FTIR diffusion experiments

Characterization. In-situ diffuse reflectance FTIR (DRIFT) experiments were carried out with undiluted samples of the zeolites in a Spectratech DRIFT cell and a Nicolet Magna 550 spectrometer. Most experiments were carried out in a flow mode, passing 0.84 ml/s of a gas mixture containing inert (He, Ar or N2) and N2O, NO, CO or mixtures of these gases continuously through the cell at atmospheric pressure. Each spectrum was recorded by addition of 256 scans and a resolution of 8 cm. ... 

Fig. 5.12. FTIR microscopy study of diffusion-in profile of Cyasorb UV531 into PP a 3D plot of the IR spectra of the stabiliser at different distances from the surface of the PP plaque after a diffusion experiment at 60° C. After Hsu et al [473]. Reprinted with permission from S.C. Hsu et al.,AppL Spectrosc. 46, 225-228 (1992).

An elegant alternative method to measure sorption into polymers is the Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR) method. It allows in situ acquisition of the kinetic data and at the same time records the changes that occur in the polymer matrix due to the influence of the diffusant. Effects such as swelling, changes in morphology and polymer solvent interactions can all be simultaneously monitored. To calculate the diffusion coefficients from ATR-FTIR data, the mass uptake equation used in gravimetric diffusion experiments has to be modified to take into account the convolution of the evanescent field with the diffusion profile. 

Samples were characterized by FTIR spectroscopy with a Perkin Elmer (Spectrum BX) spectrometer using KBr pressed disks as matrices. The DRIFT experiments were carried out with a Broker IFS 55 spectrometer equipped with a Thermo Spectra Tech reacting cell. UV-vis Diffuse Reflectance spectra were recorded on a Perkin Elmer Lambda 45 spectrophotometer equipped with a diffuse reflectance attachment. Raman spectra were collected with Perkin Elmer system 2000 NIR FT-Raman using as excitation radiation the 5th harmonic of a diode pumped Nd YAG laser (1065 nm). 

Apparatus. Preliminary experiments were carried out in a modified Kiselev-type cell [21 ] with a grating spectrometer, PERKIN ELMER model 325. Precise measurements of diffusivities were conducted by means of a fast Fourier Transform IR (FTIR) spectrometer, PERKIN ELMER model 1800 inserted in a complex set-up equipped with UHV, gas dosing and mass flow control systems. Details of the cell and experimental devices will be described elsewhere [22]. 

These are GC-IR (5), LC-IR (6), and diffuse reflectance (7). On-the-fly GC-IR systems are commercially available, and lower detection limits are being continually reported. While GC-IR may not replace GC/MS in residue and metabolism work, it can provide valuable data in these areas. On-the-fly LC-IR systems have been developed and are also commercially available. The major problem in these systems is the strong infrared absorbence of many common LC solvents. However, with proper selection of solvents and the development of LC conditions specifically designed for the LC-IR experiment, these problems may be overcome. Recent reports on diffuse reflectance measurements by FTIR indicate the technique may provide a method of examining formulated material or TLC spots with no sample preparation. While this technique is still in the development stage, it may become quite significant in the future. 

The total acidity deterioration and the acidity strength distribution of a catalyst prepared from a H-ZSM-5 zeolite has been studied in the MTG process carried out in catalytic chamber and in an isothermal fixed bed integral reactor. The acidity deterioration has been related to coke deposition. The evolution of the acidic structure and of coke deposition has been analysed in situ, by diffuse reflectance FTIR in a catalytic chamber. The effect of operating conditions (time on stream and temperature) on acidity deterioration, coke deposition and coke nature has been studied from experiments in a fixed integral reactor. The technique for studying acidity yields a reproducible measurement of total acidity and acidity strength distribution of the catalyst deactivated by coke. The NH3 adsorption-desorption is measured by combination of scanning differential calorimetry and the FTIR analysis of the products desorbed. 

A lack of CO desorption observed for surfaces with variable structures is difficult to reconcile with work on dispersed ceria powders which are known from FTIR to adsorb CO and retain it in vacuo. It would seem that CO adsorption must occur at many structurally distinct sites on the ceria surface. Appearance of surface species on reduced ceria, may indicate that in fact some CO does adsorb, but is not seen in TPD because CO dissociates and the C and O diffuse into the bulk, rather than desorbing. A lack of CO adsorption also implies inability of CO to reduce ceria surfaces, although CO readily reduces ceria powder. It is possible that the low pressure conditions of UHV experiments is partly responsible for the general lack of CO adsorption and surface reduction. Li et al have shown that CO species linearly bound to Ce are unstable in vacuum and these may be partly responsible for the reduction of CCO2/ ... 

