Electrochemically modulated infrared reflectance spectroscopy

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. 

EMIRS electrochemically modulated infrared reflectance spectroscopy... 

In the early work of Bewick and Robinson (1975), a simple monochromator system was used. This is called a dispersive spectrometer. In the experiment the electrode potential was modulated between two potentials, one where the adsorbed species was present and the other where it was absent. Because of the thin electrolyte layer, the modulation frequency is limited to a few hertz. This technique is referred to as electrochemically modulated infrared reflectance spectroscopy (EMIRS). The main problem with this technique is that data acquisition time is long. So it is possible for changes to occur on the electrode surface. 

Beden B, Bewick A, Lamy C. 1983. A study by electrochemically modulated infrared reflectance spectroscopy of the electrosorption of formic acid at a platinum electrode. J Electroanal Chem 148 147-160. 

In recent years,3 4 however, there has been renewed interest in the study of the electrode/solution interface due in part to the development of new spectroscopic techniques such as surface-enhanced Raman spectroscopy,5-7 electrochemically modulated infrared reflectance spectroscopy and related techniques,8,9 second-harmonic generation,10-12 and others which give information about the identity and orientation of molecular species in the interfacial... 

The infrared surface spectroscopic analysis was applied only to gas-solid surfaces imtil Bewick et al. succeeded ) to measure an in situ infiored spectra on electrode surfaces in electrochemical systems. They controlled the electrode potential and obtained the difference spectra between the measured and the reference potentials (EMIRS Electrochemically Modulated Infrared Reflectance Spectroscopy). This technique is employed in this theses also and discussed in detail in a later section. 

It was first shown by electrochemically modulated infrared reflectance spectroscopy (EMIRS) that the main poisoning species formed during the chemisorption and oxidation of methanol on a platinum electrode is carbon monoxide CO, either linearly bonded, or bridge bonded to the surface. The coverage degree of the electrode surface by linearly bonded CO can reach 90% on a pure platinum electrode, so that most of the active sites are blocked... 

However, after 1980 the development of in situ infrared spectroscopic techniques have allowed the direct identification of adsorbed intermediates. In terms of methanol oxidation on R, electrochemically modulated infrared reflectance spectroscopy (EMIRS) led to the unambiguous identification of adsorbed CO as the poisoning species. Two adsorbed CO species were identified as being responsible for the poisoning phenomena (i) a linearly bonded species (IR absorption band around 2060 cm ) and (ii) a bridge-bonded species (a small band around 1850-1900 cm ). These results were... 

In this paper, the basic principles of reflectance spectroscopy will be first discussed, emphasizing what is particular to each method electronic spectra for UV-visible Reflectance Spectroscopy (UVERS), vibrational fingerprints for Electrochemically Modulated Infrared Reflectance Spectroscopy (EMIRS). After a short presentation of the experimental set-up for each technique, various examples, taken mainly from our laboratory, will be given. 

IV - ELECTROCHEMICALLY MODULATED INFRARED REFLECTANCE SPECTROSCOPY f EMIRS)... 

The first in situ Infrared Reflectance Spectroscopy rmder electrochemical control of the working electrode in a three-electrode cell was realized by Beden et al. using the so-called Electrochemical-ly Modulated Infrared Reflectance Spectroscopy (EMIRS). Experimental details of this external reflection technique are fully described in text books. °... 

Polarisation modulation infrared reflection-absorption spectroscopy (PM-IRRAS or IRRAS). Potential modulation IR studies rely on switching the potential at a reflective electrode between rest and active states, generating difference spectra, However, the EMIRS technique has several drawbacks the relatively fast potential modulation requires that only fast and reversible electrochemical process are investigated the absorption due to irreversibly chemisorbed species would be gradually eliminated by the rapid perturbation. Secondly, there is some concern that rapid modulation between two potentials may, to some extent, in itself induce reactions to occur. 

One of the most commonly applied IR techniques developed to overcome these problems is the external reflectance technique. In this method, the shong solvent absorption is minimized by simply pressing a reflective working electrode against the IR transparent window of the electrochemical cell. The sensitivity problem, that is, the enhancement of the signal/noise ratio in the case of external reflectance techniques is solved by various approaches. These are, for instance, electrochemically modulated infrared spectroscopy (EMIRS), in situ FTIR (which use potential modulation), and polarization modulation infrared reflection absorption spectroscopy (PM-IRAS, FTIR) [86,117-123]. 

Electrochemically modulated infrared spectroscopy (EMIRS), polarization modulation infrared reflection-absorption spectroscopy (PM-IRRAS), and attenuated total reflectance (ATR) have also been used. (FTIR-ATR spectra are shown in Chap. 6, Fig. 6.11.)... 

Specular reflection spectroscopy has been actively used in in situ studies of the formation and optical behaviour of monolayer films on surfaces, and for detecting intermediates and products of heterogeneous chemical and electrochemical reactions. The vibrational spectra of the adsorbed species at electrode surfaces are obtained by surface-enhanced Raman scattering and infrared reflectance spectroscopies. Since the mid-1960s, modulated reflection spectroscopy techniques have been employed in elucidating the optical properties and band structure of solids. In the semiconductor electroreflectance, the reflectance change at the semiconductor surface caused by the perturbation of the dielectric properties 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]. 

Ever since the first reports of optical studies of electrochemical systems, efforts have been made to obtain infrared spectra of reaction intermediates and adsorbates. The earliest studies were based on total internal reflection using an n-type germanium electrode (transparent to IR radiation), and OTTLE systems using gold minigrids sandwiched between NaCl plates. These were not particularly successful, however, and it is only recently that these configurations have again been used, this time for Fourier Transform spectroscopy [29,30]. Undoubtedly the most successful technique has been potential modulated external reflectance IR spectroscopy [31]. 

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