In-situ spectroscopic technique

In these techniques a beam of photons is directed to the electrode such that it is transmitted or reflected. The majority of the techniques are reflective, since transmission is limited to transparent or semi-transparent electrode materials. 

Since the electrode has to be transparent, the electrode material is limited to thin films of metals or semiconductors deposited on a transparent substrate (for example a thin film of tin(IV) oxide or platinum on quartz) or to very fine grids of the electrode material, as shown in Fig. 9.13. The first of these two options is preferable, since the transmission coefficient is uniform and the electrode can be truly planar, and as such can be used as a hydrodynamic electrode, for example. The change in absorbance with time due to one of the reagents or products of the electrode reaction characterizes the mechanism. 

This type of cell and electrode can be used to photochemically activate an electrochemical system, the electrode reaction being used to detect or electrolyse the species produced (e.g. photoelectrolysis of water, Section 12.5). 

These techniques depend on the fact that electromagnetic radiation comprises two perpendicular vectors mutually perpendicular to the direction of photon transmission—the magnetic vector H and the electric vector E, Fig. 12.1. In normal non-polarized radiation, the directions of these vectors are not aligned, their sum being zero. In linearly polarized radiation the vectors are all aligned in the same direction in partially polarized radiation the alignment is partial. 

Reflection at a surface of a beam of linearly polarized photons alters the direction and amplitude of the electric and magnetic vectors. It is these differences between incident and reflected beams that give information concerning surface structure, as they depend on the interaction of the beam with the electronic distribution and with the associated local electric and magnetic fields on the surface. The phase and amplitude change for the vectors is different for the component parallel to the plane of incidence than for the component perpendicular to it. The result is a vector that follows a spiral during its propagation, and is referred to as elliptically polarized, Fig. 12.2. A deeper treatment of these optical properties can be found in Ref. 9. Such measurements are referred to as specular reflectance. 

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Plieth W, Wilson G S and de la Fe C 1998 Spectroelectrochemistry a survey of in situ spectroscopic techniques Pure Appi. Chem. 70 1395... 

Some of the transition metal macrocycles adsorbed on electrode surfaces are of special Interest because of their high catalytic activity for dloxygen reduction. The Interaction of the adsorbed macrocycles with the substrate and their orientation are of Importance In understanding the factors controlling their catalytic activity. In situ spectroscopic techniques which have been used to examine these electrocatalytlc layers Include visible reflectance spectroscopy surface enhanced and resonant Raman and Mossbauer effect spectroscopy. This paper Is focused principally on the cobalt and Iron phthalocyanlnes on silver and carbon electrode substrates. 

Considerable progress has been made recently In the development of In situ spectroscopic techniques applicable to the study of transition metal macrocycles adsorbed at submonolayer coverages onto electrode surfaces. These have been aimed at gaining Insight into the nature of the Interactions of these compounds with the surface and with 02 Most of the attention In the authors laboratory has been focused on Fe- and Co-TsPc, although some preliminary results have already been obtained for some Iron and cobalt porphyrins. The main conclusions obtained from these Investigations will be outlined In the following sections. 

It is only since 1980 that in situ spectroscopic techniques have been developed to obtain identification of the adsorbed intermediates and hence of reliable reaction mechanisms. These new infrared spectroscopic in situ techniques, such as electrochemically modulated infrared reflectance spectroscopy (EMIRS), which uses a dispersive spectrometer, Fourier transform infrared reflectance spectroscopy, or a subtractively normalized interfacial Fourier transform infrared reflectance spectroscopy (SNIFTIRS), have provided definitive proof for the presence of strongly adsorbed species (mainly adsorbed carbon monoxide) acting as catalytic poisons. " " Even though this chapter is not devoted to the description of in situ infrared techniques, it is useful to briefly note the advantages and limitations of such spectroscopic methods. 

The corrosion of iron represents an electrochemical reaction of huge economic significance, accounting literally for billions of dollars of waste every year. The phenomenon has been investigated since the time of Faraday and still presents many controversial and puzzling aspects which only the arrival of in situ spectroscopic techniques has begun to clarify. 

In situ spectroscopic measurements of a catalytic system provide a considerable opportunity to determine the chemical species present under reactive conditions. FTIR and NMR have been the two most frequently used in situ spectroscopic methods (see Chapters 2 and 3). They have been successfully used to identify labile, non-isolatable transient species believed to be involved in the catalytic product formation. Furthermore, efforts have been made to use this information in order to obtain more detailed kinetics, by decoupling induction, product formation, and deactivation. Thus, in situ spectroscopic techniques have the potential for considerably advancing mechanistic studies in homogeneous catalysis. 

In addition to characterization of intermediates, in situ spectroscopic techniques can be applied in kinetic studies, providing additional mechanistic insight. Also, isotopic labeling studies have proven very useful, especially vhen studying the individual steps of a catalytic cycle [5, 6]... 

