Kinetics of underpotential deposition

Swathirajan and Bruckenstein [113] have recently discussed the thermodynamics and kinetics of underpotential deposition phenomena on polycrystalline electrodes. Since work functions and surface structural features vary in different crystal faces, upd phenomena on single crystals have shown distinctive differences [109]. 

Swathirajan S, Burckenstein S (1983) Thermodynamics and kinetics of underpotential deposition of metal monolayers on polyciystalline substrates. Electrochim Acta 28 865-877... 

S. Taguchi, T. Fukuda, and A. Aramata, Kinetic study of underpotential deposition of zinc ions on pt( 111) in acidic phosphate solution, in press (1996). 

The ultimate case of a changing surface in electrode kinetics is that of the deposition of one metal on another where the surface changes intrinsically. The study of such systems involves processes described under the title of underpotential deposition (Section 7.12.11). 

When a metal is in contact with its metal ion in solution, an equilibrium potential is established commonly referred to as Nernst potential (Er). Metal deposition occurs at potentials negative of Er, and dissolution for E > Er. However, when a metal is deposited onto a foreign metal substrate, which will always be the case for the initial stages of deposition, it is frequently observed that the first monolayer on the metal is deposited at potentials which are positive of the respective Nernst potential [37, 38]. This apparent violation for Nernst s law simply arises from the fact that the interaction between deposit metal and substrate is stronger than that between the atoms of the deposit. This effect has been termed underpotential deposition (upd), to contrast deposition processes at overpotentials. (One should keep in mind, however, that despite the symmetrical technical terms the physical origins of both effects are quite different. While the reason for an overpotential is solely due to kinetic hindrance of the deposition process, is that for underpotential deposition found in the energetics of the adatom-substrate interaction.)... 

In order to investigate the dependence of a fast reaction on the nature of the metal, Iwasita et al. [3] measured the kinetics of the [Ru(NH,3)6]2+/3+ couple on six different metals. Since this reaction is very fast, with rate constants of the order of 1 cm s-1, a turbulent pipe flow method (see Chapter 14) was used to achieve rapid mass transport. The results are summarized in Table 8.1 within the experimental accuracy both the rate constants and the transfer coefficients are independent of the nature of the metal. This remains true if the electrode surfaces axe modified by metal atoms deposited at underpotential [4]. It should be noted that the metals investigated have quite different chemical characteristics Pt, and Pd are transition metals Au, Ag, Cu are sd metals Hg and the adsorbates T1 and Pb are sp metals. The rate constant on mercury involved a greater error than the others... 

A kinetic study of Cu underpotential deposition was carried out to determine if it is best described by adsorption processes or by nucleation processes. The nucleation growth process is classified into two categories instantaneous nucleation growth and progressive nucleation growth. In the case of instantaneous nucleation growth, where nucleation site formation is so fast that no other following nucleation sites are created, the number of nucleation sites N(t) is expressed as... 

Formation and stripping of a cobalt adlayer on/from a polycrystalline Au electrode have been studied [469] applying electrochemical methods under underpotential conditions. The kinetics of deposition fitted a model of a simultaneous adsorption and diffusion-controlled two-dimensional instantaneous nucleation of cobalt on the electrode surface. 

Underpotential deposition of metal adatoms at foreign metal electrodes shows a strong effect on the kinetics of inner sphere redox reactions such as the reduction of Cr(OH2)sCl2+ [130] due to electrostatic and specific interactions. 

C. F. Zinola, J. Rodriguez, and G. Obal, "Kinetic of Molecular Oxygen Electroreduction on Platinum Modified by Tin Underpotential Deposition," Journal of Applied Electrochemistry, 31 (2001) 1293-1300. 

The third part treats surface modification by underpotential deposition (UPD) of metals. Physical nature, thermodynamics, structural aspects, kinetics, as well as surface alloy formation are discussed. Experimental support is given based on classical electrochemical investigations as well as on some recent results from modern in situ surface analytical studies including atomic imaging by in situ STM and AFM. 

Iwasita T., Schmickler W. and Schultze J. W., (1985) The influence of metal adatoms deposited at underpotential on the kinetics of an outer-sphere redox reaction , J. Electroanal. Chem. 194, 355-359. 

Oscillatory kinetics with a surface reaction had been observed as early as in 1828 by Fechner [4] with an electrochemical system. As an example for these t) es of reactions. Fig. 7.1 shows the variation of the potential at a Pt electrode with time for the electrochemical oxidation of H2 in the presence of copper ions [5]. While the potential at low-current density j is constant (a), at higher j kinetic oscillations occur because of periodic poisoning and activation transitions of the electrode by underpotential deposition and dissolution of a passivating Cu overlayer. With further increase of , at first period doubling and then transition to an irregular situation (chaos) take place. 

It has been established that H d is the reactive intermediate in the HOR, and therefore the kinetics of the HOR is mainly determined by the interaction between H d and the Pt surface atoms. There are two different possible states of adsorbed hydrogen. One is the Hupd (the underpotentially deposited hydrogen), which is the strongly adsorbed stated formed on the surface at potentials more positive than the Nernst potential, and the other is the H pd (the overpotentially deposited hydrogen), which is the weakly adsorbed state formed close to or negative with respect to the Nernst potential. ... 

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