Abstract
Labeling in diffusion measurements by pulsed field gradient (PFG) NMR is based on the observation of the phase of nuclear spins acquired in a constant magnetic field with purposefully superimposed field gradients. This labeling does in no way affect microdynamics and provides information about the probability distribution of molecular displacements as a function of time. An introduction of the measuring principle is followed by a detailed description of the ranges of measurements and their limitation. Particular emphasis is given to an explanation of possible pitfalls in the measurements and the ways to circumvent them. Showcases presented for illustrating the wealth of information provided by PFG NMR include a survey on the various patterns of concentration dependence of intra-particle diffusion and examples of transport inhibition by additional transport resistances within the nanoporous particles and on their external surface. The latter information is attained by combination with the outcome of tracer exchange experiments, which are shown to become possible via a special formalism of PFG NMR data analysis. Further evidence provided by PFG NMR concerns diffusion enhancement in pore hierarchies, diffusion anisotropy and the impact of diffusion on chemical conversion in porous catalysts. A compilation of the specifics of PFG NMR and of the parallels with other measurement techniques concludes the paper.
Highlights
Among the techniques of diffusion measurement, pulsed field gradient (PFG) NMR, known in the literature as NMR diffusometry, PGSE NMR, and DOSY NMR, is distinguished by its capability of tracking molecular ensembles along their diffusion pathways
It was the associated loss in sensitivity that has given rise to the development of the PFG NMR [29, 67, 68], followed by a most impressive progress in the application of NMR diffusometry, making it the possibly most versatile technique of diffusion measurement quite in general
With the effective diffusivity provided by Eq (24) we have considered the limiting case that the diameter of the sphere to which diffusion is restricted is negligibly small in comparison with the mean diffusion path length that a molecule would cover without this restriction
Summary
Among the techniques of diffusion measurement, pulsed field gradient (PFG) NMR, known in the literature as NMR diffusometry, PGSE NMR, and DOSY NMR, is distinguished by its capability of tracking molecular ensembles along their diffusion pathways. PFG NMR measurements are generally performed under equilibrium conditions and provide, correspondingly, the coefficient of self- or tracer diffusion. Selfdiffusivities may in general be taken as a first-order approach of the transport diffusivities. They do even coincide in the limiting case of sufficiently small guest concentrations, as a simple consequence of the fact that a distinction between equilibrium and non-equilibrium phenomena becomes meaningless in systems with non-interacting elements [4]. A summary of existing challenges and a view on the potentials of the use of PFG NMR as a complementary technique in combination with other methods of diffusion measurement conclude the contribution
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