Dynamic nuclear polarization enhanced solid-state NMR studies of surface modification of gamma-alumina

Dynamic nuclear polarization (DNP) gives large (>100-fold) signal enhancements in solid-state NMR spectra via the transfer of spin polarization from unpaired electrons from radicals implanted in the sample. This means that the detailed information about local molecular environment available for bulk samples from solid-state NMR spectroscopy can now be obtained for dilute species, such as sites on the surfaces of catalysts and catalyst supports. In this paper we describe a DNP-enhanced solid-state NMR study of the widely used catalyst gamma-alumina which is often modified at the surface by the incorporation of alkaline earth oxides in order to control the availability of catalytically active penta-coordinate surface Al sites. DNP-enhanced 27Al solid-state NMR allows surface sites in gamma-alumina to be observed and their 27Al NMR parameters measured. In addition changes in the availability of different surface sites can be detected after incorporation of BaO

Ion exchange and binding in selenium remediation materials using DNP-enhanced solid-state NMR spectroscopy

Selenate-loaded selenium water remediation materials based on polymer fibres have been investigated by dynamic nuclear polarization (DNP) enhanced solid-state NMR. For carbon-13 a significant reduction in experiment time is obtained with DNP even when compared with conventional carbon-13 NMR spectra recorded using larger samples. For the selenium remediation materials studied here this reduction allows efficient acquisition of {1H}-77Se heteronuclear correlation spectra which give information about the nature of the binding of the remediated selenate ions with the grafted side chains which provide the required ion exchange functionality

Surface Sensitive NMR Detection of the SEI Layer on Reduced Graphene Oxide

The solid electrolyte interphase (SEI) is detrimental for rechargeable batteries performance and lifetime. Understanding its formation requires analytical techniques that provide molecular level insight. Here dynamic nuclear polarization (DNP) is utilized for the first time for enhancing the sensitivity of solid state NMR (ssNMR) spectroscopy to the SEI. The approach is demonstrated on reduced-graphene oxide (rGO) cycled in Li-ion cells in natural abundance and 13C-enriched electrolyte solvents. Our results indicate that DNP enhances the signal of outer SEI layers, enabling detection of natural abundance 13C spectra from this component of the SEI at reasonable timeframes. Furthermore, 13C- enriched electrolytes measurements at 100K provide ample sensitivity without DNP due to the vast amount of SEI filling the rGO pores, thereby allowing differentiating the inner and outer SEI layers composition. Developing this approach further will benefit the study of many electrode materials, equipping ssNMR with the needed sensitivity to efficiently probe the SEI.The work was supported by a research grant from Dana and Yossie Hollander, the Alon fellowship from Israel council of higher education and partially by the Israel Science Foundation (ISF) in the framework of the INREP project (M.L.). This project has received funding from the European Unions’s Horizon 2020 research and innovation programme under Grant Agreement No. 696656 – GrapheneCore1 (G.K. and C.P.G.). We thank Dr. Wanjing Yu (Central South University, China) for graphene synthesis and related discussions. G.K. thanks Dr. Duhee Yoon (Cambridge Graphene Centre) for Raman measurements and helpful discussions. The research is made possible in part by the historic generosity of the Harold Perlman family. We thank Dr. Frederic Mentink-Vigier for helpful suggestions. DNP experiments at 14.1 T were performed at the DNP MAS NMR Facility at the University of Nottingham, with thanks to the EPSRC for funding of pilot studies (EP/L022524/1)

Dynamics in Flexible Pillar[n]arenes Probed by Solid-State NMR

[Image: see text] Pillar[n]arenes are supramolecular assemblies that can perform a range of technologically important molecular separations which are enabled by their molecular flexibility. Here, we probe dynamical behavior by performing a range of variable-temperature solid-state NMR experiments on microcrystalline perethylated pillar[n]arene (n = 5, 6) and the corresponding three pillar[6]arene xylene adducts in the 100–350 K range. This was achieved either by measuring site-selective motional averaged (13)C (1)H heteronuclear dipolar couplings and subsequently accessing order parameters or by determining (1)H and (13)C spin–lattice relaxation times and extracting correlation times based on dipolar and/or chemical shift anisotropy relaxation mechanisms. We demonstrate fast motional regimes at room temperature and highlight a significant difference in dynamics between the core of the pillar[n]arenes, the protruding flexible ethoxy groups, and the adsorbed xylene guest. Additionally, unexpected and sizable (13)C (1)H heteronuclear dipolar couplings for a quaternary carbon were observed for p-xylene adsorbed in pillar[6]arene only, indicating a strong host–guest interaction and establishing the p-xylene location inside the host, confirming structural refinements

