Single-molecule observation of diffusion and catalysis in nanoporous solids

Abstract

Nanoporous solids, including microporous, mesoporous and hierarchically structured porous materials, are of scientific and technological interest because of their high surface-to-volume ratio and ability to impose shape- and size-selectivity on molecules diffusing through them. Enormous efforts have been put in the mechanistic understanding of diffusion–reaction relationships of nanoporous solids, with the ultimate goal of developing materials with improved catalytic performance. Single-molecule localization microscopy can be used to explore the pore space via the trajectories of individual molecules. This ensemble-free perspective directly reveals heterogeneities in diffusion and diffusion-related reactivity of individual molecules, which would have been obscured in bulk measurements. In this article, we review developments in the spatial and temporal characterization of nanoporous solids using single-molecule localization microscopy. We illustrate various aspects of this approach, and showcase how it can be used to follow molecular diffusion and reaction behaviors in nanoporous solids.

Microscopic mechanisms of cooperative communications within single nanocatalysts

“…The intrinsic heterogeneity of nanocatalysts, however, complicates understanding the mechanisms of chemical processes in these systems using ensemble bulk measurements. More microscopic information can be obtained from single-molecule experimental approaches ( 8 – 14 ). These studies showed that there is a wide distribution in the rates of product formations and dissociations from the nanocatalysts.…”

Diffusion in Nanoporous Solids in the Focus of IUPAC – A Tribute to Jens Weitkamp

“…The ergodic theorem [110] implies that, under equilibrium conditions, the time and ensemble averages, i.e., averages taken over one and the same particle during subsequent time intervals or for many particles within the same time interval, yield identical mean values. This hypothesis has been experimentally proved for guest diffusion in nanoporous glass [111] by the combined application of SPT [90] and PFG NMR [88] As a consequence of the mutual interaction between the guest molecules, the difference in the physical situations as considered under the measurement of transport and selfdiffusion gives rise also to differences in the diffusivities. It is by this very reasoning that these differences should be expected to vanish in the limit of sufficiently small concentrations, i.e., under conditions where the interaction between the guest molecules becomes negligibly small -and distinction between equilibrium and non-equilibrium conditions becomes anyway meaningless [112].…”

“…Similar information can be provided by the application of microimaging [91,127] via IR microscopy [91,127,128]. More extensive information about the process of catalytic conversion is provided by SPT [90], enabling monitoring of even the individual reaction path ways [129][130][131].…”

“…Their most prominent representative is the pulsed field gradient (PFG) NMR technique [88,89]. Its key information, the probability distribution of molecular displacements during an observation time of milliseconds to seconds, is based on the observation of incredibly large molecular ensembles (on the order of 10 10 ), in contrast to single-molecule tracking (SMT [90]), by which one is able to trace diffusion paths of the individual molecules. Changes in the [96], NMR Imaging (MRI) [97], X-Ray Computed Tomography (XCT) [98] concentration profiles of the guest molecules within the individual crystals (rather than the positions of the individual molecules) are the focus of microimaging [91].…”

“…Note that equilibrium techniques may also be applied under non-equilibrium conditions. Typical examples include the application of PFG NMR [88] and SPT [90] during transient uptake and release, just as also during chemical conversion. Vice versa, also non-equilibrium techniques may be applied to determine parameters defined under equilibrium conditions.…”