Adlayer ordering

Fig. 19 shows an example of the orientational density profile evaluated for the system 10. Most of the particles of the first adlayer assume two limiting configurations parallel to the surface and vertical to the surface. The second adlayer exhibits also well pronounced orientational ordering ... 

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. 

Iodine and bromine adsorb onto Au(l 11) from sodium iodide or sodium bromide solutions under an applied surface potential with the surface structure formed being dependent on the applied potential [166]. The iodine adsorbate can also affect gold step edge mobility and diffusion of the Au surface. Upon deposition of a layer of disordered surface iodine atoms, the movement of gold atoms (assisted by the 2-dimensional iodine gas on the terrace) from step edges out onto terraces occurs. However, this diffusion occurs only at the step edge when an ordered adlayer is formed [167]. 

Magnussen, O.M. (2002) Ordered Anion Adlayers on Metal Electrode Surfaces. Chemical Reviews, 102, 679-725. 

Magnussen OM. 2002. Ordered anion adlayers on metal electrode surfaces. Chem Rev 102 679-725. 

Chang S-C, Weaver MJ. 1991. Influence of coadsorbed bismuth and copper on carbon monoxide adlayer stmctures at ordered low-index platinum-aqueous interfaces. Surf Sci 241 11-24. 

The situation becomes more complicated by the fact that the incompletely oxidized side products are not only reactive, but may also affect the activity of the catalyst for the main reaction, for example by modifying the composition of the steady-state adlayer. This may be particularly important in the later part of the reactor, where the concentration of these side products will be higher than at the reactor inlet. Asa result, the reactions of the different reactive components in the electrolyte can no longer be considered as independent. Therefore, in order to properly describe the accumulation and further oxidation of the incompletely oxidized reaction products along the reaction cell, measurements using representative mixtures of the three Ci molecules are required. 

On warming to 300 K, the adlayer undergoes a disorder-order transition the Os states present at 120 K, together with the surface copper atoms, are highly mobile and can be considered to resemble a two-dimensional gas which at 300 K transforms into a structurally well-ordered immobile oxide adlayer.22 This is very similar to the model proposed from spectroscopic (XPS) studies and based on chemical reactivity evidence (see Chapter 2). 

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