Electrosorption valency determination

A physical model and a theory have been proposed [72], which might be helpful in comparative studies on electrocompres-sive behavior of electrodeposited chloride, bromide, and iodide monolayers on the Au(lll) electrode. The theoretical results were in good agreement with the experimental data, which evidence that the adatom-adatom interactions (especially repulsive ones) and electrosorption valency of halide anions determine the compressibility within halide adlayers. Also, Lipkowski et al. have discussed various aspects of adsorption of halide anions on Au(lll) in a review paper [36]. From this paper, we have taken quantitative data concerning adsorption of halide anions on Au(lll) (cf Fig. 3). 

Fig. 12.20. Electrosorption of Br and I" on polycrystalline gold. Plot of charge passed vs. coverage determined by EQCM. Slope is the electrosorption valency (from Ref. 60 with permission).

Another useful thermodynamic relationship that allows the potential dependence of l to be determined from the differential capacity Co, of the interphase at constant rs is readily obtained from the very definition of l. Choosing Ez as the reference potential and denoting by lz the electrosorption valency at Ez, the l value at any other applied potential E is given by ... 

This kinetic determination of the electrosorption valency l is based on the relatively mild assumption of a potential independent X. 

The electrosorption valency of several neutral aliphatic molecules on mercury was examined by Koppitz at al.54 As usual, the experimental l values were referred to the potential of zero charge in the absence of specific adsorption, Ez, and to low surface coverages. With the remarkable exception of thiourea, all these molecules do not undergo pet (A = 0). Since they are also neutral (z = 0), their electrosorption valency is exclusively determined by the two dipole terms ... 

Both thin-layer techniques were developed by Schmidt, Siegenthaler et al. [3.55, 3.67, 3.68] and allow an independent and precise measurement of q(E,p) and iXE.fi) isotherms. From the q-T plot, the electrosorption valency can be directly determined as illustrated in Fig. 3.12 [3.97, 3.105]. The electrosorption valency was found to be Y = z m both UPD systems indicating the absence of cosorption and competitive sorption processes in the systems studied. 

The term electrosorption valency was chosen because of the analogy between the value, y as defined above, and the charge on the ion, z. If we compare Eqs. (53) and (55), the only difference is that instead of F in the former A< can be found in the latter. Although F is a potential referred to a reference electrode while A< is considered as a potential difference across the interface, the physical content of the two equations is almost the same. Considering the conclusions drawn in connection with Eq. (53), the electrosorption valency values determined according to the preceding definition equation could be very misleading if the possible simultaneous specific adsorption of counterions is not taken into consideration. This question will be discussed in the next section. 

The electrosorption valency for lead adsorbed on silver is determined by plotting the adsorbed charge versus the adsorbed mass. This is shown for Pb on Ag(l 11) in Figure 4.31. 

Figure 4.31 Determination of the electrosorption valency for Pb UPD on Ag(lll) by adsorbed charge (determined by integration of the cyclic voltammogram in Figure 4.27) versus adsorbed mass shown in Figure 4.30. The electrosorption valency is zy = 2. (Reproduced with permission from Ref. [60], 1978, Elsevier.)...

Quantitative measurements of the coverage by Br d on Pt(lll) surface were obtained by purely electrochemical methods, as described in detail in Ref. [59]. Briefly, by utilizing the ring-shielding properties of the RRDE, Sect. 4.1.2.2, it was possible to determine the potential-dependent surface coverage by bromide and its electrosorption valence (y) on Pt(lll). The electrosorption valence... 

There are two excess parameters, the charge <7 = (9y/9 )y and the surface concentration r = — dy/dii)E from which the electrosorption valency may be defined and experimentally determined ... 

Whenever the lateral interaction in an adlayer is weak, we observe a linear dependence of I R/R on coverage 6 (or on the average film thickness d). Such a relation, which simply implies that the film dielectric constants, ej, are independent of 0 (Eq. 29), is conveniently used for a precise determination of coverages and hence of adsorption isotherms. Especially in cases like halide adsorption where the evaluation of surface excesses from charge measurements is hampered by an unknown electrosorption valency (unless a very elaborate concentration dependence study is made " ), the spectroscopic method of determining coverages can indeed be advantageous. Adzic et have... 

---