Cells infrared

The construction of two typical infrared cells is shown in Fig. 19.6. Such cells have to be carefully filled by using a syringe or Pasteur pipette to ensure that no air is trapped inside. To prevent evaporation the ports should be plugged with small plastic stoppers once the cell has been filled with the solution. 

When a beam of monochromatic radiation is passed through the windows of an infrared cell some reflection occurs on the window surfaces and interference takes place between radiation passing from the internal surface of the first window and that reflected back from the internal surface of the second window. This interference is at a maximum when 2d = (n + 1 /2)k, where d is the distance in yum between the inner surfaces of the two cell windows, X is the wavelength in m, and n is any integral number. If the wavelength k of the monochromatic radiation is varied continuously an interference pattern consisting of a series of waves (Fig. 19.7) is obtained. 

For infrared spectroscopy, 20-50 mg of the cobalt-exchanged zeolite was pressed into a self-supporting wafer and placed into an infrared cell similar to that described by Joly et al. [21], Spectra were recorded on a Digilab FTS-50 Fourier-transform infrared spectrometer at a resolution of 4 cm-i. Typically, 64 or 256 scans were coadded to obtain a good signal-to-noise ratio. A reference spectrum of Co-ZSM-5 in He taken at the same temperature was subtracted from each spectrum. 

Gases were supplied to the infrared cell from a gas manifold. 4.99% NO in He and 2.14% CH4 in He were obtained from Matheson. Oxygen and helium were obtained on-site. The He, NO, and CH4 cylinders were passed through an oxysorb trap, an ascarite trap, and a molecular sieve trap, in that order, for additional purification. The O2 was passed through an ascarite and a molecular sieve trap. 

Since the formation of NO2 can occur homogeneously, it was of interest to establish whether adsorbed NO could be oxidized. NO was adsorbed at 225 C, after which the infrared cell was purged with He and subsequently a stream of 10.1% O2 in He was allowed to flow over the catalyst. Prior to the introduction of the 02-containing stream, the only features evident were those for mono- and dinitrosyls. In the presence of O2 at 225 °C, the intensities of the bands for both mono- and dinitrosyl species attenuated and new features appeared at 1628 and 1518 cm-, corresponding to nitrate and nitrito species, respectively. A similar experiment carried out in the absence of O2, showed only a small decrease in the intensity of the nitrosyl bands due to NO desorption and the absence of bands for nitrate and nitrito species during a 30 min purge in He at 225 °C. 

By carrying out photolyses in liquid nitrogen- or liquid helium-cooled infrared cells using a special low-temperature apparatus (see Figure 4.2), one is often able to obtain direct spectroscopic evidence for intermediates of photochemical reactions. In this section we will briefly review how low-temperature techniques have been used to observe intermediates in type I cleavage reactions. 

Although this spectrum does not correspond to any particular ruthenium carbonyl complex, it is consistent with the presence of one or more anionic ruthenium carbonyl complexes, perhaps along with neutral species. Work is in progress with a variable path-length, high pressure infrared cell designed by Prof. A. King, to provide better characterization of species actually present under reaction conditions. 

In this paper we will first describe a fast-response infrared reactor system which is capable of operating at high temperatures and pressures. We will discuss the reactor cell, the feed system which allows concentration step changes or cycling, and the modifications necessary for converting a commercial infrared spectrophotometer to a high-speed instrument. This modified infrared spectroscopic reactor system was then used to study the dynamics of CO adsorption and desorption over a Pt-alumina catalyst at 723 K (450°C). The measured step responses were analyzed using a transient model which accounts for the kinetics of CO adsorption and desorption, extra- and intrapellet diffusion resistances, surface accumulation of CO, and the dynamics of the infrared cell. Finally, we will briefly discuss some of the transient response (i.e., step and cycled) characteristics of the catalyst under reaction conditions (i.e.,... 

The concentration of a liquid sample contained in an infrared cell of thickness 0.15 pm is 0.5 M. Calculate the molar absorptivity of the sample if the absorbance at a specified wavenumber is 0.300. 

Pearson and Mauermann/Squires—metallocarboxylate intermediates as C02 precursor over Fe carbonyl catalysts. In 1982, Pearson and Mauermann65 studied two reaction steps of the water-gas shift mechanism involving Fe(CO)5 in basic media using the infrared cell of Ford these included (a) Fe(CO)5 + OH- <-> HFe(CO)4 + C02 and (b) H2Fe(CO)4 <-> H2 + Fe(CO)4. For reaction (a), they proposed the following mechanism, as shown in Scheme 26 ... 

