Halide ion adsorption

Alkali and halide ion adsorption near metal surfaces has been investigated by a variety of groups. A simple (9-3) Lennard-Jones potential, first used by Lee et al. 

For many metals and alloys the determination of /p is complex, and its magnitude is governed by many factors such as surface finish, rate of formation, alloying constituents, and the presence of those anions, such as halides, that promote localised breakdown. In many instances the attack on passive films by halide ions shows a temperature and concentration dependence similar to the effect of hydrogen ions, i.e. the rate of film dissolution increases with concentration in accordance with a Freundlich adsorption relationship... 

In modern practice, inhibitors are rarely used in the form of single compounds — particularly in near-neutral solutions. It is much more usual for formulations made up from two, three or more inhibitors to be employed. Three factors are responsible for this approach. Firstly, because individual inhibitors are effective with only a limited number of metals the protection of multi-metal systems requires the presence of more than one inhibitor. (Toxicity and pollution considerations frequently prevent the use of chromates as universal inhibitors.) Secondly, because of the separate advantages possessed by inhibitors of the anodic and cathodic types it is sometimes of benefit to use a formulation composed of examples from each type. This procedure often results in improved protection above that given by either type alone and makes it possible to use lower inhibitor concentrations. The third factor relates to the use of halide ions to improve the action of organic inhibitors in acid solutions. The halides are not, strictly speaking, acting as inhibitors in this sense, and their function is to assist in the adsorption of the inhibitor on to the metal surface. The second and third of these methods are often referred to as synergised treatments. 

In the case of ions, the repulsive interaction can be altered to an attractive interaction if an ion of opposite charge is simultaneously adsorbed. In a solution containing inhibitive anions and cations the adsorption of both ions may be enhanced and the inhibitive efficiency greatly increased compared to solutions of the individual ions. Thus, synergistic inhibitive effects occur in such mixtures of anionic and cationic inhibitors . These synergistic effects are particularly well defined in solutions containing halide ions, I. Br , Cl", with other inhibitors such as quaternary ammonium cations , alkyl benzene pyridinium cations , and various types of amines . It seems likely that co-ordinate-bond interactions also play some part in these synergistic effects, particularly in the interaction of the halide ions with the metal surfaces and with some amines . 

Unlike cations, the adsorption activity of CT, Br", and I at Pt electrodes is appreciable806 and increases in the given sequence of anions. At a 0, the <7, A curves for LiC104, NaCl,NaBr, and Nal coincide, which indicates that complete desorption of halide ions takes place at negatively charged surfaces. The values of Ea=0 for a renewed Pt electrode have been found to be -0.18, -0.24, and -0.33 V (SCE in H20) for NaCl, NaBr, and Nal in DMSO, respectively. 

LEED has also been used to study the adsorption of halide ions, cyanide and thiocyanate ions, and organic molecules on single-crystal metal surfaces. 

Opinions differ on the nature of the metal-adsorbed anion bond for specific adsorption. In all probability, a covalent bond similar to that formed in salts of the given ion with the cation of the electrode metal is not formed. The behaviour of sulphide ions on an ideal polarized mercury electrode provides evidence for this conclusion. Sulphide ions are adsorbed far more strongly than halide ions. The electrocapillary quantities (interfacial tension, differential capacity) change discontinuously at the potential at which HgS is formed. Thus, the bond of specifically adsorbed sulphide to mercury is different in nature from that in the HgS salt. Some authors have suggested that specific adsorption is a result of partial charge transfer between the adsorbed ions and the electrode. 

The adsorption strength of anions on Au electrodes follows the sequence of C104 < S042- < Cl < Br < I. The strong specific adsorption of halide ions leads to a partial charge transfer between the adsorbate and the metal electrode [234]. 

In aqueous solutions these reactions seem to proceed via an outer-sphere mechanism on most metals. Typically such reactions involve metal ions surrounded by inert ligands, which prevent adsorption. Note that the last example reacts via an outer-sphere pathway only if trace impurities of halide ions are carefully removed from the solution otherwise it is catalyzed by these ions. 

The influence of anion adsorption on Zn U PD on Au(l 11) was further studied in the presence of halide ions [201]. The order of the adsorption strength of the anions on Zn UPD was found to be different from that of specific adsorption on a substrate Au electrode Cl04 < S04 < POr < Cl < Br < I . In phosphate solutions, chloride and bromide ions did not influence the Zn UPD, and iodide ions inhibited this process (see Fig. 5). 