Virgin cellulose pellets and cellulose chars produced in the simulated fire apparatus were both examined. Two different measurements were made. One involved measuring the reflected radiation in the mid-infrared from 2.5 to 25 pro (4000 to 400 cm l). These measurements were performed in a diffuse reflectance cell within an FTIR spectrometer. These experiments revealed some wavelength dependence of refleclivity. Reflectance was also measured in-situ in the simulated fire apparatus, by arranging the samples, a fluxmeter. and the heating lamps such that surface reflection of (he incident radiation... 

Bruker Optics has launched the ALPHA, a very small FTIR designed to save space in the laboratory. It weighs less than 7 kg and has a footprint of only 22 X 30 cm. Solid, liquid and gas samples can be analysed and transmission, attenuated total reflection (ATR) and diffuse reflection experiments can be carried out. 

Surface acid densities were estimated using pyridine adsomtion monitored by combined thermogravimetric and infi-a-red spectroscopic techniques. Thermogravimetric analyses were carried out using a PC controlled Cl microbalance attached to a conventional vacuum line fitted with rotary and diffusion pumps. Approximately 100 mg of sample as a fragmented disc (prepared as per FTIR experiments, see below) was outgassed for 2 h at 573 K, then exposed to 1 Torr pyridine and cooled to 373 K. A further... 

It was confirmed that the presence of the carrier gas (helium) did not affect the diffusion measurements. When helium was replaced by neon, argon, or krypton, no change in the results was observed only xenon caused some deviation of the results (Niessen W, private communication). The main flow could be very rapidly connected with streams of the adsorbates in helium (8 ml min ) in such a way that the total flow and pressure remained constant. The adsorbate partial pressures could be varied, i.e., increased or decreased, almost instantaneously by small jumps. The experiment was started by scanning the spectrum of the pure, activated adsorbent. After a first pressure jump, e.g., from zero to 115 Pa, at a chosen adsorption temperature, the spectrum of the adsorbate/adsorbent was monitored in short intervals. An FTIR spectrometer of Perkin-Elmer type 1800 was employed. An example with sets of spectra of ethylbenzene adsorbed into H-ZSM-5 is shown in Fig. 3. 

Experiments devoted to the phenomena of co- and counter-diffusion using the micro-FTIR technique were basically performed in the same way as... 

The FTIR technique has proven to be a powerful method for investigating adsorption, desorption, and diffusion of single components or binary mixtures in microporous solids such as zeolites. In the latter case of mixtures, the phenomena of codiffusion and counter-diffusion became accessible to measurement, which was not possible with methods of investigation based on changes of weight, volume, or pressure. Even with the powerful and most important NMR techniques (see Chap. 3 of the present volume), the study of multicomponent (e.g., H2-D2) self-diffusion rather than co- and counterdiffusion experiments is possible (see Sect. 1 and [6]). The only prerequisite for the IR method is that the IR spectra, which are contributed by the components of the mixture, can be sufficiently decomposed. This, however, was easily achieved for all systems studied so far, owing to appropriate computer programs nowadays available. Certainly, the computational methods... 

A new method for measuring diffusion coefficients by IR spectroscopy was developed by Karge and Niessen [158],Niessen [132] and Karge et al. [939]. The change in the intensities of bands typical of molecules migrating inside a zeolite structure during their uptake or desorption were measured by FTIR spectroscopy under high time-resolution as a fimction of small partial pressure jumps and at various temperatures. An appropriate method of evaluation of the time-resolved spectra was developed on the basis of solutions of Fick s second diffusion law provided by Crank [940]. As an example,Fig. 53 illustrates an uptake experiment with benzene (B) as the adsorbate and H-ZSM-5 as the adsorbent. 

FTIR experiments on diffusion were also carried out with single crystals of H-ZSM-5 as adsorbent and paraffin molecules (n-hexane, n-heptane, n-octane, neopentane, n-nonane) as diffusing species [944,945]. In these experiments an IR microscope (cf. Sect. 4.1) was employed and a cell adopted from Lercher and coworkers [862,894] after slight modifications. 

Diffuse reflectance FTIR spectroscopy has been shown to be a useful tool with which to study base desorption from siliceous surfaces in situ. Related experiments have yielded insight regarding the types and relative acidity of... 

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