In the future, (electro‐)chemical catalysts will have to be more tolerant towards a varying supply of energy and raw materials. This is mainly due to the fluctuating nature of renewable energies. For example, power‐to‐chemical processes require a shift from steady‐state operation towards operation under dynamic reaction conditions. This brings along a number of demands for the design of both catalysts and reactors, because it is well‐known that the structure of catalysts is very dynamic. However, in‐depth studies of catalysts and catalytic reactors under such transient conditions have only started recently. This requires studies and advances in the fields of 1) operando spectroscopy including time‐resolved methods, 2) theory with predictive quality, 3) kinetic modelling, 4) design of catalysts by appropriate preparation concepts, and 5) novel/modular reactor designs. An intensive exchange between these scientific disciplines will enable a substantial gain of fundamental knowledge which is urgently required. This concept article highlights recent developments, challenges, and future directions for understanding catalysts under dynamic reaction conditions.

Rapid motion of electrolyte ions is a crucial requirement to ensure the fast charging/discharging and the high power densities of supercapacitor devices. This motion is primarily determined by the pore size and connectivity of the used porous carbon electrodes. Here, the diffusion characteristics of each individual electrolyte component, that is, anion, cation, and solvent confined to model carbons with uniform and well‐defined pore sizes are quantified. As a result, the contributions of micropores, mesopores, and hierarchical pore architectures to the overall transport of adsorbed mobile species are rationalized. Unexpectedly, it is observed that the presence of a network of mesopores, in addition to smaller micropores—the concept widely used in heterogeneous catalysis to promote diffusion of sorbates—does not necessarily enhance ionic transport in carbon materials. The observed phenomenon is explained by the stripping off the surrounding solvent shell from the electrolyte ions entering the micropores of the hierarchical material, and the resulting enrichment of solvent molecules preferably in the mesopores. It is believed that the presented findings serve to provide fundamental understanding of the mechanisms of electrolyte diffusion in carbon materials and depict a quantitative platform for the future designing of supercapacitor electrodes on a rational basis.

The equilibrium and dynamic properties of fluids confined to mesoporous material have been studied using nuclear magnetic resonance (NMR) methods. Molecular diffusion of n -pentane in Vycor porous glass within closed sample tubes has been measured by means of the pulsed field gradient (PFG) NMR method for temperatures notably exceeding the boiling point of the neat liquid. It is found that the temperature dependence of the diffusivity dramatically depends on the state of the fluid surrounding the mesoporous monoliths. In an oversaturated sample, i.e., in a sample containing some amount of the liquid also outside of the porous material, the diffusivity in the mesopores followed the Arrhenius dependence. In samples with only the mesopores saturated by the liquid, i.e., without any excess fluid, with increasing temperature the diffusivity notably deviated from the Arrhenius dependence towards higher diffusivities. The analysis of the intensities of the respective NMR signals from the fluid within the porous material and in the surrounding phase has revealed that this anomaly is accompanied by the formation of a space free of liquid within the pore system. With the measured pore filling factors, the resulting overall diffusivity is estimated by a two-region approach with diffusion occurring in either the liquid phase or the free space within the pore volume. It is shown that this procedure, free of any fitting parameters, yields excellent agreement with the experimental data.