Crystal Structure Directed Catalysis by Aluminum Metal-Organic Framework: Mechanistic Insight into the Role of Coordination of Al Sites and Entrance Size of Catalytic Pocket

© 2020 American Chemical Society. The use of metal-organic frameworks (MOFs) in the field of catalysis is growing exponentially due to their high surface area and distinctive active sites. In this work, we report a novel understanding of the active sites responsible for the catalytic activity of aluminum trimesate MOFs and their crystal/framework structure dependency. Here, we have studied the nature of the active sites of Al-MOFs with two different framework structures (MIL-100 and MIL-96). We found that the MOFs with MIL-100 framework structures were highly catalytically active, while the same Al-MOFs with MIL-96 framework structures exhibited poor catalytic activity. This behavior is explained based on the effect of coordinated water molecules on their Brønsted acidity as well as the effect of the coordination of Al sites, specifically hexacoordinated Al3+6c sites and pentacoordinated Al3+5c sites, on their Lewis acidity. The different entrance sizes of the catalytic pocket of MIL-96 and MIL-100 also played critical roles in their catalytic performance

Strategies for 1H-Detected Dynamic Nuclear Polarization Magic-Angle Spinning NMR Spectroscopy

© 2020 The Authors. Published by Wiley-VCH GmbH Combining dynamic nuclear polarization with proton detection significantly enhances the sensitivity of magic-angle spinning NMR spectroscopy. Herein, the feasibility of proton-detected experiments with slow (10 kHz) magic angle spinning was demonstrated. The improvement in sensitivity permits the acquisition of indirectly detected 14N NMR spectra allowing biomolecular structures to be characterized without recourse to isotope labelling. This provides a new tool for the structural characterization of environmental and medical samples, in which isotope labelling is frequently intractable

DNP-NMR of surface hydrogen on silicon microparticles

Dynamic nuclear polarization (DNP) enhanced nuclear magnetic resonance (NMR) offers a promising route to studying local atomic environments at the surface of both crystalline and amorphous materials. We take advantage of unpaired electrons due to defects close to the surface of the silicon microparticles to hyperpolarize adjacent 1H nuclei. At 3.3 T and 4.2 K, we observe the presence of two proton peaks, each with a linewidth on the order of 5 kHz. Echo experiments indicate a homogeneous linewidth of 150 - 300 Hz for both peaks, indicative of a sparse distribution of protons in both environments. The high frequency peak at 10 ppm lies within the typical chemical shift range for proton NMR, and was found to be relatively stable over repeated measurements. The low frequency peak was found to vary in position between −19 and −37 ppm, well outside the range of typical proton NMR shifts, and indicative of a high-degree of chemical shielding. The low frequency peak was also found to vary significantly in intensity across different experimental runs, suggesting a weakly-bound species. These results suggest that the hydrogen is located in two distinct microscopic environments on the surface of these Si particles

Identifying the Structural Basis for the Increased Stability of the Solid Electrolyte Interphase Formed on Silicon with the Additive Fluoroethylene Carbonate.

To elucidate the role of fluoroethylene carbonate (FEC) as an additive in the standard carbonate-based electrolyte for Li-ion batteries, the solid electrolyte interphase (SEI) formed during electrochemical cycling on silicon anodes was analyzed with a combination of solution and solid-state NMR techniques, including dynamic nuclear polarization. To facilitate characterization via 1D and 2D NMR, we synthesized 13C-enriched FEC, ultimately allowing a detailed structural assignment of the organic SEI. We find that the soluble poly(ethylene oxide)-like linear oligomeric electrolyte breakdown products that are observed after cycling in the standard ethylene carbonate-based electrolyte are suppressed in the presence of 10 vol% FEC additive. FEC is first defluorinated to form soluble vinylene carbonate and vinoxyl species, which react to form both soluble and insoluble branched ethylene-oxide-based polymers. No evidence for branched polymers is observed in the absence of FEC