Pulsed field gradient (PFG) NMR in combination with quasielastic neutron scattering (QENS) was used to investigate self-diffusion of water and acetone in Nafion membranes with and without immobilized vanillic acid (VA). Complementary characterization of these membranes was performed by small angle X-ray scattering (SAXS) and NMR relaxometry. This study was motivated by the recent data showing that an organic acid, such as VA, in Nafion can preserve its catalytic activity in the presence of water even at high intra-polymer water concentrations corresponding up to 100% ambient relative humidity. However, there is currently no clear understanding of how immobilized organic acid molecules influence the microscopic transport properties and related structural properties of Nafion. Microscopic diffusion data measured by PFG NMR and QENS are compared for Nafion with and without VA. For displacements smaller than the micrometer-sized domains previously reported for Nafion, the VA addition was not observed to lead to any significant changes in the water and/or acetone self-diffusivity measured by each technique inside Nafion. However, the reported PFG NMR data present evidence of a different influence of acetone concentration in the membranes with and without VA on the water permeance of the interfaces between neighboring micrometer-sized domains. The reported diffusion data are correlated with the results of SAXS structural characterization and NMR relaxation data for water and acetone.
more »
« less
This content will become publicly available on July 26, 2024
Self‐Diffusion of a Chemical Warfare Agent Simulant and Water in Nafion by Pulsed Field Gradient NMR
Pulsed field gradient (PFG) NMR at high magnetic field was used to study microscopic diffusion of dimethyl methyl phosphonate (DMMP), a common chemical warfare agent (CWA) simulant, and water in Nafion membranes. PFG NMR measurements were performed for a broad range of molecular displacements. The self‐diffusivities were measured as a function of the DMMP concentration for several fixed water concentrations. The measured data suggest that DMMP and water diffuse in different regions of Nafion. While water mostly diffuses in hydrophilic regions of the membrane, viz. water channels, DMMP diffusion is mostly limited to interfacial perfluoroether regions between these water channels and the semi‐crystalline matrix.
more » « less- Award ID(s):
- 1836551
- NSF-PAR ID:
- 10435587
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Chemie Ingenieur Technik
- Volume:
- 95
- Issue:
- 11
- ISSN:
- 0009-286X
- Format(s):
- Medium: X Size: p. 1741-1747
- Size(s):
- ["p. 1741-1747"]
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Pulsed field gradient (PFG) NMR was used in combination with single crystal IR microscopy (IRM) to study diffusion of ethane inside crystals of a mixed linker zeolitic imidazolate framework (ZIF) of the type ZIF-7-8 under comparable experimental conditions. These crystals contain 2-methylimidazolate (ZIF-8 linker) and benzimidazolate (ZIF-7 linker). It was observed that the PFG NMR attenuation curves measured for ethane in ZIF-7-8 exhibit deviations from the monoexponential behaviour, thereby indicating that the ethane self-diffusivity in different crystals of a crystal bed can be different. Measurements of the ethane uptake curves performed by IRM under the same conditions in different ZIF-7-8 crystals of the bed yield different transport diffusivities thus confirming that the rate of ethane diffusion is different in different ZIF-7-8 crystals. The IRM observation that the fractions of ZIF-8 and ZIF-7 linkers are different in different ZIF-7-8 crystals allowed attributing the observed heterogeneity in diffusivities to the heterogeneity in the linker fraction. The quantitative comparison of the average ethane self-diffusivities measured by PFG NMR in ZIF-7-8 with the corresponding data on corrected diffusivities from IRM measurements revealed a good agreement between the results obtained by the two techniques. In agreement with the expectation of smaller aperture sizes in ZIF-7-8 than in ZIF-8, the average ethane self-diffusivities in ZIF-7-8 were found to be significantly lower than the corresponding self-diffusivities in ZIF-8.more » « less
-
High magnetic fields (up to 17.6 T) in combination with large magnetic field gradients (up to 25 T/m) were successfully utilized in pulsed field gradient (PFG) NMR studies of gas and liquid diffusion in nanoporous materials. In this mini-review, we present selected examples of such studies demonstrating the ability of high field PFG NMR to gain unique insights and differentiate between various types of diffusion. These examples include identifying and explaining an anomalous relationship between molecular size and self-diffusivity of gases in a zeolitic imidazolate framework (ZIF), as well as revealing and explaining an influence of mixing different linkers in a ZIF on gas self-diffusion. Different types of normal and restricted self-diffusion were quantified in hybrid membranes formed by dispersing ZIF crystals in polymers. High field PFG NMR studies of such membranes allowed observing and explaining an influence of the ZIF crystal confinement in a polymer on intra-ZIF self-diffusion of gases. This technique also allowed measuring and understanding anomalous single-file diffusion (SFD) of mixed sorbates. Furthermore, the presented examples demonstrate a high potential of combining high field PFG NMR with single-crystal Infrared Microscopy (IRM) for obtaining greater physical insights into the studied diffusion processes.more » « less
-
more » « less
Abstract In this article we shed light on newly emerging perspectives to characterize and understand the interplay of diffusive mass transport and surface catalytic processes in pores of gas phase metal catalysts. As a case study, nanoporous gold, as an interesting example exhibiting a well-defined pore structure and a high activity for total and partial oxidation reactions is considered. PFG NMR (pulsed field gradient nuclear magnetic resonance) measurements allowed here for a quantitative evaluation of gas diffusivities within the material. STEM (scanning transmission electron microscopy) tomography furthermore provided additional insight into the structural details of the pore system, helping to judge which of its features are most decisive for slowing down mass transport. Based on the quantitative knowledge about the diffusion coefficients inside a porous catalyst, it becomes possible to disentangle mass transport contributions form the measured reaction kinetics and to determine the kinetic rate constant of the underlying catalytic surface reaction. In addition, predictions can be made for an improved effectiveness of the catalyst, i.e., optimized conversion rates. This approach will be discussed at the example of low-temperature CO oxidation, efficiently catalysed by npAu at 30 °C. The case study shall reveal that novel porous materials exhibiting well-defined micro- and mesoscopic features and sufficient catalytic activity, in combination with modern techniques to evaluate diffusive transport, offer interesting new opportunities for an integral understanding of catalytic processes.
Graphical Abstract -
more » « less
Abstract Airborne observations in deep stratiform and anvil clouds showed extensive layers of 10‐ to 40‐kV/m electric fields colocated with highly stratified uniform radar reflectivity of 20 to 25 dBZ from 5‐ to 9‐km altitude. Size distributions with numerous small‐ and intermediate‐sized ice crystals (mostly aggregates) and large aggregates were observed in these regions. We infer that the uniform electric field, radar reflectivity, and broad particle size distributions were the result of mesoscale updrafts, confirmed by high‐resolution images of particles showing diffusional growth and no riming. No measurable supercooled liquid water was found in these regions from −10 to −45 °C. Calculated particle collision rates from observed distributions were >5 × 103collisions per cubic meter per second in this volume. Laboratory results show that weak charge separation occurs when ice particles collide and separate even in the absence of supercooled water. We infer that charge separation occurred in the mesoscale updrafts via a noninductive mechanism in which ice particles growing by diffusion collide and transfer charge without supercooled water being present. These regions with strong, uniform fields, stratified radar reflectivity, and broad size distributions also occurred in anvils that barely reached the melting zone. Thus, we deduce that the nonriming collisional mechanism acts at middle to upper cloud levels and is not dependent upon electrification occurring near the melting zone. This mechanism should produce two oppositely charged layers of charge with the top layer residing on smaller particles often existing near the top of the cloud.
