Transmission IR Spectroscopy

A great variety of cells appropriate for transmission IR spectroscopy on zeolites and related materials has been developed. Some of them are described in Refs. [151-159], among them also flow-through reactor cells suitable for in-situ investigations of diffusion and catalytic processes and low temperature adsorption [153-157]. Here, we reproduce just a scheme of one of them (Fig. 9,cf. [158, 159]), which has been successfully employed in many IR studies by a number of laboratories. 

Windows either glued or sealed with O-rings (Viton) high-vacuum tight up to about 250°C in gas flow tight up to 650 C (glue AgO) 

In transmission IR experiments, single crystals, pressed powder wafers or small powder samples embedded in pressed KBr or Csl pellets are used. For a possible detrimental effect of the pressure required to prepare thin IR transparent wafers or KBr pellets etc. compare the following Sect. 4.2. 

Transmission FTIR spectroscopy may be combined with a number of other experimental techniques such as, e.g., temperature-programmed reduction (TPR), oxidation (TPO) or desorption (TPD) of probes monitored by, e.g., frequency response (FR) spectroscopy (see also Sect. 4.2), electron spin resonance (ESR) spectroscopy, etc. 

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SEM and transmission electron microscopy (TEM) are employed to examine materials for the presence and distribution of impact modifiers such as polybutadiene rubber in high impact polystyrene (HIPS) and methacrylate butadiene styrene terpolymer in PVC. Quantification is either by transmission IR spectroscopy against standards or nuclear magnetic resonance (NMR) spectroscopy. 

A powerful characteristic of the cell described above is the opportunities it affords for the determination of the compositions of both the lower (denser) and upper (lighter) phases. In particular, the combination of ATR and transmission IR spectroscopy shows the distribution of the catalyst between the two phases. For example, in the homogeneously catalyzed formylation of morpholine with carbon dioxide and hydrogen by a ruthenium catalyst, a two-phase system was found at a... 

The work described in the previous section was essentially concerned with the physical rather than chemical adsorption of some highly polarizable molecules on zeolites. With pyridine, such information can sometimes be obtained as easily using infrared spectroscopy. However, transmission IR spectroscopy cannot so easily be used to study chemisorption on oxides if it is essential to obtain low frequency spectral data (e.g., adsorbent-adsorbate stretching modes) because of the opacity of most oxides over much of the low frequency spectral region. Recent work has shown that the Raman technique can be extremely useful in this context (4). 

Cavanagh and Yates (48) used transmission IR spectroscopy to study OH-OD exchange on Rh-Al203. They suggested that the alumina hydroxyls reacted with D2 by a rapid spillover from Rh to alumina, followed by a slow migration on alumina and a rapid exchange of the spiltover D with the OH s. They confirmed the role of CO as a poison as claimed by Apple and Dybowski (24). 

Information on the silica surface structure obtained specifically by transmission IR spectroscopy before 1980 has been summarized by Hair... 

All of the examples described above have involved dilute solutions, where transmission IR spectroscopy is relatively easy to use, and there were sufficient windows in the spectrum of the supercritical solvent to permit the detection of the molecules of interest. However, this is not always the case. For example the IR spectrum of SCCO2 is totally absorbing between ca. 3700-3500 and... 

The IR range of combination and overtone modes hes above ca. 4000 cm". It is also extremely difficult to investigate this region by transmission IR spectroscopy because of the weak intensities of the respective bands which usually are only 3 to 5% of that of the fundamentals. In this context one has to keep in mind that the absolute transmission of zeolite wafers in the region of fundamental OH stretching modes is sometimes only 1% or even less (vide supra, e.g.. Sect. 5.4.1.2.3). 

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