Co-adsorption of anions

The electrosorption valency usually increases as the underpotential decreases to approach the ionic charge (total discharge of the cation) close to the Nernst potential, for instance in the case of lead and thallium upd on silver [114]. However, the co-adsorption of anions may contribute to the observed apparent electrosorption valence, as rotating ring disc electrode (RDE) experiments have shown [113]. 

In the closely packed (1 X 1) Cu structure (Figure 4.33c) the copper atoms are obviously discharged and no co-adsorption of anions can be detected. The strain of an epitaxial layer... 

In order to calculate the effect of the co-adsorption of anions, Eqs. (28)-(31) should be modified as follows. First, we should use in Eq. (27) the electrochemical potential of the anions B instead of the chemical potential. The electrochemical potential of an inorganic ion B is given by... 

Stamenkovic V, Arenz M, Lucas C, Gallagher M, Ross PN, Markovic NM. 2003. Surface chemistry on bimetallic alloy surfaces adsorption of anions and oxidation of CO on Pt3Sn(lll). J Am Chem Soc 125 2736-2745. 

From gas phase measurements CO is known to prefer top sites on all three low index faces, with the CO molecule perpendicular to the surface and bonded through the carbon end of the molecule except at high coverages (27). It is likely that HCOOH and COOH are adsorbed in a similar way. It is not likely that they could "enter the "troughs , which seems to be possible for anions. For Pt(100) on the other hand, upon sweep reversal and gradual oxide reduction, anions are immediately adsorbed on that "flat" surface. They block adsorption of HCOOH. Adsorption of anions decreases as potential becomes more negative. The oxidation of HCOOH commences and the rate increases as at more negative potentials, i.e. at lower overpotential. A competition between anions and HCOOH adsorption explains this apparently anomalous behaviour. The explanation of the "anomalous behaviour of the Pt(110) surface can be also found in the data for stepped surface vicinal to the (100) and (110) orientations. 

Gold is one of the least reactive metals in bulk form. However, in recent years a considerable amount of theoretical and experimental works have studied the reactivity of small neutral and charged Au clusters towards different molecules, like H2, O2, CO, and organic radicals " . The reactivity depends on the size and charge state of the cluster. In the previous section we have studied the reactivity towards oxygen adsorption of anionic silver and gold clusters. In this section we study the reactivity of neutral gold clusters towards molecular O2 (subsection 6.1) and CO (subsection 6.2). 

Polivka had investigated the co-adsorption of carotenoid and pheophytin (111) on the surface of TiC>2 electrode and the photophysical properties of pheophytin in this film. The results demonstrated that the fluorescence of 111 was efficiently reductive quenched by carotenoid in this co-assembled film, suggesting similar mechanisms to that in the natural photosynthetic systems. The radical anion of 111 formed during the electron transfer recovered to the neutral state quickly before the charge recombination between carotenoid cation and pheophytin anion took place. It is suspected that the electron injection from the pheophytin anion to the conduction band of Ti02 was responsible for this quick recovery. This result indicated that such a self-assembling strategy may be also considered for novel DSSC constructions [108]. 

Typically, the rate of simple (outer-sphere) electron-transfer reactions, such as Fe(CN)e + e - Fe(CN)6 , is much slower at titanium dioxide than at metallic electrodes . This is consistent both with the flat shape of the voltammetric peaks in Fig. 2 and their shift to more negative potentials with increasing the sweep rate. The kinetics of the cathodic reactions at Ti02 appear to be markedly affected by the co-adsorption of some anions from the supporting electrolyte. The phosphate ions are not unique to cause such effects arsenate, fluoride and certainly other anions are expected to act in a similar way. 

The effect of anions was pronounced both on Au and on Ag. The fact that r (CO) on Au and Ag increases when halogen ions are added implies that the amount of adsorbed hydrogen may be decreased by a specific adsorption of anions to the metal electrodes. This may be the reason why t] CO) on Au and Ag depends on anions. 

Silver deposition is important for technical applications and is achieved by electroless deposition (from a silver-cyanide bath) or bulk electrodeposition from the same bath. It has been studied by a large number of techniques [59,62,63,87], and most of them on gold and platinum in sulfate or perchlorate electrolytes [88-95]. However, there is lack of information on the problem of co-adsorption of metal and anions and the surface restructuring caused by the presence of strong adsorbable anions such as halides. 

The breakthrough curves of Fig. 9.7 summarize the net effects of repulsion and specific adsorption on the relative adsorption of anions by soils. Solutions containing the anions at initial concentration Co were added to soil columns. The effluent concentration is C. The volume of water is expanded as pore volumes added to the... 

The adsorption of anions such as halides, cyanide, and sulfate/bisulfate on electrode surfaces is currently one of the most important subjects in electrochemistry [1 - 3]. It is well known that various electrochemical surface processes such as underpotential deposition of hydrogen and metal ions are strongly affected by co-adsorbed anions. Particularly, structures of the iodine adlayers on Pt, Rh, Pd, Au, and Ag surfaces have... 

In this section, we review the use of MC simulations to understand the complex reaction kinetics of CO electrooxidation near Pt(lll) surfaces in sulfuric acid solutions. We address the role of specific adsorption of anions on electrode surfaces, and its effect on CO electrooxidation. We also review some results of CO oxidation in alloy surfaces [55]. This section Is Arranged as follows. In Section 18. 3.1, we provide a brief background onanion adsorption and CO oxidation. Section 18. 3.2 outlines themodel used. In Section 18. 3.3, we briefly outline the KMC methodology. In Section 18. 3.4 we review the effect of competitive adsorption on base voltammograms (i.e. under CO-free conditions) and CO electrooxidation. We also briefly review CO diffusion effects and CO electrooxidation on PtRu alloys [55]. 

Two hmiting cases of the co-adsorption are of particular interest One is the reorientation of a solute on the electrode surface and another the effect of the specific adsorption of anions on the adsorption features of a neutral solute. 

Specific adsorption of anions must be accompanied by a sharp drop in the hydrogen overvoltage. This deduction from theory was for the first time tested by lofa, Kabanov, and co-workers and repeatedly confirmed later (see Figure 17). The value of r turns out to be extremely sensitive to anion adsorption a decrease in r is perceptible at such potentials at which the anion adsorption is not yet seen on the electrocapillary curves. 

In an early work by Petzold and Lunkwitz [9], this efficiency of recharging of the fibres using cationic complexes of poly(diallyldimethylammonium chloride), PDADMAC, and poly(maleic acid-co-a-methylstyrene), MS-a-MeSty, was used to flocculate cellulose fibres, but the actual adsorption of the complexes was not measured. The adsorption of anionic complexes of polyethyleneimine (PEI) and CMC on fibres pretreated with a cationic PDMDAAC has also been studied by Hubbe et al. [24]. These authors found that when the charge of the complexes was decreased there was an increase in adsorption, indicative of an electrosorption process, but the authors also detected signs of nonionic interaction although they were not able to establish the molecular reason for this behaviour. 

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