Dynamic Nuclear Polarisation Enhanced Solid-State Nuclear Magnetic Resonance Studies of Surface Modification of γ-Alumina

Dynamic nuclear polarisation (DNP) gives large (>100-fold) signal enhancements in solid-state nuclear magnetic resonance (solid-state NMR) spectra via the transfer of spin polarisation from unpaired electrons from radicals implanted in the sample. This means that the detailed information about local molecular environment available for bulk samples from solid-state NMR spectroscopy can now be obtained for dilute species, such as sites on the surfaces of catalysts and catalyst supports. In this paper we describe a DNP-enhanced solid-state NMR study of the widely used catalyst γ-alumina which is often modified at the surface by the incorporation of alkaline earth oxides in order to control the availability of catalytically active penta-coordinate surface Al sites. DNP-enhanced ²⁷Al solid-state NMR allows surface sites in γ-alumina to be observed and their ²⁷Al NMR parameters measured. In addition changes in the availability of different surface sites can be detected after incorporation of barium oxide.