Ex situ spectroscopic techniques

Other X-ray surface probe methods which have not as yet found widespread use but are applicable to electrode/solution interfacial studies are based on X-ray standing waves and glancing angle X-ray diffraction. 

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A number of ex situ spectroscopic techniques, multinuclear NMR, IR, EXAFS, UV-vis, have contributed to rationalise the overall mechanism of the copolymerisation as well as specific aspects related to the nature of the unsaturated monomer (ethene, 1-alkenes, vinyl aromatics, cyclic alkenes, allenes). Valuable information on the initiation, propagation and termination steps has been provided by end-group analysis of the polyketone products, by labelling experiments of the catalyst precursors and solvents either with deuterated compounds or with easily identifiable functional groups, by X-ray diffraction analysis of precursors, model compounds and products, and by kinetic and thermodynamic studies of model reactions. The structure of some catalysis resting states and several catalyst deactivation paths have been traced. There is little doubt, however, that the most spectacular mechanistic breakthroughs have been obtained from in situ spectroscopic studies. 

HT technology for catalysts-automated synthesis and testing appears to be reasonably adapted to date, but further improvements are expected for HT catalysts characterization, which is still restricted to costly and in general ex-situ spectroscopic techniques. These tools would provide the new catalyst descriptors needed to improve the ability to predict catalytic performances without testing. 

From the discussion of ex situ spectroscopic techniques earlier in the chapter it is clear that other products of the interaction between incident beam and the surface can be detected. One of these is backscattered electrons (BSE) which give an image in which heavy elements lead to high backscattering (white areas) and light elements lead to low backscattering (black areas). Thus a very qualitative form of elemental analysis can be performed by BSE detection. 

In parallel with complications induced by irreversible adsorption, new approaches to the experimental studies of surface excesses are developed. Namely, the adsorbate s state and coverage usually remain unchanged when the potential is switched off and the electrode withdrawn from solution. Moreover, some adsorbed layers remain stable even under UHV conditions, and a number of highly informative ex situ spectroscopic techniques were successfully applied to the studies of adsorption on d-metals [89-91]. 

Tafel slopes with those calculated for all feasible mechanisms. Additional information comes from cyclic voltammetry, potential step techniques, and capacitance measurements, while several in situ and ex situ spectroscopic techniques are beginning to give a detailed insight into the electrode-solution interface during electrocatalytic reactions [19] (see also Chapter 10). 

Spectroscopic techniques can be carried out in situ (low-energy photon, etc.) and ex situ or in vacuo (high-energy photon and electron techniques). Ex situ microscopic techniques have been employed for many years to examine surfaces, and are now widely used tools. However, in situ microscopic techniques with resolution approaching the atomic scale... 

Traditionally, new material characterization is performed ex situ using techniques that require environments which distort the properties of the material under consideration. Consequently, they are of little use in characterizing dynamic structures. Most spectroscopic techniques, for example, are used in air or in a vacuum. For dynamic polymer systems that are used in solution, such methods do not provide all essential information. In addition, conventional techniques do not normally allow the imposition of stimuli capable of collecting information on the molecular changes brought about by these stimuli in real time. 

The presented examples clearly demonstrate that a combination of several different teehniques is urgently recommended for a complete charaeterization of the chemieal eomposition and the atomie strueture of electrode surfaces and a reliable interpretation of the related results. Strueture sensitive methods should be eombined with spectroscopic and electrochemical techniques. Besides in situ teehniques such as SXS, XAS and STM or AFM, ex situ vacumn techniques have proven their significance for the investigation of the eleetrode/electrolyte interface. 

Br on UPD of Cu on the Pt(l 11) surface. Recently, combining ex-situ and in-situ spectroscopic techniques with traditional electrochemical methods has enabled us to resolve both the nature of the structure and the kinetics of monolayer formation of... 

Strategies for the development of novel catalytic materials and the design of highly active catalysts for DLFC applications largely depend on a detailed understanding of the reaction mechanism and, in particular, of the rate-limiting step(s) during the electrooxidation under continuous reaction conditions. The most commonly used technique in the electrochemical studies of fuel cell reaction mechanisms has been voltammetry, chronoamperometry (chronopotentiometry), in situ spectroscopic techniques, e.g., electrochemically modulated infrared spectroscopy (EMIRS) and infrared reflection-absorption spectroscopy (IRRAS), differential electrochemical mass spectroscopy (DEMS) and ex-situ techniques, e.g.. X-ray photoelectron spectroscopy (XPS) [92]. 

X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) are very useful analytical techniques to determine the electronic states of conducting polymers under different chemical environments and the quantitative chemical composition of metal oxides and peroxides. But these are ex situ spectroscopic methods, and the real compositions may differ from the measured ones. 

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. 

In contrast, the coupling of electrochemical and spectroscopic techniques, e.g., electrodeposition of a metal followed by detection by atomic absorption spectrometry, has received limited attention. Wire filaments, graphite rods, pyrolytic graphite tubes, and hanging drop mercury electrodes have been tested [383-394] for electrochemical preconcentration of the analyte to be determined by atomic absorption spectroscopy. However, these ex situ preconcentration methods are often characterised by unavoidable irreproducibility, contaminations arising from handling of the support, and detection limits unsuitable for lead detection at sub-ppb levels. 

The Scanning Tunneling Microscope has demonstrated unique capabilities for the examination of electrode topography, the vibrational spectroscopic imaging of surface adsorbed species, and the high resolution electrochemical modification of conductive surfaces. Here we discuss recent progress in electrochemical STM. Included are a comparison of STM with other ex situ and in situ surface analytic techniques, a discussion of relevant STM design considerations, and a semi-quantitative examination of faradaic current contributions for STM at solution-covered surfaces. Applications of STM to the ex situ and in situ study of electrode surfaces are presented. 

The application of ultra-high vacuum surface spectroscopic methods coupled to electrochemical techniques t21-241 have provided valuable information on surface structure/reactivity correlations. These determinations, however, are performed ex-situ and thus raise important concerns as to their applicability to electrocatalytic systems, especially when very active intermediates are involved. 

The techniques used in studying interfaces can be classified in two categories in situ techniques and ex situ techniques. In situ methods are those where a surface is probed by one or several techniques while immersed in solution and under potential control. In contrast, in ex situ methods, an electrochemical experiment is first carried out. Then the electrode is removed from solution and examined by one or several spectroscopic techniques, which generally require ultrahigh vacuum (UHV) conditions. Figures 6.10 and 6.11 show some of the most common ex situ and in situ techniques applicable to the study of the metal/solution interface. 

Two of the most common UHV-spectroscopic methods used in electrochemistry are briefly described next, and Fig. 6.10 lists other ex situ techniques, which can be reviewed in the literature by the inquisitive student. 

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