DNP-enhanced Solid-state NMR Studies of Catalysts

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.