Adsorption anion

Adsorption of simple inorganic anions, oxyanions and organic ions on iron oxides has been widely investigated (see Tables 11.1 and 11.2). Anions are ligands, i.e. they possess one or more atoms with a lone pair of electrons and so can function as the donor in a coordinate bond. Ligands may adsorb on Fe oxides either specifically or non specifically. 

Specific adsorption involves replacement of the surface hydroxyl groups by the adsorbing ligand, L, i. e. 

1987 12) Barron et al., 1988 13) Beckwith Reeve, 1963 14) Bibak Borggaard, 1994  

1998 78) Sun Donor, 1996 79) Sugimoto Wang, 1998 80) Tejedor-Tejedor Anderson, 1990  

It is also termed chemisorption (especially for gases), inner sphere adsorption and, in the case of ligands, ligand exchange. The binding constants, Kf and for the surface complexes show the same stability trend as do the constants for the equivalent complexation reactions in solution. 

Adsorption of chloroauric anions on oxide support is in principle possible, provided that the point of zero charge (PZC) of the oxide support is higher than the pH of the solution, that is the support surface is positively charged (see Table 17.1). The pH conditions are roughly the same as for impregnation with HAuCh or at slightly higher pH since usually the solutions are less concentrated. 

Whether impregnation or anion adsorption was carried out, the gold complexes deposited on oxides contain chlorides, so the gold particles are rather large 

Support Surface area (m g- ) PZC Entry 1 washing with water Entry 2 washing with ammonia  

Support Nominal Au loading (wt%) Au loading (wt%) d Au (nm) Standard deviation (nm) 

An alternative to the formation of large gold particles with HAuCLj precursor is to use other preparation methods, such as deposition-precipitation (Sections 17.2.4 and 17.2.6) the preparations are performed at higher pH, so most of the chlorides in the gold complexes are already hydrolyzed, and thorough washing with water removes the remaining chlorides. 

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Biggs S, Mulvaney P, Zukoski C F and Grieser F 1994 Study of anion adsorption at the gold-aqueous solution interface by atomic force microscopy J. Am. Chem. Soc. 116 9150... 

Horanyi G and Rizmayer E M 1984 Radiotracer study of anion adsorption at silver electrodes in acidic medium J. Electroanal. Chem. 176 339-48... 

Wang J, Ocko B M, Davenport A J and Isaacs H I 1992 In situ diffraction and reflectivity studies of the Au(111 )/electrolyte interface Reconstruction and anion adsorption Phys. Rev B 34 10 321-38... 

Wu S, Lipkowski J, TylizczakT and Hitchcock H P 1995 Effect of anion adsorption on early stages of copper electrocrystallization at Au(111) surface Prog. Surf. Sc/. 50 227-36... 

Local breakdown of passive film results from a localized increase in the film dissolution rate at the anion adsorption sites that are attacked by chloride ions, as will be discussed later, in the same manner as substrate metal dissolution. Such acceleration of the dissolution rate was ascribed to the formation of metal chlorides24 or the local degeneration of film surface by the formation of surface electron levels.7... 

From the first reaction, assuming that the anion adsorption process is rate determining, and neglecting the potential difference as shown in Eq. (75a), the average dissolution current density is written as a function of the surface concentration of the aggressive ions, (CJx, y, 0, /)), i.e.,... 

The metal ion forms a complex with the anions, so that the increase in the metal ion concentration enhances the anion adsorption onto the surface, according to the following equation,... 

Anion adsorption also inflnences the shape of the capacitance cnrves. In the region of the PZC and at positive snrface charge, the capacitance increases to valnes of 60 to 80pF/cm as EDL thickness drops to a valne of JCj. The capacitance ininimnm in dilute solutions is distorted, and its position no longer coincides with that of the PZC. 

Wang JX, Markovic NM, Adzic RR. 2004. Kinetic analysis of oxygen reduction on Pt(ll 1) in acid solutions Intrinsic kinetic parameters and anion adsorption effects. J Phys Chem B 108 ... 

Bhzanac BB, Lucas CA, Gallagher ME, Arenz M, Ross PN, Markovic NM. 2004a. Anion adsorption, CO oxidation, and oxygen reduction reaction on a Au(lOO) surface The pH effect. J Phys Chem B 108 625-634. 

Since in many cases it is not possible to unambiguously separate hydrogen from anion adsorption processes, it is convenient to write another equation similar to (7.7) for the charge density corresponding to anion adsorption on the free Pt sites ... 

In this equation, it has been assumed that the adatom exerts the same blockage on hydrogen and anion adsorption (i.e., the stoichiometric number m is the same for... 

On the other hand, in certain cases (e.g., Ad/Pt(l 11) in HCIO4 solutions), the adatom oxidation and anion adsorption on the free sites (OH adsorption in HCIO4) overlap. Then, another refinement to this analysis would be to use (7.7) and (7.10) to calculate qp according to... 

Figure 9.5 Anion adsorption on Pt in several electiolytes bisulfates and TFMSA as determined by radiotracers (RT), and OH and H as reflected in A/r, the ampUtiide of the XANES peak. The effect of 6 M TFMSA suppressing OH formation can be seen. (Reproduced with permission from Teliska et al. [2007].)...

The specific adsorption of bisulfate anions is observed in H2SO4 in both EXAFS and XANES data and, in agreement with voltammetry, is seen to impede oxygen adsorption. Significant specific anion adsorption was found in 6 M TFMSA, but not in 1 M TFMSA [Teliska et al., 2007]. As mentioned above, this specific anion adsorption suppresses OH adsorption (particularly the formation of subsurface O), causes the Pt nanoparticle to become more round, and weakens the Pt-Pt bonding at the smface. The specific anion adsorption becomes site-specific only after lateral interactions from other chemisorbed species such as OH force the anions to adsorb into specific sites. 

Based on the available atomic-level information and other relevant literature data, factors determining the effect of anion adsorption on the ORR include the following ... 

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