New Experimental Approaches for DNP Surface Enhanced Quadrupolar NMR Spectroscopy at Very High Magnetic Field

• Applicant: Monu Kaushik, Ph.D.
• Subject Area: Physical Chemistry of Solids and Surfaces, Material Characterisation
• Term: from 2019 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 426792711

NMR spectroscopy is the method of choice to characterize the atomic-scale structure of surfaces whenever possible, but the detection limit of NMR is far too low to allow many modern materials to be examined. Because it provides dramatic sensitivity enhancement, solid-state Dynamic Nuclear Polarization (DNP) NMR is currently emerging as a powerful tool to study samples previously inaccessible to NMR, and notably to selectively enhance the NMR signals from surfaces using an approach called DNP SENS (DNP Surface Enhanced NMR Spectroscopy). A vast majority of the NMR-active isotopes are subjected to quadrupolar interaction. For investigation of materials with quadrupolar probe nuclei, the use of high magnetic fields provide a quadratic gain in both resolution and sensitivity. However, most modern DNP experiments rely on a polarization transfer scheme whose efficiency scales with the inverse of the magnetic field strength B0 and is significantly reduced at fast Magic Angle Spinning (MAS). This project aims at implementing new spectroscopic approaches for the atomic-scale structural characterization of challenging surfaces by exploring new frontiers of DNP surface enhanced quadrupolar NMR spectroscopy at very high magnetic field and fast MAS. The project tackles several basic research issues in the field of characterization tools and in materials science. In particular, it will address the following challenges: i) the introduction of innovative sample preparation strategies for high-field DNP that are expected to outperform today’s protocols. ii) the implementation of advanced solid-state quadrupolar NMR methods under DNP conditions. Here we aim for implementation and the in-depth performance analysis of both J-based and dipolar based heteronuclear (1H-27Al/29Si-27Al) and homonuclear (27Al-27Al) correlation techniques, as well as the development of proton-detected experiments. iii) Characterization of surface and interface of aluminosilicates. The developed approaches for sample formulation and NMR techniques will be applied to the characterization of amorphous silica-alumina, for which the structure of the so far invisible but catalytically important highly reactive 27Al surface sites remains a matter of debate in the field of heterogeneous catalysis. Such atomic-scale structure details of surfaces in materials are usually not amenable to any other technique. The overall results of this project will provide innovative spectroscopic tools to probe with unprecedented sensitivity and resolution of molecular structures of surfaces. This proposal will lean on a unique DNP instrumentation available in Lyon at 9.4 and 18.8 T, on recent advanced in the design of radicals for DNP at very high magnetic field and on a first-class, consortium in solid-state NMR and materials science. This is a timely and ambitious project that will directly extend the areas of application of this spectroscopy, and substantially contribute to fundamental knowledge in catalysis.

• DFG Programme: Research Fellowships
• International Connection: France
• Host: Privatdozentin Anne Lesage, Ph.D.