Tracer exchange

All the above methods measure transport diffusivities rather than self-diffusivities. Self-diffusivities may, however, be measured in a modified uptake rate experiment in which the absorbent particle is exposed to a change in the concentration of an isotopically labeled tracer at constant total sorbate concentration. This method is particularly convenient when one of the isotopes is radioactive since the activity count then provides a simple and accurate means of monitoring the concentration of the labeled species. Self- 

Nonradioactive isotopes may also be used but are somewhat less convenient since it is then necessary to follow the progress of the exchange by mass spectromctvic analysis of the surrounding vapor. This method was used by Quig and Rees and Lindsley and Rees to study the self-diffusion of hydrocarbons in zeolite A and chabazitc. 

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We assume that the time resolution of the STM-measurements is sufficient to discriminate the effect of the entire random walk of one individual vacancy from that of the next, which was already shown to be the case in Section 2.2. The starting situation is shown in Fig. 13. A tracer atom is embedded in the origin of a square lattice with a vacancy sitting next to it at (1, 0). We assume that the only diffusion barrier that is modified by the tracer atom is that for vacancy-tracer exchange. 

The methods described so far for studying self-diffusion are essentially based on an observation of the diffusion paths, i.e. on the application of Einstein s relation (eq 3). Alternatively, molecular self-diffusion may also be studied on the basis of the Fick s laws by using iso-topically labeled molecules. As in the case of transport diffusion, the diffusivities are determined by comparing the measured curves of tracer exchange between the porous medium and the surroundings with the corresponding theoretical expressions. As a basic assumption of the isotopic tracer technique for studying self-diffusion, the isotopic forms are expected to have... 

While a direct tracer exchange would not be substantially influenced by Pt, the Pt-coating makes the chemical measurement more similar to the electrical measurement as in both cases the electrons are... 

In inorganic chemistry valuable information about chemical bonding in complexes has been obtained by investigation of exchange reactions and use of radio-tracers. Exchange of ligands L... 

For an estimate of the correlation between t he intracrystalline mean life time and the system properties, the exchange curve between the (labelled) molecules of a single-file system and the (unlabelled) surroundings ( tracer exchange curve ) may be assumed to be determined by a single dimensionless parameter... 

This relation is derived by considering a steady-state tracer exchange between the ascending and descending emulsions and the bubbles, and comparing the resulting equation with that of the diffusion model. Since Ee = mieE s, the above equation is equivalent to the following relation from van Deemter (V2) ... 

More recently, a detailed study of diffusion of the xylene isomers in large crystals of NaX and natural faujasite was undertaken by both sorption rate and tracer exchange.(11-14) The data obtained by both these techniques using several different crystal sizes were entirely consistent but the diffusivities were much smaller than the values derived for the same systems by NMR PFG measurements. In an attempt to resolve this discrepancy we have developed a new chromatographic technique (zero length column or ZLC) which is less sensitive than conventional sorption methods to the intrusion of external heat and mass transfer resistances and which is therefore useful for following relatively rapid diffusion processes. The method has now been applied to study the diffusion of a range of different hydrocarbons in both A and X zeolite crystals and the results of these studies are summarized here. 

Figure 2. Arrhenius plot showing comparison of NMR and sorption diffusivities for benzene and o-xylene in NaX zeolite crystals. NMR data from (1) Germanus et al. (19) and (2) Karger and Ruthven (10). Uptake (corrected diffusivity) and tracer exchange data of Goddard (11-13) (50 pm and 100 pm NaX, 250 pm faujasite). ZLC data of Eic (15,16).

In Chapter 1, Fyfe, Mueller, and Kokotailo describe the applications of solid-state NMR to the study of zeolite molecular sieve catalysts and related systems. Zeolites provide an apt arena in which to demonstrate the capabilities of modern techniques such as sample spinning, cross-polarization, and multidimensional correlation spectroscopy. In Chapter 2, Karger, and Pfeifer consider the question of molecular diffusion in catalyst systems. Applications of NMR techniques such as imaging, lineshape analysis, relaxation, pulsed field gradient echo spectroscopy, and NMR tracer exchange are described and compared with other, more traditional techniques such as radioactive tracing. In Chapter 3, Haw discusses the use of NMR to probe catalytic processes, showing how the combination of temperature control with novel NMR probes makes it possible to elucidate reaction mechanisms in situ. 

Fisitre 1 Experimental arrangement for NMR tracer exchange experiments. (From Ref. 6.)... 

Figure 2 Typical time dependence observed in NMR tracer exchange experiments (benzene on ZSM-5 crystallites of 55 x 55 x 90 p.m , 4 molecules per unit cell, 386 K. 1 - 7(0 denotes the relative amount of still adsorbed at time t. (From Ref. 6.)...

Figure 3 compares the self-diffusivities determined on the basis of NMR tracer exchange measurements involving benzene in silicalite [61 with the results... 

Figure 3 Sell-diffusion coefficients of benzene in ZSM-5 determined by the NMR tracer exchange method at 293 K ( ) and 386 K (A), plus comparison with the results of uptake measurements at 303 K (0)< 363 K ( ), and 393 K (A), transformed into self-diffusivities via Eq. (5). (From Refs. 6 and 11.)...

With the TEX-PEP technique experiments on the diffusion and adsorption of mixture of n-hexane/2-methylpentane in large silicalite-1 crystals have been performed. By modeling the experimental tracer exchange curves values of intracrystalline diffusion coefficient and adsorption constant were obtained. Slight preference for the adsorption of /t-hexane was found. Diffusivity of -hexane sharply decreases with increasing fraction of its isomer, since the last one occupies channel intersections thus blocking zeolite network. 

Fig. 12 Comparison of corrected diffusivity of benzene in silicalite-1 and H-ZSM-5 at low sorbate concentrations. Van Den-Begin et al., square wave o Eic and Ruthven, ZLC A, V Zikanova et al., piezometric silicalite-1, + H-ZSM-5, SSFR NMR tracer exchange. From Shen and Rees [69]...

Hufton JR, Brandani S, Ruthven DM (1994) Measm-ement of intracrystalline diffusion by zero length column tracer exchange. In J Weitkamp, Karge HG, Pfeifer H, Holderich W (eds) Zeolites and related microporous materials state of the art 1994. Proc 10th Int Zeolite Conf, Garmisch-Partenkirchen, Germany, 17-22 July 1994. Elsevier, Amsterdam, p 1323 Stud Surf Sci Catal 84 1323... 

A plot of the relative intensity of the broad constituent versus the observation time (i.e. the separation between the two field gradient pulses) contains information which is analogous to that of a tracer exchange experiment between a particular crystallite containing e.g. labelled molecules and the unlabelled surroundings. Therefore, this way of analysis of PFG NMR data of zeolitic diffusion has been termed the NMR tracer desorption technique [60]. The first statistical moment ( time constant ) of the NMR tracer desorption curve represents the intracrystalline mean lifetime Tintra of the molecules under study. 

Maximum kinetic energy of positrons Temporal analysis of products tapered element oscillating mass balance tracer-exchange positron emission profiling unit cell... 

Figure 6 shows a schematic diagram of the reactor system used for tracer-exchange positron-emission profiling (TEX-PEP) experiments. During these tracer exchange experiments, a constant flow of unlabeled hydrocarbons in a hydrogen carrier stream is fed into the reactor. The -hexane/... 

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