Electrochemical interface, studied

There are, however, numerous cases where electronegative additives can act as promoters for catalytic reactions. Typical examples are the use of Cl to enhance the selectivity of Ag epoxidation catalysts and the plethora of electrochemical promotion studies utilizing O2 as the promoting ion, surveyed in Chapters 4 and 8 of this book. The use of O, O8 or O2 as a promoter on metal catalyst surfaces is a new development which surfaced after the discovery of electrochemical promotion where a solid O2 conductor interfaced with the metal catalyst acts as a constant source of promoting O8 ions under the influence of an applied voltage. Without such a constant supply of O2 onto the catalyst surface, the promoting O8 species would soon be consumed via desorption or side reactions. This is why promotion with O2 was not possible in classical promotion, i.e. before the discovery of electrochemical promotion. 

Abruna, H. D., X-ray absorption spectroscopy in the study of electrochemical systems, in Electrochemical Interfaces, H. D. Abruna, Ed. VCH, Weinheim, Germany, 1991, p. 1. 

Hepel, M., Electrode-solntion interface studied with electrochemical quartz crystal nanobalance, in Interfacial Electrochemistry, A. J. Wieckowski, Ed., Marcel Dekker, New York, 1999, p. 599. 

Kizhakevariam N, Weaver MJ. 1994. Structure and reactivity of bimetaUic electrochemical interfaces Infrared spectroscopy studies of carbon monoxide adsorption and formic acid electrooxidation on antimony-modified Pt(lOO) and Pt(lll). Surf Sci 310 183-197. 

Despite the fact that the electrodeposition of copper and silver at the water-DCE and the water-dichloromethane interfaces has been generally regarded as the first experimental evidence for heterogeneous ET at externally biased ITIES [171], a very limited amount of work has dealt with this type of process. This reaction has also theoretical interest because the molecular liquid-liquid interface can be seen as an ideal substrate for electrochemical nucleation studies due to the weak interactions between the interface and the newly formed phase and the lack of preferential nucleation sites always present at metallic electrodes. 

Methods employing X-rays and y-radiation are used less often in electrochemistry. The possibility of using X-ray diffraction for in situ study of the electrode surface was first demonstrated in 1980. This technique has long been used widely as a method for the structural analysis of crystalline substances. Diffraction patterns that are characteristic for the electrochemical interface can be obtained by using special electrochemical cells and elec-... 

Electrochemical reactions are driven by the potential difference at the solid liquid interface, which is established by the electrochemical double layer composed, in a simple case, of water and two types of counter ions. Thus, provided the electrochemical interface is preserved upon emersion and transfer, one always has to deal with a complex coadsorption experiment. In contrast to the solid/vacuum interface, where for instance metal adsorption can be studied by evaporating a metal onto the surface, electrochemical metal deposition is always a coadsorption of metal ions, counter ions, and probably water dipols, which together cause the potential difference at the surface. This complex situation has to be taken into account when interpreting XPS data of emersed electrode surfaces in terms of chemical shifts or binding energies. 

More than a decade ago, Hamond and Winograd used XPS for the study of UPD Ag and Cu on polycrystalline platinum electrodes [11,12]. This study revealed a clear correlation between the amount of UPD metal on the electrode surface after emersion and in the electrolyte under controlled potential before emersion. Thereby, it was demonstrated that ex situ measurements on electrode surfaces provide relevant information about the electrochemical interface, (see Section 2.7). In view of the importance of UPD for electrocatalysis and metal deposition [132,133], knowledge of the oxidation state of the adatom in terms of chemical shifts, of the influence of the adatom on local work functions and knowledge of the distribution of electronic states in the valence band is highly desirable. The results of XPS and UPS studies on UPD metal layers will be discussed in the following chapter. Finally the poisoning effect of UPD on the H2 evolution reaction will be briefly mentioned. 

As mentioned previously, this can be attributed in part to the lack of structure-sensitive techniques that can operate in the presence of a condensed phase. Ultrahigh-vacuum (UHV) surface spectroscopic techniques such as low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), and others have been applied to the study of electrochemical interfaces, and a wealth of information has emerged from these ex situ studies on well-defined electrode surfaces.15"17 However, the fact that these techniques require the use of UHV precludes their use for in situ studies of the electrode/solution interface. In addition, transfer of the electrode from the electrolytic medium into UHV introduces the very serious question of whether the nature of the surface examined ex situ has the same structure as the surface in contact with the electrolyte and under potential control. Furthermore, any information on the solution side of the interface is, of necessity, lost. 

