SNIFTIRS spectrum

It will be clear that EMIRS and SNIFTIRS spectra are difference spectra and can be somewhat complex ( ). Typically they will contain positive absorption bands from species present in excess at potential El compared to potential E2 and negative absorption bands from species whose polulation changes oppositely with potential. In addition, bands which shift with potential will appear as a single bipolar band either with one lobe of each sign, figure 2, (or even more complex structures with three lobes). 

Figure 11. SNIFTIRS spectra from a Pt electrode in IM HClOi, + 0.5 mM p-difluorobenzene. Modulation from +0.2V to +0.4V.

Figure 6. SNIFTIR spectra of the adsorbed intermediates involved in the oxidation of 0.1 M CHjOH in 0.5 M HCIO4 on a smooth Pt electrode (p-polarized light modulation potential AE = 0.3 V averaging of 128 interferograms). Electrode potential (mV/RHE) (1) 370, (2) 470, (3) 570, (4) 670, (5) 770.

Figure 6.18 Subtractively normalized interfacial Fourier transform infrared spectroscopy (SNIFTIRS) spectra of a polished polyciystaUine Pt electrode, immersed in 0.1 M HCIO4, + 5 M CH3OH electrolyte. All spectra were normahzed to the base spectrum collected at 0 mV vs. RHE. (Reproduced from Iwasita and Vielstich [1988].)...

Figure 11.8 (a) SNIFTIR spectra of the species coming from methanol adsorption and oxidation at a Pt/C electrode 0.1 M HCIO4 + 0.1 M CH3OH 25 °C. (h) SNIFTIR spectra of the species coming from ethanol adsorption and oxidation on a Pt/C electrode 0.1 M HCIO4 + 0.1 M C2H5OH 25 °C. 

Fig. 2.11. Potential step SNIFTIRS spectra from a polycrystalline crystalline Pt electrode, in 10 2 M CH3OH/O.I M HCIO4. 2400-1700 cm 1 spectral region showing the changes in the bands for linear and multi bonded CO on Pt.

Fig. 2.12. Potential step SNIFTIRS spectra from a polished polycrystalline Pt electrode, in 3 M CHjOH/O.l M HC104. Reference spectrum taken at 50 mV. Other details as in Fig. 2.10.

Figure 7 shows SNIFTIRS spectra for isoquinoline molecules adsorbed on mercury. The reference spectrum in each case was obtained at 0.0V vs. a SCE reference electrode at this potential the molecules are believed to be oriented flat on the metal surface. The vibrational frequencies of the band structure (positive values of absorbance) are easily assigned since they are essentially the same as those reported by Wait et al. (22) for pure isoquinoline. The differences in the spectra are that the bands for the adsorbed species exhibit blue shifting of 3-4 cm" relative to those of the neat material, and the relative intensities of the bands for the adsorbed species are markedly changed. 

Fig. 36. Spectra for polycrystalline Pd electrode in 0.1 M NaC104 +25mM NaCN. The solid line is a combination of SNIF-TIRS and IRRAS spectra. Two SNIFTIRS spectra at -0.9 V (reference potential) and -1-0.7 V vs. Ag/AgCl are ratioed to obtain the spectrum shown. The dotted spectrum was obtained by a SNIFTIRS with s-polar-ized radiation. (After [124]). Reprinted by permission of Journal of Chemical Physics AIR...

Fig. 43. SNIFTIR spectra from a Ft electrode in 25 mM NaSCN+0.1 M KCIO4. Applied step, 0.4 V to -0.95 V vs. Ag/AgCl. The latter was taken as the reference potential. (Thken from [136]). Reprinted by permission of Elsevier Science.

Figure 6 SNIFTIR spectra of CO adsorbed on electrodes of nanometer-scale thin film of platinum supported on different substrate materials, 0.1 M H2SO4, = — 0.20 V, iis = 0.10 V. 

IR spectroscopy was used to obtain insights on the carbon monoxide absorption and oxidation mechanism on Pt-Ru electrocatalysts. Figure 17 shows the SNIFTIR spectra of CO on submonolayer Pt deposits on Ru(OOOl). Two bipolar bands are clearly visible at potentials from 0.1 to 0.8 V. Analyses of IR spectra (vide supra) attributed the bipolar band at lower frequencies to bhie-sliifted CO (i.e., moved to higher frequency) on polycrystalline Ru, whereas the higher-frequency bipolar band represents red-sliifted CO onPt(lll). ... 

Figure 17. SNIFTIRS spectra for a Ru(OOOl) electrode with a submonolayer of Pt in a CO-saturated 0.1 M H2SO4 solution. The reference spectrum is obtained at 0.075 V and the sample spectra are taken from 0.10 V incremented by 0.1 V up to 0.80 V. 8192 scans were co-added in 16 cycles, 512 scans each the resolution was 8 cm Spectra are offset for clarity. Reproduced with permission from Copyright (2001), The Electrochemical Society.

CO adsorption on Cu electrode surface is interfered with by specifically adsorbed anions. CO can be adsorbed below a certain definite potential, determined by the adsorption strength of CO and the anion. When CO molecules displace the specifically adsorbed anions on Cu electrode, a voltammetric peak is observed as exemplified for Cu(lOO) in CO saturated phosphate buffer solution in comparison with N2 saturated solution (Fig. 29). Subtractively normalized interfacial Fourier transform infrared spectroscopy (SNIFTIRS) spectra in Fig. 30 demonstrates that CO is adsorbed at -0.8 V vs. SHE but not at -0.4 V, and adsorbed phosphate anion vice versa. " This process is equivalent to charge displacement adsorption of CO on Pt electrode revealed by Clavilier et al The profile of the voltammogram depends greatly on the crystal... 

