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

核磁共振波谱是表征表面原子尺度结构的首选方法，但核磁共振的检测极限太低，无法对许多现代材料进行检测。由于它提供了戏剧性的灵敏度增强，固态动态核极化（DNP）核磁共振目前正在成为研究以前无法使用核磁共振的样品的强大工具，特别是使用DNP SENS （DNP表面增强核磁共振波谱）的方法选择性地增强来自表面的核磁共振信号。绝大多数核磁共振活性同位素受到四极相互作用。对于具有四极探针核的材料的研究，高磁场的使用在分辨率和灵敏度上都提供了二次增益。然而，大多数现代DNP实验依赖于极化转移方案，该方案的效率与磁场强度B0的反比成比例，并且在快速魔角旋转（MAS）下显著降低。该项目旨在通过探索DNP表面增强四极核磁共振波谱在非常高磁场和快速MAS下的新领域，实现新的光谱方法，用于具有挑战性的表面的原子尺度结构表征。该项目涉及表征工具和材料科学领域的几个基础研究问题。特别是，它将解决以下挑战：i)为高场DNP引入创新的样品制备策略，预计将优于今天的协议。ii)在DNP条件下实现先进的固态四极核磁共振方法。在这里，我们的目标是实现和深入的性能分析j基和偶极基异核（1H-27Al/29Si-27Al）和同核（27Al-27Al）相关技术，以及质子探测实验的发展。iii)铝硅酸盐的表面和界面表征。开发的样品配方方法和核磁共振技术将应用于非晶硅-氧化铝的表征，其中迄今为止不可见但催化作用重要的高活性27Al表面位点的结构在非均相催化领域仍然存在争议。材料表面的这种原子尺度的结构细节通常不适合任何其他技术。该项目的总体结果将提供创新的光谱工具，以前所未有的灵敏度和分辨率探测表面分子结构。该提案将依靠里昂在9.4和18.8 T下提供的独特的DNP仪器，在非常高磁场下DNP自由基设计的最新进展，以及固态核磁共振和材料科学领域的一流联盟。这是一个及时而雄心勃勃的项目，将直接扩展该光谱学的应用领域，并实质性地促进催化的基础知识。

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.