Surface IR spectra

Figure 14 Surface IR spectra of etched LDPE-ATR spectra recorded with a KRS-5 reflection element, at 45° angle of incidence. Times refer to chronic acid itch duration. (From Ref. 76.)...

Figure Bl.22.1. Reflection-absorption IR spectra (RAIRS) from palladium flat surfaces in the presence of a 1 X 10 Torr 1 1 NO CO mixture at 200 K. Data are shown here for tluee different surfaces, namely, for Pd (100) (bottom) and Pd(l 11) (middle) single crystals and for palladium particles (about 500 A m diameter) deposited on a 100 A diick Si02 film grown on top of a Mo(l 10) single crystal. These experiments illustrate how RAIRS titration experiments can be used for the identification of specific surface sites in supported catalysts. On Pd(lOO) CO and NO each adsorbs on twofold sites, as indicated by their stretching bands at about 1970 and 1670 cm, respectively. On Pd(l 11), on the other hand, the main IR peaks are seen around 1745 for NO (on-top adsorption) and about 1915 for CO (tlueefold coordination). Using those two spectra as references, the data from the supported Pd system can be analysed to obtain estimates of the relative fractions of (100) and (111) planes exposed in the metal particles [26].

The polarization dependence of the photon absorbance in metal surface systems also brings about the so-called surface selection rule, which states that only vibrational modes with dynamic moments having components perpendicular to the surface plane can be detected by RAIRS [22, 23 and 24]. This rule may in some instances limit the usefidness of the reflection tecluiique for adsorbate identification because of the reduction in the number of modes visible in the IR spectra, but more often becomes an advantage thanks to the simplification of the data. Furthenuore, the relative intensities of different vibrational modes can be used to estimate the orientation of the surface moieties. This has been particularly useful in the study of self-... 

Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy. Attenuated total redectance (atr) ftir spectroscopy is based on the principle of total internal redection (40). Methods based on internal redection in the uv and visible regions of the spectmm are also common in addition to those in the ir region. The implementation of internal redection in the ir region of the spectmm provides a means of obtaining ir spectra of surfaces or interfaces, thus providing moleculady-specific vibrational information. 

The ir spectra acquired in this way are extremely sensitive to the orientation of the surface molecules. Molecules must have a significant component of a molecular vibration perpendicular to the surface to be sensed by coupling with the highly directional electric field. Molecules whose dipole moments are perfectly parallel to the surface caimot couple to the existing electric fields, and therefore, are ir transparent by this method. This selectivity of the approach for molecule dipole moments perpendicular as opposed to parallel to the surface is known as the surface selection rule of irras. 

With the increase in irradiation dose, however, there is an optimum value of the surface energy at 10 Mrad irradiation. In order to explain these results, the grafting levels calculated from the IR spectra [30] are... 

It is worth noting that each Na atom appears to perturb the electron density of the Pt(lll) surface over large ( 12) atomic distances. This can explain nicely the observed long-range promotional effect of Na on Pt surfaces. It is strongly reminiscent of the IR spectroscopic work of Yates and coworkers who showed that a single adsorbed alkali atom can affect the IR spectra of up to 27 coadsorbed CO molecules.80... 

Fig. 3 shows the IR spectra of the adsorbed species generated on the surface in the presence of both Hj and Oj at different temperatures. No obvious absorption band due to the adsorbed species was observed at 298 K. When the temperature was increased to 473 K, tw o weak bands at 3740 and 3670 cm assigned to the stretching vibrations of a non-acidic and acidic OH groups, respectively, were observed. These two bands were also observed in H,... 

Scheme 2 is still oversimplified, because it does not take into consideration that the two silicon atoms directly involved in the hydroxyl condensation are also linked to other rings in a three-dimensional mode and that part of the surface strain could be localized on these rings. The appearance in the IR spectra of new vibrations in the 880-940 cm region, attributed to the modes of strained siloxane bridges in two membered rings [26,28-32], well evidences this fact. 

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