Figure 30. SNIFTIRS spectra from a Cu(lOO) electrode in CO saturated phophate solution (pH 6.8, 1.6 to 2.0°C). The spectra were acquired between -0.4 V vs. SHE (upward absorption band) and -0.8 V (downward absorption band). Spectrum (A) corresponds to adsorbed CO, and (B) to adsorbed phosphate anion. Reprinted from Ref. 224, Copyright (1998) with permission from Elsevier.

Fig. 10.9 SNIFTIR spectra obtained in the 1700-700 regionfora2mM [Cr(DMSO)g] (0104)3 solution in dimethylacetamide at an Au electrode. The reference potential was 0.16 V against an internal ferrocene-ferrocinium couple spectra were recorded at more negative potentials at intervals of 100 mV (From reference 17, with permission.)...

Figure 43 shows SNIFTIRS spectra of p-difluorobenzene taken at a Pt mirror electrode in aqueous acid solution for modulation between the base potential of -0.2 V and + 0.4V (vs. NHE). Table 2 shows the IR-active normal vibrational modes of the substrate. Of these, only the last three (the b3u modes) involve vibrations having a substantial component perpendicular to the electrode surface. From the work of Hubbard and co-workers [101], the difluorobenzenes are expected to adsorb flat for monolayer (or sub-monolay-... 

Figure 17.2.6 SNIFTIRS spectra of p-difluorobenzene in 1 M HCIO4 at a Pt electrode in different wavenumber regions. Each curve is a difference between spectra recorded at 0.2 and 0.4 V vs. NHE. The negative peaks correspond to spectral features dominant at 0.4 V, and the positive ones to features dominant at 0.2 V. [Data from S. Pons and A. Bewick, Langmuir, 1, 141 (1985). Figure reprinted from J. K. Foley, C. Korzeniewski, J. L. Daschbach, and S. Pons, Electroanal. Chem., 14, 309 (1986), by courtesy of Marcel Dekker, Inc.]...

The positions of both the end-on and the bridged adsorbate can be easily identified as a function of electrode potential. For comparison, SNIFTIR spectra were recorded in the same spectroelectrochemical setup as shown in Fig. 5.54. 

Fig. 5.54. SNIFTIR spectra of COad on a polycrystalline nickel electrode in contact with an aqueous 0.1 M KCIO4 electrolyte solution, m,RHE as indicated = 0.1 V (top spectrum), 0.2 V lower spectra 8 100 or 16 100 interferograms...

Fig. 9.10 Comparison of the SNIFTIRS spectra for pyridine adsorbed at an Au(lll) electrode acquired using CaFj prism and ZnSe hemispherical windows. Taken with permission from Ref [40].

Carbon monoxide has a large attenuation coefEcient for absorption of IR radiation. Therefore, the SNIFTIRS spectra of CO adsorbed at a Pt electrode surface are a convenient standard to test the S/N of a spectroelectrochemical setup. The left panel in Fig. 9.11 shows SNIFTIRS spectra of a monolayer of CO at Pt electrode surface recorded using a cell equipped with a Cap2 prism [91]. The right panel shows similar spectra recorded using a ZnSe hemispherical window. The IR bands of CO adsorbed at Pt are significantly Stark-shifted when the electrode potential is modulated between -200 and -f200 mV versus SCE. Consequently, the potential difference spectrum displays bipolar bands. Clearly a much better S/N for these bands is achieved when a ZnSe hemisphere is used as the window. 

Fig. 9.11 Comparison of SNIFTIRS spectra for CO absorbed at a Pt electrode surface acquired using Cap2 prism and ZnSe windows, 0.1 M FICI04 solution, potential modulated between f, = -0.2 V and E2=+0.2W versus SCE, taken with permission from Ref [91],...

Because of the subtraction, SNIFTIRS spectra are devoid of the common background signal from the aqueous electrolyte. However, they represent the difference between absorbances of organic molecules at potentials Ei and E2 (or precisely the difference multiplied by 2.3). In order to facilitate interpretation of such spectra, it is convenient to choose a value of the base potential Fi at which the molecules are totally desorbed into the bulk of the thin-layer cavity. The sample potential E2 corresponds then to the adsorbed state of the film. Consequently, SNIFTIRS spectra plot a difference between the absorbances of molecules desorbed into the thin-layer cavity at potential Ei and those adsorbed at the electrode surface at potential 2- Thus, positive bands (or positive lobes in bipolar bands) are due to absorbance by desorbed molecules and negative bands (or negative lobes in bipolar bands) are the result of absorbance by adsorbed molecules. 

Calculation of the Tilt Angle from SNIFTIRS Spectra... 

SNIFTIRS spectra do not allow direct determination of the tilt angles of adsorbed molecules because of the superposition of the bands of molecules in the adsorbed state onto the bands of molecules desorbed into the thin-layer cavity. In order to determine the tilt angle, one has to calculate the absorbance for the adsorbed film ... 

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