Ordered Promoter Adlattices and Electrochemical Promotion

The electrochemically induced creation of the Pt(lll)-(12xl2)-Na adlayer, manifest by STM at low Na coverages, is strongly corroborated by the corresponding catalyst potential Uwr and work function O response to galvanostatic transients in electrochemical promotion experiments utilizing polycrystalline Pt films exposed to air and deposited on (T -AbCb. 3637 Early exploratory STM studies had shown that the surface of these films is largely composed of low Miller index Pt(lll) planes.5 

It is worth noting that for both systems the observed AUWr value corresponding to the onset of the formation of the ordered Na adlattice is practically the same, which strongly supports the idea that this AUwr value is characteristic of the chemical potential of this structure. The fact that a small but not negligible Na coverage (0ga 0.015) preceeds the formation of the ordered Na structure on the surface of polycrystalline Pt samples (Fig. 5.54) may indicate preferential Na adsorption on stepped surfaces before Na adsorption on Pt(lll) starts taking place. 

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. 

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