Absorption cell design

There have been a number of cell designs tested for this reaction. Undivided cells using sodium bromide electrolyte have been tried (see, for example. Ref. 29). These have had electrode shapes for in-ceU propylene absorption into the electrolyte. The chief advantages of the electrochemical route to propylene oxide are elimination of the need for chlorine and lime, as well as avoidance of calcium chloride disposal (see Calcium compounds, calcium CHLORIDE Lime and limestone). An indirect electrochemical approach meeting these same objectives employs the chlorine produced at the anode of a membrane cell for preparing the propylene chlorohydrin external to the electrolysis system. The caustic made at the cathode is used to convert the chlorohydrin to propylene oxide, reforming a NaCl solution which is recycled. Attractive economics are claimed for this combined chlor-alkali electrolysis and propylene oxide manufacture (135). 

Most sensor volumes, whether in LC (e.g., a UV absorption cell) or in GC (e.g., a katharometer cell), are cylindrical in shape, are relatively short in length and have a small length-to-diameter ratio. The small length-to-diameter ratio is in conflict with the premises adopted in the development of the Golay equation for dispersion in an open tube and, consequently, its conclusions are not pertinent to detector sensors. Atwood and Golay [12] extended the theory of dispersion in open tubes to tubes of small length-to-diameter ratio. The theory developed is not pertinent here as it will be seen that, with correctly designed cells, that dispersion from viscous sources can be... 

Synchotron based techniques, such as surface X-ray scattering (SXS) and X-ray absorption spectroscopy (XAS), have found increased use in characterization of electrocatalysts during electrochemical reactions.37 These techniques, which can be used for characterization of surface structures, require intricate cell designs that can provide realistic electrochemical conditions while acquiring spectra. Several examples of the use of XAS and EXAFS in non-precious metal cathode catalysts can be found in the literature.38 2... 

A special O-ring cell design is needed for in situ infrared (IR) vibrational characterization of an electrochemical interface. The absorption of one monolayer (i.e. <1015 cm 2 vibrators) can be measured if the silicon electrode is shaped as an attenuated total reflection (ATR) prism, which allows for working in a multiple-in-ternal-reflection geometry. A set-up as shown in Fig. 1.9 enhances the vibrational signal proportional to the number of reflections and restricts the equivalent path in the electrolyte to a value close to the product of the number of reflections by the penetration depth of the IR radiation in the electrolyte, which is typically a tenth of the wavelength. The best compromise in terms of sensitivity often leads to about ten reflections [Oz2]. 

In general, it can be said that, often of necessity, the detector cell may be relative large with a low aspect ratio and thus, would theoretically produce serious band dispersion. In practice the predicted dispersion is reduced by deign of the inlet and outlet tubes, as discussed above, to ensure maximum secondary flow in the cell and thus, minimize dispersion. The success of the procedure to reduce detector cell dispersion depends on the type of detector and the principle of detection. For example.it is far easier to design a low dispersion electrical conductivity cell than a low dispersion UV absorption cell. 

Spectroelectrochemical cells that permit redox titrations of precious biological samples, require exclusion of oxygen, and allow acquisition of data from multiple spectroscopic domains have been described. A recent example of these cell designs combines electron paramagnetic resonance spectroscopy with UV-visible absorption spectroscopy [71] for studies of flavoproteins. 

Raman spectroscopy is characterized by lower sensitivity than IR spectroscopy, but in contrast to IR spectroscopy, Raman spectroscopy may be used to investigate catalysts under supercritical conditions of C02 or H20, because there are no strong absorptions by these molecules that interfere with the absorptions by the catalyst as is the case in IR spectroscopy. Griinwaldt et al. (2003) reviewed cell designs for spectroscopic experiments under supercritical conditions that either feature a window (lens) to focus the laser beam inside the cell, or fiber optics that are directly inserted into the cell (Howdle et al., 1994 Poliakoff et al., 1995). In some cases, several techniques may be combined (Addleman et al., 1998 Hoffmann et al., 2000). Such cells are designed with minimal void volume so that reliable kinetics and time-resolved analyses can be performed. 

The impact of the paper reporting this cell design should not be underrated, as it began a pattern of use of a whole new type of cell for XAFS spectroscopy of catalysts under reaction conditions. The cell enabled simultaneous measurements of X-ray absorption spectra and catalytic activity of the same material under conditions that are ideal for catalysis plug flow. However, although data obtained with a capillary-type cell will provide a more accurate measurement of catalyst performance, the spectroscopic data could be compromised. 

---