DNP-enhanced Solid-state NMR Studies of Catalysts

固态核磁共振（NMR）是研究从非均质材料到生物分子的广泛系统的分子结构和动力学的有力方法。固态NMR的灵敏度低，因为即使在高磁场下也涉及小的核自旋极化，因此需要长的采集时间或大的样品体积。对于稀释的物质，灵敏度的问题变得非常突出，因此通常无法测量表面吸附物、界面分子或天然丰度较低的同位素。弱NMR信号可以通过动态核极化（DNP）来增强，动态核极化涉及将电子自旋极化从注入样品的自由基转移到附近的原子核。这一过程需要电子塞曼跃迁的饱和，直到最近，由于缺乏高频、高功率微波源，这一过程一直局限于低磁场。然而，陀螺仪设计的最新发展使在高达800 MHz的1H NMR频率下工作的DNP光谱仪成为可能。与DNP获得的实质性增强（高达300倍），使NMR研究的稀释物种可行的第一次，并已促使新的NMR应用程序的表面和材料是多孔或微粒的微米到纳米级。在未来，DNP增强的实验将极大地改变固态核磁共振研究的范围广泛的技术上有用的材料与应用在气体储存和隔离，治疗输送，多相催化，和组织工程。化学，物理和生命科学学院在诺丁汉最近获得了300万英镑的资金，建立一个DNP增强的固态核磁共振设施在诺丁汉，仪器将于2015年6月交付。在与Johnson-Matthey的合作中，DNP增强固态NMR将应用于氧化物表面负载过渡金属催化剂的结构和机理研究。催化剂是具有挑战性的系统，研究固体核磁共振，因为低浓度的活性位点和吸附分子的支持表面。用DNP获得的大量信号增强将允许使用17 O NMR对氧化物载体中的表面位点进行自然丰度调查，使用15 N NMR对NOx捕获分子进行自然丰度调查，以及使用13 C NMR对吸附的有机分子进行自然丰度调查。此外，氧化铝载体上的有机吸附物和表面位点之间的相互作用将使用13 C-27 Al相关实验进行研究。最后，DNP增强的固态NMR提供了一个直接研究重要的催化金属，如钌（通过99 Ru NMR），其具有固有的低NMR灵敏度的机会。

Solid-state nuclear magnetic resonance (NMR) is a powerful method for studying the molecular structure and dynamics of a broad range of systems from heterogeneous materials to biological molecules. Solid-state NMR suffers from low sensitivity, because of the small nuclear spin polarizations involved even with high magnetic fields, and so long acquisition times or large sample volumes are required. The problem of sensitivity becomes overwhelming for dilute species, so that measurements of adsorbates on surfaces, molecules at interfaces or isotopes with low natural abundance are often impossible.Weak NMR signals can be enhanced by dynamic nuclear polarization (DNP), which involves transfer of electron spin polarization from radicals implanted in the sample to nearby nuclei. This process requires the saturation of the electronic Zeeman transitions and until recently has been limited to low magnetic fields because of the lack of high-frequency, high-power microwave sources. However, recent developments in the design of gyrotrons have made DNP spectrometers operating at 1H NMR frequencies up to 800 MHz possible. The substantial enhancements (up to 300-fold) obtained with DNP make NMR studies of dilute species feasible for the first time and have already prompted new NMR applications to surfaces and materials which are porous or particulate on the micro- to nanoscale. In the future DNP-enhanced experiments will dramatically transform solid-state NMR studies of a broad range of technologically useful materials with applications in gas storage and sequestration, therapeutic delivery, heterogeneous catalysis, and tissue engineering.The Schools of Chemistry, Physics and Life Sciences at Nottingham have recently obtained £3M of funding to establish a DNP-enhanced solid-state NMR Facility at Nottingham, and the instrumentation will be delivered in June 2015.In this collaboration with Johnson-Matthey DNP-enhanced solid-state NMR will be applied to structural and mechanistic studies of catalysts consisting of transition metals supported on oxide surfaces. Catalysts are challenging systems to study by solid-state NMR because of the low concentration of active sites and adsorbate molecules on the support surface. The substantial signal enhancements obtained with DNP will allow natural abundance investigations of surface sites in oxide supports using 17O NMR, of NOx trap molecules using 15N NMR, and of adsorbed organic molecules using 13C NMR. In addition, interactions between organic adsorbates on alumina supports and surface sites will be studied using 13C-27Al correlation experiments. Finally, DNP-enhanced solid-state NMR offers an opportunity to directly study important catalytic metals such as ruthenium (via 99Ru NMR) which have intrinsically low NMR sensitivities.