Potential-difference infrared spectroscopy

In this study, adsorption of acetic acid under voltammetric conditions was observed by a vibrational technique for the first time. The first work in the field was carried out using FTIR (potential difference infrared spectroscopy, PDIRS) and by radioactive labeling [Corrigan et al., 1988]. Both techniques... 

LPSIRS Linear Potential Sweep Infra-Red Spectroscopy SPAIRS Single Potential Alteration Infra-Red Spectroscopy PDIRS Potential Difference Infrared Spectroscopy EMIRS Electrochemically Modulated Infra-Red Spectroscopy SERS Surface Enhanced Raman Scattering (Surface Enhanced Raman Spectroscopy)... 

The tremendous advances that have occurred in the spectroscopic analysis of the electrode/electrolyte interface have begun to provide a fundamental understanding of the elementary processes and the influence of process conditions. Surface-sensitive spectroscopic and microscopic analyses such as surface-enhanced Raman scattering (SERS) [1], potential-difference infrared spectroscopy (PDIRS) [2], surface-enhanced infrared spectroscopy (SEIRS) [3], sum frequency generation (SFG) [4], and scanning tunneling microscopy (STM) [5,6] have enabled the direct observation of potential-dependent changes in molecular structure [2,7] chemisorption [8,9], reactivity [10], and surface reconstruction [11]. 

Figure 1. Plots of surface concentration versus potential for ImM acetic acid from radiochemistry measurements ( ) and for lOmAf acetic acid from potential-difference infrared spectroscopy (o) at a platinum electrode in aqueous 0. IM HCIO4. (Adapted from Corrigan et al.)...

The same method is sometimes called PDIRS (for potential difference infrared spectoscopy) or SPAIRS (for single potential alteration infrared spectroscopy). 

Most infrared spectroscopy of complexes is carried out in tire mid-infrared, which is tire region in which tire monomers usually absorb infrared radiation. Van der Waals complexes can absorb mid-infrared radiation eitlier witli or without simultaneous excitation of intennolecular bending and stretching vibrations. The mid-infrared bands tliat contain tire most infonnation about intennolecular forces are combination bands, in which tire intennolecular vibrations are excited. Such spectra map out tire vibrational and rotational energy levels associated witli monomers in excited vibrational states and, tluis, provide infonnation on interaction potentials involving excited monomers, which may be slightly different from Arose for ground-state molecules. 

Reaction products can also be identified by in situ infrared reflectance spectroscopy (Fourier transform infrared reflectance spectroscopy, FTIRS) used as single potential alteration infrared reflectance spectroscopy (SPAIRS). This method is suitable not only for obtaining information on adsorbed products (see below), but also for observing infrared (IR) absorption bands due to the products immediately after their formation in the vicinity of the electrode surface. It is thus easy to follow the production of CO2 versus the oxidation potential and to compare the behavior of different electrocatalysts. 

Structural effect was confirmed by in situ infrared spectroscopy which showed a high coverage of adsorbed CO on a R( 110) plane conversely, on the other single-crystal planes, a distribution of different species is clearly visible. Further, strong lateral interactions between the different adsorbed species on Pt(lOO) lead to very low activity of this electrode at low potentials. ... 

Finally, it should be kept in mind that quantification is often problematic in surface analysis and characterization. Firstly because some techniques are not really suited for quantification, but also in cases such as infrared spectroscopy where one does not really know precisely how deep into the material one is probing. Although, there are many good examples of semi-quantitative applications that involve measuring relative band intensities that relate to changes in a surface property. However, for problem solving revealing qualitative differences is often sufficient information to be able to identify cause and move on to look for a potential solution. 

Infrared spectroscopy is by far the most popular tool for the inverstigation of matrix-isolated species. By virtue of the suppression of most rotations in sohd matrices, IR spectra recorded under these conditions typically show patterns of very narrow peaks, compared to spectra obtained under normal laboratory conditions (solution, Nujol, or KBr pellets), where bands due to different vibrations often overlap to the extent that they cannot be separated. As a consequence, matrix isolation IR spectra are—at least potentially—are a very rich source of information on the species under investigation. Whether and how all this information can be used depends on the ability to assign the spectra, a subject to which we will return below. 

The ten chapters are organized into three main sections. The first four chapters (Jansson) introduce the reader to basic concepts and progress through a survey of both traditional linear and modern nonlinear methods. Chapters 5 (Jansson), 6 (Blass and Halsey), and 7 (Halsey and Blass) detail specific applications of a proven method to the fields of electron spectroscopy for chemical analysis (ESCA) and high-resolution infrared spectroscopy via three different instrumental techniques. Also included are brief examples of applications to nuclear and Raman spectroscopy. The final section, Chapters 8 (Frieden), 9 (Howard), and 10 (Howard), illustrates recent work and reveals some directions for potential future research. 

The feasibility of thermic and calorimetric detection of the absorbed radiation has been mentioned in the context of grazing-incidence experiments. This is quite close to the class of photothermal techniques with which a number of different detection schemes is employed (Coufal, 1986). Out of these, photoacoustic spectroscopy (PAS) is frequently used in infrared spectroscopy (Graham et al., 1985 Urban et al., 1990 McClelland, 1992) while inspite of its potential, thermal beam deflection has not yet found as many applications as in other spectral ranges, possibly due to the lower availability of suitable lasers (Low and Morterra, 1985). 

The technique using p-s modulation has received different names depending on the kind of IR instrument used. Thus for grating instruments it was called PMIRRAS (polarization modulation infrared reflection-absorption spectroscopy) [6]. For FT spectrometers the name FTIRRAS [8] was suggested. However this name was later used also in connection with Fourier transform spectra applying the potential difference approach. 

Model electrodes with a dehned mesoscopic structure can be generated by a variety of means, e.g., electrodeposition, adsorption from colloidal solutions, and vapor deposition and on a variety of substrates. Such electrodes have relatively well-dehned physico-chemical properties that differ signihcantly from those of the bulk phase. The present work analyzes the application of in-situ STM (scanning tunneling microscopy) and ETIR (Eourier Transformed infrared) spectroscopy in determining the mesoscopic structural properties of these electrodes and the potential effect of these properties on the reactivity of the fuel cell model catalysts. Special attention is paid to the structure and catalytic behavior of supported metal clusters, which are seen as model systems for technical electrocatalysts. 

As shown in Table 25-2 (p. 761), four types of thermal detectors are used for infrared spectroscopy. The most widely used is a tiny thermocouple or a group of thermocouples called a thermopile. These devices consist of one or more pairs of dissimilar metal junctions that develop a potential difference when their temperatures differ. The magnitude of the potential depends on the temperature difference. 

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