In this review, we will discuss the use of in situ spectroscopic techniques, in combination with kinetic and isotopic labeling studies, to obtain a detailed mechanistic insight of the rhodium catalyzed hydroformylation. 

The first term represents the classic unicyclic rhodium catalysis, while the second indicates a hydride attack on an acyl species. These spectroscopic and kinetic results strongly suggested the presence of bimetallic catalytic binuclear elimination as the origin of synergism of both metals rather than cluster catalysis. This detailed evidence for such a catalytic mechanism, and its implications for selectivity and nonlinear catalytic activity illustrate the important mechanistic knowledge that can be revealed by this powerful in situ spectroscopic technique. 

Oxidation/reduction of Pb electrode has been studied using in situ spectroscopic techniques - Raman [114, 130-132], fourier transform infrared (FTIR) [133-135], Auger [136], and photocurrent spectroscopy [131, 137-141]. El-Kpsometric studies underlined nonuniform PbS04 film growth a dissolution-precipitation mechanism with nucleation and three-dimensional growth has been proposed as a result of large oversaturation of Pb(II) ionic species [142]... 

With in situ spectroscopic techniques, critical data on the formation of such species as HONO, HN03, and N03, which are essential to understanding the chemistry of these systems, can also be obtained. Figure 16.8, for example, shows one portion of an FTIR spectrum obtained in a chamber run for a propene-NOx... 

This very short treatment of reversal techniques has the following basis. There are certainly treatments in the literature of chronopotentiometiy dealing with current reversal, or reversed-step voltammetry. However, their validity has to be diligently examined in each application. For example, is an assumption of a first-order reaction tacitly involved, when the actual solution may correspond to a fractional reaction order Another reason for the limited treatment has an eye on the future. There are those who see in the rapid development of in situ spectroscopic techniques (see, e.g., Section 6.3), together with advances in STM and AFM, the future of surface analysis in electrochemistry. If these surface spectroscopic techniques continue to grow in power, and give information on surface radicals in time ranges as short as milliseconds, transient techniques to catch intermediate radicals adsorbed on surfaces may become less needed. 

A general objective in any in situ spectroscopic technique is to maximize the signal that arises specifically from the electrode surface. Nonlinear optical techniques such as second-harmonic generation (SHG) and sum frequency generation (SFG) are of interest because they involve optical signals that by definition can only arise at the electrode-solution interface [65],... 

Surface functional groups Specific adsorption of ions or molecules, in-situ spectroscopic techniques... 

Experimental studies of combustion chemistry require measurements of species concentrations, often under conditions where in situ spectroscopic techniques are desirable or necessary. 

The application of novel in situ spectroscopic techniques for the study of Li electrodes in solutions should also be acknowledged. These include FTIR spectroscopy [108], atomic force microscopy (AFM) [109], electrochemical quartz crystal microbalance (EQCM) [110], Raman spectroscopy [111], and XRD [83],... 

In order to understand the manner in which the interfacial region influences the observed kinetics, especially in terms of the theoretical models discussed below, it is clearly important to gain detailed information on the spatial location of the reaction site as well as a knowledge of the mechanistic pathway. Information on the latter for multistep processes can often be obtained by the use of electrochemical perturbation techniques in order to detect reaction intermediates, especially adsorbed species [13]. Various in-situ spectroscopic techniques, especially those that can detect interfacial species such as infrared and Raman spectroscopies, are beginning to be used for this purpose and will undoubtedly contribute greatly to the elucidation of electrochemical reaction mechanisms in the future. 

This section describes various strategies for the immobilization of macrocycles on electrode surfaces and their characterization by both electrochemical and in situ spectroscopic techniques in solutions devoid of dioxygen. It also provides theoretical foundations involved in the analysis of the mechanisms of oxygen reduction at such interfaces based on measurements performed under forced convection. Studies involving a number of carefully selected phthalocyanines, and porphyrins, will be presented and discussed, which in our view best illustrate the nuances of the rich behavior this class of adsorbed electrocatalysts can exhibit. These examples serve to... 

For in-situ experiments most commonly used microscopic and spectroscopic techniques are environmental transmission electron microscopy (E-TEM) [207-209], In-situ vibrational spectroscopic [210-212], ambient pressure X-ray photoelectron spectroscopy [206,210,213], X-ray absorption spectroscopy [213,214-217], and Raman spectroscopy [218]. Making use of this in-situ experiments, the solar fuel generation processes will get a new dimension to the state-of-the-art beliefs. Moreover, the catalysts structure, coverage and composition also change with time, the combination of ultrafast of in-situ spectroscopic techniques reveal the structure and catalytic activity relationships (See Table 7) [217]. 

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