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Studies of Materials and Catalysts by Dynamic Nuclear Polarization Enhanced Solid-state NMR

Periodic Reporting for period 1 - DNPMAT (Studies of Materials and Catalysts by Dynamic Nuclear Polarization Enhanced Solid-state NMR)

Reporting period: 2017-05-01 to 2019-04-30

Nuclear magnetic resonance (NMR) is a powerful technique for studying the molecular-level detail of complex structures, such as advanced materials with technological uses, such as energy storage or catalysis. However, even with the high magnetic fields available today, NMR suffers from low sensitivity due to the small nuclear spin polarizations involved, so that long acquisitions or large samples are required. This problem is overwhelming for dilute species and limits the ability of NMR to draw conclusions about technologically important issues such as the nature of binding sites or the adhesion of adsorbates to surfaces. However, weak NMR signals can be enhanced at low temperatures (100 K) by dynamic nuclear polarization (DNP) in which the large electron spin polarization from an implanted radical is transferred to nearby nuclei. Recent progress with high-power microwave sources known as gyrotrons has made DNP possible at the high magnetic fields found in modern NMR instruments (up to 18.8 T), and signal enhancements > 300-fold have been achieved for frozen biomolecules, corresponding to a reduction by a factor of 100,000 in experiment time. More recently pioneering experiments on mesoporous silica with the radicals for DNP implanted from a solution impregnated into the pores obtained DNP-enhanced NMR signals for functionalizing groups at the pore surface. The sensitivity enhancement achieved with DNP means that previously unobtainable structural details can be quickly obtained by solid-state NMR, even for surfaces.
New products and devices for catalysis, energy storage or drug delivery cannot be developed without knowledge of the relationships between the structure and properties of their component materials. Molecular-level characterization is key to the rational design of new materials for technological applications with improved properties. DNP-enhanced solid-state NMR is a transformative technology offering a step-change in capability which completely overcomes the sensitivity limitations of NMR. Conventional methods for the high-resolution analysis of the surfaces of materials involve high-energy electrons or X-rays interacting with well-defined and relatively clean surfaces in a low-pressure environment. Therefore, the surface conditions during measurement differ from those prevailing in real-life applications of materials in for example catalysis or drug delivery. DNP-enhanced NMR does not suffer from these limitations, and the approach allows the power of solid-state NMR to be brought to bear for the first time on real-life technologically useful materials. The objectives of the project were the improved sample preparation protocols, new experimental approaches and proof of principle studies necessary to make DNP-enhanced solid-state NMR the method of choice for the molecular-level characterization of the surfaces of materials, including for example the catalytic converters used on vehicle exhausts.
The project was carried out at the state-of-the-art Facility for DNP-enhanced solid-state NMR operational at the University of Nottingham since November 2015 where DNP instrumentation unique in the UK is available to external users from both industry and academia. Progress towards these objectives was achieved by the combined efforts of Dr S. Chaudhari, the Marie Skłodowska-Curie fellow, an experienced researcher with a background in developing DNP, and researchers at the Nottingham involved in solid-state NMR studies of materials and catalysts.
Sample preparation is a critical aspect of DNP-enhanced solid-state NMR, but the effect on signal enhancement is not understood. For materials DNP enhancements are routinely substantially lower than the theoretical maximum limiting the utility of the technique. During the project Dr Chaudhari developed a new method of preparing samples for DNP measurements by replacing the standard glycerol/water impregnation matrix with an aqueous solution of an inorganic salt. The new matrix gave substantial improvements in DNP sensitivity via a combination of larger enhancements and reduced build-up times for a wide range of samples, including functionalized mesoporous silica materials and microcrystalline pharmaceutical compounds. A publication is in preparation about this research which will be submitted to a high-impact journal.
In addition, several proof of principle studies were carried out the demonstrate the utility of DNP, including investigations of the catalytic sites on the surface of γ-alumina which is widely used as an industrial catalyst support. Pre-treatment of the alumina surface with alkaline earth and rare earth oxides alters the availability of these sites, allowing control over the catalytic activity. The DNP-enhanced solid-state NMR results suggest that the reactive surface AlO5 environment provides a preferential nucleation site for barium, that pre-treatment does not significantly alter the local environment at the surface and possibly that the most distorted AlO5 sites are preferentially occupied. This study has been released on the Facility’s website as part of measures to disseminate the utility of DNP-enhanced solid-state NMR to a wide scientific audience.
Before the planned end of the action Dr Chaudhari left Nottingham to take up a permanent post in Mysore, India and so not all the objectives were fully achieved. Nevertheless, substantial progress was made, especially towards the development of new protocols for DNP sample preparation. The new matrix is already routinely in use at the Nottingham DNP Facility by both internal researchers and external visitors alike, and has resulted in an improvement in DNP enhancement for a diverse range of systems from catalyst materials to pharmaceutical ingredients.
Proof of Principle Study on Alumina