Formation of Adlayers

The quantity ruling the adsorption-site preference and the formation of ordered adlayers is the adatom binding energy  

Here (pi are the Kohn-Sham orbitals and f a is a properly chosen localized function, in the present case most suitably the Ru 4d and the O 2p atomic orbitals within a sphere around given atomic sites. According to the Anderson-Grimley-Newns model of chemisorption, the interaction of the O 2p level with the narrow 4d band of the TM surface gives rise to bonding states at the lower edge of the d band and antibonding states at the upper 

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Au atoms is preferred, but as discussed above, the mechanisms leading to this final state might be less favorable than those leading to alternative configurations, as shown in Fig. 9 (i.e., the highly, energetically favored formation of adlayer dimers over insertion in the surface plane at elevated temperatures). 

Furthermore, the interactions with other species in the solution can also stabilize or destabilize the molecular adsorption at solid-hquid interfaces. This includes the interaction of vrater with hydrophilic and hydrophobic groups of the molecules, which may promote the aggregation of hydrophobic groups at the interface via formation of adlayers or... 

As previously noted the constancy of catalyst potential UWr during the formation of the Pt-(12xl2)-Na adlayer, followed by a rapid decrease in catalyst potential and work function when more Na is forced to adsorb on the surface, (Fig. 5.54) is thermodynamically consistent with the formation of an ordered layer whose chemical potential is independent of coverage. 

It should be clear that, as well known from the surface science literature (Chapter 2) and from the XPS studies of Lambert and coworkers with Pt/(3"-A1203 (section 5.8), the Na adatoms on the Pt surface have a strong cationic character, Nas+-5+, where 5+ is coverage dependent but can reach values up to unity. This is particularly true in presence of other coadsorbates, such as O, H20, C02 or NO, leading to formation of surface sodium oxides, hydroxides, carbonates or nitrates, which may form ordered adlattices as discussed in that section. What is important to remember is that the work function change induced by such adlayers is, regardless of the exact nature of the counter ion, dominated by the large ( 5D) dipole moment of the, predominantly cationic, Na adatom. 

Further studies were carried out on the Pd/Mo(l 1 0), Pd/Ru(0001), and Cu/Mo(l 10) systems. The shifts in core-level binding energies indicate that adatoms in a monolayer of Cu or Pd are electronically perturbed with respect to surface atoms of Cu(lOO) or Pd(lOO). By comparing these results with those previously presented in the literature for adlayers of Pd or Cu, a simple theory is developed that explains the nature of electron donor-electron acceptor interactions in metal overlayer formation of surface metal-metal bonds leads to a gain in electrons by the element initially having the larger fraction of empty states in its valence band. This behavior indicates that the electro-negativities of the surface atoms are substantially different from those of the bulk [65]. 

Only at saturation coverage is there a certain degree of irreversibility, which is reflected in A p = Egn cai 45 mV. A new oxidation process is observed at 1.01V that leads to progressive dissolution of the adsorbed adlayer. This process has been tentatively ascribed to the formation of soluble Te(VI) species. The corresponding reduction process is observed around 0.9 V. 

It is observed that higher potential values for the adatom redox process are correlated with a lower energy of the M—O bond, i.e., lower (less negative) enthalpy of formation of the adatom oxygenated species. In this regard, the discrepant behavior of Ge-Pt(lOO) may be related to the dilute nature of this adlayer, with a maximum coverage of only 0.25. 

CO forms intermixed adlayers with most of the p-block adatoms. CO oxidation from mixed adlayers with Bi [Chang and Weaver, 1991 Herrero et al., 1995a, d]. As [Herrero et al., 1995d], Sb [Kizhakevariam and Weaver, 1994], Se [Herrero et al., 1996], and Te [Herrero et al., 1996] on Pt(l 11) has been studied. The formation of... 

In the anodic scan, the oxidation of the H adlayer formed below 0.1 V and the re-formation of OHad/Oad (both in peak A) are shifted to markedly higher potentials compared with the Oad/OHad removal and Hupd formation (peak A ) in the cathodic scan (Fig. 14.2b). Furthermore, it overlaps with the peak B (OHad oxidation) observed for a cathodic scan limit of 0.1 V. At low scan rates, peak A starts at 0.1-0.15 V and reaches up to 0.48 V. Hence, compared with a scan with a cathodic limit of > 0.1 V, the equilibration of the Oad/OHad adlayer is shifted from 0.28 to 0.48 V. The charge in peak A integrated in the range 0.1-0.48 V corresponds to 1.5 e per surface atom, which is equal to the sum of the charges in peaks B and A in the negative-going scan. 

Figure 14.7 Illustration of the formation, removal, and exchange of adlayers on Ru(OOOl) in the presence of Pt islands/sites as observed in the peaks A/A, B/B, and C/C (see also Figs. 14.2 and 14.8). Processes in the anodic/cathodic potential scan direction are shown in the upper/lower part for simplicity, is used instead of H30. ...

Based on electrochemical experiments combined with ex situ analysis by AES, LEED, and RHEED, Wang et al. (2001) suggested the formation of a (2 x 2) (2CO + O) adlayer on Ru(OOOl) at = 0.2 V in CO-samrated HCIO4, similar to the phase formed in UHV after CO adsorption on a (2 x 2)0-covered surface [Schiffer et al., 1997]. Erom the total charge density transferred after a potential step to 1.05 V in a CO-free electrolyte, they concluded that only 60% of the CO content in such an adlayer can be oxidized under these conditions [Wang et al., 2001]. 

Figure 14.11 Dlustration of CO electro-oxidation at Pt-modified Ru(OOOl) (a) mixed, non-leactive adlayer (b) Pt-assisted formation of OH d at high local adsorbate coverages on the Ru areas (c) CO oxidation at the Pt islands. For simplicity, H is used instead of H30. ...

Figure 4.3 Atomically resolved STM image (1.5 x 1.5 nm) of a clean Cu(110) surface (a) before and (b) after the formation of a fully developed (2 x 1) oxygen adlayer at room temperature. (Reproduced from Ref. 10).

That chemisorbed oxygen was active in hydrogen abstraction, resulting in water desorption and the formation of chemisorbed sulfur, was first established by XPS at copper and lead surfaces.42 An STM study of the structural changes when a Cu(110)-O adlayer is exposed (30 L) to hydrogen sulfide at 290 K indicates the formation of c(2 x 2)S strings. 

Figure 6.2 VEEL spectra when a mixture of CO and 02 was coadsorbed at a Cu(l 10)-Cs surface (ctCs — 3.5 x 1014 cm-2) at 80 K and the adlayer warmed to 298 K. Note the formation of surface carbonate (cf. Figure 6.9). (Reproduced from Ref. 6).

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