Clearly, the application of these techniques to the study of electrochemical interfaces will allow a much deeper understanding and correlation of structure/reactivity patterns. 

In this chapter, I will try to present an introduction to these various techniques with emphasis on EXAFS and X-ray standing waves and their application to the study of electrochemical interfaces. Each technique will be treated from theoretical and experimental points of view, and selected examples from the literature will be employed to illustrate their application to the study of electrochemical interfaces. 

Due to the experimental difficulties involved, there have been only three reports of XSW measurements at electrochemical interfaces. Materlik and co-workers have studied the underpotential deposition of thallium on single-crystal copper electrodes under both ex situU9 and in situ120 conditions. In addition, they report results from studies in the absence and presence of small amounts of oxygen. 

In addition to surface EXAFS and X-ray standing waves, X-ray diffraction can be employed in the study of electrochemical interfaces. Although an extensive treatment of X-ray diffraction techniques is beyond the scope of this chapter, some brief statements are appropriate. 

A number of surface diffraction techniques can be employed in the structural study of electrochemical interfaces, depending on the details of the system under study. For bulk materials or thick films (such that the X-ray beam only samples that layer) conventional diffraction experiments can be performed and, in fact, a number of in situ X-ray diffraction studies of this type have been reported.126 129 In the case of thin films or monolayers, two different techniques can be employed and these are the reflection-diffraction technique introduced by Marra and Eisenberger,3 ), 32 and the technique based on surface truncation rods.131 In the first case, the incident X-ray beam impinges on the sample at an angle below... 

The use of X rays is providing a rare glimpse of the in situ structure of electrochemical interfaces, and as these experiments become more widespread, a wide range of phenomena will be explored. I am certain that these studies will provide the basis for a better understanding and control of electrochemical reactivity. 

This definition requires some explanation. (1) By interface we denote those regions of the two adjoining phases whose properties differ significantly from those of the bulk. These interfacial regions can be quite extended, particularly in those cases where a metal or semiconducting electrode is covered by a thin film. Sometimes the term interphase is used to indicate the spatial extention. (2) It would have been more natural to restrict the definition to the interface between an electronic and an ionic conductor only, and, indeed, this is generally what we mean by the term electrochemical interface. However, the study of the interface between two immiscible electrolyte solutions is so similar that it is natural to include it under the scope of electrochemistry. 

In this chapter we introduce and discuss a number of concepts that are commonly used in the electrochemical literature and in the remainder of this book. In particular we will illuminate the relation of electrochemical concepts to those used in related disciplines. Electrochemistry has much in common with surface science, which is the study of solid surfaces in contact with a gas phase or, more commonly, with ultra-high vacuum (uhv). A number of surface science techniques has been applied to electrochemical interfaces with great success. Conversely, surface scientists have become attracted to electrochemistry because the electrode charge (or equivalently the potential) is a useful variable which cannot be well controlled for surfaces in uhv. This has led to a laudable attempt to use similar terminologies for these two related sciences, and to introduce the concepts of the absolute scale of electrochemical potentials and the Fermi level of a redox reaction into electrochemistry. Unfortunately, there is some confusion of these terms in the literature, even though they are quite simple. 

Many naturally occurring substances, in particular the oxide films that form spontaneously on some metals, are semiconductors. Also, electrochemical reactions are used in the production of semiconductor chips, and recently semiconductors have been used in the construction of electrochemical photocells. So there are good technological reasons to study the interface between a semiconductor and an electrolyte. Our main interest, however, lies in more fundamental questions How does the electronic structure of the electrode influence the properties of the electrochemical interface, and how does it affect electrochemical reactions What new processes can occur at semiconductors that are not known from metals ... 

The use of single crystal electrodes. While the polycrystalline electrodes that were used in older works gave reproducible results in studies of reactions, they did not possess a definite structure. Only recently have electrochemists learned to prepare and characterize single crystal electrodes. This has greatly extended the study of the structure of the electrochemical interface. 

The information obtained from the previously described studies may be used to establish guideposts for the rational design and synthesis of electrochemical interfaces (in particular, of polymer modified electrodes) with high catalytic activity. 

Hydrogen evolution, the other reaction studied, is a classical reaction for electrochemical kinetic studies. It was this reaction that led Tafel (24) to formulate his semi-logarithmic relation between potential and current which is named for him and that later resulted in the derivation of the equation that today is called "Butler-Volmer-equation" (25,26). The influence of the electrode potential is considered to modify the activation barrier for the charge transfer step of the reaction at the interface. This results in an exponential dependence of the reaction rate on the electrode potential, the extent of which is given by the transfer coefficient, a. 

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