. At greater coverages the heat of adsorption decreases fairly abruptly. This qualitative trend is found for the indirect mechanism computed in Figures 3 and 4. We note that the heat of adsorption decreases relatively faster with coverage as the symmetry of the coordination site increases. This suggests that when the low-coverage heat of adsorption of the on-top site is close to that of the bridge or hollow site, the on-top site will become preferred at higher coverage. This effect has been found in some experimental systems, including CO/Pd cluster^,'^ where a bridge to end-on conversion is observed, and CO/Pd(lOO)ls” and CO/Pd(lll),lsbwhere end-on bonded molecules are observed in a mixed-site overlayer at I9 > 0.5. Thus the indirect-interaction mechanism may be responsible for this effect. Not all experimental data conform to this picture, with CO/Pt(lll) showing partial bridge occupancy at higher coverage I9 L 0.5 compared to only on-top occupancy at low coverage.17b Complex mixed-site overlayers have been determined a t high coverages of CO on various metal surfa~es.’~J~ These involve coincidence lattices having occupied bridge and on-top adsorption sites. Some of these structures were examined in Figure 4. For CO at (2 X 2 ) I9 = 0.75 on Rh(lll), a structure involving one bridge molecule and two near-on-top molecules per unit cell has been determined by LEED.lQ Figure 4a considers this structure for the fcc (111)surface, where the idealized on-top site is considered. This arrangement is predicted to be more stable than either all on-top or all bridge molecules, as shown in Figure 4a. This theoretical result is in accord with experiment. The coincidence lattices examined for all on-top c ( 7 4 2 X 4 2 ) and observed experimentally for CO/Cu( or mixed bridge/on-top 4 5 4 2 X 4 2 ) and p ( 3 d 2 X 4 2 ) and (15) For a review, see: Yates, J. T., Jr.; Madey, T. E.; Campuzano, J. C. ‘The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis”; King, D. A., Woodruff, D. P., Eds.; Elsevier: Amsterdam, in press. (16)Grunze, M.; Dowben, P. A. Surf. Sci. 1984,141,455. (17)(a) Gelin, P.; Siedle, A. R.; Yates, J. T., Jr. J. Phys. Chern. 1984, 88,2978. (b) Gelin, P.;Yates, J. T., Jr. Surf. Sci. 1984,136, 21. (18)(a) Biberian, J. P.; Van Hove, M. A. Surf. Sci. 1982,118,443; (b) Ibid. 1984,138, 361. (19)Van Hove, M.A.; Koestner, R. J.; Frost, J. C.; Somorjai, G. A. Surf. Sci. 1983,129, 482.
observed experimentally for CO/Pd(100)15are quite stable relative to simple overlayer structures. The c ( 5 4 2 X d2) structure with two on-top and one bridge molecules per unit cell is computed to be more stable than the reverse species. The effect of increasing coverage is to inhibit the dissociation of diatomic molecules, as described earlier in connection with Figures 5 and 6 . This behavior is a manifestation of the ensemble effect discussed in catalysis studies on alloy surfaces. Ensembles of four or more vacant Ru metal atoms in Ru/Cu catalysts have been deduced for carbon monoxide hydrogenation.20 Monte Carlo analyses21of experimental data for O2 dissociative adsorption on Ni(100) have shown that 10 metal atoms are required as a site for dissociation. This result is in line with the computations in Figures 5 and 6.
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Conclusions Computations by a bond-order-conservation method have shown that coverage effects on the heat of adsorption and dissociation barriers can be largely explained by an indirect through-metal interaction. This effect is a result of repulsive interactions between saturated species. Surface metal atoms have their unsaturated valence filled by interacting with adspecies. These metal atoms interact with adjacent adspecies in a repulsive manner. Thus, as a surface becomes filed, the heat of adsorption is decreased by the indirect interactions. In accord with earlier models and computations,3the direct lateral interactions between adspecies allow formation of ordered islands at low coverage. The activation barrier to dissociation is strongly dependent upon the presence of an ensemble of six-eight metal atoms. Dissociation will not be possible when these ensembles are not present on a metal surface. Thus it may well be that at high pressures, corresponding to full surface coverage, the barrier to dissociation will be the energy of formation of six-eight metal-atom ensembles. Acknowledgment. I am grateful to Evgeny Shustorovich for helpful discussions on this paper. (20)Bond, G. C.; Turnham, B. D. J. Catal. 1976,45,128. (21)Brundle, C.R.;Behm, J.; Barker, J. A. J . Vac. Sci. Technol., A 1984,2,1038.
Experimental Observations of the Rate of Ascent of Bubbles in Lubricating Oils Gaudenz Furler and Sydney Ross* Department of Chemistry, Rensselaer Polytechnic Institute, Troy, New York 12181 Received June 24, 1985
The rate of ascent of a single bubble through an oil may be used to reveal whether a surface-active constituent is present therein, which is often a condition not otherwise readily detected in nonaqueous solutions. A hydrodynamic effect of adsorbed solute is to “rigidify”the interface of the rising bubble, which notably reduces its rate of ascent, thus betraying the presence of surface activity. The application of the test is illustrated with a number of synthetic lubricating oils both with and without additives. The results obtained with most of these oils confirm the theoretical calculations of drag coefficients of drops or bubbles with fluid interfaces. One additive widely used as an antioxidant was found to convert the behavior of a fluid interface to that of an interface effectively rigid under the dynamic conditions of the experiment, thus disclosing its surface activity in oil solution.
Introduction The presence of a surface-active solute in a lubricating oil may cause problems of lubrication due to emulsified 0743-7463/86/2402-0068$01.50/0
water, air entrapment, or foam. Such striking effects, when they occur, clearly imply a condition of surface activity as their necessary precursor, but surface activity in a solution 0 1986 American Chemical Society
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does not always manifest itself by conspicuous phenomena: it may be subtly present. An awareness of the condition is desirable in order to guard against the development of problems, which may arise either by further buildup of the concentration of the surface-active solute or by its synergistic combination with additives. The detection of the condition, in the absence of the clear evidence afforded by foaming or emulsification, is not as straightforward as might appear. Lubricating oils, like most organic liquids, have low surface tensions (a
