Electrosorption valency measurements

Indeed, in situ STM imaging with lateral atomic resolution of a flame-annealed Au(lOO) substrate in deaerated perchloric or sulphuric acid solutions fi ee of Ag ions shows the existence of a potential-induced surface reconstruction with an undulated quasi-hexagonal ( quasi-hex ) structure at Ew < 240 mV (cf. Figs. 2.5 and 2.7). Reconstructed domains were not observed at higher hE, which indicates that the quasi-hex reconstruction of the Au(lOO) surface is lifted by the applied positive electrode potential. In deaerated perchloric or sulphuric acid solutions containing Ag ions, Ag UPD is clearly indicated in cyclic voltammograms (Fig. 3.20a) as well as in q E) or r E) isotherms at = constant in the underpotential range 0 mV < A < 600 mV (Fig. 3.20b). Electrosorption valency measurements under ITL... 

Another parameter, the electrosorption valence. Y introduced by Vetter and Schulze (1973), is also widely used in electrochemistry. It is a macroscopic measure defined as y——(1/ (dq /dT). For most conditions, X = —y. 

Partial charge transfer during adsorption is difficult to evaluate because separation of the charge transferred to the electrode and that part of the charge transferred across the double layer to give specifically adsorbed ions cannot be done through measurements of the total charge in the external circuit. Vetter and Schultze [102] defined the electrosorption valence as... 

In this review, we will consider the adsorption of a single species coadsorption phenomena will not be considered, since it is generally impossible to divide the flow of charge among several species. We will present the thermodynamics on which the concept of the electrosorption valency is based, discuss methods by which it can be measured, and explain its relation to the dipole moment and to partial charge transfer. The latter can be explained within an extension of the Anderson-Newns model for adsorption, which is useful for a semi-quantitative treatment of electrochemical adsorption. Our review of concepts and methods will be concluded by a survey of experimental data on thiol monolayers, which nowadays are adsorbates of particular interest. 

Lorenz refers to f as the macroscopic pet coefficient and regards it as experimentally accessible. C M can be measured by the extrapolated value of the differential capacity C at infinite frequency, when the surface concentration rs is frozen, i.e., when it cannot keep up with the ac signal. Since, however, the partial charge associated with chemisorption does follow the ac signal, this is only true if A can be regarded as potential independent. It can be readily seen that l in Eq. (31) is the opposite of the electrosorption valency l. In fact, replacing from Eq. (9) into Eq. (2) yields ... 

This is due to the fact that the electrosorption valency, l, obtained from diffusion-controlled adsorption relies on measurements corresponding to the same small, initial amount of adsorbed material at all potentials. Consequently, this procedure does not suffer from the limitations involved in the use of Eq. (2) at high surface coverages. 

The most common procedure for estimating X consists in measuring the thermodynamically significant electrosorption valency l and in evaluating its extrathermodynamic contributions k vkw and g of Eqs. (26) and (27) in order to extract the X value. A more direct extrathermodynamic procedure that relies exclusively on the Gouy-Chapman theory can be applied to the important class of self-assembled thiol monolayers anchored to a metal surface. 

Electrochemistry shares many concepts with surface science, and for the last two decades there has been an exchange of methods and ideas between these two neighboring disciplines. However, the electrosorption valency has no equivalent in surface science, since experiments at the solid/gas or solid/vacuum interface cannot be performed at constant potential. However, for low coverages, and near the potential of zero charge, the electrosorption valency can be related to the dipole moment of the adsorbate, which can be measured both in surface science and, though with greater difficulty, also in electrochemistry. In the following, we point out the relation between these two quantities. 

After making an approximate estimate of the absolute potential difference y/ across the mercury/aqueous solution interphase, let us detennine the electrosorption valency of three mercury-supported thiol monolayers for which uM was measured by potential-step chronocoulometry. As already reported in Sec. IV.1, n-octadecanthiol... 

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. 

This UPD system is characterized by a significant positive Me-S lattice misfit (do.Tl = 0.3400 nm, doAg - 0.2890 nm) and by the formation of two T1 monolayers in the UPD range. Cyclic voltammograms and l E) isotherms of the system Ag(M/)/Pb, H, CIO4, measured with the FTTL technique [3.105], are presented in Figs. 3.3 and 3.10, respectively. The electrosorption valency was found to be = z = 1 in the entire UPD range (Fig. 3.12b). This means that cosorption or competitive adsorption processes of anions can be excluded in this system. 

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... 

Experimental results are available for the diffraction rods and structure of an overlayer of Pb on Ag(l 11)[23, 24]. Since these measurements are very accurate it is also possible to measure the compression of the UPD overlayer of Pb as the potential is changed[25, 26]. The compressibility is certainly related to the electrosorption valency, discussed in another section of this book. 

There is no direct way of measuring the partial charge transfer A, the most direct experimental evidence comes from NEXAFS experiments. It is related to the electrosorption valency [160]. 

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... 

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