Single adsorption/desorption techniques

Before the introduction of measuring techniques such as pulsed field gradient (PEG) NMR ([14,16,45], pp. 168-206) and quasielastic neutron scattering (QENS) [49,50], which are able to trace the diffusion path of the individual molecules, molecular diffusion in adsorbate-adsorbent systems has mainly been studied by adsorption/desorption techniques [ 16]. In the case of singlefile systems, adsorption/desorption techniques cannot be expected to provide new features in comparison to the case of normal diffusion [51,52]. In adsorption/desorption measurements it is irrelevant whether or not two adjacent molecules have exchanged their positions. But it is this effect which makes the difference between normal and single-file diffusion. 

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... 

Since that time, the method is widely developed experimental setups are improved and adjusted to many different purposes (e.g. for the investigations of oxidation and reduction reactions). Today, two main types of equipment are available those operating under ultrahigh vacuum and so-called flow systems. WeU-defined surfaces of single-crystalline samples are investigated in a continuously pumped ultra-high vacuum (UHV) chamber (this technique is often referred to as thermal desorption spectroscopy—TDS [5]). The equipment that is constructed to allow adsorption-desorption in the gas flow are most often used for the investigation of porous materials (catalysts, for example). Vacuum setups are customarily used for surface science studies, but they can be also useful for the characterization of porous materials. 

Mention was made in Section XVIII-2E of programmed desorption this technique gives specific information about both the adsorption and the desorption of specific molecular states, at least when applied to single-crystal surfaces. The kinetic theory involved is essentially that used in Section XVI-3A. It will be recalled that the adsorption rate was there taken to be simply the rate at which molecules from the gas phase would strike a site area times the fraction of unoccupied sites. If the adsorption is activated, the fraction of molecules hitting and sticking that can proceed to a chemisorbed state is given by exp(-E /RT). The adsorption rate constant of Eq. XVII-13 becomes... 

There is a wealth of information available on CO chemisorption over single-crystal and polycrystalline platinum surfaces under ultrahigh-vacuum conditions research efforts in this area have gained a significant momentum with the advent of various surface analysis techniques (e.g., 2-8). In contrast, CO chemisorption on supported platinum catalysts (e.g., 9, 10, 11) is less well understood, due primarily to the inapplicability of most surface-sensitive techniques and to the difficulties involved in characterizing supported metal surfaces. In particular, the effects of transport resistances on the rates of adsorption and desorption over supported catalysts have rarely been studied. 

The mixed mode of sorption of the dye l,l -dioctadecyl-3,3,3, 3 -tetramethylindocar-bocyanine pechlorate (Dil) at the interface of an ODS stationary phase and ACN-water mobile phase was studied by single-molecule resolution and fluorescence imaging techniques. The measurements indicated that minimally four types of adsorption sites are present on the surface of the ODS stationary phase. The desorption times of the dye are different at the different adsorption sites resulting in a deformed peak shape [152],... 

Both GC and LC may be operated in one of several modes. The principal modes currently used for large-scale separations are elution, selective adsorption or desorption, and simulated countercurrent chromatography. In addition, reaction and separation can be combined in a single column with unique advantages. Elution is the most used and best developed form of the technique and is described first. 

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