Fig. 5.13. Diagrams of commercial fixed path length infrared cells. Lead, or Teflon spacers provide the cell thickness and also seal the cell from leakage.

The optical path length of an infrared cell can be determined using the method shown in the text. Determine the optical path lengths for the following sodium chloride cells. 

Figure 1. Infrared cell used to hold extraction solutions...

It is theorized that perch oroethylene increases the desorption of unsaturated and aromatic components that would not desorb into pure Freon 113. There were several reasons why the initial desorption was accomplished with 0.5 ml perchloroethylene and then brought up to 10 ml with Freon 113. Using a 2 cm infrared cell, perchloroethylene is not completely IR inactive in the region of interest. It has a weak peak at approximately 2870 cm- , which would cause problems if pure perchloroethylene were used. Secondly, use of Freon 113 lowers the inhalation hazard to the analyst. Table II lists the recoveries from charcoal tubes at different levels using the Freon 113/perchlo-roethylene mixture. 

Figure 27-7 Combustion analyzer that uses infrared absorbance to measure C02, H20, and S02 and thermal conductivity to measure N2. Three separate infrared cells in series are equipped with filters that isolate wavelengths absorbed by one of the products. Absorbance is integrated over time as the combustion product mixture is swept through each cell. [Courtesy Leco Corp., st. Joseph, Ml.)...

For IR measurements the catalyst was compressed at 4 X 108 Pascal. The disc (18 mm diameter, 20-30 mg) was mounted in a quartz sample holder which was introduced in the adsorption/infrared cell (10). To avoid the reduction of the Pd(II) ions by hydrocarbons, the cell was grease free. Samples were activated according to treatment c. Spectra were recorded on a Perkin Elmer model 125 grating spectrometer. The reference beam was attenuated, and the instrument was flushed with air freed of H20 and C02. 

The device for deuterium exchange consisted of a circulation circuit with a 2000-ml reservoir containing 96% of the total volume of the system. D2 could be circulated over the sample in the infrared cell by using a magnetic pump. The pump speed was calculated to be high enough to eliminate... 

Techniques. Platelets of approximately 30 X 26 mm, weighing 3 to 4 mg/cm2, were prepared by pressing the powdered samples at 7 tons for 2 minutes. These were placed in the infrared cell, evacuated at room temperature under a vacuum of 10 6 torr, and heated slowly to 400°C (18). The samples were then cooled and heated again at reaction temperature. 

A gas-washing bottle (Figure 4.7B) may also be used for trapping. This technique is especially useful in conjunction with infrared analysis. The sample is simply bubbled through the anhydrous solvent as it exits the chromatographic column. The solution is then placed in a liquid sample infrared cell. A matching cell containing only the solvent is placed in the reference beam. An infrared spectrum of the sample may then be recorded. 

Heated attenuated total reflection (ATR) infrared cell (see Critical Parameters) FTIR spectrometer (see Critical Parameters)... 

The attenuated total reflection (ATR) infrared cell should be equipped with a single bounce element made of zinc selenide (ZnSe), diamond, or equivalent material, with capacity of 50 pL or less. It must be capable of maintaining a constant temperature of 65° 2°C. 

A versatile low-temperature infrared cell is presented in Fig. 9.16.20 It is suitable for mull or pressed-disk infrared spectra of solids, solid state infrared spectra of condensed vapors, and spectra of products from solid-gas reactions. Mulled or powdered samples are supported between alkali halide windows in the copper block B, which is cooled by refrigerant in the cold finger A. Before this refrigerant is added, the cell must be evacuated through the needle valve. When the cell is used for low-temperature spectra of condensables, the vapors are bled through the needle valve and squirted directly onto one surface of a cold alkali halide plate held in block B. 

Fig. 9.16, Low-temperature infrared cell (cross-seclion). An indium gasket between the window and copper block greatly increases the efficiency of heal transfer, if the temperature of the window is to be measured with a thermocouple, Ihe leads may be passed though pinholes in the upper O-ring. Under compression of Ihe joint, this type of electrical lead-through is vacuum-light. Threaded rods used to clamp Ihe end plates have been omitted for clarity. A spring holds Ihe KBr disk in place.

Low-temperature adsorption is also useful for the quantitative transfer of gases into special apparatus. For example, a small quantity of activated molecular sieve in a freeze-out tip on an infrared cell may be used for the identification of methane or, when placed in a freeze-out tip on a manometer and calibrated volume, the molecular sieves permit the measurement of the quantity of noncon-densable gas. 

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