Xenon-129 pulsed field gradient NMR and hyperpolarized xenon-129 spin tracer exchange NMR experiments were performed on polycrystalline LAla-L-Val (AV) and L-Val-L-Ala (VA) nanochannels under identical conditions of temperature and Xe pressure. Displacements of up to 3 μm were measured in specimens with mean crystallite lengths of around 50−100 μm. The combination of the two NMR techniques yielded the most definitive evidence for molecular single file diffusion to date, in which the time-scaling of the mean squared displacement is proportional to the square-root of time. The PFG-NMR echo attenuation data yield single-file mobility factors of 6 ± 0.7 × 10 −13 m 2 s −1/2 and 4.4 ± 0.2 × 10 −13 m 2 s −1/2 . The results establish Xe in AV nanochannels, in particular, as an ideal experimental model system for fundamental studies of single-file diffusion dynamics. SECTION: Physical Processes in Nanomaterials and Nanostructures D iffusive dynamics in single-file channel-particle systems has been the subject of numerous experimental studies over the past few decades and is among the key topics of modern transport theory. 1−12 Universal characteristics of diffusion in single-file channel-particle systems of all length scales include conservation of the sequential order of the particles and suppressed transport rate due to steric interactions. Instead of the normal proportionality of the mean squared displacement (MSD) to the diffusion time (i.e., ⟨z 2 ⟩ = 2Dt) that is observed for Fickian diffusion (FD), the MSD in single-file diffusion (SFD) increases as the square-root of time: 13−15The single-file mobility F is a function of the fractional occupancy θ and temperature. While eq 1 has been validated in various macroscopic single-file channel-particle contrivances, 16−19 observations of molecular diffusion in single-file systems are rare, 20−25 and in some cases controversial. 20,21,25−28 The first reported observation of molecular SFD utilized pulsed field gradient (PFG) NMR to measure the MSD of CH 4 and CF 4 loaded into several types of zeolites with unidimensional channel structure. 20,21,26 Inconsistent observations on nominally the same materials were attributed to the occurrence of high defect densities and imperfections at the channel openings, allowing mutual particle passages to occur. Quasielastic neutron scattering (QENS) and the zero length column (ZLC) tracer exchange were also applied to study CH 4 diffusion in AlPO 4 -5 channels. These techniques, which operate on very different length-scales than does PFG-NMR, revealed FD for observation times in the 10 −13 −10 −9 s and 0.1−1 s time windows, respectively. PFG-NMR echo attenuation data indicative of SFD of H 2 O in single-wall carbon nanotubes was recently reported, 25 but as noted in ref 27, the measured displacements are too large to be consistent with SFD in a finite single-file system with blocked boundaries. An alternate interpretation of the data in terms of normal diffusion in curvilinear channels has been debated. 27,28 Evidence for singlefile diff...

Urea is a versatile building block that can be modified to self-assemble into a multitude of structures. One-dimensional nanochannels with zigzag architecture and cross-sectional dimensions of only ∼3.7 Å × 4.8 Å are formed by the columnar assembly of phenyl ether bis-urea macrocycles. Nanochannels formed by phenylethynylene bis-urea macrocycles have a round cross-section with a diameter of ∼9.0 Å. This work compares the Xe atom packing and diffusion inside the crystalline channels of these two bis-ureas using hyperpolarized Xe-129 NMR. The elliptical channel structure of the phenyl ether bis-urea macrocycle produces a Xe-129 powder pattern line shape characteristic of an asymmetric chemical shift tensor with shifts extending to well over 300 ppm with respect to the bulk gas, reflecting extreme confinement of the Xe atom. The wider channels formed by phenylethynylene bis-urea, in contrast, present an isotropic dynamically average electronic environment. Completely different diffusion dynamics are revealed in the two bis-ureas using hyperpolarized spin-tracer exchange NMR. Thus, a simple replacement of phenyl ether with phenylethynylene as the rigid linker unit results in a transition from single-file to Fickian diffusion dynamics. Self-assembled bis-urea macrocycles are found to be highly suitable materials for fundamental molecular transport studies on micrometer length scales.

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Crystalline solids composed of one-dimensional channels with cross-sectional dimensions below 1 nm represent an intriguing class of materials with important potential applications. A key characteristic for certain applications is the average open channel persistence length, i.e., the ensemble average distance from a channel opening to the first obstruction. This paper introduces an NMR-based methodology to measure this quantity. The protocol is applied to polycrystalline specimens of two different dipeptide nanotubes: l-Ala-l-Val and its retro-analog l-Val-l-Ala. Persistence lengths derived from the NMR measurements are found to be comparable to the typical crystallite dimensions seen in scanning electron microscopy (SEM) images, indicating that the crystals of these AV and VA specimens are essentially hollow with practically no blockages. Applications of the method to an AV sample that has been pulverized in a mortar and pestle showed that the open channel persistence length was reduced from 50 to 6.6 μm, consistent with the crystallite sizes observed in SEM images.