Potential-difference IR spectroscopy

The range of metals for which similar results were found was expanded in 1997, when Lu et al. applied potential difference IR spectroscopy to show significant intensity enhancement for CO adsorbed from solution onto thin layers of platinum and palladium on a glassy carbon support. When CO was adsorbed on smooth Pt and Pt electrodes and the spectrum subtracted from the spectrum measured after oxidation of the CO to CO2 by raising the electrode potential, a weak absorption band was observed in the difference spectrum, see Fig. 3a. When the corresponding spectra were measured from thin layers of Pt and Pd on glassy carbon substrate, the intensity of the band due to adsorbed CO was enhanced by a little more than a factor of 20, see Fig. 3b. It can also be seen that whereas all of the CO that was adsorbed on the Pt layer is linearly bonded, when CO is adsorbed on the Pd layer,the CO... 

Both these concerns were addressed by the development of modified IR techniques. In the technique of Subtractively Normalised Fourier Transform IR Spectroscopy (SNIFTIRS) or Potential Difference IR (SPAIRS or PDIR) [37], the increased stability and sensitivity of Fourier Transform IR is exploited, allowing usable spectra to be obtained by simple subtraction and ratioing of spectra obtained at two potentials without the need for potential modulation or repeated stepping. A second technique which does not call for potential modulation, but actually modulates the polarisation direction of the incoming IR beam is termed Photo-elastically Modulated Infra-Red Reflectance Absorption Spectroscopy (PM-IRRAS) this was applied to the methanol chemisorption problem by Russell and co-workers [44], and Beden s assignments verified, including the potential-induced shift model for COads. 

It is this aspect that makes the combination of infrared (IR) spectroscopy with electrochemical reactions so attractive. The infrared spectrum in the wavelength range from 4 to 20 pm reflects the structural properties of a polypeptide molecule, both for backbone and the side chain conformations. However, the potential of IR spectroscopy is better exploited with reaction-induced IR difference spectroscopy. 

Polarisation modulation infrared rejiection-absorption spectroscopy (PM-IRRAS or JRRAS). 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. 

These several techniques for the solid-solution interface give different kinds of information. However, the one which gives most information about the nature of entities on the surface, and potentially near the surface, is fourier transform IR spectroscopy, which is not restricted to a particular metal, or, indeed, to the type of substrate (except that this must be reflecting). 

Organic and inorganic molecular species (except homonuclear molecules) absorb in the IR region. IR spectroscopy has the potential to determine the identity of an unusually large number of substances. Moreover, the uniqueness of a MIR spectrum confers a degree of specificity which is matched or exceeded by relatively few other analytical methods. This specificity has found particular applications for the development of quantitative IR absorption methods. However, these differ from quantitative UV/Vis techniques in their greater spectral complexity, narrower absorption bands, and the technical limitations of IR instruments. Quantitative determinations obtained from IR spectra are usually inferior in quality and robustness to those obtained with UV/Vis and NIR spectroscopy. In addition, univariate or linear cali-... 

A first parameter to be studied is the applied potential difference between anode and cathode. This potential is not necessarily equal to the actual potential difference between the electrodes because ohmic drop contributions decrease the tension applied between the electrodes. Examples are anode polarisation, tension failure, IR-drop or ohmic-drop effects of the electrolyte solution and the specific electrical resistance of the fibres and yarns. This means that relatively high potential differences should be applied (a few volts) in order to obtain an optimal potential difference over the anode and cathode. Figure 11.6 shows the evolution of the measured electrical current between anode and cathode as a function of time for several applied potential differences in three electrolyte solutions. It can be seen that for applied potential differences of less than 6V, an increase in the electrical current is detected for potentials great than 6-8 V, first an increase, followed by a decrease, is observed. The increase in current at low applied potentials (<6V) is caused by the electrodeposition of Ni(II) at the fibre surface, resulting in an increase of its conductive properties therefore more electrical current can pass the cable per time unit. After approximately 15 min, it reaches a constant value at that moment, the surface is fully covered (confirmed with X-ray photo/electron spectroscopy (XPS) analysis) with Ni. Further deposition continues but no longer affects the conductive properties of the deposited layer. 

