Adsorbate separation, island formation

The frequent occurrence of ordered fractional-coverage adsorption indicates that adsorbate-adsorbate interactions at close range (S 5 A) are often repulsive. Island formation can occur simultaneously, showing that at larger separations these interactions can become attractive. 

Phase separation or, more broadly, island formation in HCR is possible due to attractive adsorbate-adsorbate lateral interactions (thermodynamic mechanism)... 

The adsorbate-adsorbate interactions may be repulsive they always are repulsive at sufficiently small adsorbate-adsorbate separations. At larger separations they may be attractive, giving rise to the possibility of island formation. They may also be oscillatory, moving back and forth between attractive and repulsive as a function of adsorbate-adsorbate separation, with a period of several angstroms giving rise, for example, to non-close-packed islands. 

With 0CO > 1/3 (i.e., for coverages beyond the completion of the y/3 x y/3 R 30° structure) a Pd(lll) surface is no longer able to dissociatively adsorb oxygen. Since this is a necessary prerequisite for C02 formation, the reaction is inhibited by CO if its coverage is too high. At lower CO concentrations on the surface oxygen can be co-adsorbed. Both components then form separate domains on the surface [competitive adsorption (182)] as becomes evident from LEED observations (172). The mean domain diameter is at least of the order of 100 A i.e., the coherence width of the electrons used with this technique. This indicates the existence of repulsive interactions between Oad and COad. As can be seen from the schematic sketch of Fig. 32b, eventual product formation can then only occur along the boundaries of these islands. 

Conrad et al. (S2) studied in detail the mutual interaction of coadsorbed O and CO on a Pd(l 11) surface. Some of their relevant results are summarized here. Oxygen adsorption is inhibited by preadsorbed CO. At coverages below Oco 1/3, LEED patterns show that O and CO form separate surface domains. However, the behavior is different when O is preadsorbed. CO can be adsorbed on the Pd(lll) surface covered with O which is less densely packed than a saturated CO layer. The O adatom islands are then suppressed to domains of a (v 3 x y/l)R30° structure (0 = 1/3), with a much larger local coverage than can be reached with O alone, which orders in a (2 x 2) structure (ff = 0.25). After further exposure, the LEED patterns s uggest the formation of mixed phases of Oads and CO ads (with local coverages of ffo = Oco = 0.5) which are embedded in CO domains. When these mixed phases are present, CO2 is produced even at temperature lower than room temperature. Coadsorption studies of other noble metal surfaces are consistent with this scenario preadsorbed CO inhibits the dissociative adsorption of oxygen, whereas CO is adsorbed on a surface covered with O. 

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