The capacitance-potential dependences of Cd(OOOl) in dilute solutions of Cl04, N02, and NOs" were also studied [6]. A weak specific adsorption of anions increasing in the order Cl04 < N02 < N03 was observed. The adsorption of halides on the Cd(OOOl) single crystal electrode was studied [7], and was found to increase in the sequence Cl < Br < 1 [8]. Analysis of the impedance data does not point to the specific adsorption of Cl ions, and shows that the surface excess (T) of halide ions changes with potential and increases from Br to 1 (Fig. 1) [7]... 

Innocenti et al. have studied the kinetics [101] of two-dimensional phase transitions of sulfide and halide ions, as well as electrosorption valency [102] of these ions adsorbed on Ag(lll). The electrode potential was stepped up from the value negative enough to exclude anionic adsorption to the potential range providing stability of either the first or the second, more compressed, ordered overlayer of the anions. The kinetic behavior was interpreted in terms of a model that accounts for diffusion-controlled random adsorption of the anions, followed by the progressive polynucleation and growth. 

Table 6.2 shows the detachment energy of one water molecule from a hydrated halide ion cluster [41]. The strength of the water-halide interactions is reduced as the ionic radius increases in the order of Fspecific adsorption in an electrochemical environment. It is clear that the nonspecific adsorption behavior of F is due to its strongly bound solvation shell. Due to... 

In this equation, M represents a surface metal cation and the sum of m and n fulfills the coordination of M. Such a change in surface conditions is reflected in the pH dependence of the flat-band potential of ZnO electrodes (19J. Judging from the point of zero charge (pzc) of ZnO, which is at about pH=8.7 (20), the surface of the electrode must be charged positively in the soltuion chosen in the experiments. Then, there may arise specific adsorption of halide ions onto the cationic sites, and a mechanism is postulated that the observed pH effects of (X ) is due to contribution from the specifically adsorbed halide ions. Measurements of the flat-band potential of ZnO electrodes as a function of the concentration of iodide ions, however, gave no indication of the specific adsorption. Then, this model is ruled out. 

Another model for giving an explanation of the pH dependence of the reactivity of halide ions may be that surface cations serve as effective sites for adsorption of reaction intermediates which are produced in the course of the anodic oxidation of halide ions. Usually, the anodic oxidation of halide ions is believed to... 

If the adsorption of neutral halogen atoms occurs on the surface cation sites with replacing surface-bound water, then the amount of the adsorbed intermediates will be high in acidic solutions, as implied by Eq 6, resulting in an increase in apparent reactivity of halide ions with decreasing pH values. 

With these compounds the presence of the halogen will have been detected in the tests for elements. Most acid halides undergo ready hydrolysis with water to give an acidic solution and the halide ion produced may be detected and confirmed with silver nitrate solution. The characteristic carbonyl adsorption at about 1800 cm -1 in the infrared spectrum will be apparent. Acid chlorides may be converted into esters as a confirmatory test to 1 ml of absolute ethanol in a dry test tube add 1 ml of the acid chloride dropwise (use a dropper pipette keep the mixture cool and note whether any hydrogen chloride gas is evolved). Pour into 2 ml of saturated salt solution and observe the formation of an upper layer of ester note the odour of the ester. Acid chlorides are normally characterised by direct conversion into carboxylic acid derivatives (e.g. substituted amides) or into the carboxylic acid if the latter is a solid (see Section 9.6.16, p. 1265). 

The amount of adsorbed hydrogen decreases in the presence of halide ions [395, 396]. This is due to a decrease in the M-H adsorption energy induced by ion-specific adsorption with partial charge transfer. The decrease in M-H bond strength results in an increase of overpotential. The effect is lower for Cl and higher for I -. However two joint effects are operative one due to electronic modifications, and the other one of an electrostatic nature related to a change in the local electric potential... 

In all these electrostatic considerations the surface of the ionic crystal was idealized, as described in Sec. IV,2, as if it were cut by means of an ideally sharp razor blade. Our lack of knowledge of the structural deviations of the surface arrangements with respect to the structure inside the crystal renders it impossible for us to make any quantitative or semiquantitative statements regarding the actual adsorption energies caused by electrostatic forces. We can only say that in most ionic crystals negative ions i.e., halide ions or oxide ions, tend to form the outside (adsorbing) surface. We shall have an opportunity (see, for example, Secs. V,5 and VI,5) to revert to this phenomenon. 

Mechanistically, the transformation of the N-NDR into an HN-NDR can be explained by the fact that adsorbed halides inhibit the dissociative adsorption of H2O2. The decrease in the reaction current due to the loss of PtOH or the formation of upd-H upon a negative voltage shift is overcompensated by the increase in current density due to the desorption of halide ions. Sustained periodic oscillations appear under potentiostatic as well as galvanostatic conditions in the presence of halides [57] (Fig. 23). The oscillations that are associated with the NDR in the upd-H region were termed oscillations D, those connected to the autocatalytic adsorption of H2O2 oscillations C. 

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