An additional difficulty in the determination of actual TOF values for zeolite catalysed reactions deals with the accessibility by reactant molecules to the narrow micropores in which most of the potential active sites are located. The didactic presentation in Khabtou et al.[37] of the characterization of the protonic sites of FAU zeolites by pyridine adsorption followed by IR spectroscopy shows that the concentration of protonic sites located in the hexagonal prisms (not accessible to organic molecules) and in the supercages (accessible) can be estimated by this method. Base probe molecules with different sizes can also be used for estimating the concentrations of protonic sites located within the different types of micropores, which are presented by many zeolites (e.g. large channels and side pockets of mordenite1381). The concentration of acid sites located on the external surface of the... 

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. 

F. Ozanam, C. da Fonseca, A. V. Rao, and J.-N. Chazalviel, In situ spectroelectrochemical study of the anodic dissolution of silicon by potential-difference and electromodulated FT-IR spectroscopy, Appl. Spectrosc. 51(4), 519, 1997. 

The general view is that it is the activation of water, step (5.31), coupled with the strong adsorption of COHad (or its product, COad), that is the difficulty here (as in the case of O2 reduction, I o is only of the order of 10 A/cm ). Actually, the status of COad, i.e. whether it is a true reaction intermediate or merely a poison (as it is in the case of H2 oxidation, cf. Section 5.6.3), is quite controversial What is certain is that COad is there on the surface during the reaction — as shown by, e.g., in situ IR spectroscopy — and also that COad obtained via adsorption of CO shows different oxidation kinetics from COad derived form MeOH. What causes this latter phenomenon is not completely imderstood yet, but adsorption geometry (e.g. island formation) is surmised to be the main factor. Also, while CO can easily replace Had, MeOH cannot, and so its adsorption can only start in earnest when the Pt surface is substantially free from Had (he. at > 0.25V, whereas Eq = 0.01 V), thus constituting another reason why MeOH carmot react at low potentials. 

Mass spectrometry (22, 96,181, 530, 531) and NMR spectroscopy (65, 69,181,209,431) have been recorded. The UV spectra of the dienone alkaloids differ in the extinction maxima (proaporphine alkaloids at 220 nm (e 24800) and 285 nm (c 3000) promorphinane alkaloids 230 nm (e 24,000) and 285 nm (e 7500)). IR Spectroscopy reveals characteristic frequencies of the cross-conjugated dienone at 1655, 1630, and 1613 cm"i. On polarography the proaporphine and the promorphinane dienone compoimds are reduced (335) at a half-wave potential similar to that of aromatic aldehydes. 

A thermocouple is made by welding together at each end two wires made from different metals (Fig. 4.15). If one welded joint (called the hot junction) becomes hotter than the other joint (the cold junction), a small electrical potential develops between the joints. In IR spectroscopy, the cold junction is carefully screened in a protective box and kept at a constant temperature. The hot junction is exposed to the IR radiation, which increases the temperature of the junction. The potential difference generated in the wires is a function of the temperature difference between the junctions and, therefore, of the intensity of IR radiation falling on the hot junction. The response time of the thermocouple detector is slow thermocouples cannot be used as detectors for FTIR due to their slow response. 

The thieno[3,2-Z)]thiophene (145), selenolo[3,2-6]thiophene (146) and selenolo[3,2-Z)]selenophene (151) charge-transfer complexes based on TCNE were studied by UV and IR spectroscopy. The absorption spectra recorded in CCI4 and CH2CI2 show numerous charge-transfer bands (82CS2I4). A comparison with photoelectron spectra of other donors demonstrated that these bands appear due to transitions from two different occupied orbitals of the donor to unoccupied orbitals of the acceptor. The calculated ionization potentials of the donors are consistent with the photoelectron spectroscopic data. 

---