Spectroscopy and Anion Adsorption

The adsorption of anions on solid surfaces is of considerable interest, mainly because of its effect on the kinetics of electrochemical reactions. Several in-situ techniques have been applied toward this purpose. Infrared measurements were used to identify adsorbed species, estimate anion adsorption isotherms, and to gain information on anion interaction with electrode surfaces. Sulfuric acid anions are possibly the conunonest anion adsorbates because of their specific adsorption on metal surfaces. Depending on the metal, its surface orientation, and the concentration of anion, either sulfate or bisulfate can be specifically adsorbed on the surface. Identifying the predominant adsorbate on platinum-group metals has engendered some controversy. While STM studies show that 

Recent data confirmed that the preferred species on Pt (111), Pd and Ir is bisulfatc, while sulfate adsorbs on Ag(l 1 l) and on Au(lll). Apparently the sd metals (Rh, Pt, Pd) with (111) orientation adsorb bisulfate, whereas the sp metals (Cu, Ag, Au) adsorb sulfate. The IR study on Ru(OOOl) seemed to support this conclusion (see below), as the latter has the same orientation of surface atoms as the fee metals of (111) orientation, and the adsorbed species is bisulfate. 

As discussed in the previous Sections, electrochemical oxidation of polyerystalline Ru involves about one order-of-magnitude larger currents than that of Ru(OOOl), starting as early as 0.2 V. Polyerystalline Ru is covered with hydroxyl ions from water very early in the potential scale even in acidic solutions, thus blocking the surface from anion adsorption by the supporting electrolyte. This conclusion is supported by the in situ IR spectra presented below. 

Specific adsorption of sulfate-bisulfate generally displays spectral features that are blue shifted (i.e., to higher frequencies) with higher electrode potentials, as observed for both polyciystal-line and the Pt(lll) surface. The shift is explained in 

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