Electrochemical FTIR methods

Li et al. [278] have studied adsorption of L-phenylalanine at Au(lll) electrodes using electrochemical and subtractively normalized interfacial FTIR methods. It has been found that the adsorbed molecules change their orientation with the electrode potential. At a negatively charged surface, the compound was predominantly adsorbed in the neutral form of the amino acid. At potentials positive with respect to pzc, L-phenylalanine was adsorbed predominantly as zwitterion with —GOO ... 

Other Techniques Continuous methods for monitoring sulfur dioxide include electrochemical cells and infrared techniques. Sulfur trioxide can be measured by FTIR techniques. The main components of the reduced-sulfur compounds emitted, for example, from the pulp and paper industry, are hydrogen sulfide, methyl mercaptane, dimethyl sulfide and dimethyl disulfide. These can be determined separately using FTIR and gas chromatographic techniques. 

Abstract In the beginning, the mixed potential model, which is generally used to explain the adsorption of collectors on the sulphide minerals, is illustrated. And the collector flotation of several kinds of minerals such as copper sulphide minerals, lead sulphide minerals, zinc sulphide minerals and iron sulphide minerals is discussed in the aspect of pulp potential and the nature of hydrophobic entity is concluded from the dependence of flotation on pulp potential. In the following section, the electrochemical phase diagrams for butyl xanthate/water system and chalcocite/oxygen/xanthate system are all demonstrated from which some useful information about the hydrophobic species are obtained. And some instrumental methods including UV analysis, FTIR analysis and XPS analysis can also be used to investigated sulphide mineral-thio-collector sytem. And some examples about that are listed in the last part of this chapter. 

Unless the coverage of adsorbate is monitored simultaneously using spectroscopic methods with the electrochemical kinetics, the results will always be subject to uncertainties of interpretation. A second difficulty is that oxidation of methanol generates not just COz but small quantities of other products. The measured current will show contributions from ail these reactions but they are likely to go by different pathways and the primary interest is that pathway that leads on/y to C02. These difficulties were addressed in a recent paper by Christensen and co-workers (1993) who used in sifu FTIR both to monitor CO coverage and simultaneously to measure the rate of C02 formation. Within the reflection mode of the 1R technique used in this paper this is not a straightforward undertaking and the effects of diffusion had to be taken into account in order to help quantify the data obtained. 

Essentially, any technique applicable to the measurement of physicochemical properties of compounds may be considered for the study of reaction rates, and it is up to the imagination of the researcher to exploit the principles behind the technique and devise an experimental method. A number of extremely useful electrochemical techniques are covered in Chapter 6, and Chapter 8 includes a very promising new method combining calorimetric and FTIR measurements. Mass spectrometry, a field in constant development and with an abundant literature, is ideally suited for the study of wide-ranging reaction types in the gas phase, including those related to atmospheric investigations, but is beyond the scope of this chapter. 

The standard tools such as FTIR, NMR, MS and UV-Vis are not sufficient for the requirements of nonaqueous electrochemistry, that is, detection of impurities in the solvents at a ppm level. The best of these methods, the NMR, can detect impurities only at a subpromil level, which is too high. As discussed in the chapter dealing with the electrochemical windows of nonaqueous systems, a few tens of ppm of contaminants can considerably affect the electrochemical behavior. Therefore, GC or HPLC are the only appropriate methods available today [3-5], However, it is important to note that there are solvents that can partially decompose in the injection port or in the column of the GC. The relevant solvents are those whose normal boiling points are high, and thus their GC analysis requires high temperatures. For example, PC, EC, BL and SL are typically problematic for GC analysis. 

The chapters in this volume address challenging problems associated with the observation and interpretation of anodic dissolution of semiconductors, electrode reactions in nonaqueous solvents, and charge-transfer across the interface between two immiscible electrolytes. In-situ FTIR spectroscopy of surface reactions, and a review of electrochemical methods of pollution abatement complete the range of timely topics included. 

Among the ex situ methods that can be employed in surface analysis, low-energy electron diffraction (LEED) and x-ray photoelectron spectroscopy (XPS) can give the crystal structure and the nature of the surface ad-layers after the electrochemical and adsorption experiments as explained in this chapter [31,32]. Among the in situ non-electrochemical techniques, the radiotracer method [33] gives information about the adsorbed quantities however, infrared spectroscopy in FTIR mode [34] allows the identity of the bonding of the adsorbed molecules, and finally ellipsometry [35] makes possible the study of extremely thin films. Recently, some optical methods such as reflectance, x-ray diffraction, and second harmonic generation (SHG) [36] have been added to this list. 

Now, in order to employ a locked-in detection system, as in EMIRS, the modulation frequency of the potential at the electrode would have to be at least an order of magnitude greater than F(i). Thus, the potential modulation would have to be between 70 and 100 KHz, too great to allow sufficient relaxation time for most electrochemical processes to respond. As a consequence, lock-in detection has not been employed in in-situ FTIR studies and the sensitivity of SNIFTIRS is less than that of EMIRS. Nevertheless, the FT method does have the sensitivity necessary to detect monolayers, and submonolayers, of adsorbed species [55, 56]. This arises out of the very large improvements in S/N ratio available to FT (compared with dispersive) infrared spectrometry by the Jacquinot and Fellgett advantages. 

Such a technique is possibly even more severely limited to reversible electrochemical systems than EMIRS. Using this method, the authors obtained time-domain FTIR spectra for the reduction of adsorbed TCNE anion radical at a reflective Pt electrode in MeCN/TBAF, stepped from a base potential of +0.6V to -0.8 V vs. Ag/Ag+, as shown in Fig. 57. The positive band near 2083 cm-1 corresponds to the adsorbed TCNE radical nion and the negative feature near 2065 cm-1 to the C=N stretch of the solution-soluble dianion. The time resolution was reported to be 1 ms between spectra with the use of a rapid-scan FTIR, capable of acquiring spectra at almost 100 s-1, the authors postulated temporal resolutions of the order of 10 /is. 

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