Cells external reflectance

External reflectance. The most commonly applied in situ IR techniques involve the external reflectance approach. These methods seek to minimise the strong solvent absorption by simply pressing a reflective working electrode against the IR transparent window of the electrochemical cell. The result is a thin layer of electrolyte trapped between electrode and window usually 1 to 50 pm. A typical thin layer cell is shown in Figure 2.40. 

Figure 2.40 Schematic representation of the external reflectance cell design commonly employed in in situ IR experiments, if the working electrode is a semiconductor, then the semiconductor/ electrolyte interface can be studied under illumination with, for example, UV light by directing the beam perpendicular to the IR beam, as shown.

In the case that an organic phase contains light absorbing compounds, an external reflection (ER) absorption spectrometry is more useful than a TIR spectrometry [30,31]. Another advantage of the ER method is its higher sensitivity than the TIR method, especially as using s-polarized light. Therefore, it can be used as a universal absorption spectrometry of adsorbed species. Typical optical cells used for the TIR spectrometry and ER absorption spectrometry are shown in Fig. 3. 

Fig. 3. Total internal reflection cell used for the interfacial fluorescence lifetime measurement (left) and the external reflection absorption spectrometry (right).

The experimental aspects of the performance of in situ FTIR measurements are described in Refs. 43 and 44. Figure 13 shows a typical cell for in situ external reflectance mode (e.g., SNIFTIRS type measurements) [94,95], The experimental aspects of its use are described in Ref. 96. Figures 14 and 15 show cells for in situ internal reflectance modes multiple internal reflectance ATR and single internal... 

Figure 13 A cell for in situ FTIR external reflectance mode of thin layers (SNIFTIRS type measurements) [96]. (Reprinted with copyright from The Electrochemical Society Inc.)...

The above cells may be mounted and loaded with solution in a glove box, and are then transferred to the FTIR spectrometer, to which a potentiostat/galva-nostat is attached. The external reflectance and ATR modes are performed, using standard accessories from Harrick or Spectra-Tech for these modes. The SIR mode is performed using the horizontal FT-80 or FT-85 grazing angle reflectance accessory from Spectra-Tech, equipped with a polarizer (standard equipment, see Figure 11). 

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. °... 

Examination of the solute species by external reflectance presents far fewer challenges and a variety of cell designs have been reported. These include strategies that permit the study of air-sensitive compounds over a range of... 

Figure 1.2 (a) Cross section of an external reflectance SEC cell and (b) an example of... 

Figure 1.3 Working electrode designs for external reflectance SEC cells, (a) Single element, (b) temperature controlled, " and (c) multielectrode assembly. ...

Relative to the transmission cells described in Section 1.3, external reflectance cells are less simple to construct and require additional optics for incorporation into the optical path of the spectrometer. The advantages associated with the approach relate to the well-defined nature of the working electrode, the wider range of materials suitable for this purpose, and the greater control over the thickness of the thin layer of solution. These factors contribute to a much faster (>10x) rate of electrosynthesis for external reflectance compared to transmission cells. 

The activation of gaseous molecules such as H2, N2, O2, CO, and CO2 is important both for environmental and economic reasons and the SEC study of catalysts for activation of these species is facilitated by cells capable of operation at elevated pressures. This feature may be incorporated into external reflection SEC designs and a schematic diagram of such a cell is shown in... 

Figure 1.14 Cross-sectional (a) and top (b) schematic views of an external reflection SEC cell suitable for operation at gas pressures to 1 MPa. The details of the multielectrode assembly are shown in Figure 1.3(c).

With temperature control and anaerobic considerations in mind, we designed a variable-temperature thin-layer specular (external) reflectance spectroelectro-chemical cell (Figure 5.1). The cell design is such that the incident light from the spectrometer reflects off the working electrode, ensuring that the species detected... 

Figure 17.2.1 Diagram of the external reflectance configuration for IR-SEC. The cell window (e.g., Cap2, Si, ZnSe) must be transparent to IR radiation and insoluble in the solution of interest.

Besides cell designs that employ metals or other electrode materials in disc shape for external reflection spectroscopy, Robinson and McCreery have successfully employed cylindrical carbon fibers of 12 pm diameter [58, 59]. The carbon fiber was illuminated with a tunable dye laser. Scattered light was collected with fiber optics and guided to a photomultiplier detector. Because no thin layer arrangement and consequently poor electrochemical cell response were involved, fast experiments on a microsecond time scale were possible. Studies of polyaniline films deposited on platinum discs have been described [60]. 

A combination of transmission and external reflectance spectroscopy resulting in a cell for bidimensional UV-Vis spectroelectrochemistry has been described [61]. With an optically transparent electrode (OTL), the schematic setup shown in Fig. 5.8 illustrates the different pathways of the light. One beam passes through the electrode and the electrolyte solution in front of it and the second beam passes only through the solution in front of the electrode close to it, guided strictly in parallel to the surface. Thus the former beam carries information pertaining to both the solution and the electrochemical interface (e.g. polymer films or other modifications on the electrode surface), whereas the latter beam carries only information about the solution phase. Proper data treatment enables separation of both parts. Identification of... 

Fig. 5.39. Thin layer cell for NIR spectroelectrochemistry in the external reflection mode according to [145]...

Taking into account the general considerations outlined above the spectroelec-trochemical cell has to be of a thin layer design. A typical example is shown in a cross section in Fig. 5.48 another view is presented in Fig. 5.49, showing major features of the cell. In cases where a standard spectrometer is used, the cell has to be mounted in the optical beam using an external reflection attachment as schematically depicted in Fig. 5.47. 

Fig. 5.47. Optical path of IR light in an external reflection attachment equipped with an electrochemical cell a polarizer b, e,f plane mirrors c concave mirror d cell with